Computer communications:What is computer communications?

What is computer communications?

Computer communications is to do with the transfer of information by:

• Direct connection by cable of two computers.

• Connection of two computers via the public telephone system or other telecommuications links.

• Networking of a number of computers.

In the first part of this chapter I shall deal with the equipment, software, and procedures involved in computer communications, and in the second part I shall cover some of the telecommunications services that are available, such as telex, fax and videoconferencing.

There are a number of reasons why you might want to connect your computer with others:

• To transfer files between your computer and a different type of computer with incompatible disks. For example, you might wish to transfer a document written on a PC to an Apple Mac running a DTP system.

• To send messages to people with other computers, and to receive messages from them. This is called electronic mail, or email for short.

• To access information stored on other remote com­puters, such as sales, inventory, and other internal information in the case of your organization's com­puters, or financial and economic information in the case of public systems.

• To update your organization's computer files when you are away from base.

• To share files with other users in your office or organization.

In the case of computers within the same office, there are additional reasons why you might want them permanently connected together in a local area network. These were dealt with in Chapter 3, and include the fact that you can share resources (such as hard disks and printers), software, and files, and the fact that a centralized backup facility operated by the network manager increases the security of data.

Communications concepts

The four basic communications concepts that you need to know are RS-232, baud rate, protocol, and bandwidth.

1 RS-232. This is the main communications standard in the computer world, defining the connections to be used in the cabling that links computers. It specifies a channel for transmitting data, a channel for receiving data, and channels for control signals. It also specifies that data is transmitted serially, i.e. one bit at a time, and so the RS- 232 socket on a computer is often called the serial port.

2 Baud rate. This refers to the number of bits of data per second (bps) that are transmitted over a communications link. The RS-232 standard does not specify a single baud rate, but it allows a number of rates: 75, 150, 300, 600, up to 9600 and above. Commonly-used rates are 300 and 1200. Many on-line systems (such as bulletin boards) send and receive data at either 300 or 1200 baud. One exception, though, is Prestel - when connected to this, you send data from your computer at 75 baud, and receive it from the Prestel computer at 1200 baud. Before you can use your computer to communicate, you must set its baud rate. You will normally do this using communications software (see below).

3 Protocol. Besides the baud rate, there are other 'hand­ shaking' protocols that have to be set before communications can take place between computers. These include parity, stop bits, and other things which, fortunately, you can normally leave to your communications software to look after.

4 Bandwidth. This measures the amount of information

that can be transmitted through a medium (such as a telephone wire) in a second. Conventional communi­cations systems, such as much of the telephone network, transmit information in the form of waves, in which case the bandwidth is measured by the range of frequencies, in KHz, that the medium allows. (KHz means 1000 hertz, and 1hertz = 1cycle per second.)

To an increasing extent, though, communications systems transmit information in digital form, i.e. in pulses, and in these cases bandwidth is measured by the number of bits per second (bps) that can travel through the medium.

Hardware and software

What do you need for computer communications?

• To get your computer communicating with another, you need communications software and a suitable cable.

• If you wish to carry out these communications via the telephone network, you need a modem.

• To network your computer to others in your office, you need a local area network.

Let's look at each of these.

Communications software

A large number of communications packages are available, especially for the PC, and some of these are in the public domain (and therefore free).

Notable public domain packages are Kermit and Pro­ comm. Kermit is primarily for transferring files between different types of computer, and it is available for more than 200 computers, ranging from home micros to mainframes. It is available in this country from the Kermit Distribution Service, Lancaster University. Procomm is a popular PC package for communicating with bulletin boards or other computers via the telephone, and this is available from PC user groups or public domain software distributors such as Compulink, The Sanctuary, Oakhill Grove, Surbi­ton, Surrey.

Some popular commercial comms programs for the PC are Crosstalk, Smarterm, and Chit-Chat. Chit-Chat is a UK product marketed by Sage. It costs almost £100, but it offers some features which are not available in public domain software. For example, it enables the PC to handle view data systems such as Prestel, and it comes with a library of telephone numbers of UK bulletin boards and on-line databases.