Details
These radios are seen as obsolete and there is only one known instance of this system operating in the U.S. today (Grant County, Oklahoma). The general category of this kind of trunked system is called, "Scan-based trunking." In the U.S. and Australia, these systems used analog FM, operated in the 806-869 MHz band, and were primarily used for commercial, non-public-safety trunking. Some earlier systems offered half-duplex, (push-to-talk) telephone interconnect and later versions also offered full duplex telephone interconnect. This feature was popular before the rollout of analog cellular telephones.
Obsolescence is the state of being which occurs when an object, service, or practice is no longer wanted even though it may still be in good working order; however, the international standard EN62402 Obsolescence Management - Application Guide defines Obsolescence as being the "transition from availability of products by the original manufacturer or supplier to unavailability". Obsolescence frequently occurs because a replacement has become available that has, in sum, more advantages compared to the disadvantages incurred by maintaining or repairing the original. Obsolete also refers to something that is already disused or discarded, or antiquated. Typically, obsolescence is preceded by a gradual decline in popularity.
In telecommunications and signal processing, frequency modulation (FM) is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave.
A cellular network or mobile network is a communication network where the last link is wireless. The network is distributed over land areas called cells, each served by at least one fixed-location transceiver, but more normally three cell sites or base transceiver stations. These base stations provide the cell with the network coverage which can be used for transmission of voice, data, and other types of content. A cell typically uses a different set of frequencies from neighboring cells, to avoid interference and provide guaranteed service quality within each cell.
Radio models used in these systems included the names, "Classic", "Corona", "Centura", generally referred to as 3C radios. While most radio models were sold with a speaker and microphone, some had a telephone handset and cradle attached to the front of the dash-mount radio housing. There was an earlier trunk-mount model that looked like a MASTR Executive II with the name Mastr. A MPR-series hand-held model was offered for GE Marc V trunked systems. Later models included a Japanese-made radio with model numbers beginning with TMX (mobiles) and TPX (portables). Certain later models of PCS and MDX radios were dual-mode, supporting GE Marc V and EDACS.
A large system operator might own their radio backbone (repeaters). GE Marc V Specialized Mobile Radio systems, where subscribers paid a company to operate the trunked radio system, were present in major U.S. cities. The operators typically charged a monthly fee for each radio plus airtime charges.
Some mobile units could be programmed to work on more than one GE Marc V trunked system. For example, if the SMR operator had a system in Bloomington and another covering Chicago, many radio models offered an "area" switch. This selected the transmit area priority. Different radio models had 29- to 100-channel capacity and there were limitations on the number of areas and the number of channels per area. In its default configuration, the radios function in the same way a talk-back-on-scan option works: regardless of which area was selected, the radio would join a conversation on the system where it was taking place.
Early first generation radios were crystal controlled and had crystal oscillator modules with temperature compensators (ICOMS), and later models were synthesized and had an 82S123 32x8 (not 32Kx8) PROM which defined RF frequencies. Option settings and tones were selected by jumpers and laser-etched hybrids called Versatones. Second generation "3C" radios used an Intel MCS-48 microprocessor and programming was burned into two 82S123 PROMs. In subsequent generation radios, a PC programming interface set the radio's RF channels and options. In U.S. systems, the set of channel frequencies for a system or area were usually unique to each trunked system. The radios could scan multiple systems provided that the total number of channels in the combined systems did not exceed 20. The radios were not capable of roaming. (Roaming in this use means working with an unknown GE Marc V system the user happened to run across in their travels).
A programmable read-only memory (PROM) is a form of digital memory where the setting of each bit is locked by a fuse or antifuse. It is one type of ROM. The data in them is permanent and cannot be changed. PROMs are used in digital electronic devices to store permanent data, usually low level programs such as firmware or microcode. The key difference from a standard ROM is that the data is written into a ROM during manufacture, while with a PROM the data is programmed into them after manufacture. Thus, ROMs tend to be used only for large production runs with well-verified data, while PROMs are used to allow companies to test on a subset of the devices in an order before burning data into all of them.
How it worked
In a GE Marc V system:
- Electronics that determine which channel a conversation will occur on are inside each radio. When the user takes the microphone off-hook, the radio scans all available channels in the system to find an unused channel for a conversation. The user would hear a low-pitched error tone if they took the microphone off hook and no channels were available or the radio was out-of-range.
- When idle, radios scan to look for a tone sequence initiating a conversation from another radio in their own group.
- Repeaters are essentially stand-alone repeaters, except for tone handshaking electronics, deadbeat disable computer, telephone patch equipment, or equipment used to bill air time (talking time). Some of these were options.
- Because call set up was slower than "transmission-trunked" systems, repeaters had a long hang time in order to hold a group's mobile radios on a channel until a conversation was finished. This "conversation trunking" was the primary difference between GE Marc V and other systems.
