Message Provided by Transport Layer
| User Data |
X.25 Packet Network Layer
| Packet Header | User Data |
X.25 Frame Disk Layer
| HDLC | Packet Header | User Data | HDLC |
X.25 Packet Header Format(Modulo 8)
| bit | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
| Byte 0 | Data Qualifier (Q-bit) | Delivery Confirmation (D-bit) | Modulo Indicator (01 for modulo 8) | Logic Channel Number | ||||
| Byte 1 | Logic Channel Number | |||||||
| Byte 2 | Packet Receive Sequence Number | More Data (M-bit) | Packet Send Sequence Number | 0 | ||||
| User Data | ||||||||
X.25 Packet Header Format(Modulo 128)
| bit | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
| Byte 0 | Data Qualifier (Q-bit) | Delivery Confirmation (D-bit) | Modulo Indicator (10 for modulo 128) | Logic Channel Number | ||||
| Byte 1 | Logic Channel Number | |||||||
| Byte 2 | Packet Send Sequence Number | 0 | ||||||
| Byte 3 | Packet Receive Sequence Number | More Data (M-bit) | ||||||
| User Data | ||||||||
X.25 Link Layer Format High Level Data Link Control (HDLC)
| bit | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
| Byte 0 | Flag(=01111110) | |||||||
| Byte 1 | Address (can be extended beyond 8 bits) | |||||||
| Byte 2 Byte3 |
Control Frame (indicates whether frame type is Unnumbered, Supervisory, or Information in nature) | |||||||
| X.25 Packet Layer | ||||||||
| Byte 4 | Frame Check Sequence | |||||||
| Byte 5 | Frame Check Sequence | |||||||
| Byte 6 | Flag(=01111110) | |||||||
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© 1997 by STM&P, Inc.
Updated 2/1/98 by CAR