The Low Layer Compatibility (LLC) IE, which is very similar in form to the BCC IE, cannot, unlike the BCC IE, be modified by the network and accordingly would not be in a position to cater for interworking in the A-law to u-law case discussed in Example 1. This demonstrated how the coding in this IE could be changed to accommodate interworking between networks. In this case information in the LLC IE which conflicts with the corresponding information in the Bearer Channel IE will be ignored (see notes 1 and 2 below).

In this example if the A-law coding information was not included in the BCC IE and included only in the LLC IE then an inter-networking function could not be implemented as the LLC IE can not be changed by the network. The call would be rejected by the distant terminal.

Note 1: The BCC IE can be changed by the network to allow interworking with different networks like the PSTN, for example. (It could be a call from a non ISDN network as indicated by Progress Indicator IEs from the calling side). Accordingly, while the BCC IE will be similar to the LL IE at the originating side, the intention is that these IEs will have minimal duplication at the terminating side.

Note 2: BCC IE overrides any contradictions, which may arise due to network modifications, between itself and the LLC IE. Note that the LLC IE is optional and the BCC IE is mandatory. i.e. it is always included with the Setup Message. But it may not include the octet 5 which contains the low layer user information protocol. Low Layer Compatibility checking (for the layers, 1-3) may then be carried out in cases where the BCC IE does not contain this octet 5 information. In these cases the LLC IE may be sent as an optional Additional Information Message with the Setup Message or the Connect Message.