Analog Access to the Telephone Network

Home , Analog signal, Pulse dialing, Telephone line, TXE

Telecommunications Telephony Analog Access to the Telephone Network

Courseware Sample 32964-F0

Order no.: 32964-00

First Edition Revision level: 01/2015

By the staff of Festo Didactic

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Safety and Common Symbols

The following safety and common symbols may be used in this manual and on the equipment:

Symbol Description

DANGER indicates a hazard with a high level of risk which, if not avoided, will result in death or serious injury.

WARNING indicates a hazard with a medium level of risk which, if not avoided, could result in death or serious injury.

CAUTION indicates a hazard with a low level of risk which, if not avoided, could result in miinor or moderate injury.

CAUTION used without the Caution, risk of danger sign , indicates a hazard with a potentially hazaardous situation which, if not avoided, may result in property damage.

Caution, risk of electric shhock

Caution, hot surface

Caution, risk of danger

Caution, lifting hazard

Caution, hand entanglement hazard

Notice, non-ionizing radiation

Direct current

Alternating current

Both direct and alternating current

Three-phase alternating current

Earth (ground) terminal

Safety and Common Symbols

Symbol Description

Protective conductor terminal

Frame or chassis terminal

Equipotentiality

On (supply)

Off (supply)

Equipment protected throughout by double insulation or reinforced insulation

In position of a bi-stable push control

Out position of a bi-stable push control

We invite readers of this manual to send us their tips, feedback and suggestions for improving the book.

Please send these to [email protected]. The authors and Festo Didactic look forward to your comments. Table of Contents

Introduction ...... V

Courseware Outline

Analog Access to the Telephone Network ...... VII

Central Office Operation ...... IX

Private Automatic Branch Exchange (PABX) ...... XI

PABX Analog Trunk ...... XIII

Digital Trunk...... XV

Sample Exercise Extracted from Analog Access to the Telephone Network

Ex. 2-2 Hybrid Function ...... 3

Sample Exercise Extracted from Central Office Operation

Ex. 3-1 Call Processor Functions ...... 17

Sample Exercise Extracted from Private Automatic Branch Exchange (PABX)

Ex. 1-1 Architecture of a Digital PABX ...... 31

Sample Exercise Extracted from PABX Analog Trunk

Ex. 1-2 Analog Trunk Interface ...... 55

Sample Exercise Extracted from Digital Trunk

Ex. 1-2 Digital Trunk Interface ...... 87

Other Sample Extracted from Analog Access to the Telephone Network

Unit Test ...... 117

Instructor Guide Sample Extracted from Analog Access to the Telephone Network

Unit 1 The Telephone Set ...... 121

Bibliography

III IV Introduction

The Lab-Volt Telephony Training System (TTS), Model 8086, is a powerful learning tool that allows students to study the operation of modern telephone networks and digital private automatic branch exchanges (PABX). The TTS is built upon the Reconfigurable Training Module, Model 9431. This module, which uses state-of-the- art digital signal processor (DSP) technology, can be programmed to act as different parts of a telephone network. Interface cards that students install in the training module allow connection of real analog and digital telephone sets and trunk lines. A central office (CO) is easily implemented by inserting an analog line interface card into a training module programmed to act as a central office. Similarly, a digital PABX is implemented by inserting a digital telephone interface card into a training module programmed to act as a PABX. Furthermore, simple telephone networks can be set up quickly by adding analog and digital trunk interface cards to COs and PABXs implemented with training modules, and interconnecting the modules with trunk lines. Such telephone networks allow establishment of both intra- and inter- exchange calls as well as tandem-switched calls.

A Pentium-type host computer, connected to the Reconfigurable Training Module through a high-speed data link (Ethernet link with TCP/IP protocol), runs the Lab-Volt Telephony Training System (LVTTS) software. This Windows®-based software is used to download programs into the DSP memory of the Reconfigurable Training Module. The LVTTS software is also used to:

• display the functional block diagram of the telephony equipment (CO, digital PABX, etc.) implemented in the Reconfigurable Training Module, • change various system settings and options, such as the telephone ringing cadence, companding type, subscriber names and phone numbers, etc, • perform step-by-step observation of call routing sequences, • observe real signals throughout the system in both the time and frequency domains using modern virtual instruments, • insert faults in the system (password-protected feature) for troubleshooting purposes.

The TTS courseware material consists of a series of five student manuals, an instructor guide for each student manual, and a user guide. The following fields of telephony are covered in the TTS courseware:

• Analog Access to the Telephone Network • Central Office Operation • Private Automatic Branch Exchange (PABX) • PABX Analog Trunk • Digital Trunk

Each student manual covers one particular subject and is divided into several units. Each unit consists of a series of hands-on exercises dealing with certain aspects of telephony. The exercises contain a clearly stated objective, a discussion, a summary of the exercise procedure, a detailed exercise procedure, a conclusion, and a set of review questions. A ten-question test at the end of each unit allows the instructor to verify the knowledge gained by the student. Each instructor guide provides the measured results as well as the answers to all questions of each exercise in the corresponding student manual. It also provides the answers to the unit test questions. The user guide provides all the information required to set up and use the Telephony Training System.

V VI Courseware Outline

ANALOG ACCESS TO THE TELEPHONE NETWORK

Unit 1 The Telephone Set

Introduction to the public switched telephone network (PSTN). Brief description of the central office. Familiarization with the functions and operation of the analog telephone set.

Ex. 1-1 Telephone Ringing

Telephone ringing. AC ringing voltage specifications. The electronic telephone ringer circuit.

Ex. 1-2 The Telephone Switchhook and Handset

Operation of the telephone switchhook. The handset and speech circuit. Functions and operation of the speech circuit.

Ex. 1-3 Tone Dialing

Familiarization with DTMF tone dialing. Frequencies used in DTMF dialing signals.

Ex. 1-4 Pulse Dialing

Familiarization with pulse dialing. Pulse timing. Pulse dialing with an electronic-type analog telephone set.

Unit 2 The Line Interface

Role of the analog line interface. Block diagram of the analog line interface. Functions of the analog line interface (BORSCHT functions). Operation of the analog line interface.

Ex. 2-1 Battery Feed Power Supply

How electrical power is supplied to analog telephone sets. Subscriber loop interface circuit (SLIC) overcurrent protection. Equivalent electrical circuit. Maximum resistance (length) of the telephone line.

Ex. 2-2 Hybrid Function

Balanced transmitted and received signals on the local loop. Role of the hybrid function in the analog line interface. Implementing the hybrid function with electronic components.

VII Courseware Outline

ANALOG ACCESS TO THE TELEPHONE NETWORK

Ex. 2-3 Pulse Code Modulation

The coding function. Block diagram of a PCM CODEC. Voice digitization and recovery. Conversion of the PCM codes to serial format. Use of companding to improve voice digitization and recovery.

Ex. 2-4 Companding

Linear quantization and quantization noise. Voice signal-to-

quantization noise (S/NQ) ratio versus the voice signal level. Using non-linear quantization to implement companding. Comparing the

S/NQ ratio versus the voice signal level, with and without companding.

Ex. 2-5 Time-Division Multiplexing

Why use time-division multiplexing in telephone systems? Time- division multiplexing of digitized voice signals. The North American (DS1) and European (E1) multiplexing formats. Time slot assignment.

Ex. 2-6 Subscriber Signaling

Introduction to subscriber signaling. A typical subscriber signaling sequence. Telephone ringing. Telephone status (on-hook or off-hook) supervision. Denying telephone service to a subscriber.

Appendix A List of Equipment Required

Bibliography

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VIII Courseware Outline

CENTRAL OFFICE OPERATION

Unit 1 Signaling Circuit

Introduction to the operation and functions performed by the signaling circuit: hook status demultiplexing and storage, digitized DTMF dialing signal-to-data conversion, digitized call progress tone generation, and AC ringing voltage generation.

Ex. 1-1 Hook Status Demultiplexing and Storage

Familiarization with hook status demultiplexing and storage. Description of how the hook status demultiplexing and storage circuit makes hook status available for the call processor.

Ex. 1-2 Dialed Number Detection

Familiarization with dialed number detection. Description of how telephone numbers produced using either pulse or tone dialing are detected.

Ex. 1-3 Call Progress Tone and Ringing Generation

Familiarization with call progress tone and ringing signal generation.

Unit 2 Digital Switching

Introduction to digital switching circuit. Crossbar switching and step-by-step switching are also introduced.

Ex. 2-1 Time-Division Switching

Familiarization with time-division switching. Difference between time-multiplexed switching and time-division switching. Implementation of a digital time-division switch.

Ex. 2-2 Space-Division Switching

Familiarization with space-division switching. Description of step- by-step and crossbar switches. Difference between blocking and non-blocking switches. Implementation of a space-division switch. Description of the control register of the space-division switch used in the Telephony Training System.

IX Courseware Outline

CENTRAL OFFICE OPERATION

Ex. 2-3 Two-Dimensional Switching

Familiarization with two-dimensional switching. Practical considerations that limit the number of interconnections that a time- division switch or space-division switch can establish. Introduction to space-time-space (STS) and time-space-time (TST) architectures.

Unit 3 System Control

Introduction to the functions and operation of a call processor. Description of the events that take place in a central office during an intra-exchange call. Familiarization with central office configuration.

Ex. 3-1 Call Processor Functions

Familiarization with the control functions performed by the call processor during the processing of a call: system supervision, signaling, dialed telephone number reception and processing, connection control.

Ex. 3-2 Intra-Exchange Call Routing Sequence

Familiarization with a call routing sequence of control actions performed by the call processor during the processing of an intra- exchange (local) call.

Ex. 3-3 Central Office Configuration

Familiarization with central office configuration. Configuration of central office equipment so that it complies with the telephone standards of the country where it is installed.

Unit 4 Supplementary Services

Familiarization with the various supplementary services offered by today's telephone companies.

Ex. 4-1 Caller Identification

Introduction to the signaling protocol for caller identification, single data message format (SDMF) and multiple data message format (MDMF).

Appendix A List of Equipment Required Appendix B ASCII Conversion Table

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X Courseware Outline

PRIVATE AUTOMATIC BRANCH EXCHANGE (PABX)

Unit 1 Architecture and Basic Operation

Introduction to the private automatic branch exchange (PABX). Role played by the PABX in the telephone network. Introduction to the architecture and the basic operation of a PABX implemented using digital technology.

Ex. 1-1 Architecture of a Digital PABX

Familiarization with the architecture of a digital PABX (the Lab-Volt PABX). Resemblances and differences between the architecture of a digital PABX and that of a central office. Description of how two digital telephone sets are interconnected in the Lab-Volt PABX.

Ex. 1-2 Telephone Set Portability

Introduction to telephone set portability in a PABX environment. Identification (ID) number, terminal (extension) number, and line interface address. Description of how telephone set portability is achieved in the Lab-Volt PABX.

Ex. 1-3 Internal Call Establishment Procedure

Comparison between subscriber signaling in the PSTN and subscriber signaling in a modern digital PABX. Description of how signaling is performed between digital telephone sets and the call processor in the Lab-Volt PABX. Description of the signaling procedure used in the Lab-Volt PABX to control a basic internal call.

Ex. 1-4 Call Progress Indication

Introduction to call progress tone generation in a PABX environment. Description of how call progress tones are generated and routed to digital telephone sets in the Lab-Volt PABX.

Unit 2 Call Functions

Introduction to the various call functions commonly available in today's digital PABXs: call holding, multiple call control, call transfer, conference calling, and intercom.

Ex. 2-1 Call Holding and Multiple Call Control

Familiarization with the call holding function. Description of the signaling procedure used in the Lab-Volt PABX to hold and retrieve a call. Description of how multiple call control is performed in the Lab-Volt PABX, using call reference values (CRVs).

XI Courseware Outline

PRIVATE AUTOMATIC BRANCH EXCHANGE (PABX)

Ex. 2-2 Call Transfer

Familiarization with the call transfer function. Description of the signaling procedure used in the Lab-Volt PABX to transfer a call.

Ex. 2-3 Conference Calling

Familiarization with conference calling. Description of the function and basic operation of a digital conference bridge. Description of how conference calling is implemented in the Lab-Volt PABX. Description of the signaling procedure used in the Lab-Volt PABX to control conference calling.

Ex. 2-4 Intercom

Familiarization with the intercom function. Description of how the intercom function is implemented in the Lab-Volt PABX. Description of the signaling procedure used in the Lab-Volt PABX to control an intercom call.

Appendix A List of Equipment Required Appendix B ISDN Overview Appendix C Setting Up and Operating the Digital Telephone Set Appendix D Digital (ISDN) Telephone Set Block Diagram

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XII Courseware Outline

PABX ANALOG TRUNK

Unit 1 PABX Analog Trunk

Role of trunks in the public switched telephone network (PSTN). Analog versus digital trunks. Description of what an analog trunk is. Use of analog trunks to interconnect a PABX to the PSTN.

Ex. 1-1 Familiarization with the Lab-Volt PABX Analog Trunk

How to set up an analog trunk between a Lab-Volt PABX and a Lab-Volt central office. Making and receiving external calls using the digital telephone sets connected to the Lab-Volt PABX.

Ex. 1-2 Analog Trunk Interface

Role of the analog trunk interface in a PABX. The various functions of the analog trunk interface. Block diagram and operation of the analog trunk interface in the Lab-Volt PABX.

Unit 2 Call Routing Over a PABX Analog Trunk

Signaling over a PABX analog trunk. Conversion of the signaling information in the Lab-Volt PABX. Block diagram and operation of the analog trunk service circuit in the Lab-Volt PABX. Block diagram and operation of the trunk status demultiplexing and storage circuit in the Lab-Volt PABX.

Ex. 2-1 External Call Answering and Termination

Sequence of events that occurs in the Lab-Volt PABX when an external call is answered. Sequence of events that takes place in the Lab-Volt PABX when an external call is terminated.

Ex. 2-2 External Call Establishment (Overlap Sending Method)

Sequence of events that occurs in the Lab-Volt PABX when an external call is established using the overlap (conventional) sending method. Sequence of events that takes place in the Lab-Volt PABX when external call establishment fails because the PABX analog trunk is not available.

Ex. 2-3 External Call Establishment (En-Bloc Sending Method)

Sequence of events that occurs in the Lab-Volt PABX when an external call is established using the en-bloc sending method.

XIII Courseware Outline

PABX ANALOG TRUNK

Unit 3 PABX Configuration

Familiarization with the configuration of various options found in most digital PABX's.

Ex. 3-1 Configuring the Lab-Volt PABX

How to configure the Lab-Volt PABX using the host computer running the Lab-Volt Telephony Training System (LVTTS) software. Parameters related to the operation of the Lab-Volt PABX that can be configured.

Appendix A List of Equipment Required

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XIV Courseware Outline

DIGITAL TRUNK

Unit 1 Multiplexing Format and Basic Operation

Description of what a trunk is. Role of trunks in the public switched telephone network (PSTN). The evolution of trunks from the simple non- multiplexed analog trunk to today's digital trunks using the SONET/SDH technology. Multiplexing format and basic operation of digital trunks.

Ex. 1-1 Familiarization with the Lab-Volt Digital Trunk

Overview of the Lab-Volt digital trunk. How to set up a digital trunk between two CO's implemented with the Telephony Training System. What an inter-exchange call is. Making inter-exchange calls.

Ex 1-2 Digital Trunk Interface

Role of the digital trunk interface. TDM formats used in the Lab-Volt digital trunk. Simplified block diagram of the digital trunk interface used in Lab-Volt CO's. Operation of the transmitter and receiver in the digital trunk interface of Lab-Volt CO's.

Ex 1-3 Alarm Indication

Description of what alarm indication is. Role of alarm indication in digital trunks. Local alarm indication. Remote alarm indication. Illustration of common alarm situations that may occur between two Lab-Volt CO's interconnected through a digital trunk.

Unit 2 Inter-Exchange Signaling

Description of what common-channel signaling (CCS) is. Use of CCS in Lab-Volt CO's. Introduction to signaling system number 7 (SS7). Familiarization with the Integrated Services Digital Network (ISDN) signaling protocol used in Lab-Volt CO's to control inter-exchange calls established via the digital trunk.

Ex. 2-1 Outgoing Inter-Exchange Call Routing Sequence

Sequence of events that occurs in a Lab-Volt CO when an outgoing inter-exchange call is established. Sequences of events that can take place in a Lab-Volt CO when an inter-exchange call is terminated. Sequence of events that occurs in a Lab-Volt CO when establishment of an outgoing inter-exchange call fails.

XV Courseware Outline

DIGITAL TRUNK

Ex. 2-2 Incoming Inter-Exchange Call Routing Sequence

Sequence of events that occurs in a Lab-Volt CO when an incoming inter-exchange call is established. Outgoing inter- exchange call establishment versus incoming inter-exchange call establishment.

Ex. 2-3 Multiple Inter-Exchange Call Control

Description of what a call reference value (CRV) is. Understanding the mechanism that enables Lab-Volt CO's to control several inter- exchange calls established via the digital trunk.

Appendices A List of Equipment Required B ISDN Overview C Multiframe Structures of the DS1 and E1 TDM Formats

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XVI Sample Exercise Extracted from Analog Access to the Telephone Network

1 2 Exercise 2-2

Hybrid Function

EXERCISE OBJECTIVE

When you have completed this exercise, you will be able to explain why two-wire to four-wire conversion (2W/4W conversion) is required to interface an analog telephone set to the local central office. You will be able to demonstrate the 2W/4W conversion performed by the subscriber loop interface circuit (SLIC).

DISCUSSION

Introduction

To minimize the cost of the cables required to connect numerous subscribers to the telephone network, each analog telephone set is usually wired to the central office through a single pair of wires (the local loop). Since a telephone conversation is inherently bidirectional, the transmitted and received voice signals have to travel onto the local loop at the same time and in opposite directions, as shown in Figure 2-6.

BALANCED BALANCED TRANSMITTED RECEIVED VOICE SIGNAL VOICE SIGNAL

TRANSMITTED VOICE SIGNAL TRANSMITTER T

ANALOG LOCAL LOOP TO AND FROM TELEPHONE (TWO-WIRE CIRCUIT) CENTRAL OFFICE RECEIVED SET VOICE SIGNAL RECEIVER

Figure 2-6. Balanced, transmitted and received signals traveling on the local loop (two-wire circuit).

A local loop is known as a two-wire transmission circuit. The transmitted and received signals traveling on the local loop are balanced. This means that each of these signals travels on both the T and R wires of the local loop, the phase of the

3 Hybrid Function

signal on one wire being opposite to that of the signal on the other wire. The use of balanced signals on local loops provides good immunity against noise and interference.

In today's central offices, digital switching equipment is used to interconnect telephones. This type of equipment, however, uses a four-wire circuit to route the transmitted and received signals associated with a telephone conversation. Four- wire circuits are also used for links that interconnect central offices (trunks). In a four-wire circuit, one pair of wires is the transmit path and carries the transmitted voice signal, while a second pair of wires is the receive path and carries the received voice signal. The use of two separate paths for transmission and reception facilitates time-division multiplexing in the central office switching equipment as well as signal amplification in trunk circuits.

To interface a subscriber's telephone line (two-wire circuit) to the digital switching equipment of the central office (four-wire circuit), a two-wire to four-wire conversion (2W/4W conversion) must take place somewhere in the system. This conversion is performed in the analog line interface by the subscriber loop interface circuit (SLIC), which is also referred to as the subscriber line interface circuit.

Two-Wire to Four-Wire Conversion

Figure 2-7 illustrates 2W/4W conversion performed by the SLIC of a line interface. The wire at the SLIC TXA output and a wire connected to the interface's common terminal form the transmit path of a four-wire circuit. Similarly, the wire at the SLIC RXA input and another wire connected to the interface's common terminal form the receive path of the four-wire circuit. The SLIC couples the balanced transmitted signal from the telephone line (two-wire circuit) to its TXA output (the transmit path of the four-wire circuit). It also couples the signal received at its RXA input (the receive path of the four-wire circuit) to the telephone line. Furthermore, the SLIC prevents the signal received at the RXA input and coupled to the telephone line from being sent to the TXA output. This prevents the received signal from being echoed in the transmit path of the four-wire circuit.

The transmitted analog signal from the SLIC TXA output is converted into a digital signal by an encoder/decoder (CODEC) in the line interface, so that it can be processed by the digital switching circuit of the central office. Conversely, the received digital signal from the digital switching circuit is converted into an analog signal by the CODEC, so that it can be sent to the telephone set via the SLIC.

Traditionally, the 2W/4W conversion is referred to as the hybrid function. This comes from the special multiple-winding transformer, called hybrid transformer, that performs 2W/4W conversion in older non-electronic analog line interfaces.

4 Hybrid Function

CENTRAL OFFICE

LINE INTERFACE

TELEPHONE LINE (LOCAL LOOP) TRANSMITTED TRANSMITTED ANALOG SIGNAL DIGITAL SIGNAL TX T T TXA TRANSMIT PATH TRANSMIT PATH

TRANSMITTED ANALOG AND RECEIVED TO AND FROM TELEPHONE SIGNALS ARE SLIC CODEC DIGITAL SWITCHING SET ON EACH WIRE RECEIVED RECEIVED (BALANCED SIGNALS) ANALOG SIGNAL DIGITAL SIGNAL CIRCUIT RX OF CO RXA

RECEIVE PATH RECEIVE PATH R R

2-WIRE CIRCUIT 4-WIRE CIRCUIT

Figure 2-7. 2W/4W conversion performed by the SLIC in the analog line interface.

Implementing the Hybrid Function with Electronic Components

Figure 2-8 is a simplified diagram that shows how the hybrid function can be implemented in a SLIC using electronic components. The single-ended signal received at the SLIC RXA input (triangle-wave signal in Figure 2-8) is passed

through amplifiers A3 and A4, which are non-inverting and inverting amplifiers, respectively. This provides two signals of opposite phases that are sent to the T and R terminals of the SLIC to form the balanced received signal. The balanced, transmitted and received signals on the T and R terminals (sine-wave and triangle-

wave signals in Figure 2-8) are passed through amplifiers A1 and A2, which are non- inverting and inverting amplifiers, respectively. This provides signals that are in phase at the inputs of summing point 1. Adding these signals together provides a single-ended signal that corresponds to the sum of the transmitted and received balanced signals on the T and R terminals.

Note: A single-ended signal is available on a single wire. However, the voltage related to such a signal is measured (or sensed) by connecting an instrument (or any other electronic device) between this wire and a wire connected to the circuit's common terminal.

5 Hybrid Function

SUBSCRIBER LOOP INTERFACE CIRCUIT (SLIC)

T A 1

TRANSMITTED BALANCED TXA SIGNAL TRANSMITTED œ œ AND 1 2 RECEIVED SIGNALS

INVERTING A 2 AMPLIFIER R ECHO 4-WIRE CIRCUIT 2-WIRE CIRCUIT A 5 CANCELLATION INVERTING AMPLIFIER AMPLIFIER

RECEIVED RXA SIGNAL A 3

A 4

INVERTING AMPLIFIER

Figure 2-8. Hybrid function implemented using electronic components.

