1、 Rec. ITU-R S.579-6 1 RECOMMENDATION ITU-R S.579-6 Availability objectives for hypothetical reference circuits and hypothetical reference digital paths when used for telephony using pulse code modulation, or as part of an integrated services digital network hypothetical reference connection, in the
2、fixed-satellite service operating below 15 GHz (Question ITU-R 73/4) (1982-1986-1992-1994-1997-2001-2005) Scope This Recommendation deals with Availability objectives for hypothetical reference circuits and hypothetical reference digital paths when used for telephony using pulse code modulation, or
3、as part of an integrated services digital network hypothetical reference connection, in the fixed-satellite service operating below 15 GHz. It is based on the availability objectives specified in some ITU-T Recommendations. The Recommendation has been updated in order to appropriately reflect the ne
4、w changes in associated ITU-T Recommendations. A new section in the Annex is introduced to give guidance on the way to apply Recommendation ITU-R P.1623 to compute the statistics of fade attenuation due to propagation. The ITU Radiocommunication Assembly, considering a) that the hypothetical referen
5、ce circuit (HRC), as defined in Recommendation ITU-R S.352, and the hypothetical reference digital path (HRDP), as defined in Recommendation ITU-R S.521, in the FSS are intended as a guide to designers and planners; b) that the equipment availability (including the space station) is dependent on rel
6、iability performance, maintainability performance and maintenance support performance; c) that the availability of an HRC or an HRDP is determined by the equipment availability and the effects of propagation on the link; d) that the unavailability due to propagation has two components, the exceedanc
7、e of an attenuation threshold and the frequency of such exceedances; e) that it is desirable to apply similar availability objectives to cable, radio-relay and fixed-satellite systems; f) that ISDN traffic can be carried at rates below, at and above the primary rate (1.544 Mbit/s or 2.048 Mbit/s), r
8、ecommends 1 that the availability of an HRC or HRDP in the FSS should be defined by the following formula: Availability = (100 unavailability) % where: (1) 100timerequiredtimeeunavailabllityUnavailabi = % 2 Rec. ITU-R S.579-6 where the required time is defined as the period of time during which the
9、user requires the circuit or digital path to be in a condition to perform a required function, and unavailable time is the cumulative time of circuit or digital path interruptions within the required time; 2 that the unavailability of an HRC or HRDP in the FSS due to equipment should be not more tha
10、n 0.2% of a year; 3 that the unavailability due to propagation should be not more than: 3.1 0.2% of any month for one direction of an HRDP in the FSS (see Note 6); 3.2 0.1% of any year (referring to the term of “any year”, see Note 11 of Recommendation ITU-R S.353) for one direction of an HRC in the
11、 FSS; 4 that a link in the FSS defined between the ends of the HRC or HRDP in Recommendations ITU-R S.352 and ITU-R S.521 should be considered unavailable if one or more of the conditions in recommends 4.1 to 4.5 below exist at either of the receiving ends of the link for 10 consecutive seconds or m
12、ore (see Note 5). (A period of unavailable time begins when one of the conditions in recommends 4.1 to 4.5 persists for a period of 10 consecutive seconds. These 10 seconds are considered to be unavailable time. The period of unavailable time terminates when the same condition ceases for a period of
13、 10 consecutive seconds. These 10 seconds are considered to be available time.): 4.1 for analogue transmission the wanted signal is received at the far end at a level 10 dB or more below its expected level; 4.2 for analogue transmission, the unweighted noise power in a telephone channel at a point o
14、f zero relative level, with 5 ms integration time is higher than 106pW0; 4.3 for digital transmission the digital signal is interrupted (i.e. alignment or timing is lost); 4.4 for digital transmission below the primary rate (1.544 Mbit/s or 2.048 Mbit/s), the bit error ratio (BER), averaged over 1 s
15、, exceeds 103; 4.5 for digital 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. An SES is defined as a second which contains 30% errored blocks or at least one severely disturbed period (SDP) (see ITU-T R
16、ecommendation G.826); 5 that the following Notes should be regarded as part of this Recommendation. NOTE 1 The unavailability of analogue multiplexing equipment is not taken into account. The unavailability of digital multiplexing equipment in the earth station is included in recommends 2. NOTE 2 Th
17、is Recommendation only applies to digital traffic (below, at and above the primary rate) transported within the plesiochronous digital hierarchy (PDH) or the synchronous digital hierarchies (SDH) and combinations thereof (see ITU-T Recommendation G.823). NOTE 3 Periods of degraded performance lastin
18、g less than 10 consecutive seconds, during which conditions in recommends 4.1 to 4.5 exist, are considered available time and are taken into account in the application of error performance Recommendations. NOTE 4 All outages due to solar eclipses and interference from the Sun are included as part of
