Advanced Multicast and Broadcast Content Distribution in ... - CiteSeerX

Advanced Multicast and Broadcast Content Distribution in Mobile Cellular Networks Michael Knappmeyer

Björn Ricks, Ralf Tönjes

Mobile & Ubiquitous Systems Group, CCCS Research University of the West of England Bristol, UK

Faculty of Engineering and Computer Science University of Applied Sciences Osnabrück Osnabrück, Germany

Adel Al-Hezmi Fraunhofer Institute FOKUS Berlin, Germany Abstract—Recently 3GPP (Third Generation Partnership Project) has standardised MBMS (Multimedia Broadcast Multicast Services) enabling broadcast and multicast transmissions over GPRS (General Packet Radio Service) and UMTS (Universal Mobile Telecommunications System). Hence it makes an efficient usage of radio resources possible. 3GPP and 3GPP2 introduced the specification of IMS (IP Multimedia Subsystem) and MMD (Multimedia Domain), receptively, which both are responsible for resource, admission and charging control. It allows for cost efficient and flexible provision of enriched multimedia services over IP networks. Up to now the controlling IMS and MBMS are separated subsystems sharing no common interfaces. This paper introduces both systems and depicts ongoing standardisation activities regarding their interworking. Finally, it proposes a service provision architecture and describes required signalling flows which enable the provision of multicast streaming using the MBMS specifications not only as access bearer technology. Thus, we present a step towards the evolution of IMS enabling it with multicast and broadcast capabilities. Keywords- Multimedia Streaming, Multicast, Broadcast, Mobile Cellular Networks, MBMS IMS Integration, IMS Signalling, Service Architecture, Content Distribution

I. INTRODUCTION The evolution of mobile cellular networks focuses on the provision of enriched multimedia services and the support of QoS (Quality of Service) guarantees. The Network Operators envisage a cost efficient distribution of contents to large recipient groups and would like to add location based and context aware functionalities. MBMS and IMS are the most promising European candidates for fulfilling their requirements of ubiquitous availability, user interactivity and personalisation. Regarding the trend of distributing user generated content, they furthermore enable a subscribed user to become CP (Content Provider) himself. MBMS (as specified in 3GPP Release 6) primarily cares for efficient utilisation of radio resources while IMS was developed to control multimedia sessions based on IP delivery, i.e. it provides admission and policy control. Moreover, IMS offers the

This work was supported by the European IST project C-MOBILE which aims at enhancing MBMS for systems beyond 3G.

possibility of easy and flexible service creation by using service enablers. Each enabler offers a set of basic functionalities and by their composition and extension, novel services and applications can be generated individually. Some of them are standardised by OMA (Open Mobile Alliance), e.g. a service enabler providing broadcast functionalities entitled BCAST [1]. In this paper we present a promising approach of combining the advantages of both MBMS and IMS by designing an integrated service provisioning framework and defining common service enablers. This work has been conducted within the European IST project C-MOBILE that aims at enhancing MBMS at the RAN (Radio Access Network) and at the CN (Core Network) for systems beyond 3G. With this regard C-MOBILE is working on the provisioning of interactive and personalised mobile services with QoS and mobility support as well as ensuring a smooth migration path for the following 3GPP releases. The next section gives a brief overview of the reference technologies. Section III analyses the functionalities of both IMS and MBMS and proposes the derived integrated architecture. The most relevant signalling flows are depicted on the basis of an exemplary case study before section IV finally draws a summarising conclusion. II.

REFERENCE TECHNOLOGY OVERVIEW

A. 3GPP Multimedia Broadcast Multicast Service (MBMS) In Release 6 3GPP standardised MBMS [2][3][4] to support efficient broadcast and multicast IP packet delivery in existing UMTS and GPRS networks. Concerning the RAN point-to-multipoint transmissions in downlink direction are introduced in order to save radio resources. MBMS allows two modes of operation: the broadcast mode and the multicast mode. In broadcast mode, transmissions take place regardless of user presence in a defined area, whereas in multicast mode solely those areas are supplied where subscribers need to be served. Consequently, MBMS multicast mode is associated with subscription and authorisation prior to group joining.

