1、 Rec. ITU-R S.614-4 1 RECOMMENDATION ITU-R S.614-4 Allowable error performance for a satellite hypothetical reference digital path in the fixed-satellite service operating below 15 GHz when forming part of an international connection in an integrated services digital network*(1986-1990-1992-1994-200
2、5) The ITU Radiocommunication Assembly, considering a) that the concept of an integrated services digital network (ISDN) has been defined by the ITU-T; b) that satellites operating in the fixed-satellite service (FSS) will have an important role to play in extending the concept of the ISDN to intern
3、ational connections; c) that satisfactory error performance is an essential feature of any digital transmission system; d) that the error performance of an international digital connection forming part of an ISDN has been specified by the ITU-T in ITU-T Recommendation G.821 at 64 kbit/s; e) that the
4、 costs of establishing and maintaining digital communication satellite systems are critically dependent on overall error performance; f) that in defining error performance criteria, it is necessary to take account of all foreseeable error-inducing mechanisms, especially time-varying propagation cond
5、itions and interference, noting 1 that there are satellite hypothetical reference digital paths (HRDP) still in operation that have been designed taking into account the objectives of ITU-T Recommendation G.821, including degrading minutes parameter that was subsequently deleted in the 1996 version
6、of ITU-T Recommendation G.821, recommends 1 that the bit error ratio (BER) (see Note 2) at the output (i.e. at either end of a two-way connection) of a satellite HRDP (as defined in Recommendation ITU-R S.521) operating below 15 GHz and forming part of a 64 kbit/s ISDN connection should not exceed d
7、uring the available time the values given below: 1.1 1 107for more than 10% of any month, 1.2 1 106for more than 2% of any month, 1.3 1 103for more than 0.03% of any month (see Note 5); *This version of the Recommendation refers to ITU-T Recommendation G.821 as drafted before 1996 when the degraded
8、minute parameter was removed. All future satellite system designs should be based upon the latest version of Recommendation ITU-R S.1062. 2 Rec. ITU-R S.614-4 2 that the following Notes should be regarded as part of the Recommendation: NOTE 1 Section 1 was established utilizing the method outlined i
9、n Annex 1. It is based on, and sufficient to meet, the required fixed-satellite HRDP error performance objectives given in ITU-T Recommendation G.821 under all envisaged operating conditions. The ITU-T allocations for an FSS HRDP, which are considered to apply to the available time over a period of
10、the order of any one month, can be stated as: fewer than 2% of the 1 min intervals to have a bit error ratio worse than 1 106 ; fewer than 0.03% of 1 s intervals to have a bit error ratio worse than 1 103 ; fewer than 1.6% of 1 s intervals to have errors. NOTE 2 The BERs in 1 should be measured over
11、 a sufficiently long period of time in order to ensure that they provide a good estimate of the bit error probability (see Annex 1). NOTE 3 The BERs given in this Recommendation have been based on the assumption that contributions to severely errored seconds can be produced by two different error me
12、chanisms: those occurring randomly and those occurring in bursts. For the major portion of the time, the errors are random and are constrained by 1.1 and 1.2. Severely errored seconds are excluded from the random error measurements made to verify 1.1 and 1.2, but are included in 1.3 (see Annex 1). N
13、OTE 4 The BERs in 1 provide a margin for some burst errors that could arise from sources identified in Annex 1. NOTE 5 The value 0.03% of any month relates to the measured BER during the available time. This objective could be met, for example, by designing the satellite system to an unavailability
14、objective of 0.2% of the worst month (total time). By using a 10% availability factor (ratio of available to total time while the BER is worse than 103), this would correspond to 0.02% of the available time of any month. Further, it is necessary to include an allowance to accommodate contributions t
15、o those severely errored seconds which occur when the BER is better than 103. Taking as an example 0.01% of the worst month for this allowance, the total performance objective would be 0.03% of the available time of the worst month (see Annex 1). NOTE 6 The HRDP referred to in this Recommendation is
16、 specified in Recommendation ITU-R S.521. NOTE 7 It may be necessary to make special provision regarding the performance of satellite-to-satellite links. The extent of this provision is a matter requiring further study. NOTE 8 The Recommendation applies only when the system is considered available i
17、n accordance with Recommendation ITU-R S.579 and includes periods of high bit error ratio exceeding 103which persist for periods of less than 10 consecutive seconds. Short interruptions (less than 10 s) shall be treated as equivalent to the case in which the BER exceeds 103. NOTE 9 The error perform
