ITU Objectives

Summary

Overview

ITU-T Error Performance Recs.

Error performance in a radio link

Unavailability Objectives

Objectives vs. Propagation impairments

HERALD Lab #8

_______________________________________________________________

Summary

In this Session ITU performance objectives are discussed. Under some aspects the matter is rather complex, so a brief history and overview of ITU recommendations is first presented. Then, more details are given about ITU-T and ITU-R error performance and availability objectives and the most significant points of relevant recommendations are outlined. Finally, the impact of propagation impairments on objective compliance is discussed.

Overview

In recent years, International Telecommunication Union (ITU) committees produced several Recommendations to set Performance Objectives for Telecommunication Systems. It is not always easy to understand how different recommendations are linked together and which is the correct one to refer in particular cases.

In the following sections, we try to give a brief review of ITU performance objectives and some simple indication on the use of ITU Recs, even if it will not be possible to go in details about many related questions.

ITU-T and ITU-R Recommendations

Dealing with ITU performance objectives, it is useful to clarify the role of ITU-T and ITU-R, in the framework of ITU activities.

ITU-T is mainly involved in regulating the end-to-end service and performance of telecommunication networks and systems. On the other hand, ITU-R is devoted to radio systems only (not limited to communications systems, but including radioastronomy, earth monitoring, etc.).

As a consequence, the ITU-T recommendations on performance objectives are "media independent ", that is they are addressed to the end-to-end performance of a telecommunication link, independently of the transmission system(s) used (cable, fiber, or radio).

ITU-R recommendations on performance objectives refer to radio communications only. They derive from (and must be in agreement with) ITU-T recommendations, since a link regulated by ITU-T may be implemented (partially or totally) using radio-relay systems.

Unavailability and Error Performance Objectives

ITU objectives are organized as:

· Unavailability objectives: they refer to conditions required to get an operating link; in general terms, we can state that unavailability is caused by equipment failures or by other events that produce an enduring loss of signal or an unacceptable signal degradation.

· Error Performance (quality) objectives: they refer to the received signal quality and are evaluated during the available time only. So the definition of unavailability is fundamental both for error performance and for unavailability evaluation.

Testing the compliance of a given telecommunication link with ITU objectives, we need:

· first to identify unavailability periods and check the Unavailability Objectives;

· then to check the Error Performance Objectives during available time.

A brief history and overview of ITU Recs

The recent history of ITU performance Recs starts with the approval of ITU-T Rec. G.821, at the end of 70's.

At that time, telecommunication systems were mainly addressed to the telephone (voice) service and digital transmission was almost exclusively based on the 64 kbit/s PCM channel. Data services were expected to grow through ISDN networks, again based on the 64 kbit/s channel.

For that reason, also performance objectives in G.821 make reference to the 64 kbit/s channel (or multiples, but anyway below the primary rate), without considering the actual bit rate of transmitted signals.

Error Performance and Availability objectives, derived by ITU-R from G.821, apply to bit rates below the primary rate. Suitable rules have been used to translate error performance measurements obtained at the system bit rate to the 64 kbit/s level.

Such problems were not solved until ITU-T Rec. G.826 was approved (1990). The new recommendation introduces two significant modifications, with respect to G.821:

· objectives apply at the system bit rate, not on the 64 kbit/s channel;

· performance parameters are no longer based on bit error rate (BER), but on errored blocks.

The whole set of ITU-R Recs. had to be revised. As a first step, two recommendations were approved (F.1092 and F.1189), which apply to the national and international portion of the Hypothetical Reference Path (HRP).

The practical use of these recommendations was not straightforward, since the HRP concept does not match closely with network configurations found in the real world.

At the same time, in the ITU-T environment, it was recognized the need for performance objectives specifically issued for SDH networks. ITU-T Rec. G.828 was approved in the year 2000 and defines error performance objectives for SDH path.

To clarify the applicability of G.826 and G.828 to SDH systems, we quote from G.828: "... this Recommendation is the only Recommendation required for designing the error performance of synchronous digital paths. (...) It is not required to apply this Recommendation to SDH paths using equipment designed prior to the adoption of Recommendation G.828 in March 2000. Performance objectives for paths using equipment designed prior to this date are given in Recommendation G.826."

