In a typical communications network a wireless device, communicates via a Radio Access Network (RAN) to one or more Core Networks (CNs). The communications network may also be referred to as e.g. a wireless communications network, a wireless communications system, a communications network, a communications system, a network or a system.
The wireless device may be a device by which a subscriber may access services offered by an operator's network and services outside operator's network to which the operator's radio access network and core network provide access, e.g. access to the Internet. The wireless device may be any device, mobile or stationary, enabled to communicate over a radio channel in the communications network, for instance but not limited to e.g. user equipment, mobile phone, smart phone, sensors, meters, vehicles, household appliances, medical appliances, media players, cameras, Machine to Machine (M2M) device or any type of consumer electronic, for instance but not limited to television, radio, lighting arrangements, tablet computer, laptop or Personal Computer (PC). The wireless device may be portable, pocket storable, hand held, computer comprised, or vehicle mounted devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another wireless device or a server.
The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station. The base station may be called a Radio Base Station (RBS), evolved NodeB (eNB), NodeB, B node, Radio Network Controller (RNC), Base Station Controller (BSC), Base Transceiver Station (BTS), MCE, depending on the technology and terminology used. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site. The base station communicates with the wireless device(s) within range of the base station.
According to the 3GPP, Multimedia Broadcast Multicast Services (MBMS) “is a point-to-multipoint service in which data is transmitted from a single source entity to multiple recipients. Transmitting the same data to multiple recipients allows network resources to be shared.” MBMS offers two modes: broadcast mode and multicast mode. The MBMS architecture enables efficient usage of radio network and core network resources. evolved MBMS (eMBMS) may be described as the Evolved Packet Core (EPC) version of MBMS. The eMBMS evolution brings improved performance thanks to higher and more flexible LTE bit rates, single frequency network operations, and carrier configuration flexibility.
In MBMS, there are some network nodes or functional entities which are important. Multi-cell/multicast Coordination Entity (MCE) is a network node or functional entity which is responsible for allocation of time and frequency resources for MBMS transmission. The MCE may be co-loated with for example an eNB. Another network node is the MBMS-GW, which is the entry point for incoming broadcast/multicast data traffic. The MBMS-GW broadcasts data packets to all eNBs within an area. Broadcast Multicast-Service Centre (BM-SC) is a network node or functional entity which is necessary in order for a communications network to support MBMS. The BM-SC is in charge of providing service to the end user.
Some of the reference points in MBMS are Sn, SGmb and Sm. Sn is the reference point for the control plane between MBMS-GW and the SGSN. SGmb is the reference point for the control plane between BM-SC and the MBMS-GW. Sm is the reference point for the control plane between the Mobility Management Entity (MME) and the MBMS-GW. M3 Application Protocol (M3AP) supports the M3 interface which is between the MCE and the MBMS GW. A reference point may also be referred to as an interface. Signaling between nodes is exchanged at a reference point.
The purpose of a MBMS Session Start procedure is to request the radio access network to notify wireless devices about an upcoming MBMS Session of a given MBMS Bearer Service and to establish a MBMS Radio Access Bearer (RAB) and MBMS signalling connection for this MBMS Session. The MBMS Session Start procedure is triggered by the core network. For example, the core network initiates the procedure by sending a MBMS Session Start request message to the RNC. The MBMS Session Start request message comprises different parameters. The RNC acts according to the received MBMS Session Start request message. The RNC sends a MBMS Session Start response message or a MBMS Session Start failure message to the core network, depending on the outcome of the procedure.
According to 3GPP, the loss or corruption of the data stored in the aforementioned network entities for support of MBMS service(s) for each MBMS session context which are created by the MBMS Session Start procedure and updated by the MBMS Session Update procedure will seriously degrade the MBMS service(s) offered to mobile subscribers. It is therefore necessary to define procedures to limit the effects of such failure, and to restore the MBMS service with minimized impact to the mobile subscribers. Such restoration procedures are related to failure and/or restart of several types of network nodes and network paths/interfaces, such as e.g. MBMS-GW, MME, SGSN etc. The terms restoration and re-establishment are equivalent.
A failure may be a failure to receive a particular message, failure of a hardware or software component of a network node. A failure may be full/complete or partial. After a node has been restarted, all its bearer contexts are deleted.
3GPP has started a new project called eMBMS restoration procedures, where the objective of this project is to specify enhanced restoration procedures to explicitly define the EPS behavior and to enable restoration of the eMBMS service when possible in order to minimize the end-user service impact upon different kinds of failure over the control path. Examples of such failures are as follows:                MBMS-GW failure/restart        MME/SGSN failure/restart        MCE failure/restart        BM-SC failure/restart        Sm/Sn path failure        M3AP path failure        SGmb path failure        
It has been agreed that the principle of these restoration procedures is to try to re-establish the control path to allow a subsequent MBMS session update and/or MBMS session stop. In addition, such re-establishing may be performed before bringing down the MBMS sessions. But the specific procedures under various failure scenarios are still under discussion.
The system behaviour upon restoration of a MBMS session towards an alternative downstream node during a SGmb path failure remains undefined in current standards.