- A trunked system or area could have up to 20 channels. Some 100-channel models, for example, could accommodate five 20-channel areas.
Two-tone sequential
GE Marc V used a two-tone sequence to identify a group: what modern systems call agency-fleet-subfleet or talk groups. Each radio had at least one tone pair, which identified the group of radios it could talk with. It was similar in format to two-tone sequential paging codes except that, in a GE Marc V system, the first tone was much longer than the second. This long first tone gave a bigger time window for all the scanning radios to find and decode a two-tone sequence. The first tone was lengthened for systems with more channels.
It was possible for radios to have several tone pairs or groups. These could be used to make phone calls over a patch without all other users in the same group having to listen to the call. Some systems had hierarchies: manager groups could talk amongst one another without going out over everyone's radio.
As larger systems used up most of the available tone pairs, it became necessary to alter the tone sequence to generate a total of four tones, one longer "collect" tone followed by three group tones in rapid sequence. This was known as GE Marc V-E for "enhanced". This also reduced a falsing problem that existed in larger systems where mobile radios would open up on other users tones as well as intermodulation products occurring in large metropolitan areas.
Continuous tone squelch
Systems transmitted a continuous tone (at 3051.9 Hz by default), called Busy Tone. A low-pass filter eliminated most of the tone from speaker audio. The tone was present through the transmission, but cut off just before the repeater dropped in order to eliminate squelch tail. There were two possible continuous tones: all radios on a single system used the same tone. These were used to distinguish between sites in the same way SAT tones were used on AMPS cellular systems. The first system in an area typically used the default tone of 3051.9 Hz, while 2918.7 Hz could be used for an additional system on the same channel(s) that might have some coverage overlap. Radios using the system had to have their continuous tone set to match the desired system and have a compatible two-tone sequence for their group.
Advanced Mobile Phone System (AMPS) was an analog mobile phone system standard developed by Bell Labs, and officially introduced in the Americas on October 13, 1983, Israel in 1986, Australia in 1987, Singapore in 1988, and Pakistan in 1990. It was the primary analog mobile phone system in North America through the 1980s and into the 2000s. As of February 18, 2008, carriers in the United States were no longer required to support AMPS and companies such as AT&T and Verizon Communications have discontinued this service permanently. AMPS was discontinued in Australia in September 2000, in Pakistan by October 2004,, in Israel by January 2010, and Brazil by 2010.
Transmitter tests
Since the radio would not transmit unless the ready light was lit, checking output power, frequency, or deviation on the bench or outside the system coverage area required the technician to attach a test box. The box bypassed the radio trunking logic so adjustments could be made in a conventional single-channel mode. Antenna forward and reflected power in a vehicle on a working system was typically checked by bringing up a talk channel and transmitting normally. Once a channel was brought up it could be 'held' for up to 3 minutes (the limit of the system carrier control timer).
Operator view of the system
If the operator desired to make a radio call, they would pull the mike out of the hang-up box. This would cause the radio to look for a channel with no busy tone (3051.9 Hz) present. When a vacant channel was found, the push-to-talk relay would chatter and the transmit indicator would flicker. The radio would handshake with the repeater using a single analog tone. The handshake would identify the repeater as "in range" and the channel as "not busy." If the radio successfully performed the busy handshake, its two-tone group sequence was transmitted over the air. As soon as the tones were sent, the receiver audio turned on and the radio made a doorbell-like "ding dong" chime to tell the operator the channel was available to talk. A green "ready-to-talk" light-emitting diode on the radio lit. The speaker turned on and the operator heard idle carrier from the repeater.
A doorbell is a signaling device typically placed near a door to a building's entrance. When a visitor presses a button the bell rings inside the building, alerting the occupant to the presence of the visitor. Although the first doorbells were mechanical, activated by pulling a cord, modern doorbells are generally electric switch and the most interesting
Other radios in the same group would scan until they heard a first tone matching their own two-tone sequence. Hearing a matching first tone, the scanning would stop and wait to determine if the second tone matched its group. If the tone didn't match, the radio would silently go on searching. On hearing a matching tone, every radio in the group would do the doorbell chime, display a green, ready indicator, and the speaker audio would turn on. The user would hear the open carrier until someone talked or the repeater carrier dropped after approximately 8 seconds.
Since the doorbell sound was annoying, users tended to try to hold the repeater carrier on until the called party answered so they wouldn't have to listen to continual ding-dongs. If base was slow to answer, the transmission might sound like, "Unit four to base," <squelch tail>... <squelch tail>... <squelch tail>... <repeater drops>. On some radio models, the doorbell sound was programmable but it's not clear if those responsible for systems knew this (Note: some operators of this system knew this was possible, but did not change it as it would have confused the users).
