Signalling for IMS and MBMS Integration Filipe CABRAL PINTO1, Michael KNAPPMEYER2, Adel AL-HEZMI3 Portugal Telecom Inovação S.A., R. José F. P. Basto, P-3810-106 Aveiro, Portugal Tel: +351 234 403 200, Fax: +351 234 424 723, Email:
[email protected] 2 Mobile & Ubiquitous Systems Group, UWE, Bristol, UK BS16 1QY, England Tel: +49 541 969 3453, Fax: +49 541 969 13453, Email:
[email protected] 3 Fraunhofer Institute FOKUS, Kaiserin-Augusta-Allee 31, 10589 Berlin, Germany Tel: +49 30 3463 7109, Email:
[email protected] 1
Abstract: In the last years, the demand for multimedia services increased significantly. Mobile TV was recently lunched in several countries resulting already in commercial success. The 3rd Generation Partnership Project (3GPP) specifies two different systems for delivering multimedia content: The IP Multimedia Subsystem (IMS) and the Multimedia Broadcast Multicast Service (MBMS). IMS is the 3GPP evolved vision of the core network, providing resource, admission and charging control. MBMS enables broadcasting and multicasting packet data in UMTS networks to large user groups. Up to now, these completely independent systems share no common interfaces. In 3GPP Release 7 and 8 an efficient integration of IMS and MBMS is currently being studied. Within this paper the signalling procedures for an effective IMS and MBMS integration are presented. They support the provision of multimedia sessions utilising unicast, multicast and broadcast bearers. Keywords: Multicast, Broadcast, MBMS IMS Integration, Signalling
1
Introduction
Today, most of the mobile communication is built upon means of dedicated connections, i.e. a communication path between a sender and a receiver. This mode of communication is known as unicast. However, the high demand for the same streaming services by groups of mobile users made this type of communication obsolete due to the high resource consumption. Unicast services are not suited for weighty mass communications. In order to utilise the network resources in an efficient way, 3GPP introduced MBMS in its Release 6. MBMS is the system employed to broadcast and multicast data over the UMTS (and GPRS) infrastructure to user groups. For sure, MBMS is a promising opportunity for the provision of Mobile TV services. Since the target traffic is multimedia, it makes sense to expect some interactions with the IMS. 3GPP Release 5 introduced IMS as a controlling subsystem for IP traffic in UMTS networks. It was developed to provide multimedia services over different access technologies. The main gains of having this additional subsystem are threefold: IMS may provide Quality of Service (QoS) in the user plane since it is aware of the service requested by the user; knowing the exact service, the operators may also improve their charging schemes for multimedia sessions; finally, IMS allows integrating different services offering the customers more and enhanced services with improved flexibility. But IMS was designed to solely deal with point-to-point connections. For services involving groups of users receiving the same information this limitation may become a drawback. As an example, within a multiparty conference, a point-to-point connection is created between the media server and each user. An improved solution is to create a common bearer connecting the media server to all the participants simultaneously. By using a multicast bearer both radio and core network resources are saved. When looking into IMS and MBMS, potential synergies between both systems become obvious: IMS may
control the sessions using Session Initiation Protocol (SIP) signalling while MBMS serves for delivering multicast data efficiently. For that reason, there is a need for an enhanced and integrated architecture providing new interfaces, protocols and functionalities in order to control the multicast bearers used to deliver multicast content to a converged core network. This paper aims at enhancing the service provision procedures while taking into account an improved signalling scheme. The introduced integrated IMS/MBMS framework will allow Mobile TV services to be optimally distributed. The remainder of this paper is structured as follows. Section 2 gives an overview of the relevant reference technologies. In section 3 the proposed integration of IMS and MBMS is explained. The required signalling flows are then presented in section 4. Finally, section 5 concludes the paper.
2
Technology Overview
The following subsections present the IMS and MBMS reference technologies. Their main characteristics which need to be considered for building a combined framework are described in the following sub-sections. 2.1
IP Multimedia Subsystem
Within Release 5 of 3GPP, the IMS appeared as an extension of the UMTS architecture. It adds a set of functionalities linked by standardised interfaces. IMS utilises – among other protocols – the IETF protocol SIP in order to manage and control multimedia sessions. It is able to provide QoS by means of resource reservation and allows operators to apply new charging schemes for multimedia traffic. Finally, IMS makes the integration of different services possible, hence offering a large number of varieties and enriched services to the end customers. The IMS architecture is presented in Figure 1. Further details can be taken from [9]. 2.2
Multimedia Broadcast Multicast Service
Introduced in Release 6 of 3GPP, MBMS is the system used to do broadcast and multicast IP packets over UMTS and GPRS networks to large user groups [10]. Hence, MBMS allows the unidirectional transport of information from a single source to multiple recipients (point-to-multipoint). The specified reference architecture allows an efficient utilisation of the network resources by sending the same packets to all MBMS users simultaneously. The introduction of MBMS into the UMTS network forces the modification of several network elements and, as a result, new interfaces and protocols are defined. In particular, a new entity is specified: the Broadcast Multicast Service Centre (BM-SC). The BM-SC is responsible for the management of the broadcast and multicast content that is inserted into the UMTS network. It provides functionalities for the provision and delivery of MBMS user services. The MBMS architecture is illustrated below in Figure 2.
