Methods Used for Switching in Telephone Systems | Networking

Two different methods are used for switching in telephone systems. These are Circuit Switching and Packet Switching. Of these, circuit switching is older and currently in use with most of the telephone systems throughout the world. Packet switching is relatively new and recent advances in telephone sys­tems have permitted many telephone systems to utilise packet switching.

While it is possible—indeed likely—that sometime in the future the telephone system will completely shift to packet switching; at present circuit switching is used most commonly, because of the high cost of converting to packet switching.

We shall review each of these in the following paragraphs:

Circuit Switching that has been in use since almost the start of the use of telephones consists of the simple process of establishing a physical circuit between the calling telephone and the called telephone. This circuit is permanent for the duration of the call.

To that extent—the oxymoron notwithstanding—it can be called a “temporarily permanent” connection. Permanent because the circuit will remain fixed for the duration of the call and temporary because once the call is completed, that is when the caller hangs up, the circuit is broken. This is illustrated in Fig. 6.4.

In the earliest switching systems when the switching office received a call, the operator physically connected the caller’s circuit to the circuit of the called party by a plug. Gradually, this function was au­tomated and replaced by automatic switching known as the Strowger gear. Interestingly, Mr. Strowger, who invented this device, was not a telephone engineer. He was an undertaker by profession.

In the town in which he was an undertaker, there was another undertaker whose wife worked in the telephone switching office and apparently, when any one asked to be connected to an undertaker, she naturally connected the caller to her husband.

So pretty soon Mr. Strowger found that business was dwindling and he either had to come up with an automatic switching device or to go out of business. He selected the former option, thereby showing the world that undertakers can be useful not only in deaths but also in the birth of ideas.

All the connections shown in the diagram are the paths through which the data flows. These paths can be connected through copper wire, Fibre Optic cable, microwave or any other form of data connectivity, but the basic principle remains the same in all cases. The connection must be physically established between the two parties and the connection acknowledged, before any data can flow along the estab­lished path.

This means that before any data can be transmitted, a signal must travel from the caller to the party being called and an acknowledgement must travel in the return direction. This time period can sometimes be fairly long and undesirable and lead to congestion at call set-up time. We shall see how this problem has been solved in the case of packet switching.

Packet Switching has since been an advance in switching over circuit switching. In or­der to understand packet switching, it is first necessary to understand the reasons on account of which circuit switching was found to be wanting. Largely because of this, there was serious traffic congestion at certain times.

Also, the speed of data transfer was severely affected because although the data transfer rate was reasonable when it was being transferred, the time taken over establishing a circuit and the acknowledgement was far too high, specially over long-distance data transfer.

This led to an intermediate development called message switching. In this mode of switch­ing, the entire message was transmitted to the router (that is, the switching centre in our analogy), stored there and as an added incentive, the data received was checked for errors after which it was retransmit­ted to the addressee.

This technique of message transmission is also known as store-and-forward in a network. This method of message transmission was originally used to transmit telegraphic messages. However, the fact that there was no limitation on the size of the message often meant that interactive traffic could be blocked in case of a large message, which could lead to severe congestion.

This problem led to the development of packet switching. In packet switching each message is broken into packets of precisely fixed sizes and each packet utilizes the store and forward technique.

This will usually imply that the first packet of a message will be stored and ready by the time the second packet arrives and so on. This procedure helps in making sure that the circuit is never choked and is free for any interactive traffic.

The fact that each packet is checked and ready by the time the next packet arrives also means that there are no delays and the throughput is substantially improved. As a result of these features, packet switching has become the de facto standard for computer networks. In packet switching, since the band­width is not reserved for any user, there is no wastage and the bandwidth can be dynamically allocated.

However, on the down side, since the circuit is not dedicated, it means that occasionally it is possible that at the input router level there are too many packets, in excess of the router’s storage capacity, and as a result some packets may be lost.

However, the advantages of using packet switching far outweigh the disadvantages and as a result it is gradually replacing circuit switching. It has also led to the develop­ment of newer techniques in the telephone system.