Thực hiện chất lượng dịch vụ trong các mạng IP (P2)

2 Service Quality Requirements In this chapter, means of assessing and specifying service quality requirements are described. To lay the foundation for discussion, the following questions need to be answered: • What is a service? • What kind of services are there? • What does it mean to provide quality support for a particular service type? • Which form of representation of service quality requirements is the best one? Background for answering the first question will be sought by first studying a few typical Internet services, and then proceeding to define suitable concepts for service definition. The second and third questions will be answered by studying the factors affecting service quality, as well as the means of measuring service qual- ity. Finally, the means of specifying service quality requirements are reviewed. First, it is in order to discuss the relation of service quality sup- port to the commonly used term “Quality of Service” (QoS). QoS is an overarching term, which different schools of thought relate to Implementing Service Quality in IP Networks Vilho R ¨ ais ¨ anen  2003 John Wiley & Sons, Ltd ISBN: 0-470-84793-X 10 SERVICE QUALITY REQUIREMENTS different parts of end-to-end service quality, including user expe- rience and service quality support mechanisms. International Telecommunication Union’s Telecommunications branch’s (ITU-T’s) related definitions [G.1000] are as follows: • Quality is the totality of characteristics of an entity that bear on its ability to satisfy stated and implied needs. • Quality of service is the collective effect of service performances, which determine the degree of satisfaction of a user of service. Different viewing angles on Quality of Service are listed as fol- lowing: • QoS requirements of user or customer are a statement of the level of quality required by the applications of customers/users of a service, which may be expressed non-technically. • QoS offered or planned by provider is a statement of the level of quality expected to be offered to the customer by the ser- vice provider. • QoS delivered or achieved by provider is a statement of the level of the actual quality achieved and delivered to customer. • QoS perceived by user or customer is a statement expressing the level of quality that customers believe they have experienced. The term “QoS” is deliberately avoided in this book, except when it has a specific, well-established usage such as “IP QoS mecha- nism X” or “QoS framework of standard body Y”. The author’s use of the terms relevant for this discussion is as follows: • End-to-end service quality: end result of everything that affects the end user’s experience of service. The factors affecting this will be discussed in more detail below, but an overview is provided in Figure 2.1. End-to-end quality is not purely subjective, albeit affected by psychological factors related to the particular use of service [BSD00]. Depending on the context, this may mean either service quality planned by the provider, or service quality experienced by the end user. In this book, the viewpoint of inherent service quality requirements falls into this category. • Service quality support mechanisms: means of supporting service quality along the route assumed by the service instance. This includes, broadly speaking, service quality signalling schemes SERVICE QUALITY REQUIREMENTS 11 End user Service providerNetworkTerminal Figure 2.1 Factors of end-to-end service quality include cognitive and psy- chological factors (end user), implementation of service support in terminal, implementation of service quality support in network, and implementation of service by the service provider such as Resource Reservation Protocol (RSVP) and service qual- ity support mechanisms such as Differentiated Services (Diff- Serv). As a result of using service quality support mechanisms, one obtains QoS delivered by the provider on the service level [G.1000], which can also be characterized by network perfor- mance level on network level [I.350]. End-to-end service quality can be thought of being composed of service quality support domains (see below). The breakdown of end-to-end service quality into constituent parts is used, for example, in ITU-T’s recommendation telephony planning model, “E-model” [G.107, G.108] and the European Telecommunication Standardization Institute’s (ETSI’s) IP telephony project QoS model [TIPHON-1]. Such a division is often referred to by the term “QoS budgets”. This book attempts to be neutral with respect to Internet access technologies. The attitude towards technologies can be character- ized as “forward-looking” in the sense that the service quality support potential of different technologies is in some cases evalu- ated beyond currently used deployments. Due to the affiliation of the author, issues specific to wireless issues are considered where the author has assessed them to bring added value to the reader. IP-based Radio Access Network (RAN) is used as a case study for the