South Brunswick's Trunked Radio System |
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Table of Contents |
Notes from Author |
South Brunswick has been using a GE-Ericsson EDACS Trunked Radio System since about 1993. All municipal services, from the Police Detectives to the Bookmobile, use this system.
There is too much information to fit on just one page. A second page has been created to try to alleviate this problem. The second page contains mostly details, and is referenced in appropriate places from this, the main page. Also of interest may be a page describing South Brunswick's Emergency Services (since discontinued).
Readers already familiar with the basic operation of trunked radio systems may want to skip ahead a few sections to the Behind the Scenes of EDACS section, or further.
Trunked systems make much more efficient use of radio frequencies than traditional, even CTCSS-enhanced systems. Trunked systems can spread tens of thousands of talkgroups ("groups" here in) over a very small number of repeated frequencies. The limiting factor is most often peak demand, as the maximum number of users who can transmit simultaneously equals the number of repeater channels minus the number of control channels (see below).
The key to managing all these groups is the site controller, working on the control channel, also known as the "handshake" channel. The site controller is simply a computer that manages the system by broadcasting data over the control channel, to all the remote radios/devices. The control channel, which usually can occupy any of the channels, will be apparent; there is a continuous sound (the data) on it. The control channel and regular voice messages may avoid a channel if the system is configured to reserve that channel for data (MDT's, monitoring stations, etc.) or special voice messages (scrambled/digital, plectron paging, etc.). All radios constantly listen to the control channel, even while receiving a voice message. In it's own data format, the site controller tells the individual radios when to receive a transmission. Each radio is pre-programmed with the talkgroups appropriate to the user of the radio. The site controller will basically say, Transmission for talkgroup 0001 from radio 0001 on repeater channel 01. Upon receiving this, all radios on group 1, or who have 1 in the scan list and aren't otherwise busy, tune to repeater channel 1 and open up to receive the signal. When the transmission is over, the site controller will stop saying this, and the radios programmed for group 1 will stop listening to repeater channel 1.
For a transmission to go through, the remote radio will transmit a quick code (a "Handshake") that includes the radio number and the talkgroup or Agency/Fleet/Subfleet it is on, all to essentially request permission to transmit. The site controller will then send the approval or denial over the control channel, and the mobile radio will either switch to the dictated channel for the voice message, or give the user an error message based on the denial reason. A transmission may be denied because the system is full (user will usually get a "wait" tone in this case), the radio has been muted, and more. The first part (the data burst) is transmitted 45 MHz below the control channel on its paired input.
As mentioned above, one trunked system on a dozen frequencies can support hundreds of talk groups, many more "channels" than older-design systems. Not every talk group will be needed at once, so the site controller takes advantage of this. The system can only support as many simultaneous transmissions as it has repeater channels, not counting the control channel. If there is too high a demand, the controller can either give preference to radios with pre-programmed priority, or to the radio that keyed up first. Either way, users that are forced to wait will receive a wait tone on their end.
Trunked systems can also permit radio-to-radio communication, instead of broadcasting to the entire group. This is commonly known as Individual Call. In order for a user to individual call another, the first person has to pick the Radio Number of the person they wish to contact. The radio keys up similarly to a normal transmission, but transmits the destination radio id instead of the destination talkgroup. The site controller then tells the destination radio to listen to listen to a particular repeater channel, and tells the originating radio to transmit on the paired input to that channel. Depending on the radio model, the radio being called can display the originating radio, kind of like caller id. If done properly, no third person will receive any of this. It is possible, however, that dispatch consoles or even remote radios (ostensibly for system managers) can be programmed to receive third party individual calls.
If a radio is stolen, the site controller can be told to ignore any transmissions originating from that radio number. That way criminals can not interfere with operations. Similarly, radios not programmed by the township have a miniscule chance of transmiting on the system, regardless of compatability of the hardware. Theoretically, an outside user could key up on the input frequency and get it repeated if an approved user went through the site controller, and was assigned the same repeater channel. The outsider's signal would have to overpower the insider's, and he would have no control into which group the signal is inserted. An open carrier may not cause this, as the site controller may be programmed to check repeater frequencies for both local and outside traffic before assigning it to a new call.
Users of trunked radio systems may be able to place a phone call via their two-way radio. This requires special radios, and special configurations on the tower and site-controller end. Often repeater channels are reserved exclusively for these phone patches, rather than also containing regular communications.