19、 the unavailable time in recommends 2 when they occur during the required time. The impact of solar interference during the required time can be minimized by operational measures since these events can be accurately predicted. See Annex 1 and Recommendation ITU-R S.1525. NOTE 5 Availability calculat
20、ions should explicitly take into account mean time between failures, mean time for resumption of service and precautions taken to mitigate interruptions and impairments of satellite performance including the use of reserve channels and back-up systems. Rec. ITU-R S.579-6 3 NOTE 6 A percentage of una
21、vailability for any month is assumed to correspond to a period of any year by a conversion 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 Rec
22、ommendation ITU-R S.614. Annex 1 1 Definition of availability In the context of an end-to-end connection, availability comprises a number of component parts, and these are discussed in ITU-T Recommendation G.106. As applied to the satellite HRC and HRDP, availability is concerned only with equipment
23、 availability and effects of propagation. 2 General considerations A variety of factors may have an impact on availability: mean time between interruptions; total interruption over a long period (e.g. a year); total interruption time over a worst period (e.g. any month); mean duration of interruptio
24、n; the rate of occurrence (e.g. measured on an hourly basis); the statistical distribution of interruptions (e.g. attenuation, duration, frequency of occurrence). 3 Unavailability due to equipment A number of different causes of interruption are included under this heading. They are: satellite-relat
25、ed effects, including partial or complete failure of any of the systems on board, plus eclipse outages; earth-station related effects, including failure of any equipment as far as the terrestrial network interface, outages caused by human error, Sun transits and the effects of natural disasters. 4 U
26、navailability due to propagation Studies of the impact of propagation on availability require the separation of short breaks of less than 10 consecutive seconds which are covered by performance recommendations, and those of 10 consecutive seconds or more which contribute to unavailability. In this r
27、espect an “availability factor” has been used which can be defined as: %100occuroutagesallwhichfortimetotaloccurdurations10ofoutageswhichfortimetotalfactortyAvailabili = The meaning of “outages” depends on whether an analogue or digital circuit is considered, and a precise definition in each case is
28、 given in recommends 4. 4 Rec. ITU-R S.579-6 Recommendation ITU-R S.1323 contains information on the combined effects of interference and propagation. 5 Unavailability due to Sun transit Interference due to Sun transit is a predictable natural phenomenon that occurs for short periods twice per year.
29、 Based on the simplified Sun transit algorithm described in Annex 2 of Recommendation ITU-R S.1525, earth station operators can estimate the time and day when Sun interference will occur. With this information, they can take proactive action to mitigate the effects of Sun interference. These strateg
30、ies are described in the following paragraphs. 5.1 Links carrying public switched network (PSN) traffic Most PSN traffic routed via INTELSAT satellites is carried by earth stations with larger antennas such as Standard A (30 to 33 m), revised Standard A (16 to 18 m) in the 6/4 GHz band, or Standard
31、C (16 m) or revised Standard C (9 m) in the 14/10-11 GHz band. Because of their large size and small beamwidth (0.2 to 0.3), the impact of Sun interference is relatively small and is negligible in terms of availability. PSN circuits are designed to an annual availability standard of 99.96% or better
32、. For the antennas above, the availability due solely to Sun interference ranges from 99.997% to 99.998%. In addition, because most PSN traffic is networked through digital switches, essential traffic can be temporarily rerouted for the few minutes of the Sun interference event, or the satellite tru
33、nk circuits can be manually or automatically locked out to prevent the seizure of degraded circuits. Unlike propagation fading that can vary up and down during rain events, Sun transit events will degrade the link performance in a continuous manner until it reaches its maximum, and then improve unti
34、l the normal performance is restored. 5.2 Links carrying time division multiple access and demand assignment multiple access traffic INTELSAT operates two services that require hub stations to provide essential reference timing or bandwidth allocation to the earth station user community. To minimize
35、 the effect of Sun interference on network availability, these stations use the Sun prediction data to transfer network control from the master station to geographically separated secondary master stations. This eliminates the effect of Sun interference and ensures the continuity of network control
36、to provide the essential traffic control services. 5.3 Links carrying leased traffic Links carrying leased traffic usually operate with earth stations having small antennas, and because the same signal is received by several stations, rerouting the traffic is not always realistic or cost effective.
