Data call Routeing Mechanism

Description

The Data call Routeing Mechanism (DRM) is a legacy core network functionality defined in early 3GPP releases for GSM and UMTS circuit-switched (CS) domain. It governs the setup and routing of mobile-originated data and fax calls, which are distinct from voice calls due to their need for specific interworking functions (IWFs). When a mobile station (MS) initiates a data or fax call, the serving Mobile Switching Center (MSC) must determine how to connect this call to the external data network (like the PSTN/ISDN or a packet data network via an IWF). The DRM is the standardized procedure that enables this decision-making process.

The mechanism works through a series of queries and information exchanges between network entities. Upon receiving a data call setup request, the serving MSC analyzes the called party number and the bearer capability information element provided by the MS. The MSC then typically queries the Visitor Location Register (VLR) for subscriber-specific data. Crucially, the DRM may involve interrogating the Home Location Register (HLR) to retrieve the subscriber's data call routing information, which can include preferences or restrictions. Based on this collected information—including the requested data service type (e.g., asynchronous data, facsimile), subscriber profile, and network configuration—the MSC executes the DRM logic to select the optimal exit point from the PLMN. This often means choosing a specific Interworking Function (IWF) unit, which could be integrated within the MSC (MSCIWF) or be a standalone node, to perform the necessary protocol adaptation, rate adaptation, and modulation conversion (e.g., from modem audio tones to digital data streams).

The DRM's architecture is embedded within the CS core network protocols, primarily in the MAP (Mobile Application Part) signaling between MSC, VLR, and HLR, and in the call control procedures (e.g., TS 24.008). Its role was critical in the era of circuit-switched data services (like CSD and HSCSD) and fax services, ensuring that these calls were routed to network nodes equipped with the correct modems and interworking capabilities. With the industry's shift to all-IP networks and the predominance of packet-switched data (GPRS, EPS, 5GS), the practical relevance of DRM has greatly diminished, as data services are now natively packet-based and do not require circuit-switched interworking in the same way.

Purpose & Motivation

DRM was developed to solve the problem of efficiently routing circuit-switched data and fax calls in early digital mobile networks (GSM). Unlike voice calls, which could be routed through standard PSTN trunks, data/fax calls required specific terminal equipment (modems) and interworking functions to convert between the digital mobile network protocols and the analog modem signals used on the PSTN or the digital protocols of ISDN. Without a standardized mechanism, there was a risk of misrouting calls to network nodes lacking the required IWF capabilities, leading to call setup failures or poor quality.

The creation of DRM provided a standardized, subscriber-aware routing decision process. It allowed network operators to configure flexible routing policies based on service type, subscriber subscription (e.g., some subscribers might be restricted to certain data services), and network topology. This ensured reliable call establishment for value-added services like fax and dial-up data connectivity, which were important business services in the 1990s and early 2000s. It addressed the limitations of treating all circuit-switched calls identically and was a necessary component for the full service offering of 2G and early 3G networks before the widespread adoption of always-on packet data.