Description
The Incoming Call Gateway (ICGW) is a core network element defined within the 3GPP architecture, specifically for circuit-switched (CS) voice services. It serves as the primary entry point for calls destined for subscribers within a mobile network operator's domain. When a call originates from an external network, such as the Public Switched Telephone Network (PSTN) or another Public Land Mobile Network (PLMN), it is first routed to the ICGW. The ICGW performs the necessary signaling and protocol conversions to translate the external signaling (e.g., ISUP) into the mobile network's internal signaling protocol, primarily the Mobile Application Part (MAP) or other relevant protocols used for call control.
Architecturally, the ICGW is often integrated with or closely associated with the Gateway MSC (GMSC) function. The GMSC is the MSC that interrogates the Home Location Register (HLR) to obtain routing information for the called mobile subscriber. The ICGW can be seen as the physical or logical gateway function that interfaces with the external network. It handles the initial call setup signaling, including address analysis, number translation, and potentially charging-related functions. Its operation involves receiving an Initial Address Message (IAM) from the external network, determining the correct HLR to query, and then forwarding the call to the appropriate Visited MSC (VMSC) where the subscriber is currently located.
The ICGW's role extends beyond simple routing. It is a critical point for implementing regulatory and service features such as lawful interception, call barring checks, and specific routing policies for incoming calls. It ensures that the call setup complies with the network's policies and the subscriber's service profile before the call is extended into the mobile network. In modern networks, especially with the transition to Voice over LTE (VoLTE) and IP Multimedia Subsystem (IMS), the traditional CS-based ICGW function has evolved or been replaced by IMS-based entities like the Interrogating Call Session Control Function (I-CSCF) and the Media Gateway Control Function (MGCF) for handling incoming calls from legacy CS networks.
Purpose & Motivation
The ICGW was created to address the fundamental need for interworking between mobile networks and external fixed or other mobile networks. In the early days of GSM and UMTS, mobile networks were islands that needed to connect to the global PSTN and other PLMNs to provide universal connectivity. Without a dedicated gateway function, incoming calls from these external networks would have no defined entry point or standardized procedure for routing within the mobile network.
Prior to standardized gateways like the ICGW, inter-network communication was often ad-hoc, proprietary, and inefficient. The ICGW provided a standardized, scalable, and reliable interface. It solved problems of address translation (converting E.164 numbers to mobile network routing addresses), signaling protocol conversion (e.g., from ISUP to MAP), and centralized point of control for incoming traffic. This allowed for efficient call delivery, simplified network management, and the implementation of consistent features like number portability resolution and security screening at the network boundary.
Its creation was motivated by the commercial necessity for mobile networks to be fully integrated into the global telecommunications ecosystem. It enabled the 'anywhere, anytime' connectivity promise of mobile telephony by ensuring that calls from any network in the world could successfully reach a mobile subscriber. The ICGW became a cornerstone for billing, lawful interception, and service differentiation for incoming calls, forming a critical part of the revenue and operational infrastructure for mobile operators.
Key Features
- Acts as the standardized entry point for mobile-terminated calls from external networks
- Performs signaling protocol conversion between external (e.g., ISUP) and internal (e.g., MAP) protocols
- Interrogates the HLR to obtain routing information for the called subscriber
- Provides a central point for implementing call screening, barring, and routing policies
- Supports charging functions for incoming call handling
- Facilitates regulatory compliance features like lawful interception at the network edge
Evolution Across Releases
Introduced as a defined functional entity for handling incoming calls in the 3GPP network architecture. It established the basic role of interfacing with external circuit-switched networks, performing protocol conversion, and querying the HLR for routing information to enable call delivery to mobile subscribers.
Defining Specifications
| Specification | Title |
|---|---|
| TS 21.905 | 3GPP TS 21.905 |