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Date: 11 th October 2001 Document Filename: 54916223.docMeeting: RSSPWG Paper Number: RSSP (04-01)/35

Working Document for the RSSPWG

A Review of Error Performance & Availability

in meeting

International Obligations based upon ITU Recommendations.

Abstract

Following discussions at the 6 th FLCC Policy meeting on the 10 th September 2001, concerns were raised that theMinimum Path Length Study Report (August 2001) and its conclusions, would not enable operators to meet their international obligations in terminating traffic. At this meeting a presentation was made on UK Link Length Policyfactors and the technical considerations that would affect availability and performance. This working document tries toreview the whole issue of error performance and availabilty issues against the ITU-T Recommendation G.826 andG.828 Error Performance Objectives (EPO) in conjunction with the latest draft revision of the radio link performanceof ITU-R Recommendation P.530 for Propagation data and prediction methods required for the design of terrestial line-of-sight systems.

Bob HowellMike PineChris CheesemanHenry Taylor henry.taylor@bt.comJohn Tomlinson john.tomlinson@bt.com

mailto:henry.taylor@bt.commailto:john.tomlinson@bt.commailto:henry.taylor@bt.commailto:john.tomlinson@bt.com

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1. Background

The International Telecommunications Union (ITU) specify the performance objectives for longdistance communications links in a series of recommendations produced by the ITU TelecommunicationStandardization Sector (ITU-T). The G series of these recommendations specify the performancerequirements for a 27,500km Hypothetical Reference Path (HRP).

In particular, ITU-T Recommendations G.821 [1], G.826 [2], G.827 [3] and G.828 [4] specify theperformance required from an international digital HRP and G.829 [5] specifies that required fromSynchronous Digital Hierarchy (SDH) multiplex and regenerator sections. ITU-T RecommendationsG.821, G.826, G.827 and G.828 only specify the end-to-end performance of an HRP and applyirrespective of the physical transport mechanism (e.g. optical fibre, digital radio relay, metallic cableetc.).

The ITU-T G Recommendations cannot be used directly when designing a digital radio system andtherefore the ITU Radiocommunications Sector (ITU-R) have used the ITU-T G Recommendations toproduce a series of ITU-R Recommendations (the F series) which specify the performance requiredfrom digital radio links if they are to meet the overall HRP performance objectives.

ITU-R F series Recommendations, based on ITU-T Rec. G.821 ( Error performance of aninternational digital connection forming part of an integrated services digital network ), have beenused for many years to specify the error performance objectives for digital radio links and specify therequired error performance in terms of Bit Error Ratios (BER). ITU-T Rec. G.821 has now beensuperseded by ITU-T Rec. G.826 (1999) ( Error performance parameters and objectives forinternational, constant bit rate digital paths at or above the primary rate ) and ITU-T Rec. G.828(2000) ( Error performance parameters and objectives for international, constant bit ratesynchronous digital paths ) and new ITU-R error performance objectives based on these and applicableto digital radio links have been produced. These new recommendations specify error performance interms of error blocks rather than on BER values.

ITU-T Rec. G.826 applies to both Plesiochronous Digital Hierarchy (PDH) and Synchronous DigitalHierarchy (SDH) systems. ITU-T Rec. G.828 only applies to Synchronous Digital Hierarchy (SDH)systems. The performance requirements in ITU-T Rec. G.828 are more stringent than those in ITU-TRec. G.826 and compliance with ITU-T Rec. G.828 will, in most cases, also ensure compliance withITU-T Rec. G.821 and ITU-T Rec. G.826. It should be noted that ITU-T Rec. G.828 only applies toequipment designed after 10 th March 2000 (the date ITU-T Rec. G.828 was adopted by the ITU).Performance objectives for paths using equipment designed prior to this date are given in ITU-TRec. G.826.

A key ITU-R Recommendation in this context is ITU-R F.1491 ( Error performance objectives forreal digital radio links used in the national portion of a 27,500km hypothetical reference path ator above the primary rate ) [6]. This recommendation gives the error performance objectives for the

national portion of digital radio links of various types (Long haul, Short haul and Access) for systemsconforming to ITU-T Rec. G.826 and ITU-T Rec. G.828.

