B.4 Interworking with existing networks

Limitations in network or distant user signalling (e.g. in the case of an incoming call from a PSTN or a call from an analogue terminal) may restrict the information available to the called user in the incoming SETUP message. A called user should accept limited compatibility checking (e.g. without the High layer compatibility information element) if a call is routed from an existing network which does not support High layer compatibility information element transfer.

In cases where the network cannot provide all incoming call information, or where the network is not aware of the existence or absence of some service information (such as a compatibility information), the incoming SETUP message includes a Progress indicator information element, containing progress indicator No. 1, call is noted-to-end ISDN; further call progress information may be available in band,or No. 3, origination address is non-ISDN (see Annex G).

The terminal equipment receiving a SETUP with a Progress indicator information element shall modify its compatibility checking, the terminal equipment should regard the compatibility as successful if it is compatible with the included information, which as a minimum, will be the Bearer capability information element. A terminal equipment expecting information in addition to the Bearer capability information element in a full ISDN environment need not reject the call if such information is absent but a Progress indicator information element is included.

Table B.1/Q.931

Bearer capability compatibility checking

Table B.2/Q.931

Low layer and high layer compatibility checking -
Compatibility assured with the available description of the call

Table B.3/Q.931

Low layer and high layer compatibility checking -
Compatibility not assured with the available description of the call