The Public Safety Technology Alliance (PSTA) has released a report that recommends the use of the P25 Digital Fixed Station Interface (DFSI) for LMR/LTE interoperability.

The report was produced by the PSTA’s LMR/LTE interoperability subcommittee as part of its work to identify integration protocol standards suitable for public safety users operating across LMR and Broadband Push-to-Talk (PTT) using any wireless broadband technology.

According to the report (which can be viewed here), the primary need being addressed by this effort, is the identification of a suitable protocol and feature set for the integration of analogue FM LMR, given that this accounts for roughly 30 per cent of the North American public safety user base.

PSTA sought an affordable open standard LMR protocol capable of supporting features identified in the National Public Safety Telecommunications Council’s (NPSTC’s) report on LMR/LTE integration, while being compatible with the greatest number of deployed public safety LMR systems.

The report notes that “DFSI is inherently part of P25 conventional systems (no gateways would be needed), can be modified with simple extensions to add significant capabilities to Analogue FM systems (including PTT-ID and Emergency calls), can be used as an affordable alternative to the ISSI/CSSI for P25 trunking systems, and could possibly be leveraged to integrate some or all of the remaining 5 per cent of the [North American] market that uses other LMR technology.”

However, the PSTA is not proposing the elimination of the P25 ISSI (Inter RF Subsystem Interface)/CSSI (Console Subsystem Interface) – “which still have their place as the ideal integration standard where the system owners can afford those licenses” – and views DFSI as the best short-term option by “virtue of it being an existing extensible standard”, while potentially considering an enhanced Radio over IP option for the long-term use with the hope that it could be developed to be “entirely suitable for the intended application”. It also notes a number of FSI’s shortcomings which it says could be addressed with “manufacturer extensions”.

ATIS and the Telecommunications Industry Association (TIA) are working to develop an standard for P25-LTE interworking (see our July issue’s article on P25 for more information) that will leverage 3GPP’s generic interworking function (IWF), which is currently expected to be completed in September.

“After a thorough examination and review of several open standard LMR protocols, the PSTA LMR/LTE Interoperability subcommittee has identified the P25 Digital Fixed Station Interface (DFSI) as best suited to meet our primary goals,” said LMR/LTE Interoperability Subcommittee member Randy Richmond, JVCKENWOOD.

“Cost of implementation is always an important consideration in public safety and the subcommittee decided on DFSI as the primary recommendation from the PSTA because it is an affordable solution that fills the stated need for a viable and broad open standard,” added LMR/LTE interoperability subcommittee member Andy Seybold.

“It is critical to have seamless voice communication between legacy LMR systems and public safety LTE based communications as we see more and more first responders relying on LTE each day,” noted chief Jeff Johnson, PSTA board member.

Author: Sam Fenwick