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ITU-T-G.826


INTERNATIONAL TELECOMMUNICATION UNION

ITU-T
TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU

G.826
(12/2002)

SERIES G: TRANSMISSION SYSTEMS AND MEDIA

, DIGITAL SYSTEMS AND NETWORKS Digital networks – Quality and availability targets

End-to-end error performance parameters and objectives for international, constant bit-rate digital paths and connections

ITU-T Recommendation G.826

ITU-T G-SERIES RECOMMENDATIONS TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS INTERNATIONAL TELEPHONE CONNECTIONS AND CIRCUITS GENERAL CHARACTERISTICS COMMON TO ALL ANALOGUE CARRIERTRANSMISSION SYSTEMS INDIVIDUAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON METALLIC LINES GENERAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON RADIO-RELAY OR SATELLITE LINKS AND INTERCONNECTION WITH METALLIC LINES COORDINATION OF RADIOTELEPHONY AND LINE TELEPHONY TESTING EQUIPMENTS TRANSMISSION MEDIA CHARACTERISTICS DIGITAL TERMINAL EQUIPMENTS DIGITAL NETWORKS General aspects Design objectives for digital networks Quality and availability targets Network capabilities and functions SDH network characteristics Management of transport network SDH radio and satellite systems integration Optical transport networks DIGITAL SECTIONS AND DIGITAL LINE SYSTEM QUALITY OF SERVICE AND PERFORMANCE TRANSMISSION MEDIA CHARACTERISTICS DIGITAL TERMINAL EQUIPMENTS DIGITAL NETWORKS
For further details, please refer to the list of ITU-T Recommendations.

G.100–G.199 G.200–G.299 G.300–G.399 G.400–G.449 G.450–G.499 G.500–G.599 G.600–G.699 G.700–G.799 G.800–G.899 G.800–G.809 G.810–G.819 G.820–G.829 G.830–G.839 G.840–G.849 G.850–G.859 G.860–G.869 G.870–G.879 G.900–G.999 G.1000–G.1999 G.6000–G.6999 G.7000–G.7999 G.8000–G.8999

ITU-T Recommendation G.826 End-to-end error performance parameters and objectives for international, constant bit-rate digital paths and connections

Summary This Recommendation defines end-to-end error performance parameters and objectives for international digital paths which operate at or above the primary rate and for international digital connections which operate below the primary rate of the digital hierarchy. The objectives given are independent of the physical network supporting the path or connection. For digital paths which operate at or above the primary rate, this Recommendation is based upon a block-based measurement concept using error detection codes inherent to the path under test. This supports in-service measurements. For digital connections which operate below the primary rate of the digital hierarchy, this Recommendation is based upon bit error and bit error ratio measurements. This approach does not support in-service measurements. Annex A deals with the definition of availability of the path or connection. Annexes B, C and D give specific information concerning PDH, SDH and cell-based transmission paths. It is not required to apply this Recommendation to connections which operate below the primary rate using equipment designed prior to the adoption of this Recommendation in December 2002. This Recommendation deals with the performance of PDH paths, and of those SDH paths using equipment designed prior to the adoption of ITU-T Rec. G.828 in March 2000. ITU-T Rec. G.828 deals with the performance of SDH paths using equipment designed as of or after the adoption of ITU-T Rec. G.828 in March 2000. New Recommendation G.8201 deals with performance of ODUk paths of the OTN.

Source ITU-T Recommendation G.826 was revised by ITU-T Study Group 13 (2001-2004) and approved under the WTSA Resolution 1 procedure on 14 December 2002.

Keywords Background block error, background block error ratio, bit error, bit error ratio, block-based concept, digital connection, digital path, error detection codes, error performance objectives, error performance parameters, errored second, in-service measurements, severely errored second.

ITU-T Rec. G.826 (12/2002)

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FOREWORD The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of telecommunications. The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of ITU. ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Assembly (WTSA), which meets every four years, establishes the topics for study by the ITU-T study groups which, in turn, produce Recommendations on these topics. The approval of ITU-T Recommendations is covered by the procedure laid down in WTSA Resolution 1. In some areas of information technology which fall within ITU-T's purview, the necessary standards are prepared on a collaborative basis with ISO and IEC.

NOTE In this Recommendation, the expression "Administration" is used for conciseness to indicate both a telecommunication administration and a recognized operating agency.

INTELLECTUAL PROPERTY RIGHTS ITU draws attention to the possibility that the practice or implementation of this Recommendation may involve the use of a claimed Intellectual Property Right. ITU takes no position concerning the evidence, validity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others outside of the Recommendation development process. As of the date of approval of this Recommendation, ITU had not received notice of intellectual property, protected by patents, which may be required to implement this Recommendation. However, implementors are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database.

ITU 2003 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU.

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ITU-T Rec. G.826 (12/2002)

CONTENTS Page 1 Scope ............................................................................................................................ 1.1 Application of this Recommendation............................................................. 1.2 Transport network layers................................................................................ 1.2.1 PDH and SDH transport networks ................................................................. 1.2.2 ATM connections ........................................................................................... 1.3 Allocation of end-to-end performance ........................................................... References..................................................................................................................... Abbreviations................................................................................................................ Terms and definitions ................................................................................................... 4.5 Error performance events for paths ................................................................ 4.6 Error performance events for connections ..................................................... 4.7 Error performance parameters........................................................................ The measurement of the block...................................................................................... 5.1 In-service monitoring of blocks...................................................................... 5.2 Out-of-service measurements of blocks ......................................................... Performance assessment ............................................................................................... 6.1 Implications for error performance measuring devices.................................. 6.2 Performance monitoring at the near end and far end of a path ...................... Error performance objectives ....................................................................................... 7.1 End-to-end objectives..................................................................................... 7.2 Apportionment of end-to-end objectives........................................................ 7.2.1 Allocation to the national portion of the end-to-end path or connection ....... 7.2.2 Allocation to the international portion of the end-to-end path or connection....................................................................................................... 1 1 2 2 2 3 3 4 6 6 7 7 7 7 8 8 8 8 8 8 10 10 11 11 11 12 12 12 13 13 13 13 13
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Annex A – Criteria for entry and exit for the unavailable state............................................... A.1 Criteria for a single direction.......................................................................... A.2 Criterion for a bidirectional path or connection ............................................. A.3 Criterion for a unidirectional path or connection ........................................... A.4 Consequences on error performance measurements ...................................... Annex B – Relationship between PDH path performance monitoring and the block-based parameters..................................................................................................................... B.1 General ........................................................................................................... B.1.1 Block size for monitoring PDH paths ............................................................ B.1.2 Anomalies....................................................................................................... B.1.3 Defects............................................................................................................
ITU-T Rec. G.826 (12/2002)

B.2 B.3 B.4 B.5 B.6 B.6.1 B.6.2

Types of paths................................................................................................. Estimation of the performance parameters..................................................... In-service monitoring capabilities and criteria for declaration of the performance parameters ................................................................................. Estimation of performance events at the far end of a path ............................. Differences between this Recommendation and ITU-T Rec. M.2100 concerning path performance ......................................................................... General ........................................................................................................... Allocation methodology .................................................................................