ITU-R Recs. F.1397 and F.1491 take account of both G.826 and G.828 and apply, respectively, to international and national real radio links. The two recs. finally merged as ITU-R Rec. F.1668, with minor modifications, mainly addressed to clarify the recommendation scope ("... It is the only Recommendation defining error-performance objectives for all real digital fixed wireless links. Performance events and objectives for connections using equipment designed prior to approval of [revised] ITU-T Recommendation G.826 in December 2002 are given in ITU-T Recommendation G.821 and Recommendations ITU-R F.634, ITU-R F.696 and ITU-R F.697.").

The Table below gives a summary of the evolution of significant ITU-T and ITU-R recommendations.

ITU-T Rec.	ITU-R Rec.	Applicable to
G.821 (1978)	F.594	HRDP (64 kbit/s ch.)
G.821 (1978)	F.634, F.696, F.697	Real links (64 kbit/s ch.)
G.826 (1990)	F.1092, F.1189	HRP (PDH, SDH)
G.826 / G.828 (2000)	F.1397, F.1491	Real links (PDH, SDH)
G.826 / G.828 (Rev. 2002)	F.1668 (2004-07)	Real links (PDH, SDH)

Evolution of ITU-T and ITU-R error performance Recs.

Definitions

Primary rate: the first level of PCM multiplexing (that is E1 rate = 2.048 Mbit/s in Europe and other countries; T1 rate = 1.544 Mbit/s in the USA and other countries).

System bit rate: the gross bit rate of the transmitted signal, after multiplexing operation and including any service and controlling bits.

Block Allowance: a component of the overall threshold objective, which is allocated independently of the connection length.

Block (of bits): a set of consecutive bits associated with the signal path.

Hypothetical Reference Connection (HRX): used in the context of ITU-T G.821, a 27,500 km link including an international portion (High Grade objectives) and National portions (High, Medium, and Local Grade objectives).

Hypothetical Reference Path (HRP): used in the context of ITU-T G.826, a 27,500 km international link including the two terminal countries and up to four intermediate countries.

Hypothetical Reference Digital Path (HRDP): used in the context of ITU-R F.594, a 2,500 km radio link, subdivided in radio sections of at least 280 km.

Finally, we note that the word "Path" is used in the context of ITU-R Recs. F.1668 and F.1703, as well as in ITU-T Rec. G.826, with a different meaning with respect to the use in radio link design. As shown in the figure below, a Path is made of one or multiple Digital Links, implemented in Fixed Wireless (Radio) or Fibre Optics technologies.

A fixed wireless link forming a portion of a path

ITU-T Error Performance Recs.

G.821 - ITU-T Rec. G.821 defines error performance parameters and objectives of a Hypothetical Reference Connection (HRX), at a bit rate below the primary rate.

The error performance objectives are stated for each direction of a N x 64 kbit/s connection (1 £ N £ 32 or 24 chs.), independently of the transmission medium.

The following error performance events and parameters are defined:

· Errored Second (ES) : a one-second period in which one or more bits are in error.

· Severely Errored Second (SES) : a one-second period which has a BER ³10^-3.

· 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.

The end-to-end error performance objectives for the 27500 km Hypothetical Reference Connection (HRX) are given in the Table below, referring to the three classes (high-, medium- and local-grade).

	ESR	SESR
End-to-End	< 0.08	< 0.002
High grade	0.032	0.0004 (1)
Medium grade	0.012	0.00015 (1)
Local grade	0.012	0.00015

(1) For radio-relay systems a block allowance of 0.0005 SESR can be added to a 2 500 km HRDP

G.821 error performance objectives.

G.826 - ITU-T Rec. G.826 defines error performance parameters and objectives of a Hypothetical Reference Path (HRP), for international Constant Bit Rate (CBR) digital paths at or above the primary rate.

The recommendation is applicable to each direction of the 27500 km HRP. It applies as well for PDH, SDH and cell-based networks. It is independent of the transmission medium.

The important difference between ITU-T Rec. G.821 and ITU-T Rec. G.826 is that in G.826 the parameters are based on errored blocks and not on errored bits.

The parameters defined in ITU-T Rec. G.826 are based on the following events:

· 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.

