1、 Rec. ITU-R S.1806 1 RECOMMENDATION ITU-R S.1806 Availability objectives for hypothetical reference digital paths in the fixed-satellite service operating below 15 GHz (Question ITU-R 73/4) (2008) Scope This Recommendation deals with availability objectives for hypothetical reference digital paths (
2、HRDPs) in the fixed-satellite service operating below 15 GHz. It is based on the availability objectives specified in relevant ITU-T Recommendations and complements availability and performance objectives contained in relevant ITU-R Recommendations, as listed under References. References ITU-T Recom
3、mendation G.827: Availability performance parameters and objectives for end-to-end international constant bit-rate digital paths. Recommendation ITU-R S.579: Availability objectives for a hypothetical reference circuits and hypothetical reference digital paths when used for telephony using pulse cod
4、e modulation, or as part of an integrated services digital network hypothetical reference connection, in the fixed-satellite service operating below 15 GHz. Recommendation ITU-R S.614: Allowable error performance for a satellite hypothetical reference digital path in the fixed-satellite service oper
5、ating below 15 GHz when forming part of an international connection in an integrated services digital network. Recommendation ITU-R S.1062: Allowable error performance for a satellite hypothetical reference digital path operating below 15 GHz. Recommendation ITU-R S.1424: Availability objectives for
6、 a hypothetical reference digital path when used for the transmission of B-ISDN asynchronous transfer mode in the fixed-satellite service by geostationary orbit satellite systems using frequencies below 15 GHz. The ITU Radiocommunication Assembly, considering a) that the hypothetical reference digit
7、al paths (HRDPs), as defined in Recommendation ITU-R S.521, in the fixed-satellite service (FSS) are intended as a guide to designers and planners; b) that the availability of an HRDP is determined by the combined effects of equipment and propagation availability; c) that the equipment availability
8、(including the space station) is dependent on reliability performance, maintainability and maintenance support performance; d) that unavailability due to propagation impairments is dependent on the exceedance of an attenuation level threshold, the frequency of these exceedances and the duration of s
9、uch exceedances; 2 Rec. ITU-R S.1806 e) that satellite systems are capable of offering availability performance similar to that of terrestrial paths under service level agreements (SLAs) or service quality agreements (SQAs) for individual links (see ITU-T Recommendation E.801); f) that the availabil
10、ity of a geostationary-satellite link is essentially independent of the distance between the origination and termination points of the link, recommends 1 that the availability of an HRDP in the FSS should be defined by the following formula: %lity)Unavailabi100(tyAvailabili = where: %100timeRequired
11、timeeUnavailabllityUnavailabi = where the required time is defined as the period of time during which the user requires the digital path to be in a condition to perform a required function, and unavailable time is the cumulative time of digital path interruptions within the required time (see also r
12、ecommends 5 for a discussion of unavailable time); 2 that the outage intensity should be defined as the number of outages (i.e. a period of unavailable time as defined in recommends 5) per measurement period, where the typical measurement period is one year (8 766 h) (see also Note 5); 3 that an HRD
13、P should meet the objectives of propagation-dependent availability given in Table 1: TABLE 1 Propagation-dependent availability objectives for an FSS digital satellite link Rate (Mbit/s) Up to primary rate 1.544 to 2.048 Above primary rate Parameter Availability Outage intensity(outages/year) Availa
14、bility Outage intensity (outages/year) Mean value for HRDP 99.9% 20 99.96% 20 4 that the additional unavailability of an HRDP in the FSS due to equipment should not be more than 0.11% of a year; 5 that a link in the FSS as defined between the ends of the HRDP in Recommendation ITU-R S.521 should be
15、considered unavailable if one or more of the conditions in 5.1 to 5.3 below exist at either of the receiving ends of the link for 10 consecutive seconds or more (see Note 3): 5.1 the digital signal is interrupted (i.e. alignment or timing is lost); 5.2 for a transmission below the primary rate (1.54
16、4 Mbit/s or 2.048 Mbit/s), the bit error ratio (BER), averaged over 1 s, exceeds 103; Rec. ITU-R S.1806 3 5.3 for a transmission at or above the primary rate (1.544 Mbit/s or 2.048 Mbit/s), each second is considered to be a severely errored second (SES) event, where an SES is defined consistently wi
17、th ITU-T Recommendation G.826 as a second which contains 30% or more errored blocks or at least one severely disturbed period (SDP); 6 that the following Notes should be regarded as part of this Recommendation: NOTE 1 A period of unavailable time begins when one of the conditions in 5.1 to 5.3 persi
