3Com MSR 50 Series, Руководство по настройке

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13
HDLC C
ONFIGURATION
When configuring HDLC, go to these sections for information you are interested
in:
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“Introduction to HDLC” on page 281
■
“Configuring HDLC” on page 282
Introduction to HDLC
HDLC Overview High-level data link control (HDLC) is a bit-oriented link layer protocol. Its most
prominent feature is that it can transmit any types of bit stream transparently.
■
HDLC supports point-to-point link only and does not support
point-to-multipoint link.
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HDLC supports neither IP address negotiation nor authentication. It uses
keepalive messages to check link status.
■
HDLC can only be encapsulated on synchronous link. A synchronous
/asynchronous interface can also apply HDLC provided that it works in
synchronous mode. Currently, this protocol is applied on the Serial interface
and POS interface that work in synchronous mode.
HDLC Frame Format and
Frame Type
There are three types of HDLC frames: information frame (I frame), supervision
frame (S frame) and unnumbered frame (U frame).
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Information frame is responsible for transmitting useful data or information.
■
Supervision frame is responsible for error control and flow control.
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Unnumbered frame is responsible for the link establishment, teardown, and so
on.
An HDLC frame is composed of flag field, address field, control field, information
field and checksum field.
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The flag field, 0111111, marks the beginning and end of an HDLC frame. Each
frame begins with F and ends with F.
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The address field is eight bits; it identifies the source or destination where the
frame is sent or received.
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The control field is eight bits; it identifies the control type and defines the frame
type (control or data).
■
The information field can be an arbitrary binary bit set. The minimum length
can be zero and the maximum length is decided by the FCS field or the buffer
size of the communicating node. Generally, the maximum length is between
1000 and 2000 bits.