Apr 7, 2000 - users [5]. Due to market saturation, network service designers need to find a way to maintain user demand by differentiating their services.
Designing QoS and charging mechanisms: he who pays the service shapes the design A. Bouch & M. A. Sasse Department of Computer Science University College London Gower Street, London WC1E 6BT, UK Abstract-To accommodate applications with highly variable degrees and categories of Quality of Service (QoS) requirements, a major effort has been invested to overcome the traditional best-effort service quality of the Internet [1],[2]. Nevertheless, there has been a shift towards weak QoSassurance paradigms based on the differentiation of different QoS levels [3]. What is currently missing is a technique for specifying user preferences for QoS and network charging mechanisms. If the Internet is to remain economically viable, demand for Internet services must be maintained. Specifying users' QoS requirements therefore has potentially crucial implications for providing a degree of service that will encourage users to pay for the quality that they receive. This paper investigates the role of conceptual design in establishing users' mental representations of Internet services and their charges. We report empirical work that shows how these models can be used to predict the acceptability of Internet services. Preliminary guidelines for the design of QoS charging mechanisms are presented.
I. INTRODUCTION Internet usage is expected to grow to over 85 million users in 2002 [4]. Other researchers have suggested that there are already more than 100 million Internet users [5]. However, the current growth rate for Internet usage is considerably less than the growth rates from 1996-1997 (124%) and 1997-1998 (68%). While a vision of future networks offers the potential to break traditional barriers in communications and commerce, the current service quality is often unacceptable to users [5]. Due to market saturation, network service designers need to find a way to maintain user demand by differentiating their services. Resource provisioning for QoS support in packet-switched networks has lead to the design and development of sophisticated QoS architectures, notably ATM, IntServ or DiffServ. However, what is often overlooked in the technical community is that there are several stakeholders in the design of Internet services: server designers, network providers, advertisers, companies whose products are sold on-line, and consumers themselves. Economics is driven by demand. Failure to meet users’ QoS requirements can impact users’ perception of a company’s stature and commercial viability which, in turn, affects the business interests of service providers and advertisers [6]. The future Internet will have more users and support a greater diversity of Internet applications. It has the
potential to change the way that consumers interact with companies. Research is needed to identify users’ requirements for QoS and the schemes that are used to charge them for it. Only through such identification will it be possible to achieve the customer satisfaction that leads to the success of any commercial system [7]. Traffic produced from different applications, or service classes, can be characterized through an associated payment ([8][9][10]). For example, high-volume video may be sent as priority traffic by associating it with a high-priced stream. Most pricing schemes are based on the assumption that the amount and type of quality that is implementable within the network is identical to the quality that will be paid for by users. Previous research has shown that users judge the acceptability of pricing schemes according to a variety of dimensions [8]. The salience of these dimensions is not determined by the fact that they are technically implementable, but by their semantic value. What is clear is that users’ definition of QoS is multi-dimensional and depends on what they are trying to achieve (see e.g. [11]); users consider multiple factors when judging the acceptability of levels of QoS, and deciding whether to pay for it. In networked multimedia applications in particular, variations in quality at the network level are not directly linked to the subjective assessment of quality received by the user [12]. This paper investigates users' requirements for QoS, focusing on conceptual design principles. The contribution of this paper is threefold: 1.
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Conceptual design is successfully tailored to network QoS and charging issues, providing scope for integrating and extending this methodology to networking environments. Conceptual models are generated that can be used to predict users' acceptance of QoS and charging schemes. Guidelines are presented from which network service designers can assess the acceptability of resource allocation and QoS charging mechanisms.
Section 2 shows how conceptual design can provide understanding of users' requirements in general, and how its principles can be applied to the investigation of requirements for QoS and charging in particular. Section 3 describes empirical work designed to investigate users' models of the Internet functionality that supports QoS and charging.
