Layer 2 Ethernet Summary

As mentioned earlier in this chapter, physical layer protocols define how to deliver data across a physical medium. Data-link protocols make that physical network useful by defining how and when the physical network is used. Ethernet defines the OSI Layer 1 functions for Ethernet, including cabling, connectors, voltage levels, and cabling distance limitations, as well as many important OSI Layer 2 functions. In this section, four of these data link features were emphasized, as shown in Table 3-7.

Table 3-7 OSI Layer 2 Feature Summary: Ethernet

OSI Layer 2 Function

Ethernet Implementation


CSMA/CD algorithm


6-byte-long MAC addresses

Error detection

FCS in Ethernet trailer

Identifying the type of packet inside the frame

Protocol Type (2 bytes)—DIX Ethernet DSAP (1 byte)—IEEE 802.2

SNAP Protocol Type (2 bytes)—IEEE 802.2 with SNAP header

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