Intermediate Mobile Switching Centre

Description

The Intermediate Mobile Switching Centre (IM-MSC) is a functional entity within the Circuit-Switched (CS) core network domain of GSM and UMTS systems. It operates during the call setup phase for mobile-terminated calls (calls to a mobile subscriber). The primary scenario involves a Gateway MSC (GMSC), which receives the call from an external network, querying the Home Location Register (HLR) to obtain routing information (the Mobile Station Roaming Number, MSRN). If the MSRN points to a network where the GMSC cannot establish a direct trunk or signaling connection to the Visited MSC (VMSC) serving the target subscriber, an IM-MSC is employed as an intermediary.

Architecturally, the IM-MSC is a standard MSC that performs both call control and switching functions. It receives an Initial Address Message (IAM) from the GMSC via ISUP signaling, containing the MSRN. The IM-MSC then analyzes this MSRN and, acting as an originating switch, forwards a new IAM towards the final VMSC. It establishes a trunk connection on both sides, effectively becoming a transit switch in the call path. The IM-MSC remains in the signaling and bearer path for the duration of the call, handling subsequent call control messages like Answer (ANM), Release (REL), and Release Complete (RLC).

Its role is defined in 3GPP TS 23.119, 'Optimal Routing of Circuit-Switched calls.' The specification outlines procedures for optimal and non-optimal routing scenarios. The IM-MSC is key in non-optimal routing cases, such as when the GMSC and VMSC belong to different network operators without a direct interconnection agreement, or within a single operator's network using complex, hierarchical routing plans. The IM-MSC ensures backward compatibility and interworking between different network nodes and legacy routing schemes, maintaining call completion rates where direct routing is not feasible.

Purpose & Motivation

The IM-MSC was introduced to solve practical routing problems in large-scale, multi-operator, or geographically distributed GSM/UMTS networks. In the early days of mobile telephony, not all MSCs had direct trunk interconnections due to cost, physical limitations, or commercial agreements. A GMSC might need to route a call through one or more intermediate networks to reach the subscriber's serving MSC.

Without the defined concept of an IM-MSC, call setup would fail if the GMSC could not interpret the routing number (MSRN) or lacked a route to the network indicated by that number. The IM-MSC standardizes the behavior of a transit switch in the mobile call path, ensuring that signaling (ISUP) and user traffic can be relayed predictably. It addresses the limitation of requiring full-mesh connectivity between all MSCs and GMSCs, which is economically and technically impractical.

Historically, as networks evolved and roaming agreements expanded, the need for a standardized intermediary function became clear. The IM-MSC allows for flexible network planning, hierarchical routing, and the interconnection of networks from different vendors or operators, thereby ensuring reliable call delivery for mobile-terminated services across complex global infrastructures.