concepts developed in this book since the author has partici- pated in the work in this area. 12 SERVICE QUALITY REQUIREMENTS 2.1 SERVICES ON THE INTERNET To develop a conceptual definition of services, let us next take a look at what kind of services are known to be in use on the Internet. Examples of well-known services on the wireline Internet at the moment include: • sending and receiving E-mail; • accessing news content with a browser; • e-banking; • e-trading; • whiteboard applications; • chatting. E-mail is “the original service” of the Internet (a short summary of Internet history can be found, for example, in [Kap99]). Developed from a simple means of conveying text messages, modern e-mail clients can deliver attachments and support multiple content types. Most of the services in the above list typically make use of Hyper- text Transfer Protocol (HTTP). Whiteboard and chat applications, on the other hand, can be implemented as point-to-point applications between communication endpoints (computers). In their original form, the HTTP-type examples listed above were classical examples of the client–server type interaction on the wireline Internet. For example, when I start up my PC, launch a web browser and click the British Broadcasting Corporation (BBC) link on my Netscape browser, a HTTP request is sent to the server whose IP address results from the Domain Name Server (DNS) res- olution of “news.bbc.co.uk”. As a response to the request by the client (Netscape browser), a web server of BBC sends the news homepage back to my browser in Hypertext Mark-up Language (HTML) format. A single HTML page may include components from multiple servers, which is often the case with commercial HTML pages with embedded advertisement content. In addition to text and figures, the content provided by an HTTP server can also contain streamed audio or video such as news footage and downloadable files, for example, Moving Pictures Expert Group (MPEG) audio layer coding 3 (MP3) files. An HTTP page can be interactive with menus, type-in fields, and buttons invoking further reply/request interactions, while still conforming to the client–server paradigm. 2.1 SERVICES ON THE INTERNET 13 Common Gateway Interface (CGI) scripts can be invoked dur- ing HTML page access in the HTTP server, making it possible to implement actions such as database searches and on-request HTML page creation based on results. Downloading of Java  applets into the browser allows for local processing in the client. A variant of the client–server paradigm spontaneously came into being after the free on-line MP3 repository Napster was charged of infringing copyright of the publishers and of the music artists. Distributed versions of the service were developed, consisting of a central server containing pointers to the location of the actual music files on other Internet hosts. From the point of view of interactions, the distributed scheme is still of client–server type, the only substantial difference being distributed storage of the content and related redirections. A trend in the making is that of allowing greater freedom to select the time and place of accessing the Internet content. We are not referring to 200-metre modem cable here, but to wireless access. Many airports and hotels already provide 802.11 standard WLAN access to the Internet – all that customer needs is an 802.11 Network Interface Card (NIC) in the PC. WLAN speeds provided by the 802.11 family of standard operating at 2.4 GHz frequency band extend at the moment up to 11 Mbit/s. Similarly, high-end mobile phones have evolved into wireless terminals supporting browsing (with Wireless Access Protocol, WAP) and e-mail delivery. For cellular access, protocols have been developed with better support for Internet content, including General Packet Radio Service (GPRS) and Universal Mobile Terrestrial System (UMTS). Again, the basic scheme for browsing the Internet is of client/server type for WLAN access, GPRS, and UMTS. Because of increased support for user mobility, services dedi- cated to reaching the mobile user are becoming increasingly impor- tant. An example of this type, the client–server paradigm can be used for implementing instant messaging service, differing from e- mail by the requirement of rapid delivery of message. One of the first such services was the SMS of GSM, recently extended to MMS supporting also pictorial content. Same kind of services have been developed also for the general purpose Internet. Extending this idea beyond pairwise person-to-person messaging leads to mul- tiparty communication such as online chatting (web chat). It is 14 SERVICE QUALITY REQUIREMENTS useful to note that often also the person-to-person services make use of a server in the network, which takes care of distribution of the actual messages. A new class of services, different from the traditional client–server paradigm and related to mobile users, is emerging, called push-type services. To provide an example of this, a research engineer reading his e-mails in the airport