Introduction To Line Telecommunications


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9. Simple Telephone Devices
  9. - Simple Telephone Devices 9.4 - Modems
  9.1 - Telephone Answering Machines 9.5 - Voicemail Systems
  9.2 - Cordless Telephones 9.6 - Specialist Devices
9.3 - Facsimile Machines

9.4 - Modems

MODEM stands for MODulator-DEModulator. These devices convert data, usually from a computer, into a form, which can be transmitted across the telephone network. These are required because the PSTN is not suitable for digital data transmissions directly and even the digital networks require the data to be formatted in a specific way, different from a computer, for transmission.

For transmission of data across the network the modem encodes the data using a specific format or protocol which includes control data with the data in a similar way to FAX. A FAX device can be thought of as a scanner and modem combined.

There are a wide variety of transmission protocols available to the modem user. Each has developed from a specific requirement or development and the choice is dependant upon the type of data, speed and error correction requirements of the transmission.

The destination needs to know the protocol and transmission speed used to correctly receive the data. Many modern modems will automatically detect the speed and some software can guess the protocol, but if both parties know these parameters the transmission will be more likely to succeed.

9.4.1 - Computer Modems

Computer modems are designed to convert computer data for transmission. These were originally connected to a serial data port on the computer and housed in an external case operating as a separate unit. Recent trends are for the modem to be built on an expansion card and fitted internally to the computer. This allows a closer merging between the two devices, reduces cabling and does not require a separate power source. In conjunction with software these modem cards usually double as FAX machines and can transmit documents stored on the computer to other computers or paper copy FAX machines. Recent products provide voice functions enabling the PC to act as a TAM or even a voicemail system for a small group of users, as well as simple dial in access from remote systems.

9.4.2 - Card Readers

Card readers are a specialised modem unit. They are now found in most retail outlets. The unit will read the magnetically stored information on a plastic credit or debit card, then link up to the issuers computer over the network and get authorisation or refusal of the transaction and inform the user, usually by printed slip, of the result. If the transaction is allowed the data sent will be used to charge or debit the card holders account and transfer the funds to the account of the vendor. The customer is then required to sign as authentication and receives a copy of the transaction slip.




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Introduction To Line Telecommunications
Copyright Panasonic Business Systems UK Ltd 2000