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APPENDIX B
X.25 (1980) Frame and Packet Formats
B 1. Frame Formats
B 1.1 Data on the Circuit
7E 7E 7E 7E 7E Frame 7E Frame 7E 7E Frame 7E 7E
• At least one flag byte between frames
• Line always busy sending flags or frames
• Flag is hex 7E—binary 01111110
• Frames are of different lengths
• Frames are bit-stuffed to avoid occurrence of six binary ones
B 1.2 General Frame Layout
7E 7E 7E 7E 7E Frame 7E 7E
Either Address Control FCS
Or Address Control Information FCS
• Address is eight bits
• Address is 3 for DCE DTE commands or DTE DCE responses
• Address is 1 for DTE DCE commands or DCE DTE responses
• Control is eight bits and shows frame type (see below)
• Information field only present in I-frames and CMDR/FRMR
frames
• Information field carries the data to be transmitted and may be
any length up to an agreed maximum
• FCS is sixteen bits

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