using 802.11 WLAN access could receive on his display an unsolicited advertisement for a perfume far too expensive for his income level on offer at the nearby tax-free store. Push-type services require a way of identifying the target mobile host. Push-type services are possible also in GSM networks. An equally novel type of service, made possible by the Internet, is point-to-point real-time communication over the Internet. Voice over IP (VoIP) clients for PCs can be downloaded from the Inter- net that contain audio and/or video coding/decoding function as well as protocols for sending audio/video samples in packet- switched networks. This makes it possible for two such clients to contact each other directly, if they know each other’s DNS names or IP addresses. This kind of solution is not very scalable, how- ever, since the end user is left with the task of maintaining a “telephone directory” of callees. A Call Processing Server (CPS) such as a Session Initiation Protocol (SIP) proxy is typically used for interfacing to terminal availability information. Also, the Inter- net of today does not automatically provide good enough quality for a VoIP call without signalled quality support. Thus, a practi- cal solution typically involves a directory server as well as some means of providing service quality support for the connection. These examples only scratch the surface of possible Internet ser- vices. The ones described above are among the most well-known ones. As is evident from the examples, the Internet is a good plat- form for creating new kinds of innovative services, provided that the inherent quality requirements of services can be satisfied. The ITU-T provides a long list of Internet service types in [G.1010]. In [Y.1541], the following summary classification for Internet services is given in Table 2.1: The following types of Internet communications have been identified so far: • Delivery of real-time content such as audio or video. This includes both conferencing and streamed content. • Delivery of data-type content such as e-mail. 2.1 SERVICES ON THE INTERNET 15 Table 2.1 Draft IP QoS classes  International Telecommunication Union QoS Class Applications 0 Real-time, jitter-sensitive, high interaction (VoIP, VTC) 1 Real-time, jitter-sensitive, interactive (VoIP, VTC) 2 Transaction data, highly interactive (signalling) 3 Transaction data, interactive 4 Low loss only (short transactions, bulk data, video streaming) 5 Traditional applications of default IP networks Source: From [Y.1541]. • Interactive client–server applications. This category includes browsing and messaging involving a network server. There may be multiple degrees of urgency within this category, ranging from web browsing to real-time remote control of machinery. • Server-initiated services. This category includes “unsolicited” services such as receiving of advertisements. Although the actual content may fall within data-type or real-time type content discussed above, this is a separate category for the reason of not being requested by the terminal. Let us next take a closer look at the diverse types of content can be delivered over the Internet. A crude summary of some of the most common types is given in Table 2.2, and more elaborate discussion will follow later in this chapter. The diverse nature of the types of services sets certain requirements of the service quality support on the Internet. Definition of the conceptual framework, as well as a description of the mechanisms needed, form the fact matter of this book and are shown in Table 2.2. The mobility aspects of Table 2.2 Summary of some of the most common Internet service content types Internet service Request/reply? Content delivery time Continuous content? E-mail No Not critical No HTTP page Yes Interactivity required No Streamed content Yes Medium Yes Instant messaging/ web chat Yes Interactivity required No Music file download Yes Not critical No Push-type advertisement No Not applicable No Multimedia call No Short Yes 16 SERVICE QUALITY REQUIREMENTS service quality support are not a central topic in this book, but will be referred to where appropriate. 2.2 DEFINITION OF A SERVICE On the Internet, the definition of service in general is a difficult issue [RFC3052]. The open standards environment characteristic of the Internet makes it possible to devise services with much greater freedom than in the Intelligence Networks (IN) environment. The SIP framework of the Internet Engineering Task Force (IETF), for example, provides for building blocks for reachability and identity services, and – together with the Session Description Protocol (SDP) – makes it possible to signal application requirements between two ends of the communication. A practical consequence of the preceding facts is that the idea of standardizing services is no longer valid [RFC3052]. Thus, we shall adopt a broad view here based on classification of services. There is a long body of experience of implementing services within the telecommunications industry. The basic reason for this is