SECX1034 TELECOMMUNICATION SWITCHING SYSTEMS UNIT I EVALUATION OF PUBLIC SWITCHED TELECOMMUNICATION NETWORKS Switching system functions – stronger switching system – cross bar exchange – SPC exchange – Message switching – circuits switching – Telephone handset – four wire concept – Hybrid circuit – Echo suppressor and cancellors – PCM coders – Modems and relays – Telecommunication standard.
1.1 Switching System. The switching centers receives the control signals, messages or conversations and forwards to the required destination, after necessary modification (link amplifications) if necessary. A switching system is a collection of switching elements arranged and controlled in such a way as to setup a communication path between any two distant points. A switching center of a telephone network comprising a switching network and its control and support equipment is called a central office. In computer communication, the switching technique used is known as packet switching or message switch (store and forward switching). In telephone network the switching method used is called circuit switching. Some practical switching system are step-by-step, cross barred relay system, digital switching systems, electronic switching system etc. Signalling Systems: A signalling system in a data communication networks exchanges signalling information effectively between subscribers. The signalling systems are essential building blocks in providing the ultimate objective of a worldwide automatic telephone services standardized. Signalling provides the interface between different national systems. The introduction of signalling system was the big step in improving the PSTN.
1.2 Strowger switching systems-The Invention of Automatic Switching
Almon B. Strowger was an undertaker in Kansas City, USA. The story goes that there was a competing undertaker locally whose wife was an operator at the local (manual) telephone exchange. Whenever a caller asked to be put through to Strowger, calls were deliberately put through to his competitor. This obviously frustrated Strowger greatly and he set about devising a system for doing away with the human part of the equation.Strowger developed a system of automatic switching using an electromechanicalcal switch based around around electromagnets and pawls. In this selector, a moving wiper (with contacts on the end) moved up to and around a bank of many other contacts, making a connection with any one of them.
Selector Theory A selector starts in the 'home' position and with each 'impulse' the wiper contacts would progress round the output bank to the next position. Each output would be connected to a different subscriber, thus the caller could connect to any other subscriber who was connected to that bank, without any manual assistance from an operator.
Fig1. Diagram of a simple Selector
In Figure (above), the selector has 10 outputs, so a caller can choose to connect to any of 10 different subscribers by dialling any digit from 1 to 0 (0=10). This sort of automatic selector is known as a Uniselector, as it moves in just one plane (rotary).
Fig2. simple Selector The contact arm (wiper) moves across a fixed set of switch contacts. In the case single uniselector, each contact is connected to an outgoing channel, so a caller can choose to connect to any of 10 different subscribers by dialling any digit from 1 to 10. As this selector moves in just one plane, thus sort of automated selector is known as uniselector. An uniselector is operated by (wiper movement) is performed by a drive mechanism of a rotary switch. This mechanism contains an armature, electromagnet, Pawl, and Ratched wheel. The wiper is attached to the ratchet wheel. When the line relay detects a calling signal, the magnet is energised and operates the ratchet wheel, pawl and its associated wiper. When the electromagnet is deenergised the armature is released and returns to its rest position. Thus, if the electromagnet is energised and deenergised, (for example 3 times by applying 3 pulses), the wiper moves by three contacts.
Two motion selectors A two motion selector is a selector in which a set of wipers is moved in two different planes by means of separate mechanisms. By mounting several arcs of outlets on top of each other, the number of outlets can be increased significantly, but the wipers are then required to move both horizontally to select a bank and then vertically to move around that bank to the required outlet. Such a selector is known as a Two motion selector. By mounting several arcs of outlets on top of each other, the number of outlets can be increased significantly but the wipers are then required to move both horizontally to select a bank and then vertically to move around that bank to the required outlet. Such a selector is known as aTwo-Motion Selector. Two-motion selectors typically have 10 rows of 10 outlets, thus 100 possible outlets altogether. A two-motion selector can therefore accept two dialled digits from a subscriber and route the call to any of 100 numbers. The selector 'wipers' always start in their resting 'home' position. The first digit moves the selector veritically up to the corresponding level and then the second digit moves the wipers around the contacts of that level. This is shown in figure 3, below.
Fig. 3 - A Two-Motion "Final" Selector
The type of selector shown above is known as a Final Selector as it takes the final two digits of the number dialled. Most numbers dialled are several digits longer, and therefore pass through a chain of selectors. Selectors previous to the Final Selectors are different; they are called Group Selectors. Group selectors take only ONE digit from the caller, and step up the number of levels according to the digit dialled. The rotary movement is then automatic; the wipers search around that level to find a free outlet - i.e. the next free selector in the chain. This is covered in more depth later.
