Carriers Stress Test Their Fiber
That's one of the conclusions of a recent test carried out by France Telecom SA (NYSE: FTE), Deutsche Telekom AG (NYSE: DT), and Alcatel SA (NYSE: ALA; Paris: CGEP:PA), which showed that an existing 430-kilometer single-mode fiber link in the French carrier's network is capable of carrying eight 160-Gbit/s channels (see Alcatel Tests Fiber With FT, DT ).
That compares with the 10 Gbit/s per wavelength achieved by today's installed commercial DWDM systems, says Alcatel's CTO Niel Ransom.
"Increasing the bit rate per wavelength channel leads to potentially better equipment integration, higher capacity and lower network costs," said France Telecom's R&D director Pascal Viginier in a prepared statement. The carrier "needs to increase its network capacity at the lowest costs" as it introduces "high-bit-rate services combining voice, data and video," added Viginier (see French Say Oui to DSL TV).
And transmitting fewer wavelengths saves money, as it requires less regeneration, says Alcatel's Ransom. So while the same capacity can be achieved with multiple 10 Gbit/s or 40 Gbit/s wavelengths, that's a more expensive option.
The test results are good news for any operator with an extensive fiber network, as it's going to be expensive enough migrating to next-generation network systems without having to replace existing installed fiber. For example, BT Group plc (NYSE: BT; London: BTA) is set to spend £10 billion ($19.2 billion) on its 21st Century Network, or 21CN, project (see BT Moves Ahead With Mega Project).
It's worth pointing out that BT and Siemens Communications Group have already conducted a similar trial of 160-Gbit/s transmission technology, over BT's existing fiber infrastructure. In the Light Reading article covering the BT/Siemens demo, earlier trials of 160-Gbit/s transmission systems by Mitsubishi Electric Corp. (Tokyo: 6503), Bell Labs, and Germany's Heinrich Hertz Institute (HHI) were also cited (see Siemens Claims 160-Gbit/s Milestone and Mitsubishi Looks to 160-Gbit/s Future). Alcatel learned a lot from the test it conducted with FT and DT, according to Ransom. He says his team had to "put together all sorts of tricks to get this to work over a standard G.562 fiber," and they found that the key feature that needs to be addressed when trying to send so much data over legacy fiber is the polarization mode dispersion (PMD).
PMD is one of a bunch of phenomena that cause light pulses to spread out as they travel along fiber, so that they eventually overlap and become indistinguishable from each other. This means errors get introduced when translating the pulses back to electronic signals. A brief tutorial on the topic is given in a Light Reading Beginner's Guide: Chromatic Dispersion and Polarization Mode Dispersion (PMD)).
"We found that PMD is not constant, that it's affected by temperature and by physical stresses on the fiber, for example if a train runs overground where the fiber is installed. So we built adaptive PMD devices that constantly monitored and adjusted the optical components to cope with the variations," says Ransom.
So when can we expect to see 160-Gbit/s systems available commercially? Not any time soon, says the Alcatel man, as the electrical components for such equipment don't exist yet. For this test, Alcatel interleaved four short-pulse optical TDM 42.7-Gbit/s signals to create a single 170-Gbit/s signal (160 Gbit/s plus an overhead for bit error detection and correction).
But while that's fine for experiments, it's not something that's economically feasible in commercial networks, says Ransom, and at the moment there's no demand for 160-Gbit/s systems anyway. "But the test has shown us what's needed to transmit single 160-Gbit/s wavelengths, and the carriers now know they won't need new fiber when they need to deploy 160-Gbit/s systems."
Ransom says that, at present, there is a small number of sophisticated end users, such as universities and R&D networks, that want to generate 40-Gbit/s Ethernet signals, "but in six or seven years' time they'll be looking to do 160 Gbit/s."
— Ray Le Maistre, International News Editor, Light Reading