HDLC Operations, and Frame Types
There are three fundamental types of HDLC frames.
- Information frames, or I-frames, transport user data from the network layer. In addition they can also include flow and error control information piggybacked on data.
- Supervisory Frames, or S-frames, are used for flow and error control whenever piggybacking is impossible or inappropriate, such as when a station does not have data to send. S-frames do not have information fields.
- Unnumbered frames, or U-frames, are used for various miscellaneous purposes, including link management. Some U-frames contain an information field, depending on the type.
The general format of the control field is:
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | |
|---|---|---|---|---|---|---|---|---|
| N(R) Receive sequence no. |
P/F | N(S) Send sequence no. |
0 | I-frame | ||||
| N(R) Receive sequence no. |
P/F | type | 0 | 1 | S-frame | |||
| type | P/F | type | 1 | 1 | U-frame | |||
There are also extended (2-byte) forms of I and S frames. Again, the least significant bit (rightmost in this table) is sent first.
| 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | ||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| N(R) Receive sequence no. |
P/F | N(S) Send sequence no. |
0 | Extended I-frame | |||||||||||||
| N(R) Receive sequence no. |
P/F | 0 | 0 | 0 | 0 | type | 0 | 1 | Extended S-frame | ||||||||
Read more about this topic: High-Level Data Link Control
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