HLDC Frame

A HLDC Frame is an individual packet of data and some metadata about the packet.

Component	Purpose
Flag	The beginning and end of a frame
Address	The source secondary device
Control	The type of the frame and it’s acknowledgement identifier
Frame Check Sequence	A $16$ or $32$ bit Cyclic Redundancy Check

Types

There are a number of HLDC frame types with different purposes.

I-Frame

The I-Frame is a type of frame containing information for transfer.

Control Sequence

It’s control sequence looks like: Where:

• $N(S)$ = Sequence number of sender
• $N(R)$ = Sequence number of receiver
• $P/F$ = Poll/Final Bit

S-Frame

The S-Frame is a type of frame containing supervisory information for communication between devices. These may pertain to flow control or error control.

Control Sequence

It’s control sequence looks like: Where:

• $Code$ can be any of:
  • $00$ = Receive Ready (RR)
  • $10$ = Receive Not Ready (RNR)
  • $01$ = Reject (REJ)
  • $11$ = Selective Reject (SREJ)
• $N(R)$ = Sequence number of reciever

U-Frame

The U-Frame is a type of frame used for miscellaneous other purposes.

Control Sequence

It’s control sequence looks like: Code may be any of: