Trunking for Profits

The Growth Of Low Cost Radio Trunking Systems In Latin America
by Eliot F. Terborgh

Eliot Terborgh is a founder and former president of SmarTrunk Systems, National City (San Diego), CA. SmarTrunk manufactures an overlay scan-based trunking system, as described in this article, trademarked under the name SmarTrunk II.

While industry attention has been focused on the growth of cellular and 800 MHz trunking systems, private carriers in Latin America have been quietly building low cost VHF and UHF radio trunking systems which have created an exciting new profit opportunity for radio manufacturers, dealers and service providers alike.

For those of us in the land mobile industry, frequency spectrum, next to the air we breath, is one of our most valuable resources. As frequencies become more congested worldwide, we continually seek ways to use this spectrum more efficiently. In recent years, one of the most popular and successful methods of achieving spectrum efficiency has been radio trunking systems.

While land mobile radio trunking has proliferated at 800 and 900 MHz, systems for use on VHF and UHF channels have only recently been introduced. The early success of these systems in Latin America and other developing regions indicates that there is a great demand for trunking at lower frequencies that had not been previously addressed by the major radio manufacturers.

This article examines these low cost "scan-based" trunking systems that are becoming increasingly popular in Latin America.

What Is Radio Trunking?

The introduction of multi-frequency, synthesized radios in the late 1970's made it possible to apply telephone industry trunking concepts to land mobile radio applications. Instead of having a single radio accessing a single channel, trunking allows that same radio to search for one clear channel out of many possible channels. Trunking, therefore, refers to the automatic search for a clear (idle) channel among two or more possible channels. Gone is the requirement to monitor a channel before transmitting, since this is done automatically. Gone, also, is the possibility of eavesdropping or interference from other co-channel users, since this is not possible in a trunking system.

Scan-Based VS. Control Channel Trunking

Most trunking systems at 800 or 900 MHz use relatively sophisticated (and expensive) multi-channel radio equipment with one or more control channels and dozens, or even hundreds, of possible voice channels. The control channels may be dedicated, as in the Motorola Smartnet® and MPT1327 systems, or dynamically assigned, as in the E.F. Johnson's LTR® system. In these systems, the control channels continually pass channel assignment information and other data to the mobile unit. Because of the frequency agility of the radio equipment, channel access time is quite fast, and wide area networking can be easily supported.

By contrast, scan-based trunking systems have no dedicated control channels. All channels are used for both trunking data and voice. Channel acquisition by a mobile unit is accomplished by rapidly scanning the trunking channels in the system and locking on a vacant channel. Channel signalling data is passed at the beginning and end of a call. Once a channel is acquired, it is maintained for the duration of the call.

Scan-based systems typically use less expensive VHF and UHF radio equipment with a maximum of 16 channels. Because of the limited frequency capacity of the mobile equipment, scan-based trunking systems are most appropriate for single-site wireless local loops. While some roaming and wide area networking is possible, there are significant limitations. Despite these limitations, scan-based systems are popular because the per- channel cost of the base infrastructure equipment is one-fifth to one-tenth the cost of cellular or 800 MHz trunked systems. Similarly, the mobile subscriber equipment is typically one-half to one-third the cost of comparable 800 MHz terminal equipment.

Overview Of A Scan-Based Trunking System

One U.S. manufacturer offers a scan-based trunking system especially designed for developing countries. Using a proprietary digital signalling scheme known as "SmarTrunk II", this economical system will operate on any frequency band, is compatible with many popular mobile and portable transceivers, and supports both dispatch and radiotelephone trunking applications.

• System Configuration
  
  The SmarTrunk II system consists of two elements: a base station controller which interfaces to any full duplex base station or repeater; and a miniature mobile logic board installed in each mobile or portable transceiver. The base controller serves as a full-featured telephone interconnect panel in addition to controlling all trunking functions. One controller is required for each trunking channel. The mobile logic board controls the radio's channel scanning, monitor, squelch, and PTT functions and provides all the signalling functions. The SmarTrunk II system will support from 2 to 16 trunking channels and has a capacity of over 4,000 subscribers per system. Since there is no interconnection between base stations, the trunking channels can be placed in different locations for wider area coverage.
  
  
• System Operation
  
  Six types of calls can be made through the system: mobile to landline calls, landline to mobile calls, mobile to mobile calls, group calls, PTT dispatch calls, and operator/emergency calls. Although the system is designed primarily telephone interconnect applications, it is the mobile to mobile and group calling capability which sets SmarTrunk II apart from cellular systems. Mobile to mobile calls can be made through the base station without accessing the public telephone network (PSTN). And by dialing a special group call code, a user can call an entire group from a landline or another mobile.

Each time a call is made, regardless of the type of call, a call accounting record is generated by the system. This record shows the subscriber number, the type of call, the number called, and the time, date, and duration of the call. This information can be periodically downloaded to a central computer from each controller via an external modem. These records can then be used for subscriber billing.

The Need For Trunking in Developing Countries

In recent years, there has been an increasing amount of interest in low cost scan-based trunking systems in developing regions in Asia, Russia, the C.I.S., Eastern Europe, and Latin America. This demand exists because many developing countries have turned to interconnected radio systems to alleviate a chronic shortage of telephone lines. In many of these countries, it is not uncommon to find as few as one or two telephones per 100 people in rural areas. These same countries typically have long waiting lists for telephone lines which, even when available, are prohibitively expensive. As a result, most developing countries are moving directly to wireless solutions to save the time and expense of building a wireline infrastructure.

Applications In Latin America

Applications for the scan-based trunking systems generally fall into two categories: private "campus" systems and shared "public" systems. The private systems are owned and operated by business and governmental organizations for the exclusive use of their own personnel. These typically include factories, mines, oil fields, airports, customs facilities, and police agencies. These systems are often interfaced with the organization's office PABX, so that personnel in the field can communicate with personnel in the office, or, by dialing a "9" or "0", the user can obtain a PSTN line through the PABX.

Public shared systems are typically constructed by telephone companies or private entrepreneurs on a "for profit" basis. These include shared business dispatch systems similar to SMR service in the United States, and wireless telephone service in rural areas, often in competition with the local cellular franchise. Some operators are providing full duplex service for phone, fax, and modems at fixed site applications. Often these will be at public calling offices in a rural areas, which may be the only phone service available in an entire village.

Scan-based trunking systems are offered in Latin America by most major manufacturers of low cost VHF and UHF radios, including Alinco, Icom, Kenwood, Motorola, Standard, and Yaesu. There are over 500 systems in use throughout most of the countries in the region.

Conclusion

As the worldwide demand for wireless communications increases, so will the demand for trunked radio systems. But this demand will require new systems which better meet the needs of developing countries. Existing 800 MHz trunking systems are simply too complex and too expensive for many poorer countries of the world. Fortunately, there is an alternative which allows trunking for a fraction of the cost of cellular or conventional trunking systems. Using low cost mobile and base equipment operating on VHF and UHF frequency bands, scan-based trunking systems are rapidly gaining acceptance as the most cost-effective way to provide wireless communications in developing countries in Latin America and throughout the world.

For a printable version of this article, click the link below
Trunking For Profits (89k)