LANs

Communication systems allow people and organisations to stay in touch. They also help to make the world seem smaller. Communication devices and applications include

Telephones (analogue, digital, desktop and mobile)

·        Facsimile machines

·        Modems

·        Interactive television

·        Two-way radio

·        Teleconferencing

·        Bulletin boards

·        Electronic mail

·        Personal digital assistants (PDAs)

There are many types of modems, acoustic coupler (uses the handset in a cradle) and direct connect (internal and external). Null modem cables connect two computers through their serial ports when they are close together.

Simplex, half-duplex and full duplex are ways that data can be transmitted. Simplex is one-way, inexpensive and easy to install. Half duplex is two way but not simultaneously. Full duplex is two way and simultaneous.

Parallel transmission is used for sending data over short distances eg printer. Serial is generally used for longer distances but can be used over short distances (slower than parallel).  There is synchronous and asynchronous serial transmission. Asynchronous sends characters one at a time with special start and stop bits. It is inexpesive but slow. Synchronous sends characters at a fixed rate where the transmitter and receiver are synchronised to a "clock". It is efficient and more expensive. Error checking is better.

The baud rate is a transmission speed measurement. Rates can be 600, 1200, 2400, 4800, 9600(old fax machines), 14400(computer faxes), 19 200, 33 600 and 56 000(fastest modem). A parity bit is sometimes sent as a check. It is a zero or one. Odd parity means that all transmissions will have an odd number of ones.

Terminals can be smart(local processing), dumb(accept input and send to a processor) or intelligent(built in memory and input/output eg  computer). A home computer can EMULATE or imitate other machines eg send and receive faxes, voice transmission, transmit and receive to mainframe computers.

Handshaking allows two different computers with two different modems to come to an agreement on the method of transmission. Computer communication protocol may establish who will transmit first or set the transmission speed.

Local Area Networks (LANs) are generally confined to one or two buildings, have a high volume of data and all devices interconnected. Coaxial cable and fibre optic cable connect file and print servers, terminals, printers, modems and interfaces.

Gateways allow LANs using different protocols to communicate. Bridges allow LANs of like protocols to communicate. Servers have special responsibilities eg printing or files. Examples of LAN protocols are ethernet, arcnet, token ring and apple talk.