The Rotary Dial
A train of pulses is used to represent a digit in the subscriber number „Successive Digits are distinguished by a pulse (interdigit gap)
To dial a digit, the circuit is interrupted according to the number dialed so, for example, if you dialed a '4' then the line would be pulsed four times, quickly in succession. After a moment, it was assumed that the digit was complete and that any further pulses belonged to the next digit. In order to ensure that successive digits didn't come too soon and thus be mistaken for pulses belonging to the previous digit, the finger stop on the dial was put some way round so that after removing your finger from the dial, there was a minimum time taken for the dial to return to the home position. It is important to note here that for the purposes of dialing, the digit '0' sends TEN pulses for dialling - i.e. the selector will step around to the 10th position.
Progress Tones
With manual switching systems, there had always been an operator to advise the caller of the current status. Having removed the need for an operator, a system was required to indicate call progress to the caller. A series of distinctive tones was developed which were produced by a machine called a Ring Generator. The ring generator was entirely electromechanical; different cadences and tones were produced by rotating cams connected to a generator. As well as generating the tones, the Ring Generator machine also provided timed pulses which were used by various processes throughout the exchange. The progress tones produced were as follows :
Dial Tone (DT). This is a 33 c/s continuous note and is applied to the line after the subscriber has lifted his handset and the switching equipment has allocated him an available outlet for this call to proceed. There would have been a physical limit on the number of calls an exchange could handle so if all equipment was already in use, the subscriber would not get dial tone. The actual pitch of the dial Tone varied from exchange to exchange depending on the adjustment of the ring generator. Call in Progress Tone. Inform the calling subscriber that the call is being established Busy Tone (BT). A higher pitched note of 400 c/s interrupted to give a cadence of 0.75 seconds on, 0.75 seconds off. Busy tone indicated either that the called subscriber is already off-hook (busy) or that the route to the called subscriber is congested. In later systems, a slightly different cadence was introduced in order to distinguish between these two scenarios. Number Unobtainable Tone (NUT). Identical pitch to the busy tone but continuous. This tone is used to indicate that a number is out of service, faulty or that a spare line has been dialled. Ring Tone (RT). A tone of 133c/s which interrupted in the same cadence as the ring current which rings the telephone's bell at the called party's end : 0.4 seconds on, 0.2 seconds off.
Step-by-step Switching What does “step-by-step” mean? „ The wiper steps forward by one contact at a time and moves as many contacts as the number of dial pulses received. Construction „ *Using uniselector „ *Using two-motion selectors „ *Combination of both configuration of a step-by-step switching system
Line equipment part (Pre-selector stage) Function: Provide access to common switching resources Category: Selector hunters - There is one dedicated selector hunter for each subscriber to search and seize a free selector from the switching matrix part. Usually 24-outlet uniselectors are used as selector hunters. Suitable for large Switches with fairly heavy traffic.
Line finder-Associated with the first set of selectors in switching matrix part, there is one line finder for each selector. Usually built using uniselectors or two-motion selectors. Suitable for small Switches with low traffic.
Switching network part (group selector stage + final stage)
Control functions Performed by circuits associated with the selectors. Contact banks: Control and supervisory signals (P-wire/private~) Voice signals (positive & negative wires) Control and supervisory signals are carried from stage to stage by means of contacts in one of the banks. A selector X is said to have seized another selector Y in the next stage when positive, negative and private wires have been connected to those of Y. Selector control circuits Guarding circuit - An essential feature of all the selectors Making the selector busy as soon as it is seized. The guarding condition remains set as long as the call is not terminated.The guarding condition is indicated by an earth on the P-wire. Impulsing circuit - An essential part of all those selectors which have to respond to dialing pulses, i.e. group and final selectors The circuit is designed around 3 relays: 1 .Fast acting: respond to dialing pulses and pass them to P-wire. 2 slow acting: . Maintaining guarding conditions . Recognizing the end of a pulse train Homing circuit: All the selectors need homing circuit. Function: At the end of a conversation, release all the selectors and make them return to their home positions. Metering circuit A special feature of the final selectors. Function: Register a call against the calling party as soon as the called party answers. The circuit drives a meter containing counting mechanism. Ring-trip circuit A part of the final selectors. Function: Control the ringing current to the called party and the ringing tone to the calling party. Both the ringing current and the ringing tone are cut off by the ring-trip circuit as soon as the called party answers the call. Alarm circuit Provide visual and audible indications of any fault or undesirable condition creeping into the selector circuits. Three types of faults detected: Off-hook condition: short circuit in the subscriber line or the subscriber not having replaced his handset on the hook. Called-subscriber-held: the handset of called-subscriber is not replaced properly. Release held: sense the failure of a selector to return to home position. Disadvantages of Strawger S
Dependence on moving parts a contacts that are subject to we tear;
Difficulties in maintenance and adjustments.
