Implementation Notes

Overview

For a complete description of the frame and packet level formats used within X.25, refer to the ITU-T recommendation, or to the documentation produced by most PTT authorities for use in conjunction with their own public packet switched networks.

Each unit transmitted over an X.25 link is called a 'frame'. A frame has a header and a trailing CRC, is preceded and followed by a flag byte (binary 01111110), and may include an 'I' (information) field as well. Frames may be 'un-numbered' (link control) frames, flow control frames or information frames. The headers of flow control and information frames contain frame-level sequence numbers.

All 'packets' are transmitted as the I-field of an information frame. A packet may be for call control, packet level flow control, or to carry information over a virtual circuit either in an interrupt packet or (more often) a data packet. Packets have a header which defines the packet type and the logical channel number to which it applies. In the case of data or flow control packets, the header also contains packet level sequence numbers.

Timeout Processing

Frame level time-outs are set in the Poll and Select configuration words or through the timer statements (T1_timer, T2_timer, T3_T4_timer) in the X.25 Link Term definition. This defines the period within which a response to a Command frame must be received across the local X.25 link. This timer is referred to within the ITU-T recommendation as timer T1. Should timer T1 expire before the expected response is received, the Command frame will be re-transmitted up to a maximum of N2 times. N2 (used as defined in the ITU-T recommendation) is also set by the Poll and Select configuration words. If timer T1 expires on N2 successive occasions, the link will be reset.

An additional timer, T2, is used to control the transmission of acknowledgments to received I-frames. As the acknowledgment may be 'piggy-backed' onto a transmitted I-frame, it may not always be necessary to transmit an explicit flow control frame to send the acknowledgment. When an I-frame is received, timer T2 is started, and if an I-frame is transmitted, the acknowledgment will be sent with it and timer T2 will be cancelled. Only if T2 expires (no I-frame having been transmitted) will an explicit flow control frame be transmitted. The T2 timer value may be explicitly set by using the T2_timer statement. If this statement has not been defined in the Link Term entry, T2 is set to half of the value of T1, and for a 9600 baud link, T1 will usually be set to 400 milliseconds. N2 is usually set to 10.

Timer T4 is used at Level 2 (LAP-B) for DTEs. This is the maximum time the DTE will allow without frames being exchanged on the data link. The value of T4 is approximately 6*N2*T1. Note that for a DCE, this timer is called T3.

Packet level timers are implemented according to the ITU-T/ISO recommendation for T10, T11, T12, T13, T20, T21, T22, T23, T25, and T26. A 180-second timeout is applied to the receipt of data packet acknowledgments. At the end of the time-out, the virtual circuit will be reset.

Summary of Timer Values

The following sections shows the mandatory and optional X.25 timer values. Note that the DCE timer values are shown for completeness only.

	NOTE: The X.25 Protocol product supports some of the features of a DCE interface, however these are intended to be used for loopback configurations and direct computer to computer connections where the other end of the connection is a DTE device.

Level 2 X.25 Timers

DTE - Mandatory Timers:
	T1	- T2 * 2 (see note)
	T2	- Configurable (20 milliseconds to 630 seconds)
	T4	- N2*T1*6 approx. (see note)
DCE - Timers:
	T1	- T2 * 2 (see note)
	T2	- Configurable (20 milliseconds to 630 seconds)
	T3	- N2*T1*6 (see note)

	NOTE: T1, and T3/T4 may be independently set by the use of the T1_timer, T2_timer, and T3_T4_timer keywords in the X.25 link terminal definition. See the “X.25 Link Term Definitions” section.

Error Handling

The ZLU error counter for the X.25 link ZLU is incremented whenever an invalid frame is received, any frame needs to be re-transmitted or a FRMR (Frame Reject) frame is either transmitted or received.

These counters can be viewed via the tt and ttx commands of the ZMNTR utility.