18、sts for a period of 10 or more consecutive seconds. These 10 s are considered to be unavailable time. The period of unavailable time terminates when the same condition ceases for a period of at least 10 consecutive seconds. These 10 s are considered to be available time. Periods of degraded performa
19、nce lasting less than 10 consecutive seconds, during which conditions in recommends 5.1 to 5.3 exist, are considered available time. NOTE 2 All outages due to solar eclipses and interference from the Sun are included as part of the unavailable time in recommends 3 when they occur during the required
20、 time. The impact of solar interference during the required time can be minimized by operational measures since these events can be accurately predicted (see Recommendation ITU-R S.1525). NOTE 3 Availability calculations should explicitly take into account mean time between failures, mean time for r
21、esumption of service and precautions taken to mitigate interruptions and impairments of satellite performance including the use of reserve channels and back-up systems. NOTE 4 A percentage of unavailability, due to propagation, for any month is assumed to correspond to a period of any year by a conv
22、ersion factor of 5, i.e. 0.2% of any month would correspond to 0.04% of any year (referring to the term of “any year”, see Note 11 of Recommendation ITU-R S.353). This conversion factor is discussed in Annex 1 to Recommendation ITU-R S.614. NOTE 5 Outage intensity objectives in Table 1 are based on
23、a value of the “mean time to restore” (i.e. the average duration of unavailable service time intervals) of 4 h. Propagation impairments, giving rise to self-healing unavailability events, are usually much shorter. In this case, they should not be taken into account. NOTE 6 Additional information on
24、outage intensity objectives for digital satellite paths is given in Annex 1. Annex 1 The applicability of the availability ratio and outage intensity parameters of ITU-T Recommendation G.827 to satellite links 1 Introduction This Annex examines the availability objectives given in ITU-T Recommendati
25、on G.827 in the light of the predictions of fade duration and fade occurrence frequency that were performed using the methods described in Recommendation ITU-R P.1623, and to give consideration to their applicability to availability objectives for digital satellite connections. Specifically, the out
26、age intensity objectives of ITU-T Recommendation G.827 are considered in relation to estimates of fade depth and the number of fades predicted for a satellite link. 4 Rec. ITU-R S.1806 2 Background Recommendation ITU-R S.579 has a long history and is applicable to many “legacy” links within the FSS.
27、 It serves as a general reference on the availability of satellite links and contains information on the effects of propagation on satellite links. New satellite connections supporting Internet protocol traffic and the next generation network require additional parameters in order to specify availab
28、ility performance. This annex provides information on adapting the availability parameters contained in ITU-T Recommendations to satellite portions of HRDPs. 3 ITU-T Recommendation G.827 ITU-T Recommendation G.827 provides parameters used in the specification of availability performance. One paramet
29、er is the availability ratio (AR) that is defined in the same way as availability in Recommendation ITU-R S.579. AR is defined as: AR = Available time/Total time For these calculations, the total time is generally taken to be one year (365.25 days, 8 766 h). This parameter has been used as the basis
30、 for Recommendation ITU-R S.579. A second parameter contained in ITU-T Recommendation G.827 is outage intensity (OI) and its reciprocal, the mean time between outages (Mo). According to ITU-T Recommendation G.827: “The mean time between outages (Mo) is the average duration of intervals when the path
31、 element is available during a measurement period. The number of outages per measurement period is called the “outage intensity”. If the measurement period is one year and Mo is expressed in fractions of a year, then OI is the reciprocal of Mo.” Table 2/G.827 gives the end-to-end availability perfor
32、mance objectives for a 27 500 km international digital hypothetical reference path (HRP) at or above the primary rate. These objectives are reproduced below. “TABLE 2/G.827 End-to-end availability performance objectives for a 27 500 km international digital HRP at or above the primary rate Performan
33、ce level Availability ratio (AR) Outage intensity (OI) High priority 98% 70 Standard priority 91% 250 ” Using a 35% block allocation of the end-to-end of objectives, as given in ITU-T Recommendation G.826, for the satellite portion of a 27 500 km HRP, the corresponding satellite portion availability