Page 14 14 15 16 16 16 16 17 17 17 17 17 17 18 19 19 19 19 19 20 20 21 21 21 22 23 24

Annex C – Relationship between SDH path performance monitoring and the block-based parameters..................................................................................................................... C.1 General ........................................................................................................... C.1.1 Converting BIP measurements into errored blocks........................................ C.1.2 Block size for monitoring SDH paths ............................................................ C.1.3 Anomalies....................................................................................................... C.1.4 Defects............................................................................................................ C.1.5 Measurement of performance events using aggregate parity error counts..... C.2 Estimation of the performance parameters..................................................... C.3 Estimation of performance events at the far end of a path ............................. Annex D – Relationship between cell-based network performance monitoring and the block-based parameters ................................................................................................ D.1 General ........................................................................................................... D.1.1 Block size for monitoring cell-based paths .................................................... D.1.2 Anomalies....................................................................................................... D.1.3 Defects............................................................................................................ D.2 Types of paths................................................................................................. D.3 Estimation of the performance parameters..................................................... D.4 Estimation of performance events at the far end of the path.......................... Appendix I – Flow chart illustrating for digital paths the recognition of anomalies, defects, errored blocks, ES and SES............................................................................. Appendix II – Bit errors and block errors, merits and limitations ........................................... Appendix III – Applicability of this Recommendation to non-public networks .....................

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ITU-T Rec. G.826 (12/2002)

ITU-T Recommendation G.826 End-to-end error performance parameters and objectives for international, constant bit-rate digital paths and connections
1 Scope

This Recommendation specifies end-to-end error performance events, parameters and objectives for: 1) digital paths operating at bit rates at or above the primary rate; and 2) N × 64 kbit/s circuit-switched digital connection (1 ≤ N ≤ 24 or 31 respectively). This Recommendation also specifies allocations of the end-to-end performance objectives. 1.1 Application of this Recommendation

This Recommendation is applicable to international, constant bit-rate digital paths which operate at or above the primary rate and to international N × 64 kbit/s (1 ≤ N ≤ 24 or 31 respectively) digital connections.
NOTE – It is not required to apply this Recommendation to connections which operate below the primary rate using equipment designed prior to the adoption of this Recommendation in December 2002. Performance events and objectives for connections using equipment designed prior to this date are given in ITU-T Rec. G.821 [14].

The constant bit-rate digital paths may be based on a Plesiochronous Digital Hierarchy, Synchronous Digital Hierarchy or some other transport network such as cell-based. This Recommendation is generic in that it defines the parameters and objectives for the paths and connections independent of the physical transport network. Compliance with the path performance specification of this Recommendation will, in most cases, also ensure that a client 64 kbit/s connection will meet its requirements. Therefore, this Recommendation and ITU-T Rec. G.828 [24] are currently the only Recommendations required for designing the error performance of digital paths at or above the primary rate1. In accordance with the definition of a digital path, path end points may be located at user's premises. Paths are used to support services such as circuit switched, packet switched and leased circuit services. The quality of such services, as well as the performance of the network elements belonging to the service layer, is outside of the scope of this Recommendation. The performance objectives are applicable to a single direction of the path or connection. The values apply end-to-end over a 27 500 km Hypothetical Reference Path or Connection (see Figure 3), which may include optical fibre, digital radio relay, metallic cable and satellite transmission systems. The performance of multiplex and cross-connect functions employing ATM techniques is not included in these values. The parameter definitions for digital paths which operate at or above the primary rate are block-based, making in-service measurement convenient. In some cases, the network fabric is not able to provide the basic events necessary to directly obtain the performance parameters. In these cases, compliance with this Recommendation can be assessed using out-of-service measurements or estimated by measures compatible with this Recommendation, such as those specified in ____________________
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This Recommendation deals with the performance of PDH paths, and of those SDH paths using equipment designed prior to the adoption of ITU-T Rec. G.828 in March 2000. ITU-T Rec. G.828 deals with the performance of SDH paths using equipment designed as of or after the adoption of ITU-T Rec. G.828 in March 2000. New ITU-T Rec. G.8201 deals with the performance of ODUk paths of the OTN. ITU-T Rec. G.826 (12/2002) 1

Annexes B, C and D. The parameter definitions for digital connections which operate below the primary rate of the digital hierarchy are not block-based; rather, they are based upon bit error and bit-error ratio measurements. 1.2 Transport network layers

For paths, this Recommendation specifies the error performance in a given transport network layer. Two cases have to be considered: 1.2.1 PDH and SDH transport networks Figure 1 gives the intended scope where ATM does not form part of the end-to-end path. It should be noted that end-to-end performance monitoring is only possible if the monitored blocks together with the accompanying overhead are transmitted transparently to the path end points.
Application of this Recommendation

A

Network fabric, e.g., PDH, SDH

B
G.826_F01

NOTE – A and B are path end points located at physical interfaces, e.g. in accordance with ITU-T Rec. G.703 [1].

Figure 1/G.826 – Application of this Recommendation for a non-ATM end-to-end transmission path 1.2.2 ATM connections

Where the path forms the physical part of an ATM connection (see Figure 2), the overall end-to-end performance of the ATM connection is defined by ITU-T Rec. I.356 [16]. In this case, this Recommendation can be applied with an appropriate allocation to the performance between the path end points where the physical layer of the ATM protocol reference model (see ITU-T Rec. I.321 [15]) is terminated by ATM cross-connects or switches. ATM transmission paths in the physical layer correspond to a stream of cells mapped either into a cell-based format or into SDH or PDHbased frame structures.
Under study ITU-T Rec. I.356 AAL ATM PL AAL ATM PL ATM Adaptation Layer ATM Layer Physical Layer G.826 allocated ATM PL PL G.826 allocated AAL ATM PL
G.826_F02

Figure 2/G.826 – Architectural relationship between this Recommendation G.826 and ITU-T Rec. I.356 [16]

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ITU-T Rec. G.826 (12/2002)

1.3

Allocation of end-to-end performance

Allocations of end-to-end performance of CBR paths and connections are derived using the rules laid out in 7.2 which are length- and complexity-based. Detailed allocations of G.826 performance to the individual components (lines, sections, multiplexers and cross-connects, etc.) are outside the scope of this Recommendation, but when such allocations are performed, the national and international allocations as given in 7.2 shall be achieved. 2 References

The following ITU-T Recommendations and other references contain provisions which, through reference in this text, constitute provisions of this Recommendation. At the time of publication, the editions indicated were valid. All Recommendations and other references are subject to revision; users of this Recommendation are therefore encouraged to investigate the possibility of applying the most recent edition of the Recommendations and other references listed below. A list of the currently valid ITU-T Recommendations is regularly published. The reference to a document within this Recommendation does not give it, as a stand-alone document, the status of a Recommendation [1] [2] [3] ITU-T Recommendation G.703 (2001), Physical/electrical characteristics of hierarchical digital interfaces. ITU-T Recommendation G.704 (1998), Synchronous frame structures used at 1544, 6312, 2048, 8448 and 44 736 kbit/s hierarchical levels. ITU-T Recommendation G.707/Y.1322 (2000), Network node interface for the synchronous digital hierarchy (SDH), plus Corrigendum 1 (2001), Corrigendum 2 (2001), and Amendment 1 (2001). ITU-T Recommendation G.732 (1988), Characteristics of primary PCM multiplex equipment operating at 2048 kbit/s. ITU-T Recommendation G.733 (1988), Characteristics of primary PCM multiplex equipment operating at 1544 kbit/s. ITU-T Recommendation G.734 (1988), Characteristics of synchronous digital multiplex equipment operating at 1544 kbit/s. ITU-T Recommendation G.742 (1988), Second order digital multiplex equipment operating at 8448 kbit/s and using positive justification. ITU-T Recommendation G.743 (1988), Second order digital multiplex equipment operating at 6312 kbit/s and using positive justification. ITU-T Recommendation G.751 (1988), Digital multiplex equipments operating at the third order bit rate of 34 368 kbit/s and the fourth order bit rate of 139 264 kbit/s and using positive justification. ITU-T Recommendation G.752 (1988), Characteristics of digital multiplex equipments based on a second order bit rate of 6312 kbit/s and using positive justification. ITU-T Recommendation G.755 (1988), Digital multiplex equipment operating at 139 264 kbit/s and multiplexing three tributaries at 44 736 kbit/s. ITU-T Recommendation G.775 (1998), Loss of Signal (LOS), Alarm Indication Signal (AIS) and Remote Defect Indication (RDI) defect detection and clearance criteria for PDH signals. ITU-T Recommendation G.783 (2000), Characteristics of synchronous digital hierarchy (SDH) equipment functional blocks, plus Corrigendum 1 (2001).

