11th October 2001
RSSPWG
Date:
Meeting:
Document Filename:
Paper Number:
687289241
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 6th FLCC Policy meeting on the 10th September 2001, concerns were raised that the
Minimum 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 Policy
factors and the technical considerations that would affect availability and performance. This working document tries to
review the whole issue of error performance and availabilty issues against the ITU-T Recommendation G.826 and
G.828 Error Performance Objectives (EPO) in conjunction with the latest draft revision of the radio link performance
of ITU-R Recommendation P.530 for Propagation data and prediction methods required for the design of terrestial lineof-sight systems.
Bob Howell
Mike Pine
Chris Cheeseman
Henry Taylor
John Tomlinson
henry.taylor@bt.com
john.tomlinson@bt.com
1. Background
The International Telecommunications Union (ITU) specify the performance objectives for long
distance communications links in a series of recommendations produced by the ITU Telecommunication
Standardization Sector (ITU-T). The “G” series of these recommendations specify the performance
requirements 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 the
performance required from an international digital HRP and G.829 [5] specifies that required from
Synchronous Digital Hierarchy (SDH) multiplex and regenerator sections. ITU-T Recommendations
G.821, G.826, G.827 and G.828 only specify the end-to-end performance of an HRP and apply
irrespective of the physical transport mechanism (e.g. optical fibre, digital radio relay, metallic cable
etc.).
The ITU-T G Recommendations cannot be used directly when designing a digital radio system and
therefore the ITU Radiocommunications Sector (ITU-R) have used the ITU-T G Recommendations to
produce a series of ITU-R Recommendations (the “F” series) which specify the performance required
from 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 an
international digital connection forming part of an integrated services digital network), have been
used for many years to specify the error performance objectives for digital radio links and specify the
required error performance in terms of Bit Error Ratios (BER). ITU-T Rec. G.821 has now been
superseded by ITU-T Rec. G.826 (1999) (Error performance parameters and objectives for
international, 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 rate
synchronous digital paths) and new ITU-R error performance objectives based on these and applicable
to digital radio links have been produced. These new recommendations specify error performance in
terms of error blocks rather than on BER values.
ITU-T Rec. G.826 applies to both Plesiochronous Digital Hierarchy (PDH) and Synchronous Digital
Hierarchy (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-T
Rec. G.826 and compliance with ITU-T Rec. G.828 will, in most cases, also ensure compliance with
ITU-T Rec. G.821 and ITU-T Rec. G.826. It should be noted that ITU-T Rec. G.828 only applies to
equipment designed after 10th 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-T
Rec. G.826.
A key ITU-R Recommendation in this context is ITU-R F.1491 (Error performance objectives for
real digital radio links used in the national portion of a 27,500km hypothetical reference path at
or 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 systems
conforming to ITU-T Rec. G.826 and ITU-T Rec. G.828.
Page 2 of 13
2. Error performance and availability
2.1
Error performance
The digital bit stream in modern digital radio systems is divided into blocks. For example an SDH
STM-1 system might have 801 bits per block and carry 192,000 blocks per second. Each block is
monitored by means of an inherent Error Detection Code (EDC), e.g. Bit Interleaved Parity or Cyclic
Redundancy Check. ITU-T Rec. G.826 and ITU-T Rec. G.828 (and the ITU-R Recommendations based
on them) specify error performance objectives in terms of the rate at which blocks containing errors
occur 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 one
defect (e.g. loss of pointer LOP).
•
Severely Errored Second (SES): A one-second period, which contains 30% errored blocks
or 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 a
fixed measurement interval.
•
Severely Errored Second Ratio (SESR): The ratio of SES to total seconds in available time
during a fixed measurement interval.
•
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.
2.2
Availability
Each direction of a path can be in one of two states, available time, or unavailable time. The criteria
determining 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 time
begins 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.
Detection of unavailable state
< 10 sec
Available state
10 sec
Detection of available state
< 10 sec
Unavailable state
10 sec
Available state
T1306350-95
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 such
that neither direction becomes unavailable, and the combined period is greater than 10 seconds, then the
path 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 available
state.
It is generally accepted that for a well planned link clear air fading is largely produced by multipath
propagation which tends to produce fades lasting less than 10 seconds, whereas, rain tends to produce
fades lasting longer than 10 seconds.
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 the
national portion of the 27,500km HRP the later can be subdivided into the three sections shown in
Figure 2.
Path
end-point
Local
exchange
Switching
Centre
International
Gateway
L (km)
Access
Long haul
Short haul
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 digital
radio 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 the
Long haul inter-exchange section of the national portion of the HRP according to
ITU-T G.826
Rate (Mbit/s)
ESR
SESR
BBER
1.5 to 5
0.04A
0.0002A (*)
> 5 to 15
0.05A
> 15 to 55
0.075A
0.002A
0.0002A
* For systems designed prior to 1996 the BBER objective is 0.0003.
> 55 to 160
0.16A
> 160 to 3,500
For further study
0.0001A
Table 1b
Error performance objectives for SDH radio links belonging to the
Long 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)
ESR
SESR
BBER
Where:
2,240
(VC-12,
TC-12)
0.01A
6,848
(VC-2,
TC-2)
48,960
(VC-3,
TC-3)
0.02A
150,336
(VC-4,
TC-4)
0.04A
0.002A
0.00005A
0.0001A
If L < 50 km then L = 50 km
(1)
A = (A1 + 0.002) x L/100
for 50 km  L  100 km
(2)
A = A1 + 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 A1 ,
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 links
belonging to the Short haul inter-exchange section of the national
portion of the HRP according to ITU-T G.826
Rate (Mbit/s)
ESR
SESR
BBER
1.5 to 5
0.04B
> 5 to 15
0.05B
0.0002B (*)
> 15 to 55
0.075B
0.002B
0.0002B
> 55 to 160
0.16B
> 160 to 3,500
For further study
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 the
Short haul inter-exchange section of the national portion of the HRP
according to ITU-T G.828
Rate (Mbit/s)
ESR
SESR
BBER
1,664
(VC-11,
TC-11)
2,240
(VC-12,
TC-12)
0.01B
Where B is in the range 0.075 to 0.085.
6,848
(VC-2,
TC-2)
48,960
(VC-3,
TC-3)
0.02B
150,336
(VC-4,
TC-4)
0.04B
0.002B
0.00005B
0.0001B
3.3
Access
Table 3a
Error performance objectives for PDH and SDH radio links
belonging to the Access network section of the national portion of the
HRP according to ITU-T G.826
Rate (Mbit/s)
ESR
SESR
BBER
1.5 to 5
0.04C
> 5 to 15
0.05C
0.0002C (*)
> 15 to 55
0.075C
0.002C
0.0002C
> 55 to 160
0.16C
> 160 to 3,500
For further study
0.0001C
* For systems designed prior to 1996 the BBER objective is 0.0003.
Table 3b
Error performance objectives for SDH radio links belonging to the
Access network section of the national portion of the HRP according
to ITU-T G.828
Rate (Mbit/s)
ESR
SESR
BBER
1,664
(VC-11,
TC-11)
2,240
(VC-12,
TC-12)
0.01C
6,848
(VC-2,
TC-2)
48,960
(VC-3,
TC-3)
0.02C
150,336
(VC-4,
TC-4)
0.04C
0.002C
0.00005C
0.0001C
Where C is in the range 0.075 to 0.085. When considering the performance objectives given in the
above 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 A1 + B + C shall not exceed 0.175 (i.e. 17.5%) in accordance with the allocations to
the national portion of an international Constant Bit Rate (CBR) path given in ITU-T
Recommendations 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 and
C 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-hop
links 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 the
objectives given in the tables.

