This paper reports on the transfer of the traditional QoS notion into ... Especially continuous media ... technology with high data rates) [de Pryker 931 offers the.
Quality of Service (QoS) in Distributed Hypermedia-Systems Gerrit Kalkbrenner, Teodor Pirkmayer, Arnd van Dornik, , Peter Hofmann Technical University of Berlin Forschungsschwerpunkt PV / PRZ,TUBKOM Infrastrukturprojektfiir BERKOM-Aktivititten
Abstract This paper reports on the transfer of the traditional QoS notion into the environment of distributed hypermedia systems. We will show that the concept of QoS that has been developed in the technical environment is unprofitable for the user of a hypermedia application. A new view should emphasise the use of the QoS terminology based on the requirements of the user, who wants to express quality with regard to his subjective needs. A user interface called "Quality Query by Example" will be presented as a solution for these requirements.
In this paper we will try to give a basic overview of the concepts of QoS and to emphasize the coherence between QoS and hypermedia systems.
2. The Items of QoS In this paper we follow the definition of [QOS-Map 911: "QOS is a set of user-perceived attributes of that which makes a service what it is. It is expressed in user understandable language and manifests itself as a number of parameters, all of which have either subjective or objective values ".
(NP) Provider Performance (PP)
Figure 1 : Items of QoS
1. Introduction Research about Quality of Service aims at mapping the term "quality" known from everyday life to telecommunication objects. It can be assumed that providers and customers see the term "quality" from a different perspective. Efforts are being made to describe the quality of telecommunication products in a common language in to order be able to negotiate quality of service parameters. It it possible to develop a new class of multimedia systems [Berra 921, [Campbell 931 using the new networking technology emerging now. Especially continuous media make high demands on the network and terminals. ATM (Asynchronous Transfer Mode, a new cell-based network technology with high data rates) [de Pryker 931 offers the ability to reserve bandwidth for a network connection. The research about QoS define a framework to classify new communication requirements. There have to be provisions for the end-to-end coordination of QoS, renegotiation of QoS parameters, and the coordination of several parallel connections.
0-8186-7180-7/95 $04.00 0 1995 IEEE
The performance of a service has to take into account four objects: the provider, the customer, the agreement between the two and, last but not least, the service itself. (Figure 1). The agreement determines the financial negotiation between the provider and the customer and has to be negotiated before the service can take place. This topic is not going to be discussed in this article.
Quality of Service (QoS) is seen from the customers point of view.
Network Performance (NP) expresses the provider's point of view. ]In [QOS-Map 911 the author favours the notion Provider Performance (PP), as the quality of the service can not be limitated to network parameters. In general the service can be regarded as a set of commodities and functions that are offered by the provider and used by the customer. The customer himself can become a provider assumed that he is able to extend the service or to bundle it with further functionality.
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T h e Service is a behaviour of the service provider in favour of the customer. * The Service is defined through the interactions between provider and customer. e
ice (PP)
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We describe an object by defining attributes and by optional attributes (four wheels will define a car, Network Performance (NP) the maximum speed is not a defining feature). Figure 2: Customer, service, and hypermedia provider The quality of a service In the conclusion we have to state that there are no terms of will be determined only by the optional attributes. QoS provided at the OS1 application layer: QoS parameters are just handed down to the transport layer. 3. QoS State of the Art
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Traditionally, the notion of QoS, as it is used in the field of communication, refers to properties of network services that are evident to programmers using the interface of a transport system. These properties take into account only the aspects of the service concerning the network provider. The quality of service in terms of subjective user requirements was not expressible and for this reason, QoS parameters were not under control of the application.
The user who wants to specify individual QoS in his application has to leave the application level and communicate with underlying layers. This strategy contradicts the software engineering requirement postulating the isolated implementation of each layer.
