Coarse wave solution heads for the water peak

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CWDM in the 'water peak'

Considering mergers between Tsunami and Stratos Lightwave, and ONI and Ciena, the CWDM arena is highly active. Lightwave Europe interviewed Lars Bergström of new player Transmode about the benefits of being coarse.

At both CeBIT and OFC shows, this year, several vendors demonstrated coarse wave division multiplexing (CWDM) solutions which are becoming an efficient and economical means of squeezing more value out of the metro edge architecture. In an interview with Lars Bergström, project manager at Sweden's Transmode AB, Lightwave Europe set out to explain how the technique works and who is using it.

Matthew Peach: At a time when tunable lasers are enabling more and more channels in a narrow waveband, why would anybody want a CWDM solution that is by definition crude?

Lars Bergström: In a word, cost. The market we are targeting, the metro edge, is all about cost. The DWDM systems we are up against are well suited to long-haul or even metro core networks, but at the metro edge, they are over-performing and over-priced.

The low purchase price, together with the overall economics of our CWDM system is attractive. The cost of installation, configuration and operation of the system is equally important to customers.

MP: What type of users could benefit from Transmode's 1400nm CWDM system?

LB: Customers that would benefit from this are those with networks where the bandwidth need is fairly high, but who would like to have the cost advantages of CWDM nevertheless. This could include anyone from carriers and power utility companies to SAN and media companies.

Transmode has shipped 8-channel systems for over a year, with customers in both Europe and the US. This equipment can be upgraded to 16 channels without any traffic interruptions. Several of our customers have expressed their future need for upgrades towards 16 channels. The only problem is the water peak, which limits the channel count to 12. The remaining four channels are seeing high attenuation from O-H (the hydroxyl "arm" in a water molecule) contamination in the fibre. We have started a marketing collaboration with OFS, manufacturer of the AllWave fibre, in order to educate people of the enhanced performance the AllWave fibre and Transmode CWDM equipment brings together.

MP: Does Transmode develop and manufacture all components and software for the CWDM system?

LB: We are doing all the optical system design, as well as the hardware and software design in-house. We recognised CWDM at an early stage as being an important technology for the new metro edge networks, and we were pioneers in the field. On the component side, the Transmode CWDM equipment is based on standard transceivers available from multiple vendors. The manufacturing and assembling of the systems is out-sourced to contractors.

MP: How do these devices work and what benefits in terms of signal quality, bandwidth, data rate, and cost do they offer your customers?

LB: What we do is transmission at the physical layer. Our equipment collects the data signals from the client equipment, performs full 3R regeneration and retransmits the signal on the line side with the specific CWDM wavelength. A passive mux/demux collects the different CWDM wavelengths and combines them onto the line fibre.

Transmode also has OADMs in its product portfolio, making it possible to add and drop specific wavelengths along the way in a point-to-point network. The OADMs enable more complex network topologies, such as logical or hubbed rings.

The Transmode CWDM system is optimised for networks of metro edge type, with protocol transparency and auto-detect 3R. The uncooled, easy-to-produce lasers and the relatively simple filters bring down the channel cost to about one third of that of DWDM.

A rate of 2.5Gbit/s per channel with up to 16 channels per fibre fits well into the needs of today and for the foreseeable future.

MP: What are Transmode's expectations about the communications industries migrating to "water peak" wavelengths?

LB: The 1400nm band Common Interest Group (1400 CIG) is carrying out the standardisation work of CWDM in the water-peak wavelength region, the main task being establishing the wavelength grid. The standard is implemented in the ITU-T, which has a working standard called G.CWDM. The ITU-T body expects to have a standard on the table by its meeting in spring 2002. This means that Transmode's estimate of 1400nm system deployment in mid-2002 holds. We are working together with our suppliers of transceivers and filters to make sure the development is as swift as possible.

MP: When will commercial CWDM systems start running on the water peak wavelength?

LB: To my knowledge, there are no installed CWDM systems running on the "water peak" wavelengths as yet. As soon as the CWDM standard for the 1400nm band is in place, it shouldn't be too long before we start to see CWDM systems running over, for instance, [OFS Brightwave's] AllWave fibre. However, there may be other WDM flavours, such as Wideband WDM (WWDM), installed on those wavelengths.

MP: Do you see further CWDM developments in other parts of the spectrum; far infrared or radiowave wavelength-based optical systems with CWDM characteristics?

LB: In the wavelength window currently used - 1270 to 1610nm - there is room for 18 channels with 20nm spacing. Two of them are not used due to practical reasons associated with filtering technologies. Some companies are using CWDM in the 850nm window as well, but on multimode fibre. To increase the channel count on the singlemode fibre, some people are talking about adding DWDM channels in the 1550nm CWDM channel. But the next development step for Transmode towards higher bandwidth will be to increase the data rate on the first (current) 16 channels.

MP: What are the market implications of Transmode's CWDM for developers of tunable lasers and finely tuned systems?

LB: As people become more and more aware of the advantages of CWDM systems, the DWDM system vendors will lose more and more market shares in the metro edge networks. Economics is everything, especially in these times, and CWDM has obvious advantages in that field. The relative simplicity of a CWDM system, when it comes to system installation and maintenance, makes life easier for our customers.

CWDM is already taking market shares from DWDM in the metro edge networks, a process that will accelerate in the near future. The recent mergers we have seen in the CWDM field underline the importance of this technology.


Transmode Systems AB, Stockholm, Sweden, is a CWDM system developer, with experience in datacom and telecom networking, optical technologies and network management systems. Since 2000, Transmode has focused on CWDM as a key optical technology to fully exploit existing networking protocols. Transmode has been shipping CWDM systems since March 2001, introducing OADM in August 2001 and launching the first True Logical Ring products in November 2001.


GothNet owns and operates one of the largest active metropolitan fibre networks in Sweden, covering almost the entire city of Gothenburg. It is the first metro network in Sweden based on CWDM technology. The core of the metro network is a fibre ring based on ATM switches operating at 2.5Gbit/s (left). GothNet is completely owned by Göteborgs Energi, which in turn is completely owned by the City of Gothenburg.

The business concept is to own, operate and sell services in a public metro network, independent of operators, to the benefit of local companies and residents. Construction of GothNet started 10 years ago and it now reaches 450km across the metropolitan area.

In February 2002, Transmode Systems delivered an extension of GothNet's metro network. The established network, deployed in April 2001, was extended from 2 nodes to 12 nodes with all-optical Add/Drops. This means that one single wavelength channel can be addressed optically in one node to be sent in one direction, while the other wavelengths (channels) remain unaffected.

The CWDM equipment is primarily used on stretches where GothNet is short on fibre and needs to increase the bandwidth over the existing fibres. Even on stretches where there is plenty of fibre, the flexibility of Transmode's equipment brings advantages to GothNet's metro network.

The latest addition to this is the introduction of optical Add/Drops in the network. "CWDM provides us with an indispensable tool both for building capacity and for supporting different access technologies, especially now that we have introduced all-optical Add/Drops," says Tom Svensson, network manager at GothNet.

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