Introduction to Tandem X.25 Capabilities

Packet Types
What is X.25?
065307 Tandem Computers Incorporated 1–23
Interrupt Packets A DTE can transmit one nonsequenced interrupt packet without following normal
flow-control procedures. This type of packet is used for unusual conditions, such as a
high-priority message. Once an Interrupt packet is sent, the sender cannot transmit a
second Interrupt packet until the first one is acknowledged by an Interrupt
Confirmation packet.
Flow-Control Packets Two types of flow-control packets can be used to start and stop transmission:
Receive Ready (RR) packets
Receive Not Ready (RNR) packets
RR Packets
The RR packet is used to notify the remote end to start sending data packets, or to
acknowledge packets that have been received when the node has no data packets to
send. The RR packet uses the P(R) field to acknowledge packets that have been
received.
RNR Packets
The RNR packet is used to request that the other end stop sending packets. Like the
RR packet, the RNR packet uses the P(R) field to acknowledge packets that have been
received.
Reset Packets A Reset packet is used to reset and reinitialize the logical channel if an error or
unrecoverable condition is encountered.
Restart Packets Restart packets are used for error recovery when no other recovery procedure is
possible. A Restart packet reinitializes all the logical channels in the interface and
clears all the virtual calls that are established. Logical channels with permanent virtual
circuits are reset.
Diagnostic Packets The diagnostic packet is used by some networks to indicate certain error conditions
not covered by such methods as reset and restart. No confirmation is required for this
type of packet. (Diagnostic codes can also be sent with Clear, Reset, and Restart
packets.)