that in the communications model of telephony the services assurance is provided by the network, whereas terminals (tele- phones) are relatively simple. Subsequently, precise service defi- nitions are required to provide service quality end-to-end also in the presence of multiple commercial operators. In addition, espe- cially the service implementation of mobile telephony requires a sophisticated service quality requirement definition. For this rea- son, telecommunication services are used as a framework and point of comparison in the following. The original telecommunications service, voice telephony, has been complemented with services such as call forwarding, voice mailboxes, and A subscriber (caller) number display for the B sub- scriber (callee). The paradigm for supporting this in the POTS environment is called Intelligent Networks (IN), making possi- ble the creation of new services in the world of circuit-switched telephony. The TeleManagement Forum defines service from a telecommu- nications point of view as follows [SMH01]: Service – a telecommunication service is a set of independent functions that are an integral part of one or more business processes. This functional set consists of the hardware and 2.2 DEFINITION OF A SERVICE 17 software components as well as the underlying communica- tions medium. The above definition is a fairly high-level one. Another approach is adopted by the 3rd generation partnership project (3GPP), view- ing services from a technical support perspective. The 3GPP QoS framework provides the following set of definitions related to ser- vices [22.105]: Bearer service: A type of telecommunication service that provides the capability of transmission of signals between access points. Service Capabilities: Bearers defined by parameters, and/or mechanisms needed to realize services. These are within networks and under network control. Service Capability Feature: Function offered by service capabilities that are accessible via the standardized application interface. Services: Services are made up of different service capability features. For the present purposes, it is sufficient to know that the con- cept of “bearer” is an abstraction for service quality support class provided by the network. Comparing the two definitions above, services can be viewed as being a part of business processes from above (by “the suits” in Internet terminology), and being made possible with service quality support mechanisms from below (by the engineers). In telecommunications, the management of services has been an important issue. According to [ECN02], standardization of ser- vice management framework in telecommunications industry has not been successful in the sense that all aspects of service man- agement conform to a standard. On the other hand, standards have been created for relevant interfaces for the purpose of inter- operator service management. In this book, the viewpoint is not that of standardized services, but of management and other pro- tocol interfaces that can be used for constructing services. When the interfaces are based on open standards and the concepts used in specifying service support across business parties are involved, the lack of standardized services can be turned into an advantage for all parties involved: the customer, the network provider, and the service provider. 18 SERVICE QUALITY REQUIREMENTS 2.2.1 End user service versus provider-level services A useful conceptual division for bringing structure into the discus- sion about services is that between end user services and provider- level services. End user service means the service as experienced by an end user (single instance of a service), whereas provider-level service definition is typically concerned with an aggregated view of individual sessions. An example of studying services at end user level is a study to what extent of user experience of e-commerce is satisfactory. As will be discussed in Section 2.3, this requires the definition of suitable metrics, and a controlled means of performing the measurement. Continuing the e-commerce example, the provider-level service would view e-commerce sessions en masse, on an aggregated level. The e-commerce service provider would probably look at the overall average number of transactions per hour, broken down according to time of day, taking an example. Another example, an Internet Service Provider (ISP) providing connectivity for home customers is likely to be able to look at history data of average load in different parts of the network, including Digital Subscriber Line Access Module (DSLAM) and the link towards a Point of Presence (PoP) in backbone network. A connectivity provider for service operators would likely look at the loading level on leased lines towards service providers and towards the backbone network. Now let us assume that Mary Ann, the owner of a corner shop grocery buying her fruits in Internet auctions, wants to get an Internet connection to take care of her bidding and other financial matters of her business using Asymmetric Digital Subscriber Line (ADSL) access to the Internet. What is she to do? Mary Ann will have an agreement with her ISP saying, for example, that her access line maximum throughput is 1024 kbit/s downlink and 256 kbit/s uplink. Having experienced problems with congestion before, Mary Ann may not be satisfied with the theoretical access line maximum throughput only, but require guarantees about average throughput available between the Internet (PoP) and DSLAM also during peak hours, for example. Capacity-wise, this throughput requirement would be a component of an end user service level agreement (SLA) between the ISP and Mary Ann. The [...]