Multicast/broadcast network convergence in next generation mobile networks

Transcription

1 Available online at Computer Networks 52 (2008) Multicast/broadcast network convergence in next generation mobile networks Justino Santos a, Diogo Gomes a, Susana Sargento a, Rui L. Aguiar a, *, Nigel Baker b, Madiha Zafar b, Ahsan Ikram b a Instituto de Telecomunicações, University of Aveiro, Aveiro, Portugal b Mobile and Ubiquitous Systems Group, CCCS Research, UWE, Bristol BS16 1QY, United Kingdom Available online 8 September 2007 Abstract The 3GPP Multimedia Broadcast Multicast Service (MBMS) aims to introduce group communications into the 3G networks. One of the current key challenges is how to evolve these incipient features towards the beyond 3G vision of a converged global network where multimedia content can be delivered over one or more selected broadcast transport bearers. This paper presents potential multicast/broadcast technologies convergence and discusses the issues and challenges in moving towards this next generation network vision from the viewpoint of evolving MBMS. Ó 2007 Elsevier B.V. All rights reserved. Keywords: IMS; MBMS; Multimedia and broadcast; Service enablers 1. Introduction Multimedia applications and services range from conventional TV broadcasting to personalized content delivery, from traditional service-based multicast groups to context-aware gaming communities. Convergence of telephony, data, and video/tv services, in order to access media services over any type of network, is often referred to as triple play in fixed line telecommunications. Convergence of communications, media and broadcast industries towards common technologies has opened up significant business potential by offering entertainment media and broadcast content to mobile users. * Corresponding author. address: ruilaa@ua.pt (R.L. Aguiar). A similar mobile triple play vision exists in the mobile communications world. Here, an added attraction is that broadcast/multicast techniques offer cost efficient delivery of content to large audiences in bandwidth-limited mobile radio access networks. Wireless access networks, with the continuous technology evolution, will provide the means of efficiently delivering data to several users increasingly across several different access technologies such as MBMS, DVB-H, WiMax or WiFi. Provisioning of multimedia streaming services (e.g., live TV) can easily be offered over several access technologies in a stove pipe model. However, enabling interactive and personalized streaming service delivery in an integrated model via any access network requires a cooperative framework within the network infrastructure /$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi: /j.comnet

2 J. Santos et al. / Computer Networks 52 (2008) The aim of the Next Generation Networks (NGNs) is to handle diverse types of services across different types of access technologies. The main tenant of NGN architecture is that it allows decoupling of the network s transport and service layers. This means that, whenever a provider wants to enable a new service, it can do so by defining it directly at the service layer without considering the transport layer, thus making services independent of any transport details. This is a business environment being heavily pursued in the wired domain, limited solely by regulation restrictions. The problem is: how can current mobile communication infrastructures evolve to support these types of communication and related services? True network convergence would envisage multicast broadcast and group services delivered across several access networks with the added possibility of seamless service mobility, as illustrated in Fig. 1. The solution for this problem is engaging many telecommunications standards groups, most notably ITU NGN Focus Group [1], Telecoms and Internet converged Services and Protocols for Advanced Networks (TISPAN) [2], 3rd Generation Partnership Project (3GPP) [3], and the Open Mobile Alliance (OMA) [4]. These groups share some similar, but not coincidental, technological visions. This study departs from a Multimedia Broadcast Multicast Services (MBMS) [9] environment, and analyses how such a broadcast multicast service might evolve to meet this overall vision, and how it can be related to (other) standards architectures. Fig. 1. C-mobile vision converged broadcast multicast architecture.

3 230 J. Santos et al. / Computer Networks 52 (2008) This work is being pursued under the C-MOBILE project. The strategic objective of the C-MOBILE project [5] is to foster the evolution of multicast broadcast services and, in particular, the evolution of MBMS towards this converged network vision. In the C-MOBILE project, a clear strategy is defined for integration of MBMS into the converged architecture utilizing a converged control plane based on IMS, as depicted in Fig. 1. An integrated IMS MBMS architecture is defined to transparently support multicast and broadcast services in NGN. This paper starts from standard architectures for the support of next generation networks, and clearly identifies the issues raised by the evolution path of MBMS towards next generation networks, proposing (different stages of) evolved IMS MBMS architectures for multicast/broadcast network convergence. The paper is organized as follows. Section 2 briefly presents standard architectures that aim at the support of specific functionalities of NGN architectures in a mobile environment, such as IMS, TISPAN, OMA and MBMS. Section 3 addresses the IMS MBMS architecture, its functionalities, and integration issues in a NGN architecture. Following these integration issues, Section 4 proposes an evolved integrated approach for the interaction between MBMS and IMS, in order to build future mobile multicast/broadcast services in an NGN environment, as discussed in Section 5. Finally, Section 6 presents final conclusions. 2. Standard architectures In order to deliver services and media content across several access networks as depicted in Fig. 1, overall connectivity is required. Quality of service (QoS) and security must also be guaranteed across these networks. A further level of transport provision required comprises the functions to manage session establishment and control communication such as voice, multimedia and messaging. The complexity of realizing these tasks across technologies makes IP an essential supporting layer for these networks. This section presents standard architectures that aim at the support of specific functionalities of NGN architectures in a mobile environment. The concepts of these architectures and some of the architectures themselves will be the basis for the proposed multicast/broadcast network convergence in the mobile telecommunications environment. Given its commonality, IP layer issues will only be mentioned where strictly essential IP Multimedia Sub-system (IMS) One of the most promising NGN architectures is IP Multimedia Sub-system (IMS) [6]. IMS is a standardized NGN architecture for an Internet media-services capability defined by the European Telecommunications Standards Institute (ETSI) and 3GPP. IMS has a layered architecture, which consists of delivery, control and service planes as illustrated in Fig. 2. IMS reference architecture.

