Principles of Bit Errors – II

Bit Error Performance Events & Related Alarms

Through the BBE event, whether the receiving side of local end detects bit error and whether the problem lies in the channel between the sending of remote end and receiving of local end can be identified. Through the FEBBE event, whether the receiving side of remote end detects bit error or whether the problem lies in the channel between the sending of local end and the receiving of remote end can be identified.

Corresponding to three bit error remote performance events MSFEBBE, HPFEBBE and LPFEBBE, there are three bit error remote alarm events, which are multiplex section remote error index MS-REI, higher order path remote error index HP-REI and lower order path remote error index LP-REI. By observing these remote alarm events, we can identify whether the remote end detects bit error.

When the bit error exceeds the preset performance threshold, the bit error threshold crossing/degradation alarm event will be reported.

Bit Error Performance Detection Bytes in SDH

In the SDH frame structure, the overhead bytes B1, B2, B3 and V5 are used to monitor the bit errors of regenerator section, multiplex section, higher order path and lower order path respectively. Bit error monitor adopts the even parity of Bit Interleaved Parity (BIP). For the bit error detection point and its correspondence with the remote bit error indication, refer to the alarm signal flow. Below table lists the overhead bytes of the bit errors.

Termination Points of of Bit Error Performance

The pass through and termination of different sections are different. e.g. it is different for REG Section and MUX section. Similarly, it is different for different type of paths i.e. Higher Order Path or Lower Order Path.

Bit Error Testing

BER testing methodology and testing tools/equipment are used for testing that how much data has been corrupted during the transmission. These testing methods also indicate much about the link quality and the ability of the system to accommodate the link characteristics. So that we know in advance about a link/medium before we put our actual data on it.

The idea behind the BER Testing is very simple. We send some data over the link/medium to be tested & then compare the results with original data. This comparison then tells us how much data has been corrupted/lost during the transmission.

In order to shorten the time required for measurements, a pseudorandom data sequence is used because as the error rates goes low, it takes longer for measurements if any degree of accuracy is to be achieved. For Gigabit Ethernet (GE) that specifies an error rate of less than 1 in 10^¹², the time taken to transmit the 10^¹² bits of data is 813sec. To gain a reasonable level of confidence of the bit error rate it would be wise to send around 100 times this amount of data. This would take about 22hours. It is clearly not convenient to have measurements taking this long. Accordingly to assist making measurements faster, mathematical techniques are applied and the data that is transmitted in the test is made as random as possible – a pseudorandom code is used that is generated within the bit error rate tester. This helps reduce the time required while still enabling reasonably accurate measurements to be made.

BERT (Bit Error Tester) is a commonly used tool for Testing. BER Testing requires a bit generator or a test pattern generator, and a receiver, which is used to compare that pattern. The pattern generator sends a bit stream (stimulus) to the Device Under Test which then responds back with another bit stream. The receiver compares the actual response from the Device Under Test with the expected response which is provided by the user. The calculated result is the ratio of errored bits to the total number of transmitted bits usually shown in an exponential form, such as 2^^B, where B is the BER ratio.

There are simply two methods to test Bit Errors using BERT Meter:

End to End Test (Interrupts Service)  Use meter to stop the service for test. Use the pseudo-random binary sequence (PRBS) to replace thee service. The service can be tested bit by bit. So, the error detection capability of stopping service is better than that of online test

Online Test (No Interruption to Live Service)  The online test without interrupting service is made when the tested system enables the services. Generally, the SDH system realizes the online test with its abundant overhead bytes. The meter adopts the multiplex framing structure of different bit rates to test the overhead of path or device, parity check bit and alarm bit state, thus to test the transmission performance without interrupting services

Summary

BER (Bit Error Rate Testing) is a powerful tool for testing of Transmission Systems. Design Phase is the most important point in a project in which we have to take extreme care about BER. A nominal & controlled BER ensures smooth Network Performance in later phases while a poorly designed network always results in bad and de-stabilised service quality. This mainly depends upon the balanced recipe of interference, Transmitter Power, Lower order modulation and Bandwidth.

Principles of Bit Errors – I