ACC X.25 Protocol User's Guide

ZX25D X.25 Protocol Driver
Implementation Notes
Chapter 358
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