· Severely Errored Second (SES) : a one-second period which contains more than 30% errored blocks or at least one defect in the received signal.

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

The following error performance parameters are defined:

· 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 BBE to total blocks in available time during a fixed measurement interval. The count of total blocks excludes all blocks during SESs.

The end-to-end error performance objectives for a 27 500 km HRP are specified in the Table below. The path fails to meet the error performance requirements if any of the objectives is not met.

Mbit/s	Bits / Block	ESR	SESR	BBER
1.5 - 5	800 - 5000	0.04	0.002	2 x 10^-4
> 5 - 15	2000 - 8000	0.05	0.002	2 x 10^-4
> 15 - 55	4000 - 20000	0.075	0.002	2 x 10^-4
> 55 - 160	6000 - 20000	0.16	0.002	2 x 10^-4
> 160	15000 - 30000	t.b.d.	0.002	1 x 10^-4

G.826 end-to-end error performance objectives.

The suggested measurement interval is one month. For radio-relay systems the objectives should be respected for any month.

The end-to-end objectives are divided into one national portion and one international portion. The allocation rules are rather complex and are based on a block allocation and a distance allocation (multiples of 500 km). Each National Portion is allocated at least a 17.5% of the end-to-end objectives.

G.828 - ITU-T Rec. G.828 defines error performance parameters and objectives for SDH systems only (application of G.826 and G.828 was discussed above). G.828 uses the error performance events and parameters as defined by Rec. G.826. Two additional definitions are:

· Severely Errored Period (SEP): a sequence of between 3 to 9 consecutive SES; the sequence is terminated by a second which is not a SES

· Severely Errored Period Intensity (SEPI): the number of SEP events in available time divided by the total available time in seconds

The end-to-end objectives defined by ITU-T Rec. G.828 for a 27 500 km HRP in terms of the error parameters (ESR, SESR, BBER) are shown in the Table below (SEPI objectives are not indicated, since further study is required).

Bit rate (Mbit/s)	Bits / Block	ESR	SESR	BBER
1,664	2000	0.01	0.002	5 x 10^-5
2,240	2000	0.01	0.002	5 x 10^-5
6,848	2000	0.01	0.002	5 x 10^-5
48,960	8000	0.02	0.002	5 x 10^-5
150,336	8000	0.04.	0.002	1 x 10^-4
601,344	8000	Not spec.	0.002	1 x 10^-4

G.828 end-to-end error performance objectives.

The objectives applicable to a real path are derived using allocation principles for international and national portions. Again, this is based on block allocation and on distance allocation (multiples of 100 km).

Error performance in a radio link

ITU-R Recs on error performance, derived by ITU-T Recs introduced in the previous section, are now discussed.

Error objectives for real links using equipment designed prior to approval of [revised] ITU-T Recommendation G.826 in December 2002

The error performance objectives for real links are based on ITU-T Rec. G.821, using the same definitions of Errored Second Ratio (ESR) and of Severely Errored Second Ratio (SESR). They apply to bit rate below the primary rate and are specified in three ITU-R recommendations, referring to high-, medium- and local-grade, respectively.

A summary is given in the Table below.

	ESR	SESR
End-to-End	< 0.08	< 0.002
High grade (1) ITU-R Rec. F.634	0.0032 x L / 2500	0.00054 x L / 2500
Medium grade ITU-R Rec. F.696	0.012	0.0004
Local grade ITU-R Rec. F.697	0.012	0.00015

(1) Length L (km) in the range 280 to 2500 km. For L < 280 km, a linear scaling can be used.

Error performance objectives

for real digital radio-relay systems, below the primary rate.

Error objectives for real digital fixed wireless links in 27500 km HRP and connections (equipment designed after approval of [revised] ITU-T Recommendation G.826 in December 2002)

ITU-R Rec. F.1668 gives the performance objectives both for the international portion and for national connections of a real link. For international portions of HRP, distinction has to be made between terminating countries and transit (or intermediate) countries.

Moreover, we distinguish PDH and SDH systems, referring to ITU-T Recs. G.826 and G.828, respectively.

