Supplementary Services

Description

Supplementary Services (SS) represent a comprehensive portfolio of value-added telephony features defined in 3GPP standards that operate in conjunction with the basic telecommunication service, which is primarily the establishment and maintenance of a two-party voice call. They are a cornerstone of traditional circuit-switched (CS) mobile networks like GSM and UMTS, and their principles and many specific services have been carried forward into the IP Multimedia Subsystem (IMS) for VoLTE and VoNR. SS modify, supplement, or restrict the basic call in some way to provide users with greater control, privacy, and functionality.

Architecturally, in classic GSM/UMTS CS networks, Supplementary Services are managed by the Home Location Register (HLR) and executed by the Mobile Switching Center (MSC) or Visitor Location Register (VLR). The HLR stores the subscriber's SS subscription data, including which services are allowed and their specific parameters (e.g., the forward-to number for Call Forwarding Unconditional). When a subscriber registers with a network, this data is transmitted to the VLR serving that subscriber. The MSC/VLR then applies the logic for these services during call setup and in-call phases. Signaling for SS activation, deactivation, and interrogation uses the Mobile Application Part (MAP) protocol between HLR and VLR, and DTMF tones or unstructured Supplementary Service Data (USSD) codes are often used for subscriber interaction.

In the IMS domain for VoLTE/VoNR, Supplementary Services are implemented using SIP (Session Initiation Protocol) methods and event packages, aligning with the IP-based nature of IMS. The service logic often resides in an Application Server (AS) within the IMS core. For example, a Call Forwarding service in IMS would be handled by a SIP AS that intercepts INVITE requests, checks the subscriber's forwarding rules, and potentially redirects the call. The traditional CS-style SS are mapped to equivalent IMS communication services, ensuring feature parity and smooth migration for subscribers. The management of SS settings in IMS can be done via Ut interface (using XCAP protocol) allowing a user to configure services via a web portal.

The operation of a specific Supplementary Service involves a well-defined state model and procedures. Taking Call Waiting as an example: when a subscriber (User B) is engaged in an active call and a new incoming call arrives, the MSC (or IMS AS) determines that Call Waiting is active for User B. Instead of rejecting the new call with a busy signal, the MSC/AS applies call waiting tone to the existing connection and sends a notification (e.g., a SIP re-INVITE with alert-info header) to User B's UE. The UE then provides an indication to the user, who can choose to place the first call on hold and answer the second one, invoking the Call Hold supplementary service. This demonstrates how multiple SS can interact to create a complex user experience. Another critical category is Call Barring, which includes services like Barring of All Outgoing Calls (BAOC) or Barring of Incoming Calls When Roaming (BIC-Roam), which are crucial for security and cost control, especially in roaming scenarios.

Purpose & Motivation

Supplementary Services were created to transform the mobile phone from a simple wireless voice pipe into a sophisticated personal communications tool, matching and eventually surpassing the feature set of the fixed-line telephone network (PSTN/ISDN). The basic telecommunication service only provides connection establishment and voice transmission. SS add the intelligence and user control necessary for practical daily use, such as managing multiple calls, protecting privacy, and customizing call handling based on time or caller identity.

Historically, the development of SS in GSM (starting from Phase 1 and greatly expanded in Phase 2) was driven by the need for standardization across different operators and countries to enable seamless roaming and interoperability. Without standardized SS, a feature like Call Forwarding might work in a subscriber's home network but fail when roaming, leading to a fragmented user experience. The 3GPP specifications provided a common set of features, protocols, and interfaces, ensuring that a service like Calling Line Identification Presentation (CLIP) would work consistently regardless of the visited network.

Furthermore, SS became a significant source of revenue and differentiation for operators. They could offer tiered service packages, charging extra for advanced features like multi-party conferencing or explicit call transfer. They also provided essential tools for subscribers: Call Barring services allowed parents to control children's phone usage or businesses to manage employee calling privileges. Call Forwarding enabled "follow-me" functionality. As networks evolved to IMS and VoIP, it was imperative to preserve this rich set of features to ensure user acceptance of new technology. Therefore, a major design goal for IMS was to support equivalent or enhanced Supplementary Services, solving the problem of feature regression when migrating from CS voice to VoLTE/VoNR, thereby protecting both the user experience and the operator's service revenue.