Essential guidelines for network designers are suggested in Section 4. II. CONCEPTUAL DESIGN Quality is currently defined at a lower level, using metrics such as packet loss., Defining quality with reference to particular user activity, however, carries semantic value to the user. For example, users may conceptualize the quality they receive in terms of attributes of the media stream (e.g. smoothness), or attributes of the entire session (e.g. reliability). From a user’s perspective, quality associated with an application is a multi-dimensional, variable concept [11]. What determines quality depends on the purpose of interaction. For example, [13] identified continua within which users could describe a particular level of QoS, according to the particular context in which they were operating. Users possess mental representations of networks, and of the QoS they receive from those networks. They use these models to form expectancies about that QoS, which in turn determine whether the delivered level of service is seen as “value for money”. Our work applies the principles of an HCI design approach, conceptual design [14] ,which states that users form a users’ model of the network system through they interaction with the system image (see Fig. 1). The structure and function of this model influences what users expect from the network, including their impression of the level of QoS they receive. This means that knowledge of users' models can provide designers with essential information on how to meet users' QoS expectations.
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inelastic application, requiring a relatively rich set of performance guarantees. Elastic applications, on the other hand, are less dependent on the speed at which data is delivered [15]. However, traditional QoS metrics such as response time and delay no longer suffice to fully describe Quality of Service as perceived by users [16]. The Internet is a layered environment: It consists of many components, some of which -although integral to its operation -are hidden from end-users and not well understood by them (see below). The extent and implications of users' misunderstanding about network structure and function needs to be established.
A. Users' models Previous studies have shown that conceptual design principles can provide applicable guidelines for the design of Internet services. Thatcher & Greyling [17] [18] looked at users' models of the Internet and found that novice users' models are extremely simple, consisting of two components, the user and the Internet 'cloud'. Sheeran et al. [19] investigated intermediate users' models of a series of common Web-service breakdown situations. The authors argue that the location of the breakdown attributed by the user can determine their ability to recover from the error. Furthermore, the attribution of error to the local-level (e.g. the user has made a typing error in a URL), as opposed to server-level has potentially important implications for users' perception of network QoS. The results provided in [19] show that novice users understand and represent the network very differently than expert users. Novice users attribute poor service to an error in the Web address. This may be because they have only a limited idea of the description or function of Web servers. B. The system image
Fig. 1: Conceptual design (after Norman, 1986) Conceptual design has to date not been applied to the design of Internet QoS management and charging mechanisms We argue it is particularly needed in this areafor two resaons: 1.
In the networking community, different technical Quality of Service (QoS) requirements have been ascribed to applications: real-time multimedia is an example of an
Perry (1995) found that the amount and type of knowledge Internet users have varies widely, and that novice users had to dramatically rethink their understanding of Internet services in order to perform what had been thought of as simple mailing tasks [20]. More recent evidence suggests that the system image doesn't fit novice users' models of the Internet. Novice users are unable to identify the server or host name when provided with a URL, but both these words are often used in both Netscape Communicator and Internet Explorer's error messages and status bars [19]. This means that the system image is not intuitive to users, suggesting a mismatch between the designer's and user's model. This paper will show that, when asking users to make payments for Internet services and levels of service therein, there are crucial consequences in failing to provide a system image that fits with the user's model. The work reported in this paper differs from previous work (e.g. [17]), in that differential QoS and charging is a by nature a dynamic process. We are therefore concerned with both
structural - how should we represent the network to users, and procedural issues - how do users' tasks map onto to this structure? Additionally, [19] conclude that users' models are “patchy and inaccurate” by referring to specific error scenarios. This paper reports work that attempts to capture users' models of Internet QoS by allowing users themselves to specify their own scenarios. We argue that the models are only inaccurate because of the lack of integration between network designers’ vision of the Internet services, and users' expectancies of those services. The following section describes an attempt to investigate design principles that might guide this integration.
certain level of QoS than tasks that are frequent and in where users' expectancies have formed into habit. The implications of this is that network feedback about typical QoS levels associated with infrequent tasks is necessary to form and maintain users' QoS expectancies and their consequent acceptance of lower levels of QoS [21]. $
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III. EMPIRICAL WORK !
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Our study was structured to capture users' representations of both the structure and function of the Internet. This involved:
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Collection of statistics on the type of tasks typically performed on the Internet. Analysis of how users map these tasks to underlying Internet functionality. Collection of statistics on users' representations of current Internet charging schemes. Analysis of users' interpretation of how Internet functionality should support their preferred charging scheme.
