Packet Data Protocol Context

Description

A PDP Context is a fundamental concept in GPRS and UMTS networks, representing the session state for a packet data protocol (PDP) connection between a Mobile Station (MS) and a Gateway GPRS Support Node (GGSN). It is essentially a set of information stored in both the MS and the GSNs (SGSN and GGSN) that defines the characteristics of the data session, including the PDP address (IP address), the Access Point Name (APN), the Quality of Service (QoS) profile, and other parameters such as the PDP type (IPv4, IPv6, IPv4v6). The PDP Context is identified by a PDP Context Identifier (a numeric value) and is used to route user data between the MS and the external packet data network. Without an active PDP Context, the MS cannot send or receive IP packets.

The establishment of a PDP Context involves a signaling procedure called PDP Context Activation. The MS initiates this by sending an Activate PDP Context Request to the SGSN, which includes parameters like the APN, PDP type, and requested QoS. The SGSN validates the request, selects a GGSN based on the APN, and sends a Create PDP Context Request to the GGSN. The GGSN allocates an IP address (if not provided by the MS) and returns a Create PDP Context Response. The SGSN then sends an Activate PDP Context Accept to the MS, completing the procedure. At this point, the PDP Context is active in the MS, SGSN, and GGSN, and user data can flow. The context remains active until it is deactivated, either by the MS (e.g., when the application closes) or by the network (e.g., due to inactivity or policy).

Each PDP Context is associated with a specific QoS profile, which defines parameters like maximum bit rate, guaranteed bit rate, and traffic class. The network can modify the QoS of an active PDP Context through a PDP Context Modification procedure, which may be triggered by the MS, SGSN, or GGSN. This allows dynamic adaptation to changing network conditions or application requirements. The PDP Context also includes the PDP address, which is the IP address assigned to the MS for that session. In dual-stack scenarios (IPv4v6), a single PDP Context can have two IP addresses (one IPv4 and one IPv6), as specified in Release 9 and later. This was an enhancement over earlier releases where only one PDP address was supported per context.

The PDP Context is stored in the MS and GSNs as a set of information elements. In the MS, it is managed by the Non-Access Stratum (NAS) layer. In the SGSN and GGSN, it is part of the packet data protocol state machine. The context includes the Tunnel Endpoint Identifier (TEID) for GTP tunnels, which are used to carry user data between the SGSN and GGSN. The SGSN maintains a mapping between the PDP Context and the radio access bearer (RAB) for the MS, ensuring that data is correctly routed over the air interface. When the MS moves, the SGSN may change, but the PDP Context can be transferred to the new SGSN through a Routing Area Update procedure, maintaining session continuity.

In EPS (LTE), the PDP Context concept was largely replaced by the EPS Bearer and PDN Connection, but it still exists for interworking with legacy GPRS/UMTS networks. When a UE moves from LTE to UTRAN/GERAN, the PDN Connection is mapped to a PDP Context to preserve the IP session. The PDP Context is defined in 3GPP TS 23.060 (GPRS service description) and related specs like TS 29.060 (GTP) and TS 29.119 (GTP' for charging). Understanding PDP Context is essential for engineers working on legacy networks, as it forms the basis for packet data session management in 2G and 3G.

Purpose & Motivation

The PDP Context was created to manage packet data sessions in GPRS and UMTS networks, providing a standardized way to establish, maintain, and release IP connectivity for mobile devices. Before its introduction, circuit-switched data was the norm, which was inefficient for bursty IP traffic. The PDP Context introduced a packet-switched approach, allowing multiple users to share network resources and enabling always-on connectivity. It solved the problem of how to allocate and manage IP addresses, QoS, and routing for mobile data sessions in a scalable manner.

One of the key motivations was the need to support different QoS levels for various applications. The PDP Context includes a QoS profile that can be negotiated and modified, allowing operators to offer differentiated services. It also enabled the concept of multiple PDP Contexts per MS, so a user could have simultaneous sessions to different APNs (e.g., Internet and corporate intranet) with different IP addresses and QoS. This was a significant advancement over previous technologies.

Furthermore, the PDP Context facilitated roaming and interworking with external packet data networks by using the APN to identify the target network. It also provided a framework for charging, as each PDP Context could be tracked for data volume and duration. The evolution of the PDP Context to support dual-stack (IPv4v6) in Release 9 addressed the growing need for IPv6 transition. Although EPS introduced the PDN Connection, the PDP Context remains relevant for legacy interworking and is still defined in 3GPP specifications. In essence, the PDP Context was the foundation for mobile packet data, paving the way for the all-IP architectures of LTE and 5G.