VeEX VePAL TX300, Руководство пользователя

Страница: 391 / 421

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 difficulty 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 protocols. 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.
Go back to top Go back to TOC
Header Setup (FC-2 only)
The FC-2 Frame Header is subdivided into the fields as shown in the diagram below.
TX300/e_e-manual_D07-00-033P_RevG00