The ITU standard G.657A and G.657B treated bend insensitive monomode fibers are characterized by a much smaller bend radius compared to the monomode fibers specified in the standard
G.652. The ITU standard G.657 defines four different classes of
bend radiiand consists of the sub-standards G.657A1 and G.657A2, as well as G.657B2 and G.657B3.
G.657A1: The two single mode
fibres specified in G.657A have a similar structure to those specified in G.652D. They differed in that some fibers have a refractive index trench like bend optimized fibers (BIMMF). The minimum bend radius for the monomode fiber specified in G.657A1 is 10 mm. For the G.657A2
standard, the minimum bending radius is 7.5 mm. As for the properties specified in the G.657B standard, G.657B fibers differ in structure from G.652D fibers in that they have voids around the core glass. The minimum bending radius of G.657B2 fiber is 7.5 mm, and that of G.657B3 fiber is 5 mm.