PTT-over-cellular solutions make compelling case for LMR replacement, webinar speakers say

Newburn said that Fairfax County has experienced no latency problems and realized “better fidelity” with the PoC solution. Olbrich said the voice-quality assessment is consistent with industry tests of mean opinion scores (MOS) associated with PoC applications, noting that LMR systems use lower-level codecs that are critical to determining voice quality.

“Wideband AMR codec [the codec used for PoC] MOS scores, on average, are 4.1 or 4.2 out of 5,” Olbrich said. “With LMR P25, you can maybe get to 3—and that’s under ideal conditions.

“So, it’s not even a contest, with regard to voice quality. Even under duress and under poor conditions, you have a much wider frequency-response range—typically, the codec just operates better, even under bad conditions.”

PTT also is integrated much better into devices today than in the past, with many boasting dedicated PTT buttons like those found on LMR subscribers, Olbrich said.

 “So, it’s not like before, where you had to swipe your phone, put in your code, run the app and then push to talk,” he said. “These phone have dedicated buttons that are just like a LMR radio, insomuch as they’re always on—you push a button, you talk, it works.

“They are highly integrated and optimized for call-setup time and for battery life.”

Olbrich noted that carriers are providing prioritization and quality of service to PoC solutions in an effort to deliver greater reliability and performance to PoC users, but PoC traffic does rank behind voice-over-LTE (VoLTE) traffic in carriers’ prioritization schemes today. That will not be the case with MCPTT, which will be given the higher prioritization than VoLTE traffic, he said.

In March, 3GPPP—the standards body that oversees LTE technology—approved most of the MCPTT standard, which is designed to PoC services that would meet public-safety standards for performance and reliability, including off-network direct-mode capabilities and the ability for an LTE cell site to continue local operation even if connectivity to the evolved packet core is lost, Olbrich said.

Existing PoC offerings already meet the MCPTT standard, from a functionality standpoint, Olbrich said. The big difference is that MCPTT will have additional network requirements that are designed to provide the kind of performance and reliability that public-safety users demand, he said.

With the deployments of public-safety LTE systems such as FirstNet in the United States and Safenet in South Korea, real-world experience with MCPTT should begin as early as next year, Olbrich said.

“One thing to note is that MCPTT is here; there are early trials going on,” he said. “[South Korea] started in 2015; in 2016, they’ll be nearly done with the entire country; and by the end of next year, the entire country—plus 200 kilometers off the coast of Korea—will have mission-critical-push-to-talk LTE coverage for first responders.

“So, this is not something that is way off in the future, is difficult to instantiate, is impossible to do, or will take many, many years to happen. It’s happening now, as we see it.”