MC LTE in USA
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www.criticalcomms.com March 2020
solutions on FirstNet, but it does not
ensure operability between carriers.
“It is in the interest of public
safety to have interoperability,” says
Poltermann. “While I am of the mind
that having all users on one network
makes things a lot easier and safer,
the reality is that there’s multiple
networks. Multiple carriers with
interoperability using open standards
and allowing for flexibility in UE
and applications is best for US
public safety.” Hence he wrote in
support of BRETSA. But he is
concerned that the cheaper OTT
PTT clients are a significant threat to
the take-up of MCPTT.
“There’s a lot of unknowns
about implementation,” he says.
“We currently face interoperability
issues on the current carrier PTT
platforms based on the ‘customer’.
If one agency pays for a device and
another one pays for the other, they
can’t talk to each other through
PTT. We’re worried about it rippling
into MCPTT.”
Kennedy is more optimistic and
thinks the superior service MCPTT
will offer will ensure a good take-up.
“I believe MCPTT will be successful
since it will meet all the functional
demands being met by P25 today for
most departments. Open standard
solutions are important.”
Ken Rehbehn, directing
analyst, critical communications
at Omdia (IHS Markit), argues
that data services on LTE have
some important differences that
make the interoperability question
less pressing. “First, by moving
to LTE we automatically gain an
interoperable environment thanks to
the 3GPP protocol set. The degree
of connectedness then becomes a
political, not a technical, question.
“More fundamentally, unlike past
voice systems, the data system of
3GPP underpins access to the cloud,
which is by its nature completely
interoperable thanks to standardised
web interfaces. This diminishes the
argument about interoperability,
but it does not eliminate it
entirely. If the cloud is isolated
in a walled-off portion of the
internet, interoperability becomes a
question of cloud access instead of
radio access.
“The FirstNet core, for example,
is largely isolated from the broader
internet. A PTT voice solution
hosted on FirstNet core will, as a
result, not be interoperable to users
without access to the FirstNet core.
So, the problem is mainly political,
but there is still a potential trace of
the technical interoperability issue,”
says Rehbehn.
MC LTE at scale
Where Rehbehn sees a potential
problem in the future is with the
transition to MCPTT over LTE at
scale, as this will require LTE network
broadcast capability – 3GPP Evolved
Multimedia Broadcast Multicast
Services (eMBMS).
“We need eMBMS to enable
very large talk group communities.
Broadcast eliminates thousands of
redundant voice packets arriving at
handsets at different times. Without
eMBMS, we cannot achieve MCPTT
scale. The issue there is that
broadcast mechanisms are poorly
defined for operation across network
boundaries,” he says.
The danger is each network
becomes an island and agencies on
FirstNet for MCPTT and others on
Verizon or T-Mobile are not able
to communicate during a major
incident. “That brings us back
to a very bad starting point, and
prevents wholesale fleet migration
to MCPTT,” says Rehbehn. “In my
opinion, the missing eMBMS features
will ultimately force all public
safety agencies in the US to move to
FirstNet or remain on classical Project
25 systems forever.”
This brings us to the crux of the
matter behind the BRETSA petitions,
because as Rehbehn points out:
“For FirstNet, there is a commercial
imperative to not fix this eMBMS
limitation. This issue is not a
technical problem, it is a commercial
one. AT&T is a business and it
potentially has a captive audience, so
why would it risk losing them to a
competitor by opening up a hole in
the wall allowing other carriers with
first-responder subscribers to access
its FirstNet core and services?”
Direct mode
A further challenge to the migration
to MC LTE services, and one that
faces every country looking to follow
suit, is the lack of an adequate direct
mode, radio-to-radio solution in
3GPP – Proximity Services (ProSe) or
Sidelink, as 3GPP calls it.
As Poltermann points out, the
coverage between a P25 site and a
cellular site is distinctly different
(particularly P25 on VHF). “The
If public safety
agencies use
other mobile
operators
besides FirstNet/
AT&T then it will
be necessary
to ensure
interoperability
between carriers
to allow different
agencies to
communicate
with each other
at major incidents
– the issue is
largely political
and commercial,
rather than
technical
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