Telecommunication
The other meaning of the term "channel" in telecommunications is seen in the phrase communications channel, which is a subdivision of a transmission medium so that it can be used to send multiple streams of information simultaneously. For example, one radio station can broadcast radio waves into free space at frequencies in the neighborhood of 94.5 MHz (megahertz) while another radio station can simultaneously broadcast radio waves at frequencies in the neighborhood of 96.1 MHz. Each radio station would transmit radio waves over a frequency bandwidth of about 180 kHz (kilohertz), centered at frequencies such as the above, which are called the "carrier frequencies". Each station in this example is separated from its adjacent stations by 200 kHz, and the difference between 200 kHz and 180 kHz (20 kHz) is an engineering allowance for the imperfections in the communication system.
Reference distance
This is in turn used to define most broadcast classes for FM stations in North America. Each class (except D) is defined as having a maximum ERP and HAAT. If the HAAT of a station's radio antenna exceeds that specified for the class, it must reduce ERP so that its signal does not exceed the reference distance. The signal strength used differs by class, but generally the value is 1.0mV/m (millivolt per meter) or 60dBu (decibels over one microvolt per meter) for most of the United States, and 0.5mV/m or 54dBu in Canada, and for some U.S. stations in parts of certain areas including California, the Great Lakes region, and the Northeast. This is considered the service contour of a station by the Federal Communications Commission (FCC) and the Canadian Radio-television and Telecommunications Commission (CRTC), and by COFETEL in Mexico, according to NARBA and other international agreements among the three. The reference distances are in turn used to create mandatory minimum spacing distances among co-channel stations, and certain adjacent channels as well.