40G wave technology has been around for a couple of years and has seen some deployment in terrestrial optical networks. But various trials aside, it has yet to really gain traction in the subsea market, primarily due to the wet nature of the business. As most of the network plant is underwater, boosting wavelength bit rates has to be done using SLTE (submarine line terminal equipment) gear plugged into the dry plant at either end of a given link. A number of carriers have already done this to upgrade 2.5G waves to 10G - Pacnet did it this past May, for example, using SLTE cards from XTera to upgrade its EAC-C2C system.
Taking that to 40G has been problematic, partially in terms of the economics (as it has been generally cheaper for carriers to deploy four 10G cards than one 40G card), but also in terms of reach - 40G has to not only be able to cover relatively short links of a few hundred kilometers, but also transoceanic links that can span 9,000 km or more.
But that could be changing soon. Vendors like Nortel Networks (its bankruptcy proceedings notwithstanding), Infinera and Huawei Technologies are bringing their terrestrial optical technology to the SLTE space, challenging the big subsea suppliers (Alcatel-Lucent, Fujitsu, NEC and Tyco) with SLTE gear that they say can help 40G go the distance.
SLTE: plug it in
Nortel's 40G SLTE solution has drawn considerable attention in the last couple of months after the company publicized a trial of the solution with subsea cable operator Southern Cross Cables Network.
"Southern Cross gave us a route of around 4,500 km from Hawaii to the US west coast, Segment D on their route," says Anthony McLachlan, VP of carrier networks for Nortel Asia. "We turned that up in a matter of days. We didn't touch the wet plant, we just plugged in our OME 6500s with 40G cards, and it worked."
While Southern Cross was impressed, McLachlan continues, "They said, 'if I have to light up one segment at 40G, I have to be able to do the lot, and you'll need to run 40G on Segment C, which is 8,000 km between Hawaii and Auckland'. So we did that too."
The secret to Nortel's ability to send 40G at that reach is the use of DPBPSK (Dual Polarization Binary Phase Shift Keying) modulation, which gives it a 3dB increase in reach and 6dB more resilience to phase distortion, enabling longer distances than before.