Synchronous digital paths operating at bit rates covered by ITU-T Recommendations G.826
and 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 rate
paths, which are expected to be carried. Meeting the allocated objectives for this highest bit
rate 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 the
STM-1 section should be designed such that it will ensure that the objectives as specified in the
above 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 the
above tables are “understood to be long-term objectives”. In the case of systems conforming to ITU-T
G.826 these could be expressed in the form of the SESR, BBER and ESR averaged over any month and
in 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, and
this therefore implies that the worst month during the whole lifetime of the system must be the month to
consider, it is generally accepted in ITU-R Recommendations that an average worst month should be
used when designing radio links. With the relaxation of this to a year in ITU-T G.828 it is probably safe
to assume that this means an average year, however, clarification on this point in adopted ITU
documentation is required.
4
Recommendation ITU-R P.530 in the context of G.826 and G.828
4.1
General
The task now is to match the error performance objectives derived above to radio link performance as
predicted using the implementation of Recommendation ITU-R P.530 [7].The interface between the two
consists 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 fading
is present and the equipment signature applies when only frequency selective fading is present. It
should be noted that the equipment signature referred to throughout this document is that measured with
no flat fading i.e. at frequencies within the receiver band well away from the selective fade notch the
signal 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 be
adopted in the near future) estimate the outage probability for flat and frequency selective fading
separately 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-based
error 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 and
the Annex 1 to ITU-R P.530-9 propagation models.
The method is based on statistical relationships between the block-based parameters (SESR, BBER and
ESR) and BER. The methods in Annex 2 of ITU-R P.530-9 allow the SESR, BBER and ESR to be
determined using equipment characteristics (e.g. Residual Bit Error Ratio (RBER) and equipment
signature) 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 relationship
between SES and BER. The SESR is evaluated in two steps, one to determine the number of SES due to
EB, and a second one to obtain the number due to Loss of Pointer (LOP). The contribution from LOP
has been found to be small, and can therefore be ignored. From measurements on digital systems, it is
found that for BER values below a threshold, none of the seconds are severely errored, but above this
threshold, all the seconds are severely errored i.e. the curve of SES due to EB versus BER can be
approximated by a step function. The BER value where the SES probability changes from 0 to 1 is
denoted BERSES. Table 4 gives the BERSES value for a number of path types.
Table 4
BERSES FOR VARIOUS PATH TYPES
Path
type
Bit rate
supported
(Mbit/s)
BERSES
(Notes 1 and 2)
Blocks/s, n
(Note 2)
Bits/block, NB
(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 x
10–4
8 000
192 000
19 940
801
NOTE 1 – 
NOTE 2 – The blocks/s are defined in ITU-T Recommendation G.826 for SDH path, in ITU-T Recommendation G.829 for SDH
sections. Some STM-1 equipment might be designed with 8 000 blocks/s (19 940 bits/block), but ITU-T Recommendation G.829
defines the block rate and size to be 192 000 blocks/s and 801 bits/block, respectively.
The SESR is equal to the outage probability (PtSES) determined using a BER equal to BERSES as the
criterion for outage in the methods given in Annex 1 of ITU-R P.530-9 i.e. the flat fade margin to be
used 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 BERSES.
Hence, for an SDH STM-1 path having a block structure of 192,000 blocks/s and 801 bits/block and
assuming a Poisson distribution of errors BERSES = 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
–10
(PtR) determined using a BER equal to the residual BER (RBER) (typically in the range from 10 to
–13
10 for the bit rates of 2 to 155 Mbit/s, respectively), as the criterion for outage in the methods given in
Annex 1 of ITU-R P.530-9 and RBER as follows:
PtR = Pt (RBER)
m 
(4)
log 10 RBER  log 10  BERSES 
log 10  PtR   log 10  PtSES 
(5)
Where m is the slope of the BER distribution curve on a log-log scale for BER values in the range from
BERSES to RBER.
Then:
BBER  SESR
1

