Mạng và viễn thông P34

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Quality o Service (QOS) and f Network Performance ( N P ) The maintenance of good quality for any product or service (i.e. its ‘fitness for purpose’ and its price) isofsupremeimportance to theconsumerandthereforerequiresutmostmanagement attention. However, although it easy enoughto test a tangible product destruction, measureis to ment of the quality of a service is more difficult.

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  1. Networks and Telecommunications: Design and Operation, Second Edition. Martin P. Clark Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic) Quality o Service (QOS) and f Network Performance ( N P ) The maintenance of good quality for any product or service (i.e. its ‘fitness for purpose’ and its price) isofsupremeimportance to theconsumerandthereforerequiresutmostmanagement attention. However, although it easy enoughto test a tangible product destruction, measure- is to ment of the quality of a service is more difficult. In telecommunication the customer is left with nothing more tangible than his her own perception of how well the communication went. On or a datalink the errors that have had to be corrected automatically are barely appreciated, while in conversation unobtrusive bursts of line noise may go unnoticed. This is not to say that loss of data throughput on a datalink caused by continual error correction isofnoconcern to the customer, nor does it mean noises which disturb conversation are acceptable. What really that we mean is that in measuring service quality, due regard must be paid to anything of importancethat thecustomerperceivesandremembers.Concentrationonsettingandmeetingqualitytargets should be paramount in planning and administration. Insufficient attention to them is the roadto customer dissatisfaction and loss of business. This chapter reviews some aspects of communica- tionsquality the and practices of management, examples the with of commoner quality parameters and control measures used by the world’s major operators. 34.1 FRAMEWORK FOR PERFORMANCE MANAGEMENT Good management of whatever industry demands the use of simple, structured and effective monitoring tools and control procedures to maintain the efficiency of the internal business processes and the quality of the output. When is running smoothly all a minimum of effect should be required. However, to be able quickly to correct defects or cope with abnormal circumstances, measurable means of reporting faults or excep- tions and rapid procedures for identifying actionable tasks arerequired. The framework for doing so needs to be structured and comprehensive. In Table 34.1 a number of simple management ‘dimensions’ together with possible managementperformance As tools/parameters within each dimension are presented. you will note the dimensions cover a range of areas, some requiring more tangible monitoring measures and control procedures than others. 633
  2. 634 QUALITY OF SERVICE (QOS) AND NETWORK PERFORMANCE (NP) Table 34.1 Frameworkforperformancemanagement Dimension Measures/controls Formal plan and delivery cycle Formal presentation and agreement of company strategy, (organizational framework) policy, guidance and plans. Regular review. Framework for business ‘value’ assessment and assurance. User support User comprehension of services (by questionnaire). (fit for purpose) Swift elimination of problems (full complaint log). Supplier performance Maintaining professional attitude with suppliers. (quality of supply) Monitoring and demanding swift lead times. Meeting targets for repair performance. Quality performance Meeting user needs (number of complaints). (effectiveness) Number of lost messages (e.g. percentage of data lost) Percentage of ‘down-time’ during normal business hours. Measure of delay (e.g. propagation delay, or backlog of orders/messages). Measure of congestion (e.g. percentage calls lost - callers frustrated). Repair performance (e.g. percentage not repaired in target time). Connection quality (e.g. percentage customers satisfied). ‘Cost of poor quality’ (i.e. that correcting avoidable faults). Financial performance Return on capital investment. (efficiency) Shareholders financial return. Cost actually spent can be compared with competitors or alternative suppliers. Internal chargeout rate for telecom services (e.g. ‘pounds per bit-mile’). Technical performance Compatibility of networks can be adjudged by measuring (efficiency) the money spent on interworking or upgrading and comparing it with total system value. Resource management Asset inventory and management (needs to be accurate (efficiency) and up to date). Cost per task. Adequate resourcing to meet target service lead times. Head count and man-hours should be maintained on target. Evolution of networks Network upgrades should be properly planned and meet (responsiveness) their budget. There should be steps to give ‘benefits’ assurance. The response time of new software and computer networks should be according to plan. The networks should be adaptable.
