1、Rec. ITU-R S.1424 1 RECOMMENDATION ITU-R S.1424 AVAILABILITY OBJECTIVES FOR A HYPOTHETICAL REFERENCE DIGITAL PATH WHEN USED FOR THE TRANSMISSION OF B-ISDN ASYNCHRONOUS TRANSFER MODE N THE FSS BY GEOSTATIONARY ORBIT SATELLITE SYSTEMS USNG FREQUENCIES BELOW 15 GHz (Questions ITU-R 73/4 and ITU-R 78/4)
2、 (2000) The ITU Radiocommunication Assembly, considering a) that satellites operating in the FSS play an important role in providing reliable international digital communications: b) that the hypothetical reference digital path (HRDP) in the FSS is intended as a guide to designers and users; cl that
3、 the broadband integrated services digital network (B-ISDN) asynchronous transfer mode (ATM) semi-permanent connection availability has been specified in ITU-T Recommendation 1.357; d) ITU-T Recommendation 1.357; that it is desirable to be compliant with the concepts, terms and definitions related t
4、o availability as given in e) transmission medium; that in defining availability criterion, it is necessary to take into account all the specific characteristics of the f) aggregate effects of network congestion, transmission errors, equipment failures and propagation characteristics, that the avail
5、ability for a HRDP used for the transmission of B-ISDN ATM over satellite is determined by the recommends 1 objectives set forth in ITU-T Recommendation 1.357; that a satellite HRDP carrying B-ISDN ATM semi-permanent connection traffic meet the availability 2 that the HRDPs defined in Recommendation
6、 ITU-R S.521 and the reference model defined in Recommen- dation ITU-R S.1420 be considered as a basis for developing methods and techniques for meeting the availability objectives of ITU-T Recommendation 1.357; 3 ITU-T Recommendation 1.356 during the available time (see Annex 1); that a satellite H
7、RDP carrying B-ISDN ATM connection traffic must meet the performance objectives given in 4 Notes 1 to 3); that the yearly availability of a satellite HRDP (one direction) in the FSS should be greater than 99.85% (see 5 that the following Notes be regarded as part of this Recommendation. NOTE 1 - The
8、 satellite HRDP availability objective encompasses the availability due to propagation and the earth station and spacecraft equipment availability. Based on previous studies for 14/11 GHz and 30/20 GHz band transmission paths, the yearly availability due to disturbed propagation is 99.96% to earth s
9、tations with a GIT2 31.7 dB(K-l) and the availability of manned 14/11 GHz band earth stations with a GIT2 31.7 dB(K-l) is 99.95%. Current information provided to the ITU-R shows that the spacecraft availability is better than 99.99%. Based on these considerations, the yearly availability for a satel
10、lite HRDP within recommends 4 applies to links between manned earth stations with GIT2 31.7 dB(K-l). The yearly availability for a satellite HRDP within recommends 4 does not apply to unmanned earth stations nor to earth stations with G/T 31.7 dB(K-l). The yearly availability for links that originat
11、e or terminate at an unmanned earth station or at an earth station with G/T 31.7 dB(K-l) is for further study. 2 Rec. ITU-R S.1424 NOTE 2 - The equipment availability figures may be revised in the future once information from earth station operators and satellite system operators becomes available t
12、o the ITU. NOTE 3 - The additional impact on system availability due to satellite-specific ATM equipment used at earth stations and of spacecraft that employ on-board processing and switching is for further study. NOTE 4 - The availability requirements for HRDP that carry non-ATM traffic within a hy
13、pothetical reference connection (HRX) in the FSS are given in Recommendation ITU-R S.579-4. NOTE 5 - The availability objectives for B-ISDN ATM switched connections is for further study. However, until the specific availability requirements of these type of connections are adopted by the ITU-T, the
14、objectives given in this Recommendation may be used (see ITU-T draft Recommendation 1.35av). ANNEX 1 (Normative) 1 General considerations on B-ISDN ATM satellite connection availability The current version of ITU-T Recommendation 1.356 (B-ISDN ATM layer cell transfer performance) provides in Table 2
15、D.356 the QoS class definitions and the end-to-end network performance objectives. These objectives are given, for each performance parameter, as upper bounds that need to be met on a virtual channel (VC) or virtual path (VP) for the duration of the connection. ITU-T Recommendation 1.356 makes no re
16、ference to the B-ISDN ATM availability requirements as this is the subject of ITU-T Recommendation 1.357 (B-ISDN semi-permanent connection availability) and ITU-T draft Recommendation I.35av (3-ISDN switched virtual channel connection availability). The objectives given in Table 2D.356 had been base
17、d on some preliminary test results of QoS measurements of B-ISDN ATM services and applications. Generally, the results that were presented focused on the upper bound limits for service acceptability; that is, they should have specified the maximum cell loss ratio (CLR) and cell error ratio (CER) at