4 J. Santos et al. / Computer Networks 52 (2008) Fig. 2. It uses Session Initiation Protocol (SIP) for session establishment, control, modification, and termination of voice, video, and messaging between two or more participants. These functions are implemented in Call Session Control Functions (CSCFs), based on SIP proxies. Authentication, Authorization, and Accounting (AAA) within the IMS is based on the IETF Diameter protocol and is implemented in the Home Subscriber System (HSS), CSCFs, and various other IMS components. The different CSCFs are known as Proxy CSCF (P-CSCF), Interrogating CSCF (I-CSCF) and Serving CSCF (S-CSCF). The P-CSCF is the first access point within the core network for the terminal starting session. It behaves like a proxy to accept requests from the users and to either serve them internally or forward them. The P-CSCF is further responsible for authorizing bearer resources for the appropriate QoS level, identifying I-CSCF to forward the requests, enforcing local policies, and performing header compression and decompression. The I- CSCF is then the contact point within an operator s network for all connections to a subscriber of that network operator. It assigns an S-CSCF to each user performing SIP registration, and may also act as a Topology Hiding Interworking Gateway. The S-CSCF is the central node in the control signaling path and is assigned to a user during registration. It is the anchor point for the interconnection with IMS applications servers (AS), allowing signaling to be routed between the users and ASs. The HSS is the central repository for user related information. It stores IMS user and application server profiles. The user profiles contain location, security, user status, and individual filtering information. This filter information is obtained and used by the S-CSCF to route signaling requests from users to the desired AS. ASs enable the flexible development of multimedia applications including conversational, streaming, and messaging type, or enhanced service enablers such as presence or group management; however, the IMS standards do not specify how these applications should be developed. An AS communicates with S-CSCF and with HSS through a Diameter-based interface. The Media Resource Function (MRF) can be split up into the MRF Controller (MRFC) and the MRF Processor (MRFP). It provides media stream processing resources for media mixing, media announcements, media analysis, and media transcoding. Also important in IMS are the legacy concerns: the Border Gateway Control Function (BGCF), Media Gateway Control Function (MGCF), and Media Gateway (MG) are responsible for interworking the bearer between Real Time Transport Protocol (RTP)/IP networks and circuit switching networks. A Policy Decision Function (PDF) is also defined within IMS, which authorizes media plane resources, e.g., QoS over the media plane. It is used for policy control and bandwidth management Telecoms and Internet Converged Services and Protocols for Advanced Networks (TISPAN) Although IMS is a major step forward towards the network convergence vision of delivering any multimedia service over any network, it is still rudimentary in many aspects, such as coordination required between access networks in networked databases, admission and resource control. TISPAN, also ETSI sponsored, is in charge of addressing these convergence issues, aiming at a fixed-mobile convergence environment. As illustrated in Fig. 3, TISPAN (Release 1) architecture is based on the concept of cooperating sub-systems sharing common components. This approach allows the addition of future sub-systems and ensures that network resources, applications, and user devices are common to all sub-systems. The IMS Core, which is closely based upon 3GPP IMS Release 6, is one of these sub-systems. The Network Attachment Sub-system (NASS) and the Resource and Admission Control Sub-System (RACS) are two other relevant sub-systems, responsible for IP connectivity and QoS, respectively [8]. NASS provides address allocation, authentication and authorization functions, access network configuration and location management. RACS provides QoS control (including resource reservation, admission control and gate control), Network and Port Address Translation (NAPT) and/or Firewall (FW) traversal control functionalities over access and core transport networks. Admission control involves checking authorization based on user profiles, Service Level Agreements (SLAs), operator specific policy rules, and resource availability within access and core transport. The inter-relation between all these sub-systems is the key advance in the TISPAN architecture Open Mobile Alliance (OMA) The sought-for convergence has also implications for the top service layer of Fig. 1, as applications and

5 232 J. Santos et al. / Computer Networks 52 (2008) Application Layer NGN End Terminals UPSF Network Attachment Subsystem PSTN / ISDN Emulation Streaming Subsystem RTSP-based IMS core Subsystem Resource and Admission Subsystem Other Networks Access and Transport Layer Fig. 3. TISPAN reference architecture. services are many and diverse. Such applications may be able to monitor or control multimedia sessions and may be accessed through a session control protocol such as SIP. Thus, these services are closely coupled to the underlying network on which they are running, and therefore, a standard approach is required to deliver them across different networks. The Open Mobile Alliance (OMA) is a standardization entity responsible for specifying market driven service enablers that ensure service interoperability across devices, geographies, service providers, operators, and networks [4]. Examples of applications and services are presence, call conferencing, transcoding and billing. OMA specifies an OMA Service Environment (OSE) [7], 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. Fig. 4 illustrates the layered architecture of the OSE and OMA enablers. Applications OMA / 3 rd Party Applications Policy Enforcers Digital Rights Management Billing Framework Bindings Bindings s Presence Device Mgmt. BCAST Location Operator / Terminal / Service Provider Resources Fig. 4. OMA reference architecture.

6 J. Santos et al. / Computer Networks 52 (2008) There are a large number of enablers defined or partially defined. Some have particular relevance for our aims, such as BCAST, Presence, Transcoding, Group List Management and IMS Utilization. The later facilitates OMA services and applications to make use of IMS. It is clear from the previous sections that NGN convergence is multifaceted, requiring service convergence, session control convergence, architecture convergence and mobility convergence; OMA enablers provide solutions to the service layer Multimedia Broadcast/Multicast Services (MBMS) Besides being instrumental to IMS development, 3GPP also created the Multimedia Broadcast/ Multicast Services (MBMS) [9], a sub-system standardized since 3GPP release 6. MBMS allows delivery of IP multicast datagrams to User Equipments (UEs) with specified QoS. On the control plane, it manages bearer service activation status of the UEs, outsources authorization decisions to a newly introduced Broadcast Multicast Service Centre (BM-SC), provides control of session initiation/ termination by the MBMS user service, and manages bearer resources for the distribution of MBMS data. IP plays a key role in MBMS, being used to identify the particular instance of the bearer service (which is composed of an IP multicast address and an access point name network identifier) and to manage all MBMS multicast services. The Gateway GPRS Support Node (GGSN) serves as the entry point for IP multicast traffic as MBMS data. Upon notification from the BM-SC, the GGSN is responsible for setting up the required radio resources for the MBMS transmission inside the UMTS Terrestrial Radio Access Network/GSM/EDGE Radio Access Network (UTRAN/GERAN). The UTRAN decides on the appropriate radio bearer based on the number of users within a cell, prior to, and during a MBMS transmission. Mobility aspects are intrinsically supported in UTRAN/GERAN, but further mobility needs to be supported by the Serving GSN (SGSN), requiring the capability to store a user-specific MBMS context for each activated multicast MBMS bearer service (Fig. 5). Fig. 5. MBMS reference architecture.

