Description
Support of Optimal Routing (SOR) is a feature within the CAMEL (Customized Applications for Mobile network Enhanced Logic) protocol that optimizes the routing of Mobile-Terminated (MT) calls, particularly for roaming subscribers. Its architecture involves key Core Network nodes: the Gateway MSC (GMSC), the Home Location Register (HLR), and the Visited MSC (VMSC). When a call is placed to a roaming subscriber, the call typically first routes to a GMSC in the subscriber's home network. Without SOR, the GMSC would query the HLR, receive the Mobile Station Roaming Number (MSRN) from the VMSC in the visited network, and then route the call internationally from the home network to the visited network. However, if the calling party is located in the same visited country or a third country, this creates a 'tromboning' effect where the call leg travels unnecessarily to the home network and back, increasing latency and cost.
SOR works by enhancing the CAMEL interrogation process. When a call arrives at a GMSC, the GMSC can be configured to determine if optimal routing is possible based on the location of the caller. If the caller is determined to be in a location where a more direct route to the visited network exists, the GMSC initiates a CAMEL dialogue with a Service Control Point (SCP). The SCP, using its logic and potentially the caller's location, can instruct the GMSC to forward the call to a different network node or to use an alternative routing procedure. A key mechanism involves the GMSC sending a special parameter in the MAP_SEND_ROUTING_INFORMATION message to the HLR, indicating a request for optimal routing information. The HLR and VMSC cooperate to facilitate a more direct routing path, potentially by providing routing numbers that allow interconnection in a different, more optimal point in the network.
The primary role of SOR in the network is economic and performance optimization for international roaming scenarios. It minimizes the use of expensive international transit circuits managed by the home network operator, reducing Inter-Operator Tariffs (IOT). Technically, it reduces post-dial delay by shortening the call path, which can improve the user experience. SOR is tightly integrated with the MAP (Mobile Application Part) protocol and CAMEL service logic. Its deployment requires support in the GMSC, HLR, and potentially in the signaling network (e.g., STP configurations) to ensure the optimal routing queries and responses are handled correctly across different operators' networks, often governed by bilateral agreements.
Purpose & Motivation
SOR was created to solve the specific and costly problem of suboptimal call routing for roaming mobile subscribers, a major pain point in early international GSM roaming. Before SOR, the call routing for a roaming subscriber was rigid and followed a standard path: call to home network GMSC -> query home HLR -> contact visited VMSC for MSRN -> route call from home network to visited network. This model assumed the calling party was in the home country. However, with increasing international travel, a common scenario emerged: a caller in the same foreign country as the roaming subscriber (e.g., two tourists in Spain) would have their call routed back to the subscriber's home network (e.g., in the USA) only to be sent back to Spain, creating an inefficient and expensive 'trombone' route across international carriers.
This tromboning resulted in several problems: higher call setup times due to longer signaling paths, increased call failure rates due to more network segments involved, and significantly higher costs for both operators and end-users, as the call incurred international carriage charges on two long-haul legs. SOR addressed these limitations by introducing intelligence and flexibility into the call routing decision. It allowed the network to dynamically determine a more optimal path based on the caller's location, bypassing the home network when it was not the most efficient routing point. The motivation was primarily economic—to reduce the Inter-Operator Tariff (IOT) settlements between operators—but it also delivered a tangible improvement in service quality by making international roaming calls faster and more reliable. SOR became a crucial feature for operators with large outbound roaming subscriber bases to control costs and remain competitive.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (257 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-4, normative work from Rel-15.
In Release 15, the SOR (Support of Optimal Routing) function was newly introduced, with specifications for the SOR acknowledge message coding and updates to the DL and UL NAS Transport procedures for its support. The release also defined the mechanism for sending the SOR information over the control plane in non-3GPP access and mandated that the UE send a registration complete message for SOR. Furthermore, it included enhancements for the protection of initial NAS messages and support for sending multiple payloads via the Payload container.
