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Home , Crossbar switch, Inside plant, Stored program control, Telephone exchange

Evolution of Switching System

UNIT 2

3.1 Introduction

Switching systems, and associated signalling systems, are essential to the operation of networks. The functions performed by a switching system, or a subsystem of it, in order to provide customers with services are called facilities. Over the years, the design of switching systems has become ever more sophisticated, in order to provide additional facilities which enable networks to provide more services to customers and to facilitate operation and maintenance.

3.2 Message switching

In the early days of a customer might wish to send a message from town A to town B although there was no telegraph circuit between A and B. However, if there was a circuit between A and C and one between C and B, this could be achieved by the process known as message switching. The operator at A sent the message to C, where it was written down by the receiving operator. This operator recognised the address of the message as being at B and then retransmitted the message over the circuit to B. Subsequent technical developments enabled improvements to be made in message switching. The figure 3.1 below describes the manual process and the advanced methods in message switching.

Figure 3.1 Evolution of message switching (a) Manual transfer of hard copy (b) Manual transfer of paper tape (c) Manual transfer of paper tape with automatic route selection (d) Automatic message switching system. T = , R/P = reperforator, A/T = Automatic transfer, S= store.

First, the message received at C was automatically recorded on punched tape and subsequently torn off the receiver by the operator, who read the address from the tape. The message was then transmitted automatically from the same tape as shown in Figure 3.1(b). Later, the outgoing route was also selected automatically as shown in Figure 3.1 (c). Finally, the paper tape was eliminated by storing the messages electronically and analyzing their addresses by electronic logic as shown in Figure 3.1(d).

Message switching was the first area in telecommunications to adopt stored-program control (SPC). In message switching centre an incoming message is not lost when the required outgoing

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route is busy. It is stored in a queue with any other messages for the same route and retransmitted when the required circuit becomes . Message switching is thus an example of a delay system or a queuing system

3.3

Invention of introduced a new requirement; simultaneous both-way communication in real time. It became necessary to connect the circuit of a calling telephone to that of the called telephone on demand and to maintain this connection for the duration of the call. This is called circuit switching. If the required outgoing circuit from a is already engaged on another call, the new call offered to it cannot be connected. The call cannot be stored, as in message switching; it is lost. Circuit switching is thus an example of a lost-call system.

Difference between Message Switching and Circuit Switching

3.4 Functions of a switching system

The basic functions that all switching systems must perform are as follows.

1. Attending: The system must be continually monitoring all lines to detect call requests. 2. Information receiving: In addition to receiving call and clear signals the system must receive information from the caller as to the called line (or other services )required. This is called address signal. 3. Information processing: The system must process the information received, in order to determine the actions to be performed and to control these actions. Since both origination and terminating calls are handled differently for different customers, class of service information must be processed in addition to the address information. 4. Busy testing: After processing the received information to determine the required outgoing circuit the system must make a busy test to determine whether it is free or already engaged on another call. If a call is to a customer with a group of lines to a PBX or to an outgoing junction route, each line in the group is tested until a free one is found. In an automatic

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system, busy testing is also required on trunks between in the exchange. 5. : For a call between two customers, three connections are made in the following sequence. (a) A connection to the calling terminal (b) A connection to the called terminal (c) A connection between the two terminals 6. Alerting: Having made the connection, the system sends a signal to alert the called customer to the call. 7. : After the called terminal has answered, the system continues to monitor the connection in order to be able to clear it down when the call has ended. When a charge for the call is made by metering, the supervisory circuit sends pulses over the P wire to operate a meter in the line circuit of the calling customer. 8. Information sending: If the called customer‟s line is located on another exchange the additional function of information sending is required. The origination exchange must signal the required address to the terminating exchange.

3.5 Distribution systems

Many changes occur during the life of a . New customers join and old ones leave. Customers move from one part of the exchange area to another area. Those with PBX may increase their number of exchange lines. Total number of lines may increase from that during the initial installation. Growth of traffic may require additional switches in the exchange and more junctions to the other exchanges. Great flexibility is therefore required in of an exchange. This is obtained by inserting distribution frames into the permanent exchange cabling. These frames contain an array of terminal blocks and the terminals are linked in a less permanent fashion by wires called jumpers. The in a typical step-by-step exchange is as shown in the Figure 3.2.

(a)

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(b)

Figure 3.2 (a) Simple diagram illustrating the in an exchange. (b) Distribution Frames in Strowger exchange.

