Computer communications:Circuit switching and packet switching

Circuit switching and packet switching

At present, most telephone networks are circuit switched. This means that each device is connected to a switching centre (telephone exchange), which handles all message routing. When a circuit linking transmitter to receiver is set up, it is dedicated to the call, and cannot be used for any other purpose until the call is finished. This has two disadvantages:

• It wastes telecommunications resources, because for a significant part of most calls no information is being exchanged, and the circuit is therefore unused.

• One device cannot communicate with another if all the lines are engaged or if the latter is itself making a call.

To overcome these problems, packet switching is being introduced. As in local area networks, the data is transmit-

ted in addressed 'packets', each one up to 512 bits long. The computers controlling the switching centres read the addresses and route the packets accordingly.

Unlike circuit switching, the lines are not dedicated to individual calls, and some packets making up a call may travel by a different route to other packets. However, each packet contains information giving its position in the sequence of packets that make up the message, and so it can be properly sequenced upon arrival at its destination. If necessary, the network is able to store the complete message until the addressee is available to receive it.

This means that telephone lines can be utilized with much greater efficiency than before, and problems of engaged lines do not occur. Another change that is planned when the system is fully implemented is that the cost of using the network is dependent solely on the amount of information transmitted, not upon the distance it is sent. In this sense it will be similar to the postal services - it costs as much to send a package to the other end of the country as it does to send it to the next town.

Another advantage of digital networks is the fact that the rate at which data is transmitted can be varied in different parts of the network, so that:

• The network can respond flexibly to changes in the volume of traffic and the type of communication link.

• Devices with differing baud rates can intercommunicate via the network.

The Open Systems Interconnection (OSI) reference model

The OSI model defines a set of networking and telecommun­ ications standards. By complying with these standards, different hardware and software manufacturers ensure that their products are compatible. (It is no good attempting to link up different makes of equipment on a network if they are not compatible, as they will not be able to communicate.)

The OSI standards are widely accepted. They divide

networking issues into layers numbered 1 (the most basic) to 7 (the most complex):

• Layer 1 is the most basic, covering the physical connec­tions. Twisted-wire pairs, coaxial, and optical fibre cabling are all defined by this layer.

• Layer 2 defines protocols, i.e. how the data that travels around the network is to be encoded and decoded.

• Layer 3 defines how data should be routed through the network. Single channel networks, such as most LANs, do not need this level.

• Layer 4 deals with error handling across the network, and flow control.

• Layer 5 defines how communications sessions between network devices are to be managed.

• Layer 6 defines how menus, colours, characters, and so on are to be encoded as control sequences for trans­ mission across the network. SQL, for example, sits within this layer and conforms to it.

• Layer 7 defines how application packages are to use the network.

Telecommunications services

The remainder of this chapter covers the main telecommunications services that are of use to the office. They include the telephone and cellular radio, telex and fax, videophone, and video conferencing, as well as on-line databases and email.

Telephone

Although the telephone is one of the oldest of these services, it has recently undergone a transformation in terms of developments and related services that have enhanced its usefulness. These include:

• Low-cost answering machines for receiving and record­ing incoming calls.

• Handsets offering features such as a memory for storing and dialling frequently-used numbers and an automatic redial facility for engaged numbers.

• Computer-based facilities offered by Telecom's digital exchanges, such as automatic ring-back and tracing of malicious calls.

• Telephone conferencing so that several parties can be linked together in a single call.

• Cellular radio so that phones can be used in moving vehicles (see below).

Some new services being developed for subscribers in Amer­ica but not yet available in this country are:

• Transmission of the caller's telephone number, so that it appears on a screen attached to the recipient's phone. This enables the latter to choose to answer, reject, or forward the call to another number.

• Call block, allowing recipients to program their phones to automatically reject calls from up to 12 known numbers.

• Priority screening, which gives priority treatment to calls from up to 12 selected numbers - the phone rings in a distinctive way when a priority number calls, and if the recipient is out, the number is stored in the telephone company's computer for later access by the recipient.

• Call forwarding, which automatically switches all calls or pre-selected calls to another number.

• Call answering, which works rather like an answering machine. Subscribers can leave a pre-recorded message on the telephone company's computer, which is auto­ matically relayed to callers. They in turn can leave messages for the subscriber.

Cellular radio

Cellular radio is a computer-controlled mobile communi­cations service made possible by the change-over to digital communications. Under this system, cellular radio sets, or 'mobiles', normally installed in cars, can communicate with each other and with ordinary phone users in the following way.

The country is split up into a large number of 'cells', each one being between two and twenty miles across (depending on whether the area covered is urban or rural) and having at its centre a base with a low-powered radio transmitter. This is able to transmit to and receive from any mobiles within the perimeter of the cell, and it is connected via a computer-controlled switching centre to the telephone net­ work. Outside the perimeter the strength of the signal falls away, so that although bases in adjacent cells use different frequencies, one in a non-adjacent cell can use the same frequencies without risk of interference.

When a cellular radio subscriber keys in a telephone number, it is transmitted over a special control channel to the cell base, which passes it to the switching centre. The centre dials the number on the telephone network, and at the same time allocates a radio frequency to the mobile, which automatically switches to that frequency and so enables the user to make the call. If the subscriber drives from one cell to another while the call is in progress, the switching centre automatically switches transmitters and frequencies.

In the case of calls made to a mobile, the switching centre sends a paging signal on the control channel. The system will switch the paging transmission from cell to cell until it locates the set, the search being done in an intelligent manner by contacting first the set's home base and looking up the computer records of its last known location. The search time is never more than a few seconds.