Switching systems requiring l maintenance and adjustment installation are expected. It is achieved by using Cross bar Exchange
1.4 CROSS BAR EXCHANGE In the late 1930‘s and throughout 1940‘s, AT & T introduced various versions of the crossbar switches. This crossbar switch basically consists line link frames trunk, link frames and common control equipments. With crossbar switchies and common control equipments, the crossbar exchange achieves full access and nonblocking capabilities. Active elements called crosspoints are placed between input and output lines. In common control switching systems, the switching and the control operations are separated. This permits a particular group of common control circuits to route connections through the switching network for many calls at the same time on a shared basis. The unique features of the crossbar switches are (i) Common control allows the customer and the switch to share the common equipments used to process the call. (ii) Wire logic computer allows specific routine functions of call processing to be handwired into the switch. (iii) Flexible concentration ratios allows the system designer to select the appropriate ratio for a specific switch based on customer mix in a specific location. (iv) Crossbar switches are easier to maintain because the switch have significantly fewer moving parts than strowger switching system. Basic principle. The fundamental concept of crossbar switching is that it uses common control networks. The common control networks enables the exchange to perform event monitoring, call processing, charging, operation and maintenance. The common control also facilitates uniform numbering of subscribers in a multiexchange area like big cities and routing of calls from one exchange to another via some intermediate exchanges. The common control method of switching overcomes the disadvantages of step-by-step switching. The common control makes no call processing until it receives entire number. It receives all the number, stores, and then establishes connection. Crossbar switching matrix. The basic crossbar matrix requires atleast M × N sets of contacts and M + N or less activactors to select one of the contacts. Fig. illustrates the 3 × 4 crossbar switching. It contains an array of horizontal and vertical wires (shown as a solid line). Both wires are connected to initially separated contact points of switches. Horizontal and vertical bars (shown as dotted lines) are mechanically connected to these contact points and attached to the electromagnets.
Fig4. 3 × 4 crossbar switch. When both horizontal and vertical bars connected to the electromagnet are activated, the contact of the intersection of the two bars will close together. Thus the contact is made and continues to hold. When the electromagnets are deenergized both horizontal bar and vertical bars are released from the contact. In order to prevent the catching of different crosspoint in the same circuit, a procedure is followed to establish a connection. Accordingly, horizontal bar is energised first and then vertical bar is energised to make contact or in reverse. But while removing contact horizontal bar is deenergized first and then the vertical bar is deenergized. The crossbar switch is known as a non-blocking crossbar configuration. It requires N2 switching elements for N subscribers. Thus for 100 subscribers, 10000 crosspoint switches are required. Hence, crossbar is economic only for small private exchanges requiring small switches. For connecting and releasing the subscriber, the select magnet and bridge magnet should be energised and deenergised respectively. External switch must decide which magnet to operate. This is called marker. A marker can control many swtiches and serve many registers. Thus, even a large exchange needs few markers. In Ericsson ARF system, groups of 1000 subscribers are served by a line switch network controlled by the two markers. Diagonal crosspoint matrix. The number of crosspoints are reduced to N (N–1)/2, where N is the number of subscribers. It is also called triangular matrix or two way matrix.
Fig5. Diagonal crosspoint matrix Blocking Configurations. By blocking configuration the crosspoint switches required can be reduced significantly.For the rectangular matrices with N inputs and N outputs, the number of switches is now 2N2 compared to N2 for the single stage. Here, the random failure of a limited number of switches will not preclude connections.
Fig6.Two stage matrics.
AT & T No. 5 Crossbar System This system was developed by the Bell Telephone Laboratories and brought into service in 1948. This system is especially suitable for isolated small cities and for residential areas on the fringes of large cities. When the percentage of calls connected to subscribers in the same office is relatively high, No. 5 crossbar system is useful. Improvements and added features have widened the applications of the No. 5 equipment. It is presently being used in almost all areas including metropolitan business exchanges and rural centres of about 2000 lines or more.
Fig7. No. 5 crossbar switching structure. It consists of line link frame, trunk link frame and common control equipment. The No. 5 system employs a single switching train to handle incoming, outgoing or switch through. Also, the connection of the subscriber to the dial register circuit is also made through this switching train. Subscriber lines and incoming trunks terminate on the line link frames, trunks and orginating registers termination on the trunk link frames. The line link and trunk link frames are interconected by ‗‗junctors‘‘ which give each link frame access to all of the trunk link frames. Line link frames : The basic line link frame is a 2 bay frame work with each bay mounting ten 200 point, 3 wire switches. The ten switches on one bay are used as combined line and junctor switches and provide terminations for 100 junctors and 100 lines. The ten switches on the other bay are line switches which provide terminations for 190 additional lines and ten ‗‗no test‘‘ verticals used to obtain access to busy lines. Thus a line link frames provides 290 line terminations and 100 junctor terminations. A feature of this line link frame is that the same frame can serve customers who have various classes of service. Like coin telephone, emergency line facility, hot line facility, a maximum of thirty classes of service can be served on a frame. Junctors: Each line link frame has 100 junctor terminations which are used to connect to all the trunk link frames in the office. The number of junctors in a group depends on the number of trunk line frames in the office. For example, in an office with eight trunk link frames and sixteen line link frames, each junctor group contains either twelve or thirteen junctors. Trunk link frames: The trunk link frame is made up of trunk switches, junctor switches and relays for marker access to the frame. Trunks and originating registers, which register the called number are connected to the trunk switches. The trunk links run from vertical to vertical, the junctors being connected to the horizontals of the junctor switches and the trunks to the