[130544] in North American Network Operators' Group
Re: A New TransAtlantic Cable System
daemon@ATHENA.MIT.EDU (Chris Tracy)
Tue Oct 5 13:05:28 2010
From: Chris Tracy <ctracy@es.net>
In-Reply-To: <AANLkTimCJ4hRGMdqA539gXpy-js5hqMXnNKKY2RvbGai@mail.gmail.com>
Date: Tue, 5 Oct 2010 13:05:14 -0400
To: Heath Jones <hj1980@gmail.com>
Cc: nanog@nanog.org
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
Heath,
> I just had a thought about EFDA - please forgive my lack of
> terminology though, i'll try to explain:
> Say you have signal coming in to EFDA, the signal is just amplified
> (as you said, also noise - the whole source signal).
> Would it be possible to extract via PLL or similar the source clock
> and use that to modulate the amplifier power?
> Does it work with QPSK / whatever keying is used?
> Would that even help with the noise issue at all, or am I waaaaay off?
Although you can amplify just a single wavelength with an EDFA (has to =
be in the 1550nm range, not 1310nm), most deployments are using EDFAs in =
a DWDM environment. The C-band alone consists of ~5THz (5000GHz) of =
spectrum between 191.00-195.95 Thz. Some systems pack 40 wavelengths =
into this space at 100GHz spacing, some 80 channels @ 50GHz spacing, =
others 160 @ 25GHz. Each of these signals is independent, they can each =
be using different modulation/bitrate/etc. The amplifiers are =
completely ignorant to what is going on with each channel, only the =
devices performing conversion back to the electrical domain need to care =
about these details (after the incoming light has been demultiplexed =
into individual signals, of course).
Re: amplifier power... Amplifier gain should really stay constant =
unless new wavelengths are added/removed from the fiber. There are =
fixed-gain and variable-gain amps. VGAs have the advantage that =
engineers do not need to manually re-balance power levels whenever a =
large number of wavelengths are added or removed from a span, they =
adjust automatically. Newer DWDM systems should all have VGAs whereas a =
lot of earlier generation DWDM systems still use fixed-gain amps. With =
the older fixed-gain amps, you had to have the input power just right -- =
hence the need to re-balance if your aggregate signal changes a lot -- =
too low and the EDFA would not kick on at all, too high and you'd =
saturate the amp.
-Chris
--
Chris Tracy <ctracy@es.net>
Energy Sciences Network (ESnet)
Lawrence Berkeley National Laboratory
