The television signals are distributed as broadcast television which is modeled on the radio broadcasting systems. Broadcast television uses high-powered radio-frequency transmitters to broadcast the television signal to individual television receivers. Until the early 2000s, broadcast television was generally transmitted as an analog signal but soon countries started switching to digital signals, with the transition expected to be completed worldwide by 2020. In addition to terrestrial transmission, television signals are also distributed by cable (digital transition is ongoing) and satellite (digital only) systems.