Two transmitters can use the same frequency at the same time if they are separated.
TV channel 2 uses the 54-60 MHz band in both New York and Los Angeles. This does not cause a problem because the geographic areas are far enough apart that at allowable power levels the signals will attenuate and not cause interference. If a station exceeded its allowable power limit, it would interfere with nearby stations.
TV stations broadcast in all directions (they use omni-directional antennas). Another approach is to use antennas that confine the signal to a horizontal or vertical sector. Several of these could be mounted at a single location as long as they faced away from each other.
More sophisticated smart antennae can shift their focus under computer control.
Two transmitters can cover the same area and transmit at the same time if they use different frequencies.
This is the solution used by local television and radio stations, cellular telephones, and most other wireless communication. A regulator like the FCC grants a license to use a given frequency in a given area. The rules and cost vary from nation to nation, and in recent times many governments have auctioned spectrum.
In license-free bands, transmit power is limited, but otherwise radios and their controllers must expect and deal with interference. WiFi radios operate in license free bands. An operator does not have to obtain a license, but the radio manufacturer must have the FCC certify that their radios comply with the power limit regulations.
Two transmitters can cover the same area and use the same frequency if they transmit at different times.
For example, one could set up a point-multi-point network in which remote radios could only transmit when given "permission" by the base station radio. Since it would only give permission to one at a time, interference would be eliminated. Another approach would be to let remote radios transmit when they did not detect a signal from another one in the area. If two start transmission at the same time, an error would be generated and they could be programmed to delay for a random time before trying again.
The media access control (MAC) portion of the data link layer is responsible for minimizing simultaneous transmission in TCP/IP networks.
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