  3. QUALITY: A MAREKTING VIEW 635 34.2 QUALITY:MARKETING A VIEW To look at a network from the customer’s viewpoint need to understand his reasons we for using telecommunications services. It a mistake to generalize about all customers is as ‘wanting to convey information over long distances’, asthat takes no account of the service in use, or of each customer’s individual purpose or application. It is, after all, a Figure 34.1 A customer’s perception of quality. Get the basics right before worrying with the frills. Drawing by Patrick Wright. (Courtesy of M . P. Clurk)
  4. 636 QUALITY OF SERVICE (QOS) AND NETWORK (NP) PERFORMANCE fundamental of marketing products that should be attuned the to markets and customers they are intended to serve. Thus the motivations and interests of someone who is out for the evening in callinghome from a payphone, are quite different from the needs of a large multi-national company wishing to convey vast volumes of computer data around the world on sophisticated and dedicated networksat any hour of the day or night. It may be that the same network infrastructure can accommodate the service needs of a number of different markets customer ‘clubs’ but it must so without compromise. or do One example of a network capable of supporting a number of services is the public telephone network, and it must run at optimum quality not only for human conver- sation, but also for facsimile machine interconnection. A more advanced example of integrated services networks is the integrated services digital network (ZSDN) discussed in Chapter 10 and the B-ZSDN of Chapter 25. Customers’ communication needs can often be met in a of alternative fashions, number ranging from travelling in person, using the postal service, the telephone, telex, packet switching, and high speed data telecommunications services. Pitching a given service to meet a given need is jobof telecommunications marketing specialists who determine the its relevant qualities, expressed as a number of measurable parameters. An example of sucha‘qualityparameter’, used commonly by telephonenetwork operators is the percentage of calls completed.Not only is this a fair measureof network congestion, itis also a good predictor of the level of customer frustration. 34.3 QUALITY OF SERVICE (QOS) AND NETWORK PERFORMANCE (NP) You may have realized in our example above that the percentage of answered calls is dependent not only on network congestion, but also on the availability of somebody at the destination end to answer the telephone. Thus the quality of service enjoyed by the customer depends on a number of factors in addition to the performance of the net- work. ITU-T has also drawn this distinction, and its general recommendations on the quality of telecommunications services recognizetwoseparatecategories of perfor- mance measurement 0 qualityof service (QOS) 0 networkperformance (NP) Quality of service measurements help a telecommunications service or network pro- vider to gauge customers’ perceptions of the service. Network performance parameters, on the other hand, are direct measurements of the performance of the network, in isolation from customer and terminating equipment effects. Thus quality of service encompasses a wider domain than network performance, so that it is possible to have a case of poor overall quality of service even though the network performance may be excellent. The relationship of quality of service to network performance is shown in Figure 34.2.
  5. QUALITY OF SERVICE (QOS) AND NETWORK PERFORMANCE (NP) 637 quality of service (QOS) W network performance (NP) Figure 34.2 The relationship between quality of service (QOS) and network performance (NP) Similar parameters may be used to measure both quality of service and network per- formance (e.g. propagation delay, bit error ratio (BER), % congestion, etc.). Normally the measured quality of service is lower than the measured network performance. The differenceis duetotheperformancedegradations caused by the user’s ownend equipment (Figure 34.2). The measured quality of service will differ greatly from the measured network perfor- mance values where a connection is composed of a number of connections, traversing several different networks and enduser equipments. Although is of utmost importance it to the end user, the problem with quality of service as a performance measure that it is is difficult to measure, and would need to bemeasured for eachindividualcustomer separately. Thisis the reason for the development the conceptof networkperformance. of Network performance can be more easily measured within the network, and provides for meaningful performance targets for the technicians and network managers oper- ating the network. In short, QOS parameters are user-oriented, and provide useful input the network to design process, but they are not necessarily easy to translate into meaningful technical specifications for the