Call Party Handling

Description

Call Party Handling (CPH) is a core functional component of the CAMEL (Customized Applications for Mobile network Enhanced Logic) Intelligent Network (IN) architecture standardized by 3GPP. It defines the mechanisms by which a Service Control Function (SCF), typically residing in a Service Control Point (SCP), can instruct a Service Switching Function (SSF) within the Mobile Switching Center (MSC) or GMSC to manipulate the individual connections (or 'legs') that constitute a call. A call leg represents the connection between the network and a single participant. CPH provides the primitives to create, join, split, and release these legs, enabling the construction of complex call states.

Architecturally, CPH operates through the CAMEL Application Part (CAP) protocol, which carries the dialogue between the SCF and SSF. The SSF, embedded in the MSC, acts as a trigger point and switch resource manager. When a call event matching a subscribed CAMEL trigger occurs (e.g., call setup), the SSF suspends call processing and reports to the SCF. The SCF, hosting the service logic, can then issue CPH operations like 'Connect', 'SplitLeg', or 'MoveLeg' to the SSF via CAP messages. The SSF executes these commands by controlling the switch fabric and bearer connections, then reports the result back to the SCF. This separation of service logic (SCF) and switching (SSF/MSC) is fundamental to the IN principle.

Key components of CPH include the Call Party Handling DP (Detection Point) events and the associated operations. Detection Points like O_Answer and T_Answer allow the SCF to be notified when a call leg is answered. The primary operations are: Connect (to establish a new call leg to a party), Disconnect (to release a specific leg), SplitLeg (to separate one leg from a multi-party connection, placing it on hold), and MoveLeg (to transfer a leg from one call segment to another, essential for call transfer). The SCF maintains a Call Segment Association (CSA) model, which is a logical view of how call legs are grouped into call segments (e.g., a held leg in one segment, an active two-party call in another).

CPH's role is to provide the granular control necessary for advanced telephony services. For instance, to implement Call Waiting, the SCF uses CPH to place the existing active leg on hold (using SplitLeg) and connect the new incoming leg. For a three-party conference, the SCF would connect the third leg and merge it with the existing call segment. This level of control, standardized across vendors, allows for the creation of portable, network-centric services that are independent of the subscriber's handset capabilities, ensuring a consistent user experience.

Purpose & Motivation

CPH was created to address the limitations of traditional, switch-based call control, which was rigid and made introducing new telephony services a slow, vendor-specific, and costly process. Before Intelligent Network concepts like CAMEL, advanced features (e.g., call forwarding, multi-party) were hard-coded into individual MSC switches. Deploying a new service required software upgrades across all switches from a single vendor, stifling innovation and leading to long time-to-market. The CAMEL framework, with CPH as a key enabler, was developed to separate service logic from the switching infrastructure.

The primary problem CPH solves is providing a standardized, abstracted interface for external service logic to dynamically manipulate the participants in a call. This allows network operators and third-party service providers to create and deploy feature-rich communication services rapidly from a centralized platform (the SCP). It enables sophisticated call scenarios that involve more than two parties or dynamic changes to call topology, which are fundamental for business and premium consumer services. CPH provided the necessary toolkit within the CAMEL protocol to make these services possible in a standardized way across different network equipment vendors.

Historically, CPH was introduced in CAMEL Phase 2 (3GPP Release 5) to expand upon the basic call control of CAMEL Phase 1. Phase 1 allowed mainly call routing control (e.g., pre-paid, freephone). Phase 2 and CPH were motivated by the need to support a broader suite of value-added services like explicit call transfer, call waiting, and meet-me conferencing, which require active management of call legs during an established call. This evolution was critical for GSM networks to compete with fixed-line ISDN services and later to form the foundation for IP Multimedia Subsystem (IMS) service control, where similar principles of session and party control are applied.