IEEE 8023 Frame Format

The IEEE produced their Ethernet standard in June 1983. They changed the type field to a length field and added the Logical Link Control (LLC) layer.

Figure D-4 shows the IEEE 802.3 frame format. The remainder of this section discusses the fields in the IEEE 802.3 frame format.

Figure D-4 IEEE 802.3 Frame Format

Data+pad 43—1497 bytes

Preamble 7 bytes

SFD 1 byte

DA 6 bytes

SA 6 bytes

Length 2 bytes

LLC 3 bytes

FCS 4 bytes

The IEEE defined the preamble to be a string of 1s and 0s that is 7 bytes long. The following byte is the Start Frame Delimiter (SFD):

10101011

Following the SFD are the DA and SA fields, with MAC addresses.

The length field contains the length of the LLC plus the data field. Its values range from 3 to 1500.

The LLC layer is added by the IEEE to indicate the upper-layer protocol. It is divided into three fields, as shown in Figure D-5.

Figure D-5 IEEE LLC Layer Fields

DSAP

SSAP

Control

1 byte

1 byte

1 or 2 bytes

The destination service access point (DSAP) indicates the destination upper-layer protocol. The source service access point (SSAP) indicates the source upper-layer protocol. Examples of SAPs are as follows:

The data field (see Figure D-4) contains the upper-layer protocol information, as in other frame formats. The FCS field (see Figure D-4) contains the CRC

IEEE 802.3 SNAP Frame Format

The problem the IEEE faced when developing the IEEE frame format was, by then, protocols using DIX Version 2 had been in use for three years. To transition to an IEEE 802.3-compliant frame, the SNAP field was added to 802.3, to provide transition. The SNAP field includes the Ethernet type information used in DIX Version 2. Figure D-6 shows the IEEE 802.3 SNAP

frame format. The remainder of this section discusses the fields in the IEEE 802.3 SNAP frame format.

Figure D-6 IEEE 802.3 SNAP Frame Format

Data+pad

Preamble 7 bytes

SFD 1 byte

DA 6 bytes

SA 6 bytes

Length 2 bytes

LLC 3 bytes

SNAP 5 bytes

All fields are the same as in IEEE 802.3. The SNAP field is added. It contains two fields, including a 3-byte vendor code field that is unique to different vendors. The next field in the SNAP is the 2-byte Ethernet type field, used in Ethernet Version 2. The SNAP fields are displayed in Figure D-7.

Figure D-7 IEEE SNAP Fields

Vendor code

Type

3 bytes

2 bytes

What has happened in the years following these standards is that IP and DEC protocols use Ethernet Version 2, Novell IPX (3.x) uses its 802.3 raw format, and SNA uses 802.3. In multiprotocol networks today, there is a mix of frame formats on the network.

The following sections cover Ethernet physical specifications.

Was this article helpful?

0 -1

Post a comment