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BERT or Bit Error Rate Test is a testing method for digital communication circuits that uses predetermined stress patterns comprising of a sequence of logical ones and zeros.

A BERT Tester typically consists of a test pattern generator and a receiver that can be set to the same pattern. They can be used in pairs, with one at either end of a transmission link, or singularly at one end with a loopback at the remote end. BERT Testers are typically stand-alone specialised instruments, but can be Personal Computer based. In use, the number of errors if any are counted and presented as a ratio such as 1 in 1,000,000, or 1 in 10e06.

• QRSS (Quasi Random Signal Source) – A pseudorandom sequence which generates every combination of a 20-bit word, repeats every 1,048,575 bits, and suppresses consecutive zeros to no more than 14. It contains high-density sequences, low-density sequences, and sequences that change from low to high and vice versa. This pattern is also the standard pattern used to measure jitter.
• 3 in 24 – Pattern contains the longest string of consecutive zeros (15) with the lowest ones density (12.5%). This pattern simultaneously stresses minimum ones density and the maximum number of consecutive zeros. The D4 frame format of 3 in 24 may cause a D4 Yellow Alarm for frame circuits depending on the alignment of one bits to a frame.
• 1:7 – Also referred to as “1 in 8”. It has only a single one in an 8-bit repeating sequence. This pattern stresses the minimum ones density of 12.5% and should be used when testing facilities set for B8ZS coding as the 3 in 24 pattern increases to 29.5% when converted to B8ZS.
• Min/Max – Pattern rapid sequence changes from low density to high density. Most useful when stressing the repeater’s ALBO feature.
• All Ones (or Mark) – A pattern composed of ones only. This pattern causes the repeater to consume the maximum amount of power. If DC to the repeater is regulated properly, the repeater will have no trouble transmitting the long ones sequence. This pattern should be used when measuring span power regulation. An unframed all ones pattern is used to indicate an AIS (also known as a Blue Alarm).
• All Zeros – A pattern composed of zeros only. It is effective in finding equipment misoptioned for AMI, such as fiber/radio multiplex low-speed inputs.
• 2 in 8 – Pattern contains a maximum of four consecutive zeros. It will not invoke a B8ZS sequence because eight consecutive zeros are required to cause a B8ZS substitution. The pattern is effective in finding equipment misoptioned for B8ZS.
• Bridgetap - Bridge taps within a span can be detected by employing a number of test patterns with a variety of ones and zeros densities. This test generates 21 test patterns and runs for 15 minutes. If signal error occur, the span may have one or more bridge taps.
• Multipat - This test generates 5 commonly used test patterns to allow DS1 span testing without having to select each test pattern individually. Patterns are: All Ones, 1:7, 2 in 8, 3 in 24, and QRSS.