Page 2 of 18

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2. Error performance and availability

2.1 Error performanceThe digital bit stream in modern digital radio systems is divided into blocks. For example an SDHSTM-1 system might have 801 bits per block and carry 192,000 blocks per second. Each block ismonitored by means of an inherent Error Detection Code (EDC), e.g. Bit Interleaved Parity or CyclicRedundancy Check. ITU-T Rec. G.826 and ITU-T Rec. G.828 (and the ITU-R Recommendations basedon them) specify error performance objectives in terms of the rate at which blocks containing errorsoccur using the following parameters:-

Errored Block (EB) : A block in which one or more bits are in error.

Errored Second (ES) : A one-second period with one or more errored blocks or at least onedefect (e.g. loss of pointer LOP).

Severely Errored Second (SES) : A one-second period, which contains 30% errored blocksor at least one defect. SES is a subset of ES.

Background Block Error (BBE) : An errored block not occurring as part of a SES.

The error performance of a digital radio link can then be described in terms of:-

Errored Second Ratio (ESR) : The ratio of ES to total seconds in available time during afixed measurement interval.

Severely Errored Second Ratio (SESR) : The ratio of SES to total seconds in available timeduring a fixed measurement interval.

Background Block Error Ratio (BBER) : The ratio of Background Block Errors (BBE) tototal blocks in available time during a fixed measurement interval. The count of total blocksexcludes all blocks during SESs.

2.2 Availability

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Each direction of a path can be in one of two states, available time, or unavailable time. The criteriadetermining the transition between the two states are as follows.

A period of unavailable time begins at the onset of 10 consecutive Severely Errored Second (SES)events. These 10 seconds are considered to be part of unavailable time. A new period of available timebegins at the onset of 10 consecutive non-SES events. These 10 seconds are considered to be part of available time. Figure 1 illustrates the transitions between the availability states.

T1306350-95

Detection of unavailable state Detection of available state

< 10 sec 10 sec < 10 sec 10 sec

Available state Unavailable state Available state

SES

Non-SES

Error-free second

Figure 1

A path is available if, and only if, both directions are available. For a path to enter the unavailable state,either direction can be unavailable. But if the two directions are subject to overlapping SES events suchthat neither direction becomes unavailable, and the combined period is greater than 10 seconds, then thepath still remains in the available state.

ITU-T Rec. G.827 specifies the availability objectives for path elements of international digital paths.

It should be noted that error performance should only be evaluated whilst the path is in the availablestate.

It is generally accepted that for a well planned link clear air fading is largely produced by multipathpropagation which tends to produce fades lasting less than 10 seconds, whereas, rain tends to producefades lasting longer than 10 seconds.

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3 ITU-T Recommendation G.826/G.828 error performance objectives

The following error performance objectives are based on those given in Draft Revision of ITU-R F.1491(October 2000) [6] which in turn are based on ITU-T Recommendations G.826 (1999) and G.828(2000).

For the purposes of defining the error performance objectives of real digital radio links used in thenational portion of the 27,500km HRP the later can be subdivided into the three sections shown inFigure 2.

Access Short haul Long haul

SwitchingCentre

Localexchange

Pathend-point

L (km)

InternationalGateway

Figure 2

The error performance objectives for Access and Short haul sections are independent of radio link length. The error performance objectives for a Long haul section are a function of link length (L km)for links having lengths greater than 50 km and are fixed at the objectives for a 50 km link length for links shorter than 50 km.

The following tables give the error performance objectives, based on ITU-T Recommendations G.826(1999) and G.828 (2000), for each direction of the Long haul, Short haul and Access sections a digitalradio link in the national portion of the HRP.

3.1 Long haul

Table 1a

Error performance objectives for PDH and SDH radio links belonging to theLong haul inter-exchange section of the national portion of the HRP according to

ITU-T G.826

Rate (Mbit/s) 1.5 to 5 > 5 to 15 > 15 to 55 > 55 to 160 > 160 to 3,500ESR 0.04A 0.05A 0.075A 0.16A For further study

SESR 0.002ABBER 0.0002A (*) 0.0002A 0.0001A

* For systems designed prior to 1996 the BBER objective is 0.0003.

