Tuesday, March 19, 2013

Multi-access Systems

Multi-access Systems

A multi-access system is one which allows a number of users with on-line terminals to interact with the same computer at the same time. (See Fig 4.)

In fact, the computer can only run one program at a time. However, it is so fast compared with the terminals that it can do work for each terminal very quickly one after the other. Usually each user is able to interact with the computer as if he or she was the only user.

RESPONSE TIME

The response time of a multi-access system is the time the computer takes to start an operation which is requested. Usually it is the time between pressing the 'RETURN' key and getting a reply.

If a computer has too little processing power for the number of terminals on-line. it can happen that the response becomes very slow.

TIME-SHARING

One way of organizing a computer for multi-access is by time-sharing. Time-sharing is a system where the computer allows each terminal in turn a small amount of processing time before it goes on to the others.

Multi-access Systems 

Fig 4 Example of a multi-access system (hardware). In different systems the terminals may vary in type and number . They may be connected to the computer by any of a number of different methods . The diagram shows a system with six terminals, of which three are connected directly to a multiplexor, and three are connected using modems and telephone lines

Examples of multi-access systems

1 An airline flight-booking system. This is an example of a multi-access system which is also a real-time system. If a ticket is booked from a terminal, this will be put into effect immediately. If someone else then tries to book the same seat from another terminal, they will be told it is taken.

2- a university or polytechnic main computer with a large number of terminals . many of the users would be linked to the computer directly by cable.

3 A national network such as TINS-The Times Network for Schools. Most of the users of this system are schools using the telephone system via a modem. They usually use microcomputers programmed to act as terminals.

4 Bank cash points. These are in fact cash issuing terminals connected to the bank's main computer .

Advantages and disadvantages of multiple access

Advantages

1 Allows interactive use of a powerful computer.

2 Can be used at large distances from the computer (e.g. using modems and a telephone line).

3 Needs little action by computer operators.

Disadvantages

1 Time consuming for the user, who has to be available to run the program.

2 Response time may be too slow for many real-time operations.

3 it is often not practical to run jobs with large amounts of input or output data.

INTERACTIVE COMPUTING

Interactive computing is a method of computer use where the user and the computer communicate with one another in the form of a conversation.

Examples of interactive computing

1 Using a terminal linked to a time-sharing system.

2 Using a microcomputer via a keyboard and screen.