34、 ratio and outage intensity objectives were derived and are given in Table 2. Rec. ITU-R S.1806 5 TABLE 2 Satellite portion G.827 availability performance objectives at or above the primary rate Performance level Availability ratio (AR) Outage intensity (OI) (outages per year) High priority 99.3% 25
35、 Standard priority 96.8% 88 4 Link segment availabilities In Recommendation ITU-R S.1424 an example of space segment availability is given as 99.99%. Earth station availability, for manned earth stations with a G/T of 31.7 dB/K at 11/12 GHz, is given as 99.95% for each earth station. This leads to a
36、 composite Earth segment availability of 99.90%. The availability allotted to the combined Earth segment and space segment equipment is 99.89%. Using the ITU-T G.827 availability performance of 99.3%, for high-priority paths, as an objective, the equipment availability of 99.89% implies that the pro
37、pagation availability must be 99.4% or better. This information is summarized in Table 3. TABLE 3 Satellite portion availability performance required to meet G.827 objectives for high-priority performance level links at or above the primary rate Satellite portion of G.827 availability ratio objectiv
38、es (high priority) 99.3% Space segment equipment availability 99.99% Earth segment equipment availability (each earth station) 99.95% Implied propagation availability 99.41% It should be noted that the values of the availability ratio that are recommended in Table 3 are more stringent than the ones
39、derived from ITU-T Recommendation G.827 because an HRDP should also meet the availability objectives of ITU-T Recommendations G.821 and G.826 (see also Recommendations ITU-R S.579, ITU-R S.614 and ITU-R S.1062). In the absence of more precise data concerning the outage intensity of satellites and ea
40、rth stations, five outages per year have been allocated to the space and Earth segments based on engineering experience. Collecting reliable data would enable the development of a more precise value and may be the subject of further studies within ITU-R. 5 Outage intensity and attenuation Based on t
41、he application of Recommendation ITU-R P.1623, information is presented here on the derivation of outage intensity values predicted for satellite links with 99.4% propagation availability. For 6/4 GHz links, propagation outages of greater than about 1 dB occur rarely, therefore the only significant
42、cause of outage would be due to equipment. Modern earth stations are maintained to levels where meeting the outage intensity objectives of ITU-T Recommendation G.827 should not be a problem. 6 Rec. ITU-R S.1806 For satellite links operating above 10 GHz, outages due to propagation can be significant
43、 and are taken into account in the design of these links. Briefly describing the procedure applied here, 99.4% propagation availability corresponds to a certain fade depth that can be determined through the application of Recommendation ITU-R P.618. Using this fade-depth value, the methods given in
44、Recommendation ITU-R P.1623 can be applied to determine the probability of fades of that depth and the number of fades per year of that depth. Table 5 shows the fade depths that correspond to 99.4% propagation availability, computed using the methods in Recommendation ITU-R P.618, for three example
45、locations whose characteristics are contained in Table 4. TABLE 4 Examples of locations and associated climatic characteristics Earth station coordinates 46.222 N 6.139 E 25.81 N 279.88 E 40.773 N 286.025 E Elevation angle 30 24 23 Rain rate for 0.01% of a year 25 mm/h 96 mm/h 43 mm/h TABLE 5 Fade d
46、epths corresponding to a propagation availability of 99.4% Earth station location 46.222 N 6.139 E 25.81 N 279.88 E 40.773 N 286.025 E Path frequency (GHz) 11 14 11 14 11 14 Fade depth (dB) 0.6 1.1 2.7 4.7 1.5 2.6 Note that the fade depths given in Table 5 do not take into account the additional mar
47、gin required to overcome the increase in noise temperature of the receiving station due to cloudy sky conditions. 5.1 Number of fades resulting in an outage According to the existing Recommendations on satellite performance objectives and availability, an outage on a satellite connection occurs when
48、 the performance of the connection drops below the availability threshold for 10 s or more. Given the outage intensity numbers contained in Table 2, the fade depth corresponding to those numbers of outages can be determined. Compliance with ITU-T Recommendation G.827 implies that enough margin1must
49、be designed into the satellite link to prevent the number of fades of 10 s or more from exceeding the outage intensity objectives. Thus the fade margin for the satellite link must meet or exceed the depth of the fade of 10 s duration that appears on the ordinate of Figs 1 through 6. Table 6 summarizes the implied fade margins required to meet the outage intensity objectives of ITU-T Recommendation G.827, as well as the margins required to meet the propagation availability objectives given in Table 1, for the three locations at 11 GHz and 14 GHz. 1Recommend