18、ance objectives given in this Recommendation are designed to meet the specified end-to-end performance of a 64 kbit/s circuit switched, ISDN connection consistent with ITU-T Recommendation G.821. Performance objectives for satellite systems carrying PCM encoded telephony in a non-ISDN connection are
19、 given in Recommendation ITU-R S.522. Recommendation ITU-R S.614 may be used in the design of satellite systems carrying other forms of digital information, such as voice band data (e.g. facsimile) or low rate encoded (LRE) speech (under 64 kbit/s), until more specific studies are completed which ma
20、y lead to improved performance objectives. Rec. ITU-R S.614-4 3 NOTE 10 The objectives for BERs indicated in 1 are not unique in meeting the required performance objectives given in ITU-T Recommendation G.821. Other masks for the BERs may be used by the designer where appropriate as long as these ma
21、sks satisfy ITU-T Recommendation G.821. Examples of alternative masks are shown in Annex 1. NOTE 11 It is desirable that systems be planned on the basis of propagation data covering a period of at least four years. The performance recommended to be met for “any month” should be based on the propagat
22、ion data corresponding to the median “worst month of the year” taken from the monthly statistics of all the years for which reliable data are available. NOTE 12 The error performance described in 1 was developed based on the use of the HRDP in the “high grade” section of the hypothetical reference c
23、onnection (HRX) (see ITU-T Recommendation G.821). Other applications of the HRDP in the HRX are possible and the error performance objectives can be adjusted accordingly. NOTE 13 In order to comply with the values given in 1, for systems operating above 10 GHz, it may be advantageous to make use of
24、fade countermeasures strategies and techniques for which basic guidance is provided in Recommendation ITU-R S.1061. NOTE 14 A method of measuring BERs against percentage of time is given in Annex 1. Annex 1 Characteristics of a fixed-satellite service hypothetical reference digital path forming part
25、 of an integrated services digital network 1 Introduction This Annex discusses the performance objectives that an FSS HRDP will need to achieve when it forms part of a hypothetical reference connection (HRX) in an ISDN. The ISDN HRX for a 64 kbit/s circuit switched connection is defined in ITU-T Rec
26、ommendation G.821 (CCITT Blue Book 1988, Fascicle III.3) which uses three circuit classifications for performance quality definition: local, medium and high grade. International satellite circuits are considered part of the high grade performance section. The apportionment of the overall ISDN HRX pe
27、rformance objectives to the FSS and the impact of this apportionment on the design of FSS systems are presented in the following sections. 2 64 kbit/s satellite channels forming part of the ISDN HRX 2.1 Performance requirements of the FSS HRDP 2.1.1 Satellite system error performance objectives The
28、performance of satellite systems is generally given in terms of bit error probability while ITU-T Recommendation G.821 identifies time intervals that must have a specified error ratio for certain time percentages. These percentages are taken over a longer period, i.e. of the order of one month. This
29、 section presents the method that has been used for converting from the ITU-T specification to the form of performance objective used for satellite systems, and gives the satellite HRDP performance requirements that result from applying this method to the values quoted in ITU-T Recommendation G.821.
30、 4 Rec. ITU-R S.614-4 A careful distinction between bit error probability (BEP) and bit error ratio (BER) has been made in this Annex. BEP, which is used extensively in the following sections, is an abstract quantity used to express the theoretical performance of data communications equipment. BER i
31、s a quantity that is readily measurable (i.e. bit errors per bits transmitted). By making a sufficient number of measurements each lasting a sufficient time, the BEP can be estimated to within any desired accuracy. The approach adopted in this Annex is to assume that (at 64 kbit/s) the satellite sys
32、tem link performance is limited by mechanisms that are essentially random in nature and can be analysed by using a Poisson or binomial approach to calculate the probability of experiencing a given number of errors in a given time interval, with a given bit error probability. In practice, system desi
33、gners must also be aware of bursts of errors which would not be picked up by this approach (some of the mechanisms which could give rise to such bursts are described in 2.2) and allow sufficient margins to cover these effects. The bursty errors due to error correction techniques are treated in 5. 2.
