FYI:
Simplex - One radio direct to others on a single shared frequency without any other infrastructure in between. Can be either analog or digital.
Duplex - Transmitting on one frequency and receiving on another. Typically, a mobile/portable transmitting into a repeater on frequency 'A' and the repeater re-transmitting whatever it hears over a wide area on frequency 'B'. Duplex without a repeater is seldom used but was set up where mobiles would talk into a dispatcher on one frequency and dispatch would reply on another frequency. Also used in areas where a single base has good coverage but mobiles would manually change channels to talk into different receivers in different locations by changing different input frequencies. Can be either analog or digital.
Repeater - A base transmitter which listens for traffic on one frequency and repeats whatever it hears on another frequency over a much wider area than a mobile could cover in simplex. Most re-transmit simultaneously, or with a few milliseconds of latency. Can be either analog or digital.
PL, DPL, CTCSS, DCS, RAN and a host of other trademarked "privacy codes": An inaudible signal sent by a transmitter along with the message. Receivers can be programmed to only 'open' for messages with the right signal, and ignore all others. Receivers without code squench capability (or set in carrier squelch, "monitor", "CS" or "QSC" mode) will open on anything. Commonly used to exclude nuisance voices from others using nearby systems on the same frequency, and to qualify repeater inputs as being intended for a particular repeater. In no way foolproof, and many users wrongly think that by transmitting a privacy code their message cannot be heard by others. Analog or digital, the function is the same for all.
Squelch vs. Privacy Codes - Picture yourself at an amusement park: "Carrier squelch" (opening the speaker based on the strength of a received signal) says you cannot ride a ride unless you are this tall. "Privacy codes" (opening the speaker based on a non-audible signal) say you cannot ride the ride without a red ticket. Yellow, blue or mauve tickets (or no ticket at all) don't work, and the speaker remains blissfully silent. Can be used alone or in combination. Analog or digital, the function is the same for all.
Voting - a simplex or repeater infrastructure system with a single base transmitter and multiple receivers in different locations listening for mobiles on the same input frequency. The "Voter" (or "Comparator") automatically selects the receive site with the clearest available signal and repeats it. Still commonly used in buildings with portable input coverage issues and in large area to allow radios to 'talk into' the system reliably over a wide area, even with low power. Can also have base/repeater sites in different locations where the base in the same area as the voted receiver is selected for transmit. Can be either analog or digital.
Simulcast - two (or more) base radios or repeaters linked and precisely aligned using GPS time signals to re-transmit the same content at the same time on the same frequency over a wider area than can be covered with a single base transmitter. Simulcast base transmitters are normally within twenty miles of each other with overlapping coverage, but may be located at any distance. The timing synchronization of the transmitted signal (a function of the speed of light causing the radio waves to arrive in or out of sync) causes some areas to be gangbusters and others to be less than desirable. Can be either analog or digital.
Trunking - Instead of a user/group having their own dedicated repeater and frequency pair, a trunking system has one or more repeaters run by a trunking controller that creates virtual channels ("talkgroups), allowing more multiple users/groups to share fewer frequency pairs. When there is traffic on a talkgroup, the trunking controller tells all mobile radios associated with that talkgroup to change to a different physical repeater frequency pair for the duration of that one message. By rapidly moving radios on and off of the physical repeater frequency pairs, limited bandwidth is shared, typically by a multiplying factor of six or more. Areas with limited traffic may only have two or three physical repeaters, while metro areas may have sixteen or more. A single site can also have repeaters on different frequency bands, allowing VHF radios to seamlessly talk with 7/800 MHz radios on the same talkgroup. Can be analog, but most are digital.
Multi-Site Trunking - Trunking on steroids. Each mobile "associates" with the closest/best tower site, and (only) when there is traffic on an associated talkgroup is instructed to change to a local, physical repeater frequency pair for the duration of that talkgroup message. When it works right, mobiles hundreds of miles apart associated with different sites in the same system can talk to each other. Statewide systems handle hundreds of messages at the same time, but only talkgroups intended for one or more mobiles associated with a site can be heard on that site. Can be analog, but most are digital.
Control channel trunking - a trunking system with one repeater in transmit 24/7. That one control repeater acts a a beacon for mobiles to find, handles all mobile association, and sends instructions from the trunking controller to associated mobiles telling them which repeater frequency pair to change to so they can hear traffic on a particular talkgroup. The most reliable for public safety as mobiles can always hear the control repeater and can instantly find a better one if the signal gets weak. Can be analog, but most are digital.
Distributed control channel trunking - One "Master" repeater carries the system control data PLUS voice. The disadvantage is that when there is no traffic the Master only transmits a 'heartbeat' pulse a few times a minute to announce its presence for mobiles to find. This takes longer for a mobile to find and associate with a site, a disadvantage if you are being shot at, dove for cover, and you radio starts looking for a site to re-associate with. Less expensive and acceptable for taxis, school buses and business fleets. Always digital.
Scanning trunking systems... two methods - Down and dirty: manually enter and scan all the voice repeater output channels. Works, but you may hear several different messages without continuity. -OR- Control channel scan: your radio sits on the control channel listening for instructions from the trunking controller and automatically switches to a different frequency when a talkgroup you are following becomes active on that site. (Most trunking scanners only need to have the control channel input, and will shift frequencies up or down "X" MHz as specified by the trunking controller.) Either analog or digital.
"Bonk" - the ugly "you cannot talk now" sound made by a mobile or portable radio trying to use a trunking system when the system is saturated or the talkgroup cannot be set up. Most commonly heard during panic or emergency situations, and a fact of life for trunking users.
Latency - The delay between the mouth moving and the voice being heard. Always present in digital systems, latency is caused by the analog voice being converted by a software codec to digital and then back again. Delays in transporting voice data packets of 1's and 0's over digital networks also add to latency. Seldom an issue unless can hear the person transmitting without the radio, or your ear can hear several nearby radios with different latency delays.
