Multiprotocol trunking and networking: Analog SMR operators can meet the sophisticated requirements of today’s radio use while maintaining flexibility and control over the system. Controllers handle multiple trunking formats, wide-area, real-time networki
Analog specialized mobile radio (SMR) operators have been increasingly faced with requirements to offer more-flexible radio service to their subscribers, especially in the wake of the emerging enhanced SMR systems (ESMRs). Many SMR operators started out years ago offering radio service on a community repeater located on a tower. Over the years, as an operator’s business grew, additional channels in various frequencies were obtained, usually spread out over several towers within a certain region.
As trunking technology emerged, operators began offering trunked radio service to their customers. Because of the efficiency of trunking systems, operators were able to load many more radios onto a single channel, and customers were able to obtain the privacy they had long sought with two-way radio. The migration to the new technology had begun.
Unfortunately, the migration brought about new problems. Becuase of the proprietary nature of trunking formats, the previously available technology restricted SMR operators from loading radios with various formats onto their systems. As a result, customers have been forced to make a decision that in most cases would tie them to a specific manufacturer’s trunking radio. As trunking became more and more popular, regional trunking service being offered became segmented with proprietary systems. Customers unhappy with their service provider were usually forced to change to other radios if they wanted to move to another service provider. Service providers that were interested in soliciting their competition’s customers usually found great resistance when they informed the prospect that a complete change of existing radio equipment would be required. Standard controlling equipment also did not have wide-area capabilities, which meant that communication between a vehicle and the company’s dispatch location was usually restricted to the coverage area of a specific tower. Any travel by a vehicle roaming outside of that area meant that radio calls (or interconnect calls) to and from that vehicle were not received. Radio users were increasingly beginning to require that dispatch and interconnect calls reached their fleet regardless of how far the calls may have traveled in a region or state.
In addition to the requirements that commercial businesses had, federal, state and local governments had their own problem, and they were becoming increasingly burdened with numerous departments using different frequencies. Communication among these departments was becoming cumbersome and usually required that users carry separate radios tuned to each frequency or be manually patched through a communications center.
The analog radio industry was badly in need of equipment with the ability to handle multiple trunking formats over the same channels, along with wide-area, real-time networking and frequency crossbanding. Typically, these kinds of capabilities were only available in sophisticated, expensive equipment, limiting use of these features to the large entities that had the budgets to absorb the tremendous costs.
The multiprotocol trunking and networking system developed by our company was designed to be an affordable way for the typical SMR operator to remain competitive into the future by offering multiprotocol trunking, real-time networking, frequency crossbanding, hand-off of calls in progress and other features. The system allows SMR operators the opportunity to use their channels to provide service to all LTR, GE Marc V and Marc VE radios, as well as conventional radios.
The system is frequency-transparent, allowing repeaters in all frequencies, including 220MHz narrowband, to be brought into the system. These various types of radios are allowed to communicate with each other through simple programming in the customer database program. This feature eliminates the need for manual patching of crossbanding calls through a communications center console. It also eliminates the need for multiple radios in a vehicle to accomplish multifrequency communication, which is a typical requirement for municipal, state and federal agencies.
System abuse has also been a long-time problem for SMR operators. The networking system helps to control that problem by allowing an operator to restrict customer access to the system by time of day, three time periods by day of the week, specify the sites for which the user has access and networking rights, and other limitations. This enables the operator to offer various “levels” of service to customers based on their access to the system or whether they want to use the network feature.
As shown in Figures 1 and 2 on page 28, configuration is possible on a single-site or network basis. Real-time networking is accomplished by the system using dedicated links to the network switch location, which help provide typical network setup times of 250msec. Two different capacity networking switches are available. The smaller switch will handle as many as five sites and 50 channels. The larger switch can handle as many as 32 sites and 640 channels. There are no limitations on the number of sites that can be brought on line at any one time during a dispatch or interconnect call.
The system’s interconnect feature allows telephone lines to be installed at various sites, or just one site. If telephone lines serve only one site, all sites have the ability to access those lines if the operator permits it. This access can be restricted on a per-customer basis. A repeater interconnect (RIC) card is needed for each telco line. The card can handle virtually any type of telephone circuit, including direct inward dialing (DID), direct outward dialing (DOD), or Type 1A. It also is a shared card, meaning that any user authorized to request dial tone, regardless of which channel the user entered the system on, can use the card(s), if available. This allows the SMR operator to load all channels with dispatch customers, rather than exclusively dedicating one or more channels for interconnect, thus improving the efficiency of spectrum use.
A “follow-me” roaming feature allows all dispatch and interconnect calls to be forwarded to users wherever they may be in the network. If a user forgets to register in the system, it is possible to make a broadcast over all sites to locate him. Once the user answers the broadcast, the other sites are dropped from the call, and the user is then registered for future calls to be forwarded to him.
The system also provides for hand-off of calls in progress, a feature that is typically provided by cellular systems only. This feature allows for any dispatch or interconnect call to be handed off to another tower site as the user moves from one site to the next. Termination and re-initiation of the call is not necessary, unlike other systems.
In summary, the multiprotocol trunking and networking system offers the analog SMR operator an affordable way to complete with ESMR by offering cellularlike coverage and enhanced features previously not available.