HLDC-Data Communication-Lecture Slides, Slides of Data Communication Systems and Computer Networks

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Lecture- BS(CIS) Semester-IV

Data Communication

Tody’s Lecture

• Introduction to HDLC  HDLC Frame Structure  HDLC Operation

HDLC Station Types

• Based on the responsibilities of maintaining and controlling link function, HDLC defines three roles of a node
• Primary station  Controls operation of link  Frames issued are called commands  Maintains separate logical link to each secondary station
• Secondary station  Under control of primary station  Frames issued called responses
• Combined station  May issue commands and responses

HDLC stations types…

HDLC Transfer Modes

• Three data transfer modes  Normal Response Mode (NRM)  Asynchronous Balanced Mode  Asynchronous Response Mode
• Normal Response Mode (NRM)  Unbalanced configuration  Primary initiates transfer to secondary  Secondary may only transmit data in response to command from primary  Used on multi-drop lines  Host computer acts as primary  Terminals as secondary  Primary station polls secondary stations for their turns

HDLC Transfer Modes ..

• Asynchronous Balanced Mode (ABM)  Balanced configuration  Either station may initiate transmission without receiving permission  Most widely used  No polling overhead

Frame Structure

• Synchronous transmission
• All transmissions in frames
• Single frame format for all data and control exchanges

Header (^) Trailer

Flag Fields

• Delimit frame at both ends
• The (01111110) indicates start or end of a frame
• Can serve as end of one frame and start of the other
• Receiver hunts for flag sequence to synchronize
• Bit stuffing is used to avoid confusion with data containing 01111110  0 inserted after every sequence of five 1s  If receiver detects five 1s it checks next bit  If 0, it is deleted  If 1 and seventh bit is 0, accept as flag  If sixth and seventh bits 1, sender is indicating abort

Address Field

• Identifies secondary station that sent or will receive frame
• Usually 8 bits long
• May be extended to multiples of 7 bits  LSB of each octet indicates that it is the last octet (1) or not (0)
• All ones (11111111) is broadcast
• Not required in case of direct link

Control Field

• Different for different frame type  Information - data to be transmitted to user (next layer up) - Flow and error control piggybacked on information frames  Supervisory - ARQ when piggyback not used  Unnumbered - supplementary link control
• First one or two bits of control filed identify frame type
• Remaining bits explained later

Poll/Final Bit

• Use depends on context
• Command frame  P bit  1 to solicit (poll) response from peer
• Response frame  F bit  1 indicates response to soliciting command

control field..

Role of S field RR Receiver ready—positive acknowledgment RNR Receiver not ready—positive acknowledgment REJ Reject—negative acknowledgment, go back N frames SREJ Selective reject—negative acknowledgment, selective repeat

Information Field

• Only in information and some unnumbered frames
• Must contain integral number of octets
• Variable length

Frame Check Sequence Field

• FCS
• Error detection
• 16 bit CRC
• Optional 32 bit CRC