Main Distribution Frame (MDF): The Main Distribution Frame is a place where the cables of the customer‟s distribution network terminate. The arrangement of terminals on the line side of the MDF corresponds to the street cabling and so reflects the geography of the area. The terminals on the exchange side of the MDF are arranged in the directory-number (DN) order. Thus, the number of a line is changed by moving a jumper. Protectors and fuses are mounted on the MDF to guard exchange apparatus against any high voltage surges on the external lines. The MDF also provides a convenient point of access for testing lines. Circuits which are not switched in the exchange are strapped together at MDF as shown in the above Figure 3.2 (b).

Intermediate Distribution Frame:

Some customers originate much traffic, but others very little. The Intermediate distribution frame is used to distribute incoming traffic evenly over the groups of first selectors. On the multiple side of the IDF, lines are arranged in arranged in directory-number order. On the local side, the order can be arbitrary to obtain the desired result. The terminals on this side of the IDF can be said to correspond to equipment numbers (EN) of the lines. Customer‟s uniselectors are therefore connected to the local side of the IDF. If exchange is equipped with meters, these are required to be associated with directory numbers; they are therefore cabled to the multiple side of the IDF. Incoming calls for a customer terminate at the final selector on an outlet corresponding to the directory number. The final selector multiples are therefore cabled to the multiple side of the IDF. The Figure 3.3 below shows how the switching action is performed for the number dialled by the customer to the called person.

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Figure 3.3 Strowger switching action for the number dialled by the subscriber.

Modern systems provide directory number to equipment number translation (DN-EN) in order to enable customer‟s incoming traffic to be redistributed in addition to their outgoing traffic. Between the ranks of selectors there are trunk distribution frames (TDF). If additional selectors are needed at any switching stage, to cater for the growth in traffic these can be accommodated by rearranging connections in TDF. For a digital switching system, digital circuits are terminated on a digital distribution frame (DDF). Use of IDF is no longer necessary.

3.6 Crossbar systems Strowger switched require regular maintenance. The banks need cleaning, mechanism need lubrication and adjustment and wipers and cords wear out. This disadvantage led to the development of several other forms of switch and was one among them. Figure 3.4 shows the simple crossbar .

Figure 3.4 Matrix of crosspoints Each crosspoint represent a contact which is operated through horizontal and vertical bars by magnets at the sides of the switch. Thus, a switch with N inlet and N outlet only need 2N operating magnets and armatures instead of N2. Magnets which operate the horizontal bars are called the

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select magnets and those operating the vertical bars are called hold magnets or bridge magnets. Figure 3.5 gives a general view of the crossbar switch.

Figure 3.5 General view of the crossbar switch When an electromagnet, say in the horizontal direction is energised, the bar attached to it slightly rotates so that the crosspoints attached to the bar move closer to its facing contact points but do not actually make any contact. Now, when a magnet in the vertical direction is energised, the corresponding bar rotates causing the contact point at the intersection of the two bars to close. However the crossbar has no intelligence and something external to the switch must decide which magnets to operate. This is done with the help of marker, which can control many switches and serve many registers as shown in the Figure 3.6 below.

(a) (b) Figure 3.6 (a) & (b) Marker control of crossbar switch Functions of Marker 1. Defines the link to be used and the magnets to be operated. 2. Doesnt make connection until the busy or free condition of the outgoing trunk and relevant link has been interrogated. 3. Switches are operated only if both ends are found to be free which is known as conditional selection. 4. Since the marker has access to both ends of a connection it can test the connection for Page 7

continuity before it releases. 5. If the connection is found to be faulty, the marker can output a fault report and attempt to set up the connection over a different path through the network.

3.7 Electronic Switching In electromechanical exchanges, common controls mainly use switches and relays which were originally designed for use in switching networks. Common controls are operated much more frequently and wear out earlier whereas the life of an electronic device is almost independent of its frequency of operation. This led to the development of electronic common controls and resulted in the electronic replacement as these kinds of controls have much greater reliability compared to electromechanical controls.

Advances made in technology were incorporated and led to the development of (SPC). This enables a digital computer to be used as a central control and perform different functions with the same hardware by executing different programs.SPC can offer a wider range of facilities than earlier systems. As the processors stored data can be altered electronically, some of these facilities can be controlled by customers which include:

