Send Routeing Information

Description

Send Routeing Information (SRI) is a critical MAP (Mobile Application Part) procedure defined within the 3GPP core network architecture, primarily utilized in GSM, UMTS, and evolved packet core networks. It operates between network entities like the HLR (Home Location Register), VLR (Visitor Location Register), MSC (Mobile Switching Center), and SGSN (Serving GPRS Support Node). The procedure is invoked when a network node, such as a Gateway MSC (GMSC) for a mobile-terminated call or an SMS Interworking MSC (SMS-IWMSC) for SMS delivery, needs to determine the current serving node for a specific Mobile Station International Subscriber Directory Number (MSISDN). The requesting entity sends an SRI message to the subscriber's HLR. The HLR, which maintains the subscriber's profile and current location information, responds with routing details, which typically include the MSC Number (MSC-Number) or SGSN address. This response may also include additional information like the subscriber's status and any forwarding instructions.

The SRI procedure is foundational for mobility management and session establishment. For circuit-switched services like voice calls, the GMSC uses the MSC number received in the SRI response to route the call via an ISUP (ISDN User Part) Initial Address Message (IAM) to the correct serving MSC. For packet-switched services or SMS over GPRS, the routing address for the SGSN is provided. The HLR may also indicate if the subscriber is not reachable (e.g., detached) or has active call forwarding services, which alters the routing logic. The procedure supports both synchronous and asynchronous modes of operation and is secured using MAP security mechanisms.

Architecturally, SRI is part of the MAP protocol suite, which uses the SS7 (Signaling System No. 7) or SIGTRAN (Signaling Transport) for transport in legacy networks, and Diameter-based interfaces in later evolved packet core deployments. Its role extends beyond basic call delivery; it is integral to lawful interception, location services (LCS), and number portability queries. In modern networks, even with the transition to all-IP cores and IMS (IP Multimedia Subsystem), SRI and its evolved counterparts remain relevant for interworking with legacy circuit-switched domains and for certain administrative and regulatory functions.

Purpose & Motivation

The SRI procedure was created to solve the fundamental problem of locating a mobile subscriber within a globally distributed, cellular network. In fixed telephony, a telephone number is statically associated with a physical line and switch. In mobile networks, a subscriber can move anywhere, making dynamic routing essential. Prior to standardized procedures like SRI, there was no efficient, standardized method for a gateway switch to query the subscriber's home database (HLR) to find their current serving switch. This would have led to failed call deliveries, inefficient network flooding with location queries, or reliance on non-standard proprietary mechanisms.

Its introduction in Release 4 formalized a key component of the GSM/UMTS signaling plane, enabling scalable, automated call and message delivery. It addressed the limitation of needing prior knowledge of a subscriber's location by centralizing this intelligence in the HLR. The procedure allows the network to separate the call control function (in the GMSC) from the subscriber location database (HLR), adhering to a clear functional separation that enhances network scalability, reliability, and the ability to introduce new services like call forwarding and roaming. It is a cornerstone for enabling seamless national and international roaming, as it provides a standardized interrogation method that works across different network operators' infrastructures.