HDLC.md

HDLC

Frame Format

Flag	Address	Control	Information	FCS	Flag
01111110	1/more bytes	8/16bits	variable, 8 x n bits	16/32bits	01111110

Flag Sequence

Use the flag sequence 01111110 to delimit frames.

If the flag sequence appears in the data, it is escaped by inserting a 0 after five consecutive 1s.

Address Field

• all bits are 1 for broadcast
• can have multicast addresses.

For address field longer than 1 byte, the last byte has the last bit set to 1, and the previous bytes have the last bit set to 0.

Control Field

The control field is used to specify the type of frame.

• I for Information frames, identified by first bit 0
• S for Supervisory frames, identified by first two bits 10
• U for Unnumbered frames, identified by first two bits 11

Information Frames

	0	N(S)	P/F	N(R)
bit(s)	1	3	1	3

• N(S) is the send sequence number
• P/F is the poll/final bit
• N(R) is the receive sequence number

Supervisory Frames

	10	CODE	P/F	N(R)
bit(s)	2	2	1	3

• CODE is the command/response code
  • 00: RR (Receive Ready)
  • 01: RNR (Receive Not Ready)
  • 10: REJ (Reject)
  • 11: SREJ (Selective Reject)
• P/F is the poll/final bit
• N(R) is the receive sequence number

here are some cases of supervisory frames:

ACK:  RR | RNR(N(R)=x) | I
REJ:  REJ(N(R)=x) | SREJ(N(R)=x) | I

POLL:     RR(P/F=1)
POLL.ACK: I
POLL.NAK: RR(P/F=1)

SEL:      RNR(P/F=1)
SEL.ACK:  RR(P/F=1)
SEL.NAK:  RNR(P/F=1)

Unnumbered Frames

	11	CODE	P/F	CODE
bit(s)	2	2	1	3

• 5-bit CODE field, can represent up to 32 commands.
• P/F bit is set to 1 in most cases.

Information Field

• In an information frame, the information field contains the data to be transmitted.
• In a supervisory frame, the information field is empty.
• In an unnumbered frame, the information field contains information about the link.

HDLC uses piggybacking to send data and control information in the same frame, which is more efficient.

Frame Check Sequence (FCS)

• Use CRC to check the integrity of the frame.
• CRC-CCITT (X^16 + X^12 + X^5 + 1) is commonly used.

Example