Introduction to Tandem X.25 Capabilities

Packet Types
What is X.25?
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Call Clear Packets Call clear packets are used to clear a DTE-to-DTE session on a switched virtual circuit.
Call clear packets are not used by permanent virtual circuits. Call clear packets
contain all the information included in the call-setup packets, plus the clearing cause
field (mandatory) and the diagnostic code (optional).
Clear Request / Clear Indication / Clear Confirmation Packets
When the calling DTE has completed the sending of data packets and has received
acknowledgement for all the packets it has sent, it sends a Clear Request packet to its
DCE, which forwards the packet to the remote DTE as a Clear Indication packet.
The remote DTE must respond to the Clear Indication packet with a Clear
Confirmation Packet. When the DTE which sent the Clear Request packets receives
the Clear Confirmation Packet, the LCNs used for the virtual call become available for
another call establishment.
Clearing packets can also be sent at any time by either the DTE or DCE if operational
problems occur.
If the virtual circuit cannot be established, the DCE sends a Clear Indication packet
back to the calling DTE.
If the called DTE cannot accept the call, for example, because of lack of resources,
it issues a Clear Request Packet to its network DCE. The DCE sends this packet to
the calling DTE as a Clear Indication packet.
Figure 1-14 illustrates the exchange of packets for clearing a call from a switched
virtual circuit.
Figure 1-14. Call Clear Packets
Network
Clear Indication
DTE Clear
Confirmation
Clear Request
Calling
DTE
Called
DTE
051
Local
DCE
Remote
DCE
DCE Clear
Confirmation