ITU Sets Global Standard for Metro Networks

Standard needed to satisfy the demands of voice, data and multimedia services for low-cost short-haul transport solutions in urban centres

Geneva, 13 June 2002 — The ITU has set a global standard for Metro ‘Optical Fibre’ Networks that will expand the use of Coarse Wavelength Division Multiplexing (CWDM) in metropolitan networks. This standard is necessary to meet the increasing demand of voice, data and multimedia services for low-cost short-haul optical transport solutions, and it is expected to produce savings for telecommunications operators, which it is hoped will be passed on to consumers.

The worldwide ‘optical metro network’ market will increase from $1.1 billion in 2001 to $4.3 billion by 2005, according to a recent report by Gartner DataQuest. Bolstered by this agreed new standard, CWDM is poised to capture a substantial share of this market. CWDM applications are especially good for coverage of up to 50 kilometres. "Where the distances are shorter and the need for capacity is less, CWDM applications are able to use wider channel spacing and less expensive equipment, yet achieve the same quality standards of long-haul optical fibre systems" says Peter Wery, Chairman of ITU-T Study Group 15.

Dense WDM optical systems, which carry a large number of densely packed wavelengths, require a thermoelectric cooler to stabilize the wavelength emission and absorb the power dissipated by the laser. This consumes power while adding cost. However, for short transmission distances a ‘coarse’ wavelength grid can reduce terminal costs by eliminating the temperature control and allowing the emitted wavelengths to drift with ambient temperature changes.

"This agreed upon standard will be a stabilizing force for manufacturers of filters with wide channel spacing, for manufacturers of uncooled lasers with an expanded number of wavelengths, and for system manufacturers looking to offer low-cost short-haul optical transport solutions. Taking all this together, a well-defined grid will increase the rate of CWDM product and market development," added Wery.

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Note to Technical Editors ITU-T Recommendation G.694.2 is the most recent in the series, which specify physical layer attributes of optical interfaces. Other Recommendations in this series include:

Recommendation G.694.2 provides a grid of wavelengths for target distances up to about 50 km on single mode fibres as specified in Recommendations G.652, G.653 and G.655.

The CWDM grid is made up of 18 wavelengths defined within the range 1270 nm to 1610 nm spaced by 20 nm.

Coarse Wavelength Division Multiplexing, a WDM technology, is characterized by wider channel spacing than Dense WDM. CWDM systems are more cost-effective for metropolitan applications.

The wavelength plan contained in the new ITU-T Recommendation G.694.2 has a 20 nm channel spacing to accommodate lasers that have high spectral width and/or large thermal drift. This wide channel spacing is based on economic considerations related to costs of lasers and filters, which vary with channel spacing. To accommodate multiple channels on each fibre, the agreed wavelength grid covers most of the recently approved bands of the single-mode optical fibre spectrum, from less than 1300 nm to more than 1600 nm.

CWDM systems support transmission distances up to 50 km. Within this distance, CWDM can support various topologies—hubbed rings, point-to-point and Passive Optical Networks. CWDM is well suited for metro applications (e.g. CWDM local rings that connect central offices to major Dense WDM metro express rings) and for access applications (e.g. access rings, PONs).

CWDM systems can be used as an integrated platform for multiple clients, services and protocols for enterprise users. The channels in CWDM can have different bit-rates. CWDM also provides flexibility with regard to traffic demand changes through the ease of adding or dropping channels into or from systems.

CWDM related standardization activities in SG 15 are continuing with work on a draft new Recommendation (G.capp) specifying optical parameters and values for physical layer interfaces in CWDM applications.

ITU-T Rec. G.694.2 has been agreed under the fast-track approval procedure called AAP. Under this procedure, a comment period is initiated when a study group gives consent to approve the draft text of a recommendation which it considers mature. The combined announcement and comment period took just under two months. This standard is therefore expected to become effective before the end of the year if the comment period yields a positive outcome.

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