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.
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.
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.