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modem provide the generation/detection of the V.25 calling tone or the generation / detection
of the V.25 answering tone, the called party will immediately send the carrier in the high band.
The calling modem will detect this 2400 Hz carrier and consider it to be the indication that the
called party is connected. After sending the 1200 Hz carrier the connection is established.
B.2.
MODEM USED AS CONVERTOR IN POOL
If an analogue trunk line can be used for both voice and data calls, it is necessary to switch a
modem into the connection for data calls. Such a modem can be located in a modem pool both
for incoming and for outgoing calls. Only when such a trunk line is used for a data connection,
is the modem to be used.
The modem is connected to the MLU by both the V.24 interface and the analogue line.
Note: The projecting signalling group for the MLU always indicates that the modem can be used
for outgoing calls and for incoming calls. The MLU uses CT126 (STF) to set the modem in
the answering or calling mode. If the modem does not support CT126, it must be strapped
for one direction only. By means of the relation CV-A-CV-B-modem pool type, the modem
must be assigned to a pool with modems with one direction: incoming or outgoing. The
relation CV-trunk line-CV internal party (incoming) now uses a different modem pool type
than for the relation CV-internal party-CV-trunk line (outgoing).
B.2.1. Incoming Call
Modem pool dialling with incoming calls is only possible for trunk lines which support Direct
Dial In (DDI). If DDI is not possible, the call is routed to the operator or a Permanent Line
Extension and modem pool dialling is not used. A call to a PLE can be routed to a modem
which is determined by the CV matrix (route CV/PLE CV).
If a modem in the modem-pool is idle, all signals between the MLU and the modem are in the
off-condition.
Call processing detects an incoming call over an analogue trunk line (with CV) via the line
signals. The ISPBX retrieves the required destination number by the register signalling.
This number indicates a destination party to which a compatibility value is assigned. The
compatibility check between the trunk-CV and the destination-CV is used to find out whether
the party is a data party and which type of modem must be used for the connection. Now a
free circuit on a MLU with the correct modem type is seized.
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