Packet Switched Domain

Description

The packet switched (PS) domain is a fundamental concept in 3GPP networks, referring to the domain within GSM, UMTS, and LTE networks where data is transferred in packet switched mode. It is the counterpart to the circuit switched (CS) domain, which is used for voice and other circuit-switched services. The PS domain corresponds to the General Packet Radio Service (GPRS) and provides packet data services to mobile users. In this domain, data is divided into packets that are transmitted independently and reassembled at the destination, allowing for efficient use of network resources and support for bursty data traffic.

The PS domain architecture includes several key components, such as the Serving GPRS Support Node (SGSN) and the Gateway GPRS Support Node (GGSN) in GSM and UMTS, and the Serving Gateway (SGW) and Packet Data Network Gateway (PGW) in LTE. These nodes are responsible for routing and forwarding user data packets, managing mobility, and enforcing policy and charging rules. The PS domain also includes the Home Subscriber Server (HSS) for subscriber data management and the Policy and Charging Rules Function (PCRF) for policy control. In 5G, the PS domain concept has evolved into the 5G Core (5GC), which is entirely packet-switched and supports new capabilities like network slicing and ultra-reliable low-latency communication.

The operation of the PS domain relies on the concept of a Packet Data Protocol (PDP) context in GSM and UMTS, or an EPS bearer in LTE. These are logical connections that provide a data path between the user equipment (UE) and the external packet data network (PDN). Each PDP context or bearer has associated QoS parameters that define the treatment of packets within that connection. The PS domain uses IP as the primary protocol for data transport, although other protocols can be supported. The domain also implements mobility management to ensure that data sessions continue seamlessly as the UE moves across different cells and access technologies.

In terms of standardization, the PS domain was introduced in GSM as part of GPRS in the late 1990s and was later enhanced in UMTS with higher data rates and improved QoS. LTE was designed as an all-IP network, eliminating the CS domain entirely and relying solely on the PS domain for all services, including voice (via VoLTE). The 3GPP specifications for the PS domain are found in series 23 (technical realization) and 29 (signalling protocols), among others. The term "packet switched domain" is defined in 3GPP TR 21.905 and other specifications, and its definition has evolved to include LTE in Release 17.

The PS domain is crucial for enabling mobile broadband, Internet access, and a wide range of data services. It provides the foundation for the mobile Internet and has enabled the proliferation of smartphones and data-intensive applications. As networks continue to evolve towards 5G and beyond, the PS domain remains the core of mobile data delivery, with ongoing enhancements to support higher speeds, lower latency, and massive connectivity.

Purpose & Motivation

The packet switched domain was created to provide efficient data services in mobile networks, addressing the limitations of circuit switched data transmission. In circuit switched mode, a dedicated physical connection is established for the entire duration of a call or data session, which is inefficient for bursty data traffic and wastes network resources when no data is being transmitted. The PS domain, based on GPRS, introduced packet switching to mobile networks, allowing multiple users to share the same transmission resources and only consuming bandwidth when data is actually sent. This dramatically improved the efficiency and scalability of mobile data services.

The motivation for the PS domain was to enable new data-centric applications and services, such as Internet access, email, and multimedia messaging, which were not well-suited to circuit switched networks. The PS domain also provided a foundation for the evolution to 3G and 4G networks, which were designed to support higher data rates and more advanced services. In LTE, the PS domain became the sole domain, as the CS domain was eliminated in favor of an all-IP architecture. This simplified the network and reduced operational costs while enabling seamless integration with other IP-based networks.

Today, the PS domain concept continues to evolve with 5G, where the core network is entirely packet-switched and supports diverse use cases with varying requirements. The PS domain has thus been instrumental in transforming mobile networks from voice-centric to data-centric, enabling the mobile Internet and driving innovation in services and applications.