Re: 64/66 system benifits and ad-hoc agenda
Hi Rick,
I totally agree with you. I understand the motivation behind a
low-overhead
block code for serial links: increase operating distance on fibers, relax
specs on the
optics, low latency.
The Hari interface is a perfect match (probably was designed) for a WWDM
transceiver.
Unless the serial transceivers use simple 4:1 bit-wise muxing of the 4
lanes of Hari (which
would destroy the structure of the 8b/10b code and seems a wasteful method)
the use of
any other code requires many extra operations such as decoding/recoding
8b/10b,
deskewing bytes, encode/decode (with new code), rate conversion + FIFO,
etc.
But if the 8b/10b code is already set in Hari, then we'll have to live with
it I guess!
Thanks,
Kamran
Rick Walker wrote:
> Dear Kamran,
>
> > is the 64b/66b a proposal for line code in serial link, or could be a
> > replacement of 8b/10b also in the Hari interface? It seems like the
> > 8b/10b code is well accepted as the right choice for Hari. Is this
> > still flexible or set in stone (because many companies have already
> > started designing their high-speed I/O)?
>
> The 64b/66b code is motivated by the relative difficulty of pushing
> existing lasers system to run at 12.5 Gbaud vs 10.3 Gbaud. At the
> higher speeds we have difficult technical challenges in many areas such
> as skin loss, dielectric loss, laser relaxation-oscillation time
> constants, etc.
>
> The 8b/10b code has a 25% overhead rather than the 3.125% of the
> 64b/66b, but in return has better control over transition density and
> disparity. At speeds below ~5 Gb/s the overhead of the 8b/10b code can
> be ignored with little practical consequence.
>
> Although a code like 64b/66b could be substituted for the 4x 2.5Gb/s
> HARI links, there is little incentive because the 3.125 Gbaud
> transmission of 8b/10b is easily supported both by CMOS and by the FR-4
> PCB infrastructure.
>
> Best regards,
> --
> Rick Walker