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ITU-T Recommendation G.955 deals with digital transmission systems. The recommendation, named “Digital Line Systems based on the 1544 kbit/s and the 2048kbit/s hierarchy on optical fibre cables” provides specifications for the transmission of optical signals based on both the 1544 kbit/s and 2048 kbit/s hierarchies. ITU-T Recommendation G.955 was revised by ITU-T Study Group 15 (1993-1996) and was approved under the WTSC Resolution No. 1 procedure on the 8th of November 1996.

The aim of this Recommendation is to achieve longitudinal compatibility on elementary cable sections of different digital line systems, i.e. the possibility of installing digital line systems, produced by different manufacturers, on the same optical fibre cable. This Recommendation covers digital line systems for the transmission of signals based on both the 1544 kbit/s and the 2048 kbit/s hierarchy on optical fibre cables and includes systems conveying the following bit rates such as 1544kbit/s, 2048kit/s, 3152kit/s, 6312kbit/s, 8448kbit/s, 32064kit/s, 34368kit/sec, 44736kit/s, Nx44736kbit/s, 97728kit/s, 4x97728kit/s, 139264kit/s and 4x139264kbit/s.

Four major system characteristics defined by ITU -T G.955 to attain  longitudinal compatibility are:

– Make use of Multimdoe and Singlemode optical fibers described in TIU-T G.651 and ITU-T G.652 respectively in one of the wavelengths of 850 nm, 1300 nm or 1550 nm depending on the transmission system.

– Specify maximum bandwidth value at -3dB optical bandwidth in MHz for multimode fiber systems. Specify maximum dispersion in ps/nm at point R for singlemode fiber systems.

– Specify maximum value of attenuation in dB between the range of wavelengths at BER = 10^-10 for applications with laser and LED transmitters.

– Allow equipment margin of 3dB.

ITU-T gives much freedom to telecommunicaiton administrators. Rec.G.955 leaves a lot of space to the different administrations when considering the most appropriate attenuation value for the various transmission capacities and for the various wavelengths. Also it provides wide room when choosing the characteristic attenuation of the fiber splices, of the optical connectors and of the cable margin to guarantee a total attenuation between the points S-R greater or equal to that specified in G.955 documents.