horizontals of the trunk switches. Thus, the trunk is accessible to all the junctors on the frame but only to either, the left or right junctors on one channel test. Common control equipments: The common control equipments used in AT & T crossbar system are originating and incoming registers, markers, translators, senders and connectors. These equipments are explained briefly below. Marker: It is the most active part of common control equipment. All the markers and their associated equipment serves up to a maximum of 20,000 members make a marker group. There are three types of markers. The combined marker performs dialtone, alloting the jobs between markers. The dialtone marker is used exclusively on dialtone jobs. Completing marker performs all the other jobs. The principal functions of the marker are to respond demands for dialtone, determine the proper route, establish connection, determine the class of service, recognize the status of the connection etc. Outgoing registers: It furnish dialtone to subscribers and record the digits that are dialed, and then the called number is transmitted from register to marker. The originating circuit may be assigned to seize the pretranslator after either the second or third digit has been dialed. Pretranslator. The pretranslator determines, how many digits the register should expect before seizing a marker. A pretranslator can be placed in the outgoing register. 1.5 STORED PROGRAM CONTROL - SPC The switching procedures in SPC exchanges arc controlled by means of stored programs (a program is a sequence of instructions stored in the computer memory). The SPC uses a computer as the time-shared control. The computer is defined by the program stored in its memory. The computer is programmed to test the conditions of the inputs and old states and decide on new outputs and states. Stored program control of switching systems is based on this principle. The computer controllers provide the time-shared decision maker, expressed as programs, which can be used to change or extend the functions of a control system.
In the figure registers are connected to the processor with input/output interfaces to the register access switch. Each interface has an address and may be connected to the input/ output buses by loading that address into the address buffer. Programming of the computer is such that its memory is divided into two:
1. Program memory, for the sequence of instructions. 2. Call memory, for the state of each register.
There is a third memory called translation memory, which translate the information dialed by the subscriber into specific actions. It is part of the program but is treated independently, so that the program may be used for any similar system. The register interfaces have one or more inputs and are scanned at regular intervals. These scan points are accessed by loading the address of the required register into the address buffer. Decoding the address returns the state of the required scan points. The outputs of the interfaces are treated similarly. In operation, an external clock pulse triggers the computer. An overall control program, after loading the address and setting one of the call memories to point to the block of the memory associated with the first register, then hands over to the register program that decides whether any action is required for the current state and input. The next register is addressed, when the register program has completed its actions and the overall control memory modifies the addresses of the input/output buffer and call memory index. When all the registers have been dealt with, the computer waits for the next clock pulse before restarting a new scan. The SPC has the following advantages over other types of control: 1. By changing the program, changes in the working of the exchange can be made at any time. Thus new services for the used and new maintenance and monitoring procedures can be introduced. 2. It provides cheaper inter-exchange signaling, i.e., Common Channel Signaling. 3. No compromise between the efficiency of the switching network and the cost of a marker. Differences between Circuit Switching and Packet Switching
1.6 Message Switching 1 A message is a logical unit of information and can be of any length. In message switching, if a station wishes to send a message to another station, it first adds the destination address to the message. Message switching does not establish a dedicated path between the two communicating devices i.e. no direct link is established between sender and receiver. Each message is treated as an independent unit. In message switching, each complete message is then transmitted from device to device through the internetwork i.e. message is transmitted from the source node to intermediate node. The intermediate node stores the complete message temporarily, inspects it for errors and transmits the message to the next node based on an available free channel and its routing information. Because of this reason message switched networks are called store and forward network as shown in fig. The actual path taken by the message to its destination is dynamic as the path is established as it travels along. When the message reaches a node, the channel on which it came is released for use by another message. As shown in Figure message Ml is transmitted from A to D and M2 is transmitted C to B. Message Ml follows the route A --> I --> II --> III --> D and M2 follows the route C --> IV --> II --> B depending upon the availability of free path at that particular moment. The first electromechanical telecommunication system used message switching for telegrams. The message was punched on paper tape off-line at the sending office and then read in and transmitted over a communication line to the next office along the way, where it was punched out on paper tape. An operator there tore the tape off and read it in on tape readers.
Advantages of Message Switching The various advantages of message switching are: 1. It provides efficient traffic management by assigning priorities to the messages to be switched. 2. No physical connection is required between the source & destination as it is in circuit switching. 3. It reduces the traffic congestion on network because of store & forward facility. Each node can store the message until communication channel becomes available. 4. Channels are used effectively and network devices share the data channels. 5. It supports the message length of unlimited size. Disadvantages of Message Switching The various disadvantages of message switching are: 1. As message length is unlimited, each switching node must have sufficient storage to buffer message. 2. Storing & forwarding facility introduces delay thus making message switching unsuitable for real time applications like voice and video. Delay in message switching: • A message switched network consists of store-and-forward switches interconnected by trunks. A single trunk is usually sufficient between a pair of switches. • Multiple trunks can be provided to increase reliability. Each switch is equipped with a storage device wherein all incoming messages are temporarily stored for onward transmission.
