LTR MultiNet

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.

APCO 16 compliance

Some trunked systems queue calls if a user's attempt to transmit gets a busy signal. In other words, if someone presses their push-to-talk button and all trunked radio system channels are busy, some systems will wait-list users in the same order as their busy signals occur. When a channel becomes available, the system notifies the user. There is disagreement about MultiNet's ability to queue calls when all channels are busy.[1] Usually, the control channel is the path allowing wait-listed users to get in line. One publication says MultiNet communicates using low baud rate data multiplexed under voice if all channels are busy. One report says MultiNet users on a live system who got a busy had to either hold their push-to-talk button down continually until the system assigned them a channel or periodically check for an available channel by repeatedly pressing the push-to-talk.[2]

Typical system capabilities

Like any trunked system, LTR MultiNet allows users to be grouped to virtual channels on a system backbone. The system backbone consists of repeaters configured to support the LTR protocol. The virtual channels, called system and group, are controlled by the system electronics. In an LTR system, the electronics that set up communications between radios are housed in each radio set, (scan-based). The MultiNet system has some form of central controller in addition to the scan-based radio electronics.[3] When idle, radios scan all system channels searching for a valid system-and-group call. When a valid call is detected, all radios set to the detected system and group, (virtual channel) meet on a physical channel and can communicate. A valid call could be an individual call or a call to a group of radios.

There are group virtual channels, where groups of mobile and hand held radios are addressed based on their system and group (virtual channel selector) setting. On a commercial system, a typical group might be a small business such as a towing company. On a government system, a typical group might be a group of people in a department with similar tasks such as the city street department. When any user in the group talks, all radios with their selector set to that group meet on a virtual channel and can communicate. When the conversation ends, radios immediately begin a search for another call.

Units can be individual-called: a dispatcher can call a single mobile or hand-held radio and initiate a conversation that will not be heard by any other radios in the system. A form of individual call is the telephone patch. Some systems allow half-duplex telephone calls to be placed. The radio user must use the push-to-talk button to speak and cannot talk and receive simultaneously.

Unlike central-controlled trunked systems, LTR MultiNet does not provide a busy queue. If a user gets a busy signal, they will not automatically be called when a channel becomes available. [4]

Radios have an "emergency" capability which can alert a dispatch center that the person with the radio has pressed an emergency button and needs assistance.

San Rafael, California Police Department system

After six years of use, a consultant was hired in May 1994 to evaluate perceived problems with the Police LTR MultiNet radio system. A committee was convened to review the consultant report and devise a strategy to resolve issues. A 1995 report to the Mayor and City Council found the city's Police LTR system could not be fixed, "...without redesigning or replacement of the E. F. Johnson system and/or its components."[5] The report describes problems with the specific MultiNet system implemented by San Rafael Police including:

Estimated replacement cost for the system as of the 1995 report's date was $1,190,000. The committee recommended replacement of the trunked system with a conventional simulcast system.[15]San Rafael now uses the MERA (Marin County) Motorola ASTRO SmartZone system and got rid of the LTR-Multinet system.

Notes

  1. An article claiming MultiNet does queue calls is: "4.2.4 Logic Trunked Radio (LTR)," Arizona Phase II Final Report: Statewide Radio Interoperability Needs Assessment, (Phoenix, Arizona: Macro Corporation and The State of Arizona, 2004), pp. 46.
  2. "3.4 Radio System Deficiencies," San Rafael Police Radio Committee: Report to Mayor and City Council, (San Rafael, California: City of San Rafael, 1995,) pp. 13.
  3. From literature, it's unclear what the LTR controller does other than rudimentary tasks like deadbeat disable in commercial systems.
  4. "San Rafael Police Radio Committee: Report to the Mayor and City Council," July 1, 1995, (San Rafael, California: City of San Rafael, 1995), (page 13). It's odd the system could be APCO-16 compliant, as the originator of this article asserts, and not have this capability.
  5. "San Rafael Police Radio Committee: Report to the Mayor and City Council," July 1, 1995, (San Rafael, California: City of San Rafael, 1995), (page 3). Not every implementation of an LTR MultiNet system will experience these problems. This is simply one case study with real data.
  6. "San Rafael Police Radio Committee: Report to the Mayor and City Council," July 1, 1995, (San Rafael, California: City of San Rafael, 1995), (page 14). Loading refers to the number of licensed radio units on a system. San Rafael Police had about 100 radios total. FCC regulations require the transmitter call sign to be transmitted every 30 minutes. The Morse code identifiers did this automatically at a cost of tying up one of the two channels momentarily.
  7. "San Rafael Police Radio Committee: Report to the Mayor and City Council," July 1, 1995, (San Rafael, California: City of San Rafael, 1995), (page 10).
  8. "San Rafael Police Radio Committee: Report to the Mayor and City Council," July 1, 1995, (San Rafael, California: City of San Rafael, 1995), (page 11, 16). Many possible causes for ghost signals exist. The emergency button switch could be defective or have a conductive liquid spilled in it. The presence of a radio-frequency energy field from another nearby transmitter may cause the circuit to falsely trigger the emergency encoder circuit. A user could accidentally press the button without knowing it or another piece of equipment on the user's duty belt might depress the button. Can you imagine others?
  9. "San Rafael Police Radio Committee: Report to the Mayor and City Council," July 1, 1995, (San Rafael, California: City of San Rafael, 1995), (page 11-12, 15).
  10. "San Rafael Police Radio Committee: Report to the Mayor and City Council," July 1, 1995, (San Rafael, California: City of San Rafael, 1995), (page 4).
  11. "San Rafael Police Radio Committee: Report to the Mayor and City Council," July 1, 1995, (San Rafael, California: City of San Rafael, 1995), (page 11).
  12. "San Rafael Police Radio Committee: Report to the Mayor and City Council," July 1, 1995, (San Rafael, California: City of San Rafael, 1995), (page 13).
  13. "San Rafael Police Radio Committee: Report to the Mayor and City Council," July 1, 1995, (San Rafael, California: City of San Rafael, 1995), (page 13).
  14. "San Rafael Police Radio Committee: Report to the Mayor and City Council," July 1, 1995, (San Rafael, California: City of San Rafael, 1995), (page 12).
  15. This was an alternative based on the countywide radio plan not being completed by the 1995 decision date. San Rafael has the difficult-to-cover terrain typical of many coastal California cities. Building a system with reliable coverage would be challenging to any vendor because of the complex terrain.