For international portions of HRP, Error Performance Objectives (EPO) are expressed by a general formula:

EPO = B × (L_link / L_R) + C

where:

EPO can be replaced by SESR, ESR, or BBER, as appropriate;

B, C are numerical values, given in Rec. F.1668 tables;

L_link is the link length;

L_R is the reference length (2500 km).

The SEPI parameter, which is defined by ITU-T G.828, is ignored. The reason is that no propagation model to predict SEPI is presently available and the need for SEPI objectives is still under study.

Application of the general formula given above leads to the results shown in the tables below. In that formula, B and C are functions of parameter B_R (= block allowance ratio), to be selected by the network operator, in the range 0 to 1. That's why all the objectives are not expressed as a single value, but as a range, corresponding to B_R=0 and B_R=1.

Two link lengths (L1, L2) are considered in the tables; for any other length (in the 50 to 2500 km range), the following rules must be applied:

· up to L1, the objective is proportional to the link length;

· for a length between L1 and L2, the objective is computed by linear interpolation.

Bit rate

(Mbit/s)

Length

[km]

[s / month]

SES

[s / month]

BBER

( x 10^-6 )

1,664

(VC-11, TC-11)

1000

2500

520 - 1040

1300 - 1820

104 - 208

260 - 364

1.0 - 2.0

2.5 - 3.5

2,240

(VC-12, TC-12)

1000

2500

520 - 1040

1300 - 1820

104 - 208

260 - 364

1.0 - 2.0

2.5 - 3.5

6,848

(VC-2, TC-2)

1000

2500

520 - 1040

1300 - 1820

104 - 208

260 - 364

1.0 - 2.0

2.5 - 3.5

48,960

(VC-3, TC-3)

1000

2500

1040 - 2080

2600 - 3640

104 - 208

260 - 364

1.0 - 2.0

2.5 - 3.5

150,336

(VC-4, TC-4)

1000

2500

2080 - 4160

5200 - 7280

104 - 208

260 - 364

2.0 - 4.0

5.0 - 7.0

F.1668 error performance objectives for SDH international links,

intermediate countries.

Bit rate

(Mbit/s)

Length

[km]

[s / month]

SES

[s / month]

BBER

( x 10^-6 )

below

primary rate

1000

2500

2080 - 4160

5200 - 7280

104 - 208

260 - 364

4.0 - 8.0

10.0 - 14.0

1.5 - 5

1000

2500

2080 - 4160

5200 - 7280

104 - 208

260 - 364

4.0 - 8.0

10.0 - 14.0

> 5 - 15

1000

2500

2600 - 5200

6500 - 9100

104 - 208

260 - 364

4.0 - 8.0

10.0 - 14.0

> 15 - 55

1000

2500

3900 - 7800

9750 - 13650

104 - 208

260 - 364

4.0 - 8.0

10.0 - 14.0

>55 - 160

1000

2500

8320 - 16640

20800-29120

104 - 208

260 - 364

4.0 - 8.0

10.0 - 14.0

>160 - 400

1000

2500

NOT Applicable

104 - 208

260 - 364

4.0 - 8.0

10.0 - 14.0

F.1668 error performance objectives for PDH international links,

intermediate countries.

Bit rate

(Mbit/s)

Length

[km]

[s / month]

SES

[s / month]

BBER

( x 10^-6 )

1,664

(VC-11, TC-11)

500

2500

260 - 520

1300 - 1560

52 - 104

260 - 312

0.5 - 1.0

2.5 - 3.0

2,240

(VC-12, TC-12)

500

2500

260 - 520

1300 - 1560

52 - 104

260 - 312

0.5 - 1.0

2.5 - 3.0

6,848

(VC-2, TC-2)

500

2500

260 - 520

1300 - 1560

52 - 104

260 - 312

0.5 - 1.0

2.5 - 3.0

48,960

(VC-3, TC-3)

500

2500

520 - 1040

2600 - 3120

52 - 104

260 - 312

0.5 - 1.0

2.5 - 3.0

150,336

(VC-4, TC-4)

500

2500

1040 - 2080

5200 - 6240

52 - 104

260 - 312

1.0 - 2.0

5.0 - 6.0

F.1668 error performance objectives for SDH international links,

terminating countries.

