1、Rec. ITU-R S.1420 1 RECOMMENDATION ITU-R S. 1420 PERFORMANCE FOR BROADBAND INTEGRATED SERVICES DIGITAL NETWORK ASYNCHRONOUS TRANSFER MODE VIA SATELLITE (Questions ITU-R 78/4 and ITU-R 75/4) (1 999) The ITU Radiocommunication Assembly, considering a) digital communications; that satellites operating
2、in the fixed-satellite service play an important role in providing reliable international b) Recommendation 1.356-B-ISDN ATM layer cell transfer performance; that, therefore, satellite performance must comply with performance objectives as specified by ITU-T 4 transmission medium used for asynchrono
3、us transfer mode (ATM) transmission, that in defining performance criteria, it is necessary to take into account all the specific characteristics of the recommends 1 (B-ISDN) ATM transmission meet the objectives set forth in ITU-T Recommendation 1.356 (see Note 1); that connection portions including
4、 satellite links carrying broadband integrated services digital network 2 Note 2); that these objectives should be met only during the time that the satellite link is in the available state (see 3 methods and techniques for meeting the objectives of ITU-T Recommendation 1.356 in the above context; t
5、hat the reference model, set forth in Annex 1 of this Recommendation, be considered a basis for developing 4 part of a hypothetical reference connection Annex 1 of this Recommendation; that the allocations of ATM performance objectives to connection portions including satellite links forming for B-I
6、SDN ATM systems be according to the allocations set forth in 5 performance levels of satellite systems designed to carry ATM traffic (see Note 3); that the translation methods detailed in Annex 1 of this Recommendation be used in the assessment of 6 NOTE 1 - Satellite transmission systems that carry
7、 ATM traffic but are not part of an international connection are outside the scope of ITU-T Recommendation 1.356. In this case, performance objectives derived fiom ITU-T Recommendation 1.356 may not apply. that the following Notes are regarded as part of this Recommendation. NOTE 2 - The B-ISDN ATM
8、availability objectives for semi-permanent connections are specified in ITU-T Recommendation 1.357. The availability of B-ISDN ATM switched connections is for further study. Satellite system ATM availability parameters and objectives are the subject of Recommendation ITU-R S. 1424. NOTE 3 - Annex 2
9、contains informative material regarding the general performance of ATM over satellites. 2 Rec. ITU-R S.1420 ANNEX 1 (Normative) B-ISDN ATM layer performance objectives and translation methods for satellite links 1 Scope This Annex describes a reference model for the international portion of ATM sate
10、llite connections and details the methods for translating between the required ATM layer performance parameters and the satellite link bit error ratio (BER). Both the HRX definition and the ATM performance parameters of the various parts of the HRX are given in ITU-T Recommendation 1.356. The use of
11、 satellite systems in other connection portions is a subject for further study. 2 Reference model To interpret the allocation of performance objectives given to satellite portions of an ATM connection a reference model is provided in Fig. 1. Notice that the satellite path may comprise the earth stat
12、ions and a single transparent (bent-pipe) satellite or a series of satellites. Some satellite systems may include on-board processors (OBPs), ATM switching, and inter-satellite links (ISLs). The terrestrial segment of the ATM satellite path comprises the earth station equipment (antennas, amplifiers
13、, up-converters, down-converters, modems, etc.) and any satellite specific ATM equipment that may be used within a satellite path. The demarcation point between the domestic ATM network and the international ATM network is known as the measurement point international (MPI). The MPI can be a user-net
14、work interface (UNI) or a broadband inter-carrier interface (B-ICI). The portion between the two MPIs is known as the international inter-operator portion (IIP). FIGURE 1 Reference model for an ATM satellite path UNI or B-II i i i i i mi Satellite system UNI or B-II i i i i switch i 7 IIP satellite
15、path terminal equipment or ATM switch i 1420-0 1 One use of satellites is to provide connectivity between separate ATM networks located in different countries. In such case, the terrestrial ATM network will generally interface with the satellite subnetwork through an UNI (or B-ICI). Since this inter
16、face point may not always be Co-located at the satellite earth station, there may be a terrestrial distance between the terrestrial gateway node and the earth station. Rec. ITU-R S.1420 3 Default objectives 3 ATM performance objectives for satellite links CTD 2-pt. CDV CLRo+i CLRO CER CMR SECBR No d
17、efault No default No default No default 4 x llday i x 10-4 This section provides an interpretation of the performance objectives defined in ITU-T Recommendation 1.356 and the corresponding requirements for the satellite portion(s) of an ATM connection. The end-to-end ATM layer network performance pa
18、rameters and objectives for public B-ISDN are defined in ITU-T Recommendation 1.356. To accommodate the characteristics and the requirements of various traffic types, ITU-T Recommendation 1.356 defines various classes of service. Class 1 (stringent class) is a delay-sensitive class and it is intende
19、d to support constant bit rate (CBR) and real-time variable bit rate (VBR) services such as telephony and videoconference. Class 2 (tolerant class) is a delay-tolerant class and supports available bit rate (ABR) and non-real-time VBR services such as video and data. Class 3 (bi-level class) supports
20、 VBR and ABR services such as high-speed data. Finally, Class 4 (unspecified class) supports unspecified bit rate (UBR) services such as file transfers and e-mail. Table 1 provides the ATM layer performance objectives for the various service classes (see Note 1). These objectives may be revised in t
21、he future based on operational experience (see Note 2). NOTE 1 - During a recent meeting (June 1998) of Telecommunication Standardization Study Group 13 a new stringent bi-level quality of service (QoS) class was provisionally accepted for inclusion in the next version of ITU-T Recommendation 1.356.