While in multicast mode a group has to be joined before service data reception is possible, broadcast services are locally activated by the UE (User Equipment). Furthermore, MBMS classifies three basic types of user services according to the method used to distribute them: (1) streaming services providing a stream of continuous media (e.g. audio, video), (2) file download services used to deliver binary files and (3) carousel services whose content (either download files or streaming media) is periodically retransmitted. The MBMS reference architecture is shown in Fig. 1. Note that the three depicted planes are not part of standardisation but are included for facilitating the functional analysis as explained in section IV. In comparison to previous 3GPP releases the BM-SC (Broadcast/Multicast Service Centre) is added as completely new functional entity serving as central controlling unit. It is connected to the GGSN (Gateway GPRS Support Node) over the interfaces Gmb and Gi. The former provides access to the control plane functions, the latter to the bearer plane. The current MBMS specification defines eight phases as depicted in Fig. 2. The Subscription phase refers to establishing a contractual relation. The actual service is then subscribed by the Joining phase in which a user joins the recipient group. If the user is not interested in receiving the content any more, he is able to quit by initialising the Leaving phase. These three phases are solely needed for delivery in multicast mode. In case of broadcast mode, the remaining five phases (cp. Fig. 2) suffice. The Session start and Session stop phases build a frame around the actual Data transfer. The optional MBMS notification may inform about any forthcoming data transfer. This separation between the Joining and the Data transfer phase allows for maximum flexibility and avoids injecting the network with a high load of joining messages at a beginning delivery session. B. 3GPP IP Multimedia Subsystem (IMS) The IMS [5][6][7] was designed as overlay control subsystem for IP based multimedia applications. It supports the delivery of mobile multimedia services in a sub-domain of the packet switched domain of the GPRS/UMTS CN. Initially, the introduction of IMS in Release 5 addressed cellular networks as the basis of IP connectivity but solely unicast transmissions were considered. The layered IMS reference architecture (Fig. 3) comprises user, control and service plane and includes several reference points between the IMS core, services deployed on AS (Application Server) and access network entities. The main signalling of IMS is build upon the SIP (Session Initiation Protocol) [8]. This application layer protocol standardised by IETF is used to establish, control, modify and terminate multimedia sessions between two or more participants. S e r vice P la n e Gmb

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Figure 2. MBMS phases.

SIP is based on a request-response transaction model, whereat each transaction contains a request invoking a particular method on the server and at least one response. For service provisioning, several interfaces are defined between the AS and the IMS core. At the ISC (IMS Service Control) reference point the AS can behave either as SIP user agent, SIP proxy, back–to-back SIP user agent or third party call control agent. The Sh reference point connects the AS and the HSS (Home Subscriber Server) whose communication is based on the DIAMETER protocol. It enables the AS to obtain user data or to identify the S-CSCF (Serving Call Session Control Function) to which the SIP request is sent. To ensure content delivery with a specified QoS the IMS core connects the GGSN through the reference points Go (Release 6) and S7 (Release 7) via the PDF (Policy Decision Function) and the PCRF (Policy Control and Charging Rules Function), respectively. C. OMA Service Framework OMA (Open Mobile Alliance) specifies an OSE (OMA Service Environment) as a flexible and extensible architecture, offering support to a diverse group of application developers and service providers. OSE specifies enablers which provide standardised components to create an environment in which services may be developed and deployed. The OMA enablers, the decomposition into these elements and the interactions between them complete the OSE. One of these enablers is the BCAST (Mobile Broadcast Services) enabler that offers multicast and broadcast capabilities for other OMA enablers. OMA BCAST deals with a variety of functional areas: (1) distribution and definition of an ESG (Electronic Service Guide), Ut

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(2) methods for broadcast file distribution and stream distribution, (3) mechanisms for service and access protection with/without content protection, (4) service interaction, (5) service provisioning, (6) terminal provisioning and (7) user and application notification. Requirements for the above functional areas along with the requirements for the enabler as whole are given in the BCAST requirements specification [1]. III.

