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The two ITU error performance recommendations most referred to within the industry are G.826 and G.821. G.826 has largely replaced G.821, except for sub-rate connections (less than 2048 bps / E1).

Other ITU error performance recommendations include G.828 and G.829, which are designed to better match the capabilities of SDH systems.

G.821

G.821 error performance is based on 64 kbps connections. For connections operating at a higher rate, the results obtained are normalized to a 64 kbps channel.

However for rates in excess of 2 Mbps this normalizing process is considered flawed, and it was primarily because of this, and an industry requirement for in-service testing, that G.826 was formulated.

G.821 based measurement remains in wide use for error measurement at E1/DS1 and fractional rates.

G.826

G.826 was designed for error performance measurement on circuits with bit rates higher than E1/DS1 (higher than 2 / 1.5 Mbps), and to allow in-service

measurement. It is non-transmission-medium (radio, fiber, wire, and so on) dependent, and non-system dependent, meaning it supports measurement on PDH, SDH and cell-based systems. Its performance objectives are also higher than G.821 and requires detection of errors with at least 90% probability.

The move to in-service performance assessment was to allow for use of built-in error monitoring equipment, which required a move away from bit error measurement towards block error measurement.

Block monitoring takes advantage of the FEC (forward error correction) monitoring and correction circuits built into modern transmission equipment.

Typically the block size equates to the FEC frame size, which is specified by the number of bits or bytes per frame. (Block size = FEC frame size)

At this point it is as well to note that under G.826 recommendations the

performance objectives are for a hypothetical 27,500 km reference path with the total circuit split into national and international portions and the block size sized separately for different system rates. Block sizes typically vary between 800 and 5,000 bits per block for bit rates from 1.5 to 5 MHz, and between 4,000 and 20,000 bits per block for bit rates from 55 to 160 MHz. This is in sharp contrast with the G.826 interpretation for a typical microwave link, where the

performance objectives apply to a single hop, and for links supporting software selection of different data rates the block size is similar for all rates.

The reason for the similar block size in a multiple-rate radio is that the FEC circuit operation is common to all rates. A typical modern microwave link for PDH and SDH capacities will have a block size of 1600 to 2000 bits (200 to 250 G.821 error performance parameters are based on measurement of errored bits, giving rise to measurement of a bit error ratio. However, errored bits can only be clearly recognized if the bit sequence being monitored is known, which means the circuit to be measured must be taken out of service while the test pattern is applied.

bytes).

G.826 Monitoring and Measurement

Table 8-1 shows the four error events that G.826 monitors.

Table 8-1.G.826 Monitoring

Measurement of these events requires absolute counts. For practical reasons the preference is to use ratios. Refer to Table 8-2.

Table 8-2.G.826 Measurements

In accordance with the definition of error events, blocks occurring within severely errored seconds are not considered when computing the Background Block Error Ratio (BBER).

Event Description

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.

Severely Errored Second (SES) A one-second period which contains at least 30%

errored blocks or at least one defect.

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

Measurement Description

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.

For all three ratio definitions, only the time during which the transmission system is available is considered when forming the ratios

For the purposes of Recommendation G.826, availability ends at the start of a time interval containing at least ten consecutive severely errored seconds in at least one direction of transmission. The system becomes available again at the start of a time interval consisting of at least ten seconds that are not severely errored. Figure 8-1 shows an example of how availability is determined.

Figure 8-1.Example of Unavailability Determination

G.828 and G.829

While G.828 has essentially the same structure as G.826 in that it is a block based system, it specifies tighter error performance objectives for SDH operation. It introduced a new error parameter, the Severely Errored Period (SEP), which is defined as a period during which at least three but not more than nine consecutive severely errored seconds (SES) occur. The measurement parameter associated with this is the Severely Errored Period Intensity (SEPI). G.828 also defines tighter values for error performance, specifically ESR and BBER.

G.829 builds on G.828 to provide recommendations on error performance events for SDH multiplex and regenerator sections.

Comparing G.821, G.826, and G.828 Error Performance

A direct comparison between G.821 and G.826/G.828 is not possible due to the differences in defining the error events. G.821 is bit error oriented, whereas the others are block based. To enable a comparison requires knowledge of how the bit error ratio relates to the errored block, specifically error distribution versus time. For example, if errors occur in a burst, G.821 would count a number of bit errors, while G.826/G.828 might count only one errored block. If the errors are equally distributed vs. time, then each bit error can produce a block error. It can be verified however, that depending on the error model, the requirements of G.826 and G.828 can be much tougher than G.821.

G.826 data has a 10 second availability latency.

10 sec