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102 Chapter 3: OSI Reference Model & Layered Communication
Summary: Data Link Functions
Table 3-9 summarizes the basic functions of data link protocols:
Table 3-8
Different Choices for Encoding Protocol Types for Each of the Four Example Data Link Protocols
Data Link Protocol
Field
Header in Which It
Is Found
Size
802.3 Ethernet and
802.5 Token Ring
DSAP
802.2 header
1 byte
802.3 Ethernet and
802.5 Token Ring
SSAP
802.2 header
1 byte
802.3 Ethernet and
802.5 Token Ring
Protocol Type
SNAP header
2 bytes
Ethernet (DIX)
Ethertype
Ethernet header
2 bytes
HDLC
Cisco proprietary
protocol id field
Extra Cisco header
2 bytes
Frame Relay RFC 2427
NLPID
RFC 1490
1 byte
Frame Relay RFC 2427
L2 or L3 Protocol ID
Q.933
2 bytes each
Frame Relay RFC 2427
SNAP Protocol Type
SNAP Header
2 bytes
Table 3-9
Data link Protocol Functions
Function
Ethernet
Token Ring
HDLC
Frame Relay
Arbitration
CSMA/CD
Algorithm (part of
MAC)
Token passing
(part of MAC)
--
--
Addressing
Source and
destination MAC
addresses
Source and
destination MAC
addresses
Single 1-byte
address;
unimportant on
point-to-point links
DLCI used to
identify virtual
circuits
Error Detection
FCS in trailer
FCS in trailer
FCS in trailer
FCS in trailer
Identifying
contents of
data
802.2 DSAP, SNAP
header, or
Ethertype, as
needed
802.2 DSAP or
SNAP header, as
needed
Proprietary Type
field
RFC 1490/2427
headers, with
NLPID, L2 and
L3 protocol IDs,
or SNAP header
ch03.fm Page 102 Monday, March 20, 2000 4:58 PM