Data centre interconnect (DCI) is the fibre and optical equipment that connects one data centre to another, so that servers in separate buildings can exchange data at very high speed. Most DCI links are within one city or region, tens of kilometres long. Some cross a country. DCI is a special use of the optical transport network (OTN).
Why do data centres need to be linked at all? A large cloud provider rarely keeps everything in one building. It builds several buildings a few tens of kilometres apart, so that a fire or a power failure in one does not stop its customers' apps. Every piece of data written in one building is copied to another, often within milliseconds. The buildings must therefore be joined by links with enormous capacity and short delay.
AI training has made this demand grow faster than any other. An AI cluster can have tens of thousands of GPUs, the chips that train AI models. Often one building no longer has enough electricity for all of them. So the cluster is split across nearby buildings, and the GPUs in each building must exchange results with the others many times a minute. These are now among the busiest fibres anywhere. The AI accelerators page explains why these chips need so much power.
Consider a training job split between two data centres 40 kilometres apart near Hyderabad. Every few minutes, the job saves a checkpoint, a full copy of the model's current state, of 20 terabytes. The two buildings are joined by DCI that carries 64 wavelengths of 400 gigabits a second each, about 25 terabits a second in total. In bits, 20 terabytes is 160 terabits. So the checkpoint crosses to the second building in under 7 seconds. The light itself takes only about 0.2 milliseconds to travel the 40 kilometres.
Modern DCI is simpler than long-distance optical equipment. Short links need no amplifiers along the way. Often the optics come as small modules that plug straight into the data centre's switches, instead of separate cabinets. Inside each building, the link ends at cross-connects in a colocation hall or at the cluster's own network. Further in, co-packaged optics bring the same light right up to the switch chips.
DCI differs from the AI cluster networking inside a building. That network joins GPUs a few metres apart. DCI joins whole buildings, so delay and the cost of fibre matter much more.