Bit rate

(Mbit/s)

Length

[km]

[s / month]

SES

[s / month]

BBER

( x 10^-6 )

below

primary rate

500

2500

2080 - 4160

5200 - 6240

52 - 104

260 - 312

2.0 - 4.0

10.0 - 12.0

1.5 - 5

500

2500

2080 - 4160

5200 - 6240

52 - 104

260 - 312

2.0 - 4.0

10.0 - 12.0

> 5 - 15

500

2500

2600 - 5200

6500 - 7800

52 - 104

260 - 312

2.0 - 4.0

10.0 - 12.0

> 15 - 55

500

2500

3900 - 7800

9750 - 11700

52 - 104

260 - 312

2.0 - 4.0

10.0 - 12.0

>55 - 160

500

2500

8320 - 16640

20800-24960

52 - 104

260 - 312

2.0 - 4.0

10.0 - 12.0

>160 - 400

500

2500

NOT

Applicable

52 - 104

260 - 312

2.0 - 4.0

10.0 - 12.0

F.1668 error performance objectives for PDH international links,

terminating countries.

Note that, while ESR and BBER objectives are dependent on the bit rate, the SESR objective is applicable to any rate (from 1.5 to 160 Mbit/s) and it is common to SDH and PDH systems.

The figure below gives an example referring to the significant case of SESR objective for SDH or PDH links.

ses

SES objective for SDH and PDH international links (Rec. F.1668).

A) Block Allowance B_R = 0, Intermediate and terminating countries. B) B_R = 1, terminating countries. C) B_R = 1, intermediate countries.

For the national portion of HRP or connections, ITU-R Rec. F.1668 gives the performance objectives for a real link, which is divided in 3 sections:

· the long-haul section (from the International Gateway to the Primary, Secondary, or Tertiary Center);

· the short-haul section (from the Primary, Secondary, or Tertiary Center to the Local Exchange);

· the access section (from the Local Exchange to the path end-point).

For long-haul sections, the objectives are listed in the Tables below. A parameter A1 (= block allowance for long-haul sections) is left to be selected by the network operator, in the range 1% to 2%. That's why all the objectives are not expressed as a single value, but as a range, corresponding to A1=0.01 and A1=0.02.

Two link lengths (L1, L2) are considered in the tables; for any other length (in the 50 to 2500 km range), the following rules must be applied:

· up to L1, the objective is proportional to the link length;

· for a length above L1, the objective is computed by linear interpolation or extrapolation of the values given for L1 and L2 .

Bit rate

(Mbit/s)

Length

[km]

[s / month]

SES

[s / month]

BBER

( x 10^-6 )

1,664

(VC-11, TC-11)

100

1000

312 - 572

780 - 1040

62 - 114

156 - 208

0.6 - 1.1

1.5 - 2.0

2,240

(VC-12, TC-12)

100

1000

312 - 572

780 - 1040

62 - 114

156 - 208

0.6 - 1.1

1.5 - 2.0

6,848

(VC-2, TC-2)

100

1000

312 - 572

780 - 1040

62 - 114

156 - 208

0.6 - 1.1

1.5 - 2.0

48,960

(VC-3, TC-3)

100

1000

624 - 1044

1560 - 2080

62 - 114

156 - 208

0.6 - 1.1

1.5 - 2.0

150,336

(VC-4, TC-4)

100

1000

1248 - 2088

3120 - 4160

62 - 114

156 - 208

1.2 - 2.2

3.0 - 4.0

F.1668 error performance objectives for SDH links

in the long-haul network.

Bit rate

(Mbit/s)

Length

[km]

[s / month]

SES

[s / month]

BBER

( x 10^-6 )

below

primary rate

100

1000

1248 - 2088

3120 - 4160

62 - 114

156 - 208

NOT

Applicable

1.5 - 5

100

1000

1248 - 2088

3120 - 4160

62 - 114

156 - 208

2.4 - 4.4

6.0 - 8.0

> 5 - 15

100

1000

1560 - 2610

3900 - 5200

62 - 114

156 - 208

2.4 - 4.4

6.0 - 8.0

> 15 - 55

100

1000

2340 - 3915

5850 - 7800

62 - 114

156 - 208

2.4 - 4.4

6.0 - 8.0

>55 - 160

100

1000

4992 - 8352

12480-16640

62 - 114

156 - 208

2.4 - 4.4

6.0 - 8.0

>160 - 400

100

1000

NOT

Applicable

62 - 114

156 - 208

1.2 - 2.2

3.0 - 4.0

F.1668 error performance objectives for PDH links

in the long-haul network.