A questionnaire covering these 4 aims was sent to novice Internet users via email. The questionnaire was also given to expert users to provide a comparison group. Questions concerned users' tasks, metaphorical representations of Internet services, preferred charging mechanisms and user demographics. Open-ended Questions asked users to provide examples and drawings of their understanding of Internet services and charging. This provided results with the richness necessary to model users' understanding of these issues. 46 responses were received. A. Users' Tasks The first set of questions asked users state their primary and secondary purpose for using the Internet. The data obtained allowed us to classify users' tasks. Fig. 2 shows that there is a large difference between task frequency for some tasks. Email is a frequent task and ecommerce, chat and downloading temporary information are all infrequent tasks. However, downloading temporary information is a task that is necessarily infrequent; the information might not be on-line long enough to support multiple accesses. This is important because previous work has shown that users are much more likely to accept the level of QoS provided to them as long as it meets their expectancies (e.g. [21]). An important point is that tasks that are necessarily infrequent are less likely to maintain users' expectancy for a
Fig. 2. Frequency of tasks
B. Task Operations The level of QoS users expect for each task depends on the operations users premeditate they should perform when executing the task. We asked users to describe, in any way they felt best, the operations of the network that enable them to carry out their primary task previously specified. By asking such open-ended questions, we can elicit users’ models of Internet operations that should be mapped to designers’ models. Information was distilled through language and concept analysis*. Users split task operations into 2 stages, Access and Operation. B.1 Access Respondents focused on Internet access as the primary and most important step in achieving their task. Access was defined by respondents as a) Acquire hardware b) Acquire software ‘toolkit’ and c) Access software ‘toolkit’. There is a loop from c) to b) representing the acquisition of multiple pieces of software. Subjective attributions are associated with each of these steps. Such attributions are useful to network QoS designers as they reveal users’ expectancies. They are: • Assessment of brands: To novice users, brand names such as Internet Explorer and yahoo are the Internet services. As opposed to lower-level routing operations, novice users see brands as objects that represent and are responsible for the operations that complete their task. Brand names are Anchors to certain QoS expectancies. • Trustworthiness and Uncertainty: When acquiring a software toolkit, novice users assess the trustworthiness of each brand. At each stage of ‘See [26]for a simple description of concept analysis.
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accessing the Internet, there is no room for what can be termed Uncertainty. This means that users expect the QoS (here defined as ‘access Internet’) to be error free- where errors can be defined, for example, as delay or failure to fire up a browser. This is because certain brand-names are seen subjectively as trusted representations of good service. Users’ expectancies of the QoS of an entire future interaction are severely impaired if an error occurs at this stage. B.2. Operation The second component users perceive to be involved in task achievement is Internet Operation. Internet operation involves 3 stages a) Fire-up browser, b) Select engine and c) Enter through gateway. Fig. 4. shows these stages, together with users’ subjective attributions at each stage. These attributions are: • Gateways: Novice users believe that software represented by brands represent gateways into the Internet ‘cloud’. Indeed, novice users possess no model of the multiple component Internet; their perception of the Internet is confined to the local machine. This concurs with pervious work [17][18][19] (see Section II). • Uncertainty: At the operation stage, users will accept lower levels of QoS and potential uncertainty concerning the affordance of the Internet to fulfill their task. This largely because of novice users’ conceptions of the Internet as vast open space. For example: “The Internet is like a big sea where people from all over the world go to communicate.” •
Facilitators: One of the most interesting aspects of users’ models of Internet operations is the role of what can be called facilitators. Facilitators are realworld roles that have been transferred to an on-line environment. These facilitators take the responsibility for, and uncertainty involved in, task accomplishment. Our previous work had shown that users will accept a lower level of QoS as long as it is predictable (e.g. [21]). This suggests that Predictability is an important criteria for high QoS. Our new study shows that a relatively unpredictable level of QoS, as is inevitable in any multiplexed, dynamic environment, is more likely to be accepted if a facilitator is used. Our results show 3 types of facilitator are attributed to the task: librarian, postman and trustee. Users construct the role of a trustee when they are asked to evaluate he level of QoS they will receive against a financial penalty. The facilitators are higher-level constructs that are separate from brands that represent users’ software toolkit.
Privacy and Anonymity: Previous research has shown that, if users' privacy is invaded unexpectedly, they loose trust in the technology that afforded the invasion [22]. Our results indicate that users' perceptions of QoS depends on whether they trust the technology to keep their interaction private. In terms of the email-postal service metaphor, for example, novice users expect total privacy in their transactions-just as they expect their personal paperbased mail not to be opened by another. 50
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