IMS MBMS INTEGRATION

Considering IMS as the emerging technology for fixed and mobile convergence, the main requirement is to enable the delivery of multimedia content to a group of IMS users using a multicast/broadcast bearer where appropriate. IMS provides a common platform for various access technologies. Since IMS delivery is restricted to unicast transmissions, it would obviously benefit from using an underlying bearer such as MBMS. MBMS in Release 6 did not consider IMS and thus did not utilise any of its functionalities. However, its integration is considered as a study item within Release 7 [9][10]. Currently, 3GPP follows two different approaches to integrate MBMS in IMS. The first is to maintain the BM-SC and extend functionalities by adding SIP signalling, whereas the second approach tries to distribute the BM-SC functionalities (as specified in Release 6) among enhanced IMS entities. In C-MOBILE the latter is considered and a service provisioning architecture is developed. The identified MBMS BM-SC functionalities are mapped onto the IMS layered architecture. Hence, the BM-SC is removed from the architecture and a long term solution for the next generation multimedia content distribution is envisaged instead. A. Functional Analysis of MBMS and IMS Currently many overlapping functions are specified in MBMS and IMS. The BM-SC in MBMS provides five defined functions [2]: (1) Membership function, (2) Session and Transmission function, (3) Proxy and Transport function, (4) Service Announcement function and (5) Security function. The Membership function is responsible for authorisation and user charging. Furthermore, it manages user subscription data. The Session and Transmission function is mainly in charge of scheduling of content delivery sessions. After the CS (Content Server) is authorised and authenticated, its content is stored for optional retransmission to the UEs. The Session and Transmission function reserves resources on the delivery path in order to maintain the defined QoS. Therefore it establishes and terminates MBMS bearer resources based on service requirements and stores MBMS context information. The Proxy and Transport function acts as a gateway between the circuit and the packet switched domain. It consists of a Proxy function managing the control plane (Gmb interface) and a Transport function managing the content delivery over the multicast path (Gi interface). In addition, it generates charging records for CP charging. The Service Announcement function provides media and session descriptions prior to service establishment. 3GPP has defined several mechanisms for service announcing, e.g. SMS, HTTP, WAP or MBMS itself. The Security function is in charge of service protection, also known as digital rights management. This includes ciphering of media streams and security key distribution to authorised users.

The main functions of the IMS core can be classified into three categories: (1) Management and control of multimedia sessions covering registration, session establishment and teardown; (2) Bearer control including end-to-end quality of service, authentication, authorisation and charging; (3) Service provisioning on the AS facilitating network information and network capabilities. In conclusion the BM-SC maintains functionalities that are already covered by IMS. In particular the Membership function is implemented in the S-CSCF and HSS, e.g. the subscription data could be stored in user profiles and the S-CSCF performs the authorisation check. The QoS reservation, part of the BMSC Session and Transmission function, is provided by the PDF and GGSN. In IMS QoS reservation consists of DiffServ and IntServ mechanisms. The IMS entity MRF (Media Resource Function) is able to combine media streams or to act as a conference bridge. These responsibilities are related to the Proxy and Transport function. The MRF may be extended to support additional MBMS content processing such as transcoding. Only the Session scheduling function, the Proxy function with Gmb signalling and the Service Announcement function have to be introduced in IMS. B. Integrated Architecture A simplified version of the proposed integrated framework is depicted in Fig. 4. The Access and Transport Plane corresponds to the RAN and CN. It consists of several IP routers and the network dependent access technology. In case of UMTS this plane contains the GGSN, the SGSN (Serving GPRS Support Node) and the UTRAN (UMTS Terrestrial RAN). Therefore our proposal will reuse the existing MBMS interfaces Gmb and Gi. The MDF (Media Delivery Function) is composed of MDFC (MDF Controller) and MDFP (MDF Processor). The latter is responsible for media processing and media relaying from the CS to the UE over IP-based unicast, multicast and broadcast bearers. Thus, the MDFP is controlled by the MDFC and fulfils the tasks of the Proxy and Transport function of the original BM-SC. The MDF is placed as a mediator between the CS (Content Server) and the Access and Transport Plane. It is introduced as an evolved Media Resource Function to provide additional MBMS specific signalling. Besides, it is responsible for resource scheduling, content distribution, congestion control and content adaptation. Session control and session negotiation take place in the IMS-based Control Plane in collaboration with the Service Plane. This includes security and access control, QoS provisioning and creation of charging records. Therefore, the original MBMS Session and Transmission function is logically split into several sub-functionalities. The lower layered IP transport functionalities (e.g. IP packet scheduling, resource reservation) are maintained by subordinated entities in the control plane. The high layered functionalities (e.g. service scheduling) are realised by service enablers located in the Service Plane. The Service Plane is moreover in charge of service control covering bearer selection and service authorisation. It also offers service capabilities such as group management, announcements, content protection and location based services.

