Re: Interface reality check
Roy,
As an example consider the following segment of an octet stream:
DDDTZZZZZZZZZZZZZZZZZZZZSDDDDDDD
This gets coded up as (conceptually)
10_DDDTZZZZ 10_ZZZZZZZZ 10_ZZZZZZZZ 10_SDDDDDDD
So the 32 octets at the input gets coded into 33 octets at the output. Exactly
how
your 20 byte idle gets coded depends really on where it is embedded in the
input
octet stream. But yes, the 20 byte idle is spread across 3 66-bit frames. But
there
is no wastage of bits in these frames as any left over space is allocated to
the tail
of the preceding data packet or the head of the succeeding data packet.
Incidentally, I believe the only other possible scenario for a 20 octet
IPG, which is valid
under the currently proposed XGMII is:
DDDDDDDTZZZZZZZZZZZZZZZZZZZZSDDD
This gets coded up as (conceptually)
10_DDDDDDDT 10_ZZZZZZZZ 10_ZZZZZZZZ 10_ZZZZSDDD
Regards,
Birdy
ps: I had thought that the minimum IPG was 12 octets. I assume the 20 octet
number you used was merely for discussion purposes.
Roy Bynum wrote:
> Bhardwaj,
>
> You are right. I should have said 256 input bit boundaries produces 264
> output bit boundaries, which as you pointed out, becomes octet aligned only
> on even 32 byte boundary input data. Thank you for the correction.
>
> As a point of question, what happens to the IPG under 64B/66B? Does the 20
> byte idle become a control character 33 bytes long?
>
> Thank you,
> Roy Bynum
>
> ----- Original Message -----
> From: Bharadwaj S Amrutur <bharadwaj_amrutur@xxxxxxxxxxx>
> To: Roy Bynum <rabynum@xxxxxxxxxxxxxx>; <stds-802-3-hssg@xxxxxxxx>
> Sent: Wednesday, April 12, 2000 7:27 PM
> Subject: Re: Interface reality check
>
> > Roy,
> > A small arithmetic error:
> > Since 64/66 converts 64 bits to 66 bits, I assume you really want to
> > say that the output is octet aligned only on 32 octet boundaries
> > of the input (i.e. it outputs 33 octets for every 32 octets).
> >
> > I am still trying to understand your concern, so please bear with me as I
> > think "aloud".
> >
> > I view the sonet payload as a bucket of so many octets with which
> > I need to transport a sequence of 66 bit words. I can have a 33 octet
> > buffer into which I write in 66 bit chunks from one end and read from
> > the other end in octet chunks, 33 times ( I need to phase lock
> > 66bit word clock/4 to byte clock/33).
> > Isn't this simple, or are there other issues I am not considering?
> >
> > I believe, the XGENIE proposal envisions itself to be in between the
> > XGXS (or is it RS ?) and the PCS - I might have got the layer names
> > wrong, but essentially they want to avail of the inter packet gap and
> insert
> > some control octets. Since this is before the PCS layer, 64/66 can code
> > this up. You can refer to Rick Walkers posting on how he plans to do
> that -
> > its buried somewhere in the morass of posting.
> >
> > Thanks
> > Birdy
> >
> >
> > Roy Bynum wrote:
> >
> > > Bharadwj,
> > >
> > > I may help with an observation. While the input of the 64B/66B encoding
> is
> > > a serial octet stream, the output is not. The output of 64B/66B
> encoding
> > > achieves octet alignment only on input data frames at 256 octet
> boundaries,
> > > which outputs to 264 octet boundaries. For a LAN only PHY which does
> not
> > > have framing boundaries, that is not an issue. For the WAN compatible
> PHY
> > > with a fixed synchronization frame payload, being on octet boundaries
> with
> > > the encoded data is an issue. This might also be an issue with XGENIE,
> > > which if I understood correctly, also used fixed octet payload
> boundaries.
