Ethernet Framing

All types of Ethernet use either the IEEE-defined framing or the older Ethernet Version 2 framing that predated the standardization of Ethernet by the IEEE. With 802.3 Ethernet, there are two main variations of framing—one without the use of a SNAP header and one with the use of a SNAP header. Figure 11-13 shows the three variations of headers.

Figure 11-13 Ethernet Framing

Ethernet (DIX)

8 6 6 2 Variable 4

Figure 11-13 Ethernet Framing

Ethernet (DIX)

8 6 6 2 Variable 4

T

Preamble

Dest.

Source

y

Data

FCS

Address

Address

p

e

IEEE Ethernet (802.3)

Preamble

SD

D

S

Dest.

Source

Length

S

S

address

address

A P

Control

Data

802.3

IEEE 802.3 with SNAP Header

802.2

802.3

5 Variable 4

IEEE 802.3 with SNAP Header

5 Variable 4

D

S

Preamble

SD

Dest. address

Source address

Length

S A P

S A P

Control

SNAP

Data

802.3 802.2 802.3

One of Ethernet's greatest strengths is that each newly defined Ethernet standard has used the same MAC and LLC headers and trailers, including the same address formats. So, when a new type of Ethernet is developed, engineers have very little new information to learn before supporting the new type.

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