The continuous data channel facilitates remote programming of radios and modification of the entire system without operations grinding to a halt until radios can be brought in to the system manager for reprogramming. Similarly, some radios may easily be shut down remotely, in case of theft or abuse.
Many of the advantages of trunking also work against it in certain situations. A trunked system is the ultimate in a private, closed system. The result is a decreased ability to communicate with outside agencies, unless arrangements are made. These could be as simple (and potentially expensive) as equipping everyone with the necessary radios, or creating an uplink through a conventional channel (similar to what was done by Kendall Park FARS on SB's TRS).
The system is restricted to a finite number of frequencies. If designed well, it will rarely be a problem, but it can become troublesome in a major incident. For example, South Brunswick has 4 channels available. That allows one for EMS, one for Fire, one for Police, and one for everyone else. There is no defined allotment, but it is evident how quickly the system can be maxed out. It is also possible for a repeater channel to go down because of hardware problems, which would cut South Brunswick's system capacity by 25%.
Trunked radios don't like to work even on the fringes of reception. To receive, the radio must be able to semi-clearly (thanks to error correction, etc.) receive the control channel, and the carrier on the voice channel must break the squelch threshold even after the radio has been alerted to listen for it. If the radio can not receive the control channel for a programmed length of time, it may revert to the fail-safe system (see below). In order to transmit, the radio must be able to reach the site controller with the Handshake, and subsequently receive the site controller's channel assignment. The radio will try several times over two seconds to send the handshake, but if it can't get through, the trunked system on that particular radio is useless.
A fail-safe system utilizing the same frequencies as the primary system is designed to take over in case of system-wide failure. Radios that don't receive the control channel for a period of time may revert to this system, but will still be useless unless there is system-wide failure (since the site controller is still putting out the control channel, and trunking everything else). A secondary (not to be confused with fail-safe), point-to-point system is also in place for most TRS. These systems, intended mostly for distant talk-around, generally use totally different frequencies from the primary, and are in no way compatible.
Users of high-tech trunked systems may be lulled into a false sense of security. Many manufacturers advertise that trunked systems are more difficult for scanners to monitor. It is difficult, but not impossible (as seen below). Many users also think that individual calls are secure, but they are broadcast as standard analog waves, just like the rest of the voice communications.
EDACS, or Enhanced Digital Access Communications System, is GE / Ericsson's entry to the Trunked Radio System field. The other popular systems are by Motorola, and, to a lesser degree, LTR. Like Motorola, EDACS has a continuous control channel. There are three easy ways to distinguish between EDACS and Motorola.
The EDACS control channel is audibly different from Motorola's. [ 5s .wav of Essex County's EDACS Control Channel, ~54k ] [ 5s .wav of NJ State Police / Troop B's Motorola Control Channel, 54k ] For a much more detailed description of the EDACS control channel content and formatting, and suggestions on how to decode it and put it to use in a home-made EDACS "Trunktracker", see two postings from a scanner newsgroup. edacs1.txt and edacs2.txt Unfortunately, attribution has been lost. A compiled version of the program quoted in this posting is available at ftp://ftp.netcom.com/pub/so/source/edacs.zip. The author has not tried this program, and can not take responsibility for any resultant problems.
EDACS has the "GE Jingle." After most transmissions, the repeater broadcasts a series of 5 beeps of constant pitch and frequency (a few systems reportedly mimic the "We Bring Good Things to Life" (registered, GE Corp) jingle). In actuality, the first beep is generated by the transmitting radio, to signal the site controller/repeater of the end of the transmission. The next four beeps are generated by the repeater. For transmissions appended with only four beeps, either the first beep wasn't received (in which case there is usually a 1-2 second burst of static before the repeater decides the transmission ended), or the transmission originated at a dispatch console that is wired directly into the site controller, rather than use the repeater inputs. [ 3s .wav of SB's Jingle, ~33k ]
The most-hated distinguishing characteristic of EDACS is the anti-scanner (or scan defeat) tone. Implemented on many systems, this is an obnoxious buzzing sound generated by the site controller and transmitted on many, if not all, repeater channels. There is always a pattern to it; the buzzing lasts a constant amount of time, has a constant period, and usually goes in the opposite direction of the voice assignments (if voice ascends, the buzzing descends through the channels). [ 5s .wav of SB's anti-scanner tone, ~56k ]
It is believed that EDACS trunked radios open the squelch completely to receive the control channel, but require a signal to break the squelch threshold to receive regular communications. As a result, some radios may be receiving the the control but not the assigned repeater channel, in which the radio will display that it is receiving a message on group xxx, but will remain silent. In other circumstances, the radio may receive the opposite signals, and switch to the fail-safe system thinking there is a problem. At this point, the anti-scanner tones become a real problem, as they do the same to the two-way radios as to scanners.
