 Affiliates of the U.S. ABC, CBS, Fox, NBC, and PBS networks are also readily available in Canada, but their programming is subject to a CRTC rule known as simultaneous substitution (or simsub), which gives Canadian broadcast stations within a viewer's market the right to require that U.S. feeds of programs be substituted by BDUs with their own if they are broadcasting the same program in simulcast.This rule serves to protect Canadian advertising revenue.The majority of Canada's multichannel television industry is dominated by vertically integrated companies and their respective services, including Bell Canada's Bell TV satellite and fibreoptic Fibe TV IPTV services, Rogers Communications' cable systems (primarily in Ontario and Atlantic Canada), Shaw Communications' cable systems (primarily in Western Canada; the Shaw family also owns Corus Entertainment, a major operator of Canadian specialty channels), and Vidéotron (which operates mainly in its home province of Quebec, and is owned by local conglomerate Quebecor).