At 11:25 AM 2/1/01, =?ks_c_5601-1987?B?w7W4rr7IuN7Azw==?= wrote:
>in fddi, what is the charateristics of 4b/5b encoding?
>cisco www show me a little information.. that 4b/5b is used in multi-mode
>fiber over fddi or atm..
>and that is a encoding scheme.. and support speed up to 100Mbps..on
>multimode fiber..
>I just know some more characteristics about 4b/5b enconding over fddi or atm..
Encoding happens at the PHY layer, which is sandwiched between the Physical
Medium Dependent (PMD) layer below and the Media Access Control (MAC) layer
above. I think ATM is far enough up the layers, that a question about ATM
encoding doesn't make sense. In the case of FDDI, however, the question
makes sense. FDDI's MAC layer depends on the PHY layer to encode bits using
4b/5b for sending over interfaces defined at the PMD layer.
4b/5b coding is a way of encoding ones and zeroes along with clocking
information. The shorthand notation of 4b/5b means 4 bits are encoded into
5 code bits. In high-speed networks, it is almost always necessary to
encode data if there is no "master clock" and no separate clocking signal.
There are many ways of doing this. Original 10 Mbps Ethernet, for example,
used Manchester encoding. Fast Ethernet uses 4b/5b when fiber-optic cabling
is used. Gigabit Ethernet uses 8b/10b. T1 WAN circuits use Bipolar with 8
Zeros Substituted (B8ZS).
Zeros cause a problem when clocking is embedded in the signal. Too many
zeros are indistinguishable from no signal. FDDI deals with this by
substituting each 4-bit "nibble" with a 5-bit nibble that is guaranteed not
to have too many zeros. So, 0000 becomes 11110, for example. 0001 becomes
01001. And so. The senders and receivers use a table lookup to encode and
decode all 4-bit values. Sounds inefficient, eh? Well, it is. But the other
way to look at is that FDDI is really 125 Mbps.
Priscilla
>could you give me those?
>thanks.
>
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Priscilla Oppenheimer
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