[4] [5] [6] [7] [8] [9]

[10] [11] [12]

[13]

ITU-T Rec. G.826 (12/2002)

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[14]

ITU-T Recommendation G.821 (2002), Error performance of an international digital connection operating at a bit rate below the primary rate and forming part of an Integrated Services Digital Network. ITU-T Recommendation I.321 (1991), B-ISDN protocol reference model and its application. ITU-T Recommendation I.356 (2000), B-ISDN ATM layer cell transfer performance. ITU-T Recommendation I.3622, B-ISDN ATM adaptation layer (AAL) functional description. ITU-T Recommendations I.432.x series, B-ISDN user-network interface – Physical layer specification. ITU-T Recommendation I.610 (1999), B-ISDN operation and maintenance principles and functions, plus Corrigendum 1 (2000). ITU-T Recommendation M.60 (1993), Maintenance terminology and definitions. ITU-T Recommendation M.2100 (1995), Performance limits for bringing-into-service and maintenance of international PDH paths, sections and transmission systems. ITU-T Recommendation M.2101 (2000), Performance limits and objectives for bringinginto-service and maintenance of international SDH paths and multiplex sections. ITU-T Recommendation M.2101.1 (1997), Performance limits for bringing-into-service and maintenance of international SDH paths and multiplex sections. ITU-T Recommendation G.828 (2000), Error performance parameters and objectives for international, constant bit-rate synchronous digital paths. ITU-T Recommendation I.325 (1993), Reference configurations for ISDN connection types. ITU-T Recommendation I.340 (1988), ISDN connection types. ITU-T Recommendation G.801 (1988), Digital transmission models. Abbreviations ATM Adaptation Layer Alarm Indication Signal Asynchronous Transfer Mode Administrative Unit Background Block Error Background Block Error Ratio Bit Interleaved Parity Broadband Integrated Services Digital Network Constant Bit Rate Cell Error Control Cyclic Redundancy Check

[15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] 3 AAL AIS ATM AU BBE BBER BIP

This Recommendation uses the following abbreviations:

B-ISDN CBR CEC CRC

____________________
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Withdrawn in June 1997. ITU-T Rec. G.826 (12/2002)

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EB EDC ES ESR FAS HEC HP HRP HRX IG ISDN ISM LOF LOM LOP LOS LP MS N-ISDN NTE OAM ODUk OOS OTN PDH PEP PL RDI REI SDH SES SESR STM TE TIM TP TU

Errored Block Error Detection Code Errored Second Errored Second Ratio Frame Alignment Signal Header Error Check Higher order Path Hypothetical Reference Path Hypothetical Reference Connection International Gateway Integrated Services Digital Network In-Service Monitoring Loss of Frame Alignment Loss of Multiframe Alignment Loss Of Pointer Loss Of Signal Lower order Path Multiplex Section Narrow-band Integrated Services Digital Network Network Terminal Equipment Operation and Maintenance Optical Channel Data Unit-k Out-of-Service Optical Transport Network Plesiochronous Digital Hierarchy Path End Point Physical Layer Remote Defect Indication Remote Error Indication Synchronous Digital Hierarchy Severely Errored Second Severely Errored Second Ratio Synchronous Transport Module Terminal Equipment Trace Identifier Mismatch Transmission Path Tributary Unit
ITU-T Rec. G.826 (12/2002) 5

UAS UNEQ VC 4

Unavailable Second Unequipped (defect) Virtual Container Terms and definitions

This Recommendation defines the following terms: 4.1 hypothetical reference path: A Hypothetical Reference Path (HRP) is defined as the whole means of digital transmission of a digital signal of specified rate including the path overhead (where it exists) between equipment at which the signal originates and terminates. An end-to-end Hypothetical Reference Path spans a distance of 27 500 km. 4.2 digital paths: A digital path may be bidirectional or unidirectional and may comprise both customer-owned portions and network operator-owned portions. 4.2.1 PDH digital paths: With regard to PDH digital paths, ITU-T Rec. M.60 [20] applies. 4.2.2 SDH digital paths: An SDH digital path is a trail carrying the SDH payload and associated overhead through the layered transport network between the terminating equipment. 4.2.3 cell-based digital paths: Under study. 4.3 digital connections: The performance objectives for digital connections are stated for each direction of a N × 64 kbit/s circuit-switched connection (1 ≤ N ≤ 24 or ≤ 31 respectively). ITU-T Rec. I.325 [25] gives reference configurations for the ISDN connection types listed in ITU-T Rec. I.340 [26]. In the context of error performance of 64 kbit/s circuit-switched connection types and the allocation of performance to the connection elements, an all-digital hypothetical reference configuration (HRX) is given in Figure 3. It encompasses a total length of 27 500 km and is a derivative of the standard hypothetical reference configuration given in Figure 1/G.801 [27] and of the reference configuration given in Figure 3/I.325. 4.4 generic definition of the block: The error performance of digital paths in this Recommendation is based upon the error performance measurement of blocks. This clause offers a generic definition of the term "block" as follows3: A block is a set of consecutive bits associated with the path; each bit belongs to one and only one block. Consecutive bits may not be contiguous in time. Table 1 specifies the recommended range of the number of bits within each block for the various bit rate ranges. Annexes B, C and D contain information on block sizes of existing system designs. 4.5 4.5.1 Error performance events for paths4 errored block (EB): A block in which one or more bits are in error.

4.5.2 errored second (ES): A one-second period with one or more errored blocks or at least one defect. 4.5.3 severely errored second (SES): A one-second period which contains ≥30% errored blocks or at least one defect. SES is a subset of ES. Consecutive Severely Errored Seconds may be precursors to periods of unavailability, especially when there are no restoration/protection procedures in use. Periods of consecutive Severely Errored ____________________
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Appendix II contains information on block error versus bit-error measurements. See Appendix I, which contains a flow chart illustrating for digital paths the recognition of anomalies, defects, errored blocks, ES and SES. ITU-T Rec. G.826 (12/2002)

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Seconds persisting for T seconds, where 2 ≤ T < 10 (some Network Operators refer to these events as "failures"), can have a severe impact on service, such as the disconnection of switched services. The only way this Recommendation limits the frequency of these events is through the limit for the SESR. (See Notes 1 and 2.)
NOTE 1 – The defects and related performance criteria are listed in the relevant Annexes (B, C or D) for the different network fabrics PDH, SDH or cell-based. NOTE 2 – To simplify measurement processes, the defect is used in the definition of SES instead of defining SES directly in terms of severe errors affecting the path. While this approach simplifies the measurement of SES, it should be noted that there may exist error patterns of severe intensity that would not trigger a defect as defined in Annexes B, C and D. Thus, these would not be considered as an SES under this definition. If in the future such severe user-affecting events were found, this definition will have to be studied again.

