Envoy ACP/XF Application Programming Manual

Asynchronous Balanced Mode (ABM) Protocols

EnvoyACP/XF Application Programming Manual–132179

2-8

Timers and Number of Retries

In the ABM protocol, the primary and the secondary station use the T1TIMER similarly.

However, the secondary station does not send a P-bit. Sending the RR-frame without the

P-bit indicates that the local station is alive and ready to accept transmissions. The

secondary station sends RR-frames only if L2HEARTBEAT and IDLERESPRR are

enabled.

Both HDLC and ADCCP maintain two retry count variables: one for the RR frame

retransmissions and one for the I-frame retransmissions.

If the remote station responds to one RR frame, the local station does the following:

1. Resets the RR frame retry count to 0.

2. Retransmits the I-frame.

3. Increments the I-frame retry count.

The I-frame retry count resets to 0 only when the remote station acknowledges the I-

frame or when the WRITE operation aborts because the retry count is exhausted.

The value of the retry count is the upper limit for both RR frame and I-frame

retransmissions. Figure 2-4

shows the use of timeouts and these two retry count

variables.

Figure 2-3. T1TIMER and RR Frame Retry Count

013

Retry Count = 3

WRITE Operation Issued

Send I-Frame

Send RR,P

(No Response; Timeout)

WRITE Operation Aborted with File-System Error Code 162.

Send RR,P

RR Frame

Retry Count