CTCSS
Trunking
Motorola Trunking
LTR Trunking
Encryption
Project 25 Digitized Audio
Digital Paging
More information
Users may have to share a frequency, but they don't always have to listen to each others traffic. There are two common methods of sending subaudiable information. Receiving units look for the proper information, and if a signal comes in without it, the unit remains mute.
The most popular, refered to as CTCSS or PL, uses a constant tone between 67-250 Hz (below the human hearing range). Also widely used is DPL, a constant subaudible data stream of three octal digits. All units within a fleet will encode, and look for, the same tone or data stream.
CTCSS capability is highly desirable in a scanner to be used in an urban area. Not only are unwanted signals on a shared channel eliminated, virtually every form of spurius interference is also silenced, including intermod.
CTCSS eliminates hearing co-channel users, but when a conventional channel is in use, you still have to wait your turn.
Trunking was designed to improve spectrum efficiency and user performance. Each system will have between two and twenty eight channels available. If one channel is in use, the trunking system will simply send fleets using the system to another. This all happens transparent to the user, creating the effect of having exclusive use. On a fifteen channel system, for example, fifteen fleets will all have to be talking at the same time before anyone gets a busy signal. Unless there is an equipment failure, a trunked radio system user will never hear anyone other than his own fleet.
In the past, trunking presented a serious obsticle to the monitoring hobbyist. Today, there are many scanners on the market capable of following trunked systems.
My Frequency Lists include Northern Utah Trunked Systems, System ID's, owners names, and locations
Several trunking formats are currently in use:
Motorola Trunking systems use a control channel
(sample audio file). The radio will silently monitor this data, waiting for instructions to go to a voice channel and unmute. Some 800 MHz systems will alternate between 4 channels for control, changing every 24 hours. When not being used for control, a channel will be used for voice. On the newer public safety systems, the control channel will always remain the same.A Trunktracker scanner will work fine for most people wanting to listen to a Motorola system, but for the serious monitoring hobbyist, either the public domain Trunker or Unitrunker program is a must. Trunker is a much better program, but must be run on a DOS machine.
LTR Trunking, sometimes refered to as the EF Johnson trunking format, does not use dedicated control channels. Instead subaudiable data is sent on voice channels.
The radio will silently watch the data stream on it's assigned home channel for instructions to unmute or to go to another channel and unmute. If a unit starts to transmit when it's home channel is free, the conversation will take place on the home channel. If another user is conversing on the channel, the data stream will send a unit attempting to transmit, and the rest of it's fleet, to another channel.
Not all Trunktrackers are capable of following LTR trunking.
Passport (link) by Trident Micro Systems is an enhanced form of LTR. Unlike other attempts to rectify the shortcomings of LTR, US companies seem to be taking this format seriously, and are starting to put systems on the air. It's ability to provide wide area coverage, by linking sites, seems to be one of this format's main strong points. LTRTrunk/LTRDump is currently the only means of following a Passport system.
MA/COM's trunking formats are not used by any Utah system that I am aware of, though they are in use in Nevada. The NHP and NDOT share an 800 MHz EDACS format trunked network, and construction is nearing completion on a system using their new Opensky protocol for the Las Vegas Metropolitan Police. Most current trunktrackers will follow EDACS, but Opensky cannot be monitored using any known method
MPT-1327 has recently been installed by the UTA for comunications between busses and their dispatch center, and MariTEL has a network on the air using the old VHF Marine Public Correspondence (Marine Operator) channels (24, 25, 26, 27, 28, 84, 85, 86, and 87). No stand alone scanner will "trunktrack" MPT-1327, though programs are available to follow this type of system using a receiver under computer control. (Sample audio file of an MPT-1327 control channel).
Some operations will attempt to make their communications secure from interception by scrambling the audio (analog encryption). More sophisticated methods involve converting the audio into a data stream, encrypting the data, and sending it over the air in a noise envelope (digital encryption).
In the past, encryption required expensive hardware. An encrypted version of a $1000 radio could easily cost $3000. As technology progresses, it will soon only require an update of the radio's firmware. Post 9-11, interest in secure communications has dramatically increased.
Simple Inversion Scrambling (analog) sample audio file
Rolling code Inversion Scrambling (analog) sample audio file
Samples of digital formats now replaced by P-25 deleted (but available on request).
Digitized audio and encryption are not the same thing, and the fact that some operations are replacing analog voice technologies with digital should not be seen as the end looming for this hobby.
A few technologies are designed to employ encryption 100% of the time, but most are not. CDMA and GSM, the two technologies used by the cellular networks, are 100% encrypted. Motorola Tetra, used by public safety in Europe, is 100% encrypted. Fortunately, American public safety has chosen to standardize on the APCO Project 25 format, P-25 for short, sometimes also known as Motorola Astro. As long as it is not encrypted, P-25 can be demodulated by today's "digital scanners". All talkgroups at Hill AFB and the Utah Test and Training Range are now using unencrypted P-25, and on the UCAN System, P-25, encrypted or in the clear, is available, though 95% of users remain analog.
Project 25 Digitized Audio on the UCAN System - sample audio file
Formats developed for business systems (NXDN, MotoTRBO) are now on the market, and I am monitoring with the intent to capture some samples to post here.
Even the hams are putting up digital audio systems. P-25 is used by hams in some areas, but here there seems to be more interest in the DSTAR format.
Digital Paging
The bane of the scanner listener, we've all heard these squawks and squeals overloading the front end of our receivers. What they are is information being sent to numeric and alphanumeric pagers. Several formats are in use.
Golay (older format) sample audio file
512 baud POCSAG sample audio file
1200 baud Super POCSAG sample audio file
2400 baud POCSAG-2400 sample audio file
Flex (the newer of the 3 formats) sample audio file
Northern Utah digital paging system frequency list.
This page last revised on Thursday, 21 October 2010