Data Communication Systems

Telephone

Telephones are still the most important form of electronic communication. They provide direct person to person oral communication. The major limitation is that the person you need to speak to must be available at the other end. Recent developments include:

• Call switching where calls are automatically routed from one number to another.
• Automated switchboards where incoming calls are routed to the correct extension. The caller receives a series of messages and responds by keying digits on their hand set to select options.
• Conferencing systems that allow a number of callers to be connected at the same time.
• Mobile phones that allow people to be contacted irrespective of location.
• Text messaging is an important development in mobile phone technology.
• Answer phones. Incoming messages recorded. Stored messages can be recalled form any location using a remote control device.

Telex

Allows users to transmit and receive printed information. Both sender and receiver must subscribe to a telex service. The service began in the 1930's and it is rather dated and restricted in terms of capability. However there are over 2 million subscribers worldwide. Originally a special telex machine was used to send and receive data but now a PC can be used.

Telex is particularly important in third world countries because it does not require high quality communication links. A single transmission can be addressed to a number of telex locations.

E-Mail

Messages are prepared (usually on a microcomputer using a word-processor program) and sent to the recipients mail box in a central computer. The mailbox is a reserved storage area which is dedicated to holding that particular persons electronic mail. The recipient connects to the computer, again usually using a PC, to read the mail. Mail can be copied from the central computer onto a local disc and printed. The telephone network is usually used by sender and recipient to connect to the central computer. In addition to a PC and software needed for connection, both will also require a modem. Many portable PC's now include a built in modem so that they can connect to e-mail services anywhere that has a phone outlet. Sender and recipient must subscribe to an e-mail service.

Advantages are:

• Does not depend on recipient being available or in a specific location since mail is sent to a central computer.
• Mail can be sent to a number of users at one time by attaching a multiple address list to the message. The cost is the same as for sending one message.
• Data other than text can be sent - but the recipient must be able to use the data in the format used. An example could be sending a computer program or picture file.
• Reading of message can trigger an automatic acknowledgement to sender's mailbox so the sender knows that the message has been received.
• Speed - whilst it is not possible to guarantee delivery in a given time as there is no method of checking that the recipient is logged on, the message will reach the destination mailbox quite quickly.

Voice Mail

A callers spoken message is recorded and stored at the recipient's mailbox in digitised form. The messages can then be accessed from any location via the telephone. Similar to an answer phone but the service is centrally provided and the message is stored digitally.

Fax

This transmits a copy or facsimile of the original document. The original is scanned by the sending machine and data transmitted to the receiving machine which reproduces the document. Reproduction is done on thermal or electrostatic sensitive paper or using the same mechanism as a photocopier or laser printer on plain paper. PC's can be used instead of a fax machine and printed documents input using a scanner while documents stored as files can be transmitted directly.

Fax provides immediate transmission of both picture and text. Both sender and recipient must have a fax machine, although some shops offer a fax sending service. There may be a problem with security - fax can be read by anyone at the receiving end. There have also been problems over sensitive faxes being sent to the wrong number or cover sheets transmitted to identify the source of the material getting mixed up. Fax must be sent to a physical destination.

Advantages

• Pictures and diagrams can be transmitted as well as text
• Fax machine can continue re-dialling if line is engaged
• Immediate transmission/reception
• Cost is less than stamp. Sender pays for call recipient pays for paper/consumables to print the fax.

© LEV