Packet Flow Management

Description

Packet Flow Management (PFM) is a core functional process within the 2G (GSM/EDGE) and 2.5G (GPRS) packet-switched network architecture. It encompasses the signaling procedures and associated protocols used to control the establishment, modification, and teardown of packet data transfer resources, known as Packet Flows or Packet Data Protocol (PDP) contexts, between a Mobile Station (MS) and the network's Serving GPRS Support Node (SGSN). A Packet Flow represents a logical communication path with specific Quality of Service (QoS) attributes for transporting user data packets. The PFM procedures are primarily executed using the GPRS Mobility Management and Session Management (GMM/SM) protocols over the radio interface and the GPRS Tunneling Protocol for control (GTP-C) within the core network.

The process begins with the MS initiating a PDP Context Activation procedure, which is a key PFM function. This request, sent to the SGSN, includes desired QoS parameters. The SGSN then interacts with the Gateway GPRS Support Node (GGSN) via GTP-C to create a GTP tunnel and establish the end-to-end bearer. The SGSN also coordinates with the radio access network (BSS) to allocate necessary radio resources (Temporary Block Flows) on the Um interface. The PFM framework defines specific message flows and timers (like Packet Flow Timers, PFT) to manage the state of these contexts. For example, a network-initiated PDP Context Modification procedure allows dynamic adjustment of QoS during an active session, while a Deactivation procedure releases all associated resources.

PFM operates in conjunction with other management planes like Mobility Management (MM) and Radio Resource (RR) management. It is responsible for managing multiple concurrent Packet Flows for a single MS, each potentially with different QoS profiles, enabling services like simultaneous web browsing and voice over IP. The SGSN acts as the central controller for PFM, maintaining the PDP context states, enforcing QoS, and managing the mapping between the MS's logical PDP contexts and the physical radio and core network bearers. This management is crucial for efficient resource utilization, supporting different traffic classes (conversational, streaming, interactive, background), and providing the foundation for mobile internet connectivity in pre-3G networks.

Purpose & Motivation

Packet Flow Management was created to introduce and manage packet-switched data services in the originally circuit-switched-only GSM network. Before GPRS, GSM supported data via circuit-switched data calls, which were inefficient for bursty, internet-style traffic as they dedicated a full timeslot for the entire session duration. The development of GPRS required a new paradigm for resource management that could handle 'always-on' connectivity, dynamic resource allocation, and multiple parallel data sessions with varying quality needs.

PFM solved the critical problem of establishing and controlling the 'virtual connections' or bearers for packet data. It provided the necessary signaling framework to set up a PDP context—a fundamental concept that binds the MS's IP address to a specific QoS profile and a GTP tunnel towards the GGSN. This was a radical departure from circuit-switched permanent connections. PFM enabled the network to allocate radio and core resources on a per-packet basis (in blocks) rather than dedicating a continuous channel, dramatically improving spectral efficiency for data. Its procedures for modification and deactivation allowed the network to adapt to changing user needs and efficiently reclaim resources, which was essential for scaling early mobile data services. PFM laid the essential groundwork for the evolved packet core concepts that would later appear in 3G UMTS and 4G LTE.

Key Features

• Manages the lifecycle of PDP Contexts (activation, modification, deactivation)
• Utilizes GMM/SM signaling over the radio interface and GTP-C in the core
• Coordinates resource allocation between the MS, BSS, SGSN, and GGSN
• Supports multiple concurrent Packet Flows with independent QoS profiles
• Enables dynamic QoS negotiation and modification during a session
• Defines and uses Packet Flow Timers (PFT) for state and resource management

Evolution Across Releases

Defining Specifications