Optimal Routeing

Description

Optimal Routeing (OR) is a service feature defined within the 3GPP specifications for GSM and UMTS core networks. Its primary function is to enhance the efficiency of call setup procedures, particularly for mobile-originated calls. When a Mobile Station (MS) initiates a call, the network must determine how to route the call signaling and subsequent traffic to the correct destination, which could be another mobile user, a fixed-line subscriber, or a service node. Traditional routing might use a default or static path. OR introduces intelligence into this process by allowing the network to select the optimal route based on dynamic criteria.

The architecture for OR involves several core network entities, primarily the Mobile Switching Centre (MSC). The MSC serving the calling subscriber receives the call setup request. Utilizing OR logic, which may be based on subscriber data, current location information, and network configuration, the MSC evaluates potential routing paths. The goal is to minimize the number of network nodes (like other MSCs or transit switches) involved in the connection, reduce signaling load, and potentially lower transmission costs. The decision might involve routing the call directly to the destination MSC if possible, or via the most efficient gateway.

How OR works is integrated into the call control procedures. During the Initial Address Message (IAM) setup in SS7 signaling, the originating MSC applies OR algorithms. These algorithms can consider the called party's number, the calling subscriber's Home PLMN (HPLMN) or current Visited PLMN (VPLMN), and any specific routing policies defined by the network operator. The selection is not always the geographically shortest path but the most efficient from a network resource perspective, avoiding unnecessary inter-network links or international switches when a direct route exists. Its role is to streamline core network operations, improving overall service quality and operational economics.

Purpose & Motivation

Optimal Routeing was introduced to solve inefficiencies in early mobile telephony call setup. In initial GSM networks, call routing could be suboptimal, often following predetermined paths through multiple switching centers, even when a more direct connection was available. This led to increased signaling traffic, longer call setup times, and higher utilization of transmission resources, which in turn could affect network capacity and operational costs. The primary motivation was to optimize network resource usage and improve the user experience by reducing post-dial delay.

The historical context stems from the expansion of GSM networks and the complexity introduced by roaming. When a subscriber roams to a different network or country, the call routing path from the visited MSC back to the home network and then to the destination could become convoluted. OR addresses this by allowing the visited MSC to analyze the destination and potentially route the call more directly, perhaps via an international gateway closer to the destination, rather than always routing back to the HPLMN first. It addressed the limitations of static routing tables and simple home-routing approaches, providing a dynamic, cost-effective, and performance-oriented solution.