VeEX MTX150 Series, Руководство пользователя

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FC-2 BERT - Payload Setup
BERT Testing Tips
A BERT samples every incoming bit and looks for something that doesn't occur often. This traditional method
typically used in SONET/SDH measurements, can however take a very long time. For example, in a 1Gbps
Fiber Channel system, errors occur on average once every 1000 s (about 17 Min) for 1x10
–12
BER, so you
would need to detect at least 10 to 100 errors before you can have confidence in your measurement. Bear in
mind that for a quick measurement, you need a test pattern that repeats frequently. A PRBS-11 sequence
(2047 bits) repeats many times a second at a 1-Gbps rate, however a PRBS-31 pattern, with 2 billion bits,
repeats only every 2 s at 1 Gbps.
A general rule of thumb is to choose a PRBS that is closest to the nature of the data you will be passing through
your network. Patterns between 2
11
–1 and 2
31
–1 (such as 2
15
–1 and 2
23
–1) offer good gradual steps in dif-
ficulty that allow you to see where networks fail, or how much margin you have beyond pass/fail thresholds.
Bit errors can affect the data frames - these frames will be re-transmitted at the request of the upper-layer pro-
tocols. If the FC link suffers a lot of bit errors, you may experience a slight performance loss. These bit errors
can also affect the Receiver Ready (R_RDY) messages. A R_RDY is never repeated, so the buffer credit is
one BB_Credit short until the link is reset.
The Fiber Channel standard allows a 1 x 10E-12 maximum error rate.
Header Setup (FC-2 only)
The FC-2 Frame Header is subdivided into the fields as shown in the diagram below.