4.5.4 4.6

background block error (BBE): An errored block not occurring as part of an SES. Error performance events for connections

4.6.1 errored second (ES): It is a one-second period in which one or more bits are in error or during which Loss of Signal (LOS) or Alarm Indication Signal (AIS) is detected. 4.6.2 severely errored second (SES): It is a one-second period which has a bit-error ratio ≥ 1.10–3 or during which Loss of Signal (LOS) or Alarm Indication Signal (AIS) is detected. 4.7 Error performance parameters

Error performance should only be evaluated whilst the path is in the available state. For a definition of the entry/exit criteria for the unavailable state, see Annex A. 4.7.1 errored second ratio (ESR): The ratio of ES to total seconds in available time during a fixed measurement interval. This parameter is applicable to both paths and connections. 4.7.2 severely errored second ratio (SESR): The ratio of SES to total seconds in available time during a fixed measurement interval. This parameter is applicable to both paths and connections. 4.7.3 background block error ratio (BBER): The ratio of Background Block Errors (BBE) to total blocks in available time during a fixed measurement interval. The count of total blocks excludes all blocks during SESs. This parameter is applicable only to paths. 5 The measurement of the block

Clause 5 is applicable only to paths. 5.1 In-service monitoring of blocks

Each block is monitored by means of an inherent Error Detection Code (EDC), e.g., Bit Interleaved Parity or Cyclic Redundancy Check. The EDC bits are physically separated from the block to which they apply. It is not normally possible to determine whether a block or its controlling EDC bits are in error. If there is a discrepancy between the EDC and its controlled block, it is always assumed that the controlled block is in error. No specific EDC is given in this generic definition but it is recommended that for in-service monitoring purposes, future designs should be equipped with an EDC capability such that the probability to detect an error event is ≥90%, assuming Poisson error distribution. CRC-4 and BIP-8 are examples of EDCs currently used which fulfil this requirement. Estimation of errored blocks on an in-service basis is dependent upon the network fabric employed and the type of EDC available. Annexes B, C and D offer guidance on how in-service estimates of errored blocks can be obtained from the ISM facilities of the PDH, SDH and cell-based network fabrics respectively.
ITU-T Rec. G.826 (12/2002) 7

5.2

Out-of-service measurements of blocks

Out-of-service measurements shall also be block-based. It is expected that the out-of-service error detection capability will be superior to the in-service capability described in 5.1. 6 6.1 Performance assessment Implications for error performance measuring devices

There is a large number of devices (test equipment, transmission systems, collecting devices, operating systems, software applications) currently designed to estimate the connection parameters ESR and SESR at bit rates up to the fourth level of the PDH. For such devices, the path parameters ESR and SESR may be approximated using the criteria for connections, but an approximation of BBER is not possible from measurements based on connection events and parameters. As the block-based concept and the BBER parameter are not defined for connections, converting those devices to measure the path parameters of this Recommendation is not required. Maintenance on specific systems and transport paths may require other parameters. Parameters and values can be found in the M-series Recommendations. See, for example, ITU-T Recs M.2100 [21] and M.2101 [22]. 6.2 Performance monitoring at the near end and far end of a path

By monitoring SES events for both directions at a single path end point, a network provider is able to determine the unavailable state of the path (see Annex A). In some cases, it is also possible to monitor the full set of error performance parameters in both directions from one end of the path. Specific in-service indicators for deriving far end performance of a path are listed in Annexes B, C and D. 7 7.1 Error performance objectives End-to-end objectives

Table 1 specifies the end-to-end objectives for a 27 500 km HRX or HRP in terms of the parameters defined in 4.7. The actual objectives applicable to a real path or connection are derived from Table 1 using the allocation principles detailed in 7.2. Each direction of the path or connection shall concurrently satisfy the allocated objectives for all relevant parameters. In other words, a path or connection fails to satisfy this Recommendation if any relevant parameter exceeds the allocated objective in either direction at the end of the given evaluation period. The suggested evaluation period is one month.
NOTE 1 – For the purpose of this Recommendation, a month is understood to be any period of 28 to 31 consecutive 24-hour intervals. To be able to compare measurement results taken by different parties on the same path, start time and duration of the performance evaluation period need to be agreed between the parties concerned.

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ITU-T Rec. G.826 (12/2002)

Table 1/G.826 – End-to-end error performance objectives for a 27 500 km international digital HRX or HRP Connections Rate 64 kbit/s to primary rate (Note 4)
Not applicable 0.04 0.002 Not applicable

Paths 1.5 to 5 (Mbit/s)
800-5000 0.04 0.002 2 × 10–4 (Note 1)

> 5 to 15 (Mbit/s)
2000-8000 0.05 0.002 2 × 10–4

> 15 to 55 (Mbit/s)
4000-20 000 0.075 0.002 2 × 10–4

> 55 to 160 > 160 to 3500 (Mbit/s) (Mbit/s)
6000-20 000 0.16 0.002 2 × 10–4 15 000-30 000 (Note 2) (Note 3) 0.002 10–4

Bits/block ESR SESR BBER

NOTE 1 – For systems designed prior to 1996, the BBER objective is 3 × 10–4. NOTE 2 – As currently defined, VC-4-4c (ITU-T Rec. G.707/Y.1322 [3]) is a 601 Mbit/s path with a block size of 75 168 bits/block. Since this is outside the recommended range for 160-3500 Mbit/s paths, performance on VC-4-4c paths should not be estimated in-service using this table. The BBER objective for VC-4-4c using the 75 168 bit block size is taken to be 4 × 10–4. Digital sections are defined for higher bit rates and guidance on evaluating the performance of digital sections can be found in 7.1 and in a Recommendation dealing with multiplex section error performance. NOTE 3 – ESR objectives tend to lose significance for applications at high bit rates and are therefore not specified for paths operating at bit rates above 160 Mbit/s. Nevertheless, it is recognised that the observed performance of SDH paths is essentially error-free for long periods of time, even at Gigabit rates. Significant ESR indicates a degraded transmission system. Therefore, for maintenance purposes, ES monitoring should be implemented within any error performance measuring devices operating at these rates. NOTE 4 – It is not required to apply this Recommendation to connections which operate below the primary rate using equipment designed prior to the adoption of this Recommendation in December 2002. Performance events and objectives for connections using equipment designed prior to this date are given in ITU-T Rec. G.821 [14].

It is noted that SES events may occur in clusters, not always as isolated events. A sequence of "n" contiguous SES may have a very different impact on performance from "n" isolated SES events. Digital paths and connections operating at bit rates covered by this Recommendation are carried by transmission systems (digital sections) operating at equal or higher bit rates. Such systems must meet their allocations of the end-to-end objectives for the most demanding paths or connections which are foreseen to be carried. For example, in SDH, an STM-1 section may carry a VC-11/VC-12 path and therefore the STM-1 section should be designed such that it will ensure that the objectives as specified in this Recommendation for the bit rate corresponding to a VC-11/VC-12 path are met. Under the assumption of random error distribution, meeting the allocated objectives in Table 1 for the highest bit rate should be sufficient to ensure that all paths or connections through the system are achieving their objectives.
NOTE 2 – Objectives are allocated in this Recommendation to the national and international portions of a path or connection. In the above example, if the STM-1 section does not form a complete national or international portion, the corresponding national/international allocation must be subdivided to determine the appropriate allocation for the digital section. This is outside the scope of this Recommendation and is covered in a separate Recommendation.