• The basic operation of the store-and-forward service is similar to the telegram service. A message along with the destination address is sent from switch to switch till it reaches the destination. • Let us say end system A wants to send a message to end system B as shown in Fig. A sends its message along with the address of the destination and its own address to entry switch 1. The addresses are included in the header of the message. • Switch 1 accepts the message and analyzes the destination address. A routing table is maintained at each node. • It contains entries indicating destination nodes and the corresponding outgoing trunks from the switch. There is a separate queue for each trunk.
• Since the destination node may be accessible via more than one route, decision to send the message to a particular next switch depends on the expected delay in its queue. Let us say, the message from A is put in the queue for node 2. • The message received at switch 2 is again put in a queue of messages awaiting transmission to switch 4. When its turn comes, the message is sent to switch 4 which deliver it to the destination. • Some of the basic features of store- and-forward message switching are: • The store-and-forward service is unidirectional. After delivery of the message, the network does not send back any confirmation to the source. If end system B is required to send an acknowledgement to the message received from A, the acknowledgement is treated like any other message by the network and carries the addresses of the destination and the source. • For switch -to- switch transfer of the message, the network may employ some error control mechanism. The message may be appended with error-checking bits and if any error is detected by the receiving switch, it may request the sending switch for retransmission of the message. Therefore, the sending switch is required to keep a copy of the message till an acknowledgement is received. • Since the message is stored in a buffer at the switch at each stage of transmission, each switch- to-switch transfer is an independent operation. The trunks can operate at different data rates. Even the source and destination end systems can operate at different data rates. • In message switching every message is treated as an independent entity by the network and, therefore, destination and source addresses are repeated on each message. • Delay in Delivery. Fig shows the timing diagram for routing a message through a message switched network. The message passes through the entry switch, two transit switches and finally through the exit switch to arrive at the destination.
Message delivery time is the sum of the following components: • Time required sending the message to the entry switch • Switch delay • Transmission time at each switch. • The time required to send a message to the entry switch IS determined by the transmission data rate and the message size. • Propagation time to the entry switch is usually negligible. Switch delay is due to two factors : 1. Message processing at each switch (time required for error checking, routing etc.) 2. Waiting time in the queues at each switch. • The transmission time at each switch is determined by the transmission data rate and propagation p time for transmission across the trunk. • The total time required to deliver the message is the linear sum of all these components of time as they occur in a sequential manner. • The delivery time varies from message to message because of random waiting times in queues and alternate routes between the same pair of entry and exit switches. • Therefore, time relationship of the messages and their sequence are not guaranteed in a message switched network. • As traffic increases there is increase in message delivery time; because the queues get longer and there may be congestion on the route. 1.7 Circuit switching Circuit switching is a switching method in which a dedicated communication path in physical form between two stations within a network is established, maintained and terminated for each communication session. It has basically three phases as circuit establishment, data transfer and circuit disconnect. Once the connection is established, the data transfer is transparent. The main feature of such a connection is that it provides a fixed data rate channel and both subscribers must operate at this rate, It is considered inefficient compared to packet switching because channel capacity is completely dedicated for duration of connection. If there is no data at any moment of time, channel capacity goes wasted. Moreover, setting up of connection takes time. Circuit switching has two types of transmissions. Datagram transmissions - Datagram transmissions have individually addressed frames. Data-stream transmissions - Data-stream transmissions have a stream of data for which address checking occurs only once. The routing in circuit switching may have either static routing or dynamic routing. In case of static routing, this methodology uses the same approach all the time while dynamic routing allows alternate routing depending on traffic. The Key Point of Circuit Switching are 1. It is the simplest method of data communication in which a dedicated physical connection or path is established between the sending and receiving device. 2. In circuit switched networks, a set of switches are connected by physical links. A connection between two stations is a dedicated path made of one or more links. 3. Figure shows a circuit switched network in which computer A, B and C are connected to computer D, E, F and G via four switches. If these computers are to be connected with a point- to-point connections, 12 dedicated lines are required which will incur high line cost. 4. The four switches connecting these computers thus provide dedicated links by reducing the line cost. Here I, II, III and IV are the circuit switches or nodes. Nodes I, III, IV are connected to computers while II is only routing node. `
5. In circuit switching the routing decision is made when path is set up across the network. After the link has been set between the sender and receiver, the information is forwarded continuously over the link. 6. The dedicated path established between the sender and the receiver is maintained for entire duration of conversation. 7. This link or path is released only when data transmission between sender and receiver is over. 8. Circuit switching takes place at the physical layer. 9. Before starting communication, the stations must make a reservation of resources to be used during the communication. These resources can be switch buffers, switch processing time, switch input/output ports. These resources remain dedicated during the entire duration of data transfer. 10 Data transferred between the two stations are not packetized (i.e. in form of packets). The data are a continuous flow· sent by the source station and received by the destination station and there may be periods of silence. 11. There is no addressing involved in data transfer. The switches route the data based on their occupied band (FDM) or time slot (TDM). However, there is end-to-end addressing used during set up phase. 12. In telephone systems circuit switching is used. 13. The communication in a circuit switched network takes place in three phases: 1. Circuit establishment or setup phase 2. Data transfer phase. 3. Circuit disconnects or tears down phase. Circuit establishment or Setup Phase In circuit .switched network, before actual data transfer takes place, a dedicated circuit or path is established between the sender and receiver.For example, as shown in fig. if two communicating devices are A and D, then a dedicated path will be set up from A to I, I to II, II to III and III to D first. End-to-End addressing (i.e. source and destination address) is required for creating a connection between two end systems. Data Transfer Phase Actual data transfer between the source and destination takes place after the dedicated path is set up between them. The data flows are continuous between sender and receiver. There may be periods of silence in between. Generally all the internal connections are duplex. Circuit Disconnect or Teardown Phase. When one of the parties needs to disconnect, a signal is sent to each switch to release the resources. Circuit Switch A circuit switch is a device that creates a temporary connection between an input link and output link. A circuit switch usually has n input lines and m output lines i.e. number of input lines and number of output lines may not be equal.
