Solution Design for Efficient Distribution of Multimedia-based Services to Home Residences Liam Murphy University College Dublin Computer Science Department Belfield, Dublin 9, Ireland Phone: +353 1 716 2914 Fax: +353 1 269 7262
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Gabriel-Miro Muntean Dublin City University School of Electronic Engineering Glasnevin, Dublin 9, Ireland Phone: +353 1 700 7648 Fax: +353 1 700 5508
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Abstract. This paper presents a design problem and its solution in form of the Quality Oriented Adaptation Scheme (QOAS), the client-server system that implements it and the related architecture for the delivery of multimedia-based services to residential users. QOAS was proposed to adaptively stream high quality multimedia-based services to home residences, adjusting the content based on existing delivery conditions. QOAS principle, the system that deploys it and the architecture of the system for the delivery of multimedia-based services are briefly described. Subjective testing results are also presented and show very good performance of the implemented QOAS-based multimedia delivery system. Keywords: Software Development, Multimedia Streaming, Subjective Testing
1. Introduction The emerging broadband infrastructure [1, 2, 3] enables the delivery of very high quality multimedia-based services such as Video on Demand and Videoconferencing to residential users. The success of this trend highly depends on the popularity, quality and price of services provided, which all rely on the technical solutions involved in delivering rich media content to users. Therefore the problem relies on the efficient design of a client-server system that best balances the cost with the quality when providing these services. This paper presents a design problem and its solution in form of the Quality Oriented Adaptation Scheme (QOAS)-based client-server system. The paper also discusses the related architecture for efficient delivery of multimedia-based services to residential users. Before designing QOAS, significant issues related to various architectures for distribution of services are considered. The advantages and disadvantages associated with these architectures are assessed in relation to their influence on the QOAS’s design. QOAS principle is then presented as an end-to-end adaptive solution for streaming multimedia that offers high quality multimedia-based services to home residences via local broadband IP-networks. The paper ends with some subjective test results that show the very good performance of the clientserver system that deploys QOAS in terms of end-user perceived quality.
2. Distribution solutions for multimedia-based services Significant effort was involved in proposing different architectures for delivering information in different forms, including multimedia data, to home residences via telephone or cable TV infrastructure [4, 5, 6]. Lately these solutions were reviewed, addressing broadband connectivity and targeting especially broadband IP-networks [7, 8, 9, 10]. The principles behind the architectures used for delivering multimedia-based services are similar and very few details differ. They can also be applied for distributing services to business premises.
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In general, three main approaches for architectures aimed at delivering services to home residences and business premises were taken into account in [11]: a distributed architecture, a centralised one and a hybrid solution. Next they are discussed in the context of multimediabased service delivery. 2. 1. Centralised Architecture Figure 1 presents a typical example of a centralised architecture. This architecture includes a centralised headend and a number of distribution hubs through which the headend is connected to diverse groups of users. At the headend, a multimedia server (or a pool of servers) with access to a multimedia database provides among other services multimedia streams to residential customers via the distributions hubs. The hubs have minimal responsibilities, which mainly concern data forwarding in both directions: from the headend towards the users and from the users to the headend. The main advantage of this approach is that it requires only one multimedia server (or server farm) and only one multimedia database, with apparently low hardware costs and, mainly, reduced location and maintenance costs. Also the security is easier to be provided for this approach since a single location has to be protected. The most important disadvantage of this solution is that very much pressure is placed on the IP backbone between the headend and the distribution hubs, pressure that increases significantly with the number of customers served. Headend Multimedia Server & Database
IP Backbone
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Hub
Hub
Users
Distribution
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Figure 1 Centralised architecture distributing multimedia to residential users
2.2. Distributed Architecture Headend
IP Backbone
Distribution Hub
Distribution Hub
Multimedia Server & Database
Multimedia Server & Database
Users
Distribution Hub
Users
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Figure 2 Distributed architecture for delivering multimedia to home residences
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A typical case of a distributed architecture is presented in Figure 2. Similar to the centralised approach, the distributed architecture includes a centralised headend and a number of distribution hubs through which the services are provided to diverse groups of users. However the headend offers only other services such as Internet connectivity and does not provide any multimedia-based service. In this case the distribution hubs have an important role since at their level there is a multimedia server (or a server farm, but less likely) and a multimedia database that contains multimedia content to be provided to the residential users. The greatest advantage of this distributed solution is that it releases the pressure placed on the IP backbone in the centralised approach. In this case multimedia data, which have timing constraints and significant sizes and is expected to account for an important part of the total traffic, is served locally. The fact that multimedia-based services are being offered to a smaller group of customers helps at reducing the complexity of the multimedia server system, which could consist of a single server. However, since these simpler multimedia servers and their associated multimedia databases are placed at every distribution hubs, other issues appear which are not favourable to this solution. The disadvantages are mainly in relation to the costs involved in the maintenance of these distributed hubs (i.e. location, power, security) and to the update of the distributed multimedia databases. 2.3. Hybrid Architecture Headend Multimedia Server & Database
IP Backbone
Distribution Hub
Distribution Hub
Multimedia Server & Database
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Figure 3 Hybrid approach for distributing multimedia-based services to residential users
An example of a hybrid architecture for distributing multimedia-based services to residential homes is presented in Figure 3. It combines some of the issues described in the centralised solution with those of the distributed approach. The hybrid architecture includes a headend similar with the one that exists in a centralised solution and a number of distribution hubs with structure and functionality similar to those from the distributed case. In this solution the local multimedia server systems from the distribution hubs serve the associated group of users for the majority of their requests. If for some reason a request cannot be fulfilled, the request is re-directed towards the headend whose server system will answer to it. Other versions of this hybrid approach involve caches located at the level of distribution hubs instead of local multimedia servers and they may be very useful. However although caches are very well studied and recommended for being used with Web content, for continuous media with different characteristics and different interaction with the users for example, the advantages are not yet fully balanced against disadvantages [8]. By using the hybrid architecture, which is a combination of a centralised approach for lowdemand content and a distributed solution for high-demand content, a compromise is also made in terms of advantages and disadvantages.
