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Worldwide, technologies for broadband access networks are advancing rapidly. Among these, the technology applying single mode fiber provides for a high-capacity transmission medium which can meet the growing demand for broadband services.

The experience with the installation and operation of single mode fiber and cable based networks is huge, and ITU-T Recommendation G.652 describing its characteristics has been adapted to this experience. Nevertheless, the specific use in an optical access network puts different demands on the fiber and cable which impacts its optimal performance characteristics. Differences with respect to the use in the general transport network are mainly due to the high density network of distribution and drop-cables in the access network. The limited space and the many manipulations ask for operator friendly fiber performance and low bending sensitivity. In addition, the cabling in the crowded telecom offices where space is a limiting factor has to be improved accordingly.

It is the aim of ITU-T Recommendation G.657 to support this optimization by recommending strongly improved bending performance compared with the existing G.652 single mode fiber and cables.

This is done by means of introducing two classes of single mode fibers, one of which, class A, is fully compliant with the G.652 single mode fibers and can also be used in other parts of the network. The other class, class B, is not necessarily compliant with G.652 but is capable of low values of macro-bending losses at very low bend radii and is pre-dominantly intended for in-building use.

Class A fibers are suitable to be used in the O, E, S, C and L-band (i.e., throughout the 1260 to 1625nm range). Fibers and requirements in this category are a subset of G.652.D fibers and have the same transmission and interconnection properties. The main improvements are improved bending loss and tighter dimensional specifications, both for improved connectivity.

Class B fibers are suitable for transmission at 1310, 1550 and 1625nm for restricted distances that are associated with in-building transport of signals. These fibers have different splicing and connection properties than G.652 fibers, but are capable at very low values of bend radius.

Source: ITU-T Recommendation G.657 (12/2006)

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