7 234 J. Santos et al. / Computer Networks 52 (2008) Subscription Service Announcement Service Announcement Joining Session Start MBMS Notification Data Transfer Session Stop Session Start MBMS Notification Data Transfer Session Stop Leaving Broadcast Mode Multicast Mode Fig. 6. MBMS broadcast and multicast phases. MBMS is able to operate in two modes: broadcast and multicast (Fig. 6). The broadcast mode works in a simplified manner, since it does not involve subscriptions management. It is composed up of five phases: service announcement, session start, MBMS notification, data transfer and session stop. The service announcement is used to provide the UE with information on available MBMS services. The announced information includes parameters required for the service activation, such as service start time and content information, security parameters and associated delivery services. The session start phase is characterized by the trigger for bearer resource establishment for MBMS transfer. In the next phase MBMS notification phase the UEs are informed of forthcoming and ongoing MBMS broadcast data transfers. The following phase is the actual data transfer where UE receives the file or the announced streaming session. Finally, when the BM-SC has no more content to be delivered, the session stop phase releases the bearer resources. The MBMS multicast mode is more complex than the broadcast mode. It considers higher level mechanisms for subscription management in order to optimize the distribution. For enabling such services, the multicast mode needs three more phases: subscription, joining and leaving. In the subscription phase, the UE must explicitly establish a relationship with the service provider in order to receive the MBMS multicast service via higher level mechanisms, such as a Web portal or other defined services. Then, the UE, for each subscribed service, receives service announcements in a similar way to that of broadcast mode. Based on the received announcements, the UE may initiate the joining phase (typically through Multicast Listener Discovery (MLD) or Internet Group Management Protocol (IGMP) messages). The following phases session start, notification, data transfer and session stop are again similar to the broadcast mode. The main difference is that a UE is able to perform a leaving procedure, informing the BM-SC that it no longer wants to receive data from a specified service. 3. IMS MBMS converged architecture integration issues MBMS, via BM-SC, provides the means to deploy multicast/broadcast based on 3G technologies as an independent system. It can work as a standalone technology since it has its own mechanisms of user accounting, charging, security, QoS and others. However, to be able to provide the same services in a heterogeneous environment, in a next generation network, MBMS needs to be enhanced. The integration of the functionalities of both MBMS and IMS could help on the development of a NGN architecture. The IMS MBMS integrated architecture aims to support the following functionalities: IMS-compatible signaling for efficient multicast signaling, group management with context-aware communities and dynamic multicast group address allocation. Scheduling and congestion control with adaptive solutions based on feedback from the Radio Access Network (RAN). Session management with RAN/bearer selection in a converged environment.

8 J. Santos et al. / Computer Networks 52 (2008) QoS support for multicast/broadcast services (beyond unicast and multimedia ones). Transcoding strategies for provisioning of multilayer services supporting use-cases such as layered codecs and location-dependent transmissions. However, several issues on this integration need to be addressed to define such a converged IMS MBMS architecture Architectural issues As stated before, IMS is the chosen technology for session control and, to a certain extent, also instrumental for architectural convergence. However, IMS does not support delivery of multicast/ broadcast services, which reduces its scalability. If the MRF is enhanced to support multicast delivery, being able to either control a BM-SC or directly multicast bearer services (MBMS or others), it would be possible to improve the service provider resource usage. A converged broadcast architecture based on IMS leads to the main approach of making IMS multicast technology enabled. In other words it must be possible to send multimedia content to a group of IMS users through a multicast capable technology as a bearer, where MBMS is the most prominent technology currently regarding multicast delivery. One first architectural option is thus to have a simple IMS and MBMS integration. An IMS and MBMS integration first architectural option is to allow IMS applications to use MBMS, preserving as much of its functionality and structure as possible. This is the approach in the 3GPP study item [12]. The architecture of which is shown in Fig. 7, where the BM-SC is presented as an entity inside IMS architecture to support MBMS bearers. This study group presents technical considerations and solutions for the facilitation of IMS services over multicast bearer services with a focus on the possible enhancements to IMS functionalities and relevant charging, security and service provision procedures. However, it considers the inclusion of BM-SC in the IMS architecture without clearly specifying how to solve the duplicate functionality problems and provide integrated interfaces. In the situation where there are a number of possible multicast broadcast bearers, the IMS must decide somehow which multicast broadcast bearer to use. The decision must be based on the particular application, UE capabilities, multicast/broadcast access network and QoS parameters. This requires that the basic broadcast/multicast support functions are available at the service enabler layer and session control layer. For MBMS, this would mean that the functionalities of the BM-SC would be distributed over IMS entities and service enablers MBMS functional evolvement and integration The BM-SC provides mixed data path and control management functionalities in one functional entity. Several of the MBMS functions defined are generic to any multicast/broadcast services. Good examples of this are security, service discovery, service provisioning and user management. When con- Fig. 7. IMS architecture using MBMS bearers.

9 236 J. Santos et al. / Computer Networks 52 (2008) sidering NGN convergence, these functionalities need to be abstracted in a way that they can be reused by several technologies. Fig. 8 presents the BM-SC functional structure within a UMTS network and covers the interfaces and protocols used. The centralized BM-SC comprises key MBMS sub-functions, described in the next paragraphs, along with integration and evolution issues raised by the current status of the standards Membership function The BM-SC membership function provides authorization for UEs requesting activation of an MBMS service. This relates to Multicast join authorization, user membership management, timerelated charging and subscription-related charging. This function manages bearer service membership functions and the subscription related information (user profiles), authentication and authorization of the user. However, it is not completely clear in the current release of the standards how and where user profiles are stored or how services authorizations are described. In IMS, the HSS is the master user database: it can further support the membership function role of BM-SC, holding authentication and authorization information to join a multicast group, user related information about services subscribed and others. This would require a clear definition of the functionalities expected both from IMS and MBMS, and the respective interfaces Security function The BM-SC security function is responsible for service protection limiting access to both broadcast and especially to multicast transmissions to registered subscribers. This is done by means of data ciphering and key distribution [10]. The MBMS Key Management function is used for distributing MBMS keys to authorized UEs. Before the UE can receive MBMS keys, it needs to register at the Key Request sub-function. Once registered, the UE can request missing MBMS keys from the BM-SC by indicating the specific MBMS key ID, and deregister when desired. MBMS User Service data protection is optional, only needed if requested by service announcement. The security depends directly on the General Bootstrapping Architecture (GBA) [11] function for authorization and acquisition of base key information. UE Gi [http] Interactive Announcement Function Membership Function User Service Discovery Function Gmb Gmb Gmb GGSN GGSN UE Gi [UDP/RTP, UDP/FLUTE] Proxy and Transport Function Gi [http] Gi [UDP/RTP, UDP/FLUTE] Session & Transmission Function MBMS Delivery Functions Associated delivery Functions Gi Content Provider UE UE Gi [http] Gi [UDP, MIKEY] Key Management Function Key Request Function Key Distribution Function Fig. 8. BM-SC functional entities.