ITU-T Rec. G.826 (12/2002)

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7.2

Apportionment of end-to-end objectives

The following apportionment methodology specifies the levels of performance expected from the national and international portions of an HRX or HRP. Further subdivision of these objectives is beyond the scope of this Recommendation. (See Figure 3.)
Inter-country (e.g., Connection or Path carried over a submarine cable)

Terminating country PEP (Note 1)
IG (Note 2)

Intermediate countries (Note 3) IG IG IG

Terminating country PEP (Note 1)

IG

National portion

International portion Hypothetical Reference Connection or Path 27 000 km

National portion

G.826_F03

NOTE 1 – If a path is considered to terminate at the IG, only the international portion allocation applies. NOTE 2 – One or two International Gateways (entry or exit) may be defined per intermediate country. NOTE 3 – Four intermediate countries are assumed.

Figure 3/G.826 – Hypothetical Reference Path or Connection For the purposes of this Recommendation, the boundary between the national and international portions is defined to be at an International Gateway which usually corresponds to a cross-connect, a higher-order multiplexer or a switch (N-ISDN or B-ISDN). IGs are always terrestrially based equipment physically resident in the terminating (or intermediate) country. Higher-order paths (relative to the HRP or HRX under consideration) may be used between IGs. Such paths receive only the allocation corresponding to the international portion between the IGs. In intermediate countries, the IGs are only located in order to calculate the overall length of the international portion of the path in order to deduce the overall allocation. The following allocation methodology applies to each parameter defined in 4.7 and takes into account both the length and complexity of the international path. All paths should be engineered to meet their allocated objectives as described in 7.2.1 and 7.2.2. If the overall allocation exceeds 100%, then the performance of the path may not fulfil the objectives of Table 1. Network Operators should note that if performance could be improved in practical implementations to be superior to allocated objectives, the occurrence of paths exceeding the objectives of Table 1 can be minimised. 7.2.1 Allocation to the national portion of the end-to-end path or connection Each national portion is allocated a fixed block allowance of 17.5% of the end-to-end objective. Furthermore, a distance-based allocation is added to the block allowance. The actual route length between the PEP and IG should first be calculated if known. The air route distance between the PEP and IG should also be determined and multiplied by an appropriate routing factor. This routing factor is specified as follows: if the air route distance is <1000 km, the routing factor is 1.5; if the air route distance is ≥1000 km and <1200 km, the calculated route length is taken to be 1500 km; if the air route distance is ≥1200 km, the routing factor is 1.25.

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When both actual and calculated route lengths are known, the smaller value is retained. This distance should be rounded up to the nearest 500 km (i.e., the two national portions comprise at least 500 km each). An allocation of 0.2% per 100 km is then applied to the resulting distance. When a national portion includes a satellite hop, a total allowance of 42% of the end-to-end objectives in Table 1 is allocated to this national portion. The 42% allowance completely replaces both the distance-based allowance and the 17.5% block allowance otherwise given to national portions.
NOTE – If a path or connection comprises portions that are privately owned (private in this context means that the network portion is customer owned and not available to the public), end-to-end performance objectives apply to the portion situated between the two Network Terminal Equipment (NTE). Between the NTE and the Terminal Equipment (TE), no specific requirements are given. However, careful attention should be paid concerning this portion because overall performance depends on it. Appendix III contains details for the case of Leased Circuits.

7.2.2

Allocation to the international portion of the end-to-end path or connection

The international portion is allocated a block allowance of 2% per intermediate country plus 1% for each terminating country. Furthermore, a distance-based allocation is added to the block allowance. As the international path or connection may pass through intermediate countries, the actual route length between consecutive IGs (one or two for each intermediate country) should be added to calculate the overall length of the international portion. The air route distance between consecutive IGs should also be determined and multiplied by an appropriate routing factor. This routing factor is specified as follows for each element between IGs: if the air route distance between two IGs is <1000 km, the routing factor is 1.5; if the air route distance is ≥1000 km and <1200 km, the calculated route length is taken to be 1500 km; if the air route distance between two IGs is ≥1200 km, the routing factor is 1.25.

When both actual and calculated route lengths are known, the smaller value is retained for each element between IGs for the calculation of the overall length of the international portion. This overall distance should be rounded up to the nearest 500 km but shall not exceed 26 500 km. An allocation of 0.2% per 100 km is then applied to the resulting distance. In the case where the allocation to the international portion is less than 6%, then 6% shall be used as the allocation. Independent of the distance spanned, any satellite hop in the international portion receives a 35% allocation of the objectives in Table 1. The 35% allowance completely replaces all distance-based and block allowances otherwise given to parts of the international portion spanned by the satellite hop.

Annex A Criteria for entry and exit for the unavailable state
A.1 Criteria for a single direction A period of unavailable time begins at the onset of ten consecutive SES events. These ten seconds are considered to be part of unavailable time. A new period of available time begins at the onset of ten consecutive non-SES events. These ten seconds are considered to be part of available time. Figure A.1 illustrates this definition.

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Time 10 s Unavailability detected Unavailable period Severely Errored Second Errored Second (non-SES) Error-free Second
G.826_FA.1

<10 s

10 s Availability detected Available period

Figure A.1/G.826 – Example of unavailability determination A.2 Criterion for a bidirectional path or connection

A bidirectional path or connection is in the unavailable state if either one or both directions are in the unavailable state. This is shown in Figure A.2.
Forward direction Backward direction Path

G.826_FA.2

Unavailable state

Figure A.2/G.826 – Example of the unavailable state of a path or connection A.3 Criterion for a unidirectional path or connection

The criterion for a unidirectional path or connection is defined in A.1 above. A.4 Consequences on error performance measurements

When a bidirectional path or connection is in the unavailable state, ES, SES and BBE counts may be collected in both directions and may be helpful in the analysis of the trouble. However, it is recommended that these ES, SES and BBE counts are not included in estimates of ESR, SESR and BBER performance (see 4.5). Some existing systems cannot support this requirement to exclude ES, SES and BBE counts. For these systems, the performance of a bidirectional path can be approximated by evaluating the parameters in each direction, independently of the state of availability of the other direction. It should be noted that this approximation method may result in a worse estimate of performance in the event that only on direction of a bidirectional path becomes unavailable.
NOTE – This is not an issue for unidirectional paths or connections.

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Annex B Relationship between PDH path performance monitoring and the block-based parameters
B.1 B.1.1 General Block size for monitoring PDH paths

The block sizes for in-service performance monitoring of PDH paths are given in Table B.1. Table B.1/G.826 – Block sizes for PDH path performance monitoring Bit rate of PDH path
1 544 kbit/s 2 048 kbit/s 6 312 kbit/s 44 736 kbit/s

Block size according to Table 1
800-5000 bits 800-5000 bits 2000-8000 bits 4000-20 000 bits

PDH block size used in this Recommendation
4632 bits 2048 bits 3156 bits 4760 bits

EDC

Reference

CRC-6 CRC-4 CRC-5 Single bit parity check (Note)

2.1/G.704 [2] 2.3/G.704 2.2/G.704 1.3/G.752 [10]

NOTE – It shall be noted that single bit parity check does not satisfy the error detection probability of ≥90%.

B.1.2

Anomalies

In-service anomaly conditions are used to determine the error performance of a PDH path when the path is not in a defect state. The two following categories of anomalies related to the incoming signal are defined: a1 an errored frame alignment signal; an EB as indicated by an EDC. a2 B.1.3 Defects In-service defect conditions are used in the G.730 to G.750 series of Recommendations relevant to PDH multiplex equipment to determine the change of performance state which may occur on a path. The three following categories of defects related to the incoming signal are defined: d1 loss of signal; d2 alarm indication signal; d3 loss of frame alignment. For the 2 Mbit/s hierarchy, the definition of the LOF defect condition is given in the G.730 to G.750 series of Recommendations. For some formats of the 1.5 Mbit/s hierarchy, the definition of the LOF defect condition requires further study For both hierarchies, the definitions of LOS and AIS defect detection criteria are given in ITU-T Rec. G.775 [12].

