This clause is intended to explain the role of the D-channel signalling procedures in the support of packet communications in an ISDN. A complete description of terminal adaptor functions can be found in Recommendation X.31 [14].
According to Recommendation X.31, the user may access packet facilities by means of one of the following alternatives:
a) Circuit-switched access to PSPDN services (Case A)
by establishing a transparent circuit-switched access connection through the ISDN to the access port of a public network (e.g. PSPDN) referred to as "access unit (AU)" in the following sections. This connection may be initiated by the user or the AU. From the ISDN point of view, the circuit-switched call control procedures of 5 apply. Only the B-channel is used in this case.
b) Packet switched access to an ISDN virtual circuit service (Case B)
by establishing a packet-mode access connection to the packet handler (PH) of an ISDN. This connection may be initiated by the user or the ISDN. Both B- and D-channels may be used in this case.
A more detailed description of the protocol and text of 6.1 to 6.5 is in Recommendation X.31. Appendix II/Q.931 and Appendix III/X.31 are essentially the same.
The term "user" refers to the user equipment which may consist of an ISDN packet-mode terminal (TE1) or a combination of an existing data terminating equipment (DTE/TE2) attached to a terminal adaptor (TA). A DTE may not receive all of the information provided inQ.931 signalling messages at the user-network interface.
The ISDN TA/TE1 presents an S/T interface towards the network and therefore the TA/TE1 implementation should embody the procedures described in Recommendation Q.921 and this Recommendation for B-and D-channel connection establishment and control.
For demand access connections, 6.1 through 6.4 apply. Example message flows for demand access connections are shown in Appendix II.
Two physical types of semi-permanent connections on B- and D-channels are covered in this clause:
1) physical layer semi-permanently established between the terminal and the PH/AU, i.e. the I.430 and I.431 physical layer remains activated and the physical path through the ISDN is connected semi-permanently; and
2) X.25 data link and physical layers semi-permanently established between the terminal and the PH/AU (in this type, both the user and the network shall keep the X.25 data link layer in the established state).
When a PVC is used, there must exist a type 2) semi-permanent connection.
In semi-permanent connection type 1), the procedures of 6.3 are followed for X.25 call establishment and release.
In semi-permanent connection type 2), only the procedures of 6.3.2 are followed for X.25 call establishment and release.
When semi-permanent connection type 2) is used for PVCs, none of the following procedures apply.
Semi-permanent connections are established via a provisioning process without Q.931 procedures.