Amplifier A5 inverts the single-ended signal received at the SLIC RXA input. Summing point 2 adds this inverted signal to the output signal of summing point 1 (sum of the transmitted and received signals) to cancel the received signal, and thereby, prevent undesired echoes in the four-wire transmission circuit. The resulting signal at the output of summing point 2 (TXA output) is the single-ended transmitted signal (sine-wave signal in Figure 2-8).

Procedure Summary

In the first part of the exercise, you will set up a central office with the Telephony Training System (TTS).

In the second part of the exercise, you will establish a connection between two telephone sets and apply sine-wave sound signals to the microphones of the

6 Hybrid Function

handsets. You will observe the waveforms of the input and output signals of the SLIC to demonstrate the 2W/4W conversion.

In the last part of the exercise, you will disable the echo cancellation function of the SLIC. You will observe the effect this has on the waveforms of the signals at the SLIC inputs and outputs. You will also hear the effect this has on a normal telephone conversation.

EQUIPMENT REQUIRED

Refer to Appendix A of this manual to obtain the list of equipment required to perform this exercise.

PROCEDURE

Setting Up the Central Office

* 1. Make sure that the Reconfigurable Training Module, Model 9431, is connected to the TTS Power Supply, Model 9408.

Make sure that there is a network connection between the Reconfigurable Training Module and the host computer.

Install the Dual Analog Line Interface, Model 9475, into one of the analog/digital (A/D) slots of the Reconfigurable Training Module.

Connect two analog telephone sets to the Dual Analog Line Interface. Make sure that the tone dialing mode is selected on the analog telephone sets.

CAUTION!

High voltages are present on the standard telephone connectors of the Dual Analog Line Interface. Do not connect or disconnect the analog telephone sets when the Reconfigurable Training Module is turned on.

Connect the AC/DC power converter supplied with each analog telephone set to one of the AC power outlets on the TTS Power Supply. Connect the DC power output jack of each AC/DC power converter to the DC power input connector on either one of the analog telephone sets.

Note: The analog telephone set requires an auxiliary DC power source for the digital display to be operative.

7 Hybrid Function

* 2. Turn on the host computer.

Turn on the TTS Power Supply then the Reconfigurable Training Module.

* 3. On the host computer, start the Telephony Training System software, then download the CO program to the Reconfigurable Training Module. The CO program configures the Reconfigurable Training Module so that it operates as a central office.

Note: If the host computer is unable to download the CO program to the Reconfigurable Training Module, it may not be using the proper IP address. Have your instructor or the LAN administrator check if the host computer uses the proper IP address to communicate with the Reconfigurable Training Module.

Two-Wire to Four-Wire Conversion

* 4. On the host computer, zoom in on ANALOG LINE INTERFACE A and connect Oscilloscope Probes 1, 2, and 3 to TP3 (balanced signal on the telephone line), TP4 (SLIC TXA output), and TP7 (SLIC RXA input), respectively.

Note: Probes 1, 2, and 3 are associated with channels 1, 2, and 3 of the Oscilloscope, respectively.

* 5. Start the Oscilloscope.

Make the following settings on the Oscilloscope:

Channel 1 Mode ...... Normal Sensitivity ...... 0.5 V/div Input Coupling ...... AC Channel 2 Mode ...... Normal Sensitivity ...... 0.2 V/div Input Coupling ...... AC Channel 3 Mode ...... Normal Sensitivity ...... 0.2 V/div Input Coupling ...... AC Time Base ...... 1 ms/div Trigger Source ...... Ch 1 Level ...... 0 V Slope...... Positive (+) Display Refresh ...... Continuous

8 Hybrid Function

* 6. Lift off the handset of telephone set A and dial the number of telephone set B. Lift off the handset of telephone set B to answer the call and establish a communication.

* 7. Using miniature jack leads, connect the two speakers provided with the Telephony Training System to the low-impedance auxiliary outputs (outputs C and D) of the Reconfigurable Training Module.

Place the speakers connected to auxiliary outputs C and D beside telephone sets A and B, respectively.

Note: The telephone sets should be positioned as far apart as possible. This will provide maximum acoustical isolation between the two speakers.

Install the handset of telephone set A so that the microphone is located over the speaker connected to auxiliary output C.

Install the handset of telephone set B so that the microphone is located over the speaker connected to auxiliary output D.

* 8. On the host computer, set auxiliary outputs C and D of the Reconfigurable Training Module as follows:

Auxiliary Output C Power ...... On Frequency ...... 400 Hz Amplitude ...... minimum Auxiliary Output D Power ...... Off Frequency ...... 800 Hz Amplitude ...... minimum

* 9. Increase the amplitude of the signal at auxiliary output C while observing the Oscilloscope. Notice that a 400-Hz sine-wave signal appears at TP3 (telephone line). This signal represents the sound wave applied to the handset of telephone set A, that is, the signal to be transmitted. Set the amplitude of the signal at auxiliary output C so that the amplitude of the sine-wave signal at TP3 is about 0.5 V.

Observe the signals displayed on the Oscilloscope screen. Describe how the SLIC routes the sine-wave signal present on the telephone line.

9 Hybrid Function

* 10. Turn off auxiliary output C to remove the sound signal applied to the handset of telephone set A.

Turn on auxiliary output D.

* 11. Increase the amplitude of the signal at auxiliary output D while observing the Oscilloscope. Notice that an 800-Hz sine-wave signal appears at the SLIC RXA input (TP7). This signal represents the sound wave applied to the handset of telephone set B, which is received in the analog line interface of telephone set A via the central office switching circuitry. Set the amplitude of the signal at auxiliary output D so that the amplitude of the sine-wave signal at TP7 is about 0.2 V.

Observe the signals displayed on the Oscilloscope screen. Describe how the SLIC routes the sine-wave signal received at the SLIC RXA input.

Briefly explain why the received sine-wave signal is not routed to the SLIC TXA output (TP4), although it is present on the telephone line (TP3).

* 12. Turn on auxiliary output C to reapply the 400-Hz sine-wave sound signal to the handset of telephone set A.

Observe the signals displayed on the Oscilloscope screen. Describe what happens.

10 Hybrid Function

Effect of Disabling the SLIC Echo Cancellation Function

* 13. Turn off auxiliary output C to remove the sound signal applied to the handset of telephone set A.

On the host computer, disable the echo cancellation function of the SLIC in ANALOG LINE INTERFACE A while observing the signals on the Oscilloscope screen.

Describe what happens. Briefly explain.

* 14. On the host computer, enable the echo cancellation function of the SLIC in ANALOG LINE INTERFACE A.

Turn on auxiliary output C to reapply the 400-Hz sine-wave sound signal to the handset of telephone set A.

The signals displayed on the Oscilloscope screen should show that normal two-wire to four-wire conversion is performed.

* 15. On the host computer, disable the echo cancellation function of the SLIC in ANALOG LINE INTERFACE A while observing the signals on the Oscilloscope screen.

Describe what happens. Briefly explain.

* 16. On the host computer, enable the echo cancellation function of the SLIC in ANALOG LINE INTERFACE A. Disable the echo cancellation function of the SLIC in ANALOG LINE INTERFACE B while observing the signals on the Oscilloscope screen.

Describe what happens. Briefly explain.

11 Hybrid Function

* 17. On the host computer, enable the echo cancellation function of the SLIC in ANALOG LINE INTERFACE B.

Turn off auxiliary outputs C and D of the Reconfigurable Training Module to remove the sine-wave sound signals applied to the handsets of telephone sets A and B.

* 18. On the host computer, disable the echo cancellation function of the SLIC in ANALOG LINE INTERFACE A while you are having a normal telephone conversation.

Briefly explain what happens.

* 19. On the host computer, enable the echo cancellation function of the SLIC in ANALOG LINE INTERFACE A. Disable the echo cancellation function of the SLIC in ANALOG LINE INTERFACE B while you are having a normal telephone conversation.

Briefly explain what happens.

* 20. On the host computer, close the Telephony Training System software.

Turn off the TTS Power Supply as well as the host computer (if it is no longer required).

Disconnect the speakers from auxiliary outputs C and D of the Reconfigurable Training Module.

Disconnect the AC/DC power converters from the TTS Power Supply and the analog telephone sets.

Disconnect the analog telephone sets from the Dual Analog Line Interface.

Remove the Dual Analog Line Interface from the Reconfigurable Training Module.

12 Hybrid Function

CONCLUSION

In this exercise, you saw that the SLIC in the analog line interface performs 2W/4W conversion, which is also called the hybrid function. You learned that 2W/4W conversion is required because the transmitted and received voice signals travel on a single pair of wires (the local loop) between the analog telephone set and the line interface, while they travel on two separate pairs of wires (4-wire circuit) in the digital switching circuit of the central office. You observed that the SLIC prevents the received voice signal from being transmitted back to its point of origin in order to have echo-free telephone conversations.

REVIEW QUESTIONS

1. Why are the transmitted and received signals on the local loop referred to as balanced signals?

2. Briefly explain why 2W/4W conversion is required in the analog line interface.

3. Why do the transmitted and received voice signals travel on a single pair of wires between the telephone set and the analog line interface in the central office?

13 Hybrid Function

4. Why is 2W/4W conversion also referred to as the hybrid function?

5. Describe the 2W/4W conversion performed by the SLIC in the analog line interface.

14 Sample Exercise Extracted from Central Office Operation

Exercise 3-1

Call Processor Functions

EXERCISE OBJECTIVE

When you have completed this exercise, you will be familiar with the control functions performed by the call processor during the processing of a call. You will learn how to record and observe the control actions performed by the call processor of the Telephony Training System during the processing of a call.

DISCUSSION

Introduction

As stated in the discussion of fundamentals of this unit, all interconnections made in the switching circuit of today's central offices are under stored program control (SPC), i.e., under the control of a central computer (call processor).

Figure 3-1 shows a simplified diagram of a central office using stored program control. Each analog line interface (ALI), trunk interface, and service circuit (the service circuits are integrated to the SIGNALING CIRCUIT in Figure 3-1) is connected to both sides of the switching circuit (these connections are not shown to keep the diagram clear) to allow each of theses devices to transmit and receive digitized signals. The figure also shows that the call processor exchanges data with the analog line interfaces, the signaling and switching circuits, and the trunk interfaces to perform four control functions: system supervision, signaling, dialed telephone number reception and processing, and connection control (switching circuit control).

& System supervision is performed by reading circuit status information (telephone set hook status, trunk interface idle/busy status).

& Signaling mainly consists of transmitting commands to analog line interfaces to make telephone sets ring, and sending data to the signaling circuit to generate call progress tones which are routed to the proper telephone sets via the switching circuit.

& Dialed telephone number reception and processing is performed by reading dialed digits from the signaling circuit one by one to recover the complete number, and analyzing this number to determine the connections to be made. Note that when pulse dialing is used, dialed number reception is carried out by monitoring the circuit (hook status) status information.

& Connection control consists in sending the proper connection setup and release commands to the switching circuit.

17 Call Processor Functions

TELEPHONE LINES TRUNKS

SWITCHING TRUNK ALI CIRCUIT INTERFACE

SIGNALING TRUNK ALI CIRCUIT INTERFACE CONNECTION CIRCUIT SETUP AND STATUS RELEASE INFORMATION COMMANDS CALL TRUNK ALI PROGRESS INTERFACE TONES DIALED TELEPHONE NUMBERS TO TO TRUNK ALIs CALL PROCESSOR INTERFACE TELEPHONE RINGING

Figure 3-1. Simplified diagram of a central office using stored program control.

Control Functions Performed by the CALL PROCESSOR in the CENTRAL OFFICE of the Telephony Training System

Figure 3-2 is a simplified diagram of the CENTRAL OFFICE in the Lab-Volt Telephony Training System. This subsection explains how the CALL PROCESSOR in the CENTRAL OFFICE performs the control functions.

System Supervision

The CALL PROCESSOR supervises the system by cyclically reading the contents of the HOOK STATUS BUFFER MEMORY in the SIGNALING CIRCUIT. The hook status signals indicate the CALL PROCESSOR if a telephone set requests service, remains active or becomes inactive.

Signaling

The signaling function performed by the CALL PROCESSOR consists in sending commands to the TSAC of ANALOG LINE INTERFACEs to make telephone sets ring. The CALL PROCESSOR also sends data to TONE GENERATORs in the SERVICE CIRCUITs to control the generation of call progress tones.

18 Call Processor Functions

SWITCHING CIRCUIT

TX0 RX0 ANALOG LINE INTERFACE A SPACE-DIVISION TX1 SWITCH RX1

DC SOURCE

T TXA TX0 RING 0 V RELAY SLIC RXA RX0 R HS0 CODEC SIGNALING CIRCUIT

RING GENERATOR

DATA SERVICE CIRCUIT 1 FOR ANALOG LINE INTERFACES TX1 R / V TSAC TONE GENERATOR CODEC RX1

DTMF DETECTOR

TSAC

SERVICE CIRCUIT 2 FOR ANALOG LINE INTERFACES

TX1

CODEC TONE GENERATOR RX1

DTMF DETECTOR ANALOG LINE INTERFACE B

TSAC

DC SOURCE

HOOK STATUS DEMULTIPLEXING AND STORAGE CIRCUIT T TXA TX0 RING 0 V HS0 RELAY SLIC RXA RX0 R HOOK STATUS HS0 DEMUX BUFFER CODEC CONTROLLER MEMORY

DATA

R / V TSAC

PULSE DIALING DETECTED NUMBER DETECTOR CALL PROCESSOR DISPLAY

Figure 3-2. Simplified diagram of the CENTRAL OFFICE in the Telephony Training System.

Dialed Telephone Number Reception and Processing

Depending on the type of dialing used (tone or pulse), telephone numbers are received by reading the dialed digits from DTMF DETECTORs in the SERVICE CIRCUITs or by scanning the contents of the HOOK STATUS BUFFER MEMORY. Once a complete telephone number is recovered, the CALL PROCESSOR analyzes this number to determine the connections required.

19 Call Processor Functions

Connection Control

Connection control is performed by writing data in the SPACE-DIVISION SWITCH Control Register, and by dynamically controlling the receive (RX) time slot assigned to each ANALOG LINE INTERFACE and each SERVICE CIRCUIT in the SIGNALING CIRCUIT according to the connections to be established.

CALL PROCESSOR Log Function in the CENTRAL OFFICE of the Telephony Training System

A special function of the CALL PROCESSOR in the CENTRAL OFFICE of the Telephony Training System allows the recording of the control actions performed by the call processor during the processing of a call. This function is referred to as the Call Processor Log function.

The operation of the Call Processor Log function is similar to that of a recorder: the recording of the control actions can be started and stopped when required. A recorded sequence can be played back, or printed, to perform step-by-step observation.

This function is included in the Telephony Training System for educational purposes only.

Procedure summary

In the first part of the exercise, you will set up a central office with the Telephony Training System (TTS).

In the second part of the exercise, you will identify the paths through which the CALL PROCESSOR performs each of its control functions.

In the third part of the exercise, you will use the log function of the CALL PROCESSOR to record the control actions performed when a call is initiated without being completed. You will then relate each action recorded in the log to a specific control function of the CALL PROCESSOR. You will also relate the actions performed when you attempt to make the call to the control actions recorded in the log.

In the last part of the exercise, you will make a step-by-step observation of what happened in the Central Office for each of the actions recorded in the log.

EQUIPMENT REQUIRED

Refer to Appendix A of this manual to obtain the list of equipment required to perform this exercise.

20 Call Processor Functions

PROCEDURE

Setting Up the Central Office

* 1. Make sure that the Reconfigurable Training Module, Model 9431, is connected to the TTS Power Supply, Model 9408.

Make sure that there is a network connection between the Reconfigurable Training Module and the host computer.

Install the Dual Analog Line Interface, Model 9475, into one of the analog/digital (A/D) slots of the Reconfigurable Training Module.

Connect two analog telephone sets to the Dual Analog Line Interface. Make sure that the tone dialing mode is selected on each analog telephone set.

CAUTION!

Note: The analog telephone set requires an auxiliary DC power source for the digital display to be operative.

* 2. Turn on the host computer.

Turn on the TTS Power Supply, then the Reconfigurable Training Module.

Note: If the host computer is unable to download the CO program to the Reconfigurable Training Module, it may not be using the proper IP address. Have your instructor check if the computer is using the proper IP address to communicate with the Reconfigurable Training Module.

21 Call Processor Functions

Paths Through Which the CALL PROCESSOR Performs the Control Functions

* 4. Draw in Figure 3-3 the paths through which the CALL PROCESSOR performs each of the control functions listed below. Use the specified line symbols.

– System supervision (line symbol: • • • • )

– Signaling (line symbol: — — — — )

– Dialed telephone number reception (line symbol: + + + + )

– Connection control (line symbol: )

The Log Function of the CALL PROCESSOR

* 5. Make sure that the address of the TSAC in ANALOG LINE INTERFACE A is set to 01.

On the host computer, display the Call Processor Log window.

* 6. Start recording the control actions performed by the CALL PROCESSOR.

Lift off the handset of telephone set A and dial two digits on the keypad, then hang up. While doing this, observe that control actions are recorded in the Call Processor Log window as they are being performed.

Stop recording the control actions performed by the CALL PROCESSOR.

* 7. Display the detailed information about each control action recorded in the Call Processor Log window.

22 Call Processor Functions

SWITCHING CIRCUIT

TX0 RX0 ANALOG LINE INTERFACE A SPACE-DIVISION TX1 SWITCH RX1

DC SOURCE

T TXA TX0 RING 0 V RELAY SLIC RXA RX0 R HS0 CODEC SIGNALING CIRCUIT

RING GENERATOR

DATA SERVICE CIRCUIT 1 FOR ANALOG LINE INTERFACES TX1 R / V TSAC TONE GENERATOR CODEC RX1

DTMF DETECTOR

TSAC

SERVICE CIRCUIT 2 FOR ANALOG LINE INTERFACES

TX1

CODEC TONE GENERATOR RX1

DTMF DETECTOR ANALOG LINE INTERFACE B

TSAC

DC SOURCE

HOOK STATUS DEMULTIPLEXING AND STORAGE CIRCUIT T TXA TX0 RING 0 V HS0 RELAY SLIC RXA RX0 R HOOK STATUS HS0 DEMUX BUFFER CODEC CONTROLLER MEMORY

DATA

R / V TSAC

PULSE DIALING DETECTED NUMBER DETECTOR CALL PROCESSOR DISPLAY

Figure 3-3. Paths through which the CALL PROCESSOR performs the control functions.

* 8. In Table 3-1, classify the control actions recorded in the Call Processor Log window according to the control function of the CALL PROCESSOR which they are related to.

23 Call Processor Functions

CALL PROCESSOR CONTROL CALL PROCESSOR CONTROL ACTIONS FUNCTION

System supervision

Signaling

Dialed telephone number reception and processing

Connection control

Table 3-1. Relating the control actions recorded in the Call Processor Log window to the control functions of the CALL PROCESSOR.

* 9. In Table 3-2, relate the actions you performed when you attempted to make a call to the control actions recorded in the Call Processor Log window.

EVENT CALL PROCESSOR CONTROL ACTIONS

Handset of telephone set A is lifted

Two digits are dialed on the keypad of telephone set A

Handset of telephone set A is replaced on the cradle

Table 3-2. Relating the actions performed when attempting to make a call to the control actions recorded in the Call Processor Log window.

24 Call Processor Functions

* 10. In the control action named Service Request, what is the meaning of the following detailed information: received from HS0, 01 ? Explain.

* 11. In the control action named Service Request, what is the meaning of the following detailed information: mark ALI HS0, 01 as busy?

* 12. What is the purpose of the control action named Call Progress Tone Transmit Path Setup?

* 13. Explain why the control action named Call Progress Tone Removal is not immediately followed by the control action named Call Progress Tone Transmit Path Release.

Step-By-Step Observation

* 14. In the LVTTS window, adjust the view so as to be able to see the circuitry of both ANALOG LINE INTERFACEs, the SWITCHING CIRCUIT, the SIGNALING CIRCUIT, and the CALL PROCESSOR.

Using the playback function of the Call Processor Log, make a step-by-step observation of what happened in the Central Office for each of the control actions recorded in the log.

Note: The Previous View function of LVTTS allows you to reobtain the initial general view after you have zoomed on a particular section.

25 Call Processor Functions

From your observations, were several resources of the Central Office required to process the call, even if the call was not completed? Explain.

* 15. On the host computer, close the Telephony Training System software.

Turn off the TTS Power Supply, as well as the host computer (if it is no longer required).

Disconnect the AC/DC power converters from the TTS Power Supply and the analog telephone sets.

Disconnect the analog telephone sets from the Dual Analog Line Interface.

Remove the Dual Analog Line Interface from the Reconfigurable Training Module.

CONCLUSION

In this exercise, you became familiar with the call processor functions: system supervision, signaling, dialed telephone number reception and processing, and connection control. You learned that, in order to perform these control functions, the call processor must communicate with the analog line interfaces, as well as the signaling and switching circuits of the central office.

You learned that the Telephony Training System has a useful function that can provide a chronological record, or log, of all the control actions performed by the CALL PROCESSOR as it processes a call. You used this function to obtain a log of the control actions performed by the CALL PROCESSOR when a call is initiated without being completed. You related each control action recorded in the log to a specific control function of the CALL PROCESSOR. Finally, you made a step-by- step observation of what happened in the Central Office for each control action recorded in the log. This allowed you to see that, whenever a call is attempted by a subscriber, several resources of the Central Office are required to process the call even when the call is not completed.

REVIEW QUESTIONS

1. How does the CALL PROCESSOR of the Telephony Training System supervise the status of the telephone circuits?

26 Call Processor Functions

2. How does the CALL PROCESSOR of the Telephony Training System perform telephone number detection and processing?

3. How does the CALL PROCESSOR of the Telephony Training System perform the signaling function?

4. How does the CALL PROCESSOR of the Telephony Training System perform connection control?

5. What is the purpose of the log function of the CALL PROCESSOR of the Telephony Training System?

27 28 Sample Exercise Extracted from Private Automatic Branch Exchange (PABX)

Exercise 1-1

Architecture of a Digital PABX

EXERCISE OBJECTIVE

When you have completed this exercise, you will be familiar with the architecture of a digital PABX (the Lab-Volt PABX). You will be able to highlight major resemblances and differences between the architecture of a digital PABX and that of a central office. You will also be able to demonstrate how two telephone sets are interconnected in the Lab-Volt PABX.

DISCUSSION

Block Diagram of the Lab-Volt PABX

Today's PABX's are all-digital integrated systems as mentioned in the Unit's Discussion of Fundamentals, and the Lab-Volt PABX is no exception. It is completely digital inside, except for some analog circuitry mainly located in the trunk interface and the trunk service circuit, and uses digital telephone sets of the ISDN type. A simplified block diagram of the Lab-Volt PABX is shown in Figure 1-4.

