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

The experience with the installation and operation of single-mode fibre and cable based networks is huge, and Recommendation ITU-T 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 fibre 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 fibre 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 Recommendation ITU-T G.657 to support this optimization by recommending strongly improved bending performance compared with the existing ITU-T G.652 single-mode fibre and cables. This is done by means of introducing two categories of single-mode fibres, one of which, category A, is fully compliant with the ITU-T G.652 single-mode fibres and can also be used in other parts of the network. The other, category B, is not necessarily compliant with ITU-T G.652 but is capable of low values of macrobending losses at very low bend radii and is pre-dominantly intended for in-building use.

This edition of this Recommendation includes the addition of two sub-categories in both fibre categories A and B. These sub-categories differ in macrobending loss.