Multimedia Broadcast Multicast Service Download Session

Description

The MBMS download session is a core concept within the Multimedia Broadcast Multicast Service (MBMS) download delivery method. It represents a logical session that encompasses the time, protocols, and protocol state (i.e., parameters) required to define the sender and receiver configuration for the download of content files. In essence, it is the set of rules and parameters that govern how files are transmitted and received over an MBMS bearer. The session is established and managed by the Broadcast Multicast Service Center (BM-SC) and is announced to user equipment (UE) through MBMS service announcement procedures. The download session ensures that all participating receivers have a common understanding of the delivery parameters, enabling them to correctly receive and reconstruct the files.

Architecturally, the MBMS download session operates within the MBMS bearer service, which can be in broadcast or multicast mode. The session defines the use of the File Delivery over Unidirectional Transport (FLUTE) protocol, which is built on top of the Asynchronous Layered Coding (ALC) protocol. FLUTE provides the mechanisms for file segmentation, forward error correction (FEC), and file reconstruction. The download session includes parameters such as the session start and end times, the transport addresses (e.g., IP multicast address and port), the FLUTE session parameters (e.g., FEC scheme, file delivery table), and security parameters. These parameters are conveyed to the UEs via the MBMS service announcement, which can be done using various methods such as the Service Announcement Protocol (SAP) or the Open Mobile Alliance (OMA) Push mechanisms.

The download session is identified by a session identifier, which is used to correlate the session announcement with the actual data transmission. The session can be scheduled to occur at specific times, allowing UEs to tune in only when the session is active, thereby saving battery power. The session may also be repeated to ensure that UEs that missed the initial transmission can still receive the content. The BM-SC is responsible for managing the session lifecycle, including initiation, modification, and termination. It also handles the synchronization of the session across multiple MBMS bearers if the content is delivered over a large area.

Key components of the MBMS download session include the session description, which is typically an XML document that follows the schema defined in 3GPP TS 26.346. This description contains all the necessary information for a UE to join the session and receive the files. The session description is delivered to the UEs via the service announcement, which can be broadcast or unicast. The UEs use this information to configure their FLUTE receivers and to join the appropriate multicast group or tune to the broadcast bearer. The session also includes file repair procedures, which allow UEs to request missing file segments via unicast connections, ensuring reliable delivery.

In operation, the MBMS download session begins with the BM-SC creating a session description and announcing it to the UEs. The UEs that are interested in the content join the session by configuring their receivers according to the session parameters. The BM-SC then transmits the files using FLUTE, and the UEs reconstruct the files from the received packets. If a UE detects missing packets, it may initiate a file repair procedure to request the missing data. The session ends when all files are delivered or when the BM-SC terminates the session. Throughout the session, the network manages the radio resources efficiently, ensuring that the shared bearer is used optimally.

The MBMS download session has evolved over multiple 3GPP releases, with enhancements in areas such as improved FEC schemes, support for larger files, and integration with newer radio technologies like LTE and 5G. It remains a vital tool for efficient content distribution in mobile networks, particularly for applications that require high reliability and scalability.

Purpose & Motivation

The MBMS download session exists to provide a structured and standardized way to configure and manage the delivery of files over MBMS. Without a well-defined session concept, it would be challenging for senders and receivers to coordinate the transmission and reception of files, especially in a point-to-multipoint environment where many receivers must be synchronized. The session defines the common parameters and protocols that all parties must adhere to, ensuring interoperability and efficient use of network resources. It solves the problem of how to efficiently distribute the same content to multiple users without requiring individual unicast connections, which would be inefficient and costly.

Historically, before the introduction of MBMS, there was no standardized mechanism for group file delivery in mobile networks. Early methods like Cell Broadcast Service (CBS) were limited to short messages and lacked the flexibility and reliability required for large file transfers. The MBMS download session, introduced in Release 8, provided a comprehensive framework for file delivery, leveraging IP multicast and advanced protocols like FLUTE. It addressed the need for a scalable and reliable method to deliver files to many users simultaneously, which is essential for services such as mobile TV, software updates, and group communications.

The session concept also enables efficient use of radio resources by allowing the network to schedule transmissions at specific times and to manage the session lifecycle. It supports features such as session repetition, which improves reliability by allowing UEs that missed the initial transmission to receive the content later. Additionally, the session includes security parameters that ensure the content is protected during transmission, addressing the need for secure delivery in commercial services. Overall, the MBMS download session is a critical enabler for efficient and reliable group content delivery in mobile networks.