Like any other PABX, the Lab-Volt PABX consists of the same basic elements as a central office, that is, line interfaces for the telephone sets, a switching circuit, a signaling circuit, a call processor, and a trunk interface. Each of these elements has the same function as in a central office. However, the implementation of certain elements differs from that used in a central office (like the Lab-Volt Central Office for example), mainly because the telephone sets changed from analog to digital technology. For instance, in the signaling circuit the DIGITAL SIGNALING PROCESSOR and the CALL PROGRESS TONE GENERATOR replace the service circuits for analog line interfaces. The switching circuit still consists of a SPACE- DIVISION SWITCH but a device called a DIGITAL CONFERENCE BRIDGE is added to allow conference calling. And obviously, the analog line interfaces are replaced with digital line interfaces.

The Lab-Volt PABX also has an additional basic element, called ANNOUNCEMENT CIRCUIT, which is not found in a central office. The ANNOUNCEMENT CIRCUIT allows intercom calls to be performed using any of the digital telephone sets connected to the Lab-Volt PABX. Intercom calling is discussed in detail in the next unit of this manual.

31 Architecture of a Digital PABX

LAB-VOLT PABX

TELEPHONE LINE SWITCHING CIRCUIT TRUNK SETS INTERFACES INTERFACE TX0 RX0

TX0 TX1 RX1

123 RX2 456

7 8 9 * 0 # DUAL RX0 DIGITAL TX2 RX2 ANALOG TO AND SPACE- TX2 LINE TRUNK FROM D0TX DIVISION TX3 RX3 INTERFACE SWITCH INTERFACE CO A TS2

123 D0RX 456 TX4 RX4 7 8 9

* 0 #

TX5 RX5

TX4 RX4 DIGITAL TX1 CONFERENCE 1 2 3 TX5 RX5 4 5 6 789 BRIDGE * 0 # DUAL RX1 DIGITAL LINE D1TX INTERFACE SIGNALING CIRCUIT B D0TX TX3 1 2 3 D1RX 456

7 8 9

* 0 # D0RX ANALOG RX3 DIGITAL TRUNK SIGNALING D1TX SERVICE PROCESSOR INTERCOM CIRCUIT SPEAKERS D1RX

RX3

ANNOUNCEMENT CIRCUIT CALL TRUNK STATUS DEMULTIPLEXING TX3 PROGRESS TS2 AND TONE STORAGE GENERATOR CIRCUIT

CALL PROCESSOR

Figure 1-4. Simplified block diagram of the Lab-Volt PABX.

The DUAL DIGITAL LINE INTERFACE

Figure 1-5 shows the block diagram of the digital line interfaces used in the Lab-Volt PABX. This block diagram is totally different from that of the analog line interfaces used in the Lab-Volt Central Office. In fact, the only elements that remain are the TXn and RXn terminals that allow the digital line interface to be connected to the SPACE-DIVISION SWITCH, and the time-slot assignment circuit (TSAC) that generates the control signals used to multiplex and demultiplex the digitized voice signals (PCM signals on the TXn and RXn lines). As the analog line interfaces in central offices, the digital line interfaces in the Lab-Volt PABX are grouped in banks.

32 Architecture of a Digital PABX

DUAL DIGITAL LINE INTERFACE

TO A TX INPUT P/S TXn OF SPACE-DIVISION DIGITAL B1 8 SWITCH TELEPHONE SETS B1 TXE LINE TX IRX B2 8 DECODER DEMUX P/S

123 (ASI/NRZ) 456 7 8 9 RX * 0 # B2 TXE TO DIGITAL 2 2 D DnTX SIGNALING PROCESSOR

TX FROM DIGITAL D 2 DnRX SIGNALING

123 LINE 456 PROCESSOR 7 8 9 RX ITX * 0 # CODER MUX (NRZ/ASI) B1 8 RXn FROM AN RX OUTPUT S/P OF SPACE-DIVISION SWITCH B1 RXE B2 8

S/P

B2 RXE

B1 TXE FROM CALL PROCESSOR B2 TXE TSAC B1 RXE

B2 RXE

Figure 1-5. Block diagram of the DUAL DIGITAL LINE INTERFACE of the Lab-Volt PABX.

Each digital line interface is of the ISDN-BRI type and allows connection of two digital telephone sets to the Lab-Volt PABX. This is why each digital line interface in the Lab-Volt PABX is called a DUAL DIGITAL LINE INTERFACE.

Note: Those who are not familiar with the Integrated Services Digital Network (ISDN) can refer to Appendix B of this manual which provides information on ISDN that is relevant to the Lab-Volt PABX.

For each direction of transmission (from the digital telephone sets to the PABX and vice versa), each DUAL DIGITAL LINE INTERFACE provides two bearer channels (labeled B1 and B2), each having a 64-kb/s transmission rate, and a data channel (labeled D) having a transmission rate of 16 kb/s. Each bearer channel, or B channel, is used to transmit a digitized voice signal (voice data) while the D channel is used to transmit signaling data. This explains why each DUAL DIGITAL LINE INTERFACE can accommodate two digital telephone sets. Up to

33 Architecture of a Digital PABX

four digital telephone sets can be connected to the Lab-Volt PABX because two DUAL DIGITAL LINE INTERFACEs are available.

All data from the two digital telephone sets connected to a DUAL DIGITAL LINE INTERFACE (referred to as the ISDN layer-1 data) is time multiplexed to the B channels and the D channel. This data, which is coded using the alternate space inversion (ASI) line code, is received via the IRX terminal of the DUAL DIGITAL LINE INTERFACE. Figure 1-6 illustrates the paths through which the voice data and signaling data received at the IRX terminal are routed to the SPACE-DIVISION SWITCH and the DIGITAL SIGNALING PROCESSOR of the PABX, respectively.

VOICE DATA TO DUAL DIGITAL LINE INTERFACE SPACE-DIVISION SWITCH

VOICE DATA FROM CHANNEL B1

B1 TXn DEMUX P/S 4 5 6 7 8 IRX TIME SLOTS DATA B2 RECEIVED LINE P/S DECODER ISDN LAYER-1 (ASI/NRZ) VOICE DATA FROM CHANNEL B2 FORMAT

B1 B2 D SIGNALING DATA IN ISDN D LAYER-2 FORMAT SIGNALING DATA DIGITAL VOICE DATA TELEPHONE VOICE DATA DIGITAL CALL SETS SIGNALING PROCESSOR PROCESSOR MUX E

LINE ITX CODER (NRZ/ASI) SIGNALING DATA IN ISDN LAYER-3 FORMAT

B1 S/P RXn

B2 S/P

Figure 1-6. Paths through which the voice data and signaling data are routed to the SPACE- DIVISION SWITCH and the DIGITAL SIGNALING PROCESSOR of the PABX, respectively.

At the IRX terminal of the DUAL DIGITAL LINE INTERFACE, a line decoder converts the received ASI-coded data to non return-to-zero (NRZ) coded data. A demultiplexer (DEMUX) then separates (demultiplexes) the voice data and the signaling data from the B channels and the D channel. Voice data is available at the B1 and B2 outputs of the DEMUX while the signaling data is available at the D output. Two parallel-to-serial (P/S) converters and their respective tri-state buffer multiplex the recovered voice data to two different time slots (which are determined by the CALL PROCESSOR according to the connection to be established) for transmission to the SPACE-DIVISION SWITCH via the TX line associated with the DUAL DIGITAL LINE INTERFACE. The signaling data recovered at the D output of the DEMUX is sent to the DIGITAL SIGNALING PROCESSOR. Note that this data is coded according to the layer-2 format of the ISDN signaling protocol.

34 Architecture of a Digital PABX

Note: Variable transmit (TX) time slots are used in the Lab-Volt PABX (except for the transmission of call progress tones) to minimize the chances of having a telephone call blocked because no path is available to establish the required connection. The TX time slots are dynamically assigned by the PABX call processor according to the connections to be established. This differs from the Lab-Volt CO which uses fixed TX time slots, configured through dialog boxes, in order to facilitate the study of its operation.

In the other direction of transmission (from the PABX to the digital telephone sets), the PABX uses either one of the B channels of the interface to transmit voice data to a telephone set and the D channel to transmit signaling data to the two telephone sets. In other words, all data is time multiplexed to the B channels and the D channel to form an ISDN layer-1 data string that is transmitted to the two digital telephone sets via the ITX terminal of the DUAL DIGITAL LINE INTERFACE. Figure 1-7 illustrates the paths through which voice data from the SPACE-DIVISION SWITCH and signaling data from the DIGITAL SIGNALING PROCESSOR are routed to digital telephone sets.

DUAL DIGITAL LINE INTERFACE

DEMUX B1 P/S TXn

LINE IRX DECODER B2 P/S SIGNALING DATA IN ISDN (ASI/NRZ) LAYER-3 FORMAT

DIGITAL DIGITAL TELEPHONE SIGNALING CALL SETS PROCESSOR PROCESSOR MUX E VOICE DATA SIGNALING DATA IN ISDN VOICE DATA D LAYER-2 FORMAT SIGNALING DATA

B1 B2 D VOICE DATA TO CHANNEL B1 ISDN LAYER-1 LINE FORMAT B1 RXn CODER S/P 4 5 6 7 8 ITX (NRZ/ASI) DATA TIME SLOTS TRANSMITTED B2 S/P

VOICE DATA TO CHANNEL B2 VOICE DATA FROM SPACE-DIVISION SWITCH

Figure 1-7. Paths through which voice data from the SPACE-DIVISION SWITCH and signaling data from the DIGITAL SIGNALING PROCESSOR are routed to digital telephone sets.

In brief, voice data to be transmitted to the digital telephone sets is received through the RX line associated with the DUAL DIGITAL LINE INTERFACE. Two serial-to- parallel (S/P) converters and their respective tri-state buffer demultiplex voice data

35 Architecture of a Digital PABX

from two different time slots (which are determined by the CALL PROCESSOR according to the connection to be established). The two demultiplexed digitized voice signals are sent to inputs B1 and B2 of a multiplexer (MUX). The signaling data from the DIGITAL SIGNALING PROCESSOR is available at the D input of the multiplexer. The multiplexer combines all data available on its various inputs into a single data string, that is, the voice data at inputs B1 and B2 are multiplexed to channels B1 and B2, respectively, and the signaling data at the D input is multiplexed to the D channel. A line coder converts the data string from the NRZ format to the ASI format to obtain ISDN layer-1 data that is transmitted to the digital telephone sets.

Note that the signaling data received from the digital telephone sets connected to a DUAL DIGITAL LINE INTERFACE (data recovered at the D output of the DEMUX) is routed to the echo (E) input of the MUX as shown in Figure 1-8. This allows the signaling data to be echoed back to the digital telephone sets via echo (E) bits that are interleaved with the data in the B and D channels of the ISDN layer-1 data. This provides a means to resolve D channel contention. Additional explanation about D channel contention is provided in Appendix B of this manual.

DUAL DIGITAL LINE INTERFACE

DEMUX B1 P/S TXn

LINE IRX DECODER B2 P/S ISDN LAYER-1 (ASI/NRZ) FORMAT

B1 B2 D

DIGITAL SIGNALING DATA IN ISDN DIGITAL LAYER-2 FORMAT TELEPHONE SIGNALING CALL SETS PROCESSOR ECHO BITS INTERLEAVED E PROCESSOR WITH DATA IN MUX CHANNELS B1, B2, AND D

B1 B2 D

ISDN LAYER-1 LINE FORMAT B1 RXn CODER S/P ITX (NRZ/ASI)

B2 S/P

Figure 1-8. Signaling data is echoed back to the digital telephone sets as a means of resolving D channel contention.

As in the analog line interfaces of the Lab-Volt Central Office, a TSAC receives data from the call processor to generate pulse signals that are used to multiplex and demultiplex digitized voice signals (PCM signals on the TXn and RXn lines). However, in the Lab-Volt PABX, each TSAC provides two transmit enable (TXE) signals and two receive enable (RXE) signals (see Figure 1-5) because each DUAL

36 Architecture of a Digital PABX

DIGITAL LINE INTERFACE accommodates two digital telephone sets. The B1 TXE and B2 TXE signals determine the time slots in which voice data from channels B1 and B2 (voice data at the B1 and B2 outputs of the DEMUX), respectively, is multiplexed to the TX line of the line interface. Similarly, the B1 RXE and B2 RXE signals indicate the time slots during which voice data is to be read from the RX line for redirection to channels B1 and B2 (inputs B1 and B2 of the MUX), respectively. Figure 1-9 shows an example of the TSAC output signals when voice data received from channels B1 and B2 is to be multiplexed to the TX line during time slots 5 and 8, respectively, and voice data to be transmitted to the telephone sets via channels B1 and B2 is to be read from the RX line during time slots 6 and 7, respectively.

TIME SLOTS 2 3 4 5 6 7 8 9 10 11 12 13

B1 TXE SIGNAL TIME

B2 TXE SIGNAL TIME

B1 RXE SIGNAL TIME

B2 RXE SIGNAL TIME

Figure 1-9. Example of TSAC output signals used to multiplex and demultiplex digitized voice signals (PCM signals on the TXn and RXn lines).

The Switching Circuit

The SPACE-DIVISION SWITCH in the switching circuit operates in the same manner as the switch in the Lab-Volt Central Office. In brief, the SPACE-DIVISION SWITCH performs space connections (connections of an input [TX] line to any output [RX] line), at every time slot.

The switching circuit in the Lab-Volt PABX has an additional device called DIGITAL CONFERENCE BRIDGE. This device can combine two digitized voice signals into a single digitized signal. It is used to implement conference calling, which is studied in the next unit of this manual.

37 Architecture of a Digital PABX

The Signaling Circuit

The signaling circuit in the Lab-Volt PABX consists of the DIGITAL SIGNALING PROCESSOR, the CALL PROGRESS TONE GENERATOR, the ANALOG TRUNK SERVICE CIRCUIT, and the TRUNK STATUS DEMULTIPLEXING AND STORAGE CIRCUIT (see Figure 1-4).

The DIGITAL SIGNALING PROCESSOR is somewhat the equivalent of the service circuits for analog line interfaces used in the Lab-Volt Central Office, that is, it acts as a kind of interpret circuit. The DIGITAL SIGNALING PROCESSOR converts the signaling data coming from the telephone sets (data in the D channel), which is in the ISDN layer-2 format, to ISDN layer-3 signaling data. This data is then transferred to the CALL PROCESSOR. Conversely, the DIGITAL SIGNALING PROCESSOR converts signaling data from the CALL PROCESSOR, which is in the ISDN layer-3 format, to ISDN layer-2 signaling data that is transmitted to the telephone sets via the DUAL DIGITAL LINE INTERFACE. The exchange of signaling data between the digital telephone sets and the PABX will be studied thoroughly in other exercises of this manual.

Although digital signaling is used in the Lab-Volt PABX, call progress tones are still required to keep telephone users aware of call progression. The call progress tones are provided by the Lab-Volt PABX because the digital telephone sets used with this PABX do not produce these tones internally. The CALL PROGRESS TONE GENERATOR in the signaling circuit of the Lab-Volt PABX produces different digital call progress tones (dial tone, busy tone, etc.). These digital call progress tones are permanently available on line TX3, each tone being multiplexed to a different time slot. Any one of the digital call progress tones can be routed to a digital telephone set via the SPACE-DIVISION SWITCH and one of the B channels of the corresponding DUAL DIGITAL LINE INTERFACE. The operation of the CALL PROGRESS TONE GENERATOR as well as the routing of call progress tones to digital telephone sets is studied in detail in another exercise of this unit.

The ANALOG TRUNK SERVICE CIRCUIT and the TRUNK STATUS DEMUL- TIPLEXING AND STORAGE CIRCUIT make the link between the ANALOG TRUNK INTERFACE and the CALL PROCESSOR. The operation of the ANALOG TRUNK SERVICE CIRCUIT and the TRUNK STATUS DEMULTIPLEXING AND STORAGE CIRCUIT is studied in another manual of the Lab-Volt Telephony Training System.

The Trunk Interface

The ANALOG TRUNK INTERFACE in the Lab-Volt PABX converts voice data received from the SPACE-DIVISION SWITCH into an analog voice signal that can be transmitted on an analog trunk line. Conversely, it converts an analog voice signal received from a trunk line into voice data that is routed to the SPACE- DIVISION SWITCH. The operation of the ANALOG TRUNK INTERFACE is studied in another manual of the Lab-Volt Telephony Training System.

38 Architecture of a Digital PABX

Procedure Summary

In the first part of the exercise, you will set up a PABX with the Telephony Training System (TTS).

In the second part of the exercise, you will study the architecture of the Lab-Volt PABX. This will allow you to see the resemblances and differences between the Lab-Volt PABX and the Lab-Volt Central Office.

In the third part of the exercise, you will lift off the handset of a digital telephone set and determine which B channel is assigned to this telephone set for voice data transmission in the corresponding digital line interface of the Lab-Volt PABX.

In the last part of the exercise, you will determine how the Lab-Volt PABX routes the digitized voice signal coming from a digital telephone set to another digital telephone set.

Note: Before proceeding with the procedure below, it is strongly recommended that you go through Section 4 of the Telephony Training System User Guide (part number 32964-E0) all the way. This section, entitled "Familiarization with the Lab-Volt PABX", provides detailed information on how to set up the Lab-Volt PABX, use its essential features, and ensure that the digital telephone sets connected to it are set up to operate properly.

EQUIPMENT REQUIRED

Refer to Appendix A of this manual to obtain the list of equipment required to perform this exercise.

PROCEDURE

Setting Up the Lab-Volt PABX

* 1. Make sure that the Reconfigurable Training Module, Model 9431, is connected to the TTS Power Supply, Model 9408.

Make sure that there is a network connection between the Reconfigurable Training Module and the host computer.

Install the Digital Telephone Interface, Model 9476, into the digital (D) slot or one of the two analog/digital (A/D) slots of the Reconfigurable Training Module.

Connect two digital (ISDN) telephone sets provided with the Lab-Volt PABX to the left and right connectors of interface A of the Digital Telephone Interface. These telephone sets will be referred to as digital telephone sets A-left (AL) and A-right (AR), respectively, throughout the exercise.

39 Architecture of a Digital PABX

CAUTION!

Do not connect or disconnect the digital telephone sets when the Reconfigurable Training Module is turned on. High voltages are present on the RJ-45 connectors of the Digital Telephone Interface.

* 2. Turn on the host computer.

Turn on the TTS Power Supply, then the Reconfigurable Training Module.

* 3. On the host computer, start the Telephony Training System software, then download the PABX program to the Reconfigurable Training Module. The PABX program configures the Reconfigurable Training Module so that it operates as a private automatic branch exchange (PABX).

Note: If the host computer is unable to download the PABX program to the Reconfigurable Training Module, it may not be using the proper IP address. Have your instructor check if the computer is using the proper IP address to communicate with the Reconfigurable Training Module.

* 4. Lift off the handset of each digital telephone set connected to the Lab-Volt PABX, while observing the Lab-Volt PABX diagram in the LVTTS software window. When the handset of a digital telephone set is lifted off, an icon representing this telephone set should appear connected to the corresponding DUAL DIGITAL LINE INTERFACE in the Lab-Volt PABX diagram. If so, this indicates that this telephone set is properly programmed to be operational with the Lab-Volt PABX. Otherwise, this means that this telephone set is not properly programmed to be operational with the Lab-Volt PABX.

Note: An icon representing the digital telephone set may already be displayed in the Lab-Volt PABX diagram before you lift off the handset of this telephone set. This occurs because each digital telephone set automatically begins communication with the Lab-Volt PABX a certain time after it is powered up in order to identify itself to the Lab-Volt PABX. This indicates that this digital telephone set is properly programmed to be operational with the Lab-Volt PABX.

If a digital telephone set does not seem to be properly programmed, refer to Section 4 of the Telephony Training System User Guide (part number 32964-E0), entitled "Familiarization with the Lab-Volt PABX", to know how to properly program the digital telephone sets connected to the Lab-Volt PABX.

Hang up the handset of each digital telephone set.

40 Architecture of a Digital PABX

Basic Elements of the Lab-Volt PABX

* 5. On the host computer, examine the block diagram of the Lab-Volt PABX without zooming in on it. Notice that the Lab-Volt PABX contains the same basic elements as the Lab-Volt Central Office, which are:

– line interfaces for the telephone sets; – a switching circuit; – a signaling circuit; – a call processor; – a trunk interface.

On the other hand, notice that the Lab-Volt PABX additionally contains an ANNOUNCEMENT CIRCUIT, which is not found in the Lab-Volt Central Office.

* 6. On the host computer, zoom in on the icons representing digital telephone sets AL and AR. Using the Pan command, observe that each digital telephone set is connected to DUAL DIGITAL LINE INTERFACE A (DDLI A) of the Lab-Volt PABX, through two pairs of wires:

– one pair for transmission of the ISDN layer-1 data coming from the telephone set to the interface via one of the interface IRX terminals; – one pair for reception of the ISDN layer-1 data coming from the interface via one of the interface ITX terminals.

* 7. Pan across DDLI A to examine its contents. Notice that the contents of this interface differs markedly from that of the analog line interfaces used in the Lab-Volt Central Office. The only elements that remain are

– the TX0 and RX0 lines that connect DDLI A to the SPACE-DIVISION SWITCH of the Lab-Volt PABX;

– the TSAC.

What are the TX0 and RX0 lines used for? What is the function of the TSAC?

* 8. Observe that, in addition to line TX0, DDLI A uses another transmit line, D0TX. This line permits the routing of the signaling data coming from digital telephone sets AL and AR (data recovered at the D output of the demultiplexer of DDLI A) to the DIGITAL SIGNALING PROCESSOR of the Lab-Volt PABX. The data on line D0TX is in ISDN layer-2 data format.

41 Architecture of a Digital PABX

Also, observe that, in addition to line RX0, DDLI A uses another receive line, D0RX. This line permits the routing of the signaling data coming from the DIGITAL SIGNALING PROCESSOR to digital telephone sets AL and AR, via the D input of the multiplexer of DDLI A. The data on line D0RX is in ISDN layer-2 data format.

* 9. Go to DUAL DIGITAL LINE INTERFACE B (DDLI B) and observe that it does not use the same transmit and receive lines as DDLI A. What does this imply?

Which lines are used by DDLI B to transmit and receive multiplexed digitized voice signals respectively?

Which lines are used by DDLI B to transmit and receive signaling data to and from the DIGITAL SIGNALING PROCESSOR?

* 10. Go to the SWITCHING CIRCUIT of the Lab-Volt PABX. Observe that this circuit contains a TIME-SLOT SELECTOR and a SPACE-DIVISION SWITCH used to perform space connections, as in the case of the SWITCHING CIRCUIT of the Lab-Volt Central Office.