22、 This new class will have bounds for cell transfer delay (CTD), cell delay variation (CDV), cell loss ratio O (CLRO), cell error ratio (CER), cell misinsertion rate (CMR), and severely errored cell block ratio (SECBR), but none for CLRO + 1. NOTE 2 - Performance objectives designated by U are unspec
23、ified and the ITU will not establish an upper bound for these parameters. TABLE 1 QoS class definitions and network performance parameters The QoS class required by each application is part of the contract negotiation procedure between the user and the network. If the network can provide the request
24、ed service level, the connection will be established. If there is any performance objective that cannot be met, the connection will be denied. Once a connection is established, the network must ensure that the performance objectives of the QoS class are met during the connection. 3.1 ATM performance
25、 allocation principles ITU-T Recommendation 1.356 specifies upper bounds on ATM transmission performance objectives. It allocates parts of the end-to-end objectives to the national and international portions of an HRX ATM connection. Geostationary satellites have a special allocation based on the as
26、sumption that satellites will replace significant terrestrial distance, multiple 4 Rec. ITU-R S.1420 SECBR et CER (classes 1,2 and 3 (%I ATM nodes and transit country portions. These performance allocations are defined between ATM measuring points as they refer to ATM transmission performance and no
27、t to individual items of transmission equipment such as satellite systems. Thus the allocation to the IIP includes terrestrial plant and may include ATM switching nodes as well as the satellite system. CLR CLR (class 1) (classes 2 and 3) (%I (%I This Recommendation assumes that the terrestrial plant
28、 does not introduce significant errors and that all the allocation can be given to the satellite system. Thus the objectives given in terms of ATM layer parameters at ATM measurement points simply need translation to the BER objectives of the satellite link. This assumption may not be valid, however
29、, for all circuit configurations. Table 2 summarizes the allocation of objectives as specified in ITU-T Recommendation 1.356 to connection portions with satellite links. National WO) TABLE 2 42 35 34.5 35 25 1 AUocation of ITU-T Recommendation 1.356 objectives to connection portions with satellite l
30、inks W3) 48 42 31 9 I 30 I International transit portion I 36 11 JW) I 38 I 30 I 21 I 33 I 42 JW) I According to ITU-T Recommendation 1.356, the abbreviation IIP (x) (x = O, 1, 2 . . .) is used to indicate a virtual channel (VC) IIP with x intervening transit countries, each providing virtual path (
31、VP) switching or cross-connect functions. 3.2 ATM performance objectives for satellite systems Numerical values of ATM performance parameters for satellite systems can be derived by applying the allocations given in Table 1 to the performance objectives given in ITU-T Recommendation 1.356. As an ill
32、ustration, the ATM performance objectives for a satellite link used in the international portion that provides Class 1 service and does not contain switching or cross-connect functions (see Note 1) are shown in Table 3. NOTE 1 - The allocation of performance objectives for geostationary-satellite sy
33、stems that include ATM switching and processing is for further study. 4 Translation between ATM layer and physical layer parameters This section provides a mapping between the CLR, CER and SECBR ATM layer parameters and the BER of the satellite link. This section also discusses the other ATM layer p
34、arameters nominally: CMR, CTD and CDV and the impact of the satellite system characteristics on these parameters. Rec. ITU-R S.1420 Perfonnance parameters CLR 5 ITU objective end-to-end 3 x 10-7(l) ITU objective satellite 7.5 x 10-8 TABLE 3 ATM performance objectives for satellites (Class 1 services
35、) CER SECBR 4 x 10-6 1.4 x i x 10-4 3.5 x 10-5 CTD CDV 400 ms 320 ms (maximum) 3ms Negligible CMR 4.1 Characteristics of satellite transmission errors 1 per day 1 per 72 hourd2) When geostationary orbit (GSO) satellites and fixed earth stations are used, the satellite transmission channel is Gaussia
