Evolution of Switching System UNIT 2 3.1 Introduction Switching systems, and associated signalling systems, are essential to the operation of telecommunications 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 telegraphy 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 = Teleprinter, 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 Page 2 route is busy. It is stored in a queue with any other messages for the same route and retransmitted when the required circuit becomes free. Message switching is thus an example of a delay system or a queuing system 3.3 Circuit Switching Invention of telephone 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 switch 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 Page 3 system, busy testing is also required on trunks between switches in the exchange. 5. Interconnection: 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. Supervision : 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 telephone exchange. 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 trunking 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 distribution frame in a typical step-by-step exchange is as shown in the Figure 3.2. (a) Page 4 (b) Figure 3.2 (a) Simple diagram illustrating the Main distribution Frame 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. Page 5 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 crossbar switch was one among them. Figure 3.4 shows the simple crossbar matrix. 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 Page 6 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.