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CT108.1 (CDSL) forces the modem on-line;
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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;
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CT126 (STF) is set in the off-condition to indicate the modem must send in the low-
frequency band and receive in the high-frequency band (calling mode);
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The modem returns the signal CT107 (DSR) to the MLU circuit to indicate the modem is
ready for operation;
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If the V.25 tone sequence is used, either the modem or the MLU circuit sends calling tone
and starts waiting for the answer tone. If the answering tone is detected by the modem or
the MLU circuit, the calling tone is stopped. If the V.25 tone sequence is not used, this step
is skipped;
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The MLU circuit forces circuit CT105 (RTS) to on;
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The modem starts waiting for the backward 2400 Hz carrier. If this carrier is detected the
modem knows the opposite party is on-line. The circuit CT109 (DCD) indicates to the
DTE (MLU) that the DTE must also be ready to accept data. The modem now starts
transmitting the 1200 Hz carrier in the forward channel and returns CT106 (RFS) to the
DTE to indicate the DTE may starts sending.
The connection between the remote party and the local party is established.
Note: Depending on the projecting of the MLU it can switch the CT111 circuit (DSRS) to off or
on to indicate to the modem that the low or the high speed must be used. For more
information: see projecting aspects.
B.3.
MODEM USED FOR LEASED LINES IN POOL
A modem can also be used in a pool when leased lines to a fixed data party (with a modem)
are available. In this configuration it is not possible to send or receive dial information to or
from the opposite party.
The analogue trunk line is only used for data connections, so they can have a fixed modem
assigned to them. Only the connection between the related internal data party and a free
modem must be established. The analogue interface of the modem is not connected to the
MLU but directly to the leased line.
As the tone generators and detectors of the MLU are related to the analogue interface, the
MLU cannot offer the V.25 calling and answering tone functions to the modem.
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