03.793GPPStage 2Support of Optimal Routeing phase 1TS
SMG1 have decided that a given call shall involve no more than one international interrogation. As a result, the GMSC serving the originating subscriber cannot in general interrogate the HLR of a mobile C subscriber to route the call directly to VMSCC. Even if the IAM to the GMSC came from an MSC in a different PLMN from the GMSC, the GMSC cannot be certain that the call did not involve an international interrogation which did not result in OR, because the OR charging requirements were contravened. The only safe way to ensure that a given call shall involve no more than one international interrogation is to prohibit international interrogation for a mobile forwarded-to subscriber. A supplementary view expressed by SMG1 is that they want to keep phase 1 implementations simple, so the possibility of even an intra-PLMN interrogation to route a call to a mobile C subscriber has not been included for phase 1.
If the possibility of optimal routeing of a call to a mobile C subscriber is included, then no significant additional complexity is introduced by allowing multiple forwarding, limited only by the check of the number of forwardings which is already done in the HLR and VLR if a forwarding may be invoked.
Note that if a call is forwarded from the VMSC of the forwarding party this is treated for routeing and charging purposes as a new call. In existing GSM implementations the VMSC will analyse the forwarded-to number, and if it determines that the forwarded-to number is a subscriber of its PLMN it will interrogate the HLR for information to route the call. This justifies the definition of the procedure for the VMSC of the forwarding party to analyse the forwarded-to number and possibly interrogate an HLR in a different PLMN.