1. Call barring (outgoing or incoming)the customer can prevent unauthorized calls being made and can prevent incoming calls when wishing to be left in peace. 2. Repeat last callIf a called line is engaged, the caller can try again later without having to redial the full number 3. Reminder calls The exchange can be instructed to call the customer at a pre-arranged time. 4.Call diversionThe exchange can be instructed to connect calls to a different number when the customer is out of reach. 5. Three-way calls The customer can instruct the exchange to connect a third party to a call that is already in progress 6. Charge advice depending on the instruction sent by the caller at the start of a call the exchange calls back at the end of the call to indicate the call duration and charge. In order to develop a fully electronic exchange it was necessary to replace electromechanical controls with electronic. Considering the crossbar system the electromagnets and the mechanical assembly used for establishing the contact point is replaced by the diode as it could be used as the cross-point. Another approach is to use a multiplex system instead of multipled elements. A Frequency division multiplex system can be used as a switch by bringing two ends of the transmission path together. This proved too expensive to put into practice. Nevertheless multiplex principle was not abandoned. A new multiplex scheme known as TDM was developed which proved successful and is widely used throughout the world. 3.8 Digital switching systems Switching systems may be classified as: 1. Space-division (SD) systemsEach connection is made over a different path in space which exists for the duration of the connection 2. Time-division (TD) systemsEach connection is made over the same path in space, but at different instants in time.

TDM transmission was being introduced for trunk and junction circuits in the form of pulse-code . If Time division transmission is used with space division tandem switching as shown in the Figure 3.7(a), it is necessary to provide demultiplexing equipment to demodulate every channel to audio before switching and equipment to retransmit it after switching. If Time

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division switching is used, as shown in Figure 3.7(b), no multiplexing and demultiplexing equipment is needed. A considerable economy is thereby obtained.

(b) Figure 3.7 Tandem exchange with PCM junctions. (a) Space-division switching (b) Tim division switching

3.8.1 Evolution of Digital switching system. A tandem exchange has a mixture of PCM junctions and analog audio junctions, PCM terminal equipment is needed instead for the analog junctions. Large cost savings can be made by using time- division switching for trunk and tandem exchanges.

Incoming audio circuit PCM

Incoming digital circuit Terminal TDM Switch Outgoing audio circuit PCM Terminal Outgoing digital circuit (a)

Digital Junction

Audio Junction PCM TDM Terminal Switch Subscriber‟s line PCM Terminal Space division concentrator (b)

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Digital Junction

TDM Codec Switch Audio Junction Codec Subscriber‟s line (c)

Digital Junction TDM

Digital subscriber‟s line Switch

Codec (d)

Figure 3.8 Evolution of digital switching systems (a) Trunk or tandem exchange (b) Local exchange with space-division concentrators (c) Local exchange with codecs in customers line circuits (d) Local exchange with digital customers lines.

In the initial trunk exchanges as shown in Figure 3.8(a) the incoming signal from each of the subscribers end used to be analog and it was converted to digital from through PCM terminal equipment at the exchange end and then was applied to the Time division switch. If the signal was from a digital circuit itself, then it was directly applied to the Time division switching equipment which is an electronic switching form widely in use. In the next stage the from originating from a digital source is connected directly to the TDM switch whereas the was applied to the PCM terminal and later on applied to the TDM switch. Instead of providing each subscriber PCM terminal equipment, all the subscribers are connected to the space division concentrator whose output is connected to the PCM terminal equipment which was cost effective. With the development in technology, large scale integrated circuits appeared which enabled a PCM coder/decoder (codec) to be manufactured on a single chip and made it practicable to have one for each customer‟s line as shown in Figure 3.8 (c) & (d). This provides digital transmission over customer‟s line which can have many advantages With the developments in digital switching systems it became possible to implement all the necessary functions economically on subscriber‟s line-interface as shown in the Figure 3.9. The functions are as follows. Battery feed Over-voltage protection Supervisory signalling Coding Hybrid Testing

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Figure 3.9 Block diagram of subscriber’s line interface circuit for a digital exchange. if the line is digital, data can be transmitted by removing the codec. Moreover any form of can be transmitted at 64 kbit/s. This can include high-speed and slow-scan TV, in addition to speech data. This concept led to the development of Integrated services digital network (ISDN), in which the customer‟s terminal equipment and the local digital exchange can be used to provide many different services all using 64 kbit/s digit streams. Access to ISDN is provided in two forms: 1. Basic-rate accessThe customers line carries two 64 kbit/s „B‟ channels plus a 16 kbit/s „D‟ channel in each direction.

2. Primary-rate access Two wires are provided to carry a complete PCM frame at 1.5 Mbit/s or 2 Mbit/s en each direction. This gives the customer 23 or 30 circuits at 64kbit/s plus a common signalling channel, also at 64 kbit/s.