The described distribution of functionalities in respect to the involved level of granularity results in a flexible framework. The utilisation of service enabler structures allows an easy and fast creation of novel services. Applications usually make use of the functionalities provided by the Service Plane to offer the end-user combined services. A more detailed description of the C-MOBILE architecture is included in [11].

The CS adds information about the content profile (e.g. type of service, genre, codec, duration, data volume) and the point of time when it would like the content to be delivered. The CME in turn requests a time slice by sending a message to the SSE. Based on the forwarded content profile the SSE checks whether the desired start time is available. Therefore, it takes the up to now registered services into account and needs to estimate the network load.

C. Case Study This section gives an overview of the most important signalling flows regarding MBMS multicast mode. They enable content distribution over MBMS bearers. To evaluate our proposed IMS-MBMS integrated framework, one of the CMOBILE usage scenarios called “Content Casts” [12] was used. This service provides access to the latest information anytime and anywhere without the need for manual downloads. Users can subscribe to periodically released content such as video podcasts, weather forecasts or latest news which is automatically sent to the users’ device.

After calculating a possible start time, the SSE offers the CS this time slice. This message is forwarded by the CME. After acceptance by the CS, the SSE reserves this time slice and instructs the SAE (Service Announcement Enabler) to announce the service and make it available for UE subscriptions. When the reserved time of transmission is finally reached, the SSE triggers the SME (Session Management Enabler) to initiate the delivery session. Alternatively, the SSE may not be able to offer the desired time slot. The offer message containing an alternative point of time might be rejected by the CS and the negotiation is cancelled.

1) Service Scheduling The service scheduling tries to determine the optimal delivery time for each service and is maintained by a service enabler and hence provided by AS logic. In the distributed CMOBILE architecture, the SSE (Service Scheduling Enabler) is responsible for assigning the time of delivery. The SSE generally handles the differences between various types of services, i.e. streaming, carousel and file download services and considers their different QoS requirements. The long-term scheduling is especially important for the provision of TV-like services. Its output can be used to produce an ESG for the subscribers. As far as the delivery of small streaming clips and files is concerned, the role of the SSE is to ensure their efficient distribution and to maintain specified time boundaries when the content has to be delivered latest. Regarding the periodical redistribution in the “Content Cast” scenario, the SSE is furthermore in charge of generating an optimal carousel cycle to minimise the average access time. The CS needs to interface the CME (Content Management Enabler). As simplified in Fig. 5, it may request a service to be added to the system and make it available for user subscription or for free of charge broadcasting.

2) Service Delivery For accessing the content, the user activates the “Content Cast” service and then the user waits for the start of the data transmission. This corresponds to the Joining and Session Start phases as described above and constitutes the core of the signalling flows. These flows comprise MBMS, DIAMETER as well as SIP signalling. The files are transferred according to the FLUTE protocol. Fig. 6 illustrates the adapted Joining phase. The User Subscription, Service Announcement, PDP (Packet Data Protocol) and Context Activation procedures remain the same as specified by 3GPP in Release 6 [2]. Then the UE negotiates a session with the SME inside the AS. The SIP INVITE message is routed by the CSCF to the corresponding AS which provides the desired MBMS service. After a user has joined, specific MBMS service charging records can be created based on the business model. The process of billing is followed by the MBMS Multicast Service Activation defined in [2]. This signalling is used to establish a multicast routing tree with the involved GGSNs, SGSNs and RNCs (Radio Network Controller). Every involved entity in this multicast tree creates an MBMS PDP bearer context and changes the context state to “Standby”.

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Figure 5. Service Addition. Figure 4. IMS MBMS integrated Framework.

