Land Mobile

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Land mobile are privately maintained and operated mobile radio systems. Some connect to the public switched telephone network but placing telephone calls is not usually their main function. (In the past many people called all mobile telephone service land mobile. In that case the proper term was Public Land Mobile.) Most land mobile systems keep field workers connected to a main dispatch point, usually to a company headquarters, as well as to other mobiles. A taxi dispatch service uses land mobile as as do ambulance companies.

 

Southern Pacific Railroad uses land mobile to connect their maintenance workers along hundreds of miles of track from San Francisco to Denver. Workers communicate to headquarters and to place telephone calls from areas cellular radio will never serve. That's because SP set up their own radio network along the tracks. Their land mobile network today is simple, FM based and analog. All conversations can be heard on a scanner. As with all things, land mobile can get quite complicated, with multiplexed and digitized systems becoming common.

 

A welter of confusing and overlapping terms exist with land mobile. Keep in mind this general view by Geof Forrs as you read about the field:

"Public Land Mobile Radio Mobile Service as defined by the FCC is what is primarily covered by 47 CFR Part 90 licensing. In this realm, the FCC, despite what their web site pages say, makes no distinction between non-profit and for-profit companies. PLMRS is traditional business radio. Taxis, wreckers, plumbers and so forth."

"SMR or Specialized Mobile Radio, is a distinct term the FCC gave to 800 MHz trunking [multiplexing] when it was authorized in 1979. SMR systems use a central repeater which may or may not have telco interconnect, but they are always trunked. SMR's are 'systems' where many users share the same repeater. SMR is only 800 and 900 MHz systems; you might say that SMR is just PLMR on 800 MHz."

"The FCC needs a reality check, or at least whoever wrote their definitions for those pages listed below. SMR's have largely gone broke or out of business because of cell phones. Nextel bought large chunks of spectrum from them some years ago, and following that many closed up shop. Cell phones are so cheap and universally used that nobody wants to pay for a local-use-only hardwired telco-interconnected (or not) mobile radio. There are still many SMR's out there, serving traditional business radio users who migrated up there from their traditional radio systems of the 1960's. But it's going downhill in a hurry. Maybe that's why the FCC wants to do a license audit/study. All those PLMR and SMR licenses out there but nobody using them . . ."

Mark van der Hoek adds, "Actually, I think most of the SMRs went broke because of Nextel, not cellphones. Dispatch radio serves the needs of Joe Plumber and his 12 trucks MUCH better and more cheaply than cellphones. But Nextel can give them both. And, too, Nextel bought a lot of licenses at auction and just PUT a lot of shops out of business. They couldn't compete with Nextel at auction. No license, no business." Let's get back to this article.

Messrs Lawrence Harte, Alan Shark, Robyn Shalhoub, and Tom Steiner, have written an excellent book on land mobile, called Public and Private Land Mobile Radio Telephones and Systems. The following is from the first chapter of that book. You can download the complete chapter by clicking on the link below.

   

This is from Harte's book described below (16 pages, 174K in .pdf)

Public and Private Land Mobile Radio Telephones and Systems by Harte et. al. (external link to Amazon.com)

 

Public and Private Land Mobile Radio Telephones and Systems by Harte, Alan Shark, Robyn Shalhoub, and Tom Steiner (2000)

 

Chapter 1

 

Introduction to Land Mobile Radio

 

The mobile wireless communications industry easily ranks as one of the most dynamic and fastgrowing if not the fastest growing industries of today. Driving its popularity and growth are the wide variety of services it provides and the tremendous benefits it offers. Around the globe, convenience, improved efficiency, and enhanced productivity have become its trademarks.

 

Conventional Land Mobile Radio (Two-way radio)

 

Conventional systems dedicate a single radio channel to a specific group of users who share it. As such, privacy is limited. It is possible for a company using a channel to be overheard by other users on the same channel. Some of these listeners might even be competitors! Conventional systems, by limiting a group of users to a specific channel, also limit the total number of customers who can be served by the system. Moreover, because radios on conventional systems transmit and receive on a single channel, the user must wait if the channel is occupied by another conversation. For these reasons, conventional systems are considered spectrally inefficient when compared to trunking systems. Figure 1.1 shows a block diagram of a conventional land mobile radio system.

