Ethernet Data Packet
All communication between a local and remote station is accomplished by the transmission of a packet that contains the Ethernet headers and trailers and an encapsulated LLC protocol data unit (LPDU). This packet format is specifically designed for the SNA protocol, but other protocols can use this format as well.
The The DLCETHER Frame Encapsulation figure (Figure 1) illustrates the Ethernet data packet.
![This diagram shows the Ethernet data packet. The first line contains the following: preamble (8 bytes), destination address (6 bytes), source address (6 bytes), and type field (2 bytes), data (m bytes), CRC (4 bytes). The second line defines data as including the following: LPDU length (2 bytes), leading pad (1 byte), LPDU, and the trailing pad (which together with the LPDU equal n bytes). The third line shows that LPDU consists of the following: DSAP address, SSAP address (together with DSAP address consist of 2 bytes), control field [1 (2) byte], and the information field (p bytes).](figures/A0C0e411.jpg)
The Ethernet data packet consists of the following:
| Item | Description |
|---|---|
| LPDU | LLC protocol data unit |
| DSAP | Destination service access point (SAP) address field |
| SSAP | Source SAP address field |
| CRC | Cyclic redundancy check or frame-check sequence |
| m bytes | Integer value greater than or equal to 46 and less than or equal to 1500 |
| n bytes | Integer value greater than or equal to 43 and less than or equal to 1497 |
| p bytes | Integer value greater than or equal to 0 and less than or equal to 1493 |
Note: The Preamble and CRC identify both of these as something that
is added and deleted by the hardware adapter.