Missed calls
A major drawback to this architecture is the fact that a missed two-tone sequence at the beginning of a transmission, or a lost signal during a transmission, causes the rest of a transmission to be missed. If the mobile receiver gets interference or loses the repeater signal for the moment the second selective calling tone is sent, it remains muted, missing the entire conversation. The same is true if the repeater signal is lost at any time during a transmission. Only another two-tone sequence being sent will allow it to rejoin the conversation.
Some later systems were equipped with a 'join' feature to overcome this problem. The system would monitor all channels in a given system and, if it detected a mobile attempting to acquire a second channel, that user would be invalidated on that channel (dumped), and immediately afterwards a 2-tone signaling sequence would begin on the original channel, bringing the lost unit into the original conversation. This could be highly annoying to users already in a conversation, as the very audible collect and group tones would interrupt the conversation, and if the invalidated user persisted in attempting to bring up a channel rather than waiting for the system to signal him, the collect and group tones would continue to be sent out on the working channel repeatedly on each channel attempt.
A modern trunked system with a control channel is more costly and complicated, but sends continual messages for all in-progress conversations. If your radio is in a talk group conversing on channel 3, the control channel continually sends "<go to channel 3>" messages over and over until the transmission ends. Suppose the user is driving through a tunnel with no signal then exits and acquires the signal. If the mobile receiver acquires the control channel signal any time during the conversation, it immediately decodes the channel assignment and switches over to join the conversation.
In telecommunications, squelch is a circuit function that acts to suppress the audio output of a receiver in the absence of a sufficiently strong desired input signal. Essentially, squelch is a specialized type of noise gate designed to suppress randomized signals. Squelch is widely used in two-way radios and radio scanners to suppress the sound of channel noise when the radio is not receiving a transmission. Squelch can be opened, which allows all signals entering the receiver to be heard. This can be useful when trying to hear distant or otherwise weak signals, for example in DXing.
A radiotelephone is a communications system for transmission of speech over radio. Radiotelephone systems are very rarely interconnected with the public switched telephone network, and in some radio services, including GMRS, such interconnection is prohibited. "Radiotelephony" means transmission of sound (audio) by radio, in contrast to radiotelegraphy or video transmission. Where a two-way radio system is arranged for speaking and listening at a mobile station, and where it can be interconnected to the public switched telephone system, the system can provide mobile telephone service.
Terrestrial Trunked Radio, a European standard for a trunked radio system, is a professional mobile radio and two-way transceiver specification. TETRA was specifically designed for use by government agencies, emergency services, for public safety networks, rail transport staff for train radios, transport services and the military. TETRA is European version of trunked radio similar to Project 25.
In telecommunications, Continuous Tone-Coded Squelch System or CTCSS is one type of circuit that is used to reduce the annoyance of listening to other users on a shared two-way radio communications channel. It is sometimes referred to as tone squelch. It does this by adding a low frequency audio tone to the voice. Where more than one group of users is on the same radio frequency, CTCSS circuitry mutes those users who are using a different CTCSS tone or no CTCSS. It is sometimes incorrectly referred to as a sub-channel because no additional channels are created. All users with different CTCSS tones on the same channel are still transmitting on the identical radio frequency, and their transmissions interfere with each other, however the interference is masked under most conditions. The CTCSS feature also does not offer any security.
Professional mobile radio are person-to-person two-way radio voice communications systems which use portable, mobile, base station, and dispatch console radios. PMR radio systems are based on such standards as MPT-1327, TETRA, APCO 25, and DMR which are designed for dedicated use by specific organizations, or standards such as NXDN intended for general commercial use. These systems are used by police, fire, ambulance, and emergency services, and by commercial firms such as taxis and delivery services. Most systems are half-duplex, in which multiple radios share a common radio channel, and only one can transmit at a time. Transceivers are normally in receive mode, the user presses a push-to-talk button on his microphone when he wants to talk, which turns on his transmitter and turns off his receiver. They use channels in the VHF and UHF bands, giving them a limited range, usually 3 to 20 miles depending on terrain. Output power is typically limited to 4 watts. Repeaters installed on tall buildings, hills or mountain peaks are used to increase the range of systems.
The Improved Mobile Telephone Service (IMTS) was a pre-cellular VHF/UHF radio system that links to the PSTN. IMTS was the radiotelephone equivalent of land dial phone service. It was introduced in 1964 as a replacement to Mobile Telephone Service or MTS and improved on most MTS systems by offering direct-dial rather than connections through a live operator.
UHF CB is a class-licensed citizen's band radio service authorised by the governments of Australia, New Zealand, Vanuatu, and Malaysia in the UHF 477 MHz band. UHF CB provides 77 channels, including 32 channels allocated to repeater stations. It is similar in concept to 27 MHz CB Radio in the United States, Australia, and New Zealand.