Does the SPACE-DIVISION SWITCH of the Lab-Volt PABX has more inputs and outputs than that of the Lab-Volt Central Office?

* Yes * No

Observe that an additional device called DIGITAL CONFERENCE BRIDGE is included in the SWITCHING CIRCUIT of the Lab-Volt PABX. What is the use of this bridge?

42 Architecture of a Digital PABX

Through which transmit (TX) and receive (RX) lines are the SPACE- DIVISION SWITCH and DIGITAL CONFERENCE BRIDGE interconnected?

* 11. Go to the SIGNALING CIRCUIT of the Lab-Volt PABX and examine its contents. Observe that a DIGITAL SIGNALING PROCESSOR and a CALL PROGRESS TONE GENERATOR replace the service circuits for analog line interfaces and the hook status demultiplexing and storage circuit used in the SIGNALING CIRCUIT of the Lab-Volt Central Office.

Which two other elements of the SIGNALING CIRCUIT of the Lab-Volt PABX (possibly appearing faded to show that they are not currently available in your Lab-Volt PABX) are not included in the SIGNALING CIRCUIT of the Lab-Volt Central Office?

* 12. Examine the DIGITAL SIGNALING PROCESSOR of the SIGNALING CIRCUIT. This device converts the signaling data coming from the digital telephone sets via the DDLI's, which is in ISDN layer-2 data format, to ISDN layer-3 data format. Conversely, it converts the signaling data coming from the CALL PROCESSOR, which is in ISDN layer-3 data format, to ISDN layer-2 data format. This data is then transmitted to the digital telephone sets via the digital line interfaces.

Through which lines does the DIGITAL SIGNALING PROCESSOR receive the signaling data from the digital telephone sets?

Through which lines does the DIGITAL SIGNALING PROCESSOR transmit the signaling data from the CALL PROCESSOR to the digital telephone sets?

* 13. Examine the CALL PROGRESS TONE GENERATOR in the SIGNALING CIRCUIT. Observe that it consists of four tone generators associated with four CODEC's, and a TSAC that generates the control signals used to multiplex each digital call progress tone to a different time slot on line TX3.

43 Architecture of a Digital PABX

Any one of the digital call progress tones present on line TX3 can be routed to a digital telephone set, via the SPACE-DIVISION SWITCH and one of the B channels of the corresponding digital line interface.

What are the four call progress tones generated by the CALL PROGRESS TONE GENERATOR?

* 14. Go to the ANNOUNCEMENT CIRCUIT of the Lab-Volt PABX. This circuit is used for implementation of intercom calling. It converts the digitized voice signal coming from the intercom user's telephone set, via the SPACE- DIVISION SWITCH and one of the B channels of the corresponding digital line interface, into an analog voice signal. This signal is amplified and then routed to intercom speakers.

Through which line is the ANNOUNCEMENT CIRCUIT connected to the SPACE-DIVISION SWITCH?

Assignment of a B Channel to a Digital Telephone Set

* 15. Go to DDLI A and zoom in on its B-CHANNEL ASSIGNMENT display. While observing this display, lift off the handset of digital telephone set AL.

Notice that, as soon as the handset is lifted off, the SPID (Service Profile IDentifier) of digital telephone set AL appears in the B1 field of the display, indicating that channel B1 of DDLI A is assigned to this telephone set for voice data transmission. Is this your observation?

* Yes * No

* 16. Do not hang up. While observing the B-CHANNEL ASSIGNMENT display of DDLI A, lift off the handset of digital telephone set AR.

Notice that, as soon as the handset is lifted off, the SPID of digital telephone set AR appears in the B2 field of the display, indicating that channel B2 of DDLI A is assigned to this telephone set for voice data transmission. Is this your observation?

* Yes * No

44 Architecture of a Digital PABX

* 17. Hang up the handset of digital telephone set AL. Does this cause the B1 field of the B-CHANNEL ASSIGNMENT display of DDLI A to return to "----", indicating that channel B1 of this interface is now unused?

* Yes * No

* 18. Hang up the handset of digital telephone set AR. Does this cause the B2 field of the B-CHANNEL ASSIGNMENT display of DDLI A to return to "----", indicating that channel B2 of this interface is now unused?

* Yes * No

Routing of the Voice Data from a Digital Telephone Set to Another in the Lab-Volt PABX

* 19. Connect Oscilloscope probes 1, 2, 3, and 4 to TP5 (channel B1 P/S Con- verter output), TP11 (TSAC B1 TXE output), TP7 (TX0 line), and TP16 (FRAME SYNC. signal) of DDLI A, respectively.

* 20. Start the Oscilloscope.

Make the following settings on the Oscilloscope:

Channel 1 Mode ...... Normal Sensitivity ...... 5 V/div Input Coupling ...... DC Channel 2 Mode ...... Normal Sensitivity ...... 5 V/div Input Coupling ...... DC Channel 3 Mode ...... Normal Sensitivity ...... 5 V/div Input Coupling ...... DC Channel 4 Mode ...... Normal Sensitivity ...... 5 V/div Input Coupling ...... DC Time Base ...... 10 μs/div Trigger Source ...... Ch 4 Level ...... 1.0 V Slope...... Positive (+) Display Refresh ...... Continuous Display Mode ...... Square

45 Architecture of a Digital PABX

* 21. Lift off the handset of digital telephone set AL and dial the terminal number associated with digital telephone set AR. Lift off the handset of digital telephone set AR to answer the call and establish a communication.

Note: The terminal number associated with each digital telephone set can be found in the Addressing Cross-Reference Table of the Lab-Volt PABX.

* 22. Observe that a PCM signal appears at TP5 (channel-B1 P/S Converter output) of DDLI A on the Oscilloscope screen, especially when talking into the handset of digital telephone set AL. Does this confirm that channel B1 of DDLI A is currently assigned to digital telephone set AL for voice data transmission? Explain why.

* 23. According to the TIME SLOT NUMBER displays of DDLI A, which transmit (TX) time slot is currently assigned to channel B1 of DDLI A? Explain.

Note: Time slots 1 to 4 are reserved for transmission of the call progress tones produced by the CALL PROGRESS TOME GENERATOR. This will be discussed in detail in Exercise 1-4.

* 24. On the Oscilloscope screen, observe that a pulse occurs in the TSAC B1 TXE output signal (TP11) during time slot 5. Explain why.

Note: The pulse in the FRAME SYNC. signal at TP16 is aligned with time slot 0 of each frame.

* 25. Observe that the PCM signal at TP5 (channel B1 P/S Converter output) of DDLI A appears in the signal at TP7 (TX0 line) of this interface during time slot 5. Explain why.

46 Architecture of a Digital PABX

* 26. From the observations you have made up to this point, explain how the voice data from a digital telephone set is routed to an input of the SPACE- DIVISION SWITCH of the SWITCHING CIRCUIT in the Lab-Volt PABX.

* 27. Go to the SWITCHING CIRCUIT of the Lab-Volt PABX. Using the TIME- SLOT SELECTOR of this circuit, observe the connections made by the SPACE-DIVISION SWITCH during time slot 5.

You should observe that during time slot 5, which is the TX time slot assigned to channel B1 of DDLI A, the SPACE-DIVISION SWITCH connects line TX0 (DDLI-A transmit line) to line RX0 (DDLI-A receive line). What is the purpose of this connection?

* 28. Go to the B-CHANNEL ASSIGNMENT display of DDLI A. Which B channel of this interface is currently assigned to digital telephone set AR for voice data transmission? Explain.

* 29. According to the TIME SLOT NUMBER displays of DDLI A, which receive time slot is currently assigned to channel B2 of this interface?

47 Architecture of a Digital PABX

Does this time slot correspond to the transmit time slot assigned to channel B1 of DDLI A? What does this imply?

* 30. Disconnect Oscilloscope probes 1 through 3.

Connect Oscilloscope probes 1, 2, and 3 to TP10 (RX0 line), TP14 (TSAC B2 RXE output), and TP9 (channel-B2 S/P Converter input), respectively.

* 31. Does a pulse occur in the signal at TP14 (TSAC B2 RXE output) of DDLI A during time slot 5? Why?

* 32. Observe that a PCM signal appears at TP10 (RX0 line) of DDLI A, especially when talking into the handset of digital telephone set AL, due to the TX0-to-RX0 connection made by the SPACE-DIVISION SWITCH of the SWITCHING CIRCUIT during time slot 5. Is this PCM signal routed to the channel-B2 S/P Converter input (TP9) of DDLI A? Explain.

48 Architecture of a Digital PABX

* 33. From your observations, explain how the voice data from an output of the SPACE-DIVISION SWITCH is routed to a digital telephone set.

* 34. To summarize what you have learned in this exercise, explain how the CALL PROCESSOR in the Lab-Volt PABX established a connection between the B channels assigned to digital telephone sets AL and AR in DDLI A to permit a normal (bidirectional) conversation between the users at these telephone sets.

Note: In the preceding steps, you studied voice data transmission in a single direction only, that is, from digital telephone set AL to digital telephone set AR. To answer the above question, you can use LVTTS as you have done so far to determine how voice data transmission occurs in the other direction, that is, from digital telephone set AR to digital telephone set AL.

– CALL PROCESSOR actions performed to establish a connection for transmission of the voice data from digital telephone set AL to digital telephone set AR:

49 Architecture of a Digital PABX

– CALL PROCESSOR actions performed to establish a connection for transmission of the voice data from digital telephone set AR to digital telephone set AL:

* 35. Compare the technique used to interconnect two line interfaces in the Lab-Volt PABX to that used to in the Lab-Volt Central Office.

* 36. Hang up the handsets of digital telephone sets AR and AL.

* 37. On the host computer, close the Telephony Training System software.

Turn off the TTS Power Supply, as well as the host computer (if it is no longer required).

Disconnect the digital telephone sets from the Digital Telephone Interface.

Remove the Digital Telephone Interface from the Reconfigurable Training Module.

CONCLUSION

In this exercise, you familiarized yourself with the architecture of the Lab-Volt PABX. You saw that, similar to the Lab-Volt Central Office, the Lab-Volt PABX is made up of line interfaces, a switching circuit, a signaling circuit, a call processor, and a trunk interface. Unlike the Lab-Volt Central Office, however, the Lab-Volt PABX uses digital line interfaces instead of analog line interfaces, its switching circuit includes a digital conference bridge to allow conference calling, its signaling circuit contains

50 Architecture of a Digital PABX

a digital signaling processor and a call progress tone generator in place of service circuits for analog line interfaces and a hook status demultiplexing and storage circuit, and it includes an announcement circuit.

You learned that the digital line interfaces of the Lab-Volt PABX are of the ISDN-BRI type: for each direction of transmission (from the digital telephone sets to the Lab-Volt PABX and vice versa), these interfaces provide two bearer channels, labeled B1 and B2, that are used to convey voice data, as well as a data channel, labeled D, that is used to convey signaling data. Because of this, each digital line interface can accommodate two digital telephone sets and is thus referred to as a dual digital line interface (DDLI). You saw that, as soon as the handset of a digital telephone set is lifted off, the Lab-Volt PABX assigns a B channel to this telephone set in the corresponding digital line interface. You learned that the voice data produced by a digital telephone set is time multiplexed to the assigned B channel, while the signaling data produced by this telephone set is time multiplexed to the D channel, all the data being sent in ISDN layer-1 data format to the corresponding digital line interface in the Lab-Volt PABX. Conversely, you learned that the Lab-Volt PABX transmits voice data to a digital telephone set by time multiplexing this data to one of the B channels of the interface, and that it transmits signaling data to this telephone set by time multiplexing this data to the D channel, all the data being sent to the telephone set in ISDN layer-1 data format.

Finally, you compared the technique used to interconnect two line interfaces in the Lab-Volt PABX to that used in the Lab-Volt Central Office. You saw that, in both cases, the line interfaces are interconnected by combining time-multiplexed switching with space-division switching. The only difference lies in the way time- multiplexed switching is performed: in the Lab-Volt PABX, it is performed by dynamically assigning transmit and receive time slots to the line interfaces; in the Lab-Volt Central Office, it is performed by dynamically assigning receive time slots to the line interfaces, the transmit time slots being fixed and user-determined through the use of dialog boxes.

REVIEW QUESTIONS

1. How do the architectures of the Lab-Volt PABX and Lab-Volt Central Office resemble? How do they differ?

51 Architecture of a Digital PABX

2. Explain why each digital line interface on the Lab-Volt PABX can accommodate two digital telephone sets.

3. Describe the paths through which the voice data and the signaling data from a digital telephone set are routed to the SPACE-DIVISION SWITCH and the DIGITAL SIGNALING PROCESSOR of the Lab-Volt PABX.

4. How does the Lab-Volt PABX transmit voice data and signaling data to the two digital telephone sets associated with a digital line interface?

5. What is the use of the Bn TXE and Bn RXE signals generated by the TSAC of a digital line interface in the Lab-Volt PABX?

52 Sample Exercise Extracted from PABX Analog Trunk

Exercise 1-2

Analog Trunk Interface

EXERCISE OBJECTIVE

When you have completed this exercise, you will know the role of the analog trunk interface in a PABX. You will be able to explain the various functions of the analog trunk interface. You will be familiar with the block diagram and operation of the analog trunk interface in the Lab-Volt PABX.

DISCUSSION

Role of the Analog Trunk Interface

A PABX performs basic enterprise switching, that is, it makes the connections required to establish all calls between telephone sets located in an enterprise. A PABX also concentrates all telephone sets in the enterprise to a limited number of telephone lines that home in on the local CO. In many installations, conventional (analog) telephone lines, referred to as PABX analog trunks, are used to connect a PABX to the local CO.

Today's PABX's are all-digital integrated systems that switch digitized voice signals and use digital signaling protocols. Because of the intrinsic nature of modern PABX's, some kind of hybrid circuitry is required to interface each analog trunk (telephone line) with the digital circuitry in a PABX. This is the role of the analog trunk interfaces as shown in Figure 1-4. On one side, each analog trunk interface exchanges digital signals with the PABX circuitry. On the other side, it transmits and receives analog signals via a PABX analog trunk. In other words, the role of the analog trunk interface is to make the PABX, which is essentially a digital machine, appear as a conventional analog telephone set to the local CO. Note that one analog trunk interface is required for each PABX analog trunk. Also note that the analog trunk interfaces in a PABX are usually grouped in banks as are the analog line interfaces in central offices.

55 Analog Trunk Interface

ENTERPRISE PREMISES

DIGITAL TELEPHONE SETS PABX PABX ANALOG TRUNKS CONVENTIONAL 123

456 7 8 9 TELEPHONE LINES * 0 #

TO AND FROM THE LOCAL 123

456 7 8 9 CENTRAL * 0 # OFFICE DIGITAL BANK OF DIGITAL SIGNALING ANALOG LINE AND TRUNK INTERFACES SWITCHING 1 2 3 INTERFACES 4 5 6 EQUIPMENT 789

* 0 #

1 2 3

456

7 8 9

* 0 #

CALL PROCESSOR

Figure 1-4. Analog trunk interfaces make a link between the digital circuitry in a modern PABX and analog trunks.

Functions of the Analog Trunk Interface

To correctly link the digital circuitry in a PABX to the analog trunks and make the PABX appear as a conventional analog telephone set to the local CO, the analog trunk interface performs the following functions:

1. Detects the ringing voltage applied across the analog trunk when there is an incoming call to the PABX;

2. Provides trunk status information (analog trunk loop open or closed) to allow the PABX call processor to perform trunk supervision;

3. Closes the analog trunk loop to answer an incoming call to the PABX or initiate an external call;

4. Converts time-division multiplexed serial digital signals (voice, DTMF dialing tones) from the PABX circuitry into analog signals that can be transmitted via the analog trunk;

56 Analog Trunk Interface

5. Converts analog signals (voice, call progress tones, caller identification data) received from the analog trunk into time-division multiplexed serial digital signals before they are routed to the PABX circuitry;

6. Performs two-wire to four-wire conversion because the analog trunk is a two- wire circuit whereas the PABX circuitry uses 4-wire circuits;

7. Opens the analog trunk loop to terminate an external call.

The remaining subsections of this discussion relate the above functions to the block diagram of the ANALOG TRUNK INTERFACE in the Lab-Volt PABX.

Block Diagram of the ANALOG TRUNK INTERFACE in the Lab-Volt PABX

Figure 1-5 is a block diagram of the ANALOG TRUNK INTERFACE in the Lab-Volt PABX. The tip (T) and ring (R) terminals of the ANALOG TRUNK INTERFACE connect to the PABX analog trunk (local loop going to the Lab-Volt CO). Analog signals (voice signals, call progress tones, and DTMF dialing tones) are exchanged via these terminals. On the other side of the ANALOG TRUNK INTERFACE, the receive (RX2), transmit (TX2), sampling clock, bit clock, trunk status (TKS2), and data terminals connect to the call processor and the switching and signaling circuits of the Lab-Volt PABX. These terminals carry digital signals (digitized voice signals, DTMF dialing tones, call progress tones, etc).

Operation of the ANALOG TRUNK INTERFACE in the Lab-Volt PABX

This subsection explains the operation of the ANALOG TRUNK INTERFACE in the Lab-Volt PABX. Many of the explanations that follow are based on the block diagram shown in Figure 1-5. Furthermore, the subsection highlights the fact that the ANALOG TRUNK INTERFACE is somewhat a mixture of analog line interface circuitry (CODEC, TSAC, 2W/4W conversion circuit and loop current detector in a subscriber loop interface circuit) and analog telephone set circuitry (switchhook and ringing voltage detector in an electronic ringer circuit).

57 Analog Trunk Interface

ANALOG TRUNK INTERFACE

CALLER IDENTIFICATION ACCESS PATH

TO AND RECEIVE (RX2) TIP (T) FROM RING AND TO AND PABX DIGITAL ANSWER TRUNK FROM DUPLEXER ANALOG SWITCHING RELAY STATUS LOCAL TRANSMIT (TX2) TRUNK CIRCUIT DETECTOR CO OF PABX ENCODER/ RING (R) DECODER SAMPLING CLOCK (CODEC)

FROM PABX BIT CLOCK

FROM CALL DATA PROCESSOR TXE OF PABX

TIME-SLOT ASSIGNMENT RXE CIRCUIT 1 (TSAC 1)

ANS

TO SIGNALING TRUNK STATUS (TKS2) CIRCUIT OF PABX

DATA TSTXE FROM CALL PROCESSOR OF PABX TIME-SLOT ASSIGNMENT CIRCUIT 2 (TSAC 2)

Figure 1-5. Block diagram of the ANALOG TRUNK INTERFACE in the Lab-Volt PABX.

ANSWER RELAY

The ANSWER RELAY is a double-pole single-throw contact relay as shown in Figure 1-6. It plays a role similar to that of the switchhook in an analog telephone set. When the PABX analog trunk is not in use, the relay contacts are open as shown in Figure 1-6 (a). During a call, the relay contacts are closed to connect the analog trunk line to the DUPLEXER (via the RING AND TRUNK STATUS DETECTOR). This allows analog signals to go from the DUPLEXER to the analog trunk and vice versa. Furthermore, this causes DC current to flow through the PABX

58 Analog Trunk Interface

analog trunk loop as shown in Figure 1-6 (b). This is because the battery feed circuit of the analog line interface in the CO permanently applies DC voltage across the tip and ring wires of the analog trunk.

ANALOG TRUNK INTERFACE

ANSWER RING AND CENTRAL RELAY TRUNK OFFICE STATUS ANALOG LINE DETECTOR INTERFACE TIP (T) BATTERY DUPLEXER PABX ANALOG TRUNK FEED CIRCUIT RING (R)

COIL

FROM ANS OUPUT OF TSAC 1

(a) ANSWER RELAY open

ANALOG TRUNK INTERFACE

ANSWER RING AND CENTRAL RELAY TRUNK OFFICE STATUS ANALOG LINE DETECTOR INTERFACE TIP (T) BATTERY DUPLEXER PABX ANALOG TRUNK FEED CIRCUIT RING (R)

COIL DC CURRENT FLOWING THROUGH THE PABX ANALOG TRUNK LOOP

FROM ANS OUPUT OF TSAC 1

(b) ANSWER RELAY closed

Figure 1-6. The ANSWER RELAY plays a role similar to that of the switchhook in an analog telephone set.

59 Analog Trunk Interface

The ANSWER RELAY is controlled by the PABX call processor via TIME-SLOT ASSIGNMENT CIRCUIT 1 (TSAC 1) of the ANALOG TRUNK INTERFACE. When the PABX call processor sends data to TSAC 1 that forces its ANS output to go to logic state 1, the ANSWER RELAY closes. In brief, the ANSWER RELAY, in conjunction with TSAC 1, performs functions 3 and 7 mentioned in the previous subsection (i.e. closing the analog trunk loop to answer or initiate an external call and opening the analog trunk loop to terminate an external call).

RING AND TRUNK STATUS DETECTOR

The RING AND TRUNK STATUS DETECTOR performs the first two functions of an analog trunk interface mentioned in the previous subsection: ringing voltage detection and trunk status indication. When the ringing voltage is applied across the PABX analog trunk by the CO, it is detected by the RING AND TRUNK STATUS DETECTOR which sets its output to logic state 1. Similarly, when the ANSWER RELAY closes, DC current starts to flow in the analog trunk loop. This current is detected by the RING AND TRUNK STATUS DETECTOR which sets its output to logic state 1. Note that all analog signals exchanged via the PABX analog trunk pass through the RING AND TRUNK STATUS DETECTOR without being modified. Also note that the functions performed by the RING AND TRUNK STATUS DETECTOR are basically the same as the AC ringing voltage detection in the electronic ringer circuit of an analog telephone set and the loop current detection in the subscriber loop interface circuit (SLIC) of an analog line interface.

DUPLEXER

The DUPLEXER performs function 6 mentioned in the previous subsection: two-wire to four-wire conversion. This function is equivalent to the hybrid function performed by the SLIC of each analog line interface in a CO. Figure 1-7 illustrates the operation of the DUPLEXER.

The DUPLEXER converts the balanced signal received from the PABX analog trunk (via the ANSWER RELAY and the RING AND TRUNK STATUS DETECTOR) into a single-ended analog signal, and routes this signal to the ENCODER/ DECODER (CODEC) analog input. Conversely, the DUPLEXER receives a single- ended analog signal from the CODEC, and converts this signal into a balanced signal to be transmitted via the PABX analog trunk. Furthermore, the DUPLEXER prevents the balanced signal to be transmitted from being returned to the CODEC analog input as a single-ended signal. This prevents the signal to be transmitted via the PABX analog trunk from being echoed in the PABX.