 

Trunked systems also offer customers wider coverage areas through 1) interconnection with the public switched telephone network (PSTN), which allows trunked radio users to communicate with any user of the wireline telephone network; and 2) interconnection with other trunked systems, which may or may not be assigned to that user. Figure 1.2 shows a trunked land mobile radio system.

 

Commercial Trunked Radio

 

One relatively small, but significant, segment of the overall mobile wireless industry is commercial trunked radio, which has only recently begun to receive worldwide attention. This is because commercial trunked radio systems usually serve a very specific user group, rather than the public at large, and the major growth of the industry has occurred only within the last five years.

 

Trunked Radio

 

Trunking systems, using frequency trunked technology, were developed to use radio spectrum more efficiently, while offering companies a more sophisticated, private, and efficient way of communicating with their mobile workforce. Trunking systems are more expensive than conventional systems, but they also offer significant benefits and improvements in spectral efficiency. Unlike conventional technology, trunking allows for the automatic sharing of multiple radio channels. This means that a group of channels is assigned to a group of users who then share the channels. The advantage of this arrangement is that when a user attempts to make a call with the radio, a trunked system searches for an available channel and assigns it to the call. A different radio channel may be assigned each time the customer uses the radio; it may even switch during the same conversation. Either way, users are unaware of the swap.

 

In the event the system is fully loaded and all channels are in use, the user either receives a busy signal or calls are "queued" until a channel is free. After the channel is selected, users have private use of the channel, which reduces interference and eavesdropping. Trunking is considered much more spectrally efficient because switching between multiple radio channels allows less blocking and provides service to more radios per channel. Consider that on a 20channel conventional system, roughly 7001,000 users can be served. In contrast, those 20 channels on a trunked, dispatchtype system can service between 2,000 and 2,500 users! Figure 1.2, Trunked Land Mobile Radio System

 

Today, a wide range of commercial trunked radio users exist as well as a variety of technologies and services to meet their needs. As word spreads about the industry and regulators allow for it to exist, we will see commercial trunked radio systems being introduced in country after country with increasing opportunities. The term "commercial trunked radio" was created by the International Mobile Telecommunications Association (IMTA) in an attempt to create a universal definition encompassing the many names for the industry and to identify a specific kind of service.

 

As mentioned above, this small segment of the wireless communications industry has experienced rapid growth primarily outside the United States within the last five years. As the industry is created in each country, there are an increasing number of names and classifications governments use to identify the service. For example, commercial trunked radio is known as Specialized Mobile Radio ("SMR") in the United States and is typically referred to as Trunked Radio Systems ("TRS") in Asia and Public Access Mobile Radio ("PAMR") in Europe. Figure 1.3 shows a commercial land mobile radio system. Because the service is subject to different regulations in each country, it is difficult to create a single "name" for the service without first creating a definition. So, the following was developed. . . . the book continues

 

This is from Harte's book described herein (16 pages, 174K in .pdf)

Interested in the book? Click on the link below:

Public and Private Land Mobile Radio Telephones and Systems by Harte et. al. (external link to Amazon.com)

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Tom Farley back again, writing this paragraph. Many ask why land mobile or business information is scarce on the web. I think that's because setting up a land mobile system is the province of professional radio dealers and RF engineers. There's nothing much on-line that's informative because the industry doesn't cater to the individual user. As Geof Forrs puts it,

"No legitimate two-way customer looks for a radio on-line because the radios require licensing, frequency coordination and programming. This is is done by a professional at the local level. Hence, about the only thing you could do on-line would be to survey the available service providers. Dealers can be and have been fined by the FCC for selling equipment to unlicensed customers. It happened to one of my competitors back when I was in the business in the 1980's."

Resources

Mobile Radio Technology magazine reports on business radio:

http://iwce-mrt.com/mobile_radio_technology/index.htm

At the F.C.C., two confusing web pages: http://wireless.fcc.gov/smrs/

 

http://wireless.fcc.gov/services/plmrs/

 

U.K. landmobile magazine site with some good files: http://www.landmobile.co.uk/

Historical links to related subjects

Improved Mobile Telephone Service (IMTS) Photos and information

(1) Service cost and per-minute charges table/ (2) Product literature photos/ (3) Briefcase Model Phone / (4) More info on the briefcase model/ (5) MTS and IMTS history/ (6) Bell System (7) Outline of IMTS/ (8) Land Mobile Page 1 (375K)/ (9) Land Mobile Page Two (375K)/ (10) The Canyon GCS Briefcase Telephone