Re: 1000BASE-T PCS question
I have some additional comments below:
Jaime Kardontchik wrote:
> For simplicity, I did not mention nor I did include in the
> figures that the 4D encoded symbols are randomized before
> sending them to the transmitters. This procedure is described
> in the 1000BASE-T standard.
I believe that the randomizing is to ensure that the frequency spectrum
transmitted on each pair is well distributed and also even across all
four pairs. If this is the case, scrambling may be applicable to WWDM,
but it may not apply to MAS, Serial TDM, or parallel optics.
> The produce assures that the output levels send down the
> wires (or the fiber) are DC balanced. However, you will not
> get the nice extremely short running disparity that one could
> get with the 8b/10b encoder, since the randomization is based
> on the scrambler. The scrambler used is 33 delays long (much
> longer than the scrambler used in Fast Ethernet) and it is
> expected to produce a better short term balance than the one
> obtained in Fast Ethernet.
DC balance of signaled information in an optical link is required for
one or more of the following reasons. Please note that this may not be
an all-inclusive list:
- To enable AC-coupling onto the medium without distortion or the use of
a separate DC-balancing line code;
- To avoid short-term DC offset which may require the inclusion of DC
level-restoring circuitry which, in turn, affects receiver sensitivity
and dynamic range and make the link more susceptible to low-frequency
- To avoid the data dependent heating of the laser(s);
Note that all of the above affects of DC imbalance affect the
reliability of the link and can, therefore, be considered a link
penalty. The same requirement for DC balance may not exist for UTP as
for optical links. We need to be careful about doing away with this
requirement as it was one of the key reasons that 8B/10B won out over
4B/5B in the early days of Fibre Channel development. Recall that 4B/5B
was used by FDDI at that time and that DC Balance problems were noted
due to the unbounded nature of 4B/5B.
> The clock can be recovered (in the same way as it is recovered in
> Fast Ethernet). Many simulations were run during the development
> of the 1000BASE-T standard and presented during its meetings
> showing that this is the case. There is already a well known
> company that has 1000BASE-T transceivers on Silicon, and
> has shown that they work in the last Interop gathering.
I agree that clock recovery is a second-order concern relative to data
recovery due to the attenuation, SNR, dispersion, etc. at the receiver.
However, it is still a valid PHY architecture concern. In the presence
of gross data-dependent DC imbalance, this second-order effect may
quickly become a first-order concern.
> Jaime E. Kardontchik
> Micro Linear
> San Jose, CA 95131
> email: kardontchik.jaime@xxxxxxxxxxx
Richard Taborek Sr. Tel: 650 210 8800 x101 or 408 370 9233
Principal Architect Fax: 650 940 1898 or 408 374 3645
Transcendata, Inc. Email: rtaborek@xxxxxxxxxxxxxxxx
1029 Corporation Way http://www.transcendata.com
Palo Alto, CA 94303-4305 Alt email: rtaborek@xxxxxxxxxxxxx