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Last edited: 1:19:55 pm on 3 November 2005 by salchicha.sico.local

[*ethernet>@ethernet*|*LLC>@LLC*]

@ethernet
! Ethernet

Ethernet is a widely used data communications network standard developed by DEC, Intel, and Xerox. It uses a bus topology and CMSA/CD access method. The terms Ethernet and the IEEE 802.3 standard are often used interchangeably.

The Ethernet header structure is shown in the illustration below.

Destination address
The address structure is as follows:

|I/G | U/L |Address bits |
<tt>Table: Ethernet destination address structure</tt>

Source address
The address structure is as follows:

|0 |U/L |Address bits |
<tt>Table: Ethernet source address structure</tt>

Length/type
In the Ethernet protocol, the value (ł 0x0600 Hex) of this field is Ethernet List, indicating the protocol inside.

In the 802.3 protocol, the value (46-1500 Dec) is the length of the inner protocol, which is the LLC encapsulated inner protocol. (The LLC header indicates the inner protocol type.)

Data unit + pad
LLC protocol.

FCS
Frame check sequence.

_
@LLC
! LLC

The IEEE 802.2 Logical Link Control (LLC) protocol provides a link mechanism for upper layer protocols. LLC type I service provides a datalink connectionless mode service, while LLC type II provides a connection-oriented service at the datalink layer.

The LLC header structure is shown in the illustration below.

DSAP
The destination service access point structure is as follows:

|I/G |Address bits |
<tt>Table: LLC DSAP structure</tt>

SSAP
The source service access point structure is as follows:

|C/R |Address bits |
<tt>Table: LLC SSAP structure</tt>

Control
The structure of the control field is as follows:

|&nbsp;| 1 |8 |9 |16 bits |
|Information |0 |N(S) |P/F |N(R) |
|Supervisory |1 |0 |SS + XXXX |P/F |N(R) |
|Unnumbered |1 |1 |MM + ( P/F + MMM) |
<tt>Table: LLC control field structure </tt>

|Field|Description|
|N(S)| Transmitter send sequence number.|
|N(R)|Transmitter receive sequence number.|
|P/F|Poll/final bit. Command LLC PDU transmission/response LLC PDU transmission. S: Supervisory function bits: 00:RR (receive ready). 01REJ (reject). 10RNR (receive not ready).|
|X|Reserved and set to zero.|
|M|Modifier function bits.|

LLC information
LLC data or higher layer protocols.

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