60 Analog Trunk Interface

ANALOG SIGNAL TO BE TRANSMITTED VIA TRUNK

TO AND FROM TO AND FROM PABX ANALOG SWITCHING ENCODER/ BALANCED BALANCED TRUNK VIA CIRCUIT SIGNAL TO BE SIGNAL DECODER DUPLEXER ANSWER RELAY IN TRANSMITTED RECEIVED (CODEC) + LAB-VOLT VIA TRUNK FROM TRUNK RING AND TRUNK PABX STATUS DETECTOR

ANALOG SIGNAL RECEIVED FROM TRUNK

Figure 1-7. Operation of the DUPLEXER in the ANALOG TRUNK INTERFACE.

ENCODER/DECODER and TIME-SLOT ASSIGNMENT CIRCUIT 1

The ENCODER/DECODER (CODEC) and TIME-SLOT ASSIGNMENT CIRCUIT 1 (TSAC 1) perform functions 4 and 5 mentioned in the previous subsection. In fact these two functions are very similar to the coding function performed by the CODEC and TSAC of each analog line interface in a CO.

In brief, the CODEC converts the analog signal received from the PABX analog trunk, via the RING AND TRUNK STATUS DETECTOR, ANSWER RELAY, and DUPLEXER, into a digital signal. This signal consists of serial 8-bit PCM codes occurring at a rate of 8000 codes/s. Each PCM code is then time-division multiplexed to line TX2 which connects to the switching circuit of the PABX. TSAC 1 receives data from the call processor that determines the time slot assigned to the CODEC to multiplex each PCM code to line TX2. TSAC 1 produces a rectangular pulse signal at its TXE output that is aligned with the assigned transmit (TX) time slot. Figure 1-8 shows an example of the TSAC-1 TXE output signal when TX time slot 5 is assigned to the CODEC.

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TIME SLOTS 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

1 TSAC 1 TXE OUTPUT SIGNAL 0 TIME 8-BIT SERIAL PCM CODE HIGH-IMPEDANCE STATE 1 DIGITAL SIGNAL ON LINE TX2 0 TIME

1 TSAC 1 RXE OUTPUT SIGNAL 0 TIME 8-BIT SERIAL PCM CODE HIGH-IMPEDANCE STATE 1 DIGITAL SIGNAL ON LINE RX2 0 TIME

Figure 1-8. Multiplexing and demultiplexing signals at the TXE and RXE outputs of TSAC 1.

In the opposite direction, the CODEC receives a digital signal, to be transmitted via the PABX analog trunk, from line RX2 of the PABX switching circuit. This digital signal consists of time-division multiplexed serial 8-bit PCM codes occurring at a rate of 8000 codes/s. The CODEC demultiplexes the digital signal received via line RX2 and converts it into an analog signal that is transmitted via the PABX analog trunk. TSAC 1 receives data from the call processor that determines the time slot assigned to the CODEC to demultiplex a digital signal from line RX2. TSAC 1 produces a rectangular pulse signal at its RXE output that is aligned with the assigned receive (RX) time slot. Figure 1-8 shows an example of the TSAC-1 RXE output signal when RX time slot 7 is assigned to the CODEC.

Note that both the TX and RX time slots of the CODEC in the ANALOG TRUNK INTERFACE are dynamically assigned by the PABX call processor according to the connections to be established.

TIME-SLOT ASSIGNMENT CIRCUIT 2

The analog trunk interfaces in a PABX are usually grouped in banks as mentioned earlier in this discussion. The status of the analog trunk associated with each interface in a bank is generally time-division multiplexed so that complete status information about all trunks in the bank can be routed to the signaling circuit of the PABX via a single transmission line. A fixed time slot is assigned to each analog trunk interface for the transmission of the trunk status.

62 Analog Trunk Interface

This method is used in the ANALOG TRUNK INTERFACE of the Lab-Volt PABX, that is, one line (labeled TKS2) is used to transmit time-division multiplexed trunk status information. The PABX analog trunk status, available at the RING AND TRUNK STATUS DETECTOR output, is routed to a tri-state buffer controlled by TIME-SLOT ASSIGNMENT CIRCUIT 2 (TSAC 2). TSAC 2 produces a rectangular pulse signal at its TSTXE output that is aligned with the time slot assigned to the ANALOG TRUNK INTERFACE for the transmission of the trunk status. The assigned time slot corresponds to the TSAC-2 address. For example, when the TSAC-2 address is 5, the PABX analog trunk status is multiplexed to time slot 5, that is, the tri-state buffer is enabled during the corresponding time interval. This is illustrated in Figure 1-9. Note that a dialog box in the Lab-Volt PABX allows the TSAC-2 address to be set.

TIME SLOTS 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

1 PABX ANALOG TRUNK STATUS RING AND TRUNK STATUS DETECTOR OUPUT SIGNAL 0 TIME

1 TSAC 2 TSTXE OUTPUT SIGNAL 0 TIME MULTIPLEXED TRUNK STATUS HIGH-IMPEDANCE STATE 1 OF TRI-STATE BUFFER DIGITAL SIGNAL ON LINE TKS2 0 TIME

Figure 1-9. The status of the PABX analog trunk is time-division multiplexed to line TKS2.

CALLER IDENTIFICATION ACCESS PATH

When a central office directs a call toward an analog telephone set, it sends data about the calling party after the first burst of ringing. This feature is referred to as caller identification (caller ID). The caller ID data is transmitted to the analog telephone set via the local loop as a frequency-shift keying (FSK) signal.

Similarly, when a call is directed toward the Lab-Volt PABX, it receives the caller ID data via the analog trunk after the first burst of ringing. However, since the ANSWER RELAY remains open as long as the call is not answered, some alternate path must be provided in the ANALOG TRUNK INTERFACE so that caller ID data can reach the DUPLEXER. This is the role of the CALLER IDENTIFICATION ACCESS PATH in the ANALOG TRUNK INTERFACE. This path allows the FSK signal bearing the caller ID data to reach the DUPLEXER. On the other hand, a voltage limiter in the CALLER IDENTIFICATION ACCESS PATH clips the high-level AC ringing voltage to avoid damaging the DUPLEXER.

63 Analog Trunk Interface

Even though a voltage limiter in the CALLER IDENTIFICATION ACCESS PATH clips the high-level AC ringing voltage when the ANSWER RELAY is open, the DUPLEXER is provided with its own voltage limiter. This is because, when a call is answered while the AC ringing voltage is applied across the analog trunk, the ANSWER RELAY closes and the AC ringing voltage is applied directly to the DUPLEXER until the CO detects that the call is answered. This generally takes about 100 ms for a CO to detect that a call is answered, and this should never exceed 200 ms.

Procedure Summary

In the first part of the exercise, you will set up an analog trunk between a Lab-Volt PABX and a Lab-Volt Central Office.

In the second part of the exercise, you will study the operation of the ANSWER RELAY and RING AND TRUNK STATUS DETECTOR of the ANALOG TRUNK INTERFACE in the Lab-Volt PABX. To do so, you will observe the signals involved in the operation of these components when receiving or making an external call.

In the third part of the exercise, you will study the operation of the DUPLEXER, CODEC, and TSAC 1 of the ANALOG TRUNK INTERFACE. To do so, you will observe the input and output signals of these components during a call between a digital telephone set connected to the Lab-Volt PABX and an analog telephone set connected to the Lab-Volt CO.

In the last part of the exercise, you will observe how the status of the analog trunk associated with the ANALOG TRUNK INTERFACE is digitized and time-division multiplexed to a trunk status line.

EQUIPMENT REQUIRED

Refer to Appendix A of this manual to obtain the list of equipment required to perform this exercise.

PROCEDURE

Setting Up an Analog Trunk Between a Lab-Volt PABX and a Lab-Volt Central Office

Note: In this exercise, it is assumed that a single host computer is used to download the PABX and CO programs to two Reconfigurable Training Modules, Model 9431. This host computer is used to monitor the Lab-Volt PABX.

* 1. Make sure that two Reconfigurable Training Modules, Model 9431, are connected to the TTS Power Supply, Model 9408.

Make sure that there is a network connection between each Reconfigurable Training Module and the host computer.

64 Analog Trunk Interface

Install the Dual Analog Line Interface, Model 9475, into one of the two analog/digital (A/D) slots of a Reconfigurable Training Module. This module will be used as a central office (CO). Connect an analog telephone set to the Dual Analog Line Interface. Make sure that the tone dialing mode is selected on this telephone set.

CAUTION!

Do not connect or disconnect the analog telephone set when the Reconfigurable Training Module is turned on. High voltages are present on the standard telephone connectors of the Dual Analog Line Interface.

Connect the AC/DC power converter supplied with the analog telephone set to one of the AC power outlets on the TTS Power Supply. Connect the DC power output jack of the AC/DC power converter to the DC power input connector on the analog telephone set.

* 2. Install the Digital Telephone Interface, Model 9476, into the digital (D) slot or one of the two analog/digital (A/D) slots of the other Reconfigurable Training Module. This module will be used as a PABX. Connect two digital (ISDN) telephone sets provided with the Lab-Volt PABX to the left and right connectors of interface A of the Digital Telephone Interface. These telephone sets will be referred to as digital telephone sets A-left (AL) and A-right (AR), respectively, throughout the exercise.

CAUTION!

Do not connect or disconnect the digital telephone sets when the Reconfigurable Training Module is turned on. High voltages are present on the RJ-45 connectors of the Digital Telephone Interface.

Finally, install the PABX Analog Trunk Interface, Model 9477, into one of the two analog/digital (A/D) slots of the Reconfigurable Training Module used as a PABX. Using the analog trunk line provided with the PABX Analog Trunk Interface (two-wire cable terminated with standard [RJ-11] male telephone connectors), connect the RJ-11 female connector on the PABX Analog Trunk Interface installed in the Reconfigurable Training Module used as a PABX to the remaining RJ-11 female connector on the Dual Analog Line Interface installed in the Reconfigurable Training Module used as a CO.

* 3. Turn on the host computer.

Turn on the TTS Power Supply, then turn on the Reconfigurable Training Modules.

65 Analog Trunk Interface

* 4. On the host computer, start the Telephony Training System software, then download the PABX program to the Reconfigurable Training Module used as a PABX.

Note: If the host computer is unable to download the PABX program to the Reconfigurable Training Module used as a PABX, make sure that the proper IP address is used to communicate with this Reconfigurable Training Module.

* 5. Lift off the handset of each digital telephone set connected to the Lab-Volt PABX, while observing the Lab-Volt PABX diagram in the LVTTS software window. When the handset of a digital telephone set is lifted off, an icon representing this telephone set should appear connected to the corresponding DUAL DIGITAL LINE INTERFACE in the Lab-Volt PABX diagram. If so, this indicates that this telephone set is properly programmed to be operational with the Lab-Volt PABX. Otherwise, this means that this telephone set is not properly programmed to be operational with the Lab-Volt PABX.

Hang up the handset of each digital telephone set.

* 6. On the host computer, designate digital telephone set AL of the Lab-Volt PABX as the attendant's telephone set. Make sure that no external call restriction is applied to digital telephone sets AL and AR. Set the DTMF dialing tone duration to 1 s.

* 7. On the host computer, download the CO program to the Reconfigurable Training Module used as a CO.

Note: If the host computer is unable to download the CO program to the Reconfigurable Training Module used as a CO, make sure that the proper IP address is used to communicate with this Reconfigurable Training Module.

66 Analog Trunk Interface

Once the CO program has been downloaded, a box representing the Lab-Volt CO will appear connected to the PABX, via an analog trunk, in the Lab-Volt PABX diagram displayed on the host computer screen. Record below the telephone numbers associated with ANALOG LINE INTERFACEs A and B of the Lab-Volt CO.

ANALOG LINE INTERFACE A (ALI A):

ANALOG LINE INTERFACE B (ALI B):

Note: If you use two separate host computers to download the PABX and CO programs to the two Reconfigurable Training Modules used in this exercise, no box representing the Lab-Volt CO will appear in the Lab-Volt PABX diagram once the CO program has been downloaded. In that case, you can access the LVTTS Options dialog box (or the Call Processor Settings dialog box) on the host computer that monitors the Lab-Volt CO to know the telephone numbers associated with each analog line interface. For the rest of this exercise, however, the term host computer always refers to the computer monitoring the Lab-Volt PABX.

* 8. On the host computer, set the external call number length in the Lab-Volt PABX so that it matches the length of the telephone numbers associated with the analog line interfaces used in the Lab-Volt CO. Make sure the address of TSAC 2 in the PABX ANALOG TRUNK INTERFACE is set to 1.

ANSWER RELAY and RING AND TRUNK STATUS DETECTOR

* 9. On the host computer, zoom in on the ANALOG TRUNK INTERFACE of the Lab-Volt PABX. Connect Oscilloscope Probes 1, 2, 3, and 4 to TP1 (DC COUPLED LINE MONITOR output), TP2 (AC COUPLED LINE MONITOR output), TP12 (TSAC-1 ANS output) and TP14 (RING AND TRUNK STATUS DETECTOR output) of the ANALOG TRUNK INTERFACE, respectively.

67 Analog Trunk Interface

* 10. Start the Oscilloscope.

Make the following settings on the Oscilloscope:

Channel 1 Mode ...... Normal Sensitivity ...... 50 V/div Input Coupling ...... AC Channel 2 Mode ...... Normal Sensitivity ...... 5 V/div Input Coupling ...... DC Channel 3 Mode ...... Normal Sensitivity ...... 5 V/div Input Coupling ...... DC Channel 4 Mode ...... Normal Sensitivity ...... 5 V/div Input Coupling ...... DC Time Base ...... 5 ms/div Trigger Source ...... Ch 1 Level ...... 0.5 V Slope...... Positive (+) Display Refresh ...... Continuous Display Mode ...... Square

Note: Set the time base to 2 ms/div if the AC ringing voltage of the Lab-Volt CO is set for a UK ringing cadence and/or a frequency of 50 Hz.

* 11. On the Oscilloscope screen, observe that the signal at TP12 (TSAC-1 ANS output) of the ANALOG TRUNK INTERFACE is at logic state 0. Because of this, the contacts of the ANSWER RELAY in this interface are open, so that no DC current flows through the analog trunk loop. This is indicated by the DC LOOP CURRENT display of the ANALOG TRUNK INTERFACE, which reads "0 mA". Is this what you observe?

* Yes * No

Observe that the signal at TP14 (RING AND TRUNK STATUS DETECTOR output) of the ANALOG TRUNK INTERFACE is also at logic state 0. This occurs because there is no DC current flowing through the analog trunk loop, nor is there an AC ringing voltage applied by the Lab-Volt CO across the analog trunk (as can be observed at TP1 of the ANALOG TRUNK INTERFACE).

68 Analog Trunk Interface

What do all the conditions previously observed indicate about the current status (available/busy) of the PABX analog trunk?

Receiving an External Call

* 12. Using the analog telephone set connected to the Lab-Volt CO, make a call to the Lab-Volt PABX. Let digital telephone set AL (PABX attendant's telephone set) ring while observing the signals on the Oscilloscope screen.

What is the logic state of the signal at TP14 (RING AND TRUNK STATUS DETECTOR output) of the ANALOG TRUNK INTERFACE when the AC ringing voltage is applied across the analog trunk (TP1 of the ANALOG TRUNK INTERFACE) by the Lab-Volt CO?

What is the logic state of the signal at TP14 (RING AND TRUNK STATUS DETECTOR output) of the ANALOG TRUNK INTERFACE when the AC ringing voltage across the analog trunk is switched off for a silent period (ringing pause)?

Observe that, whenever the AC ringing voltage is applied across the analog trunk, the signal at TP12 (TSAC-1 ANS output) of the ANALOG TRUNK INTERFACE remains at logic state 0, which implies that the contacts of the ANSWER RELAY in this interface remain open. Why is it, then, that the AC ringing voltage appears at TP2 (AC COUPLED LINE MONITOR output) of the ANALOG TRUNK INTERFACE in the form of a severely clipped sine-wave signal?

69 Analog Trunk Interface

Hang up the handset of the analog telephone set.

* 13. Using the analog telephone set connected to the Lab-Volt CO, make a call to the Lab-Volt PABX. Let digital telephone set AL ring and observe that, after a short while, caller ID information (i.e. the telephone number associated with the analog telephone set and name of the user at this telephone set) appears on the display of digital telephone set AL.

Hang up the handset of the analog telephone set.

* 14. Make the following settings on the Oscilloscope:

Channel-1 Sensitivity ...... 1 V/div Channel-2 Sensitivity ...... 1 V/div Time Base ...... 0.2 s/div Trigger Source ...... Ch 4 Slope...... Negative (–) Display Refresh ...... Manual

Position the trigger point one horizontal division from the left-hand side of the Oscilloscope screen.

* 15. Using the analog telephone set connected to the Lab-Volt CO, make a call to the Lab-Volt PABX. During the first burst of ringing of digital telephone set AL, refresh the Oscilloscope display.

Hang up the handset of the analog telephone set.

Observe that the Oscilloscope has been triggered on a high-to-low transition that occurred in the signal at TP14 (RING AND TRUNK STATUS DETECTOR output) of the ANALOG TRUNK INTERFACE at the end of the first burst of AC ringing voltage applied on the analog trunk (TP1 of the ANALOG TRUNK INTERFACE). Observe that, after the end of that burst, a frequency-shift keying (FSK) signal was received at TP1 of the ANALOG TRUNK INTERFACE. What does this signal represent? Where does it come from?

Note: If the Lab-Volt CO is set for a customized ringing cadence with very short bursts of ringing, you may have to trigger the display refresh just before the first burst of ringing of digital telephone set AL in order for the Oscilloscope to be able to trigger onto the end of the corresponding burst of AC ringing voltage applied on the analog trunk.

70 Analog Trunk Interface

Also, if the bursts of ringing consist of two successive rings separated by a brief silent period, the Oscilloscope may trigger onto the end of the first ring instead of the end of the second ring of AC ringing voltage applied on the analog trunk. This causes no problem as long as you are able to observe the entire FSK signal.

On the Oscilloscope screen, observe that during the time interval the FSK signal was present at TP1 of the ANALOG TRUNK INTERFACE, the signal at TP12 (TSAC-1 ANS output) of this interface remained at logic state 0. Consequently, the contacts of the ANSWER RELAY remained open during this time interval. Why is it, then, that the FSK signal was present at TP2 (AC COUPLED LINE MONITOR output) of the ANALOG TRUNK INTERFACE, as can be observed on the Oscilloscope screen?

* 16. Make the following settings on the Oscilloscope:

Channel-1 Sensitivity ...... 50 V/div Channel-2 Sensitivity ...... 10 V/div Time Base ...... 0.1 s/div Trigger Source ...... Ch 3 Slope...... Positive (+)

* 17. Using the analog telephone set, make a call to the Lab-Volt PABX. Let digital telephone set AL ring a few times and listen to the ring back tone in the handset earpiece of the analog telephone set.

At the beginning of a ring back tone, refresh the Oscilloscope display and immediately lift off the handset of digital telephone set AL to answer the call.

Observe that the Oscilloscope has been triggered on a low-to-high transition that occurred in the signal at TP12 (TSAC-1 ANS output) of the ANALOG TRUNK INTERFACE when the call was answered. Observe that, following this transition, the Lab-Volt CO removed the AC ringing voltage from the analog trunk (TP1) within a delay of 200 ms.

71 Analog Trunk Interface

Note: Some spurious transients occur in the signals at TP1, TP2, and TP14 of the ANALOG TRUNK INTERFACE after the call is answered. These transients are normal and due to the nature of the electronic circuitry in the ANALOG TRUNK INTERFACE.

Explain why the Lab-Volt CO removed the AC ringing voltage from the analog trunk when the low-to-high transition occurred in the signal at TP12.

Explain why the signal at TP14 (RING AND TRUNK STATUS DETECTOR output) of the ANALOG TRUNK INTERFACE, after temporarily alternating between logic states 0 and 1 (spurious transients), remained at logic state 1 instead of returning to logic state 0 even if the AC ringing voltage was no longer applied across the analog trunk.

* 18. Do not hang up. On the Oscilloscope, select the continuous display refresh mode and set the time base to 5 ms/div.

* 19. According to the DC LOOP CURRENT display of the ANALOG TRUNK INTERFACE, does DC current flow through the analog trunk loop? Why? Explain by referring to the logic state of the signal at TP12 (TSAC-1 ANS output) of this interface.

72 Analog Trunk Interface

Hang up the handset of digital telephone set AL to terminate the call. According to the DC LOOP CURRENT display of the ANALOG TRUNK INTERFACE, does DC current flow through the analog trunk loop? Explain by referring to the logic state of the signal at TP12 of this interface.

Hang up the handset of the analog telephone set.

* 20. From the observations you have made up to this point, compare the role played by the ANSWER RELAY of the PABX ANALOG TRUNK INTERFACE to that played by the switchhook in an analog telephone set, and explain.

Making an External Call

* 21. On the Oscilloscope, select the manual display refresh mode and set the time base to 0.1 s/div.

* 22. Lift off the handset of digital telephone set AR. Refresh the Oscilloscope display and immediately press 9 as if to initiate an external call. Hang up the handset of digital telephone set AR.

Observe that the Oscilloscope has been triggered on a low-to-high transition that occurred in the signal at TP12 (TSAC-1 ANS output) of the ANALOG TRUNK INTERFACE when the digit 9 was dialed to initiate an external call.

73 Analog Trunk Interface

Notice that just after this transition occurred, the signal at TP14 (RING AND TRUNK STATUS DETECTOR output) of the ANALOG TRUNK INTERFACE also went from logic state 0 to logic state 1. Explain why.

Note: Some spurious transients occur in the signals at TP1 and TP2 of the ANALOG TRUNK INTERFACE when an external call is initiated. These transients are normal and due to the nature of the electronic circuitry in the ANALOG TRUNK INTERFACE.

* 23. Make the following settings on the Oscilloscope:

Channel-1 Sensitivity ...... 1 V/div Channel-2 Sensitivity ...... 1 V/div Time Base ...... 2 ms/div Trigger Source ...... Ch 1 Level ...... 0 V Display Refresh ...... Continuous

* 24. Lift off the handset of digital telephone set AR and press 9 to initiate an external call. On the Oscilloscope, observe that an analog dial tone signal coming from the Lab-Volt CO, via the analog trunk, appears at TP1 of the ANALOG TRUNK INTERFACE, and that this signal also appears at TP2 of this interface.

Hang up the handset of digital telephone set AR.

Explain why the Lab-Volt CO sent an analog dial tone to the Lab-Volt PABX, via the analog trunk.

74 Analog Trunk Interface

Explain why the analog dial tone signal was also present at TP2 of the ANALOG TRUNK INTERFACE.

* 25. Lift off the handset of digital telephone set AR and press 9 to initiate an external call.

While observing the signals at TP1 and TP2 of the ANALOG TRUNK INTERFACE, dial the first digit in the number of the analog telephone set on the keypad of digital telephone set AR. Notice that this causes the Lab-Volt CO to remove the dial tone signal from the analog trunk, and an analog DTMF dialing signal corresponding to the dialed digit to appear at TP1 and TP2 of the ANALOG TRUNK INTERFACE during 1 s approximately.

