System information

Troubleshooting X.25 Connections 19-413
Layer 2
The addressing fields that make up the X.121 address are necessary only when an SVC is used, and
then only during call setup. After the call is established, the PSN uses the LCI field of the data packet
header to specify the particular virtual circuit to the remote DTE.
Layer 3 X.25 uses three virtual circuit operational procedures: call setup, data transfer, and call
clearing. Execution of these procedures depends on the virtual circuit type being used. For a PVC,
Layer 3 X.25 is always in data transfer mode because the circuit has been permanently established.
If an SVC is used, all three procedures are used.
Packets are used to transfer data. Layer 3 X.25 segments and reassembles user messages if they are
too long for the maximum packet size of the circuit. Each data packet is given a sequence number,
so error and flow control can occur across the DTE/DCE interface.
Layer 2
Layer 2 X.25 is implemented by LAPB, which allows each side (the DTE and the DCE) to initiate
communication with the other. During information transfer, LAPB checks that the frames arrive at
the receiver in the correct sequence and free of errors.
As with similar link-layer protocols, LAPB uses three frame format types:
• Information (I) frames—These frames carry upper-layer information and some control
information (necessary for full-duplex operations). Send and receive sequence numbers and the
poll final (P/F) bit perform flow control and error recovery. The send sequence number refers to
the number of the current frame. The receive sequence number records the number of the frame
to be received next. In full-duplex conversation, both the sender and the receiver keep send and
receive sequence numbers. The poll bit is used to force a final bit message in response; this is
used for error detection and recovery.
• Supervisory (S) frames—These frames provide control information. They request and suspend
transmission, report on status, and acknowledge the receipt of I frames. They do not have an
information field.
• Unnumbered (U) frames—These frames, as the name suggests, are not sequenced. They are used
for control purposes. For example, they can initiate a connection using standard or extended
windowing (modulo 8 versus 128), disconnect the link, report a protocol error, or carry out
similar functions.
The LAPB frame is shown in Figure 19–5.
Figure 19-5 The LAPB Frame
The fields of an LAPB frame are as follows:
• Flag—Delimits the LAPB frame. Bit stuffing is used to ensure that the flag pattern does not occur
within the body of the frame.
• Address—Indicates whether the frame carries a command or a response.
• Control—Provides further qualifications of command and response frames, and also indicates the
frame format (I, S, or U), frame function (for example, receiver ready or disconnect), and the
send/receive sequence number.
Flag
Field length,
in bytes
FlagAddress FCSControl Data
1 1 1 Variable 2 1