Call Processing Language

Description

Call Processing Language (CPL) is an XML-based scripting language standardized by 3GPP for describing telephony services in IP Multimedia Subsystem (IMS) and other IP-based telephony networks. The language provides a standardized way to specify how incoming and outgoing calls should be processed, enabling service providers to implement customized call handling logic without requiring proprietary solutions. CPL scripts are executed by network elements called CPL servers, which interpret the scripts and apply the specified call processing rules to SIP signaling messages.

CPL operates through a structured XML schema that defines various call processing actions and conditions. The language includes two main types of nodes: signaling actions (like proxy, redirect, reject) and location modifiers (like location, lookup). These nodes can be combined with conditional statements based on call parameters such as caller identity, time of day, presence status, or called number. The scripts are typically stored in a user profile database and downloaded to the CPL server when needed for call processing.

Architecturally, CPL integrates with SIP-based networks through CPL servers that can be deployed as standalone entities or integrated within Application Servers (AS) or SIP proxies. When a SIP request arrives, the CPL server retrieves the appropriate script, parses the XML structure, and executes the logic sequentially. The language supports both incoming and outgoing call processing with separate script sections for each direction. Key components include the CPL interpreter, script storage repository, and interfaces to subscriber databases for retrieving user-specific scripts.

CPL's role in 3GPP networks extends beyond basic call control to enable advanced telephony services within IMS. It provides a standardized mechanism for implementing services that were traditionally hard-coded in switching equipment, allowing for greater flexibility and faster service deployment. The language's XML foundation makes it human-readable and machine-processable, facilitating both manual creation by service designers and automated generation by service creation environments. CPL scripts can be modified dynamically without affecting the underlying network infrastructure, supporting rapid service updates and customization.

Purpose & Motivation

CPL was created to address the need for a standardized, vendor-independent method of implementing telephony services in IP-based networks. Before CPL, service providers relied on proprietary scripting languages or hard-coded service logic within network equipment, which created interoperability issues and vendor lock-in. The emergence of SIP-based telephony and IMS architectures necessitated a common language that could work across different vendors' equipment while maintaining security and reliability.

The primary motivation for CPL development was to separate service logic from network infrastructure, enabling faster service deployment and greater innovation. Traditional telephony services required extensive testing and integration with specific vendor equipment, slowing down time-to-market for new features. CPL provided a standardized interface that allowed service providers to create services once and deploy them across multi-vendor networks. This was particularly important as telecommunications moved toward all-IP networks where services needed to work consistently across different access technologies.

CPL also addressed the growing demand for user-controlled services in next-generation networks. Unlike traditional Intelligent Network (IN) services that were entirely controlled by operators, CPL enabled end-users to customize their call handling behaviors through web interfaces or other management tools. This user empowerment aligned with the broader trend toward personalized communications services in 3GPP networks. The language's design prioritized safety and security, ensuring that user-created scripts couldn't cause network instability or security breaches while still providing meaningful customization capabilities.

Key Features

• XML-based scripting language for telephony services
• Supports both incoming and outgoing call processing
• Conditional logic based on call parameters and user status
• Standardized call actions including proxy, redirect, and reject
• Integration with SIP signaling in IMS networks
• User-controlled service customization capabilities

Evolution Across Releases

Defining Specifications