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RE: Comma Characters


The 1000BASE-X protocol is designed to make almost all the commas it emits
be comma+. As you can see from the following passage from, the
first idle at the end of a packet restores the disparity to negative and all
following idles end with negative disparity:

The /I1/ordered_set is defined such that the running disparity at the end of
the transmitted /I1/is opposite that of the beginning running disparity. The
/I2/ordered_set is defined such that the running disparity at the end of the
transmitted /I2/is the same as the beginning running disparity. The first
/I/following a packet or Configuration ordered_set restores the current
positive or negative running disparity to a negative value. All subsequent
/I/s are /I2/to ensure negative ending running disparity.

So, the first K28.5 after a packet may contain comma- or comma+, but all the
rest will be comma-. While sending configuration ordered sets, the K28.5s
will alternate between odd and even but that won't be true during idle.
Since sync is suppose to be able to be reacquired without going into
configuration, a SERDES has to be able to lock on comma+ but isn't
necessarily required to acquire lock on comma-. As states, the
1000BASE-X coding is designed to ensure that comma+ is transmitted with
equivalent or greater frequency than comma-.

When we are sending idles and are between A's and not sending any ordered
sets, our K28.5 disparity will alternate and we will have a 50/50 mix of
comma+ and comma-. Still we are less heavy in comma+ and we should mention
that difference.


-----Original Message-----
From: Rich Taborek [mailto:rtaborek@xxxxxxxxxxxxx]
Sent: Wednesday, December 13, 2000 11:20 AM
Subject: Re: Comma Characters


The history is that some old SerDes initially designed for Fibre Channel
but slated for use in Gigabit Ethernet only supported one version of
comma. I believe that you are correct in stating that the specific
version was the positive comma version, also referred to as comma+ and
corresponding to the bit pattern 0b0011111. The Gigabit Ethernet
1000BASE-X PCS protocol is designed to emit both comma versions in order
to be "friendly" to all SerDes parts.

Clause 48, 10GBASE-X PCS is specified to statisitically emit an equal
number of both comma versions. The PCS implicitly requires the
generation and detection of both comma versions. The big difference
between 10GBASE-X and 1000BASE-X is that the 10GBASE-X does not require
comma detection in the PMA.

Personally, I don't believe that anything needs to be added to the
Clause 48 to clarify this point since it is the "obvious" way that an
8B/10B protocol should work. Please go ahead and submit a comment if you
feel otherwise.  

Best Regards,

Brian Cruikshank wrote:
> In Clause on page 261, the /COMMA/ is referred to being
> defined in clause 36.
> In this section, both a positive and negative comma are defined.
> I believe that in 1 GE devices, usually only positive commas were
> recognized.  Is this enough to be 1 GE and 10 GE compliant?
> In a IPG over 20 bytes, both commas will probably exist.  In
> sustained minimum IPG, the positive comma occurrence may be random.
> Do positive commas occur often enough?
> Maybe this detail should be stated?
> /Brian Cruikshank
Richard Taborek Sr.                 Phone: 408-845-6102       
Chief Technology Officer             Cell: 408-832-3957
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