Subject: Re: draft-ietf-pppext-sonet-ds-00

From: Stuart Venters <[email-masked]>
Date: Wed, 26 Nov 97 14:18:56 CST

Bill,

Let me answer some of your low level Sonet physical link  questions to try
to get to an agreement on what a  multi-owner Sonet system expects.

>I do not understand the latter "level control" with regard to "constant
>valued bits".  Reference?

For a reference i would suggest looking at Sonet transmitter data sheets.

Since
laser diodes change over time and temp, most transmitters include a 
system to keep the output power constant as the laser changes.  This
includes a photodiode to monitor the actual transmitted power and a
feedback controller which controls the DC laser bias current and the AC 
modulation amplitude.  The goal of the controller appears to keep the
average output power constant over some averaging interval.  Hence my
mention of 'transmit automatic level control'.

The effect of the averaging time constant is that the transmitter is
effectively AC coupled with limits on how well it works given data
streams with low frequency content.  Enclosed is a typical warning
from a tranceiver spec:

"The module input will require a 50% duty cycle or dc balanced signal
for normal operation.  Failure to provide this may cause the optical
parameters to move out of specification.  Extinction ratio and duty
cycle distortion may be affected.  The low frequency cutoff of the 
transmitter control loop is < 20Khz."

Typical receivers appear to also be AC coupled resulting in warnings
like this one:

"The postamplifier is ac coupled to the preamplifier... The coupling
capacitor is large enough to pass the SONET/SDH test pattern at the
rated speed without significant distortion or performance penalty.
If a lower signal rate, or a code which has significantly more low
frequency content is used, sensitivity, jitter and pulse distortion
could be degraded."

>How does adding transmit power affect the bit timing?

If you have lots of power at the receiver, the eye pattern has a wide opening
and you can get away with a less perfect recovered bit timing clock.

> You mean for prophylactic octet stuffing?
Yes.

>> What you a really measuring is now much margin there is in the Sonet link.
>> What you are actually setting is how much margin to use when the nasty
>> sequence goes by.
>>
>No, the power margin is not affected in any way that I know.

The receiver data sheet quote above said 'sensitivity'.

It looks to me like fully compliant transeivers on
the market (from the best vendors) are not designed to support low frequency
components in the bit stream.  I suspect this is par for the Sonet
infrastructure you are targeting to run over.

Do you agree that given the right nasty packet, RFC-1619 with prophylactic
octet stuffing causes on the fiber bit streams with more low frequency
components than any other current official Sonet/SDH payload?

Sincerely,
Stuart Venters
(The views expressed do not necessarily reflect those of my employer.)