| Selective Calling |
Article Index for Selective |
Website Links For Selective |
Information AboutSelective Calling |
| CATEGORIES ABOUT SELECTIVE CALLING | |
| wireless communications | |
|
Selective calling is akin to the use of a lock on a door. A radio with carrier squelch is unlocked and will let any signal in. Selective calling locks out all signals except ones with the correct code. Selective calling systems can overlap; a radio may have CTCSS (group call) and DTMF individual calling. Selective calling prevents you from hearing others on a shared channel. It does not eliminate interference from ''co-channel users''. If another user is talking and you try to talk at the same time, your signal will be affected by the other party using the channel. Some selective calling systems experience Falsing . In other words, the decoder activates when a valid signal is not present. Falsing may come from a maintenance problem or poor engineering. GROUP CALLING In conventional US FM two-way radio systems, the most common form of selective calling is CTCSS. Radios made by nearly any manufacturer will work acceptably with existing systems using CTCSS. The system allows groups of radios to remain muted while other users are talking on the channel. In business and industrial systems, two companies could share the same channel without having to listen to calls for each other's staffs. In government systems, users can avoid having to hear users outside their own agency. (Government channels are usually separated by distance between users. Only one local user is assigned to a channel.) In uses where missed calls are allowable, selective calling can also hide the presence of interfering signals such as receiver-produced intermodulation. Receivers with poor specifications — such as scanners or low-cost mobile radios — cannot reject the unwanted signals on nearby channels in urban environments. The interference will still be present. It will still degrade system performance but by using selective calling the user will not have to hear the noises produced by receiving the interference. In the US, Federal Communications Commission rules require users of selective calling to switch to carrier squelch before transmitting. In other words, the user must monitor (listen) to make sure the channel is not in use by someone on another selective calling code before transmitting. To enforce this rule, base stations often have a monitor switch on the microphone. The push-to-talk button is split into two segments. One segment turns the selective calling off. The other segment of the button transmits. A mechanical interlock prevents the transmit button from being pressed until the monitor button is down. This is called, "compulsory monitor before transmit." In mobile radios, microphones are stored in a hang-up box. When the microphone is pulled out of the hang-up, the radio reverts to carrier squelch, (the selective calling feature is disabled). The user automatically monitors -- verifies no one else is using the channel -- by pulling the mike out of the hang-up box. Hand-held radios sometimes have LED indicators that show when the channel is in use. CTCSS See Also: CTCSS CTCSS (Continuous Tone-Coded Squelch System) superimposes any one of about 50 continuous audio Tone s on the transmitted signal, ranging from 67 to 254 Hz . At any time when the transmitter is on, the tone is encoded on the signal. CTCSS is often called ''PL tone'' (for ''Private Line'', a Trademark of Motorola ), or simply ''tone squelch''. General Electric 's implementation of CTCSS is called ''Channel Guard'' (or ''CG''). Tone codes may universally be described by their tone frequency, (for example: 131.8 Hz). SelCall
| ||||||||||||
| ||||||||||||
| ||||||||||||
| ||||||||||||
| ||||||||||||
| ||||||||||||
| ||||||||||||
|
||||||||||||
|
|