1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU42!.3-)33)/.G0G03934%-3G0G0!.$G0G0-%$)!%.%2!,G0G0#(!2!#4%2)34)#3G0G0/ this however may require special measuresto he applied when other, more demanding services (e.g. sound-programme transmission) are to be inc
2、orporated withinthe network or constituent parts thereof.The following distinctions may be made between different types of objectives:1) performance objectives for networks,2) performance objectives for circuits, transmission and switching equipment,3) design objectives for transmission and switchin
3、g equipment,4) commissioning objectives for circuits, transmission and switching equipment,5) maintenance/service limits for circuits, transmission and switching equipment.2 Explanation of a performance objectiveThe performance objective for a measurable transmission impairment for networks, entire
4、connections, nationalsystems forming part of international connections, international chains of circuits, individual circuits etc. often describesin statistical terms (mean value, standard deviation, or probability of exceeding stated value, etc.) the value to be aimedat in transmission network and
5、systems planning. It describes the performance which, based for example on subjective orother performance assessment tests, it is desirable to aim at in order to offer the user a satisfactory service.2 Fascicle III.1 - Rec. G.102The items (circuits, systems, equipments) making up the network are nor
6、mally assumed to have a performancerelated to that recommended by the performance objectives. Traffic weighting will, in some cases, be applied tocalculations.A powerful set of tools which may be used in analyses concerning network objectives and compliance therewithare the hypothetical reference co
7、nnections described in Recommendation G.103.3 Explanation of a design objectiveThe “design objective“ for a measurable transmission impairment (e.g. noise, error-rate, attenuation-distortion)for an item of equipment (e.g. a line system, a telephone exchange) is its value when the item is operating i
8、n certainelectrical/physical environments which might be defined by such parameters as power supply voltage, signal load,temperature, humidity, etc. Some of these parameters may be the subject of CCITT Recommendations and some maynot, and it is for the Administrations to assign values to them when t
9、hey prepare specifications. A suitable allowancemay also be made for aging. The most adverse combination of the specified parameters is often assumed.The purpose of a “design objective“ is to provide a basis for the design of an item with respect to the quantityconcerned. The significance of the des
10、ign objective for an item, and examples of the relative frequency of impairmentvalues, are illustrated in Figures 1/G.102 and 2/G.102 respectively.Design objectives will in many cases directly form the basis of a specification clause for the developmentand/or the purchase of equipments.A powerful se
11、t of tools used in connection with applying design objectives are the hypothetical reference (HR)circuits and hypothetical reference (HR) digital paths (see relevant Recommendations in the G.100 and G.700 Series).4 Explanation of a commissioning objectiveThe conditions encountered on real circuits a
12、nd installed equipment may differ from the assumptions valid forthe HR circuits and for the design of equipment. Therefore the performance to be expected at the time of commissioningcannot be deduced uniquely from Recommendations relating to HR circuits. Suitable allowances may have to be madefor su
13、ch matters as circuits being made up of equipments of different design, line systems differing substantially inlength from a homogeneous section, etc. (see for example Recommendation G.226 1 for noise on real links).Commissioning objectives are not normally the subject of CCITT Recommendations.5 Exp
14、lanations of limits for maintenance purposesIn service, the performance of an item or assembly of items may deteriorate for various reasons: aging,excessive loading, excessive environmental conditions, operations errors, components failures, etc. and there is aneconomic penalty in service costs if s
15、uch deterioration is always to be kept negligibly small. Therefore design objectivesare chosen to confer as great a margin as possible to assure a satisfactory in-service performance.Fascicle III.1 - Rec. G.102 3FIGURE 1/G.102Illustration of the significance of design objective for an itemWith trans
16、mission impairments, there is often no value which represents a clear boundary between “tolerable“and “unusable“ performance and in practice a range of impairments in excess of those provided by design objectives willgive satisfactory service to customers. This is the case for telephony but for othe
17、r services may be different.Nevertheless it is often expedient to define a particular value of impairment above which the item is deemed tobe “unusable“ and at which the item will be withdrawn from service at the first opportunity so that remedial action canbe taken to restore the performance to com
18、ply with some defined limit (e.g. limit for prompt maintenance action).It is often useful to define a performance limit at which attention is alerted but (perhaps) no action is takenimmediately (e.g. limit for deferred maintenance action).These limits are usually independent of the type of service c
19、arried by that particular entity. However, it issometimes necessary to define a performance limit for a particular type of service, beyond which the customer is nolonger offered a satisfactory service quality. This limit may differ for various services; some may coincide with a promptmaintenance lim
20、it (service limit).These limits (and others, if necessary) would fall above the design objective. These limits are illustrated inFigure 1/G.102 and a generic title for them is “maintenance limits“.4 Fascicle III.1 - Rec. G.102Such curves may be obtained for ensembles of items of equipment at the tim
21、e of commissioning.Alternatively curves may be plotted representing the performance of an item during its lifetime.Curve 1 - Example of relative frequency of occurrence of impairments at time of commissioning in which the designvalue is met with some margin. A similar distribution might be achieved
22、in service throughout the lifetime of anitem of equipment if the effect of environmental conditions etc. is negligible. An example might be theattenuation distortion of transformers.Curve 2 - Example of the relative frequency of occurrence of impairments at time of commissioning in which the designv
23、alue is exceeded with some agreed probability because the item of equipment is used in a way which is moredemanding than that in the design objectives. An example might be the effect of a repeater spacing of a radio orline system greater than anticipated.Curve 3 - Example of the relative frequency o
24、f occurrence of impairments in service in which the working environmenthas parameters more onerous than or additional to those specified. Examples might be the effect of excessive loading,component failure or operational errors.FIGURE 2/G.102Examples of the relative frequency of impairment valuesReference1 CCITT Recommendation Noise on a real link, Vol. III, Rec. G.226.
