Difference between revisions of "Logic Trunked Radio"
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Revision as of 20:26, 6 November 2018
- 1 Overview
- 2 What do I do if I cant find a LTR system in the database that I am hearing?
- 3 Can LTR be Trunktracked/Decoded?
- 4 LTR Compatible Scanners
- 5 LTR Standard
- 6 LTR Passport
- 7 LTR Standard and Passport
- 8 LTR MultiNet
- 9 LTR-Net
Logic Trunked Radio, or LTR, was originally developed by E. F. Johnson in the early 1980s. LTR systems do not have a dedicated control channel like Motorola and EDACS systems. Each repeater has its own controller and all of these controllers are coordinated together. Even though each controller monitors its own channel, one of the channel controllers is assigned to be a master and all the other controllers report to it.
Typically on LTR systems, each of these controllers periodically sends out a data burst (approximately every 10 seconds on LTR Standard systems) so that the subscriber units know that the system is there. The idle data burst can be turned off if desired by the system operator. Some systems will broadcast idle data bursts only on channels used as home channels and not on those used for "overflow" conversations. To a scanner user the idle data burst will sound like a short blip of static like someone keyed up and unkeyed a radio within about 1/2 second. This data burst is not sent at the same time by all the channels, but happens randomly throughout all the system channels.
What do I do if I cant find a LTR system in the database that I am hearing?
Can LTR be Trunktracked/Decoded?
There are a number of scanners that can track LTR Standard or LTR-Net systems (listed in the next section). None of the other LTR types can be decoded or trunked on any scanner. There are a few applications that can be used to gather data, and in some cases decode them. See our Trunked Radio Decoders article for more information.
LTR Compatible Scanners
- NOTE: These scanners can only trunktrack LTR Standard or LTR-Net
Radio Shack Scanners
LTR Standard systems have no dedicated control channel. All control data is sent as subaudible data along with voice transmissions. Systems can have any number of channels from 1 through a maximum of 20. Each channel in the system is assigned a unique number (01 through 20) and these need not be sequentially assigned. Each subscriber radio must be programmed with all channels in the system in proper logical channel order (the same requirement as EDACS systems).
Uniden scanners and Radio Shack scanners that track LTR Standard systems require that the channels be programmed in proper logical order in order to properly monitor the system, with exception of the Radio Shack Pro-92 and Pro-2067, that do not require that the LTR frequencies be in proper order but tracking is less efficiently accomplished. This ordering is referred to as the Logical Channel Number or LCN.
LTR Standard talkgroups are written in the format A-HH-GGG.
- "A" is the area code and is either 0 or 1. The area code is the same for all talkgroups in a given system and is arbitrarily chosen by the system operator; the most common use is to simply distinguish between talkgroups on multiple systems.
- "HH" is the home repeater number and has twenty possible values, 01 through 20. A talkgroup will always use its home repeater by default, unless the repeater is already in use by another talkgroup. If the home repeater is in use, the controller will assign another free repeater at random. If no repeater is free (all are in use), then the radio will receive a busy signal.
- "GGG" is the group number and has 254 possible values, 001 through 254.
Each passport site seems to have at least 3 frequencies:
- A "local" home channel for units that have this site as the home site.
- A "roaming" home channel for units that have another site as the home site.
- A "registering" channel that is used by roaming units to register with the site.
The local home channel is programmed into the radio. The roaming home channel is assigned when the unit registers with a site and seems to be the same for all units that register at that site. The registering channel is the frequency broadcast by neighboring sites in their "neighbor" information.
- Band 0 800 MHz
- Band 1 900 MHz
- Band 2 400-420 MHz
- Band 3 420-440 MHz
- Band 4 440-460 MHz
- Band 5 450-470 MHz (this is the one used by most UHF Passport systems in the United States)
- Band 6 460-480 MHz
- Band 7 470-490 MHz
- Band 8 480-500 MHz
- Band 9 490-512 MHz
- Band 10 409-430 MHz
LTR Standard and Passport
LTR Standard and Passport systems are hybrid systems that have some LTR Standard talkgroups and some LTR Passport talkgroups.
LTR MultiNet systems are APCO-16 compliant 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.
LTR-Net is an enhanced version of LTR that is backward compatible with LTR-Standard. LTR-Net sites and LCNs can be monitored with a trunking scanner capable of monitoring LTR-Standard.
LTR-Net networks consist of multiple sites and the network allows mobiles to roam from site to site. Each LTR-Net site has a Home Status Channel that sounds like an open carrier, but is broadcasting sub-audible site configuration data, mixed with normal LTR-Standard idle messages and LCN status messages. This configuration information enables roaming mobiles to self-configure to use the roaming site. In addition to the Home Status Channel, the site will have one or more traffic channels that transmit the normal LTR-Standard idle bursts, approximately every 10 seconds. The LTR signalling in each of these idle bursts is identical to what the Home Status Channel is continuously transmitting. Traffic calls are normally on the traffic channels, but can spill onto the Home Status Channel when the site is busy.
LTR-Net messages include:
- 17: Unique radio ID address
- 18: Site/Locality identifier (0 - 1023)
- 24A/B: LCN transmit frequency, per active LCN
- 25A/B: LCN receive frequency, per active LCN
- 26: Neighbor site/locality identifiers
- 28A/B: Site LCN map low (1 - 10) / high (11 - 20)
LTR-Net messages begin with a different Area Code (0 or 1) than what the site uses for LTR-Standard messages and calls, so that non LTR-Net mobiles can ignore the LTR-Net messages. LTR-Net messages use the Home Channel LCN field of the message to identify LTR-Net message types, and most of these message type numbers are higher than 20, to allow non LTR-Net mobiles to ignore the messages.
Roaming mobiles will attempt to re-use their normal home Talkgroup ID (TGID) when roaming to a new site. The mobile will use the Area Code for the roaming site and will attempt to reuse their home LCN. If there is a conflict or if the same LCN does not exist on the roaming site, the mobile will be assigned a new (temporary) home LCN to use on that site. You may see the same mobile talkgroup appear using several variations of Area Code and Home Channel across each of the sites, but the last three digits will normally remain the same. When talkgroups use limited roaming or no roaming, their TGIDs can be reused across the network.
Normal calls are simulcast/broadcast on all sites where talkgroup member radios are registered, so that all members of the talkgroup can hear all calls, no matter where they are.
Mobiles are assigned an LTR-Net unique-identifier that is unique across the network. This allows the network to individually address each radio to pass information or reprogramming. LTR-Net message #17 is the unique radio ID message and is used for assigning a unique ID or addressing a specific radio. This message is normally accompanied by an LTR-Standard start and stop call to TGID x-xx-253, to let other radios know that the LCN is temporarily busy.
TGIDs in the range of x-xx-240 to x-xx-247 are token, or temporary TGIDs that are used when a Mobile is making a landline call, auxiliary call, or data call. LCN 31 is used in the TGID when the mobile is making a Group Call. LTR-Net sites use TGID x-xx-254 when transmitting their FCC Morse Code callsign ID. TGID x-xx-255 is the standard idle message.
Some LTR-Net networks have an extension or repeater site that functions to extend network coverage into an area. Where seen, these sites use a site locality identifier of 1023. Virginia's Loudoun County School District network has one of these extender sites providing coverage in parts of Washington, DC.
See the LTR-Net Data Description article for more information.