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B.2

Types of paths

Depending on the type of in-service monitoring "ISM" facility associated with the PDH path under consideration, it may not be possible to derive the full set of performance parameters. Four types of paths are identified: Type 1: Frame and block structured paths The full set of defect indications d1 to d3 and anomaly indications a1 and a2 are provided by the ISM facilities. Examples of this type of path are: – primary rate and second order paths with CRC (4 to 6) as defined in ITU-T Rec. G.704 [2]; – fourth order paths with a parity bit per frame as defined in ITU-T Rec. G.755 [11]. Type 2: Frame structured paths The full set of defect indications d1 to d3 and the anomaly indication a1 are provided by the ISM facilities. Examples of this type of path are: – primary rate up to the fourth order paths in the 2 Mbit/s hierarchy as defined in ITU-T Recs G.732 [4], G.742 [7] and G.751 [9]; – primary rate paths in the 1.5 Mbit/s hierarchy as defined in Recs G.733 [5] and G.734 [6]. Type 3: Other frame structured paths A limited set of defect indications d1 and d2 and the anomaly indication a1 are provided by the ISM facilities. In addition, the number of consecutive errored FAS per second is available. An example of this type of path is: – second up to the fourth order paths in the 1.5 Mbit/s hierarchy as defined in Recs G.743 [8] and G.752 [10]. Type 4: Unframed paths A limited set of defect indications d1 and d2 is provided by the ISM facilities which do not include any error check. No FAS control is available. An example of this type of path is: – end-to-end path (e.g., for a leased circuit) carried over several higher order paths placed in tandem. B.3 Estimation of the performance parameters

Table B.2 gives information on which set of parameters should be estimated and the related measurement criteria according to the type of path considered.

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Table B.2/G.826 – Set of parameters and measurement criteria Type
1

Set of parameters
ESR SESR BBER

Measurement criteria
An ES is observed when, during one second, at least one anomaly a1 or a2, or one defect d1 to d3 occurs. An SES is observed when, during one second, at least "x" anomalies a1 or a2, or one defect d1 to d3 occurs (Notes 1 and 2). A BBE is observed when an anomaly a1 or a2 occurs in a block not being part of an SES. An ES is observed when, during one second, at least one anomaly a1 or one defect d1 to d3 occurs. An SES is observed when, during one second, at least "x" anomalies a1 or one defect d1 to d3 occurs (Note 2). An ES is observed when, during one second, at least one anomaly a1 or one defect d1 or d2 occurs. An SES is observed when, during one second, at least "x" anomalies a1 or one defect d1 or d2 occurs (Note 2). An SES is observed when, during one second, at least one defect d1 or d2 occurs (Note 3).

2

ESR SESR

3

ESR SESR

4

SESR

NOTE 1 – If more than one anomaly a1 or a2 occur during the block interval, then only one anomaly has to be counted. NOTE 2 – Values of "x" can be found in B.4. NOTE 3 – The estimates of the ESR and SESR will be identical since the SES event is a subset of the ES event.

B.4

In-service monitoring capabilities and criteria for declaration of the performance parameters

Table B.3 is provided for guidance on the criteria for declaration of an SES event on PDH paths. The capabilities for the detection of anomalies and defects for the various PDH signal formats are described in Tables B.2/M.2100 to B.6/M.2100 [21]. These tables also indicate the criteria for declaring the occurrence of an ES or a SES condition in accordance with ITU-T Rec. G.821 [14] criteria. While it is recommended that ISM capabilities of future systems be designed to permit performance measurements in accordance with this Recommendation, it is recognised that it may not be practical to change equipment already installed and/or designed in accordance with ITU-T Rec. G.821 [14] or the 1996 version of this Recommendation. Table B.3 lists examples of the ISM SES criteria x, for signal formats with EDC capabilities, implemented prior to this Recommendation.

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Table B.3/G.826 – Criteria for declaration of an SES event on PDH paths Bit rate (kbit/s)
Recommendation EDC type Blocks/second Bits/block SES threshold used on equipment developed prior to the acceptance of this Recommendation ISM threshold based on G.826 SES (30% errored blocks)

1544
G.704 [2] CRC-6 333 4632 x = 320 (Note 2) x = 100 (Note 3)

2048
G.704 [2] CRC-4 1000 2048 x = 805 (Note 2) x = 300 (Note 3)

44 736
G.752 [10] Single bit parity check 9398 4760 x = 45 or x = 2444 as suggested in ITU-T Rec. M.2100 [21] x = 2444 (Note 4)

NOTE 1 – It is recognised that there are discrepancies between the figures above and those given in Table B.1. This requires further study. NOTE 2 – Applicable to installations in accordance with ITU-T Rec. G.821 [14] or the 1996 version of this Recommendation and where compatibility with these installations is required. NOTE 3 – Preferred option for new installations. NOTE 4 – This figure takes into account the fact that, although 30% of the blocks could contain errors, a smaller value will be detected by the EDC due to the inability of the simple parity code to detect even numbers of errors in a block. It should be noted that such a simple EDC is non-compliant with the intent of this Recommendation. NOTE 5 – Completion of this table for other bit rates is for further study.

B.5

Estimation of performance events at the far end of a path

The available remote in-service indications such as RDI or, if provided, REI are used at the near end to estimate the number of SES occurring at the far end. B.6 B.6.1 Differences between this Recommendation and ITU-T Rec. M.2100 concerning path performance General

When looking at the differences between this Recommendation and ITU-T Rec. M.2100 [21], it shall be taken into account that the two Recommendations serve a different purpose and can therefore not be compatible in all respects. ITU-T Rec. M.2100 [21] is a maintenance Recommendation which also allows short-term measurements. It can be used to indicate that the long-term requirements of this Recommendation are met. B.6.2 Allocation methodology The allocation methodology used in this Recommendation differs from the methods applied in ITU-T Rec. M.2100 [21]. Though there are differences, in most cases the requirements of this Recommendation are satisfied if the objectives of ITU-T Rec. M.2100 [21] are met. With regard to the purpose of the intermediate IGs depicted in Figure 3, it shall be noted that they are required to calculate route length.

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Annex C Relationship between SDH path performance monitoring and the block-based parameters
C.1 C.1.1 General Converting BIP measurements into errored blocks

Clause 4.4 describes error performance events used in defining performance parameters. The method of converting BIP measurements into errored blocks is described below. Since this Recommendation defines a block as consecutive bits associated with a path, each BIP-n (Bit Interleaved Parity, order "n") in the SDH path overhead pertains to a single defined block. For the purpose of this Annex, a BIP-n corresponds to a G.826 block. The BIP-n is NOT interpreted as checking "n" separate interleaved parity check blocks. If any of the "n" separate parity checks fails, the block is assumed to be in error.
NOTE – It shall be noted that BIP-2 does not satisfy the error detection probability of ≥90%.

C.1.2

Block size for monitoring SDH paths

The block sizes for in-service performance monitoring of SDH paths as specified in ITU-T Rec. G.707/Y.1322 [3] are given in Table C.1. Table C.1/G.826 – Block sizes for SDH path performance monitoring Bit rate of SDH path
1664 kbit/s 2240 kbit/s 6848 kbit/s 48 960 kbit/s 150 336 kbit/s m × 6848 kbit/s 34 240 kbit/s 601 344 kbit/s

Path type

Block size according to Table 1
800-5000 bits 800-5000 bits 2000-8000 bits 4000-20 000 bits 6000-20 000 bits 6000-20 000 bits 15 000-30 000 bits

SDH block size used in this Recommendation
832 bits 1120 bits 3424 bits 6120 bits 18 792 bits 3424 bits 17 120 bits 75 168 bits

EDC

VC-11 VC-12 VC-2 VC-3 VC-4 VC-2-mc (Note 1) VC-2-5c (Note 2) VC-4-4c

BIP-2 BIP-2 BIP-2 BIP-8 BIP-8 m × BIP-2 BIP-2 BIP-8

NOTE 1 – Applies to virtual concatenation. NOTE 2 – Applies to contiguous concatenation.

