[130526] in North American Network Operators' Group
Re: A New TransAtlantic Cable System
daemon@ATHENA.MIT.EDU (Chris Tracy)
Tue Oct 5 10:09:42 2010
From: Chris Tracy <ctracy@es.net>
In-Reply-To: <AANLkTi=9a-zO9dERVtFr4c7w2UWo0D6h7KkmqhCOeTDw@mail.gmail.com>
Date: Tue, 5 Oct 2010 10:09:23 -0400
To: Heath Jones <hj1980@gmail.com>
Cc: nanog@nanog.org
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
Heath,
>> By the way, my recollection is the undersea regenerators do purely =
optical regeneration.
>> There is no O-E conversions undersea, only at the landing stations =
and terrestrial components.
>=20
> I'm not clever enough to know of some way that you could do optical
> regeneration without converting the signal to electrical and
> retransmitting back as optical.. How is that done?
Erbium Doped Fiber Amplifiers (EDFAs) do not re-shape or re-time the =
signals (the last 2 R's in 3R -- re-amplification, re-shaping, and =
re-timing). Raman is another popular amplification technology, widely =
used in long-haul WDM. Some systems have the flexibility of using EDFA =
and Raman amps on the same spans.
EDFAs amplify a band of spectrum (C- and/or L-band, depending on the =
device) -- signal *and* noise. The amplified noise floor is clearly =
visible if you connect an optical spectrum analyzer to the output of an =
EDFA -- you see a big wide bump across the entire amplified band with =
spikes for each wavelength. An optical signal can only go through so =
many EDFAs before it becomes too degraded to be accurately converted =
back to an electrical signal by the receiver -- either due to dispersion =
(especially if uncompensated) or noise, tolerances of which are =
different for every device...(EDFAs introduce some amount of noise, so =
OSNR before EDFA !=3D OSNR after EDFA :-) )
That being said, one can find examples of all-optical regeneration [1], =
but I do not know of any transport vendors who have integrated this =
capability into currently shipping products. (Some have developed =
various tricks like electronic dispersion compensation, but IIRC, these =
work by pre-distorting the signal.)
Getting back to the original post from this thread -- when I first read =
it, I immediately wondered whether the vendor might be using coherent =
optical receivers which have much higher dispersion tolerances, allowing =
the optical signal to travel much further without OEO conversions (see =
[2] and [3] for some background). Unfortunately, I could not find any =
evidence one way or the other about what Hibernia is doing. =20
In fact, Per Hansen from Ciena just so happens to be talking about =
coherent receiver technology [DP-QPSK encoding & DSP analysis] as I =
write this e-mail...
Cheers,
-Chris
[1] 3R optical regeneration: an all-optical solution with BER =
improvement, =
http://www.opticsinfobase.org/abstract.cfm?URI=3Doe-14-14-6414
[2] Coherent receivers enable next-generation transport, =
http://www.lightwaveonline.com/about-us/lightwave-issue-archives/issue/coh=
erent-receivers-enable-next-generation-transport-53426202.html
[3] Optical hybrid, http://en.wikipedia.org/wiki/Optical_hybrid
--
Chris Tracy <ctracy@es.net>
Energy Sciences Network (ESnet)
Lawrence Berkeley National Laboratory
