Computer Communications, Summaries of Computer Communication Systems

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HIGH LEVEL DATA LINK

CONTROL

Prepared by

J. Ajay Daniel

HDLC

 High-level Data Link Control (HDLC) is a bit oriented protocol

 HDLC was defined by ISO for use on both point-to-point and multipoint

data links.

 It supports full-duplex communication i.e receive and transmit at the

same time.

Data Transfer Mode

 Normal Response Mode (NRM)

 (^) Secondary station can send ONLY when the primary station instruct it to do so  (^) Two common configurations  (^) Point-to-Point link (one primary station and one secondary station)  (^) Multipoint link (the primary station maintain different sessions with different secondary stations)

Data Transfer Mode (Contd.)

 Asynchronous Response Mode (ARM)

 (^) More independent secondary station  (^) Can send data or control information without explicit permission to do so (note that it is still can not send commands)

 Asynchronous Balanced Mode (ABM)

 (^) Mainly used in point-to-point links, for communication between combined stations  (^) Either stations can send data, control information and commands

Types of HDLC Frames

 Unnumbered frames (U-frames) :-

 (^) These frames are used in link set-up and disconnection.

 Information frames (I-frames) :-

 (^) These frames carry the actual information.

 Supervisory frames (S-frames) :-

 (^) S-frames are used only to transport control information.

HDLC FRAMES (Contd.) Information frames (I-frames)

 I-frames carry user's data and control information about user's data.

 I-frame carries user data in the information field.

 The first bit of control field is always zero, i.e. the presence of zero at this

place indicates that it is I-frame.

 Bit number 2, 3 & 4 in control field is called N(S) that specifies the sequence

number of the frame.

 Bit number 5 in control field is P/F i.e. Poll/Final and is used for these two

purposes. It has, meaning only when it is set i.e. when P/F=1. It can

represent the following two cases.

 (^) It means poll when frame is sent by a primary station to secondary (when address field contains the address of receiver).  (^) It means final when frame is sent by secondary to a primary (when the address field contains the address of the sender).

 Bit number 6, 7, and 8 in control field specifies N(R)

HDLC FRAMES (Contd.) Unnumbered frames (U-frames)

 U-frames are reserved for system management and information carried by

them is used for managing the link

 U-frame is identified by the presence of 11 in the first and second bit position

in control field.

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