Modem Retrain
The term 'modem retrain' is used here to mean the temporary inability of a modem to respond to requests
from the attached business machine (DTE). On V.35 lines, this typically occurs when two modems lose
carrier synchronization. While 'retraining' the carrier, they are unable to provide service.
On X.21 lines, a 'retrain' means that the network or DTE places the line under test and during this time,
the line is unable to provide service.
V.35 Modem Retrain
'Modem retrain' is detected on V.35 lines as a drop of clear to send (CTS).
When this occurs:
1. The FESH starts a timer derived from the set mode enable time out value.
2. The FESH continues any transmission until the end of message is reached.
3. If a transmission is in progress when CTS drops, and the retrain timer has not expired, the following
ending status is sent to the CCU when the transmission completes:
SCF = X'22' (underrun/data transmitted)
SCF = X'20' (underrun)
SES = X'80' (modem retrain)
LCS = X'00'
4. If CTS does not recover, and the retrain timer expires while a transmission is in progress, the ending
status sent to the CCU is:
SCF bit 3 = 1 (modem check)
LCS = X'E2'
(CTS dropped)
If CTS does not recover, but there is no transmission in progress, the error will be detected and
reported on the next transmit command as:
SCF bit 3 = 1 (modem check)
LCS = X'F2'
(CTS failed to come up)
5. Data reception is not affected.
X.21 Modem Retrain
On X.21 lines, modem retrain is entered when a DCE uncontrolled not ready or
DCE controlled not ready steady state is detected. When this occurs:
1. The CSP microcode starts a 20 second timer.
2. The FESH continues any transmission until the end of message is reached.
3. If a transmission is in progress when the retrain starts, the following ending status is sent to the CCU
when the transmission completes:
SCF = X'22' (underrun/data transmitted)
SCF = X'20' (underrun)
SES = X'80' (modem retrain)
LCS = X'00'
4. If data reception is in progress when the retrain starts, the incoming frame is flushed (to avoid filling
the NCP buffers) and the receive command is ended with the following status:
SCF = X'04' (end of message)
SES = X'10' (data check)
LCS = X'00'
5. If the DCE does not return to a 'ready' or 'ready for data transfer' state before the time out expires, the
following ending status is sent to the CCU:
SCF bit 3 = 1 (modem check)
LCS = X'EE'
Chapter 5. High Performance Transmission Subsystem (HPTSS)
5-21
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