3G Packet Switched Multimedia Terminal

Description

The 3G PS multimedia terminal is a user equipment (UE) that supports multimedia telephony services over 3GPP packet-switched (PS) networks. It is based on the IETF Session Initiation Protocol (SIP) and Session Description Protocol (SDP), which 3GPP has profiled and extended to meet the specific requirements of operator-controlled multimedia telephony. This terminal is a key enabler of the IP Multimedia Subsystem (IMS) and is specified in 3GPP Technical Specifications 26.235 and 26.236.

The terminal implements a SIP user agent that handles session establishment, modification, and termination. SDP is used to negotiate media parameters such as codecs, media types (audio, video, text), and transport addresses. 3GPP has defined a minimum set of mandatory codecs and features to ensure interoperability across vendors and networks. The terminal also supports supplementary services like call forwarding, call waiting, and conference calling, which are implemented via SIP signalling and IMS application servers.

Architecturally, the 3G PS multimedia terminal connects to the PS domain through a radio access network (e.g., UTRAN, E-UTRAN) and a packet core (e.g., GPRS, EPC). It establishes IP connectivity and then registers with the IMS core using SIP REGISTER. The IMS core comprises Call Session Control Functions (CSCFs) that manage session control. The terminal may also support interworking with circuit-switched networks via the IMS Centralized Services (ICS) or Service Centralization and Continuity Application Server (SCC AS).

Key capabilities of the terminal include support for multiple media streams within a single session, quality of service (QoS) negotiation, and security mechanisms such as IPsec and TLS for SIP signalling. It also supports emergency calls, lawful interception, and charging. The terminal is designed to provide a consistent user experience across different access technologies, including 3G, LTE, and Wi-Fi.

The 3G PS multimedia terminal represents a shift from circuit-switched telephony to all-IP multimedia services. It allows operators to offer rich, converged services and to integrate with web-based applications. Its development was driven by the need for a standardized, interoperable terminal that could leverage the flexibility of IP while maintaining the reliability and feature set of traditional telephony.

Purpose & Motivation

The 3G PS multimedia terminal was created to enable multimedia telephony services over packet-switched networks, moving beyond the limitations of circuit-switched telephony. Traditional circuit-switched networks were optimized for voice but inefficient for data and multimedia. By adopting IETF SIP/SDP, 3GPP aimed to leverage the flexibility of IP to deliver integrated voice, video, and messaging services with better spectral efficiency and lower cost.

The terminal solves the problem of interoperability by defining a common set of protocols and codecs that ensure different vendors' terminals can communicate with each other and with network infrastructure. It also addresses the need for QoS and security in an IP environment, which are critical for operator-grade telephony. The 3G PS multimedia terminal is a cornerstone of the IMS architecture, which was introduced to provide a standardized, access-independent platform for multimedia services.

Historically, the transition from circuit-switched to packet-switched telephony required significant standardization effort to adapt SIP/SDP for mobile networks, including handling of mobility, radio resource management, and policy control. The 3G PS multimedia terminal specifications (26.235, 26.236) define the terminal requirements, ensuring that devices can support IMS-based services from the outset.