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.

HDLC is the default data link protocol (encapsulation) on Cisco routers serial interfaces. Frame Relay headers are coincidentally based on the HDLC specification, but Frame Relay was created for multiaccess networks (with more than two devices). The clear differences between Frame Relay and HDLC provide a good backdrop to examine the functions of the data link layer (Layer 2).

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