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.
Cell site
The "maximum" range of a mast (where it is not limited by interference with other masts nearby) depends on the same considerations.In any case the limiting factor is the ability of a low-powered personal cell phone to transmit back to the mast. As a rough guide, based on a tall mast and flat terrain, it may be possible to get between 50 and 70 km (30–45 miles). When the terrain is hilly, the maximum distance can vary from as little as to due to encroachment of intermediate objects into the wide center fresnel zone of the signal. Depending on terrain and other circumstances, a GSM Tower can replace between 2 and of cabling for fixed wireless networks. In addition, some technologies, such as GSM, have an additional absolute maximum range of , which is imposed by technical limitations. CDMA and IDEN have no such limit defined by timing.
The concept of "maximum" range is misleading, however, in a cellular network. Cellular networks are designed to support many conversations with a limited number of radio channels (slices of radio frequency spectrum necessary to make one conversation) that are licensed to an operator of a cellular service. To overcome this limitation, it is necessary to repeat and reuse the same channels at different locations. Just as a car radio changes from one local station to a completely different local station with the same frequency when you travel to another city, the same radio channel gets reused on a cell mast only a few miles away. To do this, the signal of a cell mast is intentionally kept at low power and in many cases tilted downward to limit its reach. This allows covering an area small enough not to have to support more conversations than the available channels can carry. Due to the sectorized arrangement of antennas on a tower, it is possible to vary the strength and angle for each sector depending on the coverage from other towers in the area.