network. Network performance parameters, on the other hand, providedirectly a usable technical for basis network designers operational and managers, but may not meaningful to end users. Sometimes, the same parameters are be appropriate for both QOS and NP, but this is not always the case. The distinction between quality of service ( Q O S ) and network performance ( N P ) is somewhatartificial,but ithasbecomenecessaryastheresultofrecentregulatory changes in some countries, where the public telephone operator ( P T O ) is allowed to provide service only up to a socket in the customer’s premises,and end-user equipment (i.e. terminal) manufacturers slug it out in the customer premises equipment ( C P E ) market.GovernmentregulationshouldkeepPTOnetworkperformance up tothe mark, but overall service may be of poor quality if it is let down by faulty or badly designed customer apparatus. Parameters should be chosen to reflect high quality end-to-end service and should be expressed quantitatively as far as possible. These QOS parameters should then be correlated one with or a number of directly related network performance (NP)
  6. 638 QUALITY OF SERVICE (QOS) AND NETWORK (NP) PERFORMANCE parameters, each NP parameter reflecting the performance of a component part of the network, and therefore contributing to the end-to-end quality. In this way quality of service problems can be traced quickly to their root cause. In the example of Figure 34.3, QOS criteria have laid it down that the signal loud- ness received by the listener should not beless than 14dB below the original signal transmitted. end-to-end The QOS parameter is therefore signal loudness and is measured in dB. The network between the two people in conversation comprises two telephone exchanges and three linking circuits,which are designed to individual network performance loudness losses of 1dB. The overallnetworkperformance is therefore + 3 + 1 + 3 1 + 3 = l l dB, and when the losses in the telephone handset, the end-to-end (QOS) loudness is 13 dB, i.e. giving a 1 dB margin within the maximum allowed loss of 14 dB. Considering only the network performance ( N P ) between the two telephone sockets of Figure 34.3, an acceptable end-to-end QOS is delivered even if one of the links or exchanges in the network degrades in performance by up to 1 dB (provided that all the otherpartscontinueto work at their designed losses). Inother words,anetwork performance of 12dB is sufficient to deliver an acceptable end-to-end QOS of 14dB loss. Likewise, if either of the customer telephone sets degrades in performance to the extent of adding up to ldB, then the overall QOS of 14dB is still maintained, provided other components remain stable. In normal operation itis difficult to monitor the end-to-end QOS performance of all connections, particularly when a large numberof permutated switched connections are possible, andtheconcept of networkperformanceparameters is invaluable.Each element of the network may be monitored in isolation and maintained within a pre- determined network performance range. Thus in the example of Figure 34.3 we might aim to keep the individual exchange losses below 1.5 dB, and the individual link losses below 3.5dB. Although theoretically this might mean that the overallnetwork performance could include a loss of 3.5 + 1.5 + 3.5 + 1.5 + 3.5 = 13.5dB, which is 1.5 dB outside the maximumallowed network performance of12 dB, itis highly unlikely that all the links and exchanges will simultaneously be performing in the most adverse manner. As a safeguard against suchan adverse occurrence, small occasional samples of the end-to-end quality of some representative connections will need to be taken. Sometimestherelationship between theend-to-endqualityof service parameter measurements and the network performance parameters may notbe as direct as in the example just discussed. In that example we were able to use similar parameters, all measured using the same scientific units (in our case dB, or decibels). However, in the QOS parameter percentage of callsreachinganswer, there is a much more complex relationship with measurable network performance parameters, depending not only on the state of network congestion, but also on the availability of alternative routes, the incidence of faults, and other abnormalitiessuch as misrouting or misinsertion. Some of the contributions may have negligible effect under normal conditions, so that reliable approximations may be used. For example, if the percentage of calls being answered at a given destination starts to deteriorate then the network operator’s first action is to look for any network links that are congested. If any are found, then these are more than likely thecauseofthecongestion,andextraresourcesshould be provided accordingly. If no individual link or exchange congestion is immediately apparent then the operator will have to look for less common causes and faults.
