Envoy ACP/XF Application Programming Manual

Features Common to Bit-Synchronous Protocols
EnvoyACP/XF Application Programming Manual–132179
B-7
Control Field
The control field of an S-frame contains the following parameters:
•
Control code (bit 0 = 1, bit 1 = 0)
•
Command/response code (supervisory message code)
•
Poll/final bit
•
Nr
In an S-frame, the control code that identifies the frame is 2 bits instead of 1.
The Ns parameter is unnecessary because supervisory frames are unnumbered. Ns is
replaced by a two-bit command/response code. The S-frame command and response
codes are identical to each other. See the descriptions of the S-frame command/response
codes in Table B-1
.
The Nr parameter is present and significant.
Table B-1. S-Frame Command/Response Codes
Command/Response
Code Description
Receive Ready (RR) Indicates that all frames that have sequence numbers less than the
value of Nr have been received without error and that the station is
ready to accept further transmissions.
EnvoyACP/XF automatically issues RR frames as appropriate. Your
application also can set RR explicitly by issuing the CONTROL 16
procedure call.
Receive Not Ready
(RNR)
Indicates that the station is busy and that frames that have a
sequence number which is the value of Nr or greater cannot be
accepted at present.
EnvoyACP/XF automatically issues RNR frames as appropriate
unless the SENDRNRRETRY modifier is disabled.*
Reject (REJ) Requests retransmission, starting with the frame that has sequence
number Nr. REJ implies acknowledgment of frames up to Nr.
To enable EnvoyACP/XF to send or accept REJ frames, either
specify the REJ parameter in the system configuration or use the
SETMODE 16 procedure call to enable REJ.
Selective Reject
(SREJ)
Requests retransmission of a specific I-frame without retransmission
of any subsequent I-frames.
To enable EnvoyACP/XF to send or accept SREJ frames, either
specify the SREJ parameter in the system configuration or use the
SETMODE 19 procedure call to enable SREJ. This capability is
particularly useful for extended control modes.
* You can limit the duration of the RNR state by setting the RNRTIMER with SCF. The RNRTIMER
determines how long a station can remain in RNR state before being disconnected.
Set the RNRTIMER to a value greater than the T1TIMER * L2RETRY. If you do not, the T1TIMER elapses
before the RNRTIMER is activated.