- SOR acknowledge message coding TS 24.501CR0216
- DL and UL NAS Transport procedure updates for SOR TS 24.501CR0357
- Dynamic Routing indicator update description TS 24.501CR0395
- Support for protection of initial NAS messages TS 24.501CR0425
- Support for Traffic Segregation TS 24.501CR0476
- Emergency Services Support indicator for non-3GPP access TS 24.501CR0106
+ 12 more changes
In Release 16, the key enhancement for the Support of Optimal Routing (SOR) function was the introduction of a "back-off control in case of routing failure" mechanism. This addition provides a specific procedure to manage and control re-attempts when optimal routing attempts fail, improving network reliability. Furthermore, corrections were made to the length of the SOR transparent container to ensure proper signalling and data handling.
- Adding support for SNPNs (Stand-alone Non-Public Networks) TS 24.501CR0963
- Support for Reliable Data Service in 5GS TS 24.501CR0982
- Update PDU session establishment procedure to support MA PDU session TS 24.501CR1021
- Adding support for unified access control in SNPNs (Stand-alone Non-Public Networks) TS 24.501CR1134
- MS classmark 2 and supported codec TS 24.501CR1188
- Signalling of UE support for RACS and of UE radio capability ID TS 24.501CR1356
+ 49 more changes
In Release 17, the enhancements for the Support of Optimal Routing (SOR) function specifically introduced the transport and usage of the SOR-CMCI (Steering of Roaming - Consumer Mobile Connection Information), along with procedural alignments and the addition of a security check criterion for it. Furthermore, the release expanded SOR support to SNPN (Standalone Non-Public Network) environments by defining procedures and coding for CP SoR (Control Plane SoR) and enabling the update of SOR information within a PLMN for SNPNs.
- SNN verification for SNPN supporting AAA-Server for primary authentication and authorization TS 24.501CR3137
- "List of subscriber data" handling for SNPN supporting AAA-Server for primary authentication and authorization TS 24.501CR3133
- Support of the default configured NSSAI in the SNPN TS 24.501CR3260
- Support for MA PDU Session with 3GPP access in EPC TS 24.501CR3208
- Multi-USIM UE support indications in 5GS TS 24.501CR3121
- Clarificaiton on behaviors of the UE and the network supporting Network Slice Admission Control TS 24.501CR3112
+ 66 more changes
In Release 18, enhancements for the Support of Optimal Routing (SOR) function included the introduction of a UE capability indication for supporting URSP provisioning in EPS. This release also added support for a UE to indicate its capability for non-3GPP access path switching to the network, enabling more flexible access network selection and routing. Furthermore, it introduced mechanisms for optimized handling of temporarily available network slices and support for network slice replacement during procedures like registration and PDU session release.
- Support for Unavailability Period TS 24.501CR4875
- UE to indicate its support for Slice-based N3IWF selection to the network TS 24.501CR4961
- Indicating the capability of supporting SDNAEPC during the PDU session establishment procedure TS 24.501CR4964
- Support for UE accessing SNPN services using non-3GPP access TS 24.501CR4944
- UE to indicate its support for Slice-based TNGF selection to the network TS 24.501CR5121
- Support of network slice replacement during registration procedure TS 24.501CR5070
+ 84 more changes
In Release 19, the key enhancement for the Support of Optimal Routing (SOR) function was the introduction of a "SoR count" mechanism. This addition, detailed in the Change Requests, provides a method to track the delivery of SOR information to the UE. Furthermore, corrections were made to the SOR transparent container and to the note regarding the SOR-CMCI length to ensure proper handling and interpretation of the SOR data.
- RAT utilization control support in 3GPP access TS 24.501CR6461
- (S)RTP multiplexed media information support TS 24.501CR6576
- Support of indirect network sharing TS 24.501CR6569
- Support of ECN marking for L4S for 5G-RG TS 24.501CR6626
- Support of QoS differentiation of traffic for N3GPP device behind UE or 5G-RG TS 24.501CR6618
- Support of LP-WUSPS assistance, general aspects TS 24.501CR6672
+ 16 more changes
Explore further
Broader topics and technologies where SOR plays a role.
Defining Specifications
3GPP specifications that define or reference SOR, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.066 vj00 | Mobile Number Portability Technical Realization | Rel-19 |
| TS 24.501 vj50 | 5G NAS Protocols Specification | Rel-19 |