High-level Data Link Control
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Beta Beta
OmniAccess 5740 Unified Services Gateway CLI Configuration Guide
Alcatel-Lucent
HDLC OVERVIEW
Layer 2 of the OSI model is the data link layer. One of the most common layer 2
protocols is the High-level Data Link Control (HDLC) protocol. In fact, many other
layer 2 protocols are based on HDLC, particularly its framing structure.
Note: The OmniAccess 5740 USG supports only Cisco HDLC.
The following sections describe HDLC:
• “HDLC Frame Structure”
• “HDLC Frame Formats”
• “HDLC Protocol Operation”
HDLC FRAME STRUCTURE
HDLC data units transmitted from one station to another are referred to as
"frames". The figure shown below is a graphical representation of an HDLC frame
with information field.
Figure 26: An HDLC frame with an information field
Field Name Size (in bits)
Flag Field(F) 8 bits
Address Field(A) 8 bits
Control Field(C) 8 or 16 bits
Information Field(I) Variable; Not used in some frames
Frame Check Sequence(FCS) 16 or 32 bits
Closing Flag Field(F) 8 bits
F A
C
I
FCS
F
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