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Table 1b

Error performance objectives for SDH radio links belonging to theLong haul inter-exchange section of the national portion of the HRP

according to ITU-T G.828Rate (Mbit/s) 1,664

(VC-11,TC-11)

2,240(VC-12,TC-12)

6,848(VC-2,TC-2)

48,960(VC-3,TC-3)

150,336(VC-4,TC-4)

ESR 0.01A 0.02A 0.04ASESR 0.002ABBER 0.00005A 0.0001A

Where: If L < 50 km then L = 50 km (1)

A = (A 1 + 0.002) x L/100 for 50 km L 100 km (2)

A = A 1 + 0.00002 x L for L > 100 km (3)

A1 is in the range 0.01 to 0.02. The actual value used for A 1 is constrained by the sum of the terms A 1 ,B (applicable to the Short haul section) and C (applicable to the Access section) (see Tables 2a/2b,3a/3b).

3.2 Short haul

Table 2a

Error performance objectives for PDH and SDH radio linksbelonging to the Short haul inter-exchange section of the national

portion of the HRP according to ITU-T G.826

Rate (Mbit/s) 1.5 to 5 > 5 to 15 > 15 to 55 > 55 to 160 > 160 to 3,500ESR 0.04B 0.05B 0.075B 0.16B For further study

SESR 0.002BBBER 0.0002B (*) 0.0002B 0.0001B

* For systems designed prior to 1996 the BBER objective is 0.0003.

Table 2b

Error performance objectives for SDH radio links belonging to theShort haul inter-exchange section of the national portion of the HRP

according to ITU-T G.828

Rate (Mbit/s) 1,664(VC-11,TC-11)

2,240(VC-12,TC-12)

6,848(VC-2,TC-2)

48,960(VC-3,TC-3)

150,336(VC-4,TC-4)

ESR 0.01B 0.02B 0.04BSESR 0.002BBBER 0.00005B 0.0001B

Where B is in the range 0.075 to 0.085.

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3.3 Access

Table 3a

Error performance objectives for PDH and SDH radio linksbelonging to the Access network section of the national portion of the

HRP according to ITU-T G.826

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Rate (Mbit/s) 1.5 to 5 > 5 to 15 > 15 to 55 > 55 to 160 > 160 to 3,500ESR 0.04C 0.05C 0.075C 0.16C For further study

SESR 0.002C

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BBER 0.0002C (*) 0.0002C 0.0001C

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* For systems designed prior to 1996 the BBER objective is 0.0003.

Table 3b

Error performance objectives for SDH radio links belonging to theAccess network section of the national portion of the HRP according

to ITU-T G.828

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Rate (Mbit/s) 1,664(VC-11,TC-11)

2,240(VC-12,TC-12)

6,848(VC-2,TC-2)

48,960(VC-3,TC-3)

150,336(VC-4,TC-4)

ESR 0.01C 0.02C 0.04C

SESR 0.002CBBER 0.00005C 0.0001C

Where C is in the range 0.075 to 0.085. When considering the performance objectives given in theabove tables it should be noted that:

The performance objectives apply only when the system is in the available state.

In ITU-T G.826 the period of time over which the performance of a link is to be evaluated (i.e.averaged) is any period of 28 to 31 days, however, in ITU-T G.828 this has been revised for radio-relay or satellite systems and in these cases a period of up to one year may be used i.e.the worst month concept has been abandoned in ITU-T G.828 .

The sum A 1 + B + C shall not exceed 0.175 (i.e. 17.5%) in accordance with the allocations tothe national portion of an international Constant Bit Rate (CBR) path given in ITU-TRecommendations G.826 and G.828.

The sum B + C shall be in the range 0.155 to 0.165.

Depending on the national network configuration administrations may reallocate the A, B andC allowances among the sections of the national portion of a radio path.

The objectives given in the above tables apply to the overall links. In the case of multi-hoplinks the allocation of objectives to each hop is the responsibility of the network operators.

The effect of interference and all other sources of performance degradation are included in theobjectives given in the tables.

