Introduction to Tandem X.25 Capabilities

Packet-Layer Components
What is X.25?
1–16 065307 Tandem Computers Incorporated
Figure 1-11. Packet Header Structure
011
General Format
Identifier
Logical Channel
Group Number
Logical Channel Number
Packet Type Identifier
Octet 1
Octet 2
Octet 3
Octet 4
Either User Data or Additional Control Fields
87654321Bit
Packet Header
The packet header contains the following peer-to-peer protocol information:
General Format Identifier (GFI) — Indicates the general format of the rest of the
header. It contains a qualifying bit (Q bit), a delivery confirmation bit (D bit), and
two packet-sequencing bits.
Q bit — Indicates to a higher-level protocol that this is a special control packet.
For example, the Q bit is used to distinguish between data and PAD messages,
such as messages used to transmit X.29 data.
D bit — Indicates the delivery-confirmation procedures to be used (that is, if
end-to-end acknowledgement of the data packets is desired). If end-to-end
acknowledgement is not selected, the DTE is notified only that the local DCE
has accepted the packet.
Packet sequencing bits — Indicate whether this packet is using modulo 8 or
modulo 128 for sequencing. Modulo 8 means that packets have sequence
numbers 0-7. Modulo 128 means packets have sequence numbers 0-127.
Logical Channel Group Number (LCGN) — Indicates the grouping of logical
channels for this packet. Groups of logical channels are assigned to different types
of circuits: such as PVCs, one-way incoming virtual circuits, two-way virtual
circuits, and one-way outgoing virtual circuits.
Logical Channel Number (LCN) — Indicates the specific logical channel assigned
to this packet.