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116 Chapter 3: OSI Reference Model & Layered Communication
Table 3-17 summarizes the different choices for encoding protocol types for each of the four
data link protocols covered in this chapter.
Table 3-18 summarizes the basic functions of data link protocols.
Table 3-17
Different Choices for Encoding Protocol Types for Each of the Four Data Link Protocols
Data Link Protocol
Field
Header in Which It Is
Found
Size
Ethernet and Token Ring
DSAP
802.2 header
1 byte
Ethernet and Token Ring
SSAP
802.2 header
1 byte
Ethernet and 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 1490
NLPID
RFC1490
1 byte
Frame Relay RFC 1490
L2 or L3 Protocol ID
Q.933
2 bytes each
Frame Relay RFC 1490
SNAP Protocol Type
SNAP header
2 bytes
Table 3-18
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
headers, with
NLPID, L2 and
L3 protocol IDs,
or SNAP header
ch03.fm Page 116 Monday, March 20, 2000 4:58 PM