10 J. Santos et al. / Computer Networks 52 (2008) The scheme defined by MBMS is clearly dependent on 3GPP and only applies to MBMS technologies. In our vision, the security scheme should be further generalized in such a way that it could be used by different access technologies. Therefore, one possibility for security convergence is to develop a specific enabler in IMS for key distribution, and have the actual data traffic encryption done in the data delivery layer in the MRF function Proxy and transport function The proxy and transport function acts as a gateway between core network and the transport layer. It manages routing of data and reservation of bearers across the transport layer to the registered GGSNs, while maintaining a clear separation between control layer (i.e., signaling) and transport/user plane (i.e., multicast payload). Also, it ensures QoS parameters agreed upon during session establishment by session and transmission functions. One main problem of the current MBMS architecture is that it assumes that the BM-SC is completely in charge of policy control and resource reservation. However, it is advisable to consider integration of the PDF functionalities that already exist in IMS for media resource policy control with adaptations for the multicast/broadcast service s resource authorization decisions. In [13] some service authorization issues are raised and hints about a possible solution using a PDF entity are provided, but not deeply pursued Session and transmission function The session and transmission function is the entity responsible for MBMS bearer session management, authenticating and authorizing external sources. It is also responsible for managing the content repository and interacting with the content provider. Its interface with Content Providers is not defined in MBMS. It is unclear how content is authorized to be distributed in the MBMS network or how it is stored in case of non-real time services. In IMS, Application Servers (AS) define the way it deploys hosts and executes services. This allows network operators or third party providers an easy integration and deployment of their value added services, such as group management, enhanced announcement services, or security support. Thus, it seems reasonable to have the interface with the Content Provider either directly or indirectly through IMS AS. Furthermore, in the current version of the standards, the BM-SC is likely to be involved in scheduling transmissions and resource reservation (and release) for MBMS transmissions. A BM-SC is responsible for multicast broadcast service provisioning, so it might limit resource usage to some configured level, and reschedule file transmissions for a later time in case of high load in the network. It is also responsible for allocating resources to bearer services and providing the GGSN with transport associated parameters such as QoS and MBMS service area, thus supporting location-specific transmissions. As stated before, MBMS assumes that BM-SC is in charge of policy control and could clearly benefit with the integration of IMS PDF functionality for policy control. For streaming delivery, the BM-SC may collect Quality of Experience (QoE) reports. For download delivery, the BM-SC may collect reception acknowledgements and statistical reception reports, although these mechanisms are not clearly defined. IMS could perfectly manage the reception of these reports through a specific application server responsible for session management of multicast/broadcast services. This application server, after collecting this statistical information, could then control the BM-SC, MRF or other specific entity to efficiently use the available resources. The purpose of this approach is to not restrict this mechanism to MBMS bearers Service discovery function The BM-SC also provides service announcements for multicast and broadcast MBMS user services, including the media descriptions specifying the media to be delivered as part of an MBMS user service (e.g., type of video and audio encodings). In addition, it also provides the UE with MBMS session descriptions specifying the MBMS sessions to be delivered as part of an MBMS user service (e.g., multicast service identification, addressing, time of transmission, etc.). These media and session descriptions are delivered by means of service announcements using IETF specified protocols, like Session Description Protocol (SDP) over MBMS multicast and broadcast bearer services. It is referred that an interactive announcement function may offer alternative means to provide service descriptions to the UE using HTTP or other interac-

11 238 J. Santos et al. / Computer Networks 52 (2008) tive transport methods but they are not clearly defined. In MBMS no considerations are made concerning context or location awareness in service announcement. It is possible to consider that new emerging service enablers being developed for IMS, such as group management, would be adequate tools for service announcement, bringing location and context-sensitivity into MBMS. 4. An evolved IMS MBMS architecture The challenge towards a long term evolution of multicast/broadcast services is to merge this technology within a truly IP based NGN architecture. Current work within 3GPP Long Term Evolution (LTE) and System Architecture Evolution (SAE) initiative is on-going and is likely to develop further: it is expected to bring enhancements of packet switched technology to cope with rapid growth in IP traffic through a fully IP network with simplified architecture and distributed control with heterogeneous access. Similarly, the work on a next generation IP-based converged network under TISPAN is also in progress. In these cases, the whole concept of group session inside MBMS needs to be rethought, as the transport layer may already support multicast capabilities. Taking the present status and trying to understand the basic premises of SAE, LTE and TISPAN, we developed and evaluated a possible architecture integrating IMS with MBMS over an evolved network. In this architecture we propose to have the MBMS BM-SC functions completely distributed among the existing network entities (a centralized BM-SC entity no longer exists). Also, the functions such as security, service announcement, and QoS provisioning, are not kept specific to one access technology; they are generalized to cope with any access technology, and IP multicast is assumed as common transmission layer. In Fig. 9 we propose a new layered design, mainly based on IMS, but we introduce more granularity to the picture: the delivery plane is now divided into access and delivery. Access relates to the access technology used by the end user to achieve access to the network; delivery concerns the converged IP layer. The service plane is also subdivided into application plane and service enabler plane. In the following sections, we explain in detail each plane considered in the evolved architecture and the way the existing BM-SC functions are distributed. Fig. 10 presents a detailed network entity design Access and transport plane The access and transport plane is completely distinct from the one in the MBMS standards: an evolved UMTS packet core is now considered, where the GTP tunneling mechanisms to support mobility have been substituted by enhanced IP-based multicast and mobility mechanisms. This trend closely follows the SAE evolution as defined in [15] and a similar architecture has been defined in [14] with support for multicast mobility. One visible consequence of this evolution is that the IP packet network is now closer to the radio access Fig. 9. Evolved IMS MBMS architecture.