As there are no QoS mechanisms provided by the application layer we propose a new terminology that allows the user to express his QoS requirements in common terms. In the meantime communication standards like the OS1 and This notion has to be mapped to terms of network CCITT recommendations make it possible to specify parameters. The underlying layers may accept or reject the user defined requirements. network requirements.
The OS1 application layer [Henshall 881 has only a very 4. Design of a QoS Model for distributed limited notion of QoS: QoS parameters are just handed down Hypermedia Systems to the transport layer. There is no application oriented QoS framework. In this section a notion of quality in distributed hypermedia systems is developed. The customers want to query and C C I R satisfy the needs of QoS configuration in future navigate in the hyper document pool of the service provider. ATM and B-ISDN networks by developing a plain set of She wants to display the documents found on her QoS parameters [CCITT 881. The user who wants to workstation screen. Because these documents are stored on a establish a connection passes his QoS requirements to the remote data store the documents requested (including texts, ATM network. This message must contain a precise pictures, audio and video data) have to be transferred over description of the QoS necessary for the transport of the user communication channels. data. For this purpose the QoS requirements of the user must be mapped to the terminology of the ATM transport We see the search in the provider's hyper document layer. repository and the presentation on the users display as a service where two service providers are involved: the The "connection acceptance" function is used for the network provider and the hypermedia provider. One of the reservation of the required resources. After the reservation is requirements is, however, that the networWhypermedia accepted the data transfer is controlled by the ATM system storage combination must be transparent for the user: she and excessive cells are dropped. Characteristic parmeters for
perceives QoS parameters such as delay, optical quality of
a bit-coded video are: peak rate = 50 Mbps, middle rate = 25 pictures, and if the document content is useful or not for her. Mbps, burstiness = 2 and peak duration = 10 ms. She cannot differentiate between the contributions of hypermedia provider and network provider to the overall QoS As we have shown QoS is actually used as a terminology of perceived. From these QoS parameters the "provider the network- and service provider. Nevertheless, we performance" (PP) for the hypermedia provider and the emphasize the necessity of a QoS notation adequate to the network provider has to be derived (see figure 2). user's needs. The customer is confronted with two problems regarding the specification of adequate quality parameters when requesting a hyper document:
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* the speed of picture display reaction time for queries color brilliance of pictures 0 size of pictures and moving images maximum delay of video display query result relevance * font readability the quality of hyperdocuments with regard to their content
The customer has few linguistic means to express her QoS requirements. One possibility would be to express these requirements using physical quantities (such as frames per second when describing video QoS). This approach is not very intuitive. An abstract specification of the requirement profile does not seem adequate. What does it mean to display video with a certain frame rate or pixel graphics with 8 or 24 bit colour depth? The customer's subjective impression is important. The same QoS -in terms of its quantitative description -- is perceived differently by different persons. Therefore, a customer-oriented description is necessary firom which QoS parameters can be derived. Using this description NP and PP can be determined. Because there is no special notation for QoS parameters i n the upper OS1 layers the user has to know details about the lower layers to specify adequate QoS parameters. Therefore, the user is forced to side-step the OS1 abstraction levels.
Search Method of Requests
* Search Method * Bandwidth Another method is to view e Document Variants @Jitter QoS parameters in an * Delay application-specific context. ... The presentation system prompts the user for her requirements using natural .Figure 3: Mapping of customer QoS requirements to QoS text that the user can directly understand. The user tlhen Because certain parameters are at variance (i.e. low cost vs. controls the quality level using relative commands such as high resolution) the change of one quality parameter "more" or "less". inevitably effects the adaptation of the other, contradictory parameter. The system should provide reasonable heuristics This method is easily implemented using a graphical user for these cases. interface. By manipulating the control elements of the user interface the user defines her requirements in an informal manner. The current setting is demonstrated by means of an 4.1. Application Scenario example document. If the user changes parameters setting she is provided with an immediate feedback when the The following customer quality requirements can be example document is redisplayed using the new quality identified at this point: level. the quality of hyper documents content * the speed of picture display
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Quality Query by Example
ocument Quality not available Quality Prospect Quality
16 Frames I sec
Journal Quality
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system components are implicated. We use a process consisting of several steps, explained in the next chapter:
4.3. Registration of the QoS Profile
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In the first step the client system will determine the QoS profile. This profile describes the limitations of the system Cost (this Example) in which the user can choose his QoS $230 parameters. The profile takes into consideration t h e p o t e n t i a l and limitations of the software, the hardware, the transport system, the network and the media server. Newspaper Quality
Figure 4: QQbE Dialog, good quality reaction time for queries colour brilliance of pictures size of pictures and moving images maximum delay of video display query result relevance e font readability
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The QoS profile describes the maximum ranges that need not be fulfilled for a concrete information transfer. The degree of fulfilment itself is a QoS parameter.