... discuss the 3GPP QoS model in more detail in Chapter 5 Another example of service quality requirement definition is provided by ETSI’s IP telephony project TIPHON Currently they cover only telephony The end-to-end service quality characterizations used by TIPHON are as follow [TIPHON-2]: • overall transmission quality rating (R); • listener speech quality (one-way non-interactive end-to-end speech quality);... limit the principle of subjective testing and establishment of average ordering scale for quality to voice telephony only Indeed, the MOS principle has been applied to gaming on the Internet, for example Most likely, there is room for research and development in this area in the future 2.3.2 Recency effect The user experience of quality is not constant with time when quality varies [TIPHON-5; Cla01]... temporal development of the recency effect for a quality dip of 20 units according to equation (2.1) The recency effect is useful in illustrating the effect of momentary poor quality: if the “quality dip” in Figure 2.4 does not coincide with important piece of communication, the end user will probably forget and forgive it, unless further quality dips occur in too short a time 2.3.3 Psychological factors... interactive communication between the participants of the conversation On the other hand, an e-mail message does not need to traverse the network equally quickly In an e-mail message, flipping of individual bits due to bit error in transmission cannot be tolerated if the meaning of the message is to be kept identical to that devised by the author of the message In VoIP telephony session or a GSM call from... parts of service instance having different characteristics and service quality requirements In the case of a VoIP call with application sharing, for example, the following distinct service event types can be enumerated: • connection set-up signalling (e.g., H.323 or SIP); • transmission of VoIP media stream (e.g., G.723.1 or Adaptive Multi-Rate codec, AMR) or a part thereof; • transmission of application... provided at multiple locations Cellular mobile networks have advanced signalling schemes for handling service continuity during handovers Analogously, going beyond simple best-effort implementation of Internet access using WLAN access points requires special support mechanisms This is true even if terminal mobility itself would be solved protocol-wise, for example, using Mobile IPv6 (MIPv6) Thus, in... resources, possibly due to inefficient socket programming • Transport layer protocol – TCP or UDP • TCP /IP stack at either end may be sub-optimal performancewise • Some routers or links of the IP network may be misconfigured • In long-range communications, route flapping may take place • Some parts of the IP networks connecting the two communicating hosts may be congested A full account of all the factors... 0 Class 1 Class 2 Class 3 Class 4 Class 5 performance network unparameter performance specified objective IPTD Upper bound 100 ms 400 ms 100 ms 400 ms 1 s U on the mean IPTD (Note 1) Note: The class numbers refer to Table 2.1 Source: From [Y.1541] The draft ITU-T recommendation for transfer delay (IPTD) for general Internet services is as shown in Table 2.6 Two-way services are typical of the client–server... (codec) with oft-used audio frame size as an example, an audio sample of 160 bytes would be transmitted every 20 milliseconds Transport protocol headers add further to the total IP packet size Let us assume that the Voice over IP (VoIP) paradigm is used, and that the voice samples are transmitted to the receiver over the Internet, with RTP [RFC1889] being used to aid endpoint processing by providing time... Internet services  International Telecommunication Union U = unspecified QoS Classes Network performance parameter IPDV Nature of Class 0 Class 1 Class 2 Class 3 Class 4 Class 5 network unperformance specified objective Upper bound 50 ms 50 ms U U U U on the 1–10−3 quantile of IPTD minus the minimum IPTD Source: From [Y.1541] Typically delay variation is expressed in the form of an estimator, of which two . European Telecommunication Standardization Institute’s (ETSI’s) IP telephony project QoS model [TIPHON-1]. Such a division is often referred to by the term. is point-to-point real-time communication over the Internet. Voice over IP (VoIP) clients for PCs can be downloaded from the Inter- net that contain audio