Enabling Multimedia Broadcast/Multicast Services over Converged Networks N. Baker *, M. Zafar *, A. Al-Hezmi **, M. Fuchs ***

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1 Enabling Multimedia Broadcast/Multicast Services over Converged Networks N. Baker *, M. Zafar *, A. Al-Hezmi **, M. Fuchs *** *Mobile & Ubiquitous Systems Group, CCCS Research, UWE, Bristol, UK BS16 1QY ** Fraunhofer Institut FOKUS, Kaiserin-Augusta-Allee 31, 10589, Berlin, Germany *** Bamboo Media Casting, Hasharon Road 12, P.O.B 5035, Kfar-Saba 44150, Israel {madiha.zafar, Abstract The C-MOBILE (EU, IST FP6 Call 4) project central aim is to evolve MBMS technologies towards the beyond 3G vision of a converged global network based on the usage of multiple broadcast transport bearers. Architecturally, the work targets a converged, multi-bearer, service architecture making use of one or more broadcast technologies and systems for content and service delivery. This paper presents the research objectives and discusses the issues and challenges in enabling multimedia broadcast/multicast services over converged networks. Index Terms Mobile Multimedia, Multicast/Broadcast, Service Delivery, Next Generation Networks, Convergence. I. INTRODUCTION Evolution of mobile communications is driven by many technology factors but four in particular are of significance. One is the ability of devices to be able to send and receive digital multimedia content and the second is the convergence of all types of networks towards the IP protocol stack. A third is ability of devices to offer many air interfaces and perform handovers between wireless access points. The fourth is sensor and location technologies that provide the possibility of context aware communication. The consequence of the first for mobile communications is a gradual shift from the basic model of offering point-to-point voice centric services into more complex service provision platforms that offer multimedia content from Mobile TV and digital audio to video clips and photographs. The combination of the first two leads to an interesting situation where the mobile terminal becomes more of a broadcastmulticast receiving device. The third allows the possibility of seamless streaming multimedia mobility between devices and networks. The fourth opens up the possibility of more sophisticated context aware group communication and interaction such as ad-hoc communities and ubiquitous gaming. Group communication of this type is important in the evolution of communication. The evolution of multimedia applications and services has initiated the need for a converged network from both the wireless and wired domains. These services ranging from conventional TV broadcasting to personalized file transfer, traditional service-based multicast groups to context-aware ad-hoc communities are to be delivered via IP protocol. Enabling convergence of telephony, data and video/tv services so that they can be accessed over any type of network is often referred to as triple play in fixed line telecommunications. A similar mobile triple play vision exists in the mobile communications world. Provisioning of multimedia streaming services can easily be offered over several access technologies in a stove pipe model. However, enabling interactive and personalized streaming services in an integrated model to deliver across any access network requires a cooperative framework within the network infrastructure. The problem is how can current mobile communication infrastructures evolve to support these types of communication and related services? There are ongoing activities by several organization bodies such as TISPAN [1], 3GPP [2], OMA (Open Mobile Alliance)[3] and DVB [4] to standardize an interactive and personalized streaming subsystem. The following sections cover the reference technologies and systems (e.g. TISPAN, MBMS, IMS, OMA BCT and other enablers), convergence issues across the architectures and functional layers and the critical research topics to be covered to support Broadcast/Multicast in a future converged environment. The emphasis is on 3GPP MBMS as the target multicast-broadcast bearer. II. KEY CORE NETWORK AND SERVICE ARCHITECTURES A. 3GPP IMS IMS [5] is a major milestone in the evolution and convergence of mobile networks. Its main functions can be classified into four categories: Multimedia session management. Service activation/initiation on the application server. Network information provisioning to IMS applications.

