5. Performance and availability objectives
5.6 Objectives based on G.826 and G.827
The ITU-T recommendation G.826 [3] specifies error performance parameters and objectives for international digital paths at or above the primary rate. These paths may be based on PDH, SDH or some other transport network. Future radio-relay systems, which will form part of these paths, have to comply with this recommendation. Generally G.826 specifies more stringent performance objectives than G.821 [2] does.
The ITU-T recommendation G.827 [4] specifies availability parameters and objectives for international digital paths at or above the primary rate.
The 1996 version of this recommendation specifies no availability figures, only definitions. All parameters are still under study, and consequently no figures can be included in this textbook.
5.6.1 Performance objectives
5.6.1.1 Some definitions
The main difference between G.826 and G.821 is that G.826 uses blocks instead of bits as in G.821. Consequently the following definitions are based on block errors rather than bit errors.
40 5. Performance and availability objectives
Block A block is a set of consecutive bits associated with the path;
each bit belongs to one and only one block. An errored block (EB) is a block in which one or more bits associated with the block are in error.
Errored Block
A block in which one or more bits are in error.
Errored Second
A one-second period with one or more errored blocks. SES defined below is a subset of ES.
Severely Errored Second
A one-second period which contains ≥ 30% errored blocks or at least one Severely Disturbed Period (SDP)
Background Block
Error
An errored block not occurring as part of a SES.
5.6.1.2 Parameters
Errored Second Ratio
The ratio of ES to total seconds in available time during a fixed measurement interval.
Severely Errored Second Ratio
The ratio of SES to total seconds in available time during a fixed measurement interval.
Background Block
Error Ratio
The ratio of errored blocks to total blocks during a fixed measurement interval, excluding all blocks during SES and unavailable time.
5. Performance and availability objectives 41
5.6.1.3 Performance objectives
Rate Mb/s 1.5 to 5 >5 to 15 >15 to 55 >55 to 160 >160 to 3500
Bits/block 800-5000 2000-8000 4000-20 000 6000-20 000 15 000-30 000 Note 2
ESR 0.04 0.05 0.075 0.16 Note 3
SESR 0.002 0.002 0.002 0.002 0.002
BBER 2.10-4 Note 1 2.10-4 2.10-4 2.10-4 10-4
Note 1 For systems designed prior to 1996, the BBER objective is 3.10-4.
Note 2 Because bit error ratios are not expected to decrease dramatically as the bit rates of transmission systems increase, the block sizes (in bits) used in evaluating very high bit rate paths should remain within the range 15000 to 30000 bits/block. Preserving a constant block size for very high bit rate paths results in relatively constant BBER and SESR objectives for these paths.
As currently defined, VC-4-4c (Recommendation G.709) is a 601 Mbit/s path with a block size of 75168 Bits/block. Since this exceeds the maximum recommended block size for a path of this rate, VC-4-4c paths should not be estimated in service using this table. The BBER objective for VC-4-4c using the 75168 bit block size is taken to be 4.10-4. There are currently no paths defined for bit rates greater than VC-4-4c (>601 Mbit/s). Digital sections are defined for higher bit rates and guidance on evaluating the performance FO digital sections can be found below.
Note 3 Due to lack of information on the performance of paths operating above 160 Mbit/s, no ESR objectives are recommenced at this time. Nevertheless, ESR processing should be implemented within any error performance measuring devices operating at these rates for maintenance or monitoring purposes. For paths operating at bit rates up to 601 Mbit/s an ESR objective of 0.16 is proposed. This value requires further study.
Table 2 End-to-end error performance objective for a 27 500 km international digital path at or above the primary rate.
PEP IG IG IG IG IG PEP
Hypothetical Reference Path 27 500 km
IG = International Gateway PEP = Path End Point
Figure 19 Hypothetical reference path, apportionment methodology G.826
42 5. Performance and availability objectives
The performance objectives for radio-relay systems as the carrier for the international and national portion are defined in separate ITU-R recommendations.
5.6.1.3.1 International portion using radio-relay systems
The ITU-R Recommendation F.1092-1 [25] defines the “Error performance objectives for constant bit rate digital path at or above the primary rate carried by digital radio-relay systems which may form part of the international portion of a 27 500 km hypothetical reference path”. The objectives are based on the overall recommendation given in ITU-T G.826, but adopted to radio-relay systems with reference length of about 1000 km.
Rate (Mbit/s) 1.5 to 5 >5 to 15 >15 to 55 >55 to 160 >160 to 3500
*) For systems designed prior to 1996: 3x10-4
distance allocation factor FL = 0.01 x L / 500 L(km) ~ 500 km block allowance factor
intermediate countries
BL = BR x 0.02 (L / Lref) BL = BR x 0.02
for Lmin < L ≤ Lref
for L > Lref
block allowance factor terminating countries
reference length Lref = 1000 km (provisionally)
Table 3 Error performance objectives for radio-relay systems being part of an international digital path at or above the primary rate.
The performance objective for each direction of a real radio link can be calculated according to ITU-R F.1397 [51]. The value given in the upper
5. Performance and availability objectives 43
part of table 3 should be multiplied by the ratio LLink L where LLink is the real length of the path (LLink ≥ 50 km) and L is the system length rounded to the nearest 500 km. As an example, the severely errored seconds objective is 10 SES/month for a 50 km link.
5.6.1.3.2 National portion using radio-relay systems
The ITU-R Recommendation F.1189-1 [26] defines the “Error performance objectives for constant bit rate digital path at or above the primary rate carried by digital radio-relay systems which may form part of the national portion of a 27 500 km hypothetical reference path”. The objectives are based on the overall recommendation given in ITU-T G.826, but adopted to radio-relay systems.
Rate (Mbit/s) 1.5 to 5 >5 to 15 >15 to 55 >55 to 160 >160 to 3500
Errored second ratio
0.04xZ 0.05xZ 0.075xZ 0.16xZ Under study
Severely errored seconds ratio
0.002xZ
Background block error ratio
2x10-4xZ *)
2x10-4xZ 1x10-4xZ
*) For systems designed prior to 1996: 3x10-4
Long haul section Z = A A = A1 + (L/500) A1 = 1 - 2 %
Short haul section Z = B fixed block allocation only B = 7.5 - 8.5 %
Access section Z = C fixed block allocation only C = 7.5 - 8.5 %
A1% + B% + C% ≤ 17.5% and 15.5% ≤ B% + C% ≤ 16.5%
Table 4 Error performance objectives for radio-relay systems being part of a national digital path at or above the primary rate.