Figure 1: R5 IMS Architecture
Figure 2: R6 MBMS Architecture
3
IMS and MBMS Integration
Several functionalities of IMS and MBMS overlap or are even duplicated. Potential synergies need to be mapped by the proposed integrated framework. The main deliberations regarding an optimal combination of both systems are explained below. 3.1
R7 and R8 IMS and MBMS Integration
The integration between IMS and MBMS is being considered in a study item within 3GPP Release 7 and Release 8 [6]. The idea behind is to have multicast bearers supporting IMSbased applications leading to radio and core network savings by mean of resources sharing. As stated in [6], IMS users and CPs may use the multicast bearers for delivering multimedia content simultaneously to a group of subscribed users. The 3GPP follows two approaches related with the MBMS and IMS integration. The first one, more conservative, is to have an integrated architecture where IMS is used to control MBMS bearers. The integrated architecture should preserve as much as possible both systems, making only slight enhancements in order to have some communication paths between IMS entities and BMSC. No new entities are foreseen, although there is the need for new interfaces. The IMS applications will use multicast bearers established by the BM-SC to deliver their content to subscribed users. Figure 3 presents the integrated architecture. Within the second approach, the MBMS and IMS integration is carried out by distributing the BM-SC functionalities among several IMS entities. The architecture is similar to the IMS subsystem, but some of the IMS entities are evolved to support a set of features which were out of their scope. They are enhanced in order to support all the functionalities needed to manage broadcast and multicast transmissions. The Application Server (AS) should be enhanced to support all the issues related to the service enablers, such as service announcement and group management, and the MRF should be set with multicast and broadcast capabilities. An integrated architecture encompassing all the functionalities originating from MBMS and IMS is depicted in Figure 4.
Figure 3: R7 and R8 Integration Architecture (BM-SC as an entity)
Figure 4: Integration Architecture (distributed BM-SC)
3.2
Logical Framework
IMS is considered as the emerging technology for fixed and mobile convergence. In an MBMS and IMS integrated scenario, users become allowed to use unicast, multicast and broadcast bearers to delivery multimedia content to a group of IMS users saving network resources. The main purpose of the European Information Society Technology (IST) research project “C-MOBILE” is to provide enhancements to the MBMS for systems beyond 3G at Radio, RAN and Core Network levels [2]. Hence, the integration of MBMS and IMS was an important area of research. Within C-MOBILE, a layered framework was developed, which is in line with the existing standard frameworks and architectures and offers a complete end-to-end design [8]. It comprises five layers, namely (1) Access and Transport Plane, (2) Delivery Plane, (3) Control Plane, (4) Service Enabler Plane, and finally the (5) Application Plane, along with the two end domains, namely the User Plane and the Content Provider Plane. Figure 5 illustrates this logical framework. The Access and Transport Plane encompasses both the Radio Access Network (RAN) and the Core Network (CN) providing QoS and mobility support. The Media Delivery Plane is in charge of media stream processing and media and content relaying from the CP to the user over a unicast, multicast or broadcast bearer. The Control Plane consists of evolved IMS control entities. It is responsible for session management for unicast, multicast and broadcast sessions. It also controls the Delivery Plane entities. The Service Enabler Plane provides a set of service capabilities, such as group management, presence and location. The Application Plane allows applications to make use of the capabilities provided by the enablers by means of well defined API offering personalised services to the end-users. The User Plane refers to the end-user who receives the content while being connected to the mobile network. On the other side, there is the Content Provider Plane, which refers to the responsibility of the content that is to be delivered to the end user.
Figure 5: C-MOBILE Logical Framework
4
IMS and MBMS Signalling
IMS signalling was originally developed for establishing unicast connections between users. In order to allow the utilisation of multicast and broadcast bearers for delivering multimedia content to a group of IMS users, the signalling procedures have to be revised. The next sections present an enhanced signalling for the proposed MBMS/IMS integration. 4.1
Service Activation
The multicast Service Activation is the process by which a subscribed user joins a multicast group indicating her or his interest in receiving the data of a specific multicast bearer service. MBMS standards assume a join procedure [5], which is mandatory in order to allow the network to configure the entire multicast path. That is why the network needs to store all the user and bearer contexts for a long time before the data transmission can begin. When considering the MBMS and IMS integration, a possibility was to consider the SIP Invite message as a trigger to the join procedure, as defended in [7]. After negotiating the codecs and the multicast address, the user should perform the MBMS join procedure. Although using a SIP Invite message allows integration between unicast, multicast and broadcast connections, this approach carries some drawbacks: all the MBMS contexts are created a long time before the data delivery phase, and furthermore, there may be a large set of active SIP sessions waiting for the data transfer to take place. In this paper a different approach is defended. As referred in Figure 6, a user having interest in one of the services will request it by sending a SIP Message to the AS containing only the Service Identification. This way, the network becomes aware of the users’ interest in the multicast service. The main advantage here is that all the session establishment and contexts creations are postponed to a point of time just before session start, hence saving network resources. Furthermore, it allows a deeper degree of flexibility and an easier content adaptation. Because the multicast address, which defines the content and its format, is only provided at the beginning of the session, it makes an accurate dynamic group creation possible e.g. by using algorithms based on context awareness. This way, the user belongs to a group being created just before the session start and not to a group which was created when the user joined the service (cp. Figure 6).
