Single Link Operations

Figure 2-4 shows the standard frame structure for a bit-oriented datalink layer frame. The shaded parts are supplied by the datalink layer. The Data field contains the data supplied by higher layer protocols. Sizes are given in octets

Figure 2-4 Datalink Layer Frame Structure

Datalink layer frames are structured as follows:

Flag

Frames are delimited at each end by a flag, with the value 01111110. This is necessary because X.25 is synchronous - in other words, data is transmitted as a continuous stream.

Address

This is one octet. The value varies, depending on the direction of data flow, and on whether this is a single or multilink operation.

Control

The control octet define the type of frame this is: an I(nformation)-frame containing data, a S(upervisory)-frame, which is a response frame, or an U(nnumbered)-frame, which performs control functions.

Data

The data field contains X.25 protocol information, as well as user data from higher layer protocols. A frame need not contain data.

Checksum

The two-octet checksum follows the data, and is derived from the contents of the data packet. It is usually generated automatically by the hardware.