Backhaul and optical transport are the links that carry a mobile network's traffic from each tower to the operator's core, and carry all traffic between cities and data centres. Backhaul is the link from a tower to the rest of the network. Optical transport is the long-distance part, carried as light over glass fibre.
Consider a single message sent from a phone near a tower outside Kozhikode. First, the tower must send it to a fibre point. If fibre reaches the tower, the message goes straight onto the fibre. If not, a pair of dishes sends it by radio to the next tower that has fibre. This is microwave backhaul. In the hills and in villages, many towers still depend on it.
Next, the message reaches the operator's fibre network. An optical transport network (OTN) carries it, together with millions of other messages, over long distances. It sends many separate colours of light down a single fibre, each colour carrying its own stream of data. In this way, one fibre between two cities can carry several terabits a second. The message finally reaches the packet core, which passes it to the internet.
All of this runs on optical fibre cable, the glass itself, laid in ducts along roads and railway lines. Fibre is cut often, for example when a road is dug up. So operators lay fibre in rings, so that traffic can go the other way round if one side is cut. Spare links of this kind are called redundancy.
The fastest-growing demand today is not between cities but between data centres, because AI training needs huge amounts of data moved between buildings. This is data centre interconnect (DCI). Every link in this zone is described by its bandwidth and latency.