HDLC DataLink Layer
Chapter 6
610
The Address 3 would be:
Time
1 1 0 0
0 0 0 0
In the control field, the order of bit transmission for sequence-number
values and command/response codes is low bit first.
In the FCS field, the order of bit transmission is high bit first.
In the data field, the order of bit transmission is not determined by or
relevant to the HDLC data-link layer. These bits are only passed
through; they are not interpreted by the HDLC data-link layer.
However, the order of transmission is low bit first within a byte, low
byte first within a word, and low word first within a block of words.
The control field is composed of sub-fields that conform to the following
formats:
0
Time
N (S)
P/F
N (R)
1
C/R
Code
P/F
N (R)
1
C/R
Code
P/F
C/R Code
Extension
Control Field Bit No.
Data Transfer Format
Supervisory Format
Unnumbered Format
0
1
2
3
4
5
6
7
WHERE:
N (S)
=
Send Sequence Number of Transmitting Node (Bit 1 is low bit.)
N (R)
=
Receive Sequence Number of Transmitting Node (Bit 5 is low bit.)
C/R Code =
Code that Specifies the Type of Command or Response
P/F
=
Poll Bit for Commands (1 = Poll)
Final Bit for Responses (1 = Final)
0
1
Use the data transfer format in a data frame as a command to transfer
data. The functions of N(S), N(R), and P/F are independent: each data
frame has an N(S); the N(R) may or may not acknowledge additional data
frames received from the other node; the P/F bit may or may not be set to
1.
Use the supervisory format to perform link supervisory control
functions—such as acknowledging data frames, requesting retransmission
Control Field Formats