This Recommendation provides the stage 2 descriptions for call set-up and release applicable to the packet mode virtual call services supported by an ISDN. The stage 2 descriptions are comprised of the switching functions and signalling information flows. Stage 2, which is defined in detail in Recommendation Q.65, is part of the three stage service description methodology described in Recommendation I.130.
Recommendation Q.72 describes virtual call services which provide the unrestricted bi-directional transfer of user information in a packetized manner over a virtual circuit within a B- or D-channel from one S/T reference point to another. The user information consists of layer 2 frames consistent with the layer 2 definitions of DSS 1. The data units are routed through the network on the basis of an attached label. This label is a logical identifier with local significance.