18、which the applications are still acceptable but not degraded. This aspect is important since packet-based systems are generally very robust but tend to degrade rapidly in quality from being acceptable to unacceptable with a small region of marginal acceptability. The numbers provisionally adopted in
19、 Table 2D.356 may be revised in the near future based on real operational experience and on more QoS test results of services and applications. However, to begin the interpretation of these numbers and their relationship with satellite link performance we must accept the premise that the QoS limits
20、must be provided to the end-user for the duration of the connection. This means that these thresholds should not be exceeded during the available time. The availability threshold in satellite links is somewhat arbitrary since it is a point of reference (in terms of percentage of time) at which all n
21、etwork objectives are met. There are at least two approaches to adopt a satellite link availability objective. The first approach is to develop a rigorous mathematical model to find the optimum curve (BER vs. per cent of time distribution) at which all performance and availability requirements are m
22、et. This model must account for the burst error characteristics of satellite modems and the link propagation characteristics; in addition, it must relate these characteristics to the network performance and availability definitions and objectives. The second approach is an empirical one, whereby mea
23、surements of performance and availability parameters of an actual satellite link demonstrate that all objectives are met. Although in practice it is common to use some combination of the two approaches, this Recommendation follows the second approach (see Note i). The availability due to propagation
24、 is discussed in Q 7.1 and in Q 2 of Annex 2. NOTE 1 - This approach may be amended in the future once Telecommunication Standardization SG 13 adopts objectives for ITU-T Recommendation 1.357 and if subsequent mathematical models and propagation measurements demonstrate the need to change this Recom
25、mendation. Rec. ITU-R S.1424 3 2 Definition of B-ISDN ATM connection availability The network performance parameters, objectives and measurement methods for describing B-ISDN ATM semi-permanent connection availability are discussed in ITU-T Recommendation 1.357. Section 4 of that Recommen- dation de
26、fines the availability of a B-ISDN ATM semi-permanent connection portion as the fraction of time during which the portion of an international connection is able to support a transaction. 3 Criteria for the entry and exit to the unavailable state To define criteria for the entry and exit to the unava
27、ilable state, another cell transfer outcome parameter was introduced by Telecommunication Standardization SG 13, namely, the severely errored second (SES) in the ATMI, (SESATM). Paragraph 4.3 of ITU-T Recommendation 1.357 gives the definition of the SESATM. “The onset of unavailability shall begin w
28、ith the occurrence of ten consecutive SESATM. These ten seconds are part of unavailable time. A period of unavailability shall end with the occurrence of ten consecutive seconds, none of which are SESATM. These ten seconds are part of available time. The ten-second criteria are supported using a sli
29、ding window with one-second granularity.” 4 B-ISDN ATM availability parameters ITU-T Recommendation 1.357 defines two parameters for the ATM availability of semi-permanent connection portions: the availability ratio (AR) and the mean time between outages (MTBO). The AR is defined as the proportion o
30、f time that the connection portion is in the available state over an observation period. The MTBO is defined as the average duration of a continuous time interval during which the portion is available. 5 B-ISDN ATM availability objectives The allocation of AR and MTBO objectives, according to the co
31、nnection portion (Le., national, international, international inter-operator) is still under study by Telecommunication Standardization SG 13. 6 Definition of satellite HRDP availability The following discussion (5 6 and 7) applies to the new AR parameter. Further studies are necessary to assess the
32、 MTBO figures representative of ATM satellite links. The availability of a satellite HRDP that carries B-ISDN ATM traffic, as defined by the HRDPs given in Recommen- dation ITU-R S.521 and the reference model given in Recommendation ITU-R S. 1420, comprises the propagation availability, and equipmen
33、t availability. The overall availability of the satellite portion of an end-to-end connection, as defined in Fig. 1 of Recommendation ITU-R S.521, can be expressed as follows: Asatellite HRDP = also, it includes sun interference effects. The availability of any ATM equipment that may be used specifi