> > >
> > > Thank you,
> > > Roy Bynum
> > >
> > > ----- Original Message -----
> > > From: Bharadwaj S Amrutur <bharadwaj_amrutur@xxxxxxxxxxx>
> > > To: <stds-802-3-hssg@xxxxxxxx>
> > > Sent: Wednesday, April 12, 2000 1:44 PM
> > > Subject: Re: Interface reality check
> > >
> > > > Hello Paul, Dave,
> > > >
> > > > I guess I am slightly confused by your arguments.
> > > >
> > > > I believe 64/66 provides a general (almost - see *)
> > > > mechanism for transporting a serial octet stream which
> > > > consists of contiguous octets(bytes) of data separated by
> > > > contiguous stream of non-data octets. It doesnt really impose
> > > > any constraint on how you interpret the packet of data,
> > > > except that if the packet of data is CRC32 protected in ethernet
> > > > style, then its 3-bit detection strength is not degraded.
> > > > (One can easily determine/verify which other CRCs share this
> > > > special relationship with 64/66 - there are two scrambler polynomials
> > > > in consideration for 64/66 and maybe one is friendlier to many more
> > > > CRCs?)
> > > >
> > > > So my question is, why should there be any problem in
> > > > interpreting/allowing/encapsulating other format types within
> > > > this framework?
> > > >
> > > > Isnt the main issue then how important the 3.125% overhead is for
> > > > WAN transport?
> > > >
> > > > Please tell me if there are other issues I have misunderstood or
> > > > if the limitations in (*) below, are severe enough to curtail its
> > > > applicability in other areas.
> > > >
> > > > I would also appreciate if you can share any insights on burst error
> > > > statistics.
> > > >
> > > > Thanks,
> > > > Birdy
> > > >
> > > > * : 64/66 is really tuned to 10GE-standards proposals in the
> following
> > > > ways:
> > > > 1) Data octet streams should start on a special quad octet boundary.
> > > > 2) It allows for only 8 different non-data octets with 3-bit
> protection
> > > >
> > > > to be transported and four types of ordered sets.
> > > > 3) Verification of nondegradation of 3-bit detection strength of
> > > > ethernet
> > > > CRC32 has been done.
> > > >
> > > > other constraints
> > > > 4) Data stream must be atleast 8 octets long.
> > > > 5) Non data stream must be atleast 11 octets long
> > > > ( I might be off a bit in the above two numbers)
> > > >
> > > > -----------------------------------------------------------
> > > > >Rick:
> > > >
> > > > >Using the Ethernet TYPE field does not work in a practical design.
> > > > First,
> > > > >the effect of errors in the TYPE field will alter the location of the
> > > > CRC,
> > > > >in effect producing a huge burst error. The large error will break
> the
> > > > >misdetected error probabilities. Second, the Ehternet TYPE can not
> > > > >eliminate the overhead of the SA and DA before generating a new
> frame.
> > > >
> > > > >Cheers,
> > > >
> > > > >Paul
> > > > -----------------------------------------------------------
> > > > >Rick,
> > > >
> > > > >Your suggestion amounts to mapping L2 payloads into another L2
> payload
> > > > >(i.e. Ethernet MAC frames) prior to mapping into SONET/SDH. This
> would
> > > > >make the SONET/SDH ANSI/ITU standards activity dependent on IEEE.
> > > > >Such a cross-coupling would hinder progress in both IEEE and ANSI/ITU
> > > > >and is therefore undesirable.
> > > >
> > > > >...Dave
> > > >
> >
> > --
> > Bharadwaj Amrutur
> > Agilent Technologies
> > 3500 Deer Creek Road, MS 26U-4
> > Palo Alto, CA 94304-1392
> > USA
> >
> > Phone : (650) 813 3357
> > Fax : (650) 857 3637
> > Email : bharadwaj_amrutur@xxxxxxxxxxx
> >
> >
> >
--
Bharadwaj Amrutur
Agilent Technologies
3500 Deer Creek Road, MS 26U-4
Palo Alto, CA 94304-1392
USA
Phone : (650) 813 3357
Fax : (650) 857 3637
Email : bharadwaj_amrutur@xxxxxxxxxxx
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