36、n and transmission errors have a bursty nature owing to the scrambling and coding used in the satellite modems. Generally, the errors that emerge fiom a decoder tend to cluster in bursts according to the decoding algorithms employed. An error burst is defined by two parameters: the average burst len
37、gth (L) and average number of bit errors per burst (N). Table 4 provides values of L and N for typical satellite error correction codes. Error correcting code TABLE 4 Average burst length (L) Average bit errors per burst (N) Values of L and N for commonly used satellite error correcting codes R 314
38、convolutional coding with Viterbi decoding 29 11.5 The above values were obtained fiom empirical measurements of burst errors. Analytical derivations are possible for complex coding schemes but it is difficult to derive the results, and therefore measurements are needed to confirm the outcome. 4.2 C
39、LR The header error control (HEC) mechanism of ATM cells can correct single errors and detect almost all multiple-errors in the 5 bytes that contain the header of an ATM cell. When the HEC detects errors that it cannot correct, the whole cell is discarded and its payload is lost. These discarded cel
40、ls are the main component of the overall CLR parameter. In the presence of randomly distributed errors, the single-bit error correcting code of the HEC is capable of correcting many of the errors encountered. In the presence of burst errors and assuming that the burst affects more than one bit of th
41、e header, then, no correction is possible since the HEC is capable of correcting only single bit errors. However, some patterns of multiple errors in the header can be misinterpreted by the HEC as a single error, and therefore an improper 6 Rec. ITU-R S.1420 correction, or miscorrection, of the head
42、er may occur thus resulting in the cell not being discarded. In this case, the ATM cell is either eliminated or transmitted to the wrong destination by the next ATM node (misrouted). Although it is lost fiom an end-to-end connection, it may be counted as a correct cell by an ATM tester which is only
43、 adjusted to measure corrupted cells at the output of a satellite link. Misdirected cells will also contribute to the CLR if they are detected. 4.2.1 ATMHEC The last octet in the 5-octet ATM cell header is an HEC byte which is used for header error detection and correction. As shown in Fig. 2, the A
44、TM layer employs an ATM HEC mechanism that can operate in either correction mode or detection mode. The ATM layer receiver normally operates in the correction mode, whereby all single bit errors are detected and corrected. All double and quadruple bit errors are only detected, and result in the cell
45、 being discarded. A small fiaction of triple bit errors is mistakenly corrected as single bit errors. When in the correction mode, if a header error is detected (or corrected), the receiver switches to detection mode. In the detection mode, the ATM layer receiver is capable of detecting all single,
46、double, and triple-header bit error patterns. It can also detect most quadruple bit error patterns. In the detection mode, no error correction is performed; all detected header errors result in cell discards. If no header errors are detected, when in the detection mode, the receiver switches to corr
47、ection mode. FIGURE 2 ATM cell header modes of operation Undetected n Multi-bit error detected corrupted hei (cell misinsa (cell discarded) No error detected (cell accepted) T Error detected (cell discarded) No error detected (cell accepted) Singlebit error detected and corrected (cell accepted) 142
48、0.02 4.2.2 In order to calculate how CLR relates to the BER, the following assumptions have been made about the cell loss outcome in the correction and detection modes: - In correction mode, all multiple header error events result in cell loss. This assumption ignores the probability that undetected
49、 multiple errors, or multiple errors which are mistakenly corrected as single errors, may result in valid values in the header (and consequently a cell misinsertion outcome). In detection mode, all single or multiple header error events result in cell loss. Calculation of relationship between CLR and BER - Based on the above assumptions: Pcell loss I correction mode s 1 - Pno errors - Pl error Pcell loss I detection mode s 1 - Pno errors and Pcell loss = Pcorrection mode x Pcell loss I correction mode + Pdetection mode x Pcell loss I detection mode Rec. ITU-R S.1420 Since t
