Optical fiber is a hair-thin strand of glass that carries data as pulses of light. In financial markets it is the main physical medium of communication: nearly all orders and market data travel over fiber between exchanges, data centres, brokers and trading firms, and inside a data centre a cross connect between two parties is normally a fiber cable. A single fiber has far more bandwidth than any wireless link.
Its weakness for trading is speed. Light in glass travels at about two-thirds of its speed in a vacuum, which works out at roughly 5 microseconds of delay per kilometre, and cables follow roads and railways instead of straight lines. Firms pursuing low latency lease the shortest available routes, and on the most contested paths they supplement fiber with a microwave network or a laser network, keeping the fiber as the high-capacity backup. Hollow-core fiber, which guides light through air inside the strand, narrows the speed gap.