This document refers to Call packets and Call Confirm packets. These names differ from those standardized by X.25. The standard
distinguishes between a Call packet sent by the data terminal equipment (DTE) station (a Call Request) and one sent by the
data communications equipment (DCE) station (an Incoming Call), and similarly between a Call Confirm packet sent by the DTE
(a Call Accepted) and one sent by the DCE (a Call Connected).
The packets are encoded identically, and in many cases the processing that X.25 does is identical; however, there are cases
where the behavior is predicated on the station type that is receiving or sending the packet.
An X.25 switched virtual circuit (SVC) is established between two stations through the exchange of a Call and a Call Confirm
packet. The X.25 standards specify that Call packets include source and destination addresses. Call Confirm packets might
also encode source and destination addresses, depending on the circumstances.When the source address is encoded in a Call
Confirm packet, the X.25 standards require that it be the same address that was specified in the Call packet. When the destination
address is encoded in a Call Confirm packet and is different from the destination address in the Call packet, the newer X.25
standards (those after ITU-T 1980 X.25) require that the reason for the difference be signaled by the encoding of the Called
Line Address Modified Notification (CLAMN) facility.
For example,when an X.25 Call is routed through a configured hunt group, a Call Redirection/Call Deflection Notification (CRCDN)
facility is encoded in the forwarded call along with the original destination address. This encoding notifies the receiver
that the Call packet was redistributed by a hunt group. If such a Call is accepted by a returned Call Confirm packet, a CLAMN
facility and the destination address of the accepting station will be encoded in the Call Confirm packet. This encoding notifies
the originator that the accepting destination was reached by distribution through a hunt group.