Advertisement
V.24 +
V.25 (bis)
Calling DTE
CT108 on (CDSL)
Off-hook
Dial tone
Transfer dial information
End-of-dialling
Detect off hook
Send calling home
Detect answering tone
Stop calling tone
Detect end answering tone
CT107 on (DSR)
Data transfer phase
CT105 on (RTS)
CT109 on (DCD)
CT106 on (RFS)
Figure B-3 V.25 (bis) Calling and Answering Tones.
Note: If the calling modem does not provide the generation of the V.25 calling tone and/or the
detection of the V.25 answering tone, the called party will send the answering tone for the
maximum defined duration of 3.3 sec. followed by the carrier in the high band. The calling
party will not respond to the answering tone but will detect the 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.
If the called modem does not provide the detection of the V.25 calling tone and/or the
generation of the V.25 answering tone, the called party will not respond to the calling tone and
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. If neither the called modem nor the calling
74
MODEM
Call establishment
Carrier 2400 Hz
Carrier 1200 Hz
NETWORK
V.24 +
V.25 (bis)
MODEM
Called DTE
CT125 on (ring indicator)
CT108 on (CDSL)
Off hook
Detect calling tone
Send answering tone
Detect end calling tone
Stop answering tone
CT107 on (DSR)
Data transfer phase
CT105 on (RTS)
CT109 on (DCD)
CT106 on (RFS)
Advertisement
