ACC HDLC-NRM (SDLC) User's Guide

Using HDLC-NRM (SDLC) Protocols
Timeout Processing
Chapter 332
Timeout Processing
Timers are used to control several mechanisms in this implementation of
the HDLC-NRM (SDLC) protocol.
Response Timer
The principal timer used to control the polling cycle is the T1 timer,
which is a parameter for each primary station. Its value is configured as
described in the next chapter. When the primary station sends a frame
with the Poll bit set to the secondary, the T1 timer is started. It is
restarted when an I-frame is received without the Final bit. The timer is
stopped when a frame is received from the secondary station with the
Final bit set. If the T1 timer expires, the primary station will issue
another poll (to the same or another secondary station).
If timeouts continue to occur, up to the configured retry limit, without a
good response being received, the primary will send a mode setting
command (Set Normal Response Mode or SNRM) with the Poll bit set to
the secondary to cause a reset. The SNRM command also uses the T1
timer to ensure that it receives a response. If no response is received to
the SNRM command for the configured retry limit, the terminal is
marked down, and an unsolicited status message is sent to the
application.
When down, the primary protocol software will conserve line time by
polling the station only once in every slow poll cycle, unless the 'No Slow
Poll' option is set for the terminal. The slow poll interval for the
HDLC-NRM (SDLC) protocol is 33 seconds. Once the secondary station
responds to a poll, it will be reinstated to the normal poll cycle. At the
time when it successfully responds to a SNRM command after being
down, the protocol software will issue an unsolicited status message to
the application with reason code “No error detected”, indicating that the
terminal is now operational.