12 J. Santos et al. / Computer Networks 52 (2008) Ut Application Plane APIs Ut Ut Service Plane Content Mgmt Context-based Group Mgmt Service Announcement Session Management Service Protection Service Scheduling Ut Content Manager Sh/Dh ISC/Ma User Plane HSS M* -Membership Gm Gq/Rx PDF/PCRF Cx/Dx MDFP Controller P/I/S-CSCF MDFP Location IMS-based Control Plane M*- Session & Transmission M* - Session Man ager Gm Congestion Controller Content Adaptation Session Schedul ing Controller Go/S7 Gmb Gi Gmb Mb Evolved Packet Core Media Delivery Function Processor (MDFP) M*- UE BS Context Managment Media Delivery Function Controller (MDFC) or MRFC Delivery Session Manager M* - Proxy And Transport Delivery Group Manager Media P rocessor/transcoder MDFP or MRFP Gi M* - Transport Media Delivery Plane Access and Transport Plane Mb Streaming and Download Source Content Providers Plane M* Distributed BMSC Functionality (M - Multicast) Fig. 10. IMS and MBMS integration details. network, which allows IP convergence closer to the user, reducing the access (technology specific) to a smaller area. Also, another important aspect is that this architecture is designed to cope with different types of access networks. It is possible to consider not only a 3GPP evolved packet core, but also other technologies such as WiFi, WiMax or DVB-H (the last one is very interesting in particular to multicast/broadcast services). The evolved packet network architecture includes an access technology dependent element, the PCEF (Policy Control Enforcement Function), to control the (technology dependent) allocation of resources, the mapping of QoS parameters, and the enforcement of charging and policy. Generic interactions with this entity are performed over the Gx interface as defined in [16] (however, [16] is still under heavy development in 3GPP and still lacks the multicast/ broadcast support). Our proposal for a multicast/ broadcast enabled architecture redefines this Gx interface in order to cope with enhanced support to multicast bearer creation, allowing a user to join a multicast group, set multicast related QoS parameters, and provide generation and transmission of charging vectors for online or offline charging, among other functions Media Delivery Plane Transport and pre-processing of content takes place in this layer. Therefore several Media Delivery Function Processor (MDFP) entities are designed. MDFP extends the MRFP defined within IMS. The MDFP, in combination with Media Delivery Function Controller (MDFC), provides the ciphering of media (broadcast/multicast security), error correction coding, mixing of different media streams, and transcoding. MDFP is placed as a gateway between the Content Provider and the Evolved Packet Core. In addition, it also handles the enhanced group management and session management delivery functions. There are several candidates for the protocol used between MDFC and MDFP, such as SIP or

13 240 J. Santos et al. / Computer Networks 52 (2008) H.246/MEGACO depending on the flexibility of the required features. The interface between MRFC and MRFP also needs to be enhanced according to the implications over the control of multicast bearers Control Plane The developed IMS-based control plane consists of CSCF proxies, HSS, PCRF and MRF (constituted by MDFC and MDFP). The Policy Control Resource Function (PCRF) is introduced in SAE/ LTE as an extension of the IMS PDF component for harmonization of admission, charging and QoS mechanisms. It is mainly responsible for QoS aspects, policy management and charging functions. Through the Rx interface, it receives from the CSCF requests from different Application Functions (such as AS or UEs), using a Diameter based interface, for admission and authorization of calls. This interface is defined in [17]. However, SAE/LTE vision of PCRF is not yet multicast-capable and needs to be enhanced to cope with multicast/broadcast support. Our proposal is to enable the CSCF capability to request policy decisions from the PCRF entity for multicast/broadcast services announced by application functions. The PCRF entity also needs the support of an extended user database in order to obtain the user profiles for policy decisions. The HSS needs small enhancements to fulfill these functions. Joining of services is feasible with initial filter criteria placed in user profiles. The S-CSCF contains service-triggered information in the form of initial filter criteria. When the user equipment establishes a connection to our architecture, the S-CSCF transfers the corresponding user profile from the HSS. The user profile contains the subscribed service groups. Whenever the connection is set up, the S-CSCF executes service triggers and sends SIP messages to the group server in the application layer. These SIP messages result in service activation and thus allow service group delivery. Scheduling and resource control is an aspect of the MDFC (the enhancement of the 3GPP MRFC). MRFC provides services for conferencing, announcements to a user or media transcoding in the IMS architecture. To control the sources of content, the MDFC has to support Content Providers to deliver specific content to a specific unicast, multicast or broadcast address. MDFC functions mainly are reservation and administration of multicast addresses e.g., allocating unused multicast address for every multicast delivery session, finding an appropriate MDFP depending on users location and multicast and multicast/broadcast service scheduling Service Plane In SAE/LTE specifications no detailed information is given about the Service and Application Layer. Most of the features and enablers along with interfaces will be evolved from IMS and OMA specifications and architectures to function over an SAE/LTE network. In order to provide multicast/ broadcast enabled services, we propose to define the MB-SE (multicast/broadcast service enabler) as shown in Fig. 10 some of these functions are already found in an early stage in the R6 architecture BM-SC. These include: Security management functions including registration for key updates, service key updates. Service description and service guide aggregation for broadcast and multicast services. High level content scheduling. Statistics collection for streaming and download deliveries. QoE statistics collection for streaming. Group management (a separate service enabler is defined and designed for group management). The Content Management (CME) serves as an interface enabler between a service provider and a content provider. It provides control and policing of the type and amount of content allowed as part of a service and it allows the service provider to interface to the content provider. The Context-based Group Management (CGME) not only handles the traditional multicast broadcast group management, but also takes it a step further by incorporating context-awareness. It is responsible for creation, deletion and management of user groups. The Session Management (SME) is the contact point for the end user. At the SME the users register for the service they are interested in receiving. All signalling flows between the MB-SE and the end user are handled by the SME. Therefore, it manages session start and session stop signaling. Bearer selection is another responsibility of the SME. The SME decides to use a multicast or

