Format

An IEEE 802.3 packet (or more properly, Medium Access Control Frame) consists of eight (8) fields. A packet starts with a Preamble, and ends with a Frame Check Sequence. One of the fields, the Protocol Data Unit, is used to implement the Logical Link Control protocol defined by the IEEE 802.2 Standard. This is illustrated in Figure 2"3. Under the IEEE 802.2 Standard, the Protocol Data Unit field is further divided into additional fields, as illustrated in Figure 2"4.

 

 

IEEE 802.3

 

 

 

 

 

Last Field

 

Logical Link

 

 

 

 

First Field

Transmitted

 

Control

 

 

 

 

Transmitted

 

 

 

 

 

 

 

 

Frame

Pad

Protocol

 

Source

Destination

Start of

 

Check

(if

Data

Length

Address

Address

Frame

Preamble

Sequence

needed)

Unit

 

 

 

Delimiter

 

4

(0-43

3-1500

2

6

6

1

7

bytes

bytes)

bytes

bytes

bytes

bytes

byte

bytes

 

 

 

 

 

 

 

 

IEEE 802.3

Medium Access Control Frame

Figure 2%3. IEEE 802.3 Frame and Location of IEEE 802.2 Sublayer

 

IEEE 802.2 PROTOCOL DATA UNIT

 

 

 

 

 

 

 

 

 

 

INFORMATION

 

SSAP

 

DSAP

 

 

CONTROL

ADDRESS

ADDRESS

 

 

 

 

 

 

 

 

 

Integral number

8 bits

8 bits

 

8 bits

 

 

of Bytes

 

 

 

(Type 1)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Last Byte

 

 

 

First Byte

Transmitted

 

 

 

Transmitted

Figure 2%4. IEEE 802.2 Sublayer Fields

Standard or custom communication protocols between higher layer processes are employed in the Information field of the Protocol Data Unit. For example, Node Manager software communications between a local and remote node is accomplished through a Hewlett"Packard Network Management protocol embedded in the Protocol Data Unit Information field.

Excluding the Preamble and Start Frame Delimiter, a packet must be at least 64 bytes, and can be up to 1518 bytes long.

2%4 General Information