Envoy ACP/XF Application Programming Manual

Features Common to Bit-Synchronous Protocols
EnvoyACP/XF Application Programming Manual–132179
B-5
Control Field
•
For ABM data-link protocol:
•
The primary/combined station sets the P bit to 1 in
command frames to solicit a response or sequence of
response frames from the remote secondary/combined
station.
•
F bit is set to 1 by a secondary/combined station to
indicate a response frame sent in reply to a command
with the P-bit.
Nr Number-received variable. Nr specifies the link-level sequence
number (modulo-8 or -128) of the next I-frame that the
transmitting station expects to receive. All I-frames and S-frames
contain Nr.
Nr acknowledges the receipt of all I-frames with sequence
numbers less than Nr; the next I-frame that Nr receives should
have an Ns sequence number equal to Nr.
Because of Ns/Nr sequence checking, two stations do not need to
acknowledge each frame individually. When the sequence
numbers are modulo-8, EnvoyACP/XF must acknowledge frames
at least once for every 7 frames received, while modulo-128
sequencing forces an acknowledgment at least once every 127
frames.
Figure B-4
shows the use of frame-sequence numbers in modulo-
8 sequencing. Note that once transmission begins, the Nr values
specify the number of the next frame to be received, and that a
single Nr value can acknowledge the receipt of several successive
frames. For example, an Nr value of 5 implies that frames 0
through 4 have been received even though they might not have
been acknowledged. (The acknowledgment pattern shown in
Figure B-4
is somewhat simplified in that it does not assume
overlapped transmissions in a full-duplex environment.)