PUSH TO TALK (PTT)
PTT is the reason LMR systems exist, and is a very
effective way for a dispersed group of people to stay in
touch with central dispatch and each other. All-informed
group modes reflect the Standard Operating Procedures
(SOP) of mission-critical agencies.
Using it correctly demands some process. The system
works best with short, coherent exchanges – long
rambling conversations are not ideal. LMR delivers
near-real time PTT. This is an important feature
for safety and operational efficiency in mission-critical operations.
Of course LMR systems provide a lot more than
voice, and with GPS, can easily be used for location
monitoring. Many lives have been saved through the
Voice was originally the reason that cell phones existed,
but the iPhone changed that. While advanced data
services existed well before Apple ventured into the
phone business, the combination of a powerful mobile
processing platform coupled with an innovative user
interface changed the landscape forever.
Ironically, it is the modern smartphone’s visually-rich
display that can get in the way of mission critical
communication. Our eyes are drawn to the display and
we need both hands to operate the touch screen. This is
the very reason the use of cell phones in cars is restricted.
Cellular technology created another pathway for PTT
but using the original carrier core voice system proved
difficult to match the speed of LMR, and all-informed
group calls proved challenging. In 1994 Nextel-Sprint
released iDEN, which combined the benefits of LMR and
cellular and also gave each handset its own IP-address.
iDEN could achieve a call setup and volley time close to
LMR. By comparison VoIP (Voice over IP) systems of
that time were hopelessly slow – with 5-10 second setups
and 2-5 second volley times. Modern cellular systems
provide the data bandwidth to deliver an effective
standards-based VoIP PTT solution. A big benefit is that
users do not waste expensive airtime minutes and in the
era of unlimited data plans, agencies could implement
low cost dispatch systems.
A bigger advantage is re-use of existing, inexpensive
cellular handsets. Rich user interfaces on smartphones
continued to drive this by adding features not easily
possible on LMR, such as simultaneously following and
recording different groups from anywhere on the web.
Shifting calls from the analog domain into digitised
packets has allowed a vast array of added features,
although not all suit mission-critical users.
On balance, PTT for mission-critical communications
seems better on a purpose-built device because it has
a simple interface (a button) that you can use without
taking your eyes or hands off your task. However PTT
The other critical aspect of deploying smart
phone technology that needs attention for
mission-critical users is the way users
interact with the device. In
a critical situation, officers
can’t be expected to take
their hands and eyes off
their task to operate the
device. Adapting hands-
free interfaces will make
smartphones come very
close to a purpose-built
LMR radio without even
starting to consider the
richer functionality and
applications you can use
So a narrow win for LMR,
with the realization that
a re-run of the race may
tell a different tale.
WHICH BEARER IS BEST? COMPARING LMR AND CELLULAR FOR MISSION-CRITICAL COMMUNICATIONS