Motorola recently announced the first-ever deployment of Inter RF Subsystem Interface (ISSI) gateways between live P25 networks in Arizona, a test that could prove to be a significant step in the “network of networks” vision that many in public-safety communications believe would enhance interoperability and radio coverage.

Although Motorola has participated in ISSI demonstrations conducted at trade shows during the past year, the Arizona deployment marks the first time that ISSI gateways have been used to link two live networks — in this case, Motorola P25 networks owned by the city of Phoenix and the county of Yuma — said Bob Schassler, corporate vice president for Motorola's government and public-safety products organization.

With both entities having P25 systems, Phoenix and Yuma personnel could talk on each system. However, if Yuma users entered the Phoenix footprint and began talking on that network, they would not be able to talk to users operating on the Yuma network, Schassler said. Installing an ISSI gateway — as well as two additional sites paid for by the state that linked the networks — resolved that issue, he said.

No mission-critical calls were delivered by the prototype ISSI gateways, but about 20 users conducted “extensive drive testing” to evaluate the technology, “which did exactly what they wanted it to do, as far as interoperability between the two systems and handoffs,” Schassler said. Motorola plans to make the ISSI gateway used in the test commercially available during the fourth quarter of 2009.

While the demonstration used the ISSI link to connect two Motorola P25 networks, the company is designing the ISSI gateway to work with P25 networks built by any vendor, Schassler said.

For states that cover a large geographic area, using ISSI gateways to link networks built by local entities may be the most economical way for statewide systems to be built in the future, Schassler said.

“Conceptually, where this could go is that the dozen-plus P25 networks throughout the state of Arizona all get linked up with ISSI links,” he said. “Then, you could have one major footprint.”