The error objectives for long-haul sections are the same defined for the international portion.

For short-haul and access sections, the objectives are listed in the Tables below. Block allowances for short-haul sections and for access sections are parameters left to the network operator, in the range 7.5% to 8.5%. That's why all the objectives are not expressed as a single value, but as a range, corresponding to block allowance 0.075 and 0.085.

Bit rate (Mbit/s)	ES [s / month]	SES [s / month]	BBER ( x 10^-6 )
1,664 (VC-11, TC-11)	1950 - 2210	390 - 442	3.75 - 4.25
2,240 (VC-12, TC-12)	1950 - 2210	390 - 442	3.75 - 4.25
6,848 (VC-2, TC-2)	1950 - 2210	390 - 442	3.75 - 4.25
48,960 (VC-3, TC-3)	3900 - 4420	390 - 442	3.75 - 4.25
150,336 (VC-4, TC-4)	7800 - 8840	390 - 442	7.50 - 8.50

F.1668 error performance objectives for SDH links

in the short-haul or access network.

Bit rate (Mbit/s)	ES [s / month]	SES [s / month]	BBER ( x 10^-6 )
below primary rate	7800 - 8840	390 - 442	NOT Applicable
1.5 - 5	7800 - 8840	390 - 442	15.0 - 17.0
> 5 - 15	9750 - 11050	390 - 442	15.0 - 17.0
> 15 - 55	14625 - 16575	390 - 442	15.0 - 17.0
> 55 - 160	31200 - 35360	390 - 442	15.0 - 17.0
>160 - 400	NOT Applicable	390 - 442	7.5 - 8.5

F.1668 error performance objectives for PDH links

in the short-haul or access network.

Again, we note that, while ESR and BBER objectives are dependent on the bit rate, the SESR objective is applicable to any rate (from 1.5 to 400 Mbit/s) and it is common to SDH and PDH systems.

Practical rules in applying ITU-R Recs

Two problems may arise in applying ITU-R Rec. F.1668:

· selecting to use SDH or PDH objectives;

· apportioning section objectives on a single hop or on a distance basis.

How to identify SDH and PDH sections - Practical rules are:

· a radio link composed by two terminals with optical and electrical STM-N interface is a SDH section, so SDH objectives shall be used.

· a radio link composed by two terminals with optical or electrical PDH interface at both terminal is a PDH section, independently from the rate carried over the radio channel, so the PDH objectives shall be used.

· a radio link composed by one terminal with optical or electrical PDH interface and one terminal with STM‑n interface is a SDH section, so the SDH objectives shall be used.

How to apportion Section objectives to each hop - Performance objectives given by ITU-R Recs apply to the overall link, even in the case of multi-kop links. Allocation of the objectives to each hop is under the responsibility of the network operator.

The network operator can allocate objectives in a way proportional to the hop lengths, or different criteria can be followed, to overcome particular problems. This gives the network operator some flexibility in link design.

Note that multipath propagation models indicate that predicted outage is not proportional to the hop length, but approximately to the third power of the hop length.

As an example, let us suppose that a three-hop link has to be designed, with one over-the-sea hop. The designer is free to decide to allocate most of the overall objective to the hop over the sea (in order to reduce cost and to overcome propagation problems), while the objectives assigned to the other two hops are less then proportional to the hop lengths.

How to apportion Short-haul or Access section objectives on a distance basis - Objectives for short-haul and access sections are given by ITU-R Rec. F.1668 (national portion) on a block allowance basis (independent on section length).

Objective apportioning on a distance basis is left to the network operator. This is mainly based on the fact that the apportioning process is strongly dependent on:

· network architecture, network protection policy, maintenance policy, etc.;

· access network structure (from high density urban environment to rural environments);

· guaranteed Quality of Service.

All the above factors are defined or managed by the network operator.

The suggested procedure is to identify a "typical" length of short-haul or access sections and then to use the typical length to scale down the objectives for sections of any length.