Advantages of Circuit Switching The advantages of circuit switching are: 1. The dedicated path/circuit established between sender and receiver provides a guaranteed data rate. 2. Once the circuit is established, data is transmitted without any delay as there is no waiting time at each switch. 3. Since a dedicated continuous transmission path is established, the method is suitable for long continuous transmission. Disadvantages of Circuit Switching The various disadvantages of circuit switching are: 1. As the connection is dedicated it cannot be used to transmit any other data even if the channel is free. 2. It is inefficient in terms of utilization of system resources. As resources are allocated for the entire duration of connection, these are not available to other connections. 3. Dedicated channels require more bandwidth. 4. Prior to actual data transfer, the time required to establish a physical link between the two stations is too long. 1.8 Telephone handset It is a device which converts human speech in the form of sound waves produced by the vocal cord to electrical signals. These signals are then transmitted over telephone wires and then converted back to sound waves for human ears. Microphone Earphone Signaling functions
Microphone (mouthpiece) It consists of a movable speaker diaphragm that is sensitive to both amplitude and frequency . The diaphragm contains carbon particles that can conduct electricity. As the human voice spoken into the transmitter varies, the amount of carbon granules that strike the electrical contacts in the mouthpiece also varies—thereby sending varying analog electrical signals out into the voice network. Earphone (earpiece) Acts in an opposite direction to the mouthpiece. The electrical signal/waves produced by the transmitter are received at an electromagnet in the receiver. Varying levels of electricity produce varying levels of magnetism—that, in turn, cause the diaphragm to move indirect proportion to the magnetic variance.The moving diaphragm produces varying sound that corresponds to the sound waves that were input at the transmitter.
1.9 Four Wire Circuits A four-wire circuit is defined as two pairs of two communications wires that permit transmission of signals in both directions at the same time. Simultaneous communication is called a full- duplex system, where two people can speak and be heard at the same time. Many home and business data lines use these circuits to optimize data transmissions. An electric circuit to provide both 120- and 240-volt power to an appliance using two hot or current-carrying wires, as well as a neutral and ground, is also called a four-wire circuit
A four-wire circuit is defined as two pairs of two communications wires that permit transmission of signals in both directions at the same time. Simultaneous communication is called a full-duplex system, where two people can speak and be heard at the same time. Many home and business data lines use these circuits to optimize data transmissions. An electric circuit to provide both 120- and 240-volt power to an appliance using two hot or current-carrying wires, as well as a neutral and ground, is also called a four-wire circuit.
Normal home phone equipment uses two-wire circuits for voice calls. Conversations can take place in both directions, but normally only a few phones may be hooked up to a single two- wire line. Data communications would be limited in this type of circuit.
Adding two more wires to create a four-wire circuit allows the system to send data to a computer or modem in one direction with two wires, and receive data from the device with the other two wires. Data speeds are greatly improved with less errors and signal loss. This system is called a full-duplex line, common since the 1980s for data systems such as Integrated System Digital Networks (ISDN). As the demand for data speed increased, digital systems expanded to Ethernet digital systems, which is a four-wire circuit pair, or eight wires, that can handle voice and data communications.