Synchronous digital paths operating at bit rates covered by ITU-T Recommendations G.826and G.828 are carried by transmission systems (digital sections) operating at higher bit rates.Such systems must meet their allocations of the end-to-end objectives for the highest bit ratepaths, which are expected to be carried. Meeting the allocated objectives for this highest bitrate path should be sufficient to ensure that all paths through the system are achieving their objective. For example, in SDH, an STM-1 section may carry a VC-4 path and therefore theSTM-1 section should be designed such that it will ensure that the objectives as specified in theabove tables for the bit rate corresponding to a VC-4 path are met.

Hence, using the appropriate table the error performance objectives for a digital radio link can be

obtained. It should be noted that ITU-T G.828 states that the error performance objectives given in theabove tables are understood to be long-term objectives. In the case of systems conforming to ITU-TG.826 these could be expressed in the form of the SESR, BBER and ESR averaged over any month andin the case of ITU-T G.828 the SESR, BBER and ESR averaged over a year.

Although ITU-T G.826 clearly states that the evaluation (i.e. averaging) period shall be any month, andthis therefore implies that the worst month during the whole lifetime of the system must be the month toconsider, it is generally accepted in ITU-R Recommendations that an average worst month should beused when designing radio links. With the relaxation of this to a year in ITU-T G.828 it is probably safeto assume that this means an average year, however, clarification on this point in adopted ITUdocumentation is required.

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4 Recommendation ITU-R P.530 in the context of G.826 and G.828

4.1 GeneralThe task now is to match the error performance objectives derived above to radio link performance aspredicted using the implementation of Recommendation ITU-R P.530 [7].The interface between the twoconsists of two system parameters:

The flat fade margin (or thermal fade margin).

The equipment signature.

These parameters determine the rate at which the three types of errors, i.e. ES, SES and BBE, occur under two different fading conditions. The flat fade margin applies when no frequency selective fadingis present and the equipment signature applies when only frequency selective fading is present. Itshould be noted that the equipment signature referred to throughout this document is that measured withno flat fading i.e. at frequencies within the receiver band well away from the selective fade notch thesignal level is at the nominal fine weather receive level (the un-stressed signature).

The methods in Annex 1 of ITU-R P.530-9 (the revised version of ITU-R P.530-8 expected to beadopted in the near future) estimate the outage probability for flat and frequency selective fadingseparately and then combine the two (together with other outages if applicable) to get the total outage.The term outage in this context refers to the ITU-T G.821 error performance objectives and in particular a BER of 10 -3. Annex 2 of ITU-R P.530-9 extends the prediction methods to cover the block-basederror performance objectives given in ITU-T G.826 and G.828. In particular it allows the radio link SESR, BBER and ESR to be calculated using the system flat fade margin, the equipment signature andthe Annex 1 to ITU-R P.530-9 propagation models.

The method is based on statistical relationships between the block-based parameters (SESR, BBER andESR) and BER. The methods in Annex 2 of ITU-R P.530-9 allow the SESR, BBER and ESR to bedetermined using equipment characteristics (e.g. Residual Bit Error Ratio (RBER) and equipmentsignature) that are BER based and do not require the direct measurement of the block-based parameters.

4.2 Prediction of SESR The procedure in Annex 2 of ITU-R P.530-9 for predicting SESR is based on the statistical relationshipbetween SES and BER. The SESR is evaluated in two steps, one to determine the number of SES due toEB, and a second one to obtain the number due to Loss of Pointer (LOP). The contribution from LOPhas been found to be small, and can therefore be ignored. From measurements on digital systems, it isfound that for BER values below a threshold, none of the seconds are severely errored, but above thisthreshold, all the seconds are severely errored i.e. the curve of SES due to EB versus BER can beapproximated by a step function. The BER value where the SES probability changes from 0 to 1 is

denoted BER SES . Table 4 gives the BER SES value for a number of path types.

Table 4

BER SES FOR VARIOUS PATH TYPES

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Pathtype

Bit ratesupported(Mbit/s)

BER SES (Notes 1 and 2)

Blocks/s, n(Note 2)

Bits/block, N B (Note 2)

VC-11 1.5 5.4 x 10 4 2 000 832 VC-12 2 4.0 x 10 4 2 000 1 120

VC-2 6 1.3 x 10 4 2 000 3 424

VC-3 34 6.5 x 10 5 8 000 6 120

VC-4 140 2.1 x 10 5 8 000 18 792

STM-1 155 2.3 x 10 5

1.3 x 10 5 x 2.2 x104

8 000 192 000

19 940801

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NOTE 1 1 indicates a Poisson distribution of errors.