37、However, leased network operators typically plan around these Sun interference outage periods by informing customers that these short outages will occur and non-essential traffic be carried at that time. With foreknowledge of Sun interference, operators can schedule the operations to minimize the im
38、pact and, similarly to planned maintenance and repair, customers have accepted such outages when warned in advance. Based on the definition of unavailability in recommends 1, and the preceding description of the practical steps taken by satellite operators and customers which would affect the requir
39、ed time, it is clear that Sun interference does not contribute to the unavailable time in a manner similar to propagation and interference outages. Rec. ITU-R S.579-6 5 6 Effect of propagation on unavailable time This section summarizes the information available to date on the way propagation effect
40、s contribute to unavailable time. Much of the information has been studied by Radiocommunication Study Group 3 who have analysed the data in terms of available time (with attenuation events less than 10 s, corresponding to “severely errored seconds”) and unavailable time (with attenuation events gre
41、ater than 10 s), in accordance with the definition of unavailable time given in this Recommendation. The limited information available is presented in Tables 1 and 2, and Fig. 1 as percentages of worst month. Table 1 is derived from satellite beacon measurements, Table 2 from radiometer measurements
42、. TABLE 1 Percentage of the worst month for which the indicated values of attenuation were exceeded A division into available and unavailable time (see recommends 4) has been made at each attenuation value Denmark (I, II)(1)Elevation angle = 26.5 Denmark (III)(1)Elevation angle = 12.5 11.8 GHz 14.5
43、GHz 11.4 GHz Attenuation level exceeded (dB) Single site (% of month) Single division (% of month) Single site (% of month) Single site (% of month) Available time Unavailable time Availabletime Unavailabletime Available time Unavailable time Available time Unavailabletime 2 4 6 8 10 15 0.0070 0.000
44、53 0.00028 0.00047 0.000096 0.00017 0.112 0.0222 0.0106 0.0056 0.0033 0.00054 0.0110 0.143 0.0165 0.0038 0.00070 0.0013 0.00014 0.213 0.0462 0.0138 0.0039 0.00070 0.0343 0.00355 0.00035 0.201 0.0215 0.00305 0.00131 (1)See Fig. 1. UK (IV, V)(1) Elevation angle = 29.9 Japan (VI)(1) Elevation angle = 6
45、.6 11.8 GHz 14.5 GHz 11.5 GHz Attenuation level exceeded (dB) Single site (% of month) Single site (% of month) Single site (% of month) Available time Unavailable time Available time Unavailable time Available time Unavailable time 2 3 4 6 8 10 15 0.015 0.0022 0.0008 0.0005 0.16 0.035 0.014 0.006 0
46、.03 0.009 0.0022 0.0009 0.03 0.10 0.033 0.016 0.96 0.16 0.027 0.008 5.7 1.84 0.52 0.17 (1)See Fig. 1. 6 Rec. ITU-R S.579-6 TABLE 2 Percentage of the worst month for which the indicated values of attenuation were exceeded in Canada Climate K (VII, IX)(1)13 GHz Climate E (VIII)(1)13 GHz Attenuation le
47、vel exceeded (dB) Site 1 Elevation angle = 20 Site 2 Elevation angle = 29 Elevation angle = 31 Available time Unavailable timeAvailable time Unavailable timeAvailable time Unavailable time2 3 4 6 8 10 0.017 0.007 0.0039 0.0022 0.0011 0.0007 1.10 0.54 0.36 0.16 0.089 0.056 0.0081 0.0042 0.0028 0.0017
48、 0.0017 0.0007 0.51 0.31 0.22 0.16 0.12 0.099 0.014 0.0046 0.003 0.0004 0.0005 0.0004 0.68 0.22 0.11 0.058 0.041 0.031 (1)See Fig. 1. The following general conclusions have been drawn from Table 1: For elevation angles in the range 26-30 and for attenuation values of 2-8 dB, the ratio of attenuation
49、 time during available time to that during total time was found to be between 3% and 10%. At greater values of attenuation, this proportion tended to increase, since event duration would decrease as the attenuation approached its maximum value. At lower elevation angles, 6-12, the ratio of attenuation time during available time to that during total time was found to be about 14% at the 3 dB attenuation value, decreasing to about 5% at values in the range 10-15 dB. For even greater values of attenuation, the above ratio is likely to increas
