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received over this `trunk line' (not an ATU-circuit), call processing does not wait for an answer
signal from the `trunk line' but immediately establishes the transparent connection between
the remote and the local modem. The rest of the connection set-up must be performed by
the MLU and the modem:
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CT111 (DSRS) is sent to the modem to indicate if the high or the low speed must be used-
depending on the projecting of the MLU circuit;
-
CT126 (STF) is sent to the modem to indicate if the modem must receive in the high
frequency band and send in the low frequency band (calling mode) or receive in the low
frequency band and send in the high frequency band (answering mode)-depending on the
projecting of the MLU circuit;
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The MLU circuit forces circuit CT105 (RTS) to on;
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The modem starts waiting for the carrier from the remote party. If this carrier is detected
the modem knows the opposite party is on-line. The circuit CT109 (DCD) indicates to the
DTE that the DTE must be ready to accept data. Now the modem starts transmitting the
carrier in the opposite channel. The modem returns CT106 (RFS) to the DTE to indicate
the DTE may start transmitting data.
The connection between the remote party and the local party is established.
B.3.2. Incoming Call
For an incoming call over a leased line, the procedure is quite different from the convertor
situation. Because there is no way of detecting an incoming call when the line is connected to
the telephone set (unattended modem), the CDSL (CT108.1) signal must be continuously
active. The modem is continuously connected to the telephone line. The CT107 signal (DSR)
is returned to the MLU to indicate the modem is connected to the telephone line.
An incoming seizure on the MLU circuit is only possible if the circuit is projected for NO
constant carrier-see Projecting Aspects.
If the pool-modem is idle and detects an incoming carrier, the signal DCD (CT109) to the
MLU is activated and the MLU circuit sends a message to the SOPHO-S2500 CPU to indicate
an incoming seizure.
If the send and receive frequency of the modem is controlled by the MLU by signal CT126
(STF), the incoming carrier can only be in the high frequency band. If the modem is idle the
signal CT126 is in the off-condition. The modem is in the calling mode and detects only the
carrier in the high-frequency band.
If the send and receive frequency is controlled by the modem itself (switches and/or straps)
the incoming carrier can be in the high (modem in calling mode) or in the low (modem in
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