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Two data paths are established through the switching network:
-
From the ATU circuit to the MLU:
The analogue input signal from the trunk line is digitised (A/D convertor) on the ATU
board into a standard 64 kbps signal. This digitised analogue signal is sent to the MLU, via
the switching network, in the USO-line. The analogue signal is retrieved from the USO line
and sent to the modem.
-
From the MLU to the destination party:
After demodulation the resulting user information is sent back in V.24 format to the MLU.
The V.24 user information is transferred to the internal party, via the data rate adaptor, in
64 kbps format in a timeslot in the USI-line. The user information is retrieved and
transferred to the SOPHO-SET, SOPHO-LAM or LAM, via the related DLC circuit, in a
64 kbps signal. Here the V.24 user information is retrieved from the 64 kbps signal and
sent to the data party.
For full duplex data communication, the reversed path must also be established through the
switching network.
Call processing sends ringing current to the destination party and starts waiting for the
answering of the call. The call establishment is guarded by timers.
When the destination party answers the call, manually or automatically, the pool-modem is
activated:
-
CT108.1 (CDSL) forces the modem on-line.
-
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 set in the on-situation to indicate the modem must send in the high-
frequency band and receive in the low-frequency band (answering mode).
-
The modem returns the signal CT107 (DSR) to the MLU circuit to indicate that the
modem is ready for operation.
-
If the V.25 tone sequence is used, either the modem or the MLU circuit sends answering
tone for 3.3 sec. If the V.25 tone sequence is not used, this step is skipped.
-
The MLU circuit forces CT105 to on and the modem starts sending the 2400 Hz carrier
(answering mode).
-
If the modem receives the 1200 Hz carrier from the remote modem, CT109 (DCD) goes
on to indicate that the DTE must be ready to accept data. The signal CT106 (RFS) from
the modem indicates that the opposite party is on-line and that the DTE may start sending
information.
The connection between the remote party and the local party is established.
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