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Introduction
Optimal Routing is a cost-saving roaming service which allows subscribers in the mobile network to make local calls to any roaming subscriber in that network. The Optimal Routing system keeps track of all roaming subscribers registered in the mobile network. When the roaming subscriber received an inbound call, a GSM enabled short-cut call is made to the subscriber. This brings cost savings to the roaming subscriber as IDD charges are avoided.
Advantages of Optimal Routing:
1.Generates revenue because inbound roamers are more likely to remain on the mobile network when they benefit from the cost savings they realize when receiving mobile-originated calls from their contacts that also are logged onto same network.
2.Generates increased revenue from mobile-originated calls to inbound roamers because those route optimized calls bypass the inbound roamers home network and the associated IDD charges when the roamer remains on the local network.
3.The mobile network is set apart from competition by providing inbound customers an easy-to-use service that will save them money on roaming calls. By eliminating international trombone and HPMN handling, international call handling costs to the VPMN are reduced. Operators can offer flexible tariffs to the caller/called parties and create strong incentives for use.
For a call to be optimally routed, the following conditions must apply:
All PLMNs involved must support OR
Financial benefits for that call must be obtained
Acceptable quality of service must be maintained
Following are the different categories of optimally routed calls:
1.Mobile originated calls
2.Mobile Terminated calls
3.Terminal to Terminal (TtT) calls
Optimal routing in various GMR systems
The GMR-1 system viz. thuraya allows MO calls to be optimally routed to a preferred GS, different than the local GS to which the MES is registered. Criteria for optimal routing can include the subscribers service provider, the called party number and/or the MES position. The network functionality to support this feature resides in the GS.
The GMR-2 viz. AceS supports optimal routing of MO calls, so that the landline segment of the end to end connection is minimized. During call set-up, the Network Control Centre (NCC) determines the gateway closest to the called PSTN terminal and then the call is assigned to that gateway. The network functionality to perform the common signaling in order to establish and clear the call is residing in the NCC. Paging of the terminals for call set-up is centralized at the NCC and the terminals will listen for pages on the common channel from the NCC.
Scope of this write up is to discuss optimally routed TtT call setup in GMR-1 systems
Network Architecture for an optimally routed TtT call in GMR-1 systems
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Technical Realization of an optimally routed TtT call
The complete cycle of an optimally routed CS call is as follows:
Initial channel access
Gateway selection
Resource Negotiation
Mobile origination
Re-registration to the local GS
Initial Access:
The originating mobile station sends a channel request message to the originating GS that shall contain the following:
Subscribers service provider ID
Numbering Plan Identifier (NPI)
Type of Number (TON)
GPS position of the calling subscriber
Dialed digits of the called partys telephone number.
Based on the information sent in the channel request, the originating GS chooses the optimal GS on receipt of this request.
Gateway Selection:
The intention is to select the optimal GS that provides the shortest route and this includes the following:
Dialed digit analysis
Selecting the optimal GS
Checking permissions
For OR case, first step is to identify the type of the called party i.e. whether it is a PSTN user, PLMN user or a GMR1 user. This is done on the basis of NPI, TON and the dialed digits. A list of optimal GS is made at the GS on the following basis:
In case of PSTN or non-co-operating PLMN calls, choose the optimal GS based on the operator configured data via dialed digit analysis.
In other cases, on getting the ATI/SRI-SMS response (which shall contain the roaming number from terminating VLR) from the HLR, GS determines whether the terminating mobile is registered at a GS within GMR1-system. If yes, then this is the optimal GS, else, the optimal GS is determined again by dialed digit analysis.
If the ATI/SRI-SMS response returns a VMSC address not supported in roaming agreement, then the terminating mobiles local GS is selected.
Once this list is ready, following permissions need to be checked for each GS in this list:
The originating mobile should have access permissions to this optimal GS
The originating mobile should have roaming permissions to this optimal GS
The local GS and the selected optimal GS should have a radio resource sharing agreement thus allowing the use of radio resource allocated to one GS by the other and vice versa.
If none of the GS in the list satisfy the above criterion, then the local GS is chosen as the optimal GS.
Resource Negotiation:
If a remote GS is selected as optimal GS, then dedicated resources are allocated from the local GS to the optimal GS. The originating GS validates the radio resource agreement between the local GS and optimal GS. If there are no restrictions, then the originating GS identifies the call as TtT call and allocates traffic channels and transfers this information to the selected gateways GS. The terminating GS validates and assigns the loaned resource to its GSC which further activates the resources and acknowledges to the originating GS.
The terminating GS thus treats the assigned resource as its own and acknowledges the same to the originating GS. After getting this acknowledgement, the originating GS sends an immediate assignment message with pause indication set to false and an indication that a location update is needed by the originating mobile. The originating mobile needs to re-register with the optimal GS selected.
Mobile origination:
The originating mobile initiates a link establishment procedure by sending an SABM to the selected GS. The selected GS sends back the UA response in return thus completing the link establishment procedure.
The originating mobile then sends a location update message to the MSC of the optimal GS. The MSC sends an AUTHENTICATION REQ to which the originating mobile replies. The MSC then sends a ciphering after which it sends the Location update accept on the originating mobile. The originating mobile then updates its SIM wit the new value of TMSI received in the Location update accept message. This way the originating mobiles registration information moves from the local VLR to the optimal GSs VLR.
Finally the mobile-originated call proceeds at the optimal GS.
Re-registration to the local GS:
After the call ends, the originating mobile initiates a re-registration back to the local GS. During initial access, the GS sets the PLMN re-registration flag in the immediate assignment message. The MES stores this flag value as well as the LAI. On completion of the MO call, the MES checks this flag value and then performs the location update accordingly.
References
GMPRS-1 03.297: Optimal Routing technical realization.
System and method for implementing TtT connections via a Geo-synchronous earth orbit satellite, Chandra Joshi.
3GPP TS 23.079: Support of optimal routing.
Satellite Mobile system Architectures report of ASMS task force.
Optimal routing: A compilation of cellular networking perspectives, David Crowe.
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