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When the routing tree is established the SGSN notifies the UE about the success. Then the UE completes the Service Joining procedure with a SIP Acknowledge Message sent to the AS. Afterwards, the UE is able to receive the content and waits for incoming data transfer. The signalling of the Session Start phase is depicted in Fig. 7. Opposed to Joining, the Session Start procedure takes place for every data transmission not for every user. Behaving as a 3rd Party Call Control User Agent, the SME establishes a delivery session to the CS and the MDF. Upon receiving the SIP INVITE message the MDF begins the MBMS Session Start procedure. In this procedure the existing 3GPP scheme is reused. All MBMS PDP context states are set to “Active” and both CN and RAN resources are reserved. Additional SIP signalling between the IMS core and the UE is avoided since this would result in network congestion if a large number of users are addressed (Each user would require a point-to-point bearer to be established). A successful MBMS signalling is indicated by a 200 OK Response. Afterwards the SME sends an Acknowledge SIP Message to the CS and to the MDF. The CS waits for a configurable time (e.g. 23 sec) before it begins to transmit the content. After completing the Session Start signalling, a CS is allowed to send its content. The content is transmitted per unicast to the MRF, where it is optionally transcoded and afterwards transformed to multicast transport for an MBMS bearer. IV.

CONCLUSTION AND FUTURE WORK

In this paper we presented an overview of the existing technologies and architectures for IP based media transport in UMTS networks. Combining the MBMS and IMS subsystems we presented a solution to enable the distribution of enriched multimedia content to large user groups. Therefore we analysed the standardised architectures and revealed overlapping functions. UE

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Figure 7. IMS Signalling for MBMS Session Start Phase.

Finally, a converged MBMS/IMS framework was introduced and an exemplary scenario was described. The integration of MBMS in the IMS sub-domain will ease the development and management of interactive mobile broadcast services. In the future, IMS could evolve towards a common control layer to integrate other IP based broadcast bearers in a converged IP based network architecture. This paper represents the results of the analysis and design phase of the C-MOBILE project. The evaluation of the architecture and the signalling flows is envisaged for the next project period. REFERENCES [1]

Open Mobile Alliance, “Mobile Broadcast Services Architecture”, Version 1.0, March 2006 [2] 3GPP TS 23.246: “Multimedia Broadcast/Multicast Service (MBMS); Architecture and functional description (Release 6)”, V6.12.0, June 2007 [3] S. Parkvall, E. Englund, M. Lundevall, J. Torsner, “Evolving 3G mobile systems: broadband and broadcast services in WCDMA”, IEEE Communications Magazine, Volume 44, Issue 2, Feb. 2006, pp. 30-36 [4] R. Rummler, A.H. Aghyami, Yun Won Chung, “A multicast mechanism for UMTS”, IEEE Wireless Communications, Volume 13, Issue 4, Aug. 2006, pp. 94-103 [5] 3GPP TS 23.228, “IP Multimedia Subsystem (IMS); Stage 2 (Release 6)”, V6.16.0, Mar. 2007 [6] A. Cuevasm, J.I. Moreno, P. Vidales, H. Einsiedler, “The IMS service platform: a solution for next-generation network operators to be more than bit pipes”, IEEE Communications Magazine, Volume 44, Issue 8, Aug. 2006, pp. 75-81 [7] G. Camarillo and M.-A. Garcia-Martin, “The 3G IP Multimedia Subsysstem (IMS): Merging the Internet and the Cellular Worlds”, Hoboken, NJ: Wiley, 2004 [8] IETF RFC 3261, “SIP: Session Initiation Protocol” [9] 3GPP TR 23.847, “Study on Enhancements to IMS Service Functionalities Facilitating Multicast Bearer services; (Release 8)”, V1.0.0, Feb. 2007 [10] 3GPP TR 23.882, “3GPP System Architecture Evolution: Report on Technical Options and Conclusions (Release 7)”, V1.8.0, Feb. 2007 [11] C-MOBILE, Deliverable 4.2, “Draft Specification & Requirements of Enhanced Core Network Elements”, Dec. 2006 [12] C-MOBILE, Deliverable 2.2, “Scenarios and Market Requirements”, Mar. 2007