2 Enable applications to make use of network functionalities through open standardized interfaces. IMS has a layered architecture, which consists of delivery control and service planes as illustrated in figure 1. RAN Fig. 1. IMS Reference Architecture CS Networks External IMS Network WLAN, ADSL cable, etc. B. 3GPP Multimedia Broadcast Multicast Service MBMS In December 2005, 3GPP (3rd Generation Partnership Project) completed Release 6 of its specification including MBMS (Multimedia Broadcast/Multicast Services)[6]. MBMS is primarily regarded as a unidirectional point to multipoint bearer service which allows data to be transmitted from a single source entity to multiple recipients and is loosely based on IGMP. 3GPP has defined two modes of operation: the broadcast mode and the multicast mode. In the case of broadcast mode, transmissions take place regardless of user presence in a given area while in multicast mode, transmissions take place in areas where there are subscribers to serve. As a result MBMS multicast mode is associated with subscription and authorization prior to group joining. Broadcast services are locally activated on the terminal while in multicast mode a multicast group has to be joined before service data reception is possible. Furthermore, 3GPP classifies two types of MBMS User Services according to the method used to distribute services. Streaming services provide a stream of continuous media (e.g. audio, video). File download services are used to deliver binary file data over an MBMS bearer. File delivery may use carousels in order to deliver file data repeatedly. (Note: As an example this mode is used in delivery of the Electronic Service Guide as defined by OMA BCT). MBMS extends the existing UMTS (Universal Mobile Telecommunications System) and GPRS (General Packet Radio Service) architecture. Almost all affected nodes can be enhanced by software updates to provide the necessary functionalities of MBMS. Figure 2 shows the MBMS reference architecture. A completely new functional entity is added: the (Broadcast/Multicast Service Centre) serving as central controlling unit. It is responsible for providing membership functions, security functions, proxy and transport functions and controls session establishment and data delivery over the two added reference points Gmb and Gi. [6] [8]. UE Um UTRAN Iu/Gb UE Uu GERAN Iu Gn/Gp Fig. 2. MBMS Reference Architecture C. OMA Service Framework Gmb BMSC Gi Content Provider PDN (e.g. Internet) The Open Mobile Alliance (OMA) specifies an OMA Service Environment (OSE), which is a flexible and extensible architecture that offers support to a diverse group of application developers and service providers. OSE specifies enablers, which provide standardized components to create an environment in which services may be developed and deployed. The OMA enablers, the decomposition into these components and the interactions between them comprise the OSE. Figure 3 illustrates the layered architecture of the OSE and OMA Enablers. Applications Policy Enforcers Bindings Enablers Digital Rights Management Fig. 3. OMA Reference Architecture OMA / 3 rd Party Applications Billing Framework Bindings Presence Device Mgmt. BCT Location Operator / Terminal / Service Provider Resources There are a large number of enablers defined or partially defined. There are a number of particular relevance BCT, Presence, Transcoding, Group List Management and IMS Utilisation.[9] D. TELECOM AND INTERNET CONVERGED SERVICES AND PROTOCOLS FOR ADVANCED NETWORKS ( TISPAN) The architectures and service frameworks discussed so far have been very much mobile communication centric. The Telecom and Internet converged Services and Protocols for Advanced Networks (TISPAN) delineates a generic multiservice, multi-protocol, multi-access IP-based framework of converged networks as Next Generation Network (NGN) architecture. The fabrication of TISPAN NGN is based on various sub-systems which allow integration of other existing and/or emerging sub-systems and is illustrated in figure 4. The first release of the TISPAN architecture [10] specified several subsystems on the transport layer and services layer. The two new and relevant sub-systems;

3 Network Attachment Sub-System (NS) and the Resource and Admission Control Sub-System (RACS) are responsible for IP connectivity and Quality of Service (QoS), respectively. On the service layer the 3GPP IMS is adopted to support broadband access (DSL) and the cellular network. The PSTN/ISDN Emulation Subsystem is defined to emulate PSTN/ISDN legacy services over IP infrastructure with IMS core as its base sub-system. Support for advanced context-aware group services such as ad-hoc communities and interactive broadcast services are also explored A logical service framework distilled from each of the above architectures is required on which to hang our research and development programme. The following needs to be taken into consideration while proposing such a logical framework. These are: Offer a complete end-to-end design Consider and cater for the NGN principles; requirements and constraints Inline with existing standard frameworks and architectures With these considerations in mind together and utilising the key layers from the standard frameworks the logical architecture within which to organize this work materializes as shown in figure 6. Application Plane Service Enabler Plane Fig. 4. TISPAN NGN Reference Architecture [10] The next release of TISPAN NGN will introduce further infrastructure and service standardization to enable service convergence at the data plane; voice, video and data to support the triple play vision. Client RACS NS Broadcast/Multicast Access Networks (MBMS, DVB-H, etc) Content Provider III. C-MOBILE: AN EVOLVED MULTICT BROADCT ARCHITECTURE True network convergence would envisage multicastbroadcast and group services delivered across several access networks with the added possibility of seamless service mobility as illustrated in figure 5. Fig. 5. C-Mobile Vision: Converged Broadcast/Multicast Architecture The strategic objective of C-MOBILE [11] project is to foster the evolution of multicast-broadcast services and in particular MBMS towards this converged network vision. Fig. 6. C-MOBILE Logical Broadcast/Multicast Service Architecture The layered framework model clearly partitions the major functions inline with the key standard architectures. These layers are: Access Network Plane: Corresponds to any access network such as illustrated in figure 5. For resource reservation and admission control into these networks the core interacts with the TISPAN NS (Network Attachment Sub-System) and the TISPAN RACS (Resource and Admission Control Sub-System) and provides resource feedback through NS. : responsible for delivery to the access networks and implemented using base IMS core functions. It primarily initiates and terminates IP-based unicast, multicast and broadcast bearer with quality of services support. For resource reservation and admission control the core interfaces with the RACS. It is also responsible for resource scheduling and content distribution, access congestion control and content adaptations along with the control plane. It provides the multi access convergence to support access mobility and exposes interfaces to the underlying heterogeneous access technologies for signaling and data delivery. : Control across all access networks again using IMS. It handles the registration of the endpoints and routing of signaling messages to the appropriate application server while guaranteeing QoS across all services. It also maintains the unique service profile for each end user.

