SmarTrunk Systems, Inc.

Scan-Based Trunking

Overlay Scan-based trunking
below 800 MHz
by Eliot F. Terborgh

Scan based trunking systems for UHF and VHF channels below 800 MHz already comply with FCC requirements to monitor channels before transmission without the need for dedicated data channels for system control.

*Reprinted with permission from Mobile-Radio Technology:
January 1998, r1998 Intertec Publishing Corp., Overland Park, KS 66212. All rights reserved*

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.

There are several advantages for the private carrier licensee that is considering converting conventional 450MHz repeaters to trunked operation. Operators of shared community repeaters can achieve higher customer loading (and greater profits) from the existing system. Trunking is also advantageous in a. private "campus" system of two or more VHF or UHF conventional channels used for plant security or maintenance.

Converting conventional channels to trunked operation will increase system capacity while providing privacy, system security and advanced features, such as selective and emergency calling.

The efficiencies and features of trunked radio are well known to the experienced 8OO MHz system operators. The primary advantages of any trunked radio system can be summarized as follows:

Automatic clear channel selection - In a conventional radio system, the user may only have access to a single channel. By FCC regulation, if the user wishes to make a call, he must first monitor the channel to make sure it is clear. If the channel is busy, the user must continue to monitor the channel until the co-channel user has terminated the conversation. By contrast, in a trunked system the channel selection is automatic. When the user initiates a call, the trunking system electronically "monitors" each channel and selects one clear (unused) channel from many possible channels.
Channel privacy - In a conventional system, other co-channel users can easily eavesdrop on conversations. In fact, channel monitoring is required by the FCC when initiating a call, as described above. In a trunked system, other system users cannot listen in other conversations. A radio can only join a conversation when directed by the trunking controller.
Channel exclusivity - In a conventional system, discourteous co-channel users can "jump" on a channel in the middle of another conversation, interrupting the call in progress. In a trunked system, once a channel is selected, it becomes exclusive for the duration of the transmission. Other users cannot interrupt or interfere with the call.
Selective calling - In most conventional radio systems, selective calling is an expensive add-on option or may not be available at all. On the other hand, some trunked radio systems provide extensive selective calling capabilities as a standard feature. This means that users can selectively call different groups or individuals in the system. Each user is typically assigned a unique individual ID code and one or more group ID codes. These codes can be used by other users in the system to specify the groups or individuals with whom they wish to communicate.

Growth Of Trunked Radio

During the past 15 years, the specialized mobile radio (SMR) industry has experienced tremendous growth. Most of this activity has been limited to portions of the 800 MHz and 900 MHz frequency bands that have been allocated for trunked radio by the FCC. The popular trunking protocols have been various formats offered by Motorola (Smartnet, Smartzone, Privacy Plus), the LTR format developed by E.F. Johnson, and GE Mark V and EDACS, offered by Ericsson.

In recent years, however, most of these SMR channels at 800/900 MHz have been acquired by a few large carriers, most notably Nextel Communications. In the process of acquiring these channels, Nextel has bought out hundreds of small entrepreneurs who pioneered this industry in the 1980s. As a result, many of these same entrepreneurs (at least those who are not ready to retire) are now "looking for an encore." But with Nextel's market dominance and the freeze on new SMR license applications, there is simply no more spectrum (and no more opportunities) available at 800/900 MHz.

Changes in 1997 to the FCC Part 88 rules allow trunking in the 150 MHz and 450 MHz bands. By acquiring conventional UHF repeaters and converting them to private carrier status, displaced SMR entrepreneurs can now establish multi-channel trunked networks, just as they did a decade earlier at 800 MHz.

The Trunking Advantage

If you operate two community repeaters, and each channel is busy 20% of the time, there is a 20% chance that the channel assigned to a given customer will be busy when one of his mobile groups wishes to make a call. But, if the two channels are trunked together, the trunking system will select an open channel when a mobile group wants to make a call.

As shown in Figure 1, the odds that both channels will be busy at the same time is only 4%. This means:
• Existing customers will receive a much higher level of service, or
• 2.5 times as many customers can be added to the system with the same level of service.

Part 88 Trunking Rules

In its changes to Part 88, the FCC authorized the use of "centralized" trunked systems for VHF and UHF frequency bands. Centralized trunking refers to trunking systems that use dedicated (exclusive) control channels for data communication between the mobiles and the trunked repeaters. In such systems, the mobiles constantly monitor the control channel for channel assignment instructions. When a group call is initiated, the trunking controller transmits instructions telling the mobiles in the group to switch to a voice channel assigned for that conversation. Thus, the "brains" of the system are in the system controller at the repeater site.

Most trunking systems, such as MPT-1327, Motorola's Smartnet, and E.F. Johnson's LTR are centralized systems. Because they are designed to use clear frequencies, these systems are widely used in the 800/900 MHz frequency bands where the FCC has granted the use of exclusive frequencies for trunked SMR service. However, these trunking formats are inappropriate for use at VHF and UHF frequencies, because of the lack of exclusive channels in those bands.

