Support of Optimal Routing

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.