For optical transmission links, the ITU distinguishes between Long Haul( LH), Very Long Haul (VLH) and Ultra Long Haul( ULH). All use single mode fibers. Very Long Haul transmission links can bridge two stations at a distance of about 60 km at a wavelength of 1,300 nm and 120 km at 1,550 nm without intermediate amplification, depending on the wavelength. At a distance of about 600 km, 3R regeneration should be performed.
Extremely long optical transmission distances can be bridged with Ultra Long Haul (ULH).