IBM CTI 2572 guide

CTI 2572 Technical Overview Manual

Frame w/ SNAP Header

Figure 10 802.3

= 000

03 = UI

0800 = IP

170 = SNAP

Code

Control

SAP

Organization

EtherType

LLC

Source

Destination

1 Byte

2 Bytes

3 Bytes

Data

IEEE 802.2 LLC Header

Data

CRC

Data

Length

Source Address

Destination

Address

4 Bytes

2 Bytes

6 Bytes

Variable (up to 1500 Bytes)

Since IEEE 802.3 does not contain any information which identifies the upper level protocol being used, it is usually combined with the 802.2 Logical Link Control (LLC) specification. The LLC header does provide information about the upper layer protocol and is placed in the first few bytes of the 802.3 data area.

However, the 802.2 specification allows only one byte for the upper level protocol identifier (called a Service Address Point). Thus, a special version of 802.2 LLC called SNAP (Sub Network Access Protocol) was created to contain the 2 byte TYPE field used in the Ethernet II packet.

Even though the Ethernet II and 802.3 packet formats are different, there are generally accepted conventions for placing information in the Type/Length field which allows communications software to differentiate between the two. As a result, both packet types can co-exist on the same network. However, a node which sends and receives only Ethernet II packets cannot communicate with a node which sends and receives only 802.3 packets.

CRC

4 bytes

The Ethernet Version 2 standard defines the packet as follows:

Ethernet Destination Address

6 bytes

Ethernet Source Address

6 bytes

Type

2 bytes

Data

Variable

CRC

4 bytes

The type field is used to define the upper level protocol being used (e.g. TCP/IP, DEC LAT, etc.).

The IEEE 802.3 specification substitutes a length field for the type field and has the following format:

Ethernet Destination Address

6 bytes

Ethernet Source Address

6 bytes

Length

2 bytes

Data

Variable