C.1.3

Anomalies

In-service anomaly conditions are used to determine the error performance of an SDH path when the path is not in a defect state. The following anomaly is defined: a1 an EB as indicated by an EDC (see C.1.1). C.1.4 Defects In-service defect conditions are used in ITU-T Recs G.707/Y.1322 [3] and G.783 [13] relevant to SDH equipment to determine the change of performance state which may occur on a path. Tables C.2 and C.3 show the defects used in this Recommendation.

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Table C.2/G.826 – Defects resulting in a near-end severely errored second Near-end defects
LP UNEQ LP TIM TU LOP TU AIS HP LOM (Note 1) HP PLM HP UNEQ HP TIM AU LOP AU AIS NOTE 1 – This defect is not related to VC-3. NOTE 2 – VC AIS defect is not included above as it only applies to a segment of a path. NOTE 3 – The above defects are path defects only. Section defects such as MS AIS, RS TIM, STM LOF and STM LOS give rise to an AIS defect in the path layers. NOTE 4 – When a near-end SES is caused by a near-end defect as defined above, the far-end performance event counters are not incremented, i.e., an error-free period is assumed. When a near-end SES is resulting from ≥30% errored blocks, the far-end performance evaluation continues during the near-end SES. This approach does not allow reliable evaluation of far-end data if the near-end SES is caused by a defect. It should be noted in particular, that the evaluation of far-end events (such as SES or Unavailability) can be inaccurate in the case where far-end SESs occur in coincidence with near-end SESs caused by a defect. Such inaccuracies cannot be avoided, but are negligible in practice because of the low probability of the occurrence of such phenomena. Applicable to higher-order paths Applicable to lower-order paths

Kind of path

Table C.3/G.826 – Defects resulting in a far-end severely errored second Far-end defects
LP RDI HP RDI

Kind of path
Applicable to lower-order paths Applicable to higher-order paths

C.1.5

Measurement of performance events using aggregate parity error counts

This clause offers guidance for equipment designed to sum individual Bit Interleaved Parity violations over the entire second instead of using the BIP-n block to detect and count errored blocks as recommended in C.1.1. The following text should not be interpreted as a basis for future equipment design. Aggregate counts of Bit Interleaved Parity (BIP) violations can be used to estimate the number of G.826 errored blocks. As a simplifying assumption, the aggregate count of individual Bit Interleaved Parity violations in a second can be taken to be roughly equivalent to the number of G.826 errored blocks in that second. The following relationship is recommended for both BIP-2 and BIP-8, even though it may tend to overestimate errored blocks in case of BIP-8: E≈P where: E P
18

number of errored blocks in the measurement period number of individual parity violations in the measurement period

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C.2

Estimation of the performance parameters

For SDH transmission paths, the full set of performance parameters shall be estimated using the following events: ES: An ES is observed when, during one second, at least one anomaly a1, or one defect according to Tables C.2 and C.3 occurs. For the ES event, the actual count of EBs is irrelevant; it is only the fact that an EB has occurred in a second which is significant. SES: An SES is observed when, during one second, at least 30% EBs; derived from anomaly a1 or one defect according to Tables C.2 and C.3, occur (see Note). BBE: A BBE is observed when an anomaly a1 occurs in a block not being part of an SES.
NOTE – The errored block threshold resulting in an SES is shown in Table C.4 for each SDH path type.

Table C.4/G.826 – Threshold for the declaration of a severely errored second Path type
VC-11 VC-12 VC-2 VC-3 VC-4 VC-2-5c VC-4-4c

Threshold for SES (Number of errored blocks in one second)
600 600 600 2400 2400 600 2400

NOTE – It is recognised that there are discrepancies between the figures above and those given in Table B.3. This requires further study.

C.3

Estimation of performance events at the far end of a path

The following indications available at the near end are used to estimate the performance events (occurring at the far end) for the reverse direction: Higher and lower order path RDI and REI (ITU-T Rec. G.707/Y.1322 [3]). Higher- or lower-order path REIs are anomalies which are used to determine the occurrence of ES, BBE and SES at the far end. Higher or lower order path RDIs are defects which estimate the occurrence of SES at the far end.

Annex D Relationship between cell-based network performance monitoring and the block-based parameters
D.1 D.1.1 General Block size for monitoring cell-based paths

The block sizes for in-service performance monitoring of cell-based paths are given in Table D.1.

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Table D.1/G.826 – Block sizes for cell-based path performance monitoring Bit rate for cellbased path
51 Mbit/s 155 Mbit/s 622 Mbit/s

Block size according to Table 1
4 000-20 000 6 000-20 000 15 000-30 000

Number of cells/block (as defined in ITU-T Rec. I.432) [18]
15 27 54

Cell-based block size used in this Recommendation
6 360 bits 11 448 bits 22 896 bits

The operation and maintenance function for the transmission path is provided by the F3 flow as defined in ITU-T Rec. I.610 [19] which deals with the general OAM principles for B-ISDN. The F3 maintenance flow corresponds to the ISM facilities and is defined in ITU-T Rec. I.432 [18]. For the 51 Mbit/s cell-based path, each F3 OAM cell monitors 15 cells. For the 155 Mbit/s and the 622 Mbit/s cell-based paths, each F3 OAM cell monitors 8 blocks of 27 (or 54) contiguous cells. The block – as defined in this Recommendation – corresponds to a set of contiguous cells monitored by a BIP-8 EDC. For the purpose of this Recommendation, the BIP-8 is not interpreted as checking 8 separate interleaved parity check blocks. One BIP-8 interleaved parity check cannot lead to more than one errored block. Within one BIP-8 check, if any of the 8 separate parity checks fails, the overall block is assumed to be in error. D.1.2 Anomalies The following categories of anomalies related to the incoming signal on an ATM transmission path are defined: a1 errored payload of an idle or ATM cell (detected by an EDC in the F3 OAM cell) (see Note 1); errored or corrected header of an idle or ATM cell; a2 a3 corrected F3 cell header; a4 loss of a single F3 cell (Note 2) or error detected by the cell error control of a valid F3 cell.
NOTE 1 – An ATM cell is provided by the ATM layer. NOTE 2 – Loss of a single F3 cell is declared when no valid F3 OAM cell is received x cells after the last valid F3 OAM cells. The value of x is given below.