NOTE 2 The blocks/s are defined in ITU-T Recommendation G.826 for SDH path, in ITU-T Recommendation G.829 for SDHsections. Some STM-1 equipment might be designed with 8 000 blocks/s (19 940 bits/block), but ITU-T Recommendation G.829defines the block rate and size to be 192 000 blocks/s and 801 bits/block, respectively.

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The SESR is equal to the outage probability ( P tSES ) determined using a BER equal to BER SES as thecriterion for outage in the methods given in Annex 1 of ITU-R P.530-9 i.e. the flat fade margin to beused is the flat fade depth relative to the fine weather receive level which produces a BER of BER SES

and the equipment signature to be used is the non-faded (non-stressed) signature for a BER of BER SES .Hence, for an SDH STM-1 path having a block structure of 192,000 blocks/s and 801 bits/block andassuming a Poisson distribution of errors BER SES = 2.33 x 10 -4 (compared to the G.821 value of 10 -3).

4.3 Prediction of BBER

The BBER can be calculated using the SSER calculated above together with the outage probability(P tR) determined using a BER equal to the residual BER (RBER) (typically in the range from 10

10 to1013 for the bit rates of 2 to 155 Mbit/s, respectively), as the criterion for outage in the methods given inAnnex 1 of ITU-R P.530-9 and RBER as follows:

P tR = P t (RBER) (4)

( ) ( )( ) ( )tSES tR

SES

P P

BE RRBERm

1010

1010

loglog

loglog

= (5)

Where m is the slope of the BER distribution curve on a log-log scale for BER values in the range fromBER SES to RBER.

Then:

( ) 321

18.2 +

=

RBERN m

SESRBBER B (6)

where:

1 = 10 to 30, average number of errors per burst for the BER in the range from 1 x 10 3

to BER SES

2 = 1 to 10, average number of errors per burst for the BER in the range from BER SESto RBER

3 = 1, number of errors per burst for the BER lower than RBER

N B : number of bits/block (see Table 4)

4.4 Prediction of ESR

Finally, the ESR can be calculated using the outage probability, P tR , for a BER equal to the RBER (as

determined above) , SESR, RBER, m and N B as follows:

3+=

RBERN nnSESRESR Bm (7)

where:

3 = 1, number of errors per burst for BER lower than RBER

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n = number of blocks/s (see Table 4)

The prediction methods for SDH paths given above can also be used for the Plesiosynchronous DigitalHierarchy (PDH) with the following choices:

use the BER SES closest to the transmission rate (Mbit/s), e.g. a VC-12 for a 2 Mbit/s PDHradio;

use the BER SES as given in Table 4 (the BER SES is under study for PDH, but only minor differences are expected).

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4.5 Equipment characteristics

The equipment characteristics required to design a radio link to conform to the error performanceobjectives of ITU-T G.826 or G.828 are:

Flat fade margins for BER values of:

BER SES

RBER

Signatures (non-faded i.e. unstressed) for BER values of:

BER SES

RBER

RBER

1 = Average number of errors per burst for the BER in the range from

1 x 10 3 to BER SES (10 to 30).

2 = Average number of errors per burst for the BER in the range fromBER SES to RBER (1 to 10).

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5 References

1. ITU-T Recommendation G.821, 1996, Error performance of an international digital connectionforming part of an integrated services digital network.

2. ITU-T Recommendation G.826, 1999, Error performance parameters and objectives for international, constant bit rate digital paths at or above the primary rate.

3. ITU-T Recommendation G.827, 2000, Availability parameters and objectives for path elementsof international constant bit rate digital paths at or above the primary rate.

4. ITU-T Rec. G.828, 2000, Error performance parameters and objectives for international, constantbit rate synchronous digital paths.

5. ITU-T Recommendation G.829, 2000, Error performance events for SDH multiplex andregenerator sections.

6. ITU-R F.1491, Error performance objectives for real digital radio links used in the nationalportion of a 27,500km hypothetical reference path at or above the primary rate.

7. Draft revision of ITU-R P.530-8, 2000, Propagation data and prediction methods required for thedesign of terrestrial line-of-sight systems.