34、cally in a satellite connection is for further study total availability (equipment reliability) of the spacecraft including eclipse outages. The availability of any on-board ATM processing and/or switching equipment is for further study Aspacecraft : Asatezlite HRDP : product of all availability com
35、ponents on a satellite link. 4 Rec. ITU-R S.1424 NOTE 1 - In addition to the parameters mentioned above, ITU-T Recommendation 1.357 includes, as part of its scope, another component of availability due to traffic congestion. Once this parameter is formally adopted we would expect that a clear defini
36、tion of this parameter will be provided by Telecommunication Standardization SG 13. As applied to satellite systems, this availability parameter may include the availability due to traffic congestion or queuing from on- board ATM equipment and/or any ATM earth station equipment that may be used spec
37、ifically in a satellite connection. General comments - A discussion of propagation effects on unavailable time, for satellite links operating at frequencies below 15 GHz, is given in Recommendation ITU-R S.579-4. The impact of the use of inter-satellite links (ISL) on the satellite system availabili
38、ty is for further study. The use of site diversity to improve the earth segment availability is for further study. Further studies are also required for the availability of the satellite portion of an end-to-end connection for the HRDPs defined in Figs. 2 to 4 of Recommendation ITU-R S.521. - - - 7
39、Satellite HRDP availability objectives As described in Q 6, the total B-ISDN ATM satellite HRDP availability is defined by the product of each availability component. The material presented below, based on measurement results obtained from various systems, proposes individual availability figures fo
40、r each of the components of the satellite HRDP availability figure (Asatezlite mp). However, these figures are not mandatory since the apportionment of individual availability values is left to the designer as long as the overall satellite system availability is met. 7.1 Link availability (Azink) Sa
41、tellite links that carry B-ISDN ATM traffic must meet the availability objectives given in ITU-T Recommen- dation 1.357 and hence they must comply with ITU-T Recommendation 1.356 objectives (CLR, CER, severely errored cell block ratio (SECBR), cell misinsertion rate (CMR), cell transfer delay (CTD)
42、and cell delay variation (CDV) during the per cent of time defined as the satellite link availability. Specifically, the B-ISDN ATM performance objectives need to be met during the available time. In accordance with Recommendations ITU-R S.1062 and ITU-R S.614, applicable to systems operating below
43、15 GHz, it is proposed that the ITU-T Recommendation 1.356 objectives should be met for 99.8% of the time (any month), or equivalently, 99.96% of the year. - Since some B-ISDN ATM classes of service (e.g. Class-1) will carry real-time voice and interactive video and since it is envisioned that some
44、PSTN traffic may be transported over B-ISDN ATM networks it is proposed that B-ISDN ATM satellite links that are designed to carry all QoS classes should be designed to support the performance and availability objectives of the most stringent service class. The availability objectives for B-ISDN ATM
45、 satellite links that carry other (less stringent) service classes (Le. either Class-2, Class-3, or unbounded class) are left for further study. The link availability specified above should be met by all existing and future systems that will carry B-ISDN ATM traffic at any bit rate. - 7.2 Earth stat
46、ion availability (Aearth station) The availability objective for manned 6/4 GHz band earth stations is 99.95%. Availability objectives for other earth station types are for further study. 7.3 Spacecraft availability (Aspacecraft) The availability objective for spacecraft used in GSO systems is 99.99
47、%. Availability objectives for other non-GSO systems or for GSO systems with ISL are for further study. 7.4 Satellite HRDP availability (Amp) For a link that operates at 6/4 GHz or 30/20 GHz band, that comprises two earth stations with a GIT2 31.7 dB(K-l) and a single GSO spacecraft, the satellite s
48、ystem availability is 99.85%. Rec. ITU-R S.1424 ANNEX 2 5 (Informative) Measurements of availability of satellite links carrying B-ISDN ATM traffic 1 Introduction This Annex contains some measurement results of the various availability components that comprise the satellite HRDP availability (Asatez
49、lite HRD), as defined in Q 6 of Annex 1. Further information is requested on the availability figures of other satellite systems. 2 Link availability (Aik) The material presented below is based on a study of propagation characteristics of a current satellite system. The results represent the link availability and hence do not include the availability of the spacecraft or the transmit and the receive earth station equipment. Also, no satellite-ATM equipment was included at the earth stations as the signal was simply processed by a conventional modem-codec. Furthermore, the spacecraf
