Although both are positioned as low-cost optical access alternatives, passive optical networks (PON) and coarse wavelength division multiplexing (CWDM) are complementary technologies that together can maximize fibre efficiency, analysts and vendors say.
PON is a point-to-multipoint optical network that uses existing fibre. Its cost savings come from using passive devices in the form of couplers and splitters, rather than higher-cost active electronics.
CWDM is a type of wave division multiplexing in which uncooled lasers are used to overlay optical light channels in a pipe. The use of uncooled lasers reduces cost, compared with dense wavelength division multiplexing (DWDM), which uses cooled lasers. In CWDM, channels are spaced farther apart than in DWDM systems. But CWDM can only scale up to five or six wavelengths, vendors and analysts say.
“Access is a special domain,” says Barry Moon, senior analyst at RHK. “It is usually single-customer focused, and the cost of service has to be paid for by the specific revenue from the services. Although CWDM is non-standard, it is the most cost-effective method of adding bandwidth to a building or group of buildings.”
But Moon adds that CWDM may not be for everyone. Although CWDM can be used to add channels to a PON in cases of limited bandwidth, its non-standard status will not appeal to some users.
“Every vendor has its own version,” Moon says. “Some of the initial PON vendors changed their minds [about using CWDM] after some thought and are using DWDM as an option, but at a much higher cost.”
Even though CWDM is being marketed as a low-cost optical access alternative, it will be more costly for users to deploy over time because of its inability to scale beyond five or six lambdas, says PON vendor Quantum Bridge Communications Inc.
“CWDM is so limited in its scalability, that in the long term it will cost more,” says Jeff Gwynne, a vice-president at Quantum Bridge. “PONs expand the number of endpoints and increase the capacity of the fibre. PONs leverage existing fibre to deliver high-bandwidth services to businesses. WDM over PON expands the number of wavelengths on the fibre. That’s where the real competition is. It’s between CWDM and WDM.”
But PONs are limited in the amount of bandwidth they can support, admits PON vendor Terawave Communications Inc. Users requiring a gigabit or more of bandwidth will have to opt for some type of WDM, the company says.
“CWDM is only good for big businesses with a need for high-bandwidth services beyond one gigabit,” says Josh Chen, a marketing executive at Terawave.
PONs were designed specifically for access while CWDMs can play in the metropolitan area as well. PONs are exclusively point-to-multipoint, but CWDM can be configured in point-to-point and Ethernet or SONET ring topologies, Tsunami Optics Inc.’s CTO James Campbell says.
Indeed, CWDM is 30 per cent of the system cost of DWDM, he says. “I think the most compelling case for CWDM is that it’s an extension of DWDM and can deliver high bandwidth over shorter distances with more connections,” he says. “PONs are the economical way to deliver bandwidth to the last mile. It’s clear that these two technologies service different portions of the market.”
