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Calling party:
Called party:
Calling party:
Called party:
Calling party:
Called party:
Calling party:
Called party:
The transparent connection is established.
The procedure on the line is illustrated in figure
Tones.
If the calling and answering tones are not used, the calling modem regards the received 2400
Hz carrier as the answering signal and starts sending the forward 1200 Hz carrier.
When the calling party has fully dialled the number of the destination
party, the network control establishes the call through the network
and sends ring current to the called party.
The receiving modem detects the ring current and sends the signal
ring indicator (CT125) to the DTE. The DTE switches the modem on
line via CT108.
The calling modem detects that the called party is off-hook and starts
sending calling tone.
The called modem detects the calling tone and sends back the
answering tone.
When the calling modem detects the answering tone, it stops sending
the calling tone.
The called modem detects the end of the calling tone and stops the
answering tone. Now also the signal CT107 is returned to the DTE to
indicate the modem is ready for operation. The DTE sends CT105 to
the modem and the modem starts sending the carrier in the backward
channel.
When the calling modem detects the end of the answering tone, it
returns CT107 to the DTE to indicate the modem is ready for
operation. At the same time the carrier in the backward channel is
received and CT109 indicates that the called party is present.
The DTE sends CT105 to the modem and the modem starts sending
the carrier in the forward channel. CT106 indicates that the called
party is on-line and the DTE may start sending information.
When the called modem detects the carrier in the forward channel
CT109 indicates that the calling party is on-line. CT106 indicates that
the DTE may start sending information.
Figure B-3 V.25 (bis) Calling and Answering
73
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