Figure 6: Service Activation Procedure
4.2
Session Start
The Session Start is the phase when the data is ready to be sent and is performed for every data transmission. All the sessions are established during this phase; furthermore, the network resources are reserved. The Session Start procedure is presented in Figure 7. Before sending the data to the UEs (User Equipment), the network needs to first create the multicast path. The AS first triggers the MDF (Media Delivery Function) to allocate the required resources. For the purpose of adaptation content might be delivered over multiple
multicast addresses with different qualities. With this context, the AS sends an invite message to the MDF for each multicast session (IP multicast). Then the AS starts inviting the joined users by sending a SIP Invite message containing the Session Description Protocol (SDP) for the session, which should include the multicast address that is allocated to deliver the content with a specific quality. The AS knows the terminal capabilities by checking the HSS, where the terminal is registered at the moment of the transmission. This will increase the degree of system flexibility because it enables a user to enjoy the service with different types of devices. If the UE accepts the parameters, it proceeds with the MBMS Service Activation process, which creates the UE contexts associated to one of these multicast addresses over the involved network elements. The authorisation for the UE is now given by the Policy and Charging Rules Function (PCRF) instead of the BM-SC, as in the equivalent MBMS procedure. After the activation of the MBMS context, the UE sends the SIP 200 OK message including the activated multicast address to the AS, which in turn acknowledges this by sending an ACK message. In order to avoid scalability problems, the network should not invite all the users at the same time. A random procedure should be used for solving this issue. After inviting all subscribed users the AS acts as a third party call control between the Content Provider (CP) and the MDF in order to establish a session between them. Therefore, the AS starts by sending a SIP Invite message without any SDP to the CP. The CP answers by sending a 200 OK message containing its SDP. This SDP is forward by the AS to the MDF using a SIP Invite message. At this time the MDF starts the MBMS Session Start Procedure as described in [5]. The MBMS Packet Data Protocol (PDP) Contexts are set to Active and the network resources become unavailable for other services. A multicast bearer is activated between the MDF and all the signed UEs. Thereafter the MDF sends the 200 OK message to the AS including the SDP parameters. The AS acknowledges both the MDF and the CP entities. At this moment the session between the CP and the MDF is created. 4.3
Session Stop
When there is no more data to send for a specific period of time, the multicast bearer is released, freeing the network resources (cp. Figure 8). As in the Session Start phase, the AS acts as a third party call control between the CP and the MDF in order to release the session between them. The AS sends a Bye message for both the CP and MDF closing the existing sessions. The CP may also trigger session stop procedure by sending first the bye message and accordingly the AS release the session as described before. The MDF shuts down the multicast bearer by triggering the MBMS Session Stop Procedure as described in [5]. The network resources are released, although the user SIP dialogs remain active until the user Service Deactivation phase takes place, or until the GGSN performs the deregistration triggering the end of the SIP dialogs with the users. 4.4
Service Deactivation
The Service Deactivation phase [5] takes place every time a user intends to leave a specific service, i.e. whenever the user stops being member of a multicast group. Therefore, the user becomes blocked to receive multicast data of the related MBMS bearer service. For leaving the service, the UE starts by sending a Bye SIP message addressed to the public SIP URI of the service (cp. Figure 9). The AS receives this message and responds by sending a 200 OK indicating the end of the session. If the user is the last one subscribed to the multicast service, the AS instructs the MDF to release the multicast bearer. In the meanwhile, the UE starts the MBMS Service Deactivation signalling in order to release the established UE context. If the UE is the last UE using the service, the PCRF should be informed about the end of the multicast session just after the MBMS deregistration procedure.
Figure 7: Session Start Procedure
Figure 8: Session Stop Procedure
Figure 9: Service Deactivation Procedure
5
Conclusions
In this paper, an extension of the IMS with multicast support was proposed. Therefore, the required signalling flows between the involved IMS entities were described in detail. The presented approach allows saving network resources by creating all the sessions and contexts just in time instead of being created much earlier than the actual content delivery. We introduced the C-MOBILE layered framework where the MBMS functionalities are distributed amongst evolved IMS components instead of having two separated subsystems. This approach reduces the investment of deploying new network components and optimises the usage of network resources by delivering multimedia content over multicast transmission mode. Furthermore, extending the IMS with multicast and broadcast capabilities facilitates the delivery of interactive and personalised multimedia applications like Mobile-TV. Currently an end-to-end demonstrator is under development. Based on that, we will consider the evaluation of this approach as future work. Critical key issues are the delay of the end-to-end session establishment and the overall system performance.
6
References
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