An optical transport network (OTN) is the equipment that sends very large amounts of data as light over fibre cables, between cities, exchanges and data centres. It is also called optical transport, long-haul optical or DWDM. Strictly, OTN is the name of one international standard for packing data into these light signals, but the industry uses it for the whole system.
The main idea is to send many colours of light down one fibre at the same time. Each colour, called a wavelength, carries its own stream of data, and the colours do not disturb each other. This method is called dense wavelength division multiplexing, or DWDM. A transmitter at one end puts each data stream onto its own wavelength. A device then combines all the wavelengths into the single fibre. At the far end, another device separates the colours again, and a receiver turns each one back into data.
Light grows weaker as it travels through glass. So along a long route, an amplifier strengthens all the wavelengths together about every 80 kilometres. At junctions along the route, a device called a ROADM can drop some wavelengths off for a local city and add new ones, without turning the rest back into electrical signals.
Take, for example, an operator's fibre route between Mumbai and Delhi, about 1,400 kilometres long. The operator lights the route with 64 wavelengths. Each wavelength carries 400 gigabits a second. So one pair of fibres carries 64 × 400, or 25,600 gigabits a second: more than 25 terabits. Along the way, amplifiers sit in small huts beside the highway, about 17 of them in all. At Ahmedabad and Jaipur, ROADMs drop some wavelengths for those cities' traffic. When demand grows, the operator adds new wavelengths at the two ends. It does not need to dig a new trench.
Traffic reaches this network from many directions. Mobile traffic arrives from microwave backhaul and fibre-fed towers on its way to the packet core. Home broadband traffic arrives from the passive optical network (PON) equipment at local exchanges. At a data centre, the network connects through cross-connects to clouds and other operators, and to the building's switches and routers.
The network is only as good as the optical fibre cable it runs on. A cut cable stops every wavelength on it at once. So operators build routes as rings, and the equipment moves traffic onto the other side of the ring within a fraction of a second. These spare routes are a form of redundancy.