Two-Wire versus Four-Wire All subscriber loops in the telephone network are implemented with a signal pair of wires Both directions of transmission Conversations are superimposed on the wire pair Two directions of longer distances are separated
Two-Wire-to-Four-Wire Conversion
Basic conversion function is provided by hybrid circuits Impedance matching is important Impedance mismatch causes ―echo‖ 1.10 Echo suppressor and cancellers Echoes result from an impedance mismatch at telephone exchange hybrids where the subscriber's 2-wire line is connected to a 4-wire line. Hybrid echo is the main source of echo generated from the public-switched telephone network (PSTN). Echoes on a telephone line are due to the reflection of signals at the points of impedance mismatch on the connecting circuits. Conventionally, telephones in a given geographical area are connected to an exchange by a 2-wire twisted line, called the subscriber's lineline, which serves to receive and transmit signals. In a conventional system a local call is set up by establishing a direct connection, at the telephone exchange, between two subscribers‘ loops. For a local call, there is usually no noticeable echo either because there is not a significant impedance mismatch on the connecting 2-wire local lines or because the distances are relatively small and the resulting low-delay echoes are perceived as a slight amplification and ―livening‖ effect. For long-distance communication between two exchanges, it is necessary to use repeaters to amplify the speech signals; therefore a separate 2-wire telephone line is required for each direction of transmission. To establish a long-distance call, at each end, a 2-wire subscriber's line must be connected to a 4-wire line attached to the exchange, as illustrated in Figure
The device that connects the 2-wire subscriber's loop to the 4-wire line is called a hybrid, and is shown in Figure.
As shown the figure. hybrid is basically a three-port bridge circuit. If the hybrid bridge were perfectly balanced then there would be no reflection or echo. However, each hybrid circuit serves a number of subscribers‘ lines. The subscribers' lines do not all have the same length and impedance characteristics; therefore it is not possible to achieve perfect balance for all subscribers at the hybrids. When the bridge is not perfectly balanced , some of the signal energy on the receiving 4-wire lines becomes coupled back onto itself and produces an echo. Echo is often measured in terms of the echo return loss (ERL); the higher the echo return loss the lower will be the echo. Telephone line echoes are undesirable, and become annoying when the echo amplitude is relatively high and the echo delay is long.
Echo Suppression
Echo suppresser is primarily a switch that lets the speech signal through during the speech-active periods and attenuates the line echo during the speech-inactive periods. A line echo suppresser is controlled by a speech/echo detection device. The echo detector monitors the signal levels on the incoming and outgoing lines, and decides if the signal on a line from,say, speaker B to speaker A is the speech from the speaker B to the speaker A, or the echo of speaker A. If the echo detector decides that the signal is an echo then the signal is heavily attenuated. There is a similar echo suppression unit from speaker A to speaker B. The performance of an echo suppresser depends on the accuracy of the echo/speech classification subsystem. Echo of speech often has a smaller amplitude level than the speech signal, but otherwise it has mainly the same spectral characteristics and statistics as those of the speech. Therefore the only basis for discrimination of speech from echo is the signal level. As a result, the speech/echo classifier may wrongly classify and let through high-level echoes as speech, or attenuate low-level speech as echo. For terrestrial circuits, echo suppressers have been well designed, with an acceptable level of false decisions and a good performance. The performance of an echo suppresser depends on the time delay of the echo. In general, echo suppressers perform well when the roundtrip delay of the echo is less than 100 ms. For a conversation routed via a geostationary satellite the round-trip delay may be as much as 600 ms. Such long delays can change the pattern of conversation and result in a significant increase in speech/echo classification errors. When the delay is long, echo suppressers fail to perform satisfactorily, and this results in choppy first syllables and artificial volume adjustment. A system that is effective with both short and long time delays is the adaptive echo canceller introduced next.
Adaptive Echo Cancellation
The speech signal on the line from speaker A to speaker B is input to the 4/2 wire hybrid B and to the echo canceller. The echo canceller monitors the signal on line from B to A and attempts to model and synthesis a replica of the echo of speaker A. This replica is used to subtract and cancel out the echo of speaker A on the line from B to A. The echo canceller is basically an adaptive linear filter. The coefficients of the filter are adapted so that the energy of the signal on the line is minimised. The echo canceller can be an infinite impulse response (IIR) or a finite impulse response (FIR) filter. The main advantage of an IIR filter is that a long-delay echo can be synthesized by a relatively small number of filter coefficients. In practice, echo cancellers are based on FIR filters. This is mainly due to the practical difficulties associated with the adaptation and stable operation of adaptive IIR filters. 1.11 Pulse Code Modulation (PCM) Pulse Code Modulation Pulse Code Modulation (PCM) is an extension of PAM wherein each analogue sample value is quantized into a discrete value for representation as a digital code word.Thus, as shown below, a PAM system can be converted into a PCM system by adding a suitable analogue-to- digital (A/D) converter at the source and a digital-to-analogue (D/A) converter at the destination. Modulator Analogue PCM Parallel Digital Input A to D Binary Output Sampler to Serial Pulse Converter Coder Converter Generator
Demodulator PCM Analogue Serial to Input D to A Output Parallel LPF Converter Converter
PCM is a true digital process as compared to PAM. In PCM the speech signal is converted from analogue to digital form. PCM is standardised for telephony by the ITU-T (International Telecommunications Union - Telecoms, a branch of the UN), in a series of recommendations called the G series. For example the ITU-T recommendations for out-of-band signal rejection in PCM voice coders require that 14 dB of attenuation is provided at 4 kHz. Also, the ITU-T transmission quality specification for telephony terminals require that the frequency response of the handset microphone has a sharp roll-off from 3.4 kHz. In quantization the levels are assigned a binary codeword. All sample values falling between two quantization levels are considered to be located at the centre of the quantization interval. In this manner the quantization process introduces a certain amount of error or distortion into the signal samples. This error known as quantization noise, is minimised by establishing a large number of small quantization intervals. Of course, as the number of quantization intervals increase, so must the number or bits increase to uniquely identify the quantization intervals. For example, if an analogue voltage level is to be converted to a digital system with 8 discrete levels or quantization steps three bits are required. In the ITU-T version there are 256 quantization steps, 128 positive and 128 negative, requiring 8 bits. A positive level is represented by having bit 8 (MSB) at 0, and for a negative level the MSB is 1. Quantization This is the process of setting the sample amplitude, which can be continuously variable to a discrete value. Look at Uniform Quantization first, where the discrete values are evenly spaced.
