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HDLC

High-level Data Link Control (HDLC) is a bit-oriented protocol for communication over point-to-point and multipoint links, utilizing ARQ mechanisms. It operates in two transfer modes: Normal Response Mode (NRM) with an unbalanced configuration and Asynchronous Balanced Mode (ABM) with a balanced configuration. HDLC defines three types of frames—information frames (I-frames), supervisory frames (S-frames), and unnumbered frames (V-frames)—to manage different types of messages during transmission.

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13 views2 pages

HDLC

High-level Data Link Control (HDLC) is a bit-oriented protocol for communication over point-to-point and multipoint links, utilizing ARQ mechanisms. It operates in two transfer modes: Normal Response Mode (NRM) with an unbalanced configuration and Asynchronous Balanced Mode (ABM) with a balanced configuration. HDLC defines three types of frames—information frames (I-frames), supervisory frames (S-frames), and unnumbered frames (V-frames)—to manage different types of messages during transmission.

Uploaded by

Judy Hopps
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© © All Rights Reserved
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High-level Data Link Control (HDLC)


High-level Data Link Control (HDLC) is a bit-oriented protocol for communication over point-
to-point and multipoint links. It implements the ARQ mechanisms.

Configurations and Transfer Modes


HDLC provides two common transfer modes that can be used in different configurations:
normal response mode (NRM) and asynchronous balanced mode (ABM).

Normal Response Mode


In normal response mode (NRM), the station configuration is unbalanced. We have one
primary station and multiple secondary stations. A primary station can send commands;
a secondary station can only respond. The NRM is used for both point-to-point and
multiple-point links, as shown in Figure 11.25.

Asynchronous Balanced Mode


In asynchronous balanced mode (ABM), the configuration is balanced. The link is point-to-point,
and each station can function as a primary and a secondary (acting as peers), as shown in Figure
11.26. This is the common mode today.
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Frames
To provide the flexibility necessary to support all the options possible in the modes and
configurations just described, HDLC defines three types of frames: information frames (I-
frames), supervisory frames (S-frames), and unnumbered frames (V-frames). Each type of frame
serves as an envelope for the transmission of a different type of message. I-frames are used to
transport user data and control information relating to user data (piggybacking). S-frames are
used only to transport control information. V-frames are reserved for system management.
Information carried by V-frames is intended for managing the link itself.

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