So, does this mean it should really be called 80BaseTX/FX because of the
20% overhead introduced by 4B/5B? :)
Prof. Tom Lisa, CCAI
Community College of Southern Nevada
Cisco Regional Networking Academy
Priscilla Oppenheimer wrote:
4B/5B is a signal encoding scheme. It's used in 100BaseTX and
100BaseFX
also, so it's rather important to know. I'm going to have all the
gory
details in an upcoming white paper at
http://www.certificationzone.com.
Here's quick preview.
100BaseFX uses Non Return to Zero, Invert on One (NRZI), as does
FDDI.
100BaseTX uses a variation of NRZI called either NRZI-3 or Multiple
Level
Transition - 3 (MLT-3).
A disadvantage of NRZI and MLT-3 is that a steady stream of zeros,
not
uncommon in data, is represented as no transition, which is
indistinguishable from no signal or a dead link. Another problem with
no
transitions is that the phase-locked loop that the receiving station
uses
to recover the clock signal can drift. If enough drift is introduced,
the
station cannot accurately receive data. To avoid this problem, the
physical
coding sublayer (PCS) first encodes data using 4B/5B translation.
With 4B/5B translation, each possible 4-bit pattern is assigned a
5-bit
code. The PCS maps four-bit nibbles in the data into five-bit codes,
and
vice versa, using a 4B/5B translation table. Every 5-bit code has at
least
two transitions to ensure proper clocking.
Isn't that fascinating! Just kidding! ;-)
Priscilla
At 09:51 PM 4/25/01, William Wong wrote:
>Dear all
>
>I couldn't find any information on this. Can you guys tell me what
is this?
>
>Thanks.
>
>William
>FAQ, list archives, and subscription info:
>http://www.groupstudy.com/list/cisco.html
>Report misconduct and Nondisclosure violations to
[EMAIL PROTECTED]
________________________
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http://www.priscilla.com
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