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CS601 - Data Communication - Lecture Handout 41

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Bit-Oriented Protocols

  • In character-oriented protocols, bits are grouped into predefined patterns forming characters
  • By comparison, bit-oriented protocols can pack more information into shorter frames

Bit-Oriented Protocols

A lot of bit-oriented protocols have been developed over the years:

  • One of these HDLC is the design of the ISO and has become the basis for all bitoriented protocols in use today
    • In 1975, IBM gave Synchronous Data Link Control (SDLC)
    • In 1979, ISO answered with High Level Data Link Control (HDLC)
  • Since 1981, ITU-T has developed a series of protocols called Link Access Protocols
  • LAPs: LAPB, LAPD, LAPM, LAPZ etc. all based on HDLC

HDLC is basis for all protocols, so we will study it in detail:

High Level Data Link Control (HDLC)

  • Bit-oriented data link protocol designed for:
    • Full Duplex and Half Duplex
    • Point-to-point And Multipoint Links

Characterization of HDLC

HDLC can be characterized by:

  • Station Types
  • Configurations
  • Response Modes


HDLC differentiates b/w 3 types of stations:

  • Primary Station
  • Secondary Station
  • Combined Station


Primary Station

  • Primary station works in the same way as primary devices in the discussion of flow control
  • The primary is a device in point-to-point or multipoint line configuration that has complete control of the link


Secondary Station

  • The primary sends commands to the secondary stations
  • A primary issues commands and a secondary issues responses


Combined Station

  • A combined station can both command and respond
  • A combined station is one of a set of connected peer devices programmed to behave either as a prim
    ary or as a secondary depending on the nature and the direction of the transmission

Configuration (1)

  • Configuration refers to the relationship of the hardware devices on a link
  • Primary , secondary and combined stations can be configured in three ways:
    • Unbalanced Configuration
    • Symmetrical Configuration
    • Balanced Configuration

Configuration (2)

Unbalanced Configuration

  • Also called Master/Slave Configuration
  • One device is a primary and others are secondary
  • Unbalanced configuration can be point to point if only two devices are involved
  • Most of the times it is multipoint with one primary controlling several secondaries

Configuration (3)

Symmetrical Configuration

  • Each physical station on a link consists of two logical stations, one a primary and the other a secondary
  • Separate lines link the primary aspect of one physical station to the secondary aspect of another physical station

Configuration (4)

Balanced Configuration

  • Both stations in a point-to-point topology are of combined type
  • HDLC does not support balanced multipoint

Station Types & Configurations

Station Types & Configurations


  • A mode in HDLC is the relationship b/w two devices involved in an exchange
  • The mode describes who controls the link
  • HDLC supports 3 modes of communication b/w stations:
    • Normal Response Mode (NRM)
    • Asynchronous Response Mode (ARM)
    • Asynchronous Balanced Mode (ABM)


Normal Response Mode (NRM)

  • Refers to the standard primary-secondary relationship
  • Secondary device must have permission from primary device before transmitting
  • Once permission has been granted, the secondary may initiate a response transmission of one or more frames containing data

Asynchronous Response Mode (ARM)

  • A secondary may initiate a TX w/o permission from the primary whenever the channel is idle
  • ARM does not alter the primary secondary relationship in any other way
  • All transmissions from the primary still go to the secondary and are then relayed to the other devices

Asynchronous Balanced Mode (ABM)

  • All stations are equal and therefore only combined stations connected in point-topoint are used
  • Either combined station may initiate TX with the other combined station w/o permission

HDLC Frames

HDLC Frames

HDLC defines 3 types of Frames:

  • Information Frames (I-Frames)
  • Supervisory Frames (S-Frames)
  • Unnumbered Frames(U-Frames)

I-Frames are used to transport user data and control information relating to user data
S-Frames are used only to transport control information
U-Frame are reserved for System Management

Each frame in HDLC may contain up to six fields

  • A beginning Flag Field
  • An address field
  • A control field
  • An information Field
  • A frame check sequence (FCS)
  • An ending Flag Field

Flag Field

Flag Field

  • •The flag field of an HDLC frame is an 8-bit sequence with a bit patter 01111110 that identifies both the beginning and the ending of the of a frame
  • It serves as a Synchronization pattern for the receiver
  • Fig. shows placement of 2 flag fields in an I-Frame

HDLC Address Field

HDLC Address Field

  • The second field of HDLC frame contains the address of the secondary station that is either the originator or the destination of the frame
  • If a primary station creates Frame it includes a ‘To’ address and if a secondary creates the frame, it contains a ‘From’ address
  • Can be of one byte or several bytes depending upon the network
  • If the address field is only 1 byte, the last bit is always a 1
  • If the address is of several bytes, all bytes but the last one will end with 0 , and the last will end with a 1
  • Ending each intermediate byte with 0 indicates to the receiver that there are more address bytes to come

HDLC Control Field

HDLC Control Field

  • The control field is a one o two byte segment of the frame used for flow management
  • The two byte case is called the Extended Mode

Control fields differ depending on the frame type:

  • If the control field is a 0, the frame is an I-Frame
  • If the first bit is 1 and the second bit is a 0 , it is S-Frame
  • If both first and second bits are 1’s, it is U-Frame


  • P/F bit is a single bit with dual purpose
  • It has meaning only when it is ‘1’ and it can mean Poll or Final
  • When the frame is sent by a primary to secondary, it means POLL
  • When the frame is sent by a secondary to a primary, it is FINAL

HDLC Control Field –EXTENDED

HDLC Control Field –EXTENDED

  • Control field in the I-Frame and S-Frame is two bytes long to allow seven bits of sending and receiving sequence
  • However the control field in the U-Frame is still one byte

HDLC Control Field –EXTENDED 1

  • Information field contains the user’s data in an I-Frame and Network Management information in a U-Frame
  • An S-Frame has no information field
  • Its length can vary from one network to another but remains fixed within each network
  • It is possible to send Control information in the information field of the I-Frame along with data.
  • This process is called Piggybacking

HDLC Control Field –EXTENDED 2

  • The FCS is HDLC’s error detection field
  • It can contain a two- or four byte CRC

HDLC Control Field –EXTENDED 3

HDLC Control Field –EXTENDED 4

Link Access Procedures

  • LAPB
    • Link access procedure, balanced
  • LAPD
    • Link access procedure for D- channel
  • LAPM
    • Link access procedure for modems


  • Synchronous Protocols
    • Bit-Oriented Protocols
  • HDLC
    • Link Access Protocols (LAPs)

Reading Sections

Section11.4, 11.5

“Data Communications and Networking” 4th Edition by Behrouz A. Forouzan