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2.4. Discussion A significant decision for the software engineers as well as network operators and service providers is whether to use a centralised, a distributed or a hybrid solution. A centralised architecture avoids the costs of installing and maintaining multimedia servers in remote distribution hubs, but offers limited scalability because of the additional load that is placed on the IP backbone network. If a centralised solution may be acceptable for voice and data-based services, for video-based services at least 10 times more bandwidth over the IP backbone is necessary for each subscriber, making congestion more likely to occur. The consequent delay, jitter and packet loss may severely affect the quality of remotely delivered video services. The requirements of bandwidth at the IP backbone level are minimised by locating the video servers as near as possible to the subscribers, like in the distributed approach. The claims that a distributed solution may cost more than a centralised one are contradicted by a comparison performed taking into account the distributed connectivity costs, core network bandwidth, headend and distributed server costs, storage and set top box costs. The results reported in [7] found out that a distributed approach may cost less than a centralised solution, while it could have supplementary benefits in terms of performance for the end-users such as reducing delays and delay variations in video deliveries, for example. Although the hybrid solution seems to make a compromise between the advantages and disadvantages of the previous two approaches, it also relies very much on the multimedia servers from the distribution hubs to serve a large majority of the requests in order to avoid the congestion of the backbone, which may affect both multimedia and other services. In consequence this research focuses on the local delivery of multimedia-based services in broadband IP-networks as they carry the very large majority of the overall traffic.
3. Multimedia distribution via local broadband multi-service IP-networks Different architectural designs allow for multimedia-based services delivery from distribution hubs to residential homes [5, 7, 9]. Among them, for instance Figure 4 shows a pure horizontal distribution structure whereas Figure 5 presents a tree-like structure. Distribution Hub Multimedia Server & Database
Figure 4 Horizontal solution for local distribution of services to home residences Distribution Hub Multimedia Server & Database
Figure 5 Local service distribution to home residences in a tree-like manner
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Regardless of the chosen solution for the distribution of services to residential users, the infrastructure that connects the distribution hub with the users has to support all the traffic exchanged by them. A schematic representation of the architecture of the problem is presented in Figure 6. Delivery Infrastructure Distribution Hub
User User
Multimedia Server
User
Multimedia Database
Figure 6 Architecture for local multimedia delivery to residential customers
The multimedia server located at the level of the distribution hub was named in [8] local video server, name that will be also used in this paper. It is in the interest of network operators, service providers and the customers to reduce the effort for providing these services. A significant source for decreasing it is to raise the number of customers served from a fixed infrastructure, while maintaining a good perceived quality. Therefore, in order to reduce the costs it is assumed that the delivery system is designed such that - at least for periods of times - the infrastructure is overloaded, having potential for congestion. In this context the problem of delivering high quality multimedia with little effort to home residences can be reduced at two simpler problems. First the solution has to allow for multimedia streaming in increased traffic delivery conditions. The traffic can be of different types and could have various variation patterns as it originates from different services not only multimedia-based ones. Therefore the solution must be adaptive to existing delivery conditions. Secondly the multimedia delivery solution must maintain the high end-user perceived quality in spite of the adjustment to the changes in the delivery situation. In response to this the Quality-Oriented Adaptation Scheme (QOAS) was proposed as a unicast adaptive solution for delivering high quality multimedia to the users. Its deployment involves placing components at both the level of an Adaptive Server Application (Ad.Srv.App.) and an adaptive client application (Ad.Cli.App.), as shown in Figure 7. The infrastructure that links the server and the client QOAS components is a local multi-service IP network with short propagation delays and high potential for congestion. IP-Network Ad. Srv. App.
Ad. Cli. App.
QOAS
QOAS
Multimedia Database
Figure 7 QOAS deployment at the level of an adaptive client-server system
4. Proposing Quality-Oriented Adaptation Scheme for streaming multimedia QOAS’s goal is to maximise the quality of a multimedia stream that is transmitted over a local IP network, as perceived at the receiver. This end-user perceived quality is mainly determined by the transmitted quality of the stream but is also directly affected by eventual 5
problems that may occur during multimedia transmissions. Among the causes of these streaming-related problems are networks overloaded with traffic that determine significant changes in the value or variation of transmission-related parameters such as loss, delays, jitter, etc. as mentioned in [12, 13]. The perceived quality at the receiver (QR), the servertransmitted quality of the multimedia stream (QT) and a set of network-related parameters, as measured at the receiver, for instance in number of N, (Pi, 1
