Envoy ACP/XF Application Programming Manual

Asynchronous Balanced Mode (ABM) Protocols
EnvoyACP/XF Application Programming Manual–132179
2-10
Selective Reject (SREJ)
You enable or disable the REJ command/response by setting bit 7 of parameter 1 in a
SETMODE 16 call, as follows:
CALL SETMODE (fnum,16,parameter1,parameter2)
parameter1.<7> = 0: disable REJ command/response
= 1: enable REJ command/response
You enable or disable the SREJ command/response by setting bit 0 of parameter 2 in a
SETMODE 116 call, as follows:
CALL SETMODE (fnum,116,parameter1,parameter2)
parameter2.<0> = 0: disable SREJ command/response
= 1: enable SREJ command/response
Selective Reject (SREJ)
The selective reject (SREJ) command or response indicates that the specified I-frame
was received in error and requests the retransmission of only that frame. EnvoyACP/XF
still must acknowledge succeeding I-frames. You can use SREJ with a basic control
field, but this action is not necessary. However, always use SREJ with an extended
control field.
This type of error recovery is very efficient. For example: suppose you use an extended
control field (modulo-128 frame sequencing) and send 100 I-frames in succession. All
100 I-frames are received without error except frame #3. If the remote station uses the
SREJ command/response, you only need to retransmit frame #3. (By contrast, if the
remote station used the REJ command/response, you would need to retransmit 97 frames
— all but frames #0, #1, and #2.)
If SREJ is disabled, EnvoyACP/XF responds with a FRMR to any incoming SREJ
command.
Activating a Line
After both stations open the communications line, they activate the link by exchanging
the SABM or SABME command and a UA response. Only one such exchange must be
completed to activate the link. The exchange can occur in either direction.
An application program within a Tandem system activates the link by issuing a
CONTROL (fnum,13,2) call. This call causes EnvoyACP/XF to transmit the SABM or
SABME command.
If the remote station responds with a UA, both stations go online and the link is
activated. If the remote station responds with anything else (such as a DM), the local
system remains disconnected.
If the remote station is supposed to activate the link, the Tandem system must be offline
(as opposed to disconnected) so that it can respond with a UA. An application program
Note. When CONTROL (fnum, 13,2) call is issued to send the SABM, EnvoyACP/XF
implicitly puts the local station in an online state.