The ITU-T created a series of recommendations — G. 657 — each representing a different single mode fiber design. 655 is optimized for long-distance, high-speed. G. Each fiber type is engineered with different refractive index profiles, dispersion properties, and bending performance to support specific applications—from long-distance. Two commonly used single mode fiber specifications are G. 65x series and the IEC 60793-2-50, this post will only introduce the simpler ITU-T G.