14 J. Santos et al. / Computer Networks 52 (2008) broadcast bearer, depending on the registered user to a service, if the service provider has not defined a fixed bearer. And, based on the selection, appropriate address allocation is ensured by this enabler. The Service Scheduling (SSE) is responsible for organizing the optimal schedule (order) of service/content deliveries in order to efficiently utilize the available resources in the Core Network (CN) and Radio Access Network (RAN). For this purpose it takes into account the spatial distribution of the users among the cells (to the extent possible) as well as the current and the estimated RAN capacity (cell/area based). It further differentiates the various types of service (carousel, streaming, file download) and considers the defined QoS requirements (jitter, delay, bandwidth). The Service Announcement (SAE) provides service announcements describing the service to the end-user and is responsible for the distribution of a service guide (ESG or EPG) to the endusers. The Service Protection/Key Management entity is responsible for delivering service key updates (session keys or MSKs as in [10]) based on previous UE registrations to receive these updates. This is done in R6 within the BM-SC based on HTTP. In the proposed architecture it is suggested to use Subscribe/Notify allowing the UEs to register to receive key updates and acquire these updates when available from the BM-SC. Other BM-SC security functions are distributed across the IMS core (authentication) and the MDFP (ciphering and traffic key generation) Issues and Consequences for future convergence The usage of this approach for future convergence presents some constrains and problems. The key point here is the increased integration and distributed implementation of the MBMS architecture and concepts into the IMS sub-system. From the point of view of 3GPP, this is actually a change in the existing deployment ideas: equipment manufacturers are now developing BM-SC and IMS boxes, mostly with low inter-relation. In fact, not even the simpler integrated architecture presented in Section 3 is now supported in any implementation. The distributed BM-SC approach breaks this deployment structure, and impairs the evolution of existing/planned products. The impact on TISPAN is not less, unfortunately. TISPAN advocates a clear sub-systems approach; here, IMS and MBMS seem to fit naturally. The distribution of the multicast/broadcast functionalities across the several entities breaks this approach, merging unicast and multicast services in a common platform. On the other hand, the proposed approach is more naturally adequate for a flexible environment, and especially to share the commonalities provided at the transport and control level. User personalization, media description, resource management and IP transport can be integrated, providing optimum service provision regardless of the type of service being provided Delivery issues This section addresses some of the issues relevant for an integrated NGN environment Scheduling/admission across RANs Future NGN will have to cope with scheduling and admission problems across different RANs and these problems will be more complex when entertainment media is being handled. Admission is always an issue associated with the user, through the HSSC and the PCRF, but network aspects are more complex than this. Considering traditional cellular environments, broadcast radio sub-carriers may (or may not) be allocated. This will depend on the interest/number of users currently in the cell. The PCEF will have to manage this process, including considering the dynamic change of carrier, depending on the total cell available bandwidth for group communications, and on the total consumption at the moment. This considerably affects admission decisions, as algorithms will now need to consider these potential changes also. The problem may be even more complex as broadcast RANs are considered, such as DVB. Here, admission is immediate (a matter of allocating the user with the correct keys, at the multicast ) but the scheduler needs to know that the UE has the capability to receive information via such interfaces. Subscription is then not an immediate process (user subscribes to the interface that is active), but a process where the interfaces able to receive the specific content are explicitly mentioned. When discussing session mobility, this brings an

15 242 J. Santos et al. / Computer Networks 52 (2008) added problem, in deciding which the best interface to continue a session is Resource usage The previous paragraph clearly identifies issues associated with resource management at the radio access. Changing streaming across technologies or changing the type of logical channel at the radio access may bring added efficiency to the network. Naturally, the PCEF needs to handle this, but when the capability of switching technologies becomes important (e.g., all devices have WiFi, 3G and DVB interfaces), a new concept for PCEF is required: cross-technology optimization needs to be in place. A further aspect of the problem is related with the wired-side optimization. Typically this is considered the core, but in most cellular systems, Base Stations (BS) are not directly in the core, but are connected to this via a limited bandwidth line (often rented to a wired provider). This brings a clear incentive to distribute to each BS only the groups that are needed and thus multicast tree optimizations at the transport level are also important. In our view, this is essentially handled at the IP level, and inherent to the whole process: the context management guarantees that required groups are subscribed at a given BS Transcoding The facilities of dynamically switching the delivery of entertainment media can be optimally exploited when content adaptation is present in the network. Delivery over a CDMA or WiFi channel, or a DVB stream, presents quite different technical problems, and is able to exploit quite different bandwidths. The content adaptation function in Fig. 10 can be dynamically invoked to provide to the UE the stream coded with the best characteristics for the current carrier. The content provider will be oblivious of this, as transcoding can be dynamically provided as an added value of a true NGN. The proxy and transport function distributed at access and delivery layers (Fig. 10) enables this feature for the converged architecture. While sending data to the transport layer, delivery-proxy and transport function checks for the carrier in use and transcodes accordingly Security Security is needed at several levels in NGN: access control, confidentiality, and integrity, amongst others. The network needs to avoid unauthorized access which it inherently does: our proposal for integration of multicast services in NGN handles access control in a similar way as unicast services, with the user profile being stored in the same entities (HSS), but now with an added support from PCRF for QoS support. Nevertheless, broadcast services (in RANs such as DVB) need a paradigm extension to handle access control through key management. Note that the access to this key is made through the bidirectional RANs, providing a simple and controllable mechanism to distribute these access keys. On the same token, information confidentiality (e.g., adult services cannot be transmitted in open channels in most countries) is also guaranteed by the same method, for broadcast RANs. When non-broadcast logical channels are used (such as a dedicated channel in 3G), confidentiality could be assured by the same mechanisms for unicast services (that is, through link-level security): however, the potential session mobility that could occur at any instant as the user moves, argues in favor of always retaining the same key-management based security mechanisms for group services, regardless of aspects of radio resource management. 5. Evolving multicast broadcast services The convergence of the multicast broadcast services and IP-based environments, as proposed for NGN, aims to open new business opportunities. Although many of these will depend on regulatory aspects of merging broadcast and telecommunication services, their significance for an open communications market will eventually lead to their emergence Multicast services One of the areas where the integration of these two areas may be simpler to support is associated with all aspects of closed-group services being delivered in a heterogeneous network Evolved group services Currently groups are handled inside telecom operators mostly as a profile for certain customers, which have subscribed to different services. Our approach provides the flexibility for deploying much more complex intelligence in group service provision. Access to groups now requires a specific request (as before), but which can eventually be

16 J. Santos et al. / Computer Networks 52 (2008) transferred to AS, or to the content provider. Group services can now go through specific logic, and provide different content to different users, depending on many aspects: transcoding of information according to the access network (as discussed above); selection of the content according to the user preferences (e.g., on transmitting a football match, the transmission may push different cameras to the user according to its team preferences this can be done easily by transmitting multiple flows, accessible by different keys) or subscriptions (a premium user can have access to all the flows being transmitted from the game). An advantage of our proposal is the fact that the specific method to deploy these features could be potentially different; however, for an optimum deployment in terms of wireless resource management, different multicast trees should be created inside the network, reaching only cells where those resources are needed Key service enablers As discussed in the previous subsection, one of the main motivations to pursue IMS MBMS integration is to enable interactive, group-based and context-aware multicast services. Also, as discussed in Section 4, in our proposed architecture some of the BM-SC functionality is relocated at the service enabler layer in the converged architecture; we present this ported functionality as an enabler called Multicast Broadcast Service (MB-SE). Fig. 11 shows the structure of MB-SE and its interaction with other enablers provided by the converged framework. As shown, multicast services interact with other IMS OMA enablers to carry out functions like scheduling, service discovery, charging and security. It should be made clear at this point that multicast transmission is a scenario made possible by this converged architecture with added flavors of groupbased and context-aware services. Therefore, for an end-to-end multicast transmission model, our MB- SE enabler makes use of other enablers specified in 3GPP and OMA specifications Context-aware Group Management This optimality of tree creation for streaming distribution creates an added dimension to group services, most especially to subscribed services related to information. Context (such as location) may lead to the creation of different groups: a video-server may provide information about local points of interest, according to location (e.g., all restaurants may have a short promotional video distributed in the food information service, and depending on the location, users will be allocated to different multicast groups, and thus receive different streams). This can be also different depending on other context aspects, such as: e.g., current weather (receiving video promotion about a nice café in the middle of Content Provider Device Management Group Management Service Discovery Content Mgmt Context-based Group Mgmt Multicast Broadcast - Service (MB SE) Service Announcement Session Management Service Protection Service Scheduling Charging Security Presence Session Management Other s MB-SE Converged IMS-MBMS Core Fig. 11. Broadcast/Multicast Service functional structure.