On the customer side these requirements have to be mapped to QoS parameters. This is done in several steps. To describe this process a look at the application scenario is necessary. The hypermedia archive is a distributed system based on a client-server architecture. A client wanting to display hypermedia documents uses the services provided by the hypermedia server. These service consist of the transmission of hyper documents, navigation and query in the hyper document pool and the provision of functions to manipulate hyper documents. The data flow between client and server is organised by a transport system.
The maximum ranges of the QoS profile will be presented to the user, so that he is able to estimate lower user quality parameters for his session. Mostly the maximum possibility of quality isn’t desired because of financial constraints. These user quality parameters will be applied to all future document fetching processes, until the user changes them another time.
4.4. Interactive Query of Customer uality Requirements (Quality Query by Example)
When determining QoS following constraints can be For interactive request of the user requirement we use the method of “Quality Query by Example”. This method takes identified: ideas from the interactive development of database queries e customer quality requirements (Query by Example), in which the user can approximate his limitations of the transport system query based on examples generated by the system. * limitations of hard- and software * cost constraints With “Quality Query by Example” the user controls some e the assortment of hyper documents offered by the quality parameters on a graphical user interface in a non hypermedia provider technical manner (see figure 4). The system demonstrates The server offers documents and media in different the chosen quality with example documents, with some representations, i.e. different quality levels. The user has to graphical controls like slider or checkboxes. One field weigh cost aspects against the desired presentation quality. indicates the presumable cost for retrieval and presentation of Possibly a change of requirements or limitations may occur this example document. We think that a linear quality scale in a later stage if further processing of the hyper document is will not be practical so we quantify the quality spectrum desired (e.g. to print out a hyper document larger resolution into ranges. Taking pattern from the classification of printmedia we choose the quality classes “Newspaper”, fonts are requested) (see figure 3). “Journal” and “Prospect”.
4.2. QoS Negotiation As we can see user requirements have end-to-end significance. To fulfil the QoS agreement most of the
Quality parameters can be contradictory like cost and resolution. The change of one parameter can cause the adaptation of other parameters. The system should be able to control the relations between these parameters i n a
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meaningful manner. There should be a possibility for the user to control these heuristics. With this heuristic the client will always try to attain the best possible presentation quality of each medium with The quality example is generated by the client system regard to cost. A catalogue gives some criteria about the without accessing the network. This way, the system is able meaning of “best” for each medium. There is also a to present quality classes that are not actually possible with meaningful priorisation about the quality of text, image, this system configuration. [QOS-Map9 1] requires that the video and audio, if there are some of this media together in comparison with other telecommunication services is one document. The user can give additional priorisation. possible thus inciting the user to buy better end systems With this the system generates a ranking list. andor use better (and therefore higher-priced) services. 4.5.2. 66Best-Fit” Heuristic With this interaction the system asks the user about his requirement. From this requirement the system can derive The second important heuristic, the “Best-Fit” heuristic, the set of QoS parameters. This set of QoS parameters is takes a selection from this ranking list. This selection valid for the whole session. In combination with should not cross over the QoS profile limit. This selection information about the document pool offered by the will be done according to the selected user quality and a set Hypermedia provider we we now able to calculate “Network of fixed rules. Some of these rules describe the minimal Performance” (NP) and “Provider Performance” (PP) for requirement for meaningful presentation. document transfer. One example: audio and video must be transferred and presented isochronously. With limitations of bandwith still 4.5. Heuristics images must be transferred first. The QoS profile enables several quality possibilities for one concrete transfer event. The system must choose between These rules take care of response time, cost and other user requirements. these variants. The document server holds documents in different resolution variants, which obtain different requirements in available bandwidth, resolution and end user system configuration. The server is also able to calculate other image resolutions than stored. The client is able to minimise the overall cost considering the partial cost for transfer, calculating, licence and other. The criteria foir minimization are influenced by user priority (Figure 5).
