X.25 PCI
X.25 is a network protocol (OSI layer 3) virtually used only on WAN links. Access to X.25 networks is handled by LAPB or LAPD protocols.
A faster successor of X.25 is 'Frame Relay'. Since X.25 has error recovery procedures which are necessary on (copper) lines with high bit error rates (BER), Frame Relay has not because it is commonly used on fiber links which are mostly error-free.
X.25 can be used in different ways. All type of X.25 use the same 3-byte header, but the first 4 bits (GFI) and the last 8 bits may be used different.
This document shows the common X.25 packet format as well as the X.25 data packet format.
X.25 packet format:
X.25 data packet:
Literature/Sources: