The first phase of development in the fiber optic communication was started in 1980s. At the beginning, 850nm was used for optical transmission. This optical window is still used for multimode fiber transmission. The Gallium Arsenide lasers injected light pulses at 850 nm to a multimode fiber. ITU-T recommendation G.651 was the standard followed for multimode fibers. The current version of this recommendation is G.651.1 which specifies a 50um multimode fiber.
The characteristics of a fiber optic transmission system operating at 850 nm suitable for PDH are specified in ITU-T G 956. This recommendation is now G.955 (Digital line systems based on the 1544 kbit/s and the 2048 kbit/s hierarchy on optical fibre cables). These two recommendations, G.651 and G.956, represent two parallel developments happened in both Optical fiber manufacturing technology and optical transmission systems.
ITU-T G.956 (Digital line systems based on the 2048 kbit/s hierarchy on optical fibre cables) is withdrawn from use and is being replaced with ITU-T G.955.The first optical networks developed were based on the Recommendation G.956. Those Transport networks based on PDH transmission systems were built with an optical capacity of 2.048 Mbps, 8.448 Mbps, 34.368 Mbps, 139.264 Mbit/s and Nx139.264 Mbps (∼565 Mbit/s PDH non standard level) for which the multiplexing structure was recommended. ITU-T recommendations G.956 intends to obtain the system longitudinal compatibility between the optical reference points S-R in a bi-directional point to point link.