A land mobile radio system (LMRS), also called public land mobile radio or private land mobile radio, is a person-to-person voice communication system consisting of two-way radio transceivers which can be mobile, installed in vehicles, or portable (walkie-talkies). Public land mobile radio systems are made for use exclusively by public safety organizations such as police, fire, and ambulance services, and other governmental organizations, and use special frequencies reserved for these services. Private land mobile radio systems are designed for private commercial use, by firms such as taxis or delivery services. Most systems are half-duplex, with multiple radios sharing a single radio channel, so only one radio can transmit at a time. The transceiver is normally in receiving mode so the user can hear other radios on the channel; when a user wants to talk he presses a push to talk button on his microphone, which turns on his transmitter. They use channels in the VHF or UHF bands giving them a limited range, usually 3 to 20 miles depending on terrain, although repeaters installed on tall buildings, hills or mountain peaks can be used to increase the coverage area. Older systems use AM or FM modulation, while some recent systems use digital modulation allowing them to transmit data as well as sound.
An amateur radio repeater is an electronic device that receives a weak or low-level amateur radio signal and retransmits it at a higher level or higher power, so that the signal can cover longer distances without degradation. Many repeaters are located on hilltops or on tall buildings as the higher location increases their coverage area, sometimes referred to as the radio horizon, or "footprint". Amateur radio repeaters are similar in concept to those used by public safety entities, businesses, government, military, and more. Amateur radio repeaters may even use commercially packaged repeater systems that have been adjusted to operate within amateur radio frequency bands, but more often amateur repeaters are assembled from receivers, transmitters, controllers, power supplies, antennas, and other components, from various sources.
A radio repeater is a combination of a radio receiver and a radio transmitter that receives a signal and retransmits it, so that two-way radio signals can cover longer distances. A repeater sited at a high elevation can allow two mobile stations, otherwise out of line-of-sight propagation range of each other, to communicate. Repeaters are found in professional, commercial, and government mobile radio systems and also in amateur radio.
Selcall is a type of squelch protocol used in radio communications systems, in which transmissions include a brief burst of sequential audio tones. Receivers that are set to respond to the transmitted tone sequence will open their squelch, while others will remain muted.
Motorola Type II refers to the second generation Motorola trunked radio systems that replaced fleets and subfleets with the concept of talkgroups and individual radio IDs. There are no dependencies on fleetmaps, therefore there are no limitations on how many radio IDs can participate on a talkgroup. This allows for greater flexibility for the agency. When scanning Motorola IDs, each Type II user ID appears as an even 4- or 5-digit number without a dash.
LTR MultiNet Systems are APCO-16 compliant LTR Trunked Radio Systems and thus are mostly found in use as public safety systems. LTR MultiNet systems usually have one or more "status channels" that act like a control channel in a Motorola or EDACS system, however these channels can also carry voice transmissions simultaneously.
MPT 1327 is an industry standard for trunked radio communications networks.
In a conventional, analog two-way radio system, a standard radio has noise squelch or carrier squelch, which allows a radio to receive all transmissions. Selective calling is used to address a subset of all two-way radios on a single radio frequency channel. Where more than one user is on the same channel, selective calling can address a subset of all receivers or can direct a call to a single radio. Selective calling features fit into two major categories — individual calling and group calling. Individual calls generally have longer time-constants: it takes more air-time to call an individual radio unit than to call a large group of radios.
The Motorola Minitor is a portable, analog, receive only, voice pager typically carried by fire, rescue, and EMS personnel to alert of emergencies. The Minitor, slightly smaller than a pack of cigarettes, is carried on a person and usually left in selective call mode. When the unit is activated, the pager sounds a tone alert, followed by an announcement from a dispatcher alerting the user of a situation. After activation, the pager remains in monitor mode much like a scanner, and monitors transmissions on that channel until the unit is reset back into selective call mode either manually, or automatically after a set period of time, depending on programming.
Remote controls are used any time a two-way radio base station is located away from the desk or office where communication originates. For example, a dispatch center for taxicabs may have an office downtown but have a base station on a distant mountain top. A Tone remote, also known as an EIA Tone remote, is a signaling system used to operate a two-way radio base station by some form of remote control.
Specialized Mobile Radio (SMR) may be an analog or digital trunked two-way radio system, operated by a service in the VHF, 220, UHF, 700, 800 or 900 MHz bands. Some systems with advanced features are referred to as an Enhanced Specialized Mobile Radio (ESMR). Specialized Mobile Radio is a term defined in US Federal Communications Commission (FCC) regulations. The term is of US regulatory origin but may be used in other regions to describe similar commercial systems which offer a radio communications service to businesses.