On digital telephone set AR, slowly dial the other digits in the number of the analog telephone set, while observing the analog DTMF dialing signals corresponding to these digits at TP1 and TP2 of the ANALOG TRUNK INTERFACE. Notice that once the last digit has been dialed, an analog ring back tone signal coming from the Lab-Volt CO, via the analog trunk, appears at TP1 of the ANALOG TRUNK INTERFACE, and that this signal also appears at TP2 of this interface.

Lift off the handset of the analog telephone set to answer the call. Observe that this causes the Lab-Volt CO to remove the ring back tone from the analog trunk, as can be observed at TP1 and TP2 of the ANALOG TRUNK INTERFACE.

Talk into the handsets of digital telephone set AR and the analog telephone set, while observing the signals at TP1 and TP2 of the ANALOG TRUNK INTERFACE. Can a normal conversation now take place between these telephone sets?

* Yes * No

Hang up the handsets of digital telephone set AR and the analog telephone set.

* 26. From your observations, does the PABX act exactly like a tone dialing telephone set when an external call is made via the PABX analog trunk?

* Yes * No

* 27. On the Oscilloscope, select the manual display refresh mode.

75 Analog Trunk Interface

DUPLEXER, CODEC, and TSAC 1

* 28. Disconnect Oscilloscope Probes 3 and 4 and connect them to TP3 (CODEC analog output) and TP6 (CODEC analog input) of the ANALOG TRUNK INTERFACE, respectively.

Make the following settings on the Oscilloscope:

Channel-3 Sensitivity ...... 1 V/div Channel-4 Sensitivity ...... 1 V/div Time Base ...... 1 ms/div Display Refresh ...... Continuous

* 29. Using the analog telephone set, make a call to the Lab-Volt PABX. Lift off the handset of digital telephone set AL to answer the call.

Talk into the handset of the analog telephone set while observing the corresponding analog voice signal received from the analog trunk (via the RING AND TRUNK STATUS DETECTOR and the ANSWER RELAY) at TP2 (AC COUPLED LINE MONITOR output) of the ANALOG TRUNK INTERFACE. Notice that this signal is also present at TP6 (CODEC analog input) of the interface. Explain why.

Note: The signals observed at TP2 and TP6 of the ANALOG TRUNK INTERFACE might not be in phase. This is inherent to the way the DUPLEXER operates.

* 30. Talk into the handset of digital telephone set AL while observing the corresponding analog voice signal at TP3 (CODEC analog output) of the ANALOG TRUNK INTERFACE. Notice that this signal is also present at TP2 (AC COUPLED LINE MONITOR output) of the interface. Explain why.

76 Analog Trunk Interface

* 31. Talk into the handset of digital telephone set AL and observe that the corresponding analog voice signal at TP2 of the ANALOG TRUNK INTERFACE is not returned to the CODEC analog input (TP6) of this interface. Explain why.

Note: A small residue of the signal at TP2 might be present at TP6 of the ANALOG TRUNK INTERFACE, since the DUPLEXER in this interface and the SLIC in the ANALOG LINE INTERFACE associated with the analog trunk in the Lab-Volt CO cannot achieve perfect echo cancellation.

* 32. From your observations, what is the function of the DUPLEXER in the ANALOG TRUNK INTERFACE of the Lab-Volt PABX? Is this function equivalent to the hybrid function performed by the SLIC of each analog line interface in a CO? Explain.

* 33. Do not hang up. On the Oscilloscope, select the manual display refresh mode.

Disconnect all the Oscilloscope Probes. Connect Oscilloscope Probes 1, 2, 3, and 4 to TP6 (CODEC analog input), TP7 (CODEC digital output), TP10 (TSAC-1 TXE output), and TP15 (FRAME SYNC. signal) of the ANALOG TRUNK INTERFACE, respectively.

77 Analog Trunk Interface

Make the following settings on the Oscilloscope:

Channel-2 Sensitivity ...... 5 V/div Channel-3 Sensitivity ...... 5 V/div Channel-4 Sensitivity ...... 5 V/div Trigger Source ...... Ch 4 Level ...... 1 V Display Refresh ...... Continuous

* 34. Talk into the handset of the analog telephone set while observing the corresponding analog voice signal received from the analog trunk (via the RING AND TRUNK STATUS DETECTOR, ANSWER RELAY, and DUPLEXER) at TP6 (CODEC analog input) of the ANALOG TRUNK INTERFACE.

While talking into the handset of the analog telephone set, decrease the Oscilloscope time base by steps until it is equal to 5 μs/div and observe the analog voice signal at TP6, as well as the serial PCM signal at the CODEC digital output (TP7). What does this PCM signal represent? Explain.

* 35. On the Oscilloscope screen, observe that a pulse occurs in the TSAC-1 TXE output signal (TP10) during time slot 5. Explain why.

Note: The pulse in the FRAME SYNC. signal at TP15 is aligned with time slot 0 of each frame.

* 36. Disconnect Oscilloscope Probe 3 from TP10 of the ANALOG TRUNK INTERFACE and connect it to TP8 (TX2 line) of this interface.

78 Analog Trunk Interface

* 37. While talking into the handset of the analog telephone set, observe the signals on the Oscilloscope screen. Is the digitized voice signal at the CODEC digital output (TP7) multiplexed to time slot 5 on transmit line TX2 (TP8) of the ANALOG TRUNK INTERFACE? Why?

* 38. On the Oscilloscope, select the manual display refresh mode.

Disconnect Oscilloscope Probes 1, 2, 3 and connect them to TP5 (RX2 line), TP4 (CODEC demultiplexed output), and TP11 (TSAC-1 RXE output) of the ANALOG TRUNK INTERFACE, respectively.

Make the following settings on the Oscilloscope:

Channel 1 Sensitivity ...... 5 V/div Coupling ...... DC Display Refresh ...... Continuous

* 39. On the Oscilloscope screen, observe that a pulse occurs in the TSAC-1 RXE output signal (TP11) during time slot 5. Explain why.

* 40. On the Oscilloscope screen, observe that a serial PCM signal appears at TP5 (RX2 line) of the ANALOG TRUNK INTERFACE during time slot 5, especially when talking into the handset of digital telephone set AL. Is this PCM signal (voice data from digital telephone set AL) extracted from time slot 5 at the CODEC demultiplexed output (TP4)? Explain.

* 41. Disconnect Oscilloscope Probe 3 and connect it to TP3 (CODEC analog output) of the ANALOG TRUNK INTERFACE.

79 Analog Trunk Interface

Make the following settings on the Oscilloscope:

Channel-3 Sensitivity ...... 1 V/div Time Base ...... 0.1 ms/div

While observing the signals on the Oscilloscope screen, talk into the handset of digital telephone set AL. Is the demultiplexed PCM signal at TP4 (CODEC demultiplexed output) converted into an analog voice signal at the CODEC analog output (TP3)? Explain.

* 42. From your observations, could several analog trunk interfaces be grouped in a bank connected to the Lab-Volt PABX switching circuit, through a single transmit line and a single receive line, as in the case of the analog line interfaces used in the Lab-Volt CO? Explain.

* 43. Do not hang up. Proceed with the next part of the exercise.

Analog Trunk Status Multiplexing

* 44. On the Oscilloscope, select the manual display refresh mode.

Disconnect Oscilloscope Probes 1, 2, and 3 and connect them to TP14 (RING AND TRUNK STATUS DETECTOR output), TP16 (TSAC-2 TSTXE output), and TP13 (TKS2 line) of the ANALOG TRUNK INTERFACE, respectively.

Make the following settings on the Oscilloscope:

Channel-3 Sensitivity ...... 5 V/div Time Base ...... 5 μs/div Display Refresh ...... Continuous

* 45. On the Oscilloscope, observe that the analog trunk status signal at TP14 of the ANALOG TRUNK INTERFACE is at logic state 1. This occurs because DC current is flowing through the analog trunk loop.

80 Analog Trunk Interface

Observe that a pulse occurs in the signal at TP16 (TSAC-2 TSTXE output) of the ANALOG TRUNK INTERFACE during time slot 1. Explain why by referring to the current reading of the TRUNK STATUS TX TIME SLOT NUMBER display of this interface.

Note: The pulse in the FRAME SYNC. signal at TP15 is aligned with time slot 0 of each frame.

Observe the signal at TP13 (TKS2 line) of the ANALOG TRUNK INTERFACE. To which time slot is the analog trunk status signal at TP14 multiplexed on line TKS2? Explain.

Hang up the handsets of digital telephone set AL and the analog telephone set.

* 46. Select the manual display refresh mode on the Oscilloscope.

* 47. Change the address of TSAC 2 in the ANALOG TRUNK INTERFACE to 5.

* 48. Select the continuous display refresh mode on the Oscilloscope. According to the signals on the Oscilloscope screen, does the pulse in the TSAC-2 TSTXE output signal (TP16) still occur during time slot 1? Explain.

* 49. Using the analog telephone set, make a call to the Lab-Volt PABX. While observing the signals on the Oscilloscope screen, let digital telephone

81 Analog Trunk Interface

set AL ring a couple of times, then lift off the handset of this telephone set to answer the call.

From your observations, is the analog trunk status signal at TP14 of the ANALOG TRUNK INTERFACE now multiplexed to the time slot recorded in the previous step on line TKS2 (TP13)?

* Yes * No

Hang up the handsets of digital telephone set AL and the analog telephone set.

* 50. Is the status of the analog trunk associated with the ANALOG TRUNK INTERFACE time-division multiplexed to a trunk status line, the same way the hook status signal of an analog telephone set is time-division multiplexed to a hook status line in the Lab-Volt CO? What does this permit?

* 51. On the host computer, close the Telephony Training System software.

Turn off the TTS Power Supply, as well as the host computer (if it is no longer required).

Disconnect the AC/DC power converter from the TTS Power Supply and the analog telephone set.

Remove the analog trunk line (two-wire cable terminated with standard [RJ-11] male connectors) connecting the PABX Analog Trunk Interface installed in the Reconfigurable Training Module used as a PABX to the Dual Analog Line Interface installed in the Reconfigurable Training Module used as a CO.

Disconnect the digital telephone sets from the Digital Telephone Interface installed in the Reconfigurable Training Module used as a PABX. Remove the Digital Telephone Interface and the PABX Analog Trunk Interface from this module.

Disconnect the analog telephone set from the Dual Analog Line Interface installed in the Reconfigurable Training Module used as a CO. Remove the Dual Analog Line Interface from this module.

82 Analog Trunk Interface

CONCLUSION

In this exercise, you became familiar with the role and operation of the analog trunk interface. You learned that this interface consists of hybrid circuitry that links an analog trunk to the digital circuitry of a PABX, thereby making the PABX appear as a conventional analog telephone set to the CO. You learned one analog trunk interface is required for each analog trunk used.

You learned that an analog trunk interface performs several functions. They are: detecting the ringing voltage, providing trunk status information, opening and closing the analog trunk loop, converting time-division multiplexed PCM signals into analog signals and vice-versa, and performing two-wire to four-wire conversion.

You familiarized yourself with the block diagram and operation of the ANALOG TRUNK INTERFACE in the Lab-Volt PABX. You learned that this interface is somewhat a combination of analog line interface circuitry and analog telephone set circuitry. Thus, you saw that the ANALOG TRUNK INTERFACE contains:

– a ring and trunk status detector that detects the ringing voltage and indicates the trunk status (similar to the electronic ringer circuit of an analog telephone set and the loop current detection in the SLIC of an analog line interface); – an answer relay that opens and closes the analog trunk line (comparable to the switchhook in an analog telephone set); – a duplexer that performs two-wire to four-wire conversion (comparable to the hybrid function in the SLIC of an analog line interface); – a CODEC and its associated TSAC for converting TDM-PCM signals into analog signals and vice-versa (comparable to the CODEC and TSAC in an analog line interface); – a tri-state buffer controlled by another TSAC for trunk status multiplexing (comparable to the tri-state buffer and TSAC used to multiplex the hook status in an analog line interface).

REVIEW QUESTIONS

1. What is the role of the analog trunk interface? How does it achieve this role? Explain.

83 Analog Trunk Interface

2. What two functions are performed by the ANSWER RELAY of the ANALOG TRUNK INTERFACE in the Lab-Volt PABX?

3. How does the Lab-Volt PABX CALL PROCESSOR make the contacts of the ANSWER RELAY in the ANALOG TRUNK INTERFACE close? What happens when these contacts close?

4. State three cases when the signal at the RING AND TRUNK STATUS DETECTOR output of the ANALOG TRUNK INTERFACE in the Lab-Volt PABX goes from logic state 0 to logic state 1.

5. In a PABX that contains a bank of analog trunk interfaces, why is it advantageous to time-division multiplex the status of the analog trunk associated with each analog trunk interface in the bank?

84 Sample Exercise Extracted from Digital Trunk

Exercise 1-2

Digital Trunk Interface

EXERCISE OBJECTIVE

When you have completed this exercise, you will be able to explain the role of the digital trunk interface in a central office. You will be familiar with the DS1 and E1 TDM formats that can be used in the Lab-Volt digital trunk. You will be able to describe the operation of the digital trunk interface used in Lab-Volt CO's using the simplified block diagram of this interface.

DISCUSSION

Role of the Digital Trunk Interface

As mentioned before in this unit, a digital trunk is a link between two switching offices of the PSTN that can carry many digitized voice signals at a same time and in both directions. This is achieved by multiplexing in time digitized voice signals according to one of the various TDM formats available in the older North American and European hierarchies of digital transmission systems (DS1 to DS4 and E1 to E5 digital signals) and the SONET/SDH hierarchy of digital transmission systems (STS-1 to STS-192 and STM-1 to STM-64 electrical signals).

The role of the digital trunk interface is to provide a link between the digital circuitry of the CO (call processor, signaling circuit, and switching circuit) and a digital trunk. This is illustrated in Figure 1-4.

87 Digital Trunk Interface

DIGITAL TRUNK INTERFACE

PAIR OF COPPER WIRES DIGITIZED VOICE SIGNALS FROM SWITCHING CIRCUIT TRANSMITTED SERIAL TO DIGITAL SIGNAL OTHER CO SIGNALING DATA FROM CALL PROCESSOR (VIA SIGNALING CIRCUIT)

DIGITAL TRUNK (FOUR WIRES)

DIGITIZED VOICE SIGNALS TO SWITCHING CIRCUIT RECEIVED SERIAL FROM DIGITAL SIGNAL OTHER CO SIGNALING DATA TO CALL PROCESSOR (VIA SIGNALING CIRCUIT) PAIR OF COPPER WIRES

Figure 1-4. The digital trunk interface provides a link between the digital circuitry of the CO and a digital trunk.

In brief, the digital trunk interface receives many digitized voice signals (64-kb/s PCM signals) from the switching circuit and signaling data from the call processor (via the signaling circuit). It time multiplexes these signals and data, according to a certain TDM format (DS1, E4, STS-1, STM-1, etc), to form a serial digital signal that is transmitted via the digital trunk line. Conversely, the digital trunk interface receives a serial digital signal corresponding to a certain TDM format (DS1, E4, STS-1, STM-1, etc) from the digital trunk line. It extracts (demultiplexes) the various digitized voice signals (64-kb/s PCM signals) and the signaling data it contains, and sends these signals and data to the switching circuit and the call processor (via the signaling circuit) of the CO, respectively. Note that depending on the TDM format used, the transmission media can be pairs of copper wires (as shown in Figure 1-4), coaxial cables, or optical fibers.

TDM Formats Used in the Lab-Volt Digital Trunk

Two TDM formats are available for the digital trunk used to interconnect Lab-Volt CO's. When the digitized voice signals are multiplexed in time over 24 time slots in Lab-Volt CO's, the DS1 format from the North American hierarchy of digital transmission systems is used. When time-division multiplexing is made over 32 time slots in Lab-Volt CO's, the E1 format from the European hierarchy of digital transmission systems is used.

Figure 1-5 illustrates the DS1 TDM format. This format divides time into intervals of equal duration that are referred to as frames. The duration of each frame is 125 μs, which exactly corresponds to the reciprocal of the sampling frequency (8 kHz) used in CO's to digitize voice signals. Each frame is subdivided into 24 time intervals that are called time slots. These time slots are numbered 1 to 24. Each time slot can carry a digitized voice signal (8-bit PCM code). An extra bit (F bit) is added at the

88 Digital Trunk Interface

beginning of each frame to convey framing information (the use of this bit is covered later in this discussion). The F bit and the digitized voice signals in the 24 time slots form a serial digital signal which consists of 193 bits that repeat every frame, thereby leading to a bit rate of 1.544 Mb/s.

DIGITIZED VOICE SIGNAL (8-BIT SERIAL PCM CODE)

SIGN BIT SIGNAL MAGNITUDE

b7b6 b5 b4 b3b2 b1 b0 MSB LSB

1 2 3 4 5 6 7 8 9 101112131415161718192021222324

24 TIME SLOTS CONTAINING 24 EIGHT-BIT SERIAL PCM CODES OR 23 EIGHT-BIT SERIAL PCM CODES PLUS 1 EIGHT-BIT WORD OF SIGNALING DATA (192 BITS)

FRAMING BIT (F BIT)

FRAME = FRAMING BIT + 24 EIGHT-BIT TIME SLOTS (193 BITS)

125 μs

Figure 1-5. DS1 time-division multiplexing (TDM ) format.

Note that one of the 24 time slots in each DS1 frame can be used to convey signaling data (instead of a digitized voice signal) related to the digitized voice signals transmitted. This is the case in the Lab-Volt digital trunk where time slot 24 conveys signaling data and the remaining 23 time slots can carry digitized voice signals. Digital signaling between CO's is covered extensively in the next unit of this manual.

Figure 1-6 illustrates the E1 TDM format. This format also divides time into frames having a duration of 125 μs. Each frame is divided into 32 time slots that are numbered 0 to 31. Each of time slots 1 to 15 and 17 to 31 can carry a digitized voice signal (8-bit PCM code), for a maximum of 30 digitized voice signals. Time slot 0 is used to convey framing information (the use of this information is covered later in this discussion). Time slot 16 is used to convey signaling data related to the digitized voice signals transmitted. The 32 time slots form a serial digital signal which consists of 256 bits (32 time slots x 8 bits) that repeat every frame, thereby leading to a bit rate of 2.048 Mb/s.

89 Digital Trunk Interface

DIGITIZED VOICE SIGNAL (8-BIT SERIAL PCM CODE)

SIGN BIT

SIGNAL MAGNITUDE

b7b6 b5 b4 b3b2 b1 b0 MSB LSB

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

15 TIME SLOTS CONTAINING 15 TIME SLOTS CONTAINING 15 EIGHT-BIT SERIAL PCM CODES 15 EIGHT-BIT SERIAL PCM CODES

TIME SLOT CONTAINING TIME SLOT CONTAINING FRAMING INFORMATION SIGNALING INFORMATION

FRAME = 32 EIGHT-BIT TIME SLOTS (256 BITS)

125 μs

Figure 1-6. E1 time-division multiplexing (TDM) format.

Simplified Block Diagram of the DIGITAL TRUNK INTERFACE

Figure 1-7 is a simplified block diagram of the DIGITAL TRUNK INTERFACE used in Lab-Volt CO's. This diagram is divided in two major sections that are referred to as the RECEIVER and the TRANSMITTER. The RECEIVER receives a serial digital signal in DS1 or E1 format from the digital trunk line, extracts (demultiplexes) the various digitized voice signals and the signaling data it contains, and sends these signals and data to the switching circuit and the call processor (via the signaling circuit) of the CO, respectively. Conversely, the TRANSMITTER receives many digitized voice signals from the switching circuit and signaling data from the call processor (via the signaling circuit), time multiplexes these signals and data to form a DS1 or E1 digital signal that is transmitted via the digital trunk line.

The remaining two subsections in this discussion describe the operation of the circuitry in the RECEIVER and TRANSMITTER of the DIGITAL TRUNK INTERFACE.

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DIGITAL TRUNK INTERFACE

RECEIVER

RECOVERED DATA TO SWITCHING TX2 RECEIVE CIRCUIT OF CO RECOVERED FRAME SYNC. LINE IRX LINE DECODER FRAMING TIME CIRCUIT SLOT RECOV. MF SYNC. AND INTERCHANGER FROM ALARM DATA/ 1 RECOV. TS CLOCK DETECTOR CLOCK OTHER LFA RECOVERY CO TO CALL CIRCUIT DORX PROCESSOR RECOVERED BIT CLOCK OF CO

VIA SIGNALING LOS CIRCUIT

BIT CLOCK AIS RAI

FRAME SYNC. DIGITAL TRUNK

TRANSMITTER TRANSMIT ITX LINE FROM RX2 SWITCHING FRAMING TO AND LINE CIRCUIT OF CO TIME SIGNALING CODER OTHER SLOT CIRCUIT INTERCHANGER CO 2

BIT CLOCK

FRAME SYNC.

FROM CALL DOTX PROCESSOR OF CO VIA SIGNALING TX BIT CLOCK CIRCUIT TX FRAME SYNC.

TX MF SYNC.

Figure 1-7. Simplified block diagram of the DIGITAL TRUNK INTERFACE used in Lab-Volt CO's.

Operation of the RECEIVER

The RECEIVER consists of a DATA/CLOCK RECOVERY CIRCUIT, a LINE DECODER, a FRAMING CIRCUIT AND ALARM DETECTOR, and TIME SLOT INTERCHANGER 1. The operation of each of these circuits is explained in this subsection. The explanations sometimes refer to the simplified block diagram in Figure 1-7.

DATA/CLOCK RECOVERY CIRCUIT

The DATA/CLOCK RECOVERY CIRCUIT receives a serial digital signal from the digital trunk line. This signal usually exhibits some distortion caused by the transmission over the digital trunk line. The DATA/CLOCK RECOVERY CIRCUIT

91 Digital Trunk Interface

recovers the bit clock from the received serial digital signal. The bit clock is a square-wave signal having a frequency that corresponds to the bit rate (1.544 or 2.048 Mb/s) of the received digital signal. The recovered bit clock is used to synchronize the operation of the FRAMING CIRCUIT AND ALARM DETECTOR and TIME SLOT INTERCHANGER 1 with the incoming serial digital signal.

The DATA/CLOCK RECOVERY CIRCUIT uses the recovered bit clock to regenerate a "clean" serial digital signal from the distorted signal received from the digital trunk line. The regenerated serial digital signal is sent to the LINE DECODER.

Figure 1-8 shows a slightly distorted serial digital signal received from a digital trunk line and the RECOVERED BIT CLOCK signal and regenerated serial digital signal at the outputs of the DATA/CLOCK RECOVERY CIRCUIT. Notice that the rising edges in the RECOVERED BIT CLOCK signal are aligned with the middle of the pulses in the distorted serial digital signal to make the regeneration of the serial digital signal easier.

