1.1. MBMS Functional Layer and Delivery Method Overview
MBMS functional layers
- Three distinct functional layers are defined for the delivery of MBMS-based service. They are Bearers, Delivery method and User service. Figure below depicts these layers with examples of bearer types, delivery methods and applications
- Functional Layers for MBMS User Service:
- User service / Application: The MBMS User service enables applications. Different applications impose different requirements when delivering content to MBMS subscribers and may use different MBMS delivery methods. As an example a messaging application such as MMS would use the download delivery method while a streaming application such as PSS would use the streaming delivery method
- Delivery Method: When delivering MBMS content to a receiving application one or more delivery methods are used. The delivery layer provides functionality such as security and key distribution, reliability control by means of forward-error-correction techniques and associated delivery procedures such as file-repair, delivery verification. Two delivery methods are defined, namely download and streaming. Delivery methods may be added beyond release 6. Delivery methods may use MBMS bearers and may make use of point-to-point bearers through a set of MBMS associated procedures.
- Bearers: Bearers provide the mechanism by which IP data is transported. MBMS bearers as defined in 3GPP TS 23.246  and 3GPP TS 22.146  are used to transport multicast and broadcast traffic in an efficient one-to-many manner and are the foundation of MBMS-based services. MBMS bearers may be used jointly with unicast PDP contexts in offering complete service capabilities.
Two Delivery Methods in MBMS:
- Two delivery methods are defined in the present document - the download delivery method and the streaming delivery method.
- MBMS delivery methods make use of MBMS bearers for content delivery but may also use the associated procedures.
- Use of MBMS bearers by the download delivery method: The File Repair Procedure and the Reception Reporting Procedure may be used by the download delivery method.
Overview of Download Delivery Method
- The MBMS Download Delivery Method allows the error-free transmission of files via the unidirectional MBMS Bearer Services.
- The files are
- The network triggers the transmission since the users are registered to the download service. There is no further user request necessary after the service registration.
- Files may contain multimedia components or any other binary data.
- The MBMS Download Delivery Method allows the transmission of an arbitrary number of files within a single data transfer phase.
- The file transmission events are organized in MBMS Download Sessions.
- Each session is started with a File Delivery Table (FDT) instance, which describes in this example each file within the MBMS Download Session in terms of file name and file type (MIME Content Type).
- The service operator and the actual service would determine the timing of MBMS Download Sessions.
- Users of the download-based MBMS User Service are not required to recognize the MBMS.
- Downloading process itself, the reception may be happening completely in background. The user may be only informed about the completion of a MBMS download and that there is new content available on the terminal.
- Depending on the service, the MBMS Download session may require time-critical delivery of content.
Transport of MBMS download data
- The files can be constructed for and transported during a FLUTE session.
- The BM-SC takes a file, e.g. a video clip or a still image, which is used as the transport object for FLUTE.
- The BM-SC constructs source block by breaking the file into contiguous portions of approximately equal size.
- Each source block is broken into source symbols. One or more encoding symbols are carried as the payload of a FLUTE packet, thus the encoding symbol size must divide the FLUTE packet size.
- The target FLUTE packet size is configured by the BM-SC and, together with the file size, is used to determine the encoding symbol length.
- Figure : Constructing FLUTE packets
- The BM-SC communicates the transport object length, the encoding symbol length and the file size to the receivers within the FLUTE session transmission. Thus the receiver can also calculate the source block structure in advance of receiving a file.
- The FLUTE packet is constructed from FLUTE header and payload containing one or more encoding symbols.
- The distinction between file and transport object is that the file is the object provided to the BM-SC and played-out or stored at the MBMS UE.
The usage of the MBMS session Identity in file download delivery
- The usage of the MBMS Session Identity is optional. The MBMS Session Identity is only applicable to MBMS download delivery sessions.
- The MBMS Identity field can be used by the BM-SC and terminal to establish an n-to-m mapping between the files of a file download sessions and their corresponding MBMS bearer sessions over which the files are transported.
- The Session Identity of the MBMS session is provided with the MBMS session start procedure from the BM-SC to the GGSN via the Gmb protocol in the MBMS Session Identity information element.
- According to 3GPP TS 23.246, the UEs are notified about the starting (or ongoing) data transmission through MBMS notification procedure. Both the TMGI and an optional Session ID are paged to the terminals, independent of their current state (idle or connected), to inform them about the starting data transmission. The UEs use the TMGI and the Session ID to decide whether they are interested in the MBMS session or not.
- In case of session repetitions, the BM-SC assigns the same TMGI and Session ID to the MBMS session. This allows the UEs to recognize that the session is repeated and decide not to receive the data in case they already received it correctly.
- The usage of the Session ID field is optional. Both UE and BM-SC may decide to ignore the Session ID field. The UE will then assume that the MBMS session is a new session and make its decision independently. The BM-SC may decide not to use the Session ID field, in which case it will not signal it (Session ID) to the UEs within the FDT (FLUTE File Delivery Table)nor during the notification.
- The BM-SC uses the FDT of a file download session to indicate for a file or a file group the Session ID value of the MBMS session to carry the file(s). The UE establishes a table to save the mappings between the Session IDs and the files. Each record has a validity time, which is signalled by the
Download Delivery Method
- FLUTE usage for MBMS download (26.346)
- FLUTE session setup and control with RTSP (26.346)
- SDP for Download Delivery Method (26.346)
- OMA Push usage for MBMS Download (26.346)
- The MBMS Streaming Delivery Method aims at continuous transmission of data and the immediate play-out via the display and/or the loudspeaker.
- Mobile terminals retrieve transmission details like Multicast IP Address, TMGI and the used ports before the MBMS UEs can activate the reception.
- Upon interaction of the user and when all parameters are known, the UE
- For Release 6 no dynamic bandwidth adaptation is foreseen. The transmission bandwidth is determined by the BM-SC once and not changed during the life-time of the MBMS streaming session.
- The MBMS Streaming receiver application may be started upon user interaction. In the case of immediate play-out upon reception, the user would be fully aware of the MBMS Streaming reception.
- The number of streams per MBMS User Service depends on the multimedia components of the Service.
- Output of the MBMS User Service Receiver is in case of the MBMS Streaming User Service a number of continuous media streams (e.g. audio and video stream).
- The required codecs for reception of MBMS Streaming User Service are provided during the MBMS Service Discovery phase.
Streaming Delivery Method
§ Transport protocol (26.346)
- RTP payload formats for media (26.346)
- FEC mechanism for RTP (26.346)
- General RTP Header Extension Mechanism (26.346)
§ Session description (26.346)
- SDP Parameters for MBMS streaming session (26.346)
- SDP Example for Streaming Session (26.346)
§ Quality of Experience (26.346)
§ Using MBMS Streaming delivery on Unicast (26.346)