Session

Description

In 3GPP, a session is a fundamental concept that represents a logical connection or an active period of interaction between two or more parties, or between a user and the network. It provides the necessary context for managing resources, state, and data flows. The term is used across various domains, from core network packet-switched connections to application-level multimedia sessions and MBMS bearer sessions. The definition has evolved significantly from a general trusted relation to a precise resource in the xMB API.

In the context of the IP Multimedia Subsystem (IMS) and Session Initiation Protocol (SIP), a session is established using SIP signalling. It involves a set of multimedia senders and receivers and the data streams flowing between them. The Session Description Protocol (SDP) is used to describe the session parameters, such as media types, codecs, and IP addresses. A session can comprise multiple media streams, and a user can be invited to the same session multiple times. The session is identified by a combination of elements from the SDP origin field, including the user name, session ID, network type, address type, and address.

For packet-switched (PS) communications, a session is defined as a logical connection between parties. This term is preferred over 'call' which is associated with circuit-switched systems. This logical connection enables the exchange of data packets and is managed by protocols like GPRS Tunnelling Protocol (GTP) and Policy and Charging Control (PCC) rules. The session context includes bearer information, QoS parameters, and charging data.

In the context of MBMS and the xMB API, a session is a resource exposed by the RESTful API. It represents one or more time intervals during which an MBMS bearer is active for transmitting service content from the Broadcast Multicast Service Centre (BM-SC) to the UE. Each session instance contains properties such as session type, state, data rate, permitted delay, user plane ingestion mode, targeted delivery area, and reporting parameters. This allows content providers to schedule and manage multicast/broadcast transmissions efficiently.

Sessions are also central to WAP and MExE, where a session is established between the mobile and the WAP gateway, lasting from a second to years, and can be associated with a subscription. Overall, the session concept provides a stateful context that enables authentication, authorization, accounting, mobility, and QoS management across diverse 3GPP technologies.

Purpose & Motivation

The session concept exists to provide a stateful context for communication and service delivery in telecommunications networks. It solves the problem of managing resources and state for ongoing interactions, which is essential for authentication, authorization, accounting, and policy enforcement. Without sessions, each transaction would be independent, making it impossible to apply consistent QoS, charging, or security policies over time.

Historically, circuit-switched systems used the term 'call' to denote a connection. As networks evolved to packet-switched and IP-based technologies, the term 'session' became necessary to describe logical connections that are not tied to a physical circuit. This shift allowed for more flexible, multimedia-capable communications, as exemplified by SIP and SDP in IMS.

In the context of MBMS and the xMB API, the session resource was introduced to allow content providers to manage broadcast/multicast transmissions programmatically. This addresses the need for efficient delivery of popular content to many users simultaneously, reducing network load. The session resource encapsulates all necessary parameters for the BM-SC to schedule and transmit content, enabling dynamic and scalable multicast services.