4 Service Enabler Layer: corresponds to the OMA OSE and the key enablers such as Presence, Location, B-CT and Policy. The enablers can be concatenated to develop more complex services and applications. Application Layer: Applications will usually make use of the functionality provided by the service enabler layer. User might interact with the application directly for configuration purpose. Content Provider Plane: operations are to mange and provide life or stored content. The role of content provider should be dynamic and flexible. So that, a normal user might be assigned to the role of content provider when he/she offers content; e.g. a user wants to stream a live captured content to his community. Therefore related interfaces have to be defined and specified. User/client Plane: presents the user connected via fixed or mobile access technologies. Client Service Enabler Plane Control Plane RACS NS Scheduling & Congestion Control Application Plane Multicast Group Management Transcoding Broadcast/Multicast Radio Access Technologies Session Management Location based Delivery IMS MBMS Integration Content Provider In Scope Out of Scope Fig. 7. C-MOBILE WP4 Key Work Items IV. EMERGING RESEARCH ISSUES Although converged, next-generation, IMS based service technology and mobile multiple broadcast technologies have received a great deal of attention there still appear to be numerous points that warrant investigation and research. It is beyond the resources of one team to analyse all the issues. The approach is based on developing next-generation usage scenarios which will most likely require new or modified protocols, interfaces, delivery capabilities and service enablers. One or two scenarios will be selected to implement, develop and test on a demonstrator system built using partners subsystems, services and content. The objective is to identify enablers that will lead to the goal of IMS integration with MBMS and towards delivery of multicast-broadcast services across access networks. Consequently the identified research objectives can be categorized into two. The first focuses on expanding the understanding of the overall converged broadcast-multicast architecture. The need to reach a more detailed view of the overall architecture and answer questions such as how broadcast functions are mapped to IMS architecture, how IMS architecture can assist content services with additional session information, what entities and interfaces are required. The second category tries to identify key points, problems, issues and opportunities that require enhancement either due to issues with current technology or resulting from the assumptions under-taken in the newly defined architecture. These include issues such as new scheduling capabilities, group and session management as well as allowing for fine-grained (e.g. location-based) content adaptations. Some of these are stand alone studies which may not be included in the demonstrator. Figure 7 illustrates the possible span across layers of these topics. The following sections discuss some of these issues in both of these two categories in more detail. A. MBMS converged Architecture Integration Issues Considering IMS as the emerging technology for fixed and mobile convergence it follows that the main requirement is to enable the sending of multimedia content to a group of IMS users using a multicast-broadcast bearer wherever possible. IMS delivery is basically unicast so the possible benefits of using an underlying bearer such as MBMS are obvious. 3GPP have also recently identified this as an important study item [12]. The integrated architectural implications are evident when studying the separate architectures of 3GPP MBMS and IMS; see figure 8. 3GPP MBMS Planes NodeB RNC Bearer Plane Mapping Fig. 8. 3GPP IMS and MBMS Planes 3GPP IMS Planes A first architectural option is that IMS applications use MBMS preserving as much of its functionality and structure as possible. This is the approach in the 3GPP study item [7] the architecture of which is shown in figure 9.