Uniform Quantization Output
-mp +mp Input
A sample amplitude value is approximated by the midpoint of the interval in which it lies. The input/output characteristics of a uniform quantizer is shown. Companding In a uniform or linear PCM system the size of every quantization interval is determined by the SQR requirement of the lowest signal to be encoded. This interval is also for the largest signal - which therefore has a much better SQR. . The PCM decoder expands the compressed value using an inverse compression characteristic to recover the original sample value. The two processes are called companding. There are 2 companding schemes to describe the curve above: 1. -LawCompanding(also called log-PCM) This is used in North America and Japan. It uses a logarithmic compression curve which is ideal in the sense that quantization intervals and hence quantization noise is directly proportional to signal level (and so a constant SQR). 2. A- Law Companding This is the ITU-T standard. It is used in Europe and most of the rest of the world. It is very similar to the -Law coding. It is represented by straight line segments to facilitate digital companding.
The advantages of PCM are: Relatively inexpensive digital circuitry may be used extensively. PCM signals derived from all types of analog sources may be merged with data signals and transmitted over a common high-speed digital communication system. In long-distance digital telephone systems requiring repeaters, a clean PCM waveform can be regenerated at the output of each repeater, where the input consists of a noisy PCM waveform. The noise performance of a digital system can be superior to that of an analog system. The probability of error for the system output can be reduced even further by the use of appropriate coding techniques.
1.12 MODEM (mōdem) (n.) Short for modulator-demodulator. A modem is a device or program that enables a computer to transmit data over, for example, telephone or cable lines. Computer information is stored digitally, whereas information transmitted over telephone lines is transmitted in the form of analog waves. A modem converts between these two forms. Aside from the transmission protocols that they support, the following characteristics distinguish one modem from another: bps : How fast the modem can transmit and receive data. At slow rates, modems are measured in terms of baud rates. The slowest rate is 300 baud (about 25 cps). At higher speeds, modems are measured in terms of bits per second (bps). The fastest modems run at 57,600 bps, although they can achieve even higher data transfer ratesby compressing the data. Obviously, the faster the transmission rate, the faster you can send and receive data. Note, however, that you cannot receive data any faster than it is being sent. If, for example, the device sending data to your computer is sending it at 2,400 bps, you must receive it at 2,400 bps. It does not always pay, therefore, to have a very fast modem. In addition, some telephone lines are unable to transmit data reliably at very high rates. voice/data: Many modems support a switch to change between voice and data modes. In data mode, the modem acts like a regular modem. In voice mode, the modem acts like a regular telephone. Modems that support a voice/data switch have a built-in loudspeaker and microphone for voice communication. auto-answer :An auto-answer modem enables your computer to receive calls in your absence. This is only necessary if you are offering some type of computer service that people can call in to use. data compression :Some modems perform data compression, which enables them to send data at faster rates. However, the modem at the receiving end must be able to decompress the data using the same compression technique. flash memory : Some modems come with flash memory rather than conventional ROM, which means that the communications protocolscan be easily updated if necessary. Fax capability: Most modern modems are fax modems, which means that they can send and receive faxes. The Relay Switch Circuit Relays are electromechanical devices that use an electromagnet to operate a pair of movable contacts from an open position to a closed position. The advantage of relays is that it takes a relatively small amount of power to operate the relay coil, but the relay itself can be used to control motors, heaters, lamps or AC circuits which themselves can draw a lot more electrical power. The Electro-mechanical Relay is an output device (actuator) which come in a whole host of shapes, sizes and designs, and have many uses and applications in electronic circuits. But while electrical relays can be used to allow low power electronic or computer type circuits to switch relatively high currents or voltages both ―ON‖ or ―OFF‖, some form of relay switch circuit is required to control it. The design and types of relay switching circuits is huge, but many small electronic projects use transistors and MOSFETs as their main switching device as the transistor can provide fast DC switching (ON-OFF) control of the relay coil from a variety of input sources so here is a small collection of some of the more common ways of switching relays.