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6178-2203
ARQ
Automatic Repeat reQuest. When incorrect data is detected a request to retransmit
the data is made to the remote modem.
V.42bis.
Data Compression technique used by modems rather than MNP5, because it offers
better transmission on already compressed data.
Data Rate
In modems this is often different to the baud rate. For instance the Data Rate of V.32bis is 14,400 bps
and the baud rate is 2 400 symbols/second.
Direct Mode
The Data to be transmitted is sent directly to the data pump (the modem circuit). No compression,
error correction or buffering occurs, allowing the data to be transmitted across the link unaltered by
the modem.
Duplex
Means that the communication is bi-directional. In half duplex, the devices take turns sending and
receiving. In full duplex, sending and receiving can take place simultaneously.
Handshaking
Confirmations and status signals sent between communicating devices in order to check the data
stream. There are two general types of handshaking, hardware (RTS/CTS) and software
(XON/XOFF). In hardware handshaking the RS-232/V.24 status lines are used to control data flow,
whereas with the software method characters are transmitted to control the data.
Hayes commands
A set of commands for controlling PTT modems. Often referred to as the AT command set, since all
commands are started with AT. Most modems support these commands, but there are variations and
commands that are specific for a certain modem.
LED
Light-Emitting Diode. A semi-conductor which emits light when it receives an electrical current.
In modems they are used as indicators for data and status signals.
Leased Line
A private point to point connection provided by a national or local telecommunication company, or
someone else who has cable available, for instance power stations, waterworks or railway compa-
nies. These can be 2 or 4 wire point to point.
