Types
Conventional Vs. Trunked
Conventional
- Conventional radios operate on fixed RF channels. Channel is selected by a user
- In multi-channel systems channels are used to separate purposes.
- Specific function
- Channel 1 - road crews talk to road maintenance office
- Channel 2 - road crews talk to state highway dept. crews.
- Geographic location
- Taxi company uses one channel to communicate in the Boston, MA and a separate channel to communicate in the Providence, RI area.
- Scanning - Some conventional radios scan more than one channel for a valid transmission.
- Primary selected channel at full volume and other channels in a scan list at reduced volume.
- Scanning can be defined and preset. Channels not changeable by radio user.
- Some radios allow an option for user-selected scan.
- Examples of some conventional radio types:
Trunked
- The system logic automatically picks the physical radio frequency channel. A protocol defines the relationship between the radios and the radio backbone which supports them.
- Digital trunked systems carry multiple conversations on one physical channel, also called multiplexing.
- Instead of channels, radios are related by groups which may be called:
- Groups, talk groups, fleet, subfleet or agency-subfleet.
- Systems make arrangements for handshaking and connections between radios by one of these two methods:
- A computer assigns channels over a dedicated control channel. The control channel sends a continual data stream. All radios in the system monitor the data stream until commanded by the computer to join a conversation on an assigned channel.
- Electronics embedded in each radio communicate using a protocol of tones or data in order to establish a conversation.
- If all physical channels are busy, some systems include a protocol to queue or stack pending requests until channel is available.
- Visual cues of trunked system:
- Lack of a squelch knob or adjustment
- No monitor button or switch
- A chirp (made infamous by Nextel), shows channel is available and ready the moment push to talk is pressed.
- Examples of trunked radio types:
- Logic Trunked Radio
- EDACS
Simplex Vs. Duplex
Simplex - uses a single channel to transmit and receive.
- Typical of aircraft VHF AM and marine radios.
- Often “legacy” systems, which have existed for years or decades.
- Architecture is simple enough for old radios to communicate with new ones in a single network.
- Advantage: Simplest system configuration, reliability from the fact that only two radios are needed to establish communication between them.
- Disadvantages: The simplex configuration offers communication over the shortest range or distance.
Duplex - transmits and receives on different discreet channels.
- Disadvantage: Systems where equipment cannot communicate without some infrastructure such as a repeater base station.
- Advantage: Repeater configurations are common in the US base station is configured to re-transmit the audio received from mobile units
larger area than simplex system.
- Two types:
- Half-duplex – push to talk switch is required to communicate.
- Full duplex – systems like mobile phones capability to simultaneously transmit and receive conversations.
Analog Vs. Digital
Analog examples – AM aircraft radios used to communicate with control towers and air traffic controllers, Family Radio Service walkie – talkie. Equipment is less complex than digital.
- Advantage: In high-quality equipment, better ability to communicate in cases where a received signal is weak or noisy.
- Disadvantage: Only one conversation at a time can occur on each channel.
Digital examples: APCO Project 25, a standard for digital public safety radios, and Nextel’s iDEN.
- Advantage: More simultaneous talking paths possible and information such as unit ID, status buttons, or text messages can be embedded into a single digital radio channel.
- Disadvantage: Radios must be designed to the same compatible standard, radios can become obsolete quickly, cost more to purchase, and may seem more complicated.
Engineered Vs. not engineered
Engineered
- Engineered systems are designed to perform close to specification or standard.
- Integral systems with all equipment matched to performed together.
- Example: Cellular telephone system.
- Designers use a series of models to help choose equipment, locations, antennas, and estimate how signals will penetrate buildings.
Not Engineered
- Legacy systems are not engineered. Never designed to meet a system performance objective.
- May have started as a base station and a group of mobile radios. Over years, the equipment added on in a building block style.
- May perform well even though not professionally designed as a coherent system.
Life of Equipment
Usage, atmosphere and environment plays a major role to decide its life term. Average life of two way radio is 5 – 7 years and 1 – 2 years for its accessories.
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