HDLC (High-Level Data Link Control)

FSM (Finite State Machine)

• Machine with a finite number of states

Simple protocol

• niether flow nor error control
• receiver can immediately handle any frame it receives
• the receiver can never be overwhelmed with incoming frame

Simple protocol

Stop and Wait protocol

• use both flow and error control
• The sender sends one frame at a time and waits for an ACK before sending the next one
• Add CRC to each frame
• If the timer expires, the sender resends the previous frame, assuming that the frame was either lost or corrupted

stop and wait protocol

HDLC (High-Level Data Link Control)

• bit oriented protocol for P2P and multipoint links
• NRM (normal response mode), ARM (asynchronous balanced mode)

NRM

ARM

Framing

I-frame (Information frame)

• to data-link user data and control information(piggy backing)

S-frame (Supervisory frame)

• to transport control information

U-frame (unnumbered frame)

• reserved for system management

HDLC frames

Flag field

• • 01111110, inentifying start and end

Address field

• • Primary station : it contains 'to' adress
  • Secondary station : it contains 'from' adress
  • one byte or more

user information field

• • containing Data

FCS field

• • containing CRC

Control field format

N(S)

• define the sequence number of the frame

N(R)

• ACK number when piggy backing is used (in the case of S-frame, it contains NAK)

P/F (poll/final)

• poll : primary station -> secondary station
• final : secondary station -> primary station

code

• defining the type of frame itself
• in the case of S-frame, having four types of S-frame
  • Receive Ready : 00
  • Receive not Ready : 10
  • Reject : 01
  • Selective reject : 11
• U-frame has 32 types (including above the fours)