Routers are a class of network controller that determines the best route for data and voice transmissions between a transmitter (sender) and a receiver. They are typically controlled by software and can be programmed to provide the most inexpensive, fastest or least-busy routes available. Routers operate at Layer 3 of the International Organization for Standardization (ISO)/Open Systems Interconnection (OSI) model. This definition applies to service provider routers only.