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174
SERIES II INTELLIGENT DATA/FAX MODEMS
Appendix G - V.25bis Operation
Chapter 5 described a set of commands which let the Series II Modem
dial, hang-up, and be configured for various applications. However,
these commands, the AT command set, are only functional when the
DTE transmits data asynchronously. That is, they cannot be used with
synchronous equipment such as that found in IBM's Binary Synchro-
nous Communications (BSC) and Synchronous Data Link Control
(SDLC)* environments.
The ITU V.25bis commands provide you with an alternate set of com-
mands and responses to those described in Chapter 5 for applications
in which the DTE is synchronous. V.25bis commands support dialing
functions in asynchronous or synchronous mode according to the rec-
ommendations of the ITU; however, the AT commands are understood
to reign in the asynchronous world, while the V.25bis commands are
relegated to the synchronous world of datacomm. If you need to
establish a datacomm link over Public Switched Telephone Network
(PSTN) dial-up services, and your equipment outputs a synchronous
data stream, V.25bis commands are essential. V.25bis mode AT
commands do not include any modem configuration commands. You
can execute normal AT commands in V.25bis mode for modem
configuration.
Delayed and Forbidden Numbers
V.25bis provides the facility to delay failed call retry attempts by putting
numbers that failed to connect on a special Delayed Number list. Subse-
quent dialing of these numbers will be delayed (time specified by a
country regulation) and an appropriate message displayed. If the
number is retried more than the maximum allowed number of times
(number is also specified by country regulation), it is placed on the
Forbidden Numbers list, and no further retries will be allowed. When
the Forbidden Numbers list is full, no dialing is allowed and a CFIFF
indication is given. The modem in AT command mode will respond
with NO CARRIER. If country regulations require that the Forbidden
Numbers list be checked in AT mode, then NO CARRIER will be the
response to a dial attempt in AT mode. Numbers will also be put on the
Forbidden Numbers list if the Delayed Numbers list is full and a new
number fails for the first time. In that case, the new number will be