It is a network operator option whether to implement optimal routeing for basic mobile-to-mobile calls. The existing UMTS and GSM architectures support the primary technical requirement of optimal routeing for mobile-to-mobile calls (basic OR): that a GMSC can interrogate an HLR in a different PLMN to obtain routeing information for a mobile terminated call (see GSM 03.04 [1]). Three logically distinct PLMNs are involved in the handling of an optimally routed mobile-to-mobile call:

• the IPLMN, which is also the VPLMN of the calling mobile subscriber;
• the HPLMN of the called mobile subscriber (HPLMNB);
• the VPLMN of the called mobile subscriber (VPLMNB).

Any two or all three of these PLMNs may be identical; in Figure 1 they are shown as distinct. Figure 1 shows the communication between the IPLMN, HPLMNB and VPLMNB for an optimally routed mobile to mobile call.

In Figure 1 and throughout the present document, the term ISUP is used to denote the telephony signalling system used between exchanges. In this architecture the VMSC of the calling mobile subscriber (VMSCA) is integrated with the GMSC; communication between them is over an internal interface. A gsmSSF may also be associated with VMSCA, to support CAMEL functionality. This is one way to support optimal routeing of mobile-to-mobile calls. Another way is to set up routeing tables in GMSCA appropriately. If the originating subscriber has a CAMEL subscription, then when VMSCA receives the setup message it sends an Initial DP message, containing the address digits received in the setup message, via the gsmSSF to the gsmSCF. If the gsmSCF determines that the destination defined by the address digits belongs to a GSM or UMTS PLMN, it responds to the Initial DP with a Connect or Continue With Argument message to VMSCA containing an indication that the call is eligible for optimal routeing. This causes VMSCA to route the call to the associated GMSC (GMSCA). If the GMSC is in the VPLMN of the calling mobile subscriber, is in a different PLMN from HLRB, it requests routeing information from HLRB using the MAP protocol. If HLRB determines that the call can be routed directly from the GMSC to VMSCB without contravening the charging requirements for optimal routeing given in clause 9.1, it requests a roaming number from VLRB using the MAP protocol, and VLRB returns a roaming number in the Provide Roaming Number ack. HLRB returns the roaming number to the GMSC in the Send Routeing Info ack. The GMSC uses the roaming number to construct an ISUP IAM, which it sends to VMSCB. The call is then handled according to the procedures defined in TS 23.018, except that if the call is answered GMSCA relays the answer event to VMSCA and includesthe destination address which it used to route the call, to allow VMSCA to generate the correct charging record.

Some cases of call forwarding on mobile subscriber not reachable (CFNRc) are handled in the IPLMN, without the call being extended to the VPLMN of the forwarding subscriber. For these cases, referred to in the present document as early call forwarding, the forwarding is already optimally routed. When a call has been extended from the GMSC to VMSCB, the procedures defined in TS 23.018 lead to any conditional call forwarding being routed from VMSCB to the forwarded-to destination; this is referred to in this specification as late call forwarding. Optimal routeing for late call forwarding (ORLCF) allows VMSCB to return control of the call to the GMSC, which can then route the call to the forwarded-to destination. Figure 2 shows the architecture for ORLCF. Phase 1 of SOR does not include optimal routeing of forwarding to a mobile subscriber, so optimal routeing of the forwarding leg is not considered.

After the call has been extended from the GMSC to VMSCB, if the VMSC/VLR determines that the call should be forwarded it requests the GMSC to resume call handling. The GMSC uses the forwarding information received in the request to resume call handling, or interrogates HLRB for forwarding information, depending on the indication received from the HLR with the roaming number. If the GMSC determines that the call can be routed directly to the forwarded-to destination without contravening the charging requirements for optimal routeing given in clause 9.1 it acknowledges the request, clears the traffic connection to VMSCB and sends an ISUP IAM to the forwarded-to local exchange.