From its beginning two decades ago, the Project 25 suite of standards has offered the promise of interoperability. To date, most of this has been achieved via common air interface that allow P25 radios from different manufacturers to communicate with each other directly. Just as anticipated has been the P25 Inter Subsystem Interface (ISSI) standard, which is designed to let users on disparate LMR networks communicate.

While the capabilities of ISSI gateways have been discussed at length and demonstrated numerous times, only recently has the technology been used to provide a link between active mission-critical LMR networks built by different vendors. The first such ISSI link is located in Dallas, connecting the city’s Motorola network and DFW Airport’s Harris network, according to Mrinalini Lakshminarayanan, senior product manager for Motorola.

“It has been tested by the customer and validated last month,” Lakshminarayanan said. “It shows the capability of connecting multiple-vendor systems together using ISSI.”

Using ISSI between networks is not new, as networks from the same vendor have used ISSI to interoperate in the past. But the Dallas-DFW Airport scenario provides the industry demonstrates that ISSI can provide connectivity between networks from disparate vendors, Tim Grabacki of Motorola said.

“The message is that ISSI is real,” Grabacki said.

Indeed, major vendors have had commercially available ISSI solutions available for months. And, as more P25 systems are deployed with ISSI gateways, similar stories hopefully will become commonplace throughout the nation, marking another step in public safety’s quest for interoperability.

Meanwhile, a next logical step will be interoperability between LMR networks and the broadband LTE networks that many public-safety agencies hope to deploy during the next year.
Grabacki said the LMR-LTE link via ISSI could be at least 18 months away. Regardless how long it takes, such a development that delivers connectivity between the two primary networks for first-responder communications for the next couple of decades promises to be welcomed with open arms by the public-safety community.

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