  7. QUALITY OF SERVICE (QOS) AND NETWORK PERFORMANCE (NP) 639 m v c m a W 2 *) 1----- m V m
  8. 640 QUALITY OF SERVICE (QOS) AND NETWORK (NP) PERFORMANCE There are no hard-and-fast rules as to which parameters should be used to measure quality of service and network performance, although ITU-T has created a generic model and a set of generic parameters. 34.4 QUALITY OF SERVICE PARAMETERS Quality of service parameters focus on the user’s likely perception of the service, rather than on thetechnical cause of specific degradations of service. For this reason, different quality of service parameters should be tailored for each different type of network application. QOS parameters are not comprehensively defined by ITU-T standards. For certain types of services (e.g. leaselines, telephone service, etc.), ITU-T defines QOS parameters, and thesesameparametersshould be used to measureend-to-endquality of such services, independent of what type of transport network is used. Thus, for example, the quality of service of a digital leaseline could be measured in terms of the bit error ratio (BER), the jitter, the accuracy of the bitrate, the propagation delay, the availability of the connection (the percentage of a one-month or three month period for which the circuit was not out-of-service) and the accuracy of the invoice. In addition, the network operator has an interest to conceive network performance parameters which would help him meet the user’s expected QOS level. In this respect, to he has a little help, as ITU-T has defined a number of standard network performance parameters for some types of networks. The network operator’s task thus becomes relatingthese to the specific expectations of individual services’ QOS, and setting appropriate network performance targets. 34.5 GENERIC NETWORK PERFORMANCE PARAMETERS For connection-oriented networks (such as the telephone network, packet and frame- switched data networks and ATM networks), ITU-T defines a set of three different types of NP parameters, and recommends that these should be used as measures of three different functional aspects of the connection (access (connection set-up), user information transfer and disengagement). Table 34.2 shows the nine different types of Table 34.2 The generic primary performance parameters Performance criterion functionAccuracy Speed Dependability Access AccessAccess accuracy speed Access dependability User information Information transfer Information transfer Information transfer transfer accuracy speed dependability Disengagement Disengagement Disengagement speed Disengagement dependability accuracy
  9. PARAMETERS PERFORMANCE GENERIC NETWORK 641 Table 34.3 Example primary performance parameters relevant to telephone, ISDN, data and ATM networks and services Performance criterion function Accuracy Speed Dependability Access Connection set-up Incorrect set-up Probability of set-up delay probability denial (connection (misrouted connection set-up denial ratio) ratio) User information Successful transfer Bit error rate (BER) Probability of rate transfer Packet, frame or cell information loss Propagation delay misinsertion rate Packet, frame or cell Cell transfer delay (CMR) loss ratio (CLR) Cell delay variation Packet, frame or cell (CDV) error ratio (CER) Cell transfer capacity Severely errored Jitter frame or cell block ratio (SECBR) Errored seconds Disengagement Delay in connection Premature release Release failure ratio clearing ratio Incorrect release ratio network performance parameter which result. These are termed the generic primary performance parameters. Similar performance parameters could also be conceived for connectionless networks. Examples of specific primary performance parameters within each the generic per- of formance parameter classes are given in Table 34.3. A subset (perhapsall) of these para- meters should be regularly measured by network operators (as network performance measures). The target values for the chosen parametersneed to be set to ensurerelevant customer satisfaction with respect to quality of service. Where no direct relationship exists between quality of service and network performance parameters, experience will help to determine acceptable threshold values for NP parameters. In addition to primary performance parameters, ITU-T also defines the concept of derived performance parameters. The most important derived performance parameters are those of availability and acceptability. Availability is a measure of the cumulative outage (i.e. non-service) time of the network asa whole (or of a given customer’s part of the network) during a given measurement period (e.g. one month or three months). Availability is measured as the percentage of the total period for which the service was not in outage. Thus thehigher the measured availability, the lower the network outage. Typical target values are 99%, 99.5% and 99.9%. Acceptability hasalso been proposed as a derived performanceparameter.This would be intended to give a qualitative measureof likely subjective customer opinion of the service level. This gives the potential for inclusion of other more general factors in measuring overall customer satisfaction with the network.