3. 9 Switching system Fundamentals

3.9.1 Digital switching system analysis

System analysis and design is defined as the process of developing user requirements and designing systems to achieve them effectively. An exchange or central office is a large, complex system comprising many subsystems each with unique characteristics and functionality. A basic understanding of these systems and their interaction with the rest of the system is needed for a digital switching system to be effectively analyzed.

Purpose of Analysis

The reliability of a digital switching system is becoming increasingly important for users of telephone services. Currently, most access takes place through digital switching systems. Almost all electronic money transfers depend on the reliability of digital switching systems.Hence, reliability of a digital switching system is a very serious matter as it can impact nation‟s commerce, security, and efficiency. The current is becoming very complex; it has multiple owners and is equipped by many different suppliers.

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The analysis of reliability of a complex digital switching system can be done by fully understanding the workings of a digital switching system. If one needs to conduct only the hardware reliability assessment of a digital switching system, then data pertaining to hardware components are needed. If the analysis includes software assessment, then the process requires the understanding of software design methodologies, software architecture, software quality control, software testing, etc.

4.0 Basic Central Office Linkages

Defining the extent of a central office and its linkages to other facilities helps in the analysis of a digital switching system. Figure 3.10 shows a typical central office with some important facilities.

Figure 3.10

Main distributing frame (MDF) It‟s the location where all lines and other related links are cross-connected to a central office switch, also referred to as the line side of a switch. All lines from subscribers terminate in the MDF. The MDF has two sides a vertical and a horizontal. The subscribers cable terminate on the vertical side whereas the wiring from the digital switching system end referred to as the line equipment will get terminated over the horizontal side. The assignment process for subscribers to line equipment is usually automated. Trunk Distribution frame (TDF) It‟s the location where all trunks and other related links are cross connected to a central office switch, also referred to as the trunk side of a switch. The TDF is usually smaller than the MDF. All trunk cabling from different locations terminates in the TDF. The TDF has two sides the vertical and horizontal. The trunk cables terminate on the vertical side. The wiring from the digital switching system, referred to as trunk equipment, terminates on the horizontal side. Based on the assignment of cable to trunk equipment, the vertical cable pair are connected to the horizontal trunk equipment pair. The assignment process for trunks to trunk equipment is usually automated. Power Plant This is a combination of power converters, battery systems, and emergency power sources which supply the basic -48V and + 24 V direct-current power and protected alternating-current power to a CO switch or a group of switches.

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Carrier facilities Facilities which employ carrier or multiplex transmission mode between central offices and with other parts of the network. These facilities typically employ coaxial cables and and satellite systems. The carrier facilities usually terminate on the TDF for cross connection to the digital switching system. Digital X-connect Digital cross-connect provides automatic assignments and cross-connections of trunks to digital switching systems. It can be considered a small switching system for trunks. Special Services Includes those services which require special interfaces or procedures to connect central office facilities to a customer, e.g., data services and services. versus Most of the telephone companies classify their equipment as outside plant or inside plant. This classification becomes important during the analysis of a switching system. Any element of telephony equipment outside the CO box, such as MDF and carrier systems, is classified as outside plant. CO equipment, such as central processors, switching fabric, and tone generators, are considered inside plant. 4.1 Evolution of Digital Switching Systems Many design concepts for the modern digital switching systems come from the electro-mechanical telephony switching systems of the past. 4.1.1Stored Program Control Switching With the advent of software controlled central processors, the control of switching functions was programmed into memory and actions were executed by the controlling processor. The early version of the switching systems had temporary memory for storing transient-call information and semi permanent memories that carried programming information and could be updated. The Figure below depicts the simplified view of a telephony switch which uses stored program control switching system.

Figure 3.11 Basic control structure of a central office The basic function of SPC system is to control line originations and terminations and to provide trunk routing to other central or tandem offices. The SPC system also provided control of special features and functions of a central office, identified as ancillary control. The intelligence of an SPC resided in one processor, and all peripherals were controlled by this single processor. These processors were duplicated for reliability. A modern digital switching system employs a number of processors and uses distributed software and hardware architectures.

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4.2 Digital Switching System Fundamentals A switching system is called digital when the input to and output from the switching system network can directly support digital signals. A digital signal can be defined as coded pulses that can be used for signalling and control. However analog signals can still be processed through the digital switching system via analog-to-digital (A/D) or digital-to-analog (D/A) converters. The evolution of digital switching from analog switching is shown in the Figure 3.12 (a) to (d).