5 NodeB RNC Bearer Plane MFRP Fig. 9. Architecture of IMS applications using MBMS as a bearer There are a number of issues to be resolved but perhaps the most prominent is how the IMS knows that MBMS is supported by and where the UE is located. The IMS should be able to find out the capabilities of the UE. New signaling procedures will be required to enquire about MBMS and UE capabilities and initiate an MBMS bearer alongside the corresponding tear down procedures. There will be consequences for IMS functionalities and service provisioning. In the situation where there are a number of possible multicast-broadcast bearers, as shown in the generic architecture of figure 5, then the IMS must decide some how which multicast-broadcast bearer to use. The decision must be based on the particular application, UE capabilities, and QoS parameters. The implications of the Long Term Evolution (LTE) [13] and Systems Architecture Evolution (SAE) [14] raise more issues concerning integration of MBMS into an all IP evolved core. The evolution towards a common IP/IMS transport over all access networks will most likely see core network type functionality migrate to this layer. Particularly this applies to the role of the new MBMS BMSC entity introduced under Release 6 of 3GPP. The is concerned with user service provision and delivery. It provides service announcements, authorization for UEs requesting to activate an MBMS service, it makes available to the transport associated parameters such as QoS and it is in charge of initiating and terminating MBMS bearer resources prior to and following transmission of MBMS data. The is able to send MBMS data, taking into account the integrity and confidentiality of the information. A is able to schedule MBMS session transmissions and retransmissions. Further responsibilities are securing access to service data by means of ciphering and key-distribution together with authentication and authorisation of content from external sources. It would seem reasonable to suppose that this functionality would be handled by IMS network entities as illustrated in figure 10. How this is achieved and the issue involved is another area of study. Bearer Plane 3GPP MBMS Planes NodeB RNC Functions Membership Function Security Function Service Announcement Function Proxy & Transport Function Session & Transmission Function 3GPP IMS Planes Fig. 10. Function Abstract Mapping over IMS Entities A discussion on the impact of next generation networks would not be complete without reference to TISPAN. Of particular interest are the Network Attachment Sub-System (NS) and Resource and Admission Control Sub-System (RACS) functions highlighted in figures 11. The functionalities in these entities enable an IMS to connect to any access network or multicast broadcast access network in our case. Similar functionality exists in 3GPP core network but of course the scope is narrowed to that domain. The objective of TISPAN is to enable convergence so that telephony, data and video/tv services can be accessed over any type of network. An objective for 3GPP LTE is mobile triple play. However these end-to-end streaming subsystem architectures are not yet standardized. Work is in progress in TISPAN on streaming subsystem architectures with groups working on an IMS based IPTV architecture. TISPAN NGN Reference Architecture Network Attachment Subsystem Resource & Admission Control Subsystem NS IP Addr. Provisioning 3GPP IMS Architecture Authentication RACS Admission Control Resource Reservation NAT Traversal Authorization Policy Control N/W Configuration Figure 11: TISPAN Entities Abstract Mapping over IMS Entities So in conclusion to this section the convergence and integration research objectives in this activity may be summarized as the follows: To investigate how IMS applications can use MBMS and to clarify interfaces and interactions. To investigate the design of common functions and generic access to different multicast-broadcast bearers.

6 To investigate end to end streaming subsystems To investigate support for interactive mass feedback. Personalisation support for multicast. B. Service Enabling and Provisioning Issues Multicast-Broadcast service enabling issues are spread across all layers and levels. The OMA have paid particular attention to application and service enabler layers. These together with a number of other provisioning issues have been identified as follows:- 1) OMA Service Environment and Enablers A major objective is to investigate if selective OMA enablers will be sufficient to support our applications, what are the implementation issues and what additional enhancements are required. 2) Scheduling & Congestion Control MBMS provides an efficient mean to distribute data simultaneously to large user groups. However, the network resources, and in particular the capacity of the radio link, is rather small compared to the bandwidth capabilities at the network core. The ability to schedule transmissions when possible and to adapt content and data to the network's present resources could improve the cost/performance of MBMS and other broadcast technology usage. Adaptive transmissions can offer best-effort minimally acceptable quality in congested areas with better quality transmissions in areas where more bandwidth is available. Scheduling of transmissions for non real-time content can smooth resource usage peaks based on predetermined policies. These can assume the ability of handsets to store and play content. Finally there is the issue of scheduling taking place at the radio network where packets from different services with different QoS parameters, priorities, bit-rates and error-rates need to be scheduled on the radio bearer. 