In the last few weeks, UK competitive operator Virgin Media has performed a trial of Nortel's 40 Gbit/s kit on its core optical network. The trial involved carrying live traffic at 40 Gbit/s over a 350 km fibre span between Virgin Media's points of presence (POPs) at London and Manchester
Virgin Media is already a customer for Nortel's CPL (Common Photonic Layer) optical transmission equipment, and simply swapped in 40 Gbit/s line cards, the company says. The client-side cards were hooked up to Juniper T-series core routers located in the POPs, which had native 40 Gbit/s interfaces fitted. The line-side cards, which send signals out over the transmission network, contained Nortel's 40G Adaptive Optical Engine WDM transponder.
The trial was a first for Nortel in the UK because it took place over a commercial network carrying live traffic, and because it used native 40G interfaces on the routers. More importantly, perhaps, is the fact that it wasn't purely a technology demonstration, but represented a customer evaluating the equipment to see if it makes economic sense to deploy it in the network.
"One aspect that they [Virgin Media] are keen to use is a 4 to 1 combiner, putting four 10 Gbit/s onto 40 Gbit/s to make more efficient use of wavelengths," says Yash Kanabar, senior marketing manager, carrier networks, at Nortel. "We think this is a big part of the story, because not everyone wants 40G [router] interfaces today."
The trial also highlights what Nortel says is the key advantage of its technology, namely that 40 Gbit/s wavelengths can be deployed on existing networks using the same design rules and amplifier spacings.
"Doing it our way, we can use the same network design requirements [for 40 Gbit/s] as we use for 10 Gbit/s," says Kanabar. "It’s not a separate network, it coexists with the 10 Gbit/s network."
One consequence is that both 40 Gbit/s wavelengths and 10 Gbit/s wavelengths can run side by side on the same network, and indeed did during the trial. This will be a key requirement in any network operators' upgrade plans, according to Nortel, because it allows individual wavelengths to be upgraded as and when required.
Nortel achieves this by making a 40 Gbit/s signal look like a 10 Gbit/s signal, through the use of a modulation scheme called dual-polarization QPSK (quadrature phase shift keying). QPSK transmits two bits of data per cycle, thus halving the baud rate; adding dual-polarization allows the baud rate to be halved again. "The baud rate remains the same [as 10 Gbit/s traffic], and that is unique," claims Kanabar.
Most other systems vendors are using 40 Gbit/s modules from Mintera and StrataLight, which employ versions of DPSK (differential phase shift keying). Nortel, on the other hand, says its solution was developed entirely in-house, including the "adaptive optical engine", which is a complex digital signal processing chip that handles detection of the polarization multiplexed signals.
Nortel says its 40G product reached general availability status in early May, in other words, in the past week.
• A spokesperson for Virgin Media says that following the success of the trial, the operator is looking to deploy 40G services, but did not give a timescale.