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94 Chapter 3: OSI Reference Model & Layered Communication

OSI Data Link Layer Functions

As a CCNA, you'll need to understand both the abstract concepts about the OSI layers and
particular instances of such protocols. This section focuses on more of the abstract concepts.
Chapter 4, "Bridges/Switches and LAN Design," and Chapter 8, "WAN Protocols and
Design," provide more details about particular data link protocols, as well as their configuration
in the IOS.

This section examines four different protocols: Ethernet, Token Ring, HDLC, and Frame Relay.
A generalized definition of the function of a data link protocol will be used to guide you through
the comparison of these four data link protocols. This definition could be used to examine any
other data link protocol. The four components of this definition of the functions of data link
(Layer 2) protocols are as follows:

Arbitration--Determines when it is appropriate to use the physical medium.

Addressing--Ensures that the correct recipient(s) receives and processes the data that
is sent.

Error detection--Determines whether the data made the trip across the medium
successfully.

Identifying the encapsulated data--Determines the type of header that follows the data
link header. This feature is included in a subset of data link protocols.

Ethernet and Token Ring are two popular LAN Layer 2 protocols. These protocols are defined
by the IEEE in specifications 802.3 and 802.5, respectively. Because 802.3 and 802.5 define
how a station accesses the media, the IEEE calls these protocols Media Access Control (MAC)
protocols. Also, both 802.3 and 802.5 call for the use of another IEEE specification as a separate
part of the data link layer, namely 802.2 Logical Link Control (LLC). 802.2 is purposefully
designed to provide functions common to both Ethernet and Token Ring, whereas 802.3 and
802.5 were designed specifically for data link functions pertinent to either Ethernet or Token
Ring topologies, respectively.

The Ethernet standards before the IEEE created 802.3 have been called DIX Ethernet for quite
a while (the letters DIX represent Digital, Intel, and Xerox). DIX Version 2 defines similar
functions to both the 802.3 and 802.2 specifications.

Table 3-6

Flow-Control Methods--Summary

Name Used in This Book

Other Names

Example Protocols

Buffering

N/A

Congestion Avoidance

Stop/Start, RNR, Source Quench

SDLC, LAPB, LLC2

Windowing

N/A

TCP, SPX, LLC2

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