34、1.2 ITU-T Recommendation G.821 requirements Table 1 summarizes the end-to-end performance objectives given in ITU-T Recommendation G.821 and the satellite HRDP objectives. For each performance classification the overall end-to-end requirement is given, along with the requirement on a satellite HRDP.
35、 2.1.3 BEP models required to meet ITU-T Recommendation G.821 Section 3 outlines the method by which a given BEP versus percentage-of-time distribution can be analysed in terms of the parameters given in Table 1. Using this procedure, it has been possible to derive a number of distributions, or mode
36、ls, based on the general characteristics of satellite systems, which meet or exceed the objectives given in ITU-T Recommendation G.821. Of the models given in 3, one is summarized here. This model strikes a compromise between the requirements of propagation limited systems typically operating above
37、10 GHz, and those of interference limited systems typically operating below 10 GHz and can be met by high-capacity, state-of-the-art satellite systems. The BEP requirements of this model as indicated by the breaking points of the curve shown in Fig. 3 are as follows: BEP = 1 107for 90% of the worst
38、month, BEP = 1 106for 98% of the worst month. The performance of this model is summarized in Table 2 in terms of degraded minutes, errored seconds and severely errored seconds. The performance is listed in terms of both total time and available time in order to show the relationship between system d
39、esign calculations and the objectives of Recommendation ITU-T G.821. Rec. ITU-R S.614-4 5 TABLE 1 Overall end-to-end and satellite HRDP error performance objectives for international ISDN connections NOTE 1 The terms “degraded minutes”, “severely errored seconds” and “errored seconds” are used as a
40、convenient and concise performance objective “identifier”. Their usage is not intended to imply the acceptability, or otherwise, of this level of performance. NOTE 2 The 1 min intervals mentioned above are derived by removing unavailable time and severely errored seconds from the total time and then
41、 consecutively grouping the remaining seconds into blocks of 60. NOTE 3 For practical reasons, at 64 kbit/s, a minute containing four errors (equivalent to an error ratio of 1.04 106) is not considered degraded. However, this does not imply relaxation of the error ratio objective of 1 106. NOTE 4 Ov
42、erall end-to-end and satellite HRDP performance objectives are expressed in terms of available time (see 2.1.5). The short-term (see Note 1) breakpoint (i.e. BEP 103) used in these models was 0.2% of the month (total time) with a propagation availability factor of 10% ( 2.1.5 and 3). NOTE 1 The phra
43、se “short-term” refers to the period of time when the satellite portion of the connection is experiencing extremely degraded performance (i.e. error performance 1 103). The words “long-term” refer to the period of time when the satellite portion of the connection is not experiencing degraded perform
44、ance (i.e. error performance 1.0 106). TABLE 2 Performance Objectives Total time (%) Available time (%) Degraded minutes 2.05 1.87 Errored seconds 1.74 1.56 Severely errored seconds 0.204 0.024 2.1.4 Satellite transmission considerations The performance of a satellite digital transmission link is a
45、function of various factors. One highly significant factor is the effect of propagation disturbances on transmission. Using methods developed by Study Group 3 (ex SG 5), the effects of propagation disturbances on digital transmission performance can be predicted. Section 4 gives the results of calcu
46、lations comparing the performance of three different international digital satellite systems. These calculations are included to provide insight into the effects of propagation on the short-term BEP, as a function of time for practical systems. Performance limits included in the various models are s
47、hown in Fig. 4. Performance classification Overall end-to-end objectives (Note 4) Satellite HRDP objectives (Note 4) (a) Degraded minutes (Notes 1, 2) Fewer than 10% of 1 min intervals to have a bit error ratio worse than 1 106(Note 3) Fewer than 2% of 1 min intervals to have a bit error ratio worse
48、 than 1 106(Note 4) (b) Severely errored seconds (Note 1) Fewer than 0.2% of 1 s intervals to have a bit error ratio worse than 1 103Fewer than 0.03% of 1 s intervals to have a bit error ratio worse than 1 103(c) Errored seconds (Note 1) Fewer than 8% of 1 s intervals to have any errors (equivalent
49、to 92% error-free seconds) Fewer than 1.6% of 1 s intervals to have any errors (equivalent to 98.4% error-free seconds) 6 Rec. ITU-R S.614-4 It should be noted that the performance of a satellite digital transmission channel can be designed to meet virtually any performance specification. However, the use of forward error correction, power control and site diversity which can significantly improve system performance has penalties of decreased capacity and/or increased