  10. 642 QUALITY OF SERVICE (QOS) AND NETWORK (NP) PERFORMANCE 34.6 PERFORMANCEMONITORINGFUNCTIONS OF MODERN NETWORKS In the most modern network technologies (e.g. SDH, synchronous digital hierarchy (Chapter 13) and ATM, asynchronous transfer mode (Chapter 26)), extended perfor- mance measurement and monitoring techniques are built-in. Thus, forexample, it may be possible to measure the instantaneousbit error ratio ( B E R ) of a given connection, or the propagation delays being experienced in a live network. This is done by inserting extra performance management traffic into the network and monitoring the experienced performance. This is intended to be a good predictor of likely network performance as perceived by end customers. The performance monitoring function of ATM operates by sending performance monitoring cells on an end-to-end basis after each block of N user cells. This function can be used to monitor end-to-end errored blocks, misinsertion(incorrectly sequenced or incorrectly delivered cells), cell tranqfer delay and other performance parameters on a specific connection. As an example of the network monitoring and diagnosis tools available withinATM, it is, for example, possible to apply a loopback to O A M (operations, administration and management) cells connections withinan ATM network without affecting other virtual connections sharing the same physical connection. This may useful to confirm circuit be continuity and to measure loop propagationtimes, without having to interrupt thelive user connections. 34.7 NETWORK PERFORMANCEPLANNINGANDMEASUREMENT The remainder of the chapter proposes a pragmatic approach for establishing and QOS NP parameters fortelecommunications services. The approach is designed to generate a balanced mix of parameters that will prove valuable in monitoring all aspectsof service quality,includingnotonlythequalityoftheconnectionsthemselves,butalsothe important support activities such as maintenance and provision of service. By defining astandard set of categories relatingto telecommunications service quality, we can ensure that selected parameters within each category give a balanced picture of current network performance and quality of service as against target. Without such discipline inthe past, many network operators made extensive measurementsof the have technical aspects of performance with too little respect for the broader service attributes giving them a distorted view of their quality, and they have suffered accordingly. For although they may have been returning exemplary technical performance statistics, their customers may have been far from happy because they could never a reply from the get fault reporting bureau, or could never get assistance about how to use a service. A pos- sible set of categories from which to choose a balanced setof performance parameters is shown below. Each category is explained, and some examples of actual parameters are given in the paragraphs that follow 0 customer service 0 service availability
  11. PERFORMANCE PLANNING NETWORK AND MEASUREMENT 643 0 provision and alteration of service 0 service reliability 0 making calls (and cleaving them) 0 quality of conversation or information transfer 0 fairness, reliability and accuracy of service charges 34.7.1 Customer Service Parameters in this category measure general customer perceptions of the ‘helpfulness’ of network operator staff, their courteousness, their ‘understanding’ of problems and their ‘willingness to help’ Some parameters might additionally reflect customer feeling about the availability and usefulness of literature, operating instructions and advertisements. Parameters in this category have historically been the most overlooked, perhapsbecause it is difficult to measure them accurately by market research, and perhaps because they are sensitive to swings in public opinion. Typical parameters might include ‘percent- well age of satisfied customers’. Great care is required when selecting the precise wording of market research questions. 34.7.2 Service Availability A customer’s perception of the quality of service may depend on its availability. For example, the geographic coverage may too restricted, either prohibiting the customer be from service subscription in the first place, or being too constraining in the number of egress points available. One’s view of a payphone service, for example, is much affected by the time it takes to find a kiosk at the time when wanting to make a call. It is also influenced by the number of payphones available at busy sites such as stations and air- ports where customers face intolerable delays. A third factor influencing the quality of payphone service might be the destinations which are avaulable from it. For example, payphones which only accept small-denomination coins have little appeal for foreign tourists making international calls to their home country. Typical parameter measures in this category might thus be 0 percentage of customers desiring service who are not in the coverage area (waiting list length) 0 average queueing time (for a payphone) 0 percentage of desired destinations not available (from market research) 34.7.3 ProvisionandAlteration of Service Having ordered a service, the customer expects it to be provided promptly and con- veniently. The time taken to complete customer orders, and perhaps the percentage of