Unavailability Objectives

Unavailability is defined by ITU-T Rec. G.826. An unavailability period begins when ten consecutive SES events are observed (the ten seconds are part of the unavailability period). When ten consecutive non-SES events are observed, the unavailability period closes (the ten seconds are part of the new availability period).

The SES definition is the same given in the context of Error Performance Recs.

ITU-T Recs. G.826 and G.827

The following availability parameters are defined by ITU-T Rec. G.826:

· Availability ratio (AR) : ratio of total available time to the duration of the observation period;

· Mean time between outages (Mo) : average duration of any continuous interval during which the link is available.

The Unavailability Ratio (UR) is the complementary parameter to AR, so that AR+UR=1.

From the Mo parameter, the "Outage Intensity" (OI) is derived as:

OI expresses the average number of unavailability periods in one year. Mo and OI have been recently introduced; this new parameters are needed since the Quality of Service may be affected not only by the total unavailable time, but also by the number of unavailability periods.

Availability objectives for International and National digital systems are given by ITU-T Rec. G.827, as media independent objectives. Two types of objectives are specified:

· mean values, that is the ensemble average of all paths of a given category in a country;

· worst-case values, that is the minimum acceptable value for individual paths.

The objectives apply to observation periods of one year. They are intended for the following purposes:

· Network design / planning: both mean and worst-case objectives are appropriate;

· Operational objectives: worst-case objectives are appropriate and are applicable to each individual path.

Radio Link Availability Objectives

In applying the most recent ITU-T availability Recs to radio links, the following points have been taken into account:

· Radio propagation models allow to evaluate worst-case performance predictions. So, ITU-R recommends worst-case objectives only and the mean-value objectives, as given by ITU-T G.827, are not considered.

· If availability objectives for radio systems were scaled from ITU‑T Rec. G.827, then previous ITU-R objectives would be relaxed in some measure. Therefore, it was decided to be coherent with previous (more stringent) figures, even in the context of new recs.

Unavailability objectives for National links (in the long-haul, short-haul, and access networks), as recommended by ITU-R Rec. F.1703, are shown in the figure below (instead of the UR parameter, the objective is expressed in hours per year).

Una

Unavailability objective (Rec. F.1703).

A) Long-haul sections. B) Short-haul sections. C) Access sections.

The Outage Intensity (OI) objective is reported in the Table below (in long-haul sections, the objective for length between 250 and 2500 km can be computed by linear interpolation of the values in the table).

Section	Length [km]	OI [events/year]
Long-haul	250	65
Long-haul	2500	155
Short-haul		100
Access		120

F.1703 outage intensity (OI) objectives.

Unavailability objectives for International links are given by ITU-R Rec. F.1703 and are the same as for long-haul networks in the national portion (see the previous figure and table; linear extrapolation for any section length exceeding 2500 km).

For practical application of availability objectives, two points have to be considered:

· availability objectives should be partitioned in order to take into account unavailability events due to propagation, equipment failures, human interventions and other causes;

· for the case when the radio link is composed of more than one hop, the objectives are applicable for the whole link. The scaling of objectives to each individual hop is under the network operator responsibility.

Objectives vs. Propagation impairments

Finally, we try to identify how propagation impairments, discussed in previous sessions, may affect the compliance with performance objectives.

The Table below indicates the expected duration of system degradation, caused by various propagation events. Based on degradation period, the relevant performance objective is identify.

Propagation

Impairment

Degradation

Period

Performance Objective

· Multipath Fading

· Short Term

Uncorrelated Interf.

< 10 seconds

Error Performance

(SESR)

· Rain

· Obstruction Fading

(Sub-Refractivity)

· Interference

(Super-Refractivity)

³ 10 seconds

Availability

· Long Term

Correlated Interf.

Permanent

Error Performance

(ESR and BBER)

Relation between propagation impairments

and applicable performance objectives.

This classification, even if it is rather rough, is quite useful to have practical objectives in the system design stage, so that we can compare performance predictions based on propagation models to objectives set by ITU Recs.

In the most simplifying terms, referring to the main propagation impairments, rain outage predictions will be compared to unavailability objectives, while multipath outage predictions will be compared to error performance (quality) objectives.