3) Multicast Group Management There are two main objectives in this activity. One is to investigate dynamic and ad-hoc creation of groups based on context and to ascertain the feasibility of offering such services and how. The second is to investigate existing enablers to build group based services. The focus is more on multicast group type applications rather than broadcast applications. The multicast group management research items can be roughly categorized as follows:- Context-aware Ad-hoc Communities User-centric Multicast Source Management Dynamic Multicast Address Allocation Multicast Service Announcement Efficient Multicast Delivery Model Robust multicast/unicast switching mechanism It is interesting to explore the synergies between these apparently independent research items. The novel concept of context-aware ad-hoc communities correlate multiple group management tasks like group creation and deletion based on group membership rules while keeping it inline with the group s profile. However, in order to support such service an underlying service for multicast address allocation and service announcement is mandatory. An enriched user experience can be achieved if the multicast service delivery model is efficient and the network intelligently switches between unicast and multicast transmission modes. 4) Session Management This activity deals with the signalling supporting service delivery in an integrated MBMS-IMS environment supporting network convergence. This activity plans to consider how to control multi-bearer streaming sessions over a reference access network or even several access networks. To that end, session management activity will manage and maintain session context (e.g. user feedback) that may also be used by other mechanisms. The work is divided into two main sub-activities. The first is to evaluate the set of IETF protocols comprising the IMS architecture with regards to MBMS architecture and decide whether it is possible to use these protocols for session management in MBMS within the integrated MBMS-IMS environment. The second activity has to do with bearer and RAN selection mechanisms necessary in the reference multibearer, multi-access network. 5) Transcoding Management MBMS lacks in flexibility of supporting multiple streams for either location-specific transmissions or potentially for layered applications scalability such as layered video codecs. Providing the ability to multiplex multiple streams on a bearer would open the way to usage of layered applications and allow for their use, where appropriate. The objective of this research item is to identify the scenarios in which layered applications may be used and to "fill the gaps" in MBMS to allow for layered application by providing a unified multiplexing approach. V. CONCLUSION The C-MOBILE project central aim is to evolve MBMS technologies towards the beyond 3G vision of a converged global network based on the usage of multiple broadcast transport bearers. Within C-mobile activities, Work-package 4 has been defined with the goal of putting together the "key ingredients" for a converged, multi-bearer, service architecture while identifying a number of key issues and problems to be researched, optimized or resolved. The work is done with reference to inputs from work-package 2 on the usage scenarios and requirements for future mobile services using mobile broadcast. The work so far has been directed at identifying the key issues, existing technologies, architectural framework and research objectives. Architecturally, the work targets a converged, multibearer, service architecture making use of one or more broadcast technologies and systems for content and service delivery. The project has chosen to focus on usage of IMS integration with MBMS looking at existing efforts such as TISPAN and OMA for possible directions. A layered functional framework mapping functional areas to different layers is provided.

7 Research objectives to be dealt with include detailed architecture work, resource scheduling and congestion control, dynamic group management, session management for broadcast/multicast sessions in a multi-ran environment and enhancements to MBMS delivery capabilities. Work continues on a more detailed architectural description and plans for a working demonstrator. ACKNOWLEDGMENT This work was performed as part of WP4 EU-IST project C-MOBILE (Advanced MBMS for the Future Mobile World) ( under the contract IST We are thankful to all the partners for their contributions. This project is also working on MBMS and RAN enhancements. REFERENCES [1] Telecoms & Internet converged Services & Protocols for Advanced Networks [2] 3 rd Generation Partnership Project [3] Open Mobile Alliance [4] Digital Video Broadcasting [5] 3GPP, TS IP Multimedia Subsystem; (stage 2), May [6] 3GPP TS , Multimedia Broadcast/Multicast Service; Stage 1, Release 6 [7] 3GPP TS , Multimedia Broadcast/Multicast Service; Protocols and Codecs, Release 6 [8] 3GPP TS , Multimedia Broadcast/Multicast Service; Architecture and Functional Description, Release 6 [9] Mobile Broadcast Service Architecture, Version 1.0, Open Mobile Alliance, CT/Permanent_documents/ [10] DTR 00001, Telecommunications and Internet Converged Services and Protocols for Advanced Networking (TISPAN); Release 1 Definition [11] C-MOBILE project web site: [12] 3GPP TR , Enhancements to IMS service functionalities facilitating multicast bearer services, September 2006 [13] 3GPP TR "Requirements for Evolved UTRA (E-UTRA) and Evolved UTRAN (E-UTRAN)", March 2006 [14] 3GPP TS "Service requirements for evolution of the system architecture", October 2006