New ITU standards make fat pipes fatter
CWDM spec allows carriers to optimize use of fibre optics

Geneva, 5 November 2003 — ITU delegates from government and industry have agreed on a new global standard that will allow network operators to increase the capacity of optical fibre. The standard, created in response to industry needs, was developed under ITU’s fast track approval process — AAP (Alternative Approval Process).

The standard — ITU-T Recommendation G.695 — applies to a technology called Coarse Wave Division Multiplexing (CWDM), used most often in metropolitan networks. In today's cost-conscious telecommunications market CWDM is seen as a cheaper and simpler alternative to DWDM (Dense Wavelength Division Multiplexing). Less expensive uncooled lasers may be used in CWDM products because of wide channel spacing. These lasers require less precise wavelength control, as well as lower-cost passive components.

Experts estimate that carriers with sufficient deployed fibre could make savings of up to 30 per cent deploying a CWDM solution compared with the DWDM alternative. The growing demand for bandwidth in this area has created a need to better utilize existing infrastructure and for a new standard to ensure interoperability. Operator interest and investment in CWDM is already significant.

Peter Wery, Chairman of ITU-T Study Group 15 responsible for the Recommendation: "CWDM systems have the flexibility to be deployed in point-to-point connections and in rings. Their suitability to carry Ethernet traffic and to interconnect Storage-Area-Network (SAN) islands make these systems of interest to large and medium-sized carriers, but also to cable TV companies and for enterprise network operators."

The meeting at which agreement on this standard and other work in the fibre-optics field was completed had the highest participation level of any standardization meeting at ITU since 2001 — a sign of industry's interest in this area and its commitment to the work of ITU.

ITU-T G.695 aims to promote vendor interoperability by specifying transmitter/multiplexer characteristics at one end of a CWDM link and the demultiplexer/receiver at the other end. ITU-T G.695 complements the existing ITU-T G.694.2 Recommendation which defines a wavelength grid with 20 nm channel spacing which includes 18 wavelengths between 1271 nm and 1611 nm. CWDM solutions standardized in ITU-T G.695 can be installed both on the already laid and widely deployed single-mode G.652 optical fibres and on the recent ‘water peak free’ versions of the same fibre. The type of fibre used will have an impact on the reach of the systems and on the number of allowed optical channels.

In order to respond to the needs of the market the new standard foresees flexible and scalable solutions moving from 8 to 16 optical channels using two fibers for the two directions of transmission and from 2+2 up to 8+8 optical channels using only one fibre for the two directions. Support for a bit rate of 1.25 Gbit/s has been added, mainly for Gigabit-Ethernet applications. This is offered alongside support for 2.5 Gbit/s. Two indicative link distances are covered in G.695: the first is for lengths up to around 40 km and the second one for distances up to around 80 km.

ITU-T Recommendation G.695 (Optical interfaces for coarse wavelength division multiplexing applications) is the most recent in the G-series which specify physical layer attributes of optical interfaces. Other Recommendations in this series include:

• ITU-T G.691-2000 — Optical interfaces for single channel STM-64, STM-256 systems and other SDH systems with optical amplifiers
• ITU-T G.692-1998 — Optical interfaces for multichannel systems with optical amplifiers
• ITU-T G.693-2001 — Optical interfaces for intra-office applications
• ITU-T G.959.1-2003 — Optical transport networks physical layer interfaces
• ITU-T G.694.2-2002 — Spectral Grids for WDM Applications: CWDM Wavelength Grid

ITU-T G.695 provides optical interface specifications for multichannel CWDM systems on target distances of 40 km and 80 km. Unidirectional and bidirectional applications are included in the Recommendation. Applications use all or part of the wavelength range from 1270 nm to 1610 nm. The main deployment is foreseen on single mode fibres as specified in Recommendation G.652.

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