4.6. Exception Handling Somethes the negotiated QoS profile can not be guaranteed during one transfer or until the end of session. The server can indicate that a medium isn’t available any more in a required resolution. The transport system can indicate the reduction of available network performance caused by a hardware fault. These cases are violations of the QoS profile that must be handled softly according to the user.
Priority Heuristic cost
Video
Colour Resolution
First some exceptional presentation for the related media are chosen to stabilise the system on a lower level of quality. This can be acheved by reducing the resolution of pictures or frame rate in videos with the goal of reducing the required bandwidth on the network. Instead of colour images there is the possibility to transfer grey scale images.
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The user will be informed about the exceptional presentation and can refuse this choice.
4.7. Renegotiation In some cases renegotiation of QoS will be necessary.
5. Summary
Figure 5: priority for mapping QoS to PP and NP
This paper reports on the transfer of the traditional The is carried in two First the QoS notion into the environment of distributed hypermedia variants will be evaluated, then one variant will be selected. systems. For this selection we use two heuristics: The application and presentation layer of OS1 gives no 4.5.1. “Best-Result’’ Ikieuristic sophisticated model for quality of service projection from
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application down to the transport system. There are no mechanisms to change quality of service parameters during a session. In the system proposed in this paper the user gets the possibility to express his quality criteria using an example. With the “Quality Query by Example” dialog the user can configure a quality profile, that takes into consideration the potentials and limitations of the software, the hardware, the transport system, the network and the media server. Based on this it is possible to map the user quality profile to network performance and provider performance.
6. References [de Pryker 931 Martin de Pryker: “Asynchronous Transfer Mode, Solution for Broadband ISDN”, Series in “Computer Communication and Networking”, ISBN 0-13-178542-7, Ellis Honvood, New York 93 [Berra 921 P.B. Berra, C-Y. R. Chen, A. Ghafoor, and T.D.C. Little: “lssues in Network and Data Management of Distributed Multimedia Systems”, in: Proceedings of the First International Symposium on High-Performance Distributed Computing (HPDC-I) September 1992, IEEE Computer Society Press 1992, ISBN 0-81 86-2970-3 [Campbell 931 A. Campbell, G. Coulson, F. Garcia, D. Hutchison, H. Leopold: Integrated Quality of Service for Multimedia Communications, Technical Report MPG-93-17, Department of Computing, Lancaster University [CCI’IT 881 CCI’IT, General Aspects of Quality of Service and Network Performance in Digital Networks, including ISDN, CCITT Recommandation 1.350 Blue Book Fasc. 111.8 (1988) [Henshall 881 Henshall, J. , and S. Shaw: “OS1 Explained: Endto-end Computer Communication Standards“, ISBN 074580253-2. Ellis Horwood, New York, 1988. [QOS-Map 911 RACE Project R1082: Mapping of QoSParameters to Network Performance Parameters Initial Parameters and Analysis, QOSMIC, Del. I . 3A, 82/ KT/ LM/ DS/ B/ 004/ bl, June 1991.
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