P25 ISSI’s Importance for MCPTT
Tuesday, May 29, 2018 | Comments
Many have criticized the Project 25 (P25) standards suite for various reasons — slow to develop and high cost, among others. However, one P25 standard, the Inter RF Subsystem Interface (ISSI), looks likely to play a large role in the integration of legacy networks with Long Term Evolution (LTE) and mission-critical push to talk (MCPTT).

At the March Third Generation Partnership Project (3GPP) meetings, the Stage 2 technical specification for LMR and MCPTT interworking was completed. Work on the interworking standard, which will allow other technologies such as P25 to plug into MCPTT to operate from a system-to-system perspective, will continue for a few months but will eventually let digital radio networks interface with MCPTT.

The 3GPP’s interworking function (IWF) will adapt LMR data and signaling to MCPTT data flows. It is the interface that connects a P25 ISSI to the nationwide public-safety broadband network. The actual specification from an LMR network to the LMR IWF gateway is not being defined by industry standards organizations, but many in the industry see the ISSI and possibly the P25 Console Subsystem Interface (CSSI) as the technology to interface with MCPTT definitions from 3GPP.

Industry consultant Emil Olbrich says use of an ISSI hub would be an efficient way to interconnect MCPTT to state and local P25 networks; however, there is a significant cost for hardware, software and maintenance of a such a solution.

ISSI is intended to interconnect P25 networks and was not designed for interoperability to disparate technologies such as LTE. But ISSI gateway vendors have taken the IP interfaces from an LTE network and emulated a P25 network via the ISSI. Given the long time lead time to develop an intranetworking solution, using ISSI is likely the most pragmatic solution.

One of the keys to future MCPTT over LTE is to ensure open-standard PTT solutions are used. “For integration of ISSI networks, a centralized ISSI gateway needs a secure and routable IP address,” Olbrich says. “Any over-the-top (OTT) PTT application that is being considered should have direct support for ISSI. Additionally, any OTT PTT app ideally should be software upgradeable to MCPTT.”

As MCPTT comes closer to reality, public-safety leaders must remember the lessons they learned from LMR and ensure every product they purchase is standards based. Moreover, AT&T and the First Responder Network Authority (FirstNet) should consider adding P25 ISSI equipment in their labs to ensure compatibility and a solid migration path.

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