BINARY SEQUENCE 110101001

SERIAL DIGITAL SIGNAL RECEIVED

RECOVERED BIT CLOCK SIGNAL

REGENERATED SERIAL DIGITAL SIGNAL

Figure 1-8. Input and output signals of the DATA/CLOCK RECOVERY CIRCUIT.

Note that the DATA/CLOCK RECOVERY CIRCUIT detects whether or not a signal is received from the digital trunk line. When a serial digital signal of sufficient magnitude is received from the digital trunk, the signal at the loss-of-signal (LOS) output of the DATA/CLOCK RECOVERY CIRCUIT is at logic level zero. Conversely, the signal at the LOS output goes to logic level 1 when no signal is received from the digital trunk line.

92 Digital Trunk Interface

LINE DECODER

The serial digital signal received from the digital trunk line is coded to ensure a minimal amount of transitions in this signal, and thereby, facilitates clock recovery. When the DS1 TDM format is used, a line code referred to as bipolar with eight-zero substitution (B8ZS) is applied to the serial digital signal. When the E1 TDM format is used, another line code called high-density bipolar of order 3 (HDB3) is applied to the serial digital signal. In brief, the B8ZS and HDB3 line codes are modified versions of another line code called alternate mark inversion (AMI). When the AMI line code is used, each binary zero is transmitted as no signal on the line and each binary one is transmitted as a rectangular pulse whose polarity alternates from one binary one to the next. When the B8ZS line code is used, the serial digital signal is still AMI coded but each sequence of 8 successive binary zeroes in the digital signal is replaced with a sequence of pulses containing two bipolar violations, that is, a sequence where the alternation in pulse polarity is not respected. Similarly, when the HDB3 line code is used, the serial digital signal is also AMI coded but each sequence of 4 successive binary zeroes is replaced with a sequence of pulses containing one bipolar violation. Figure 1-9 shows a sequence of binary ones and zeros in non-return-to-zero (NRZ) format and the digital signals that result from AMI, B8ZS, and HDB3 line coding. Notice that the return-to-zero (RZ) format is used in each coded digital signal, that is, the duration of each pulse representing a binary one is equal to one half the bit interval.

1000000001 1 BINARY SEQUENCE (NRZ FORMAT)

AMI CODED DIGITAL SIGNAL

B8ZS CODED DIGITAL SIGNAL

BIPOLAR VIOLATION

HDB3 CODED DIGITAL SIGNAL

BIPOLAR VIOLATION

Figure 1-9. Serial digital signals coded using the AMI, B8ZS, and HDB3 line codes.

93 Digital Trunk Interface

The LINE DECODER in the RECEIVER of the DIGITAL TRUNK INTERFACE receives the serial digital signal regenerated by the DATA/CLOCK RECOVERY CIRCUIT, and converts this signal to the NRZ format. The resulting serial digital signal (recovered data) is sent to the FRAMING CIRCUIT AND ALARM DETECTOR and TIME SLOT INTERCHANGER 1. Figure 1-10 is an example of signals at the input and output of the LINE DECODER.

BINARY SEQUENCE 0110001010

LINE DECODER INPUT SIGNAL

LINE DECODER OUTPUT SIGNAL

Figure 1-10. Signals at the input and output of the LINE DECODER.

FRAMING CIRCUIT AND ALARM DETECTOR

The FRAMING CIRCUIT analyzes the framing information contained in the serial digital signal coming from the LINE DECODER (recovered data) to find the beginning of each frame. This process is referred to as frame alignment, that is, the RECEIVER aligns itself with the serial digital signal received. Frame alignment is absolutely essential to correctly recover the digitized voice signals and the signaling data contained in the various time slots of each frame. When the FRAMING CIRCUIT is able to achieve frame alignment, a rectangular pulse signal appears at its RECOVERED FRAME SYNC. output, each pulse in this signal being aligned with the first time slot of each frame as shown in Figure 1-11. Furthermore, the signal at its loss-of-frame-alignment (LFA) output is at logic level zero. Conversely, the signal at the LFA output goes to logic level 1 when frame alignment is lost.

94 Digital Trunk Interface

BEGINNING OF A FRAME

FRAME INTERVAL (125 μs)

TIME SLOT INTERVALS

RECOVERED FRAME SYNC. SIGNAL

RECOVERED TIME-SLOT CLOCK SIGNAL

RECOVERED MULTIFRAME SYNC. SIGNAL

Figure 1-11. Output signals of the FRAMING CIRCUIT.

The FRAMING CIRCUIT also produces a recovered time-slot clock (RECOVERED TS CLOCK) signal and a recovered multiframe synchronization (RECOVERED MF SYNC.) signal. The RECOVERED TS CLOCK signal is a square-wave signal aligned with the time slot intervals. The RECOVERED MF SYNC. signal consists of a rectangular pulse that occurs at the beginning of each multiframe, the duration of this pulse being equal to one frame (125 μs).

Note: Appendix C of this manual provides information related to the multiframe structures of the DS1 and E1 digital signals.

The ALARM DETECTOR analyzes the serial digital signal coming from the LINE DECODER (recovered data) to detect the presence of remote alarm signals. When an alarm signal is detected, either the AIS or RAI output of the ALARM DETECTOR goes to logic level 1. Alarms are discussed extensively in the next exercise of this unit.

TIME SLOT INTERCHANGER 1

TIME SLOT INTERCHANGER 1 (TSI 1) receives a serial digital signal in NRZ format (recovered data) from the LINE DECODER. It performs time slot interchange, that is, it transfers digitized voice signals contained in certain time slots of the RECOVERED DATA signal to other time slots in the serial digital signal at its output (line TX2 of the DIGITAL TRUNK INTERFACE). For example, a digitized voice signal received in time slot 4 of the RECOVERED DATA signal can be made available in time slot 1 of the serial digital signal on line TX2, as shown in

95 Digital Trunk Interface

Figure 1-12. The use of time slot interchange in the DIGITAL TRUNK INTERFACE allows a digitized voice signal to be received from the digital trunk in a certain time slot while being space-division switched in another time slot in the switching circuit (space-division switch) of the CO. This flexibility greatly reduces the chances of having an inter-exchange call blocked because no communication path can be established. Data coming from the call processor of the CO determines what time slot interchanges are to be performed by TSI 1.

DATA TO SWITCHING TIME SLOT CIRCUIT OF CO INTERCHANGER 1 RECOVERED DATA MEMORY FROM LINE DECODER TIME SLOTS TIME SLOTS TX2 1 2 3 4 5 6 123456

DATA FROM CALL PROCESSOR OF CO RECOVERED FRAME SYNC.

SIGNALING DATA DORX TO CALL PROCESSOR RECOVERED BIT CLOCK OF CO

BIT CLOCK FROM CO

FRAME SYNC. FROM CO

Figure 1-12. TIME SLOT INTERCHANGER 1 (TSI 1) performs time slot interchange and extracts signaling data contained in the RECOVERED DATA signal.

TSI 1 also extracts the signaling data contained in either time slot 16 or 24 of the RECOVERED DATA signal. The extracted signaling data, available at the DORX output of TSI 1, is sent to the call processor of the CO (via the digital signaling processor in the signaling circuit of the CO) where it is analyzed.

Notice that two bit clock signals and two frame synchronization signals are provided to TSI 1 shown in Figure 1-12. The RECOVERED BIT CLOCK and RECOVERED FRAME SYNC. signals, which are synchronized with the RECOVERED DATA signal, are used to write the contents of the RECOVERED DATA signal into the memory of TSI 1. Conversely, the BIT CLOCK and FRAME SYNC. signals from the CO are used to read the contents of the memory in TSI 1.

Operation of the TRANSMITTER

The TRANSMITTER consists of TIME SLOT INTERCHANGER 2, a FRAMING AND SIGNALING CIRCUIT, and a LINE CODER. The operation of each of these circuits is explained in this subsection. The explanations sometimes refer to the simplified block diagram in Figure 1-7.

96 Digital Trunk Interface

TIME SLOT INTERCHANGER 2

TIME SLOT INTERCHANGER 2 (TSI 2) receives digitized voice signals from the switching circuit (space-division switch) of the CO via line RX2. These digitized voice signals are to be transmitted to another CO via the digital trunk. TSI 2 performs time slot interchange to place the digitized voice signals to be transmitted in the proper time slots of the serial digital signal sent to the remote CO via the digital trunk. For example, if a digitized voice signal is received in time slot 3 and is to be transmitted in time slot 5 on the digital trunk, TSI 2 makes the digitized voice signal received from line RX2 during time slot 3 available during time slot 5 of the serial digital signal at its output. Data coming from the call processor of the CO determines the time slot interchanges that must be carried out by TSI 2 for the digitized voice signals to be transmitted via the digital trunk during the proper time slots. The BIT CLOCK and FRAME SYNC. signals from the CO are used to to read the contents of the memory in TSI 2.

Notice that two bit clock signals and two frame synchronization signals are provided to TSI 2 shown in Figure 1-7. The BIT CLOCK and FRAME SYNC. signals from the CO are used to write the contents of the serial digital signal on line RX2 into the memory of TSI 2. Conversely, the TX BIT CLOCK and TX FRAME SYNC. signals, which are synchronized with the TDM format used in the digital trunk, are used to read the contents of the memory of TSI 2.

The serial digital signal at the output of TSI 2 is sent to the FRAMING AND SIGNALING CIRCUIT. Note that when a time slot is not used to carry a digitized voice signal, the output signal of TSI 2 is held at logic level one. This adds a sequence of binary ones in the serial digital signal to be transmitted, and thereby, increases the number of transitions in the coded signal transmitted via the digital trunk. This feature of TSI 2 and the use of B8ZS or HDB3 line coding ensure easy clock recovery in the receiver at the other end of the digital trunk.

FRAMING AND SIGNALING CIRCUIT

The FRAMING AND SIGNALING CIRCUIT (FSC) adds framing information and signaling data to the serial digital signal coming from TSI 2.

The framing information is generated locally by the FSC. When the DS1 TDM format is used, the FSC inserts the framing information in the F bit position of each frame (bit located just before time slot 1). When the E1 TDM format is used, the FSC inserts the framing information in time slot 0 of each frame.

The FSC receives signaling data from the call processor of the CO, via the digital signaling processor in the signaling circuit of the CO and the DOTX input of the DIGITAL TRUNK INTERFACE. The signaling data is information exchanged between two CO's that allows control of inter-exchange calls. When the DS1 TDM format is used, the FSC inserts the signaling data in time slot 24 of each frame, one octet at a time. When the E1 TDM format is used, the FSC inserts the signaling data in time slot 16 of each frame. Note that the FSC inserts binary ones in time slot 24 or 16 whenever there is no signaling data to be transmitted. This adds a sequence of binary ones in the serial digital signal to be transmitted, and thereby, increases the number of transitions in the coded signal transmitted via the digital

97 Digital Trunk Interface

trunk. This helps in ensuring easy clock recovery in the receiver at the other end of the digital trunk.

Note that the FSC requires the TX BIT CLOCK and TX FRAME SYNC. signals from the CO in order to be able to insert the framing information and the signaling data into the correct time intervals of each frame.

LINE CODER

The LINE CODER applies a line code to the serial digital signal coming from the FRAMING AND SIGNALING CIRCUIT, which is in NRZ format, before transmission via the digital trunk. When the DS1 TDM format is used, the LINE CODER applies the B8ZS line code to the serial digital signal. When the E1 TDM format is used, the LINE CODER applies the HDB3 line code to the serial digital signal. The use of these line codes ensures a minimum amount of transitions in the serial digital signal transmitted via the digital trunk, and thus, enables easy clock recovery by the receiver at the other end of the trunk.

Procedure Summary

In the first part of the exercise, you will set up a digital trunk between two Lab-Volt Central Offices (CO's).

In the second part of the exercise, you will observe how framing and signaling are performed in the DIGITAL TRUNK INTERFACE (DTI) used in Lab-Volt CO's. To do so, you will observe the signals involved in the operation of the FRAMING AND SIGNALING CIRCUITs in the DTI TRANSMITTER and RECEIVER. You will then observe the serial digital signals exchanged via the digital trunk when an inter- exchange call is initiated.

In the last part of the exercise, you will determine how time slot interchange is performed in the DTI. To do so, you will observe the signals at the input and output of the TIME SLOT INTERCHANGERs in the DTI TRANSMITTER and RECEIVER when an inter-exchange call is established.

EQUIPMENT REQUIRED

Refer to Appendix A of this manual to obtain the list of equipment required to perform this exercise.

98 Digital Trunk Interface

PROCEDURE

Setting Up a Digital Trunk Between two Lab-Volt Central Offices

Note: In this exercise, it is assumed that a single host computer is used to download the CO program to two Reconfigurable Training Modules, Model 9431. This host computer is used to monitor one of the two Lab-Volt CO's (the one designated as CO A throughout the exercise).

* 1. Make sure that two Reconfigurable Training Modules, Model 9431, are connected to the Power Supply, Model 9408.

Make sure that there is a network connection between each Reconfigurable Training Module and the host computer.

* 2. Install a Dual Analog Line Interface, Model 9475, into one of the two analog/digital (A/D) slots of a Reconfigurable Training Module. This module will be used as CO A. Connect two analog telephone sets to this Dual Analog Line Interface. Make sure that the tone dialing mode is selected on each telephone set.

CAUTION!

Do not connect or disconnect the analog telephone sets when the Reconfigurable Training Module is turned on. High voltages are present on the standard telephone connectors of the Dual Analog Line Interface.

Install a Digital Trunk Interface, Model 9478, into the digital (D) slot or the remaining analog/digital (A/D) slot of the Reconfigurable Training Module used as CO A.

* 3. Install a Dual Analog Line Interface, Model 9475, into one of the two analog/digital (A/D) slots of the other Reconfigurable Training Module. This module will be used as CO B. Connect two analog telephone sets to this Dual Analog Line Interface. Make sure that the tone dialing mode is selected on each telephone set.

Install a Digital Trunk Interface, Model 9478, into the digital (D) slot or the remaining analog/digital (A/D) slot of the Reconfigurable Training Module used as CO B.

* 4. Using the digital trunk line provided with one of the Digital Trunk Interfaces (multi-wire cable terminated with RJ-45 male telephone connectors), connect the RJ-45 female connector on the Digital Trunk Interface installed in the Reconfigurable Training Module used as CO A to the RJ-45 female connector on the Digital Trunk Interface installed in the Reconfigurable Training Module used as CO B.

99 Digital Trunk Interface

* 5. Connect two of the AC/DC power converters supplied with the analog telephone sets to the AC power outlets on the Power Supply. Then, connect the DC power output jack of each AC/DC power converter to the DC power input connector on either of the analog telephone sets A of CO's A and B.

Note: If another Power Supply, Model 9408, is available, connect the two other AC/DC power converters supplied with the analog telephone sets to the AC power outlets on this Power Supply. Then, connect the DC power output jack of each AC/DC power converter to the DC power input connector on either of the analog telephone sets B of CO's A and B.

* 6. Turn on the host computer.

Turn on the Power Supply, then turn on the Reconfigurable Training Modules.

* 7. On the host computer, start the Telephony Training System software, then download the CO program to the Reconfigurable Training Module used as CO A.

Note: If the host computer is unable to download the CO program to the Reconfigurable Training Module used as CO A, make sure that the proper IP address is used to communicate with this Reconfigurable Training Module.

* 8. On the host computer, download the CO program to the Reconfigurable Training Module used as CO B, using the function in the Tools menu that allows a functionality to be downloaded to another RTM.

Note: If the host computer is unable to download the CO program to the Reconfigurable Training Module used as CO B, make sure that the proper IP address is used to communicate with this Reconfigurable Training Module.

Furthermore, if you downloaded the CO program to the Reconfigurable Training Module used as CO B from a second host computer, make sure that the digital trunk TDM format (DS1 or E1) selected in CO B is the same as that selected in CO A. Also make sure that the number of CO B differs from the number of CO A. These parameters, which are accessed via the LVTTS Options dialog box, must be set as mentioned above to enable establishment of inter-exchange calls via the digital trunk. These parameters are set automatically when a single host computer is used to download the CO program to the two Reconfigurable Training Modules.

100 Digital Trunk Interface

Once the CO program has been downloaded, a box representing CO B will appear connected to CO A, via a digital trunk, in the diagram of CO A displayed on the host computer screen.

Record below the telephone numbers associated with ANALOG LINE INTERFACEs A and B of CO B.

ANALOG LINE INTERFACE A (ALI A) of CO B:

ANALOG LINE INTERFACE B (ALI B) of CO B:

* 9. On the Digital Trunk Interfaces installed in the Reconfigurable Training Modules used as CO's A and B, make sure that the alarm indicators (red and yellow LED's) are not lit, thereby confirming that the digital trunk is now ready to carry inter-exchange calls.

* 10. On the host computer, make sure that the addresses of the TSAC's in ALI's A and B are set to 01 and 02, respectively.

Also, make sure that the addresses of the TSAC's of SERVICE CIRCUITs 1 and 2 in the SIGNALING CIRCUIT are set to 01 and 02, respectively.

Framing and Signaling in the DIGITAL TRUNK INTERFACE

* 11. On the host computer, zoom in on the DIGITAL TRUNK INTERFACE (DTI) of CO A, then zoom in on the TRANSMITTER of this interface.

Connect Oscilloscope Probes 1, 2, 3, and 4 to TP8 (TX FRAME SYNC. signal), TP10 (TX TS CLOCK signal), TP7 (TX BIT CLOCK signal), and TP11 (LINE CODER input) of the DTI, respectively.

* 12. Start the Oscilloscope.

Make the following settings on the Oscilloscope:

101 Digital Trunk Interface

Channel 4 Mode ...... Normal Sensitivity ...... 5 V/div Input Coupling ...... DC Time Base ...... 20 μs/div Trigger Source ...... Ch 1 Level ...... 1 V Slope...... Positive (+) Display Mode ...... Square Display Refresh ...... Continuous

Position the horizontal trigger point two divisions from the left-hand side of the Oscilloscope screen.

* 13. On the Oscilloscope screen, observe the TX FRAME SYNC. signal at TP8 of the DTI. This signal consists of a rectangular pulse that occurs every frame.

The TX FRAME SYNC. signal is used to control the read operations in the memory of TIME SLOT INTERCHANGER 2 (TSI 2). It also allows the FRAMING AND SIGNALING CIRCUIT (FSC) to insert framing information and signaling data into the serial digital signal coming from TSI 2.

Measure the period of the TX FRAME SYNC. signal. This period corresponds to the duration of one frame.

From your measurement, do frames occur at the same rate as voice signals are sampled in the Lab-Volt CO's, i.e., 8000 times per second? Explain.

* 14. Observe the TX TS CLOCK signal at TP10 of the DTI. Each cycle in the TX TS CLOCK signal is equal to the duration of one time slot, that is, 5.2 μs when the DS1 TDM format is used, or 3.9 μs when the E1 TDM format is used.

Determine the number of cycles that occur in the TX TS CLOCK signal within one complete frame. This corresponds to the number of time slots that occur within one complete frame.

102 Digital Trunk Interface

* 15. Set the Oscilloscope time base to 5 μs/div.

Observe that the width of the pulse in the TX FRAME SYNC. signal at TP8 is equal to the duration of one time slot. What is the purpose of this pulse? Explain.

* 16. Set the Oscilloscope time base to 1 μs/div.

Observe the TX BIT CLOCK signal at TP7 of the DTI. In conjunction with the TX FRAME SYNC. signal, the TX BIT CLOCK signal is used to control the read operations in the memory of TSI 2. It also allows the FSC to insert framing information and signaling data into the serial digital signal coming from TSI 2.

Determine the number of cycles that occur in the TX BIT CLOCK signal within one time slot. This corresponds to the number of bits that can be conveyed in each time slot.

From your observation, is each time slot perfectly suited to carry a digitized voice signal (8-bit PCM code)?

* Yes * No

* 17. Measure the period of the TX BIT CLOCK signal at TP7 of the DTI. This corresponds to the time interval between transmission of two successive bits via the digital trunk.

Determine the bit rate of the digital trunk (i.e., the number of bits per second which can be transmitted via the digital trunk). To do so, divide 1 by the period of the TX BIT CLOCK signal.

103 Digital Trunk Interface

* 18. (Skip this step if the E1 TDM format is used.)

Position the horizontal trigger point seven divisions from the left-hand side of the Oscilloscope screen.

Observe that, with the DS1 TDM format, the cycle of the TX TS CLOCK signal that occurs just before the beginning of the pulse in the TX FRAME SYNC. signal lasts a little longer than the other cycles (9 bits instead of 8 bits). Explain why.

Position the horizontal trigger point two divisions from the left-hand side of the Oscilloscope screen.

* 19. Set the Oscilloscope time base to 20 μs/div. Observe the serial digital signal at TP11 (LINE CODER input) of the DTI.

Notice that this signal seems to be at logic level 1 most of the time. This occurs because all time slots that are not used to carry a digitized voice signal are filled with binary ones by TSI 2, while the time slot used to carry signaling data is filled with binary ones by the FSC when there is no signaling data to be transmitted.

* 20. Decrease the Oscilloscope time base to 5 μs/div. Observe that the serial digital signal at TP11 sometimes goes to logic level 0 during brief time intervals, since framing information appears during a short segment of this signal in the form of continuous bit transitions between logic states 0 and 1:

– When the DS1 TDM format is used, the framing information appears just before time slot 1 (F bit position) of each frame.

– When the E1 TDM format is used, the framing information appears in time slot 0 of each frame.

Is this your observation?

* Yes * No

* 21. Set the Oscilloscope time base to 10 μs/div.

Using telephone set A of CO A, place a call to telephone set A of CO B and let it ring. While doing this, observe that signaling data momentarily appears in one time slot of the serial digital signal at TP11 (LINE CODER input) of the DTI.

104 Digital Trunk Interface

Note: Since the signaling data appears for a very short period of time in the serial digital signal at TP11, you might have to hang up and repeat the above step several times in order to be able to observe this data on the Oscilloscope screen.

Using the TX FRAME SYNC. signal and the TX TS CLOCK signal, determine in which time slot of the serial digital signal at TP11 the signaling data appears.

Note: If necessary, hang up and repeat this step as many times as required to be able to answer the above question.

Hang up the handset of telephone set A of CO A.

* 22. On the Oscilloscope, select the manual display refresh mode.

Disconnect all the Oscilloscope Probes. Connect Oscilloscope Probes 1, 2, 3, and 4 to TP3 (RECOVERED FRAME SYNC. signal), TP5 (RECOVERED TS CLOCK), TP6 (RECOVERED BIT CLOCK signal), and TP2 (LINE DECODER output) of the DTI RECEIVER, respectively.

On the Oscilloscope, set the time base to 20 μs/div and select the continuous display refresh mode.

* 23. On the Oscilloscope screen, observe the signals produced by the FRAMING CIRCUIT in the DTI RECEIVER, as a result of successful frame alignment:

– Observe the RECOVERED FRAME SYNC. signal at TP3 of the DTI. This signal consists of a rectangular pulse that occurs every frame, each pulse being aligned with the first time slot of each frame.

