Page 117 - DCAP104_EXPOSURE_TO_COMPUTER_DISCPLINES

Exposure to Computer Disciplines

Notes While the modem interfaces are standardized, a number of different protocols for formatting
data to be transmitted over telephone lines exist. Some, like CCITT V.34, are official standards,
while others have been developed by private companies. Most modems have built-in support
for the more common protocols at slow data transmission speeds at least, most modems can
communicate with each other. At high transmission speeds, however, the protocols are less
standardized.
Aside from the transmission protocols that they support, the following characteristics distinguish
one modem from another:
BPS : How fast the modem can transmit and receive data. At slow rates, modems are measured
in terms of baud rates. The slowest rate is 300 baud (about 25 cps). At higher speeds, modems are
measured in terms of bits per second (bps). The fastest modems run at 57,600 bps, although they
can achieve even higher data transfer rates by compressing the data. Obviously, the faster the
transmission rate, the faster you can send and receive data. Note, however, that you cannot receive
data any faster than it is being sent. If, for example, the device sending data to your computer
is sending it at 2,400 bps, you must receive it at 2,400 bps. It does not always pay, therefore, to
have a very fast modem. In addition, some telephone lines are unable to transmit data reliably
at very high rates.

Voice/data: Many modems support a switch to change between voice and data modes. In data
mode, the modem acts like a regular modem. In voice mode, the modem acts like a regular
telephone. Modems that support a voice/data switch have a built-in loudspeaker and microphone
for voice communication.

Auto-answer: An auto-answer modem enables your computer to receive calls in your absence.
This is only necessary if you are offering some type of computer service that people can call
in to use.
Data compression: Some modems perform data compression, which enables them to send data
at faster rates. However, the modem at the receiving end must be able to decompress the data
using the same compression technique.

Flash memory: Some modems come with flash memory rather than conventional ROM, which
means that the communications protocols can be easily updated if necessary.
Fax capability: Most modern modems are fax modems, which means that they can send and
receive faxes.
Modems grew out of the need to connect teletype machines over ordinary
phone lines instead of more expensive leased lines which had previously been
used for current loop-based teleprinters and automated telegraphs. George
Stibitz connected a New Hampshire teletype to a computer in New York City
by a subscriber telephone line in 1940.

6.3.1 Narrow-Band/Phone-Line Dialup Modems
A standard modem of today contains two functional parts: an analog section for generating the
signals and operating the phone, and a digital section for setup and control. This functionality is
often incorporated into a single chip nowadays, but the division remains in theory. In operation
the modem can be in one of two modes, data mode in which data is sent to and from the computer
over the phone lines, and command mode in which the modem listens to the data from the
computer for commands, and carries them out. A typical session consists of powering up the
modem (often inside the computer itself) which automatically assumes command mode, then
sending it the command for dialing a number. After the connection is established to the remote
modem, the modem automatically goes into data mode, and the user can send and receive data.

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