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RE: wis scrambler vs 64/66 scrambler


I think Eric was referring to a real difference in the scramblers. 
The 64B/66B scrambler is a self-synchronizing scrambler and the 
WIS scrambler is a side-stream scrambler.


There are two basic types of scramblers. 

A self-syncyhronizing scrambler feeds the xor of the scrambler
terms and the data into the transmit scrambler. That is, the
transmitted stream is also the input to the transmit scrambler
shift register. The receive descrambler feeds the same stream
into its shift register. The advantage of this type of scrambler
is that the receive descrambler becomes synchronized to the
transmit scrambler as soon as n bits (where n is the length
of the scrambler) have been received. The disadvantage of this
type of scrambler is that a single received bit error can cause
an error in the received data each time that bit error hits
a descrambler tap. For example, a single bit error received by
the x^58 + x^39 + 1 scrambler will produce 3 errors in the received
data. By careful selection of the scrambler polynomial with respect
to the CRC polynomial, the CRC coverage can be maintained and the
effect of this error multiplication effect on system robustness

A side-stream scrambler feeds back only the scrambler terms into
its input. The data to be scrambled has no effect on the scrambler
contents. The advantage of this is that it avoids error multiplication.
The disadvantage of this is that some mechanism must be provided
to synchronize the transmit and receive descrambler contents.
For the WIS scrambler, the mechanism is to start the scrambler
with a known seed at the beginning of each Sonet frame. That is why
the WIS clause calls it a frame synchronous scrambler. I notice that
the WIS clause does not describe this since it covers much of the 
scrambler behavior through a reference to the Sonet spec.

I hope this helps. It is a fairly simplified explanation as a great
many considerations go into choosing appropriate scramblers.


-----Original Message-----
From: Ben Brown [mailto:bbrown@xxxxxxxx]
Sent: Wednesday, October 11, 2000 5:52 PM
To: 802.3ae
Subject: Re: wis scrambler vs 64/66 scrambler


The x7+x6+1 scrambler is the SONET scrambler within the WIS.
The x58+x19+1 (or 1+x39+x58 as it is now defined) scrambler,
is within the PCS (64B/66B). They are not the same thing and,
when the WIS is enabled, they are both used.


Eric Ng wrote:
> Hi,
> In the blue book the wis scrambler is x7+x6+1, only the
> 6th and 7th term are xor and feedback to the beginning
> of the 7-bit shift register.
> However, in the 64/66PCS section x58+x19+1 is used but
> this time the scrambled output xor 58th xor 19th and
> feed back to the beginning of the 58-bit shift registers.
> The two equations are very similar but the drawing in the
> bluebook suggested otherwise.  Also, base on the drawing,
> the wis scrambler/descrambler would be the same while the
> 64/66 scrambler and descrambler are different. I must have
> misunderstood some fundamental.  I hope someone could clarify
> this for me.
> Thanks in advance.
> regards,
> --
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