This document provides an overview of X.25, including its layers in relation to the OSI model, frame and packet layer domains, virtual circuits, logical channel numbers, packet formats, control packets, addressing format, and related triple-X protocols. Figures are included to illustrate concepts like frame and packet formats, addressing, phases of operation, and categories of packets.

1. Chapter 17 X.25

2. Figure 17-1 X.25

3. Figure 17-2 X.25 Layers in Relation to the OSI Layers

4. Figure 17-3 Format of a Frame

5. Figure 17-4 Addressing at the Frame Layer

6. Figure 17-5 Three Phases of the Frame Layer

7. Figure 17-6 Frame Layer and Packet Layer Domains

8. Figure 17-7 Virtual Circuits in X.25

9. Figure 17-8 LCNs in X.25

10. Figure 17-9 LCN Assignments

11. Figure 17-10 PLP Packet Format

12. Figure 17-11 Categories of PLP Packets

13. Figure 17-12 Data Packets in the PLP Layer

14. Figure 17-13 RR, RNR, and REJ Packets

15. Figure 17-14 Other Control Packets

16. Figure 17-15 Control Packet Formats

17. Figure 17-16 X.25 Address Format

18. Figure 17-17 Triple-X Protocols