Cross-connects and interconnection are the short, direct cables inside a data centre that join one company's equipment to another company's equipment in the same building. One such cable is called a cross-connect. Interconnection is the wider business of offering these links. Data centre operators charge a fixed monthly fee for each one.
A cross-connect is usually a single fibre optic cable. It runs from a tenant's cage, along trays above the racks, to a room called the meet-me room. There, it is joined to a cable that leads to the other company's cage. The operator's staff install it, often within a day or two of the order.
Consider an online travel company whose servers sit in a colocation data centre in Noida. Its booking site must ask airline systems for fares, and it keeps customer data with a cloud provider. Without a cross-connect, this traffic travels over the public internet. It passes through several networks, and each round trip might take 20 to 40 milliseconds. The cloud provider has equipment in the same building, so the company orders a cross-connect to it. Now each round trip takes about one millisecond. The company also pays a fixed monthly fee for the cable, instead of paying for the amount of traffic it sends over the internet. The delay of a round trip is called latency; the page on bandwidth and latency explains it.
Cloud providers sell private connections of this kind, often called cloud on-ramps. The cross-connect is the last few metres of such a connection.
A tenant can also connect to an internet exchange. An internet exchange is a switch at which many networks pass traffic directly to each other, instead of paying a larger network to carry it. In India, the National Internet Exchange of India (NIXI), set up by the government, runs exchanges in many cities. Private exchanges also run inside large data centres.
Some operators now sell virtual cross-connects. The tenant has one physical cable to the operator's own switches. It then opens new links to other companies from a web portal, in minutes. A company that uses several clouds can join them all this way, often with multi-cloud networking software on top.
Traffic for another city leaves the building on data centre interconnect fibre, carried by an optical transport network. Interconnection is the reason why tenants prefer some buildings. A building with many networks inside attracts more tenants, and those tenants attract more networks.