Continuous Tone-Coded Squelch System - Example

Example

As a simple example, suppose a two-way radio frequency is shared by a pizza delivery service and a landscape maintenance service. Conventional radios without CTCSS would hear all transmissions from both groups. The landscapers would have to listen to the pizza shop. The pizza shop would have to hear about landscape customer complaints. If both installed CTCSS, units from each group would only hear radios from their own group. This is supposed to reduce missed messages and the distraction of unnecessary radio chatter.

Note that in the example above there are only two co-channel users. In dense two-way radio environments a large number of groups may be present on a single radio channel.

A disadvantage of using CTCSS in shared frequencies is that since users cannot hear transmissions from other groups, they may assume that the frequency is open when it is not and transmit simultaneously with another user, thus accidentally overriding or interfering with the other group's transmission. For example, in the above situation a landscaper might be communicating with another landscaper. Meanwhile, a pizza delivery driver—not hearing any transmissions—assumes that the frequency is clear and calls his dispatch office. Depending on several factors (locations, power, etc.), the two simultaneous transmissions could easily interfere with each other—resulting in one or both not being clearly understood. The more separate groups that share a single frequency and the more frequently that they transmit, the more likely that this accidental interference will occur. A "Busy Channel Lockout" feature will prevent transmitting in this case.