Project 25
The idea is that as far
as the P25 system is
concerned, the 3GPP system
will just look like another
P25 system and vice versa
services, and it will also allow P25 to
do key management of 3GPP devices
in one way or another.
“We have identified a couple of
ways as to how that is possible to do,”
says Davis. “Some of this architecturelevel
work has identified that there is
some additional work for 3GPP to
do. We will move onto Phase 2 in the
next month or two. We may see some
more challenging areas where we or
3GPP will have to modify parts of the
existing standards.”
As expected, the P25 ISSI (Inter
Sub-System Interface) will provide the
primary interface to the main function
for P25-LTE interworking. “The ISSI
interface defines how you connect
two discrete P25 systems. We will
now make modifications to the ISSI
standard to define how you connect a
P25 system with a 3GPP system. The
idea is that as far as the P25 system is
concerned, the 3GPP system will just
look like another P25 system and vice
versa,” explains Davis.
Leveraging ISSI and CSSI
TIA is also continuing to work with
the Department for Homeland
Security on its P25 Compliance
Assessment Program (CAP). CAP is
a programme for testing P25 critical
functionality and reporting the
results in a consistent way. CAP has
now added ISSI and CSSI (Console
Sub-System Interface) tests to the
programme. In June, it released a draft
bulletin outlining the proposed tests,
which is now out for public review.
Davis observes: “ISSI and CSSI are
fairly complex to set up and configure,
as we have tried to accommodate the
pre-ISSI/CSSI operations that already
exist. But now customers want to
interconnect systems. The features
are standard, but each deployment of
ISSI or CSSI has a unique network
configuration and may use different
sets of standard features, so there is still
some uncertainty.”
Harris’s Elder agrees there is a
definite trend for more customers to
leverage ISSI to integrate P25 systems.
“Nearly every major system we are
shipping now has a requirement for
an ISSI system. We are seeing it being
used for mutual aid, so other agencies
coming in to help or when there are
big natural disasters. There is a lot of
uptake on this now.”
Motorola Solutions’ Lonnett
says: “We certainly sell ISSI, but I
think customers have struggled to
implement it in a meaningful way.
Hence the release of our Critical
Connect product last year. Critical
Connect is an ISSI-based solution, but
managed in the cloud by Motorola.
“It takes the power and potential
of ISSI, but limits the customer’s
responsibility to just one connection
to our cloud service, and we can
manage as many interconnections to
neighbouring systems as the customer
wants, and to commercial carriers’
PTT systems.”
Delm stresses it is important to
note that it is an on-demand service.
“ISSI is hard to set up, but with
Critical Connect you can connect in
minutes if you require a multi-agency
response, and then you can tear down
the connections as soon as they are no
longer required.”
Harris’s Teel adds: “We are
also seeing ISSI as a precursor for
interoperability with the broadband
world and we have seen some leverage
of the ISSI to facilitate this.” He cites
the city of Parma in Ohio, an existing
customer with a Harris VIDA (Voice,
Interoperability, Data and Access)
core. Parma has taken advantage of the
ISSI in the Harris core to interconnect
with AT&T’s Enhanced PTT solution,
so P25 users and AT&T users with the
ePTT app on their smartphone can
talk to each other.
Statewide systems
According to PTIG’s figures (see box),
there is a definite trend towards larger,
statewide systems. In July, the State of
Michigan announced that there were
now more than 100,000 missioncritical
radio users on the Michigan
Public Safety Communications System
(MPSCS) from almost 1,900 agencies.
Ken Rehbehn, directing analyst,
critical communications at IHS
Markit, says: “What is important
about P25-wide systems in states
like Michigan is that it illustrates a
significant trend, namely the growing
importance of regional networks.
Historically P25 has been deployed
by local agencies with a fairly