Applicable bit rate
51.840 Mbit/s 155.520 Mbit/s 622.080 Mbit/s

Value of x
14 215 431

When at least one anomaly a1 to a3 occurs in a given block, an Errored Block should be counted. If more than one anomaly occurs for a given block, only one EB is counted. When an a4 anomaly occurs, all blocks monitored by the F3 OAM cell are errored (1 block for 51 Mbit/s and 8 blocks for the bit rates of 155 and 622 Mbit/s respectively). D.1.3 Defects The following categories of defects related to the incoming signal on an ATM transmission path are defined: loss of two consecutive OAM cells, in accordance with ITU-T Rec. I.432 [18]; d1

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d2 d3 d4 D.2

transmission path alarm indication signal (TP-AIS); loss of cell delineation; loss of signal. Types of paths

Two types of ATM transmission paths are identified: – Type 1: Paths corresponding to a stream of cells mapped in a cell-based format. – Type 2: Paths corresponding to a stream of cells mapped into SDH or PDH-based frame structures. The full set of performance parameters of this Recommendation and corresponding objectives is applicable to the ATM transmission path of type 1. The performance parameters and corresponding objectives are applied to underlying SDH or PDH paths which support ATM transmission paths of type 2. The applicability of the performance parameters for type 2 ATM transmission paths requires further study. D.3 Estimation of the performance parameters

For type 1 ATM transmission paths, the full set of G.826 performance parameters should be estimated using the following events: ES: An ES is observed when, during one second, at least one anomaly a1 to a4, or one defect d1 to d4 occurs. SES: An SES is observed when, during one second, at least 30% EBs, derived from anomalies a1 to a4, or one defect d1 to d4 occur. BBE: A BBE is observed when one anomaly a1 to a4 occurs in a block not being part of an SES. D.4 Estimation of performance events at the far end of the path

The TP-RDI defect (see ITU-T Rec. I.432 [18]) and REI indications are used at the near end to estimate the G.826 performance events occurring at the far end. REIs are anomalies which are used to determine the occurrence of ES, BBE and SES at the far end of the path. TP-RDIs are defects which estimate the occurrence of SES at the far end of the path.

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Appendix I Flow chart illustrating for digital paths the recognition of anomalies, defects, errored blocks, ES and SES
Monitored second

Defects?

Yes

No

No

Anomalies?

Yes

%EB ≥ 30? ES (but not a SES) No

Yes

SES (and therefore an ES)

Path in available State? Yes cES = cES + 1

No

Path in available State? Yes cES = cES + 1

No

cBBE = cBBE + EB(s)

cSES = cSES + 1

G.826_FI.1

End

Figure I.1/G.826 – Flow chart illustrating the recognition of anomalies, defects, errored blocks, ES, SES and BBE

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Notes to Figure I.1
NOTE 1 – The determination of unavailability time introduces a delay of ten seconds. This delay should be considered when counting BBE, ES and SES. NOTE 2 – cES, cSES and cBBE represent counts of ES, SES, BBE respectively. These counts are reset at the start of a measurement period. NOTE 3 – EB is the count of errored blocks within an ES whilst %EB represents the proportion of errored blocks within an ES compared to the number of blocks per second. NOTE 4 – G.826 parameters can be evaluated during, or at the end of, a measurement period P as follows, taking into account Unavailable Seconds (UAS): BBER = cBBE/[(P UAS cSES) × blocks per second] ESR = cES/(P UAS) SESR = cSES/(P UAS) NOTE 5 – In the simplified diagram, no action is taken if the path is in the unavailable state. This is because the diagram does not consider the transition between availability states when, in fact, event counters must be modified retrospectively. In practice, the status of a second (i.e., error-free, ES or SES) must always be determined before a test is made on the status of path availability. In other words, error events are always detected regardless of whether the path is available or not – only the counting of events is inhibited during unavailability periods for the purposes of long-term performance monitoring. This process is reflected in the flow chart although consequent actions on changes of availability state are not.

Appendix II Bit errors and block errors, merits and limitations
In digital transmission technology, any bit received in error – a Bit Error – may deteriorate transmission quality. It is obvious that quality will decrease with an increasing number of erroneous bits. Therefore, the ratio of the number of errored bits referred to the total number of bits transmitted in a given time interval is a quantity which can be used to describe digital transmission performance. The quantity is called Bit-Error Ratio (BER) and is a well-known error performance parameter (see definition in Fascicle I.3 of the CCITT Blue Book). Bit-Error Ratio can only be measured if the bit structure of the evaluated sequence is known. For this reason, bit error ratio measurements are mostly carried out using well-defined Pseudo-Random Bit Sequences (PRBSs). In practice, the PRBS replaces the information sent in-service. This means that BER can only be measured correctly out-of-service because the bit structure of an arbitrary message is normally unknown. It was one of the prime objectives of this Recommendation to define all performance parameters for digital paths in such a way that in-service estimation is possible. Thus, parameter definitions for digital paths based upon Bit Error Ratios were not chosen in spite of their merits. In-service detection of errors in digital transmission is possible, however, using special error detection mechanisms (Error Detection Code, EDC) which are inherent to certain transmission systems. Examples of those inherent EDCs are Cyclic Redundancy Check (CRC), Parity Check and observation of Bit Interleaved Parity (BIP). EDCs are capable to detect whether one or more errors

ITU-T Rec. G.826 (12/2002)

23

have occurred in a given sequence of bits – the block. It is normally not possible to determine the exact number of errored bits within the block. Block Errors are processed in a similar way as Bit Errors, i.e., the term "Block-Error Ratio" is defined as the ratio of the number of errored blocks referred to the total number of blocks transmitted in a given time interval. The basic philosophy of this Recommendation is based upon the measurement of errored blocks for digital paths, thus making in-service error estimation possible for paths. It should be noted that the measurement of Bit-Error Ratio and Block-Error Ratio yields comparable results for small Bit-Error Ratios. It should also be noted that for some specific error models it is possible to calculate Bit-Error Ratio from a Block-Error Ratio. It is the drawback of this procedure that error models describe the situation found in practice only imperfectly and may be strongly media-dependent. Therefore, the result of such a calculation is not very reliable

Appendix III Applicability of this Recommendation to non-public networks
Figure III.1 depicts a typical leased circuit situation where a path is composed of three independent networks: two private networks at both path ends and a public network connecting them.
Intermediate countries (Terrestrial link) Terminating country 1 TE NTE IG IG IG IG Terminating country 2 NTE TE

Private network National portion International portion Leased circuit End-to-end path National portion

Private network

IG International Gateway NTE Network Terminal Equipment TE Terminal Equipment

G.826_FIII.1

Figure III.1/G.826 – Digital path or connection composed of two private networks and a leased circuit provided by a public network operator The public network provides a leased circuit to connect the two private networks. However, the problem is not restricted to the case shown in Figure III.1 but is of more general nature. For

24

ITU-T Rec. G.826 (12/2002)

instance, similar considerations are applicable if only one side of the path or connection ends in a private network. Taking into account that a public operator can only control the public network from NTE to NTE (Network Terminal Equipment), no performance objectives can be given for the portion between NTE and TE. It may also be that the public network operator provides the connection by other means than a leased circuit.

ITU-T Rec. G.826 (12/2002)

25

SERIES OF ITU-T RECOMMENDATIONS
Series A Series B Series C Series D Series E Series F Series G Series H Series I Series J Series K Series L Series M Series N Series O Series P Series Q Series R Series S Series T Series U Series V Series X Series Y Series Z Organization of the work of ITU-T Means of expression: definitions, symbols, classification General telecommunication statistics General tariff principles Overall network operation, telephone service, service operation and human factors Non-telephone telecommunication services Transmission systems and media, digital systems and networks Audiovisual and multimedia systems Integrated services digital network Cable networks and transmission of television, sound programme and other multimedia signals Protection against interference Construction, installation and protection of cables and other elements of outside plant TMN and network maintenance: international transmission systems, telephone circuits, telegraphy, facsimile and leased circuits Maintenance: international sound programme and television transmission circuits Specifications of measuring equipment Telephone transmission quality, telephone installations, local line networks Switching and signalling Telegraph transmission Telegraph services terminal equipment Terminals for telematic services Telegraph switching Data communication over the telephone network Data networks and open system communications Global information infrastructure and Internet protocol aspects Languages and general software aspects for telecommunication systems

*23371*
Printed in Switzerland Geneva, 2003


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