Figure 3.12 Digital switch evolution Figure 3.12(a) shows a typical analog switch with analog lines and trunks. Figure 3.12(b) shows the next step in the evolution of digital switching system which uses analog lines and trunks but employs A/D and D/A converters for digital processing of calls. The switching element is a digital switch, which means that digitized signals are sent through the switch. Figure 3.12(c) shows the next step in this process, in which digital switches can “talk” to other digital switches via digital trunks while simultaneously supporting analog lines and trunks. Figure 3.12(d) shows the ultimate, in this arrangement there is no analog lines or trunks involved; all communication between digital switches is via digital signalling. 4.2.1 Building Blocks of a Digital Switching System The development of a digital switching system model is described in four stages. The first stage looks at the very basic kernel of a digital switching system, with the switching matrix, which is called switching fabric. The switching fabric switches the lines and trunks under the control of a central processor and network controller. Stage 1 At this stage, all inputs and outputs to a digital switching system are defined. The central processor controls the network controller, which in turn controls the switching fabric. Stage 2 The concept of line modules and trunk modules is introduced here. The line and trunk modules are the building blocks of a modern digital switching system, and they represent some lines or trunks grouped together on circuit packs, termed line or trunk equipment, and connected to the switching fabric through a controlling interface. Modern digital switching systems use various schemes to terminate lines on the line module. Some digital switching systems allow termination of only one line on one line module, while other allow termination of multiple lines on a single line module. If a line module becomes defective, this may impact a number of lines, if line module carries multiple lines. However if a line equipment becomes defective, the line can be easily assigned new line equipment if LM carried multiple pieces of line equipment. In a modern digital switching system, line and trunk modules are designed to be modular, which means that a number of these units can be added on an as-needed basis without reengineering the system. This allows for the easy growth and offers flexibility in offering new services.

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(a) (b) Figure 3.13 (a) Stage 1 of Digital Switching System (b) Stage 2 of Digital Switching System (c) Stage 3 of Digital Switching System (d) Stage 4 of Digital Switching System

(c)

(d)

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Stage 3 This stage is shown in the above Figure 3.13(c).The concept of distributed processing in a digital switching system was developed here. The Network controller was replaced by control processor and addition of an interface controller for LM‟s and TM‟s. The task of controlling the switching fabric is usually assigned to a series of Network control processors that control a part of switching fabric and a group of LM‟s and TM‟s. The central processor controls the actions of the network control processors. This type of architecture is very flexible and allows the construction of different sizes of central office by increasing the number of network control processors. This type of architecture is used by many commercial digital switching systems. Stage 4 As shown in the above Figure 3.13(d) shows stage 4 of the digital switching system. This may appear to be the final stage of a digital switching system model but it is not. This basic model introduces the duplicated scheme which is now commonly used in modern digital switching systems. Since telephony processing is a nonstop process requiring high reliability, a duplicated scheme for processing units and associated memory units is almost mandatory. This basic model also shows the attachment of interface controllers and service circuits to the line and trunk modules. The purpose of the service circuits is to provide , ringing, and other associated functions. 4.2.2Basic Call Processing Some basic types of calls that are usually processed through a digital switching system are Intra-LM calls Inter-LM calls Incoming calls Outgoing calls

Intra-LM calls

Figure 3.14 (a) Calls within a line module

When a customer dials from a telephone that is connected to a specific line module and calls another customer who is also connected to the same line module, this type of call is classified as an intra-

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LM call. A call path for this type of a call is shown in the Figure 3.14(a). The off-hook (line origination) condition is detected by the line module, and service circuits area attached to supply a dial tone to the calling customer. The line modules request for a path through the switching fabric is processed by the interface controller, which in turn works with the network control processor to make a path assignment. Consequently, a path through the switching fabric for the called line, and a service circuit is attached to ring the line. Since this is an intra-LM call, the same line module will be involved in controlling the origination and termination of a call.

Inter-LM calls

Figure 3.14 (b) Calls outside a line module

The working of an inter-LM call is similar to those of an intra-LM call, except that the terminating line equipment is located in another line module. Figure 3.14(b) shows for such a call.

Outgoing Calls

When a LM processes a call which has terminating equipment outside CO the LM requests a path through the switching fabric to a trunk module via the interface controller. The interface controller works with the network control processor to establish a path to an outgoing trunk. Once a path is established through the switching fabric the TM connects service circuit for controlling the call to the called CO or a tandem office. An outgoing call from an originating office is an incoming call to a terminating office.

Incoming Calls

When a TM detects an incoming call, it attaches service circuits to control the call and requests a path through the switching fabric from the interface controller and network control processor. Once a path is found through the switching fabric to a LM that has the terminating line, service circuits are attached to ring the called telephone.

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Figure 3.14 (c) Incoming/Outgoing trunk call

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