Agilent Technologies N4906B Serial BERT User Manual page 21

Setting up Patterns
What is PRBS?
The most common type of industry-standard pattern is the Pseudo-
Random Binary Sequence (PRBS). PRBSs have a number of desirable
properties, including the ability to simulate random data with a
balanced number of 1's and 0's.
They are referred to by their pattern lengths, which are powers of 2.
Longer pattern lengths give a better approximation of random data
and provide a more rigorous test for the device. However, the pattern
repetition time is significantly longer.
PRBS patterns provide a means to simulate the type of traffic that a
system is likely to see: random traffic. They also enable you to stress
your system to its limits, because large patterns that include long
strings of 1's or 0's are more stressful.
The patterns are easy to generate and can measure at high speeds,
because they are sent repeatedly and can be predicted. Pattern
synchronization, for example, is very fast. When you compare the
sequence with itself, you get 50% errors in all positions, except when
the pattern is exactly aligned.
Hardware-generated PRBSs are non-inverted PRBS 2^n –1 patterns
that allow high-speed measurement of random traffic. They enable
stressful testing of devices, because large patterns contain long strings
of 0's and 1's.
Software-generated PRBSs are non-inverted PRBS 2^n patterns. They
are also called PRBN patterns. These patterns are useful for
applications where the pattern length must be an even number. The
Serial BERT provides variations of software-generated PRBS patterns,
allowing you to set a pattern's mark density (ratio of 1's) and zero
substitution. These features make it easier for you to run specific
stress tests on your DUT.
The following figure illustrates the PRBS-based patterns available with
the Serial BERT:
PRBS
2^n PRBS 2^n-1 PRBS
Software-Generated Hardware-Generated
Pure 2^n PRBS Mark 2^n PRBS
2^n PRBS Density Zero Substitution
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Agilent N4906B Serial BERT, User's Guide, October 2005