Glossary
FP Modem-EU Technical Manual
7.1 Technical Terms
Answer mode
Setting of a modem, S0 register is set to 1, resulting in automatic answering after a ring (RING).
The modem that was dialed up enters the ANSWER mode. The calling modem enters the
ORIGINATE online mode.
ARQ
Message that a connection with error correction has been established. It is attached to the
CONNECT message.
ASCII
During data transmission via modem, a definite combination of 8 bits is used to represent the
individual characters. This standard that was developed over many years is called ASCII code
(American Standard Code). The original ASCII table contains only 128 characters (letters,
digits, special characters) and could also be represented by a group of 7 bits instead of 8 bits:
the most significant bit is always 0. In the ASCII representation, this eighth bit is superfluous,
however it is generally transmitted since the computer is processing the data internally on an
8-bit basis (BYTE). When using your modem, you might ascertain that there is a possibility to
choose between 7 bits or 8 bits ASCII in some transmission modes.
Asynchronous
Connection without the transmission of a synchronizing frequency. The beginning and the end
of a word have to marked by a start bit and a stop bit. Sometimes this is also called anisochron.
The opposite is called synchronous connection.
AT commands
A standard for modem commands, defining the control of modems from the PC. All Hayes
commands begin with the ASCII character AT (Abbreviation for: Attention). See Hayes.
Autoretrain
Upon trouble, the transmission parameters are “retrained”, i.e. adjusted by the modems during
the connection. See also Retrain.
Break signal
Break signal for the calling modem that aborts the connection.
Busy
Busy signal. The distant modem cannot take your call because it is busy with a different
connection.
Carrier
Carrier signal that is sent by modems for the identification of the sender and the receiver.
Check sum
By comparing the results of the mathematically calculated check sums for the transferred data
blocks, it can be determined whether data transmission was carried out error-free.
CLIP decoder
Calling Line Identification Presentation Decoder
Compression
Compression reduces data volume. In most procedures (e.g. V.42bis, MNP 5) the most
frequent characters and sequences of characters are coded in short bit sequences, while the
less frequent ones are coded in long bit sequences. Certainly, this only works for data in which
characters occur with a least slightly differing frequency. But even then data is compressed two-
or three-fold. If the bytes are spread almost evenly (e.g. for data already compressed),
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