Provide Roaming Number

Description

The Provide Roaming Number (PRN) is a core network procedure and associated parameter defined within the 3GPP specifications for GSM and UMTS networks. It operates as part of the Mobile Application Part (MAP) protocol, specifically within the MAP Send Routing Information (SRI) operation. When a call is destined for a mobile subscriber who is roaming in a visited network, the Gateway Mobile Switching Center (GMSC) in the home network queries the Home Location Register (HLR) for routing information. The HLR, in turn, sends a PRN request to the Visited Mobile Switching Center (VMSC) or its associated Visitor Location Register (VLR) where the subscriber is currently registered. The primary purpose of this request is to obtain a temporary, network-specific number that can be used to route the call to the visited MSC.

Upon receiving the PRN request, the VLR/VMSC allocates a Mobile Station Roaming Number (MSRN) from a pool of numbers dedicated for this purpose. This MSRN is a temporary E.164 number that looks like a regular telephone number but is used only for internal network routing. The VLR/VMSC returns this MSRN to the HLR in the PRN response. The HLR then forwards this MSRN to the requesting GMSC. The GMSC uses this MSRN to route the call through the Public Switched Telephone Network (PSTN) or the IP-based core network to the correct VMSC. Once the call is established or if it fails, the VMSC releases the MSRN back into the pool for future use, making it a dynamic and efficient resource.

The PRN mechanism is architecturally critical because it decouples the subscriber's permanent identity (the MSISDN) from the physical routing path required to reach them. It enables location privacy and efficient use of numbering resources. The procedure involves several key network elements: the HLR as the central subscriber database, the VLR as the temporary visitor database, the VMSC as the switching point in the visited network, and the GMSC as the entry point for incoming calls. Signaling is carried over SS7 or IP-based SIGTRAN networks using the MAP protocol. The PRN operation includes parameters such as the subscriber's IMSI, the MSC number, and supported network capabilities, ensuring that the routing information is accurate and that the call can be delivered with the appropriate services.

Purpose & Motivation

The PRN procedure was created to solve the fundamental problem of routing telephone calls to a mobile subscriber who is outside the geographical coverage of their home network operator. In early cellular systems, without such a mechanism, it would be impossible for a home network switch to know how to direct a call to a subscriber whose location is dynamically changing and managed by a different network. The PRN provides a standardized, secure method for networks to exchange temporary routing numbers without exposing the subscriber's exact location or requiring permanent number portability between operators.

Historically, before the standardization of GSM and the MAP protocol, proprietary solutions or simpler fixed routing were used, which limited seamless international roaming. The PRN mechanism, introduced in 3GPP Release 4 as part of the core GSM network architecture, enabled automated, real-time acquisition of routing information. It addressed limitations such as static routing tables, which were infeasible for mobile users, and the need for manual intervention by network operators to set up call paths. By providing a dynamic, on-demand routing number, it allowed for efficient use of the international telephone numbering plan (E.164) and formed the basis for the commercial roaming agreements between operators that define modern global mobile communication.

The procedure also supports essential supplementary services like call forwarding and optimal routing. It solves the problem of how to connect a call without requiring the calling party to know the subscriber's temporary location or dial a different number. From a network perspective, it enables load distribution and scalability, as MSRNs are pooled and reused. The creation of the PRN procedure was motivated by the need for a scalable, interoperable, and efficient global roaming infrastructure, which was a key selling point for GSM technology over earlier cellular systems.