FDDI Data Packet
All communication between a local and remote station is accomplished by the transmission of a packet that contains FDDI headers and trailers, as well as an encapsulated LLC link protocol data unit (LPDU).
The DLC FDDI Frame Encapsulation figure (Figure 1) describes the FDDI data packet.
![This diagram shows the FDDI data packet containing the following: SFS (3 bytes), FC (1 byte), destination address (6 bytes), and source address (6 bytes), routing information (m bytes), LPDU length (n bytes), FCS, and EFS. Another 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 (i bytes).](f5M1g92kevi.jpg)
The FDDI data packet consists of the following:
| Item | Description |
|---|---|
| SFS | Start-of-frame sequence, including the preamble and starting delimiter |
| FC | Frame control field |
| LPDU | LLC protocol data unit |
| DSAP | Destination service access point (SAP) address field |
| SSAP | Source SAP address field |
| FCS | Frame-check sequence or cyclic redundancy check |
| EFS | End-of-frame sequence, including the ending delimiter and frame status |
| m bytes | Integer value greater than or equal to 0 and less than or equal to 30 |
| n bytes | Integer value greater than or equal to 3 and less than or equal to 4080 |
| p bytes | Integer value greater than or equal to 0 and less than or equal to 4077 |
Note:
- SFS, FCS, and EFS are added and deleted by the hardware adapter. Three bytes of alignment always precede the FC field when located in memory buffers.
- The maximum byte length of a transfer unit has been set to 4096 bytes to align to the size of an mbuf cluster (where a transfer unit is defined as fields FC through LPDU, plus a three-byte front alignment pad).