ITU-T Standards

What is the ITU-T Standards？

ITU-T standards, also known as ITU-T Recommendations, have developed standards for various optical fibers used in telecommunications. These standards define the specifications and characteristics of optical fibers to ensure compatibility, performance, and interoperability in telecommunication networks.

What are the Common ITU-T Recommendations?

ITU-T G.651.1 was developed based on the ITU-T G.651 standard which was withdrawn in 2008. It defines the 50/125µm graded-index multimode fiber used in the region of 850nm band or 1300nm band, or alternatively may be used in both wavelength regions simultaneously. As shown in the following table, this fiber features a 15mm bend radius. Since there is no other multimode fiber that defines a tighter bend radius performance, this fiber can be deemed as a bend-insensitive multimode fiber. Recently, G.651.1 fiber is mainly applied for multi-tenant / dwelling buildings in FTTH networks, as well as functions in enterprise networks, such as the Fiber to the Zone (FTTZ) architecture.

ITU G.652 is the first single-mode fiber standard specified by the ITU-T. It includes four revisions which are G.652.A, G.652.B, G.652.C, and G.652.D. Among them, G.652.A and G.652.B fibers are rarely used now due to inferior performance in modern WDM applications. While G.652.C and G.652.D fibers feature a reduced water peak (ZWP - Zero Water Peak), which allows them to be used in the wavelength region between 1310nm and 1550nm supporting Coarse Wavelength Division Multiplexed (CWDM) transmission. G.652.D fiber is the most up-to-date technology today, which provides not only the maximum return on your investments but also affords the best protection and is recommended as the fiber of choice when deploying single-mode optical fiber in most of application cases currently.

ITU G.653 defines the dispersion-shifted single-mode fiber which exhibits a zero-dispersion value around the 1550nm wavelength where the attenuation is minimum. There exist two fiber categories for Recommendation G.653: G.653.A and G.653.B, both function in the wavelength 1550nm region, and could function around 1310 nm on condition that the attenuation coefficient performs below 0.55 dB/km. Now G.653 fiber is rarely deployed anymore and has been superseded by G.655 fiber for WDM applications, the reason is channels allocated near 1550 nm in G.653 fiber are seriously affected by noise induced as a result of nonlinear effects caused by Four-Wave Mixing (FWM is a non-linear effect in WDM systems).

ITU-T G.654 covers cut-off shifted single-mode optical fibers which are optimized for operation in the 1500 nm to 1600 nm region. It includes five revisions which are G.654.A, G.654.B, G.654.C, G.654.D, and G.654.E. G.654.A, G.654.B, G.654.C, and G.654.D fibers are suitable for extended long-haul undersea applications. While G.654.E fiber is designed for high-speed long-haul terrestrial optical networks. It is considered as a promising candidate to optimize the transmission performance for next-generation ultra-high-speed long-haul optical networks. Learn more about the G.654.E fiber at High Speed Long-Haul Optical Fiber Solution - G .654.E Single-Mode Fiber.

ITU-T G.655 defines the non-zero dispersion-shifted single-mode optical fiber with performance specified at 1550 nm and 1625 nm. It covers five categories: G.655.A, G.655.B, G.655.C, G.655.D, and G.655.E. These fibers were originally intended for use at wavelengths in the range of 1530 to 1565 nm, but provisions can be made to support at wavelengths up to 1625 nm and down to 1460 nm. G.655 fibers were popular before 2005 for WDM and long-distance cable runs, suited for long-haul and backbone applications. But it falls into disuse and is replaced by G.652.D fiber.

ITU-T G.656 optical fiber has been dedicated for use in broadband systems using both DWDM and CWDM, intended to operate in 1460 nm to 1625 wavelength windows. The attenuation of G.655 fiber is low at 1460nm -1625nm, but when the wavelength is less than 1530nm, the dispersion is too low for the WDM system. So G.656 fiber is not suitable for applications operating from 1460nm to 1530nm. The

ITU-T G.657 is the latest edition of single mode optical fiber standard and specifies the characteristics of bend-insensitive single-mode optical fibers. G.657 fibers are mainly applied for broadband optical access networks in telecom offices and customer premises in apartment buildings and single dwelling houses. There exist two categories of the ITU-T G.657: G.657.A and G.657.B. G.657.A fiber is compliant with the existing ITU-T G.652.D, but it provides a roughly ten times better macrobending performance. G.657.B is a truly bend-insensitive class, with hundreds of times better than traditional single-mode fibers and about tens of times better than class G.657.A. G.657.B fiber does not conform with any former ITU-T standard. G.657.A and G.657.B can be further divided into G.657.A1, G.657.A2, G.657.B2, and G.657.B3, distinguished by their macrobending requirements. G.657.A2 and G.657.B3 have better macrobend performance than G.657.A1 and G.657.B2 fibers.

What are the Differences Between Different Common ITU-T Recommendations?

G.651.1, G.657.A, and G.657.B all define bend-insensitive fibers made for FTTH systems. However, G.651.1 multimode fiber has higher data rates for short-distance communications. Therefore it offers better performance than any single-mode fiber on campuses and other enterprise networks or wherever a majority of network links are less than 100m or 250m. G.657.A fiber is backward compatible with the existing G.652.D fiber, which helps to save installation and deployment cost through seamless and transparent integration with the already installed G.652.D fiber cabling. A full installation is needed when deploying G.651.1 and G.657.B fibers, thereby an increase in cost. The advantage of G.657.B fiber is its superior bend-insensitivity. For completely new applications to be installed, it is better to use G.657.B fiber, especially in multi-tenant buildings where installers face tough environments where the bend diameter is very low.

G.652.D, G.655, and G.656 fibers support either CWDM or DWDM systems, each fiber has its weaknesses and strengths in long-haul transmission. G.652.D is a low water peak fiber with improved attenuation performance, supporting the use of CWDM systems. G.655 fiber has low chromatic dispersion and supports long-haul systems that use CWDM in the wavelength range from 1550 nm to 1625 nm. G.656 fiber has medium chromatic dispersion and is optimized for long-haul systems with both DWDM and CWDM systems throughout 1460 nm to 1625 nm.