Channel Freq |
Frequencies: Repeaters 1-5 and their paired inputs (Repeater Fx -45 MHz= Input Fx) are for the primary, trunked system. The control channel can occupy any of them, and has recently been heard on 1 and 2.
Channel Assignment: The site controller assigns voice transmissions to the next channel in ascending order (this changed from descending in the early part of 1998). Example: 1, 2, 3, 4, 1, 2, 3, 4, 1, 2, ... (In this example the control channel is on channel 5 so the voice transmissions skip it.) Between the voice and scan defeat, the defeat used to have priority. This is probably still the case, but has not been confirmed since the change. The voice transmission would have to skip ahead if the next channel is occupied by the buzzing.
Anti-scanner Tones: The tones (5s .wav, ~56k, full length of one tone) are assigned in ascending order, last 5 seconds, and are spaced out about 15 seconds between. They appear to have priority over voice transmissions in channel assignments, but will be aborted by the site controller if the system approaches being overloaded. The anti-scanner tones used to be set to change frequencies and resume immediately, so there was always a "garbage" carrier. During the Iron Mountain Warehouse fires (~March 97) the scan defeat was turned off; when re-activated, it was in the current sequence.
The GE Jingle: South Brunswick's jingle is the more popular kind; the four beeps plus whatever the radio adds. The clip of the jingle (3s .wav, 33k) is from the end of a console transmission; you'll hear a pronounced audio drop, then the four beeps the repeater adds.
Individual Call: It is believed that there may only be one individual call at a time. At times the entire function has been shut off, apparently during heavy usage and when the system manager decides the privilege is being abused.
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With Scan Defeat
Program in the five repeater frequencies in ascending order (or tell the radio to scan in ascending order). Lock out the control channel, and turn off delay for all. Every time the scanner stops on the scan defeat buzzing, hit the scan button to check the other 3 voice channels. You can also use the 'No Scan Defeat' method, but you may miss a transmission when the defeat is active on the channel you're waiting on. |
Without Scan Defeat
Determine which frequency is being used for the control channel and note down the other four. Program those four into a bank in ascending order, filling the bank if possible. Turn off delay. Scan until you hear a transmission, and hit manual. Immediately after the voice transmission is finished, hit the manual button to go to the next channel. Wait for the next transmission. If the system is busy, you may have to play it by ear. It is possible that two or three, or even four voice channels will be active at once. This makes it difficult if not impossible to track individual conversations. |
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Alternate Methods
The scan defeat does it's job well by making it very annoying and time consuming to monitor the system. It may be more convenient to monitor the EMS and Fire plectron dispatch, neighboring police channels, and county-wide mutual aid channels (like the SPEN's or county Hotline) to get a hint of something in South Brunswick. Then tune in to the trunked system. A sure-fire way of avoiding all scan defeat is to monitor only the inputs to the repeater. To do this, first program in the five inputs. Then determine what frequency the control channel is using, and lock out the input to it (the input channel will only be used for the data bursts, no voice). Turn off delay and scan the remaining four. You will not hear the dispatcher or units out of range of you. |
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Probe Users:
If you have an OptoScan 456 or 535 with Probe software, there is a way to squelch out the scan defeat. My Probe/535 combo seems to think that the anti-scanner tones are broadcast with a CTCSS tone of 67.0 Hz. I'm not sure that this is the case, or if the anti-scanner tones themselves fool the 535, but either way you can take advantage of this. The voice messages are broadcast with no CTCSS or DCS tone/code, so program all the repeater channels in with a 'tone' value of 1.0 (so that Probe squelches any signal that does have a tone/code). Lock out the control channel, and you're set to go. The jingle still comes through, but you may even be able to work around that by setting 'templock' to 4-5 seconds, and activating it at the end of every voice transmission. This is labor-intensive to do, but the worst will be taken care of by by the tone squelch function of Probe. |
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This page is intended to give general information about trunked radio systems, and apply it to South Brunswick's. The author is in no way representing South Brunswick Township, or any other organization with which he may be affiliated.