Computer communications:Local area networks
Local area networks
Local area networks, or LANs, are used to link up a number of computers and other peripheral devices on the same site. They enable users to share applications software, data files, peripherals such as printers, and common services provided by the network manager such as daily file backup.
A LAN consists of a single cable laid around the site, with sockets along its length for connecting the computers, printers, and other devices. Data from the devices is sent round the network in small chunks or packets, each of which includes the 'address' of the destination machine.
As explained in Chapter 3, one of the computers in a network is the file-server, meaning that it handles the network filing system and directly controls the hard disk containing the application software and files. The other computers are called terminals, stations, or clients. Any network operating system command issued at a station (e.g. to retrieve a file from the hard disk) is passed along the cable to the file-server, which acts on the command and transmits the output - for instance data read from a file - along the cable to the terminal.
A major problem with networks is ensuring that messages from one station do not interfere with those from another, resulting in both messages getting scrambled and lost. There are a number of possible solutions to this problem, resulting in different types of network system. The two most popular are Ethernet and IBM's Token Ring network.
• In the Ethernet system, a station wishing to send information checks if there are any messages (packets) travelling round the cable. If there are not, it sends its message. If two stations, by chance, do this simultaneously, their messages interfere with each other and are lost. However, the fact that this has happened is signalled back to the two stations, and they each then wait a short random length of time and try again.
• In the Token Ring system, an electronic signal called a token circulates around the network, passing from station to station. A station cannot send a message until it has the token, so avoiding the possibility of two stations transmitting simultaneously.
As with most things in the computer world, messaging takes place very rapidly, whichever network system is used. Ethernet has the advantage that it is relatively inexpensive to install, since it requires a simpler cabling system. It has also become the major networking standard, being accepted by a large number of manufacturers. It is probably the best system for small networks.
The Token Ring system, in contrast, requires more elaborate cabling and a central PC dedicated to controlling the network. It is, however, a more robust system (i.e. it is less likely to fail) than Ethernet, and it is faster. It is probably the most suitable system for large networks with long lengths of cabling.
A variety of network operating systems are available for PCs, most of which will run on both the Ethernet and Token Ring systems. The most popular is Netware (from Novell), which will work on most types of network. To the user, there is little difference running software over a network to running it from floppy disk under DOS, except that some things happen rather more slowly.
Most application packages will work over a network, though problems can arise if more than one user is trying to write to the same data or text file. Many packages have special versions for networks that provide file or record 'locking' to ensure that only one person at a time can write to a file or record.
Local area networks can be linked via telecommunications
networks (see below), to become a wide area network (WAN).
'Telecommunications' means 'communications at a distance'. Telecommunications technology embraces radio waves travelling through the air or through space, electrical waves flowing along a telephone wire, and laser pulses travelling along optical fibres (see below).
In the case of wave-based telecommunications, a single frequency carrier wave is modulated with the waveform of the speech or other transmitted message. Many carrier waves, each of a different frequency, can travel along a wire or through the air, and so many conversations can take place at the same time along the same wire or over radio waves. Your radio or TV receiver, for example, allows you to pick up any of these by tuning it to the appropriate carrier wave frequency.
To use this system for computer communications, the digital signal from the computer has to be converted to a wave form at the transmitting end, and back to digital form at the receiving end hence the need for modems, as explained earlier.
With the steady computerization of all aspects of our lives, more and more computer data is travelling along telephone wire. In addition, the telephone network itself is becoming computer controlled. It makes sense, therefore, to convert the network to handle data in the same way as computers do, i.e. as digital pulses rather than as waves.
This is now being done, spurred on by developments in telecommunications technology which enables far more data to be transmitted along digital circuits than is possible over the analogue (wave form) circuits. Because these new circuits have such a wide bandwidth, they are sometimes called broadband circuits.
Optical fibre cables are one example. To cope with ever increasing computer communications, British Telecom is laying these across the country to link its main switching centres. Data will travel along these thin glass fibres in the form of pulses of laser light. The bandwidth of this medium is so high that a single fibre can carry almost half of the voice telephone traffic of the country.
(If the whole telephone network were converted to digital communications, ordinary telephone conversations would have to be digitized to use the system, and then be converted back to sound waves at the receiving end. The device that does this is called a codec - short for 'coder-decoder' - and in a digital system one of these is needed in every telephone handset.)
The advantages of digital over analogue communications are as follows:
• All information, whether voice, image, text, or data, is sent in a common digital form that can be handled by computer-based equipment. Every kind of transmission, whether phone, video, or computer data, can therefore share the same network. Modems, essential for sending data over the analogue network, are unnecessary. ISDN, short for 'integrated services digital network', is the name given to this system.
• Since the information sent over the network can be handled by computer, a number of new facilities are possible. British Telecom's digital service offers over 50 new facilities that were not previously available.
• Information in digital form is less prone to line noise and degradation than information in analogue form. This is because the equipment can easily distinguish a pulse from the absence of a pulse, and so separate it
from the accompanying noise. So vmce calls, for example, will be much clearer.
• Digital communications offer the possibility of broad band communications (e.g. using optical fibre technology) so enabling more information to be sent at lower cost.
Around 160 countries intend to support the ISDN standard. British Telecom's ISDN service has been around for some time and its use is growing. It offers a data transmission rate of 64,000 bits per second, much faster than the highest rates that can be achieved using a modem over the analogue network. However, BT's ISDN service is expensive at present, and only large corporations can justify its use. One day, no doubt, it will be much cheaper and will completely replace the ubiquitous analogue telephone network.
(Note that computers are not able to connect directly to the ISDN network, as computer data, though digital, is not in the form required for transmission over the network.
What's called network terminating equipment (NTE) is needed. In the case of PCs, NTE cards are available that can be slotted inside the computer's casing.)
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