– Set the Oscilloscope time base to 5 μs/div. Observe the RECOVERED TS CLOCK signal at TP5 of the DTI. Each cycle in this signal is equal to the duration of one time slot.

– Set the Oscilloscope time base to 1 μs/div. Observe the RECOVERED BIT CLOCK signal at TP6 of the DTI. This signal consists of a square wave whose frequency is 8 times higher than that of the RECOVERED TS CLOCK signal (8 cycles per time slot).

The RECOVERED FRAME SYNC., RECOVERED TS CLOCK, and RECOVERED BIT CLOCK signals are all synchronized with the RECOVERED DATA signal at TP2 (LINE DECODER output) of the DTI.

* 24. Set the Oscilloscope time base to 20 μs/div.

105 Digital Trunk Interface

Observe the RECOVERED DATA signal at TP2 of the DTI. Notice that this signal seems to be at logic level 1 most of the time, since neither a digitized voice signal nor any signaling data is currently received from the digital trunk line.

* 25. Decrease the Oscilloscope time base to 5 μs/div. Observe that the RECOVERED DATA signal at TP2 sometimes goes to logic level 0 during brief time intervals, since framing information appears during a short segment of this signal in the form of continuous bit transitions between logic states 0 and 1:

– When the DS1 TDM format is used, the framing information appears just before time slot 1 (F bit position) of each frame.

– When the E1 TDM format is used, the framing information appears in time slot 0 of each frame.

Is this your observation?

* Yes * No

* 26. Set the Oscilloscope time base to 10 μs/div.

Using telephone set A of CO A, place a call to telephone set A of CO B and let it ring. While doing this, observe that signaling data momentarily appears in one time slot of the RECOVERED DATA signal at TP2 of the DTI.

Note: Since the signaling data appears for a very short period of time in the serial digital signal at TP2, you might have to hang up and repeat the above step several times in order to be able to observe this data on the Oscilloscope screen.

Using the RECOVERED FRAME SYNC. signal and the RECOVERED TS CLOCK signal, determine in which time slot of the RECOVERED DATA signal at TP2 the signaling data appears.

Hang up the handset of telephone set A of CO A.

Time Slot Interchange in the DIGITAL TRUNK INTERFACE

* 27. On the Oscilloscope, select the manual display refresh mode.

Disconnect all the Oscilloscope Probes. Connect Oscilloscope Probes 1, 2, 3, and 4 to TP16 (RX2 line), TP17 (TSI-2 output), TP10 (TX TS CLOCK signal), and TP8 (TX FRAME SYNC. signal) of the DTI TRANSMITTER, respectively.

106 Digital Trunk Interface

Make the following settings on the Oscilloscope:

Trigger Source ...... Ch 4 Display Refresh ...... Continuous

* 28. Using telephone set A of CO A, place a call to telephone set A of CO B. Answer the call and have a normal telephone conversation.

While talking into the handset of telephone set A of CO A, observe the PCM codes representing the voice signal that originates from this telephone set at TP16 (RX2 line) and TP17 (TSI-2 output) of the DTI. Notice that these codes appear in different time slots of the signals at TP16 and TP17.

Using the TX FRAME SYNC. signal and the TX TS CLOCK signal, find in which time slot of the signal at TP16 the PCM codes appear. Explain why the PCM codes appear in this time slot.

Using the TX FRAME SYNC. signal and the TX TS CLOCK signal, find in which time slot of the signal at TP17 the PCM codes appear. Explain why the PCM codes appear in this time slot.

* 29. From your observations, what is the function of TIME SLOT INTERCHANGER 2 (TSI 2) in the TRANSMITTER of the DTI? What signals are required for TSI 2 to achieve this function? Explain.

107 Digital Trunk Interface

* 30. Display the control register of TIME SLOT INTERCHANGER 2 (TSI-2 Control Register) of the DIGITAL TRUNK INTERFACE.

Observe that this register indicates 6 in the cell at the intersection of the row labeled INCOMING TIME SLOT (LINE RX2) 1 and the column labeled OUTGOING TIME SLOT (TP17). This indicates that TSI 2 performs time slot interchange from time slot 1 to time slot 6, as observed in the previous steps.

Close the TSI-2 Control Register.

* 31. Do not hang up. On the Oscilloscope, select the manual display refresh mode.

Disconnect all the Oscilloscope Probes. Connect Oscilloscope Probes 1, 2, 3, and 4 to TP2 (LINE DECODER output), TP15 (TX2 line), TP5 (RECOVERED TS CLOCK signal), and TP3 (RECOVERED FRAME SYNC. signal) of the DTI RECEIVER, respectively.

On the Oscilloscope, select the continuous display refresh mode.

* 32. While talking into the handset of telephone set A of CO B, observe the PCM codes representing the voice signal that originates from this telephone set in one time slot of the RECOVERED DATA signal at TP2.

Using the RECOVERED FRAME SYNC. signal and the RECOVERED TS CLOCK signal, determine in which time slot of the signal at TP2 the PCM codes appear. Is this time slot the same as that used for transmission, via the digital trunk, of the PCM codes representing the voice signal that originates from telephone set A of CO A? Explain.

* 33. While talking into the handset of telephone set A of CO B, observe that PCM codes representing the voice signal that originates from this telephone set also appear in one time slot of the signal at TP15 (TX2 line).

* 34. On the Oscilloscope, select the manual display refresh mode.

Disconnect Oscilloscopes Probes 3 and 4 and connect them to TP29 (TS CLOCK signal) and TP30 (FRAME SYNC. signal) of the SIGNALING CIRCUIT, respectively.

108 Digital Trunk Interface

On the Oscilloscope, select the continuous display refresh mode. The FRAME SYNC. signal of CO A is now used to trigger the Oscilloscope sweep.

* 35. While talking into the handset of telephone set A of CO B, observe the PCM codes representing the voice signal that originates from this telephone set in one time slot of the signal at TP15 (TX2 line).

Using the FRAME SYNC. signal and the TS CLOCK signal, determine in which time slot of the signal at TP15 the PCM codes appear. Is this time slot the same as that in which the PCM codes appear in the RECOVERED DATA signal at TP2 of the DTI? Explain.

* 36. From your observations, what is the function of TIME SLOT INTERCHANGER 1 (TSI 1) in the RECEIVER of the DTI? What signals are required for TSI 1 to achieve this function? Explain.

* 37. Display the control register of TIME SLOT INTERCHANGER 1 (TSI-1 Control Register) of the DIGITAL TRUNK INTERFACE.

Observe that this register indicates 3 in the cell at the intersection of the row labeled INCOMING TIME SLOT (TP2) 6 and the column labeled OUTGOING TIME SLOT (LINE RX2). This indicates that TSI 1 performs time slot interchange from time slot 6 to time slot 3, as observed in the previous steps.

109 Digital Trunk Interface

Close the TSI-1 Control Register.

Hang up the handset of telephone sets A of CO's A and B.

* 38. On the host computer, close the Telephony Training System software.

Turn off the TTS Power Supply, as well as the host computer (if it is no longer required).

Disconnect the AC/DC power converters from the TTS Power Supply and the telephone sets.

Remove the digital trunk line (multi-wire cable terminated with RJ-45 male connectors) connecting the Digital Trunk Interface installed in the Reconfigurable Training Module used as CO A to the Digital Trunk Interface installed in the Reconfigurable Training Module used as CO B.

Disconnect the telephone sets from the Dual Analog Line Interface installed in the Reconfigurable Training Module used as CO A. Remove the Dual Analog Line Interface and the Digital Trunk Interface from this module.

Disconnect the telephone sets from the Dual Analog Line Interface installed in the Reconfigurable Training Module used as CO B. Remove the Dual Analog Line Interface and the Digital Trunk Interface from this module.

CONCLUSION

In this exercise, you learned that the digitized voice signals and signaling data transmitted via a digital trunk can be multiplexed according to various TDM formats. Thus, you saw that the digital trunk used to interconnect two Lab-Volt CO's supports two TDM formats: the North American DS1 format and the European E1 TDM format. You learned that both formats divide time into intervals of equal duration called frames, each frame being subdivided into time intervals called time slots. You saw that the DS1 frame uses 24 time slots, while the E1 frame uses 32 time slots. In both cases, one of the time slots is used exclusively to convey signaling data, and framing information is enclosed in a specific bit position (in DS1 TDM format) or time slot (in E1 TDM format) of the frame. The remainder of the time slots in the frame are used to convey digitized voice signals (8-bit PCM codes).

You became familiar with the role of the digital trunk interface in a CO. You learned that this interface provides a link between the digital circuitry of the CO and a digital trunk. You saw that the digital trunk interface consists of two major sections: a transmitter and a receiver. In the transmitter, the digitized voice signals and signaling data coming from the digital circuitry of the CO are time multiplexed to form a serial digital signal that is transmitted on the digital trunk line. In the receiver, the digitized voice signals and signaling data contained in the serial digital signal received from the digital trunk line are demultiplexed (extracted) from this signal and sent to the digital circuitry of the CO.

110 Digital Trunk Interface

You familiarized yourself with the block diagram and operation of the DIGITAL TRUNK INTERFACE (DTI) used in Lab-Volt CO's. You saw that the DTI TRANSMITTER contains:

– a time slot interchanger that places the CO digitized voice signals in the proper time slots of the serial digital signal to be transmitted on the digital trunk line; – a framing and signaling circuit that adds framing information and signaling data to the serial digital signal coming from the time slot interchanger; – a line coder that applies the B8ZS or HDB3 line code to the serial digital signal coming from the framing and signaling circuit, before transmission on the digital trunk line.

On the other hand, you saw that the DTI RECEIVER contains:

– a data/clock recovery circuit that recovers the bit clock from the serial digital signal received from the digital trunk line and regenerates this signal so as to eliminate the effect of distortion; – a line decoder that converts the regenerated serial digital signal from the B8ZS- or HDB3-format to the NRZ format (recovered data); – a framing circuit that finds the beginning of each frame and an alarm detector that detects the presence of remote alarm signals; – a time slot interchanger that transfers digitized voice signals contained in certain time slots of the recovered data signal to other time slots in the signal at its output.

REVIEW QUESTIONS

1. What is the role of the digital trunk interface in a CO? How does it achieve this role? Explain.

111 Digital Trunk Interface

2. What are the two TDM formats available for the digital trunk used to interconnect Lab-Volt CO's? How do these formats resemble each other? How do they differ?

3. What is the use of time slot interchange in the DIGITAL TRUNK INTERFACE of a Lab-Volt CO? What device determines what time slot interchanges are to be performed in the DIGITAL TRUNK INTERFACE?

4. What circuit in the DIGITAL TRUNK INTERFACE of a Lab-Volt CO recovers the bit clock from the serial digital signal received from the digital trunk line? What are two uses of the recovered bit clock?

112 Digital Trunk Interface

5. What is the function of the FRAMING AND SIGNALING CIRCUIT in the TRANSMITTER of the DIGITAL TRUNK INTERFACE of a Lab-Volt CO? How does this circuit achieve this function and what two signals does it require to do this? Explain.

113

Other Samples Extracted from Analog Access to the Telephone Network

Unit Test

1. The pair of wires that connects a telephone set to a central office is referred to as ...

a. a bidirectional transmission line. b. a twisted coaxial cable. c. a local loop. d. the Tip and Ring lines.

2. When number key "5" of a telephone set is depressed, ...

a. a DTMF dialing signal with frequency components at 770 and 1336 Hz is output to the telephone line. b. a DTMF dialing signal with frequency components at 770 and 1209 Hz is output to the telephone line. c. the loop current is momentarily interrupted five successive times. d. either A or C.

3. The public switched telephone network mainly consists of ...

a. telephone sets, twisted coaxial cables, central offices, trunks, and DC power plants. b. telephone sets, telephone lines, central offices, trunks, and higher-level switching offices such as toll offices. c. telephone sets, telephone lines, trunks, and DC power plants. d. telephone sets, coaxial cable transmission lines, central offices, and trunks.

4. Which one of the following functions is not performed by a basic analog telephone set?

a. It sends a service request when the handset is lifted off the cradle. b. It signals an incoming call by an audible sound. c. It displays the caller identity and phone number when a call is received. d. It converts speech at the handset microphone into an electrical signal for transmission to the called party.

5. A central office is a ...

a. telephone company building that is located at the center of a city. b. system that connects a telephone set to another telephone set wired to the same central office or to a trunk homing on this office. c. building owned by a telephone company to house its employees as well as networks of personal computers. d. telephone system specially designed for long distance calls.

117 8QLW7HVW FRQW G

6. Device on a telephone set that converts speech into an electrical signal and vice versa.

a. Speech circuit. b. Switchhook. c. Handset. d. None of the above.

7. To make a telephone set ring, the ...

a. central office applies an AC ringing voltage across the telephone line. b. calling party telephone applies an AC ringing voltage across the telephone line. c. central office applies a DC voltage across the telephone line. d. central office applies an AC ringing voltage to the switching circuit.

8. When a telephone handset is lifted off the cradle, the central office is notified of a service request because ...

a. the switchhook opens, thereby interrupting the DC current in the local loop. b. the switchhook closes, thereby applying an AC ringing voltage across the telephone line. c. dialing signals are output to the telephone line. d. the switchhook closes, thereby causing DC current to flow in the local loop.

9. To maintain a constant voice level despite an increase in the telephone line length, the speech circuit ...

a. increases its line side impedance. b. maintains its line side impedance at a constant level. c. decreases the sidetone level. d. Both A and C.

10. An analog telephone set ...

a. provides an analog access to the public switched telephone network which, by today's standards, is mainly a digital system. b. provides an analog access to the public switched telephone network which, by today's standards, is also an analog system. c. provides an analog access to the public switched telephone network as well as the Internet. d. None of the above.

118 Instructor Guide Sample Extracted from Analog Access to the Telephone Network

Analog Access to the Telephone Network

UNIT 1 THE TELEPHONE SET

EXERCISE 1-1 TELEPHONE RINGING

ANSWERS TO PROCEDURE QUESTIONS

* 6. The AC ringing voltage is a sine wave.

* 7. AC Ringing Voltage RMS Value: 86 V, can also be set to 45 or 75 V in the Telephony Training System.

AC Ringing Voltage Frequency: 16.7 Hz, 20 Hz (North America), 25 Hz or 50 Hz (UK), depending on country.

* 10. The AC ringing voltage is applied momentarily across the telephone line at regular intervals. The telephone set rings whenever the AC ringing voltage is applied across the telephone line, thereby producing a certain ringing cadence.

* 11. The AC ringing voltage is immediately removed from the telephone line to stop telephone ringing.

* 14. Telephone set A stops ringing because the increase of the telephone line resistance makes the amplitude of the AC ringing voltage across the Tip and Ring terminals decrease below the ringing threshold voltage.

* 15. Ringing Threshold Voltage: 40 V

ANSWERS TO REVIEW QUESTIONS

1. To make an analog telephone set ring, the central office applies an AC ringing voltage to the corresponding telephone line via the analog line interface associated with that line.

2. In North America, the frequency and RMS voltage values of the AC ringing voltage are 20 Hz and 86 V, respectively.

3. The telephone rings during 2-s intervals separated by 4-s pauses.

4. Electronic ringer circuits compare the amplitude of the AC ringing voltage to a fixed threshold voltage to prevent telephone ringing triggered by undesired voltage spikes on the telephone line (such as those induced by lightning for example).

5. The threshold voltage of the electronic ringer circuit in the analog telephone sets of the Telephony Training System is approximately 40 V.

121 Analog Access to the Telephone Network

EXERCISE 1-2 THE TELEPHONE SWITCHHOOK AND HANDSET

ANSWERS TO PROCEDURE STEP QUESTIONS

* 6. DC Voltage Across the Telephone Line: 45 V

Note: In the Lab-Volt Telephony Training System, the DC voltage across the telephone line may be a little less than the nominal value of 48 V.

The DC voltage across the telephone line comes from a battery feed circuit in the central office.

* 7. No current is flowing through the telephone line because the handset of telephone set A is on the cradle, thereby leaving the switchhook contacts open. Therefore, the telephone set is like an open circuit.

* 8. DC Loop Current: 36 mA (handset off the cradle)

Lifting off the telephone handset from the cradle closes the switchhook contacts. This connects the dialing and speech circuits of the telephone set to the telephone line. These circuits require DC power to operate, hence the DC current flowing through the telephone line.

* 10. The signal at TP3 of ANALOG LINE INTERFACE A represents an electrical equivalent of the voice of the person who is talking into the handset of telephone set A.

* 12. The signal at TP3 of ANALOG LINE INTERFACE A represents an electrical equivalent of the voice of the person who is talking into the handset of telephone set B.

The handset microphone converts speech sound waves into an electrical signal. Conversely, the handset earpiece converts an electrical signal into sound waves. The speech circuit performs 2W/4W conversion, that is, it directs the voice signal to be transmitted from the handset microphone to the telephone line, and the received voice signal from the telephone line to the handset earpiece.

* 13. The voice sound level in the handset earpiece does not change significantly as the telephone line resistance (length) increases. This is because the telephone speech circuit is able to increase its line side impedance, thereby preventing the electrical voice signals from being attenuated as the telephone line resistance (length) increases.

122 Analog Access to the Telephone Network

ANSWERS TO REVIEW QUESTIONS

1. A switchhook is a DPST switch that closes when the telephone handset is lifted off the cradle.

2. The telephone switchhook connects the dialing and speech circuits to the telephone line when the handset is off-hook. It is also a means to notify the central office of service requests.

3. The speech circuit properly routes the electrical voice signals on the two wires of the telephone line to the four wires of the handset (this process is referred to as two-wire to four-wire conversion). It also adjusts the level of the voice signals so as to compensate for different telephone line lengths.

4. Loop length equalization is a process that adjusts the line side impedance of the speech circuit so that it is inversely proportional to the DC current flowing in the telephone line. This makes the speech circuit impedance virtually proportional to that of the telephone line, thereby maintaining the voice signals at a nearly constant level.

5. The portion of the voice signal of a telephone user that is sent back to the handset earpiece is a sidetone. It gives feedback to the telephone user to help judge the proper volume at which to speak.

EXERCISE 1-3 TONE DIALING

ANSWERS TO PROCEDURE STEP QUESTIONS

* 7. A DTMF dialing tone is output to the telephone line whenever a key is depressed on the telephone set. These tones can be heard through the handset earpiece.

The waveform of the DTMF dialing signals resembles a distorted sine wave that fluctuates.

* 9. The frequency spectrum of each DTMF dialing signal contains two frequency components.

123 Analog Access to the Telephone Network

* 10.

FREQUENCY COMPONENTS KEY (Hz)

1 700 and 1207

2 700 and 1340

3 700 and 1480

4 773 and 1207

5 773 and 1340

6 773 and 1480

7 853 and 1207

8 853 and 1340

9 853 and 1480

a 940 and 1207

0 940 and 1340

# 940 and 1480

Table 1-2. Frequency components of the DTMF signal associated with each key of a telephone set using tone dialing.

* 11.

TELEPHONE FREQUENCY KEYPAD (Hz)

ROW 1 123 700

ROW 2 456 773

ROW 3 7 8 9 853

ROW 4 * 0# 940 COLUMN 3 1480

COLUMN 2 1340

COLUMN 1 1207

Figure 1-13. Frequencies associated with the rows and columns of the telephone keypad.

Yes.

124 Analog Access to the Telephone Network

ANSWERS TO REVIEW QUESTIONS

1. DTMF tone dialing is a means of transmitting telephone numbers to the central office using dual-tone audio signals.

2. DTMF tone dialing assigns a specific frequency to each row and column of the telephone keypad. When a key is depressed, a dual-tone audio signal consisting of the frequencies associated with the corresponding row and column of the keypad is output to the telephone line.

3. The frequencies of the DTMF dialing signal associated with number key "8" of a telephone set are 852 and 1336 Hz.

4. The frequencies associated with the rows of the keypad on a telephone set that produces DTMF dialing signals are 697 (upper row), 770, 852, and 941 Hz (lower row).

5. The frequencies associated with the columns of the keypad on a telephone set that produces DTMF dialing signals are 1209 (leftmost column), 1336, and 1477 Hz (rightmost column).

EXERCISE 1-4 PULSE DIALING

ANSWERS TO PROCEDURE STEP QUESTIONS

* 7. Whenever a number key is depressed on the telephone set, the loop current is briefly interrupted a certain number of times to create a series of current pulses. These pulses can be heard through the handset earpiece.

* 9. When the digit 6 is dialed, the loop current is interrupted briefly six successive times since the voltage across the telephone line briefly decreases to about 45 V six successive times. This produces a series of current pulses that will be interpreted as the digit 6 in the central office.

* 11. Whenever a digit is dialed on the telephone set, the loop current is interrupted briefly. The number of current interruptions depends on the digit dialed: one interruption for the digit 1, two interruptions for the digit 2, and so on up to the digit 0 which results in 10 interruptions.

* 12. Dial Pulse Period: 100 ms

Duration of Current Interruptions: 60 ms

Duration of Current Pulses: 40 ms

125 Analog Access to the Telephone Network

* 13. Inter-Digit Interval: 820 ms

ANSWERS TO REVIEW QUESTIONS

1. Pulse dialing is a means of transmitting telephone numbers to the central office through a series of momentary loop current interruptions.

2. Pulse dialing momentarily interrupts the loop current several successive times to transmit a dialed digit to the central office as a series of short current pulses. The number of momentary loop current interruptions that are produced depends on the digit dialed: one interruption for the digit 1, two interruptions for the digit 2, and so on up to the digit 0 which results in 10 interruptions.

3. Digits dialed with a pulse dialing telephone set are separated by time intervals of at least 300 ms (nominal duration of 700 ms in North America) during which the loop current is not interrupted.

4. In North America, the duration of the loop current interruptions is 60 ms. In other countries, the duration is about 67 ms.

5. Tone dialing is preferred to pulse dialing because it allows telephone numbers to be dialed more rapidly.

ANSWERS TO UNIT TEST

Unit 1: 1. c, 2. d, 3. b, 4. c, 5. b, 6. c, 7. a, 8. d, 9. d, 10. a

126 Bibliography

Bellamy, John, Digital Telephony, 3rd edition, Wiley-Interscience, New York, 2000. ISBN: 0-471-34571-7

Bigelow, Stephen J., Fike, John L., Friend, George E., Understanding Telephone Electronics, 3rd edition, Macmillan Computer Publishing, Indiana, 1991 ISBN: 0-672-27350-0

Freeman, Roger L., Telecommunication System Engineering, 3rd edition, Wiley- Interscience, New York, 1996

Thompson, Richard A., Telephone Switching Systems, Artech House Inc., Boston, 2000. ISBN: 1-58053-088-5

Van Bosse, John G., Signaling in Telecommunication Networks, Wiley-Interscience, New York, 1998. ISBN: 0-471-57377-9