RE: WWDM vs. 10Gb/s serial
- To: firstname.lastname@example.org
- Subject: RE: WWDM vs. 10Gb/s serial
- From: email@example.com (Ed Grivna)
- Date: Mon, 10 May 1999 13:54:21 -0500
- Sender: firstname.lastname@example.org
Paul A. Bottorff wrote:
> I certainly agree that the PHY must provide fast failure detection (<10
> msec). In addition, it would be nice if the PHY layer could inform the
> transmitting end of failures.
> 10 GigE has a broad range of uses for both LAN and WAN applications. The
> design tradeoffs for photonics in the WAN and LAN are different. In the WAN
> a major component of cost is the optical reach. The data which I've seen
> indicates that transmission frequency very significantly affects reach. To
> get the lowest cost solutions 10 GigE should move away form group codes
> systems like 8/10 into scrambler code systems. Scrambling provides an NRZ
> efficient line encode giving 10 gigabits of data at 10 gigabaud.
While scrambling is an efficient way of dealing with data, it is also
non-deterministic in how that data is handled. Given any scrambling
polynomial and an uncontrolled data stream, it is always possible to
zero out the scrambler. Once this happens, there are NO transitions
in the serial stream until new data containing 1's is present to
re-seed the scrambler.
The SMPTE-259 serial interface recognized this problem a long time ago
and requires the characters in the video field to be between 004h
and 3FBh. Even with these limitations, they can wind up with long
repeating patterns of 19 zero bits followed by a single 1-bit (or 19
one bits follwed by a single 0-bit). These signals have a very high
DC content (making it difficult to send through an AC-coupled channel).
An alternate pattern that they see consists of an alternating 20
0-bits followed by 20 1-bits (which generates a square wave). These
are all referred to as the SMPTE pathalogical patterns (see ANSI/SMPTE
RP-178-1996). They are nasty to handle, both for the interface
circuitry AND for the PLLs.
Since there is no control over the content of the data field in Ethernet
packets, and the fact that many records are padded with trailing zeros,
I feel that the usage of a scrambler would not be condusive to good
engineering practices for THIS application. As the data rate is increased,
the time penalty and system-level impact required to recover from
data errors becomes much more significant. If the scrambler ever did
zero out, the PLL would most surely drift by at least one bit.
Once that happend framing must occur again to start processing data.
But frameing is no longer simple either. Unlile a block code (such as
8B/10B) where not all characters are valid and extra chracters are
present for in-band signalling, scrambled codes use all possible
characters. This carries numerous implications.
First, framing must be done using combinations of data characters.
In telecom environments they identify a specific sequnce oc data
characters and the specific period in which they must occur. Once these
characters are found numerous times in that location, framing is
declared to be achieved. Unfortunately, nothing prevents these specific
characters from being part of the data field, where they may also
occur on the same bopundaries.
Since Ethernet is also not based on fixed length records or records that
are sent on a continuous basis on the same boundary, performaing framing
with a scrambled codeing will be quite difficult. Because it requires
multiple recognitons of the pattern to validate the framing, it also
takes a lot of time. All this to handle errors that are GUARENTEED
to happen because of the uncontroled nature of the data streem contents.
This makes the handling of errors much more onerous on the system. Its
bad enough that the lost data may have to be resent, but the system
level impact is much more than just the time necessary to re-transmit.
802.3z chose a block coded interface for mutiple reasons, including those
listed here. Yes there is a penalty in symbol rate to deal with this,
but generally the 20% adder is significantly easier to handle than the
baggage that comes with a scrambled interface.
By changing to a scrambler, this also makes the optics much more difficult
to design and more expensive to build. Most cannot handle even a limited
unbalance in the data stream. They are usually AC-coupled to limit the
noise gain in the receiver. The clock/data recovery PLLs are also more
difficult and must be much more stable.
> Paul A. Bottorff
> Director Switching Architecture, Bay Architecture Lab
> Nortel Networks, Inc