2.8  2 m  1
NB RBER
3
(6)
where:
1 = 10 to 30, average number of errors per burst for the BER in the range from 1 x 10–3 to
BERSES
2 = 1 to 10, average number of errors per burst for the BER in the range from BER SES to
RBER
3 = 1, number of errors per burst for the BER lower than RBER
NB : number of bits/block (see Table 4)
4.4
Prediction of ESR
Finally, the ESR can be calculated using the outage probability, PtR, for a BER equal to the RBER (as
determined above), SESR, RBER, m and NB as follows:
ESR  SESR m n 
n NB RBER
3
(7)
where:
3 = 1, number of errors per burst for BER lower than RBER
n
= number of blocks/s (see Table 4)
The prediction methods for SDH paths given above can also be used for the Plesiosynchronous Digital
Hierarchy (PDH) with the following choices:
–
use the BERSES closest to the transmission rate (Mbit/s), e.g. a VC-12 for a 2 Mbit/s PDH
radio;
–
use the BERSES as given in Table 4 (the BERSES is under study for PDH, but only minor
differences are expected).
4.5
Equipment characteristics
The equipment characteristics required to design a radio link to conform to the error performance
objectives of ITU-T G.826 or G.828 are:


Flat fade margins for BER values of:

BERSES

RBER
Signatures (non-faded i.e. unstressed) for BER values of:

BERSES

RBER

RBER

1 = Average number of errors per burst for the BER in the range from
1 x 10–3 to BERSES (10 to 30).

2 = Average number of errors per burst for the BER in the range from
BERSES to RBER (1 to 10).
5
References
1.
ITU-T Recommendation G.821, 1996, Error performance of an international digital connection
forming 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 elements
of 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, constant
bit rate synchronous digital paths.
5.
ITU-T Recommendation G.829, 2000, Error performance events for SDH multiplex and
regenerator sections.
6.
ITU-R F.1491, Error performance objectives for real digital radio links used in the national
portion 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 the
design of terrestrial line-of-sight systems.