Re: 64B66B bit ordering
Birdy,
I tend to like the first proposal but I can follow the
second. Also, I think the second is the one that matches
most closely the mapping you have in the presentation.
Changing now would probably cause more confusion.
More confusing is maping the Z fields into the frame.
You need to be very explicit in showing exactly how
the bits of each Z field maps into the frame. My suggestion
would be to pick a method for mapping the D fields into
the frame then show exactly how the Z fields map into
the frame in the exact same format.
Your "Code Summary" slide is a great picture of how the
D frames and the Z frames map to each other. What's missing
is exactly the order of the bits. I've attached a picture.
See what you think.
Ben
"Bharadwaj (Birdy) Amrutur" wrote:
>
> Rick,
> >
> >Let me try again. I'm referring to the order of the words
> >received from the XGMII.
> >
> >Let's call the 32-bit word transferred on clock tick N,
> >WORD[N] and the word transferred on clock tick N+1,
> >WORD[N+1]. In your picture on the slide titled "Bit ordering
> >sequence", I would say that WORD[N] would wind up in
> >BYTE3 thru BYTE0 and I would say that WORD[N+1] would wind
> >up in BYTE7 thru BYTE4. Do you agree?
> >
> >In your "State machine notation conventions" slide, your
> >definition of tx_tobe_coded states: "the most recently
> >received TXD word in the 35:0 bit locations". To me, the
> >"most recently received TXD" would correspond to WORD[N+1]
> >in my definition above.
> >
> >I guess what I'm missing is a bit to byte map. I picture
> >bits [35:0] corresponding to BYTE3 thru BYTE0 but these
> >should hold WORD[N].
> >
> >Thanks,
> >Ben
>
> Ben,
> Hopefully the following will clarify the picture.
> Consider the XGMII data flow:
> (pg 8: http://grouper.ieee.org/groups/802/3/10G_study/public/nov99/frazier_1_1199.pdf)
>
> lane# / time -> N N+1
> -------------------------
> 0 D0 D4
> 1 D1 D5
> 2 D2 D6
> 3 D3 D7
>
> We have two ways of describing what to do next (ignoring control
> flag processing)
>
> Description method 1
> ------------------------
> Assemble into a 64 bit register
>
> _______________________________________________________
> | |..| | |..| | |..| | |..| | |..| | |..| | |..| | |..| |
> -------------------------------------------------------
> 7 0 7 0 7 0 7 0 7 0 7 0 7 0 7 0
> D7 D6 D5 D4 D3 D2 D1 D0
>
> Add the two bit prefix
>
> 65 .................. 0
> ___________________________________________________________
> | |..| | |..| | |..| | |..| | |..| | |..| | |..| | |..| |1|0|
> ------------------------------------------------------------
> 7 0 7 0 7 0 7 0 7 0 7 0 7 0 7 0
> D7 D6 D5 D4 D3 D2 D1 D0
>
> Now serially shift from bit 0 through bit 65.
>
> Advantage: The 66bit register holds the data in the order it will
> be shifted.
> Disadvantage: The data is shifted from right to left, while a human
> would read octets from left to right.
>
> Description method 2 (pg 7:http://grouper.ieee.org/groups/802/3/ae/public/may00/walker_1_0500.pdf)
> --------------------
> Assemble into a 64 bit register
>
> _______________________________________________________
> | |..| | |..| | |..| | |..| | |..| | |..| | |..| | |..| |
> -------------------------------------------------------
> 7 0 7 0 7 0 7 0 7 0 7 0 7 0 7 0
> D0 D1 D2 D3 D4 D5 D6 D7
>
> Add the two bit prefix
>
> 65 .................... 0
> ____________________________________________________________
> |0|1| |..| | |..| | |..| | |..| | |..| | |..| | |..| | |..| |
> ------------------------------------------------------------
> 7 0 7 0 7 0 7 0 7 0 7 0 7 0 7 0
> D0 D1 D2 D3 D4 D5 D6 D7
>
> Now serially shift bit 65, 64. and the octets from the left
> to right. But within each octet, shift from 0 .. 7.
>
> Advantage: Octet shifting matches with our tendency to read from
> left to right.
> Disdvantage: Describing the shifting becomes a bit more complicated.
>
> User feedback will help identify the most convenient representation.
>
> Regards,
> Birdy
--
-----------------------------------------
Benjamin Brown
Router Products Division
Nortel Networks
1 Bedford Farms,
Kilton Road
Bedford, NH 03110
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bitorder.pdf