To get around this problem, the FCC requires potential system operators ". ..to obtain some form of exclusivity in their respective service areas,". This requires the operator to obtain written permission from all other licensees within a 70-mile radius of the proposed base station, similar to the FCC rules for telephone interconnect in major metropolitan areas. Due to the frequency congestion in these bands, this requirement is extremely difficult to meet. All other users on the proposed trunked channels and immediately adjacent channels would have to agree to convert to trunked operation. Since, by definition, centralized trunked systems require exclusive channels, the other affected users would have to agree to a simultaneous conversion of their radio systems, which would require scrapping their conventional equipment and purchasing all new mobile radios. As independent businesses, it is extremely unlikely that they would all agree to such a plan.

Scan-Based Trunking Systems

Unlike centralized trunking systems, scan- based systems do not require exclusive channels. Because the channel selection intelligence is in the mobiles, scan-based systems can co-exist with conventional users on the same channels. When a call is initiated by a mobile unit, the channel assignment is determined by the logic in the mobile, not by a controller at the repeater site. So, if the channel is busy with a conventional user (or even a co-channel user on another system), the mobile will consider the channel to be busy and will select another channel.

The advantages of scan-based system can be summarized as follows:

Since they are not considered "centralized" systems, the onerous FCC frequency coordination requirements described above do not apply.
They have always been "legal" for use below 800MHz because they fully comply with the FCC requirements to monitor the channel before transmitting and do not use dedicated data channels for system control.
Scan-based trunking protocols will operate on non-exclusive channels. This not only allows the system to operate in the presence of conventional users on the same channel, but it also allows a system operator to gradually convert conventional users to trunked operation.
They are economical due to the lower cost of both the trunking controllers and the mobile radio equipment .
They require less technical training to install and maintain.

Overlay Scan-Based Trunking

An overlay scan-based system means that, in many cases, the system can be installed with an existing two-way radio system. This means it may be unnecessary to buy new radios or repeaters to achieve the benefits of trunking. The system requires a proprietary controller connected to each repeater and a mobile logic board installed in each mobile or portable radio in the system. The system will accommodate as many as 16 channels, which may be gradually added as your system expands. Customized logic boards are available for a variety of brands and models of popular mobile and portable transceivers. In many cases, the logic board simply plugs into the radio.

The overlay scan-based system allows you to trunk repeaters together for more efficient use in either a dispatch-only mode or a combination of dispatch and telephone interconnect service. If you choose the combination mode, you can limit telephone access to only certain mobiles while denying access to other mobiles. You can further limit those mobiles with telephone access to local calls only, or you can allow certain mobiles access to toll calls or long distance calls.

The system keeps track of all calls that are made on the system, whether they are radio dispatch calls or telephone calls. It records both incoming calls and outgoing calls. It does this through call accounting records that record the date, time, type of call, number being called and the number of the mobile subscriber who is initiating the call. The call accounting records provide the data for airtime billing.

System Security And Privacy

Each mobile subscriber is assigned a confidential paging code, which is transmitted via a proprietary, digital signaling protocol. To allow any mobile to work on the system, this paging code must be installed into the memory of the trunking controller. Without it, a mobile is not allowed access to the system. This feature prevents the system pirates from adding mobiles to their fleet and not advising the system operator.

Once a mobile has acquired a channel, only the mobiles that have been called can hear the conversation. Other mobiles cannot gain access to the channel to hear any of the conversation. This privacy applies whether the call is a dispatch call or a telephone call.

Gradual Migration

One of the key features of overlay scan-based trunking for applications below 800 MHz is the ability to gradually convert conventional users to trunked operation. In most major metropolitan areas, channels in the 450 MHz band may have two or more conventional repeaters with a dozen or more user groups operating within the range of each other. A scan-based system with the channel selection intelligence in the mobile can co-exist with these conventional users. This is done by cross-connecting the controller with a conventional CTCSS tone panel on each channel. When a subscriber is using the channel, the conventional tone panel is "locked out," preventing conventional users from accessing the channel. On the other hand, when a conventional user has the channel, the trunked users are locked out.

In logic-trunked radio (LTR), the subscriber radio must receive instructions from its home channel before it can initiate a call. As a result, all home channels must be assigned to a clear channel. Otherwise, whenever a co-channel user is on the air, all radios assigned to that home channel will be unable to receive instructions and, consequently, unable to initiate or receive calls of any kind.

Obviously, the efficiency and benefits of any trunking system will be reduced by the presence of conventional co-channel users. But this ability of the system allows the private carrier licensee to gradually convert conventional users to trunked operation over a reasonable time.

Since the 450 MHz spectrum is heavily populated in the United States, obtaining a clear channel may not be an option. With scan-based trunking, all of the channels in the system may be shared with co-channel users since all the mobiles in the system monitor for a foreign carrier before selecting a channel.

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Overlay Scan-Based Trunking below 800 mhz (140k)