  12. 644 QUALITY OF SERVICE (QOS) AND NETWORK PERFORMANCE (NP) appointmentskeptareworth measuring. Otherparameters couldalsomeasurethe standard of installation work. Typical measures might be 0 percentageof contractsforthe provision of servicecompletedwithinatarget number of days 0 percentage of appointments for provision of service kept 0 percentage of installations meeting customers’ quality expectation 0 number of installation revisits required 34.7.4 Service Reliability Having had aline established on the premises, a customer will wish to make and receive messages or calls according to whim.Loss of service occurs each time the user unable is to make orreceive messages or calls for any reason other than customer mis-operation. Furthermore, when he has suffered a loss of service, the customer is anxious to be restored to fullservice within an acceptable period of time, and with minimum inconvenience. Thus time to repair or time for repair are important. Other measures in this category might be 0 total period of lost service within one year 0 number of occasions of service loss within one year 0 average time lapse between reporting a fault and satisfactory; restoration of service 0 percentage of faults cleared within target time period 0 average number of faults per customer line per year 0 percentage of payphone time out of order 0 percentage of payphone time out of order due to full coinbox 34.7.5 MakingCalls (andClearingThem) Perhaps the most over-indulged category from the historical point of view. Having had the line and terminal equipmentinstalled, the customer will be interested in establishing calls or sending messages over the network. The customer expects prompt service, and correctdelivery of messages, or connection of calls to therightnumber.Network congestion is a frustration. A plethora of easily quantifiable and measurable parameters are available 0 percentage of calls receiving dial tone within target time (fractions of a second) 0 percentage o calls connected to the correct number f
  13. PERFORMANCE NETWORK PLANNING AND MEASUREMENT 645 0 percentageof calls failed duetonetwork congestion (a networkperformance measure) 0 average time taken between commencement of dialling and receipt of ringing tone 0 average delivery time required (for a packet data message) 0 percentage of calls not clearing correctly at the end of a call 0 answer bid ratio (ABR): the ratio of calls answered to the total number of call bids 0 connection accessibility (availability) objective (see ITU-T Rec G.180). 34.7.6 ConversationorInformationTransfer The prime reason for the service to convey conversation or other information. The customer demands clarity of speech, lack of noise, sufficient loudness, absence of inter- ruptions or interference, and security of the connection (from cut-off,or intrusion by a third party). For data, accurate conveyance of information is required (e.g. low jitter (variance in speed), low bit error ratio ( B E R ) (introduced errors), etc.). Measurements of parameters in this category have historically been madeby taking a small statistical sample of human service observations. A small percentage of calls are listened into by human operators, to confirm intelligibility of speech, to detect any apparent dissatis- faction of customers, and to trace the cause of call failures during set up. With the growth in telephone traffic and the increased sophistication of services there is now a demand for exchange manufacturers to develop more accurate computer-controlled sampling methods. Typical parameters in this category include 0 percentage of calls with too faint conversation 0 percentage of calls with excessive distortion 0 percentage of data calls with excessive jitter 0 percentage of calls cut off prematurely 0 average time to deliver a data packet 0 proportion of misroutedcalls, undelivered or misinserted datapackets, frames or cells 34.7.7 Service Charges The charge invoiced to a customer reflects the accuracy and adequacy of the invoicing procedure as well as that of the exchange call meteringdevice. Errors can be introduced by the mechanism of collecting call data, by the transfer of that data to the billing
  14. 646 QUALITY OF SERVICE (QOS) AND NETWORK (NP) PERFORMANCE system, by theaggregation of individualcallcharges, and by thetransfer of that informationfrom electronic to printedform. All thesefunctionsmustwork at an acceptable standard. Typical check measures might be 0 percentage of (correctly) disputed bills 0 percentage of individual calls incorrectly charged 0 percentage bill inaccuracy on a per customer basis 0 probability of an individual customer’s bill being wrong 34.8 A FEW PRACTICAL TIPS Best network performance is achieved when the networkitself is designed from thestart to specific quality of service and network performance targets. As far as possible these should be directlymeasurableparameters,andthethresholdvaluesshouldalso be defined, if possible together with remedy actions to be taken. The remedy may not always be a simple one. As we learned in the various chapters on data networking, a slow propagation time of packets or framesacrossanetwork andthe associated response time of a given computer application may not be significantly improved simply by increasing the bitrate of the line. Indeed, aswe learned in Chapter 30, the upgrading of a line in a data network to a higher speed may lead only to greater problems. Heterogeneous networks (i.e. those comprising many sub-networksof different tech- nologies and topologies) are particularly hard to manage, because the monitoring of end-to-end connections is difficult to achieve without considerable effort and external measurement equipment. The task of narrowing down the exact source of a problem may be nearly impossible, being difficult to re-create in simulated conditions within the test laboratory. Simple network design guidelines and defined network performance measures are the bestformulaforsuccess.Complexnetworks,withcomplicatedtopologiesand very sophisticated automatic control and routing mechanisms (e.g. the Internet) are very difficult to monitor and manage. Providing a better guarantee of successful message delivery and acceptablepropagation times will beamajorchallenge forthemany network operators and service providers involved with the Internet. 34.9 SUMMARY This chapter has stressed the importance of good quality in telecommunictions services and outlined two has the concepts quality of service and network performance, categorizing both to allow the easy establishment of a balanced set of parameters for continuous monitoring and correction of service performance.
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