17 244 J. Santos et al. / Computer Networks 52 (2008) the park is probably not very effective during a thunderstorm). On the other hand, users may also push their content locally the development of environments like youtube is a good indication of this interest. Communities in the sense of groups of interrelated users can be dynamically supported, as long as a group management service is in place. This service, providing simply key management and a traffic rendezvous point, may allow closed groups to be created, and user-originated content to be distributed inside that group Broadcast Services Broadcast services, in its most basic function, means the same content to all users, regardless of the user. Nevertheless, broadcast services also benefit from synergies inside NGN Evolved Broadcast services If the content of broadcast services aims to be the same to all interested users, there are two aspects that benefit from an individual separation: (i) advertising; (ii) content authorization. On the first point, the integration of broadcast services inside NGN allows for a much more effective exploitation of advertising. Today, marketing already positions its publicity in different channels, at different times, according to the presumed user stratification. It seems obvious in our structure to allow advertising time to be fine-tuned to users, depending on location, context, or even their preferences. Broadcasters would then resort to multicast groups during advertising time. The second point is associated with content authorization. Broadcasters are having increased limitations on content distribution according to issues such as violence, profanity or adult content. With an integrated environment, all these content types could be transmitted without restrictions: viewing is individual mobile devices are not for common viewing, and as such the user may choose to see whatever content he wants, as if the transmission were made in a paid-channel. Age restrictions can be placed at the subscription time, with users being allowed (or not) to access restricted content based on their age Key service enablers In terms of functional components and enablers required, broadcast can be seen as a subset of multicast transmission. Referring back to Fig. 11, for broadcast transmission, key service enablers would be the same, apart from key management, key distribution and membership functions which are not required for a broadcast transmission setup. 6. Prototype implementation In order to evaluate the benefits of the proposed architecture, a testbed was implemented in the scope of C-MOBILE project. The testbed elements can be subdivided into 3 classes: Emulators, Open Source Components and Custom Developed Software. The Emulators class is required by RAN testing issues: a RAN emulator is used, which enables the testing of the architecture over a common wireless network without the need of expensive 3G radio equipment. The Open Source Components chosen for the testbed comprise OpenIMS, 1 GStreamer, 2 Video- Lan, 3 MAD Flute, 4 Sofia SIP 5 C/C++ stack, and NIST-SIP 6 JAVA stack. The platforms use Nokia N800 7 Linux and desktop computers running Linux (Ubuntu 7.04) and FreeBSD, providing with the basic building blocks to construct the network and platform. Additionally, several components were needed to be developed from scratch such as the User Equipment Interface, and the Application Server with all its Service s. Some other components were also significantly modified from existing private software, as was the case of the Media Delivery Function and Content Provider. The deployment view of the test bed components and their implementation and functions is described as depicted in Fig OpenIMs is a public availaiable IMS stack. See GStreamer is an Open Source Media Stack. See gstreamer.freedesktop.org/. 3 VideoLan is a highly portable multimedia player. See MAD-FLUTE is a multicast file transfer tool based on FLUTE protocol. See 5 Sofia-SIP is an Open Source User-Agent library. See opensource.nokia.com/projects/sofia-sip/. 6 NIST-SIP is an Open Source User-Agent. See snad.ncsl.nist.gov/proj/iptel/. 7 Nokia N800 is an Internet Tablet device. See

18 J. Santos et al. / Computer Networks 52 (2008) Fig. 12. Demonstrator components Components The RAN emulator takes as input the outcome of C-MOBILE simulations and reflects these results in terms of L3 behavior, as configurable parameters to a transparent shaping software placed in between the Access Point and the last IP Multicast router (see Fig. 12). This emulator allows the setting of IP QoS parameters (through-put, delay, loss) on a per flow basis providing a good reference to what the real behavior of traffic would be, under different radio channel conditions. In the core network the OpenIMS is used since it provides a relatively stable and compliant IMS platform capable of being extended. On top of this IMS platform a simplified Application Server (AS) was developed. The AS is basically composed by the Session Management (SME); the Context based Group Management Enable (CGME); and Session Scheduler (SSE). The simplified AS makes available a protocol abstraction API so that service enablers such as the SME can send SIP messages without actually knowing SIP resembling as much as possible a Resource Adapter. The Media Delivery Function (MDF) is implemented by wrapping MAD Flute and VideoLan delivery engines for download and streaming, with control logic and communication abilities on top of them. The Content Provider is the least complex of the prototyped entities and consists of a simple file repository that can be accessed by the MDF and Service s through HTTP. Finally, the User Equipment is Nokia s N800 Internet tablet device, which albeit slightly larger than 1st generation UMTS User Equipments, provides easy to program connectivity and multimedia features (such as interface and hardware base codec support through the Open Source Media Framework GStreamer). The testbed is able to support both IPv4 and IPv6 Multicast through the SIP Stacks used and OpenIMS dual stack support and the availability of Multicast Routing Daemons for Linux and FreeBSD.

19 246 J. Santos et al. / Computer Networks 52 (2008) Testbed Scenarios and Results The testcase scenarios evaluated consist of Interactive TV and Content Casting. The Interactive TV scenario is a very common scenario and is intended to demonstrate that the architecture not only fully supports current business cases, but also provides added value such as context based interactivity (e.g., location specific programming). The Content Casting scenario context is again explored (e.g., based on user presence) together with Scheduling of Services (e.g., files are transmitted only when network usage is low) and Sessions can be dynamically shifted from between Multicast and Unicast through the Session Management in order to optimize network resources. One should notice that the great advantage of this architecture lies in the service enabler capabilities. Network-wise, the performance of the demonstrator is not different from usual IP networks: we have results in the range of ms for basic (no Service s involved) Multicast IMS Session Setup. However, the processing logic on the service enablers is the dominant aspect here. It is not surprising that this logic takes a couple of seconds depending on the use case, but this is not really relevant from the user point of view, since this is the initial service request time. Afterwards, the user notices the service actual delivery, which in the tests achieved good user experience for both scenarios. Prototype evaluation of Service s is very difficult to be performed in terms of metrics, as it is very dependent on the current Context of the Network. User experience and the ability to experience new services, therefore, become the main vehicles to classify the success of our architecture in real environments. 7. Future work and conclusion This paper presents an architecture which integrates group communications in a NGN environment. This architecture is shown in an evolutionary way, with a first instantiation simply bringing the IMS and the MBMS sub-systems together. In a second more evolved approach, resorting to an All-IP concept, our architecture is developed in terms of deployment, leading to a distribution of all group-functionalities by different entities. This integrated approach exposes new approaches to group and broadcast services, where the resource usage efficiency of broadcast/multicast techniques can be merged with individualized interests, creating new and more efficient paradigms both for network and content providers. The prototype was functionally evaluated, and this provides insights for commercial deployment of this solution. Acknowledgements This work was performed as part of EU-IST project C-MOBILE (Advanced MBMS for the Future Mobile World) ( home.html) (IST ). We are thankful to all the partners for their contributions. References [1] ITU Focus Group on Next Generation Networks (FGNGN). < [2] Telecoms & Internet converged Services & Protocols for Advanced Networks. < [3] 3rd Generation Partnership Project. < [4] Open Mobile Alliance. < org>. [5] C-MOBILE project web page: < ptinovacao.pt>. [6] 3GPP TS , IP Multimedia Subsystem (IMS); Stage 2. [7] OMA WID_0075 OMA Service Provider Environment (OSPE) for Improving Integration, Deployment and Management. [8] R. Brennan et al., TISPAN NGN Architecture Overview, TISPAN-3GPP Workshop, Washington, March [9] 3GPP TS , Multimedia Broadcast/Multicast Service (MBMS); Architecture and functional description. [10] 3GPP TS , 3G Security; Security of Multimedia Broadcast/Multicast Service (MBMS). [11] 3GPP TS , Generic Authentication Architecture (GAA); Generic Bootstrapping Architecture. [12] 3GPP TR , Enhancements to IMS service functionalities facilitating multicast bearer services. [13] J. Ogunbekun, A. Mendjeli, MBMS service provision and its challenges, 3G Mobile Communication Technologies, G 2003, in: 4th International Conference on (Conf. Publ. No. 494), June 2003, pp [14] G. Leoleis, L. Dimopoulou, V. Nikas, I.S. Venieris, Mobility management for multicast sessions in a UMTS-IP converged environment, Computers and Communications, 2004, in: Proceedings ISCC Ninth International Symposium on, vol. 1, 28 June 1 July 2004, pp [15] 3GPP TR , System architecture evolution (SAE): Report on technical options and conclusions. [16] 3GPP TS , Policy and charging control over Gx reference point. [17] 3GPP TS , Policy and charging control over Rx reference point.

20 J. Santos et al. / Computer Networks 52 (2008) Justino Santos received his degree in Computers and Telematics Engineering from the University of Aveiro on The graduation also included a six month internship at NEC Europe Labs, Germany where he worked in Vehiclular Ad-hoc Networks and Location Privacy. In September 2005, he joined the Institute of Telecommunications of the University of Aveiro, where he was a researcher. His main topics of interest are multicast/broadcast technologies in next generation heterogeneous networks. Diogo Gomes graduated in Computers and Telematics Engineering from the University of Aveiro in 2003 with first class honors, and has since been working towards a PhD degree in hhqos signaling in broadcast enabled technologiesii at the same university. Since his graduation year, he has participated in several IST Projects such as IST-Mobydick, IST- Daidalos and IST-Akogrimo where he handled QoS and mobility related issues as well as prototype implementations. Recently he participates in IST-C-MOBILE where he holds leadership responsibilities on the deployment of a demonstrator for the project. His research interests include Multicast, Mobility and QoS. Susana Sargento graduated in Electronics and Telecommunications Engineering from the University of Aveiro in 1997, and concluded her PhD in In September 2002, she joined the Department of Computer Science of the Faculty of Sciences of the University of Porto, where she also led the Computer Networks Group at LIACC-UP. She returned to the University of Aveiro and the Institute of Telecommunications in February Her main research interests are in the areas of next generation and heterogeneous networks, infrastructure, mesh and ad-hoc networks. Rui L. Aguiar received a Ph.D. degree in electrical engineering in 2001 from the University of Aveiro, Portugal. He is currently an assistant professor at the University of Aveiro and is also leading a team at the Institute of Telecommunications, Aveiro, on next-generation network architectures and protocols. His current research interests are centered on the implementation of advanced wireless networks, systems, and circuits, with special emphasis on QoS and mobility aspects, areas where he has more than 150 published papers. He is currently TPC-CoChair of ISCC 07. Nigel Baker is Head of the Mobile and Ubiquitous Systems Group, Co-Director of CCCS Research, Associate Professor (Reader) in Computer Science and until 2006 Motorola Fellow. His first degrees were in Physics and Nuclear and Particle Physics. His specialties in the last twenty years have been Real Time Systems, Computer Networks, Distributed Systems and in the last decade Mobile Communications. He has written numerous Journal and conference papers and been a member of many conference and workshop committees covering these topics over the years. He was a visiting researcher at CERN Geneva for six years and worked on several projects; the two most notable were CICERO and CRISTAL. He developed and led the Mobile Applications of Software Technologies (MAST) Programme. Madiha Zafar is a researcher at UWE, Bristol with the Mobile and Ubiquitous Systems Group. She graduated from National University of Sciences and Technology (NUST), Pakistan in 2004 with distinction. From 2005 to mid 2006 she was a Research Associate at Motorola Ltd., Swindon, where she was involved in research related to mobile multicast and broadcast technologies and participated in the development, testing and demonstration of MBMS-enabled UMTS simulator for EU IST FP6 project B-BONE. Currently she is part of the EU IST FP6 project C-MOBILE. Her research interests lies in the domain of ubiquitous systems, smart spaces and context-aware communication. Ahsan Ikram is a post graduate student and researcher with Mobile and Ubiquitous Systems Group at University of the West of England (UWE), Bristol, UK. He received his BS (Software Engineering) in 2004 from National University of Sciences and Technology (NUST), Pakistan. Before joining UWE he has worked at NUST, Pakistan and the California Institute of Technology (Caltech), USA, as a Research Associate. His main areas of research have been distributed computing, mobile computing and mobile application development. His research interests are next generation mobile networks, applications and sensor networks. Currently, he is working on EU IST FP6 project C-MOBILE.