Making fat pipes fatter

International Telecommunication Union (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 – 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% deploying a CWDM solution compared with the DWDM alternative. The growing demand for bandwidth in this area has created a need to better utilise existing infrastructure and for a new standard to ensure interoperability. Operator interest and investment in CWDM is already significant.

The 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 20nm channel spacing which includes 18 wavelengths between 1271nm and 1611nm.

CWDM solutions standardised 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.

The new standard foresees flexible and scalable solutions moving from 8 to 16 optical channels using two fibres 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/sec has been added, mainly for Gigabit-Ethernet applications. This is offered alongside support for 2.5 Gbit/sec. 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.

‘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,’ said Peter Wery, the Chairman of the ITU-T Study Group responsible for the new Recommendation.