Nope, the new fiber will be high dispersion compared to the old fiber and also will not have dispersion compensation along its length. The old cable was designed for OOK signaling. Previously optical signaling was 1 bit per symbol OOK (on-off-keying) and direct detection meaning you did not have any optical phase information to make dispersion corrections so the dispersion compensation was done in a specially designed fiber with the opposite sign slope for dispersion (DCF).
This fiber was also relatively high loss and has a narrower core which leads to higher nonlinearities in the link. The old cable tries to keep the dispersion within the range that OOK technologies can operate error-free (post FEC) so there's a lot of it typical at each repeater (EDFA). The newer coherent optical technology can transmit multiple bits per symbol (BPSK, QPSK) by encoding the bits in the optical phase. Since the phase is recovered at the receiver the dispersion accumulated in the fiber can be undone in DSP with a long enough FIR filter. So the need for dispersion compensation is gone with coherent optical. Taking out the DCF also reduces loss along the link reducing EDFA (amplifier) count and increasing spacing. Also nonlinear penalties on the newer higher dispersion fiber are lower which improves something called cycle slips that can punch through the FEC and cause you to take post-FEC errors.
The net result is that you should be able to transmit QPSK at 32GBd in 2 polarizations in maybe 80 waves in each direction.
2bits x 2 polarizations x 32G ~128Gb/s per wave or nearly 11Tb/s for 1 fiber. If this cable has 6 strands, then it could easily meet the target transmission capacity.
That jumped out at me too. My wild guess is that the older fiber is a 10G system, the new one is 40G.
From [1]
> Unity cable system consists of eight fiber pairs, has design capacity up to 7.68 Tbps, with each fiber pair operating at 96x10G DWDM system.
From [2]
> the SJC cable system consists of 6 fiber pairs with the initial design capacity of 28 terabits per second,
Taking a guess that there are 96x40Gb/s x 6 fibers gets you to 23 Tb/s, so in the right ballpark. (Wavelength spacing on a fiber is different between 40G and 10G, so this is a bit of a shot in the dark.)
Caveat: 40G used to be near and dear to my heart (Big Bear Networks), so everything pretty much looks like that nail to my hammer.
Nope this will be 100G per wave not 40G. 40G was a stop-gap technology that never really shipped in large volume. With the advent of coherent optical, everyone just went to 100G (eg. Infinera, Ciena, Alcatel-Lucent)
EDIT: One caveat, depending on a particular link many of these systems will run at half-rate. A lot of legacy cables today are running BPSK at 50G in 2 waves (25G/wave) due to nonlinearities.
Do you have a link for this? Interesting news if true. Also: Things running at 50G used to be OC768 with error correction, ie 40G of data + 10G of overhead. Has this changed? At some point, the framers have to deal with standardized bitstreams, so is the 50G one part of an inverse mux or combined up from 10G?
Edit: It's been a while. Sorry for the bazillion questions, but curiousity is getting the better of me. Are folks really running 100G coherent undersea currently?
So you have to separate the "wet" plant from the terminal gear. The speed of the terminal gear is completely disconnected from the wet plant these days. Nobody replaces wet plant to upgrade capacity. They run Ciena, Infinera, Alcatel gear over Tyco's old line system.
Essentially the issue with upgrading over the wet plant is basically the presence of nonlinearities on the fiber. The links are not noise limited. Some of these fibers are still running 10G OOK in half the band and that on NZ-DSF that's used for submarine cables basically causes huge nonlinear penalties. The new subsea fiber is 22ps/nm-km and essentially larger effective diameter for reducing nonlinear penalty.
Thanks for the link. I confess I'm a little amazed that Infinera is the basis for running 100G coherent single wavelengths. That's great progress. (Edit- See below)
> Essentially the issue with upgrading over the wet plant is basically the presence of nonlinearities on the fiber
Yes, and there's great incentive to utilize legacy fiber if possible.
(BTW I managed to screw up my comment above when I edited. I had written: Usually the undersea guys are a generation behind, partly because of the need to send a destroyer-looking ship out for any repairs.)
Yes. It's a 100G service though. Infinera runs on 25G spacing so at 25G dual wave dual polarization is 100G in the same spectral efficiency as single wave.
This fiber was also relatively high loss and has a narrower core which leads to higher nonlinearities in the link. The old cable tries to keep the dispersion within the range that OOK technologies can operate error-free (post FEC) so there's a lot of it typical at each repeater (EDFA). The newer coherent optical technology can transmit multiple bits per symbol (BPSK, QPSK) by encoding the bits in the optical phase. Since the phase is recovered at the receiver the dispersion accumulated in the fiber can be undone in DSP with a long enough FIR filter. So the need for dispersion compensation is gone with coherent optical. Taking out the DCF also reduces loss along the link reducing EDFA (amplifier) count and increasing spacing. Also nonlinear penalties on the newer higher dispersion fiber are lower which improves something called cycle slips that can punch through the FEC and cause you to take post-FEC errors.
The net result is that you should be able to transmit QPSK at 32GBd in 2 polarizations in maybe 80 waves in each direction.
2bits x 2 polarizations x 32G ~128Gb/s per wave or nearly 11Tb/s for 1 fiber. If this cable has 6 strands, then it could easily meet the target transmission capacity.