X.25 Packet Layer

Within layer 3, the Packet Layer Protocol manages the exchange of data packets between DTE and DCE. This layer establishes, maintains, and terminates user sessions, handles addressing and carries out fault-management.

The general format of an X.25 packet header is shown below:

Figure 2-7 X.25 Packet Header Format

The components are:

General Format Identifier

This specifies the format of the rest of the header, which varies according to packet-type.

Group and Channel

Together, the Group and Channel fields identify the virtual circuit to be used for a call.

Type

In a control (non-data) packet, the type field identifies the type of the packet. In a data packet, the type field is used for setting flow control, sequencing, and piggybacking information.

Control

The Control field specifies whether this is a control or a data packet. It is set to 1 for control packets and 0 for data packets.

In data packets, the header is followed by data. In control packets, the header is followed by additional information that specifies, among other things, addressing and any special facilities that are to be used.