Envoy ACP/XF Application Programming Manual

Features Common to Bit-Synchronous Protocols
EnvoyACP/XF Application Programming Manual–132179
B-6
Control Field
I-Frame Control Field
An I-frame transfers data between two stations. The control field of an I-frame contains
the following parameters:
•
Control code (bit 0 = 0)
•
Ns
•
Poll/final bit
•
Nr
S-Frame Control Field
An S-frame performs link supervisory-control functions, such as:
•
Acknowledging I-frames
•
Requesting retransmission of I-frames
•
Indicating temporary inability to receive I-frames or U-frames
Figure B-4. Frame-Sequence Acknowledgment
Primary
Ns
0
1
2
3
Nr
0
0
0
0
Ns
0
1
2
Nr
4
4
4
Ns
4
5
6
7
0
Nr
3
3
3
3
3
Ns
3
4
5
Nr
1
1
1
Secondary
010