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ITU Standard Gives Operators Brighter Future
Latest Recommendation Lights Up More Optical Fibre
Geneva, 13 May 2004 — ITU has set a global standard for a new optical fibre that
will make it easier for network operators to deploy bandwidth to maximize technology in their
core networks. The development of standards in this area is important if network operators are
to reduce costs and provide more innovative services to customers.
"G.656 is another significant step in the evolution of optical networks, because it
allows a more economical deployment of optical transport networks", says Peter Wery,
Chairman of ITU-T Study Group 15 responsible for the Recommendation.
The new standard — Recommendation G.656 — permits easier deployment of Coarse Wave
Division Multiplexing (CWDM) in metropolitan areas, and increases the capacity of fibre in Dense
Wave Division Multiplexing (DWDM) systems. Wave Division Multiplexing (WDM) increases the data
carrying capacity of an optical fibre by allowing simultaneous operation at more than one
G.656 allows operators using CWDM to deploy systems without the need to compensate for
chromatic dispersion, a phenomenon that at low levels counteracts distortion but at high-levels
can make a signal unusable. Although complicated, the management of chromatic dispersion is
crucial as the number of wavelengths used in WDM systems increase. ITU has a history of
providing the specifications that allow operators to most efficiently handle this.
G.656 also means that at least 40 more channels can be added to DWDM systems. In this case
chromatic dispersion is used to control harmful interference over this — unprecedented —
range of the optical spectrum.
Note to technical editors:
The most important new feature in Recommendation G.656 fibre is the chromatic dispersion
coefficient. In G.656 this coefficient has an allowed range of 2 to 14 ps/nm*km in the 1460-1625
nm band, compared to 1 to 10 ps/nm*km for G.655.B and G.655.C which is only related to the
1530-1565 nm band. This low value of the chromatic dispersion coefficient in the S-C-L bands is
the real novelty of G.656 because it allows the utilization of a larger wavelength band.
The other characteristics are very similar to previous Recommendations. The range of mode
field diameter permitted in G.656 of 7 to 11 Ám compares to 8 to 11 Ám in the G.655 non-zero
dispersion-shifted fibre. G.656 fibre has a maximum PMD link design value of 0.20 ps/sqrtkm,
which is the lowest value recommended by ITU-T (the same value that ITU-T recently adopted for
G.655.C). G.656 has the same cable cut-off wavelength and cable attenuation coefficients in the
C and L bands as G.655.
ITU-T G.656 (Characteristics of a fibre and cable with Non-Zero Dispersion for Wideband
Optical Transport) is the most recent in the G-series which specifies the geometrical, physical,
mechanical and transmission characteristics of the optical fibres. Other Recommendations in this
ITU-T G.652 — Characteristics of a single-mode optical fibre and cable
ITU-T G.653 — Characteristics of a dispersion-shifted single-mode optical fibre and
ITU-T G.654 — Characteristics of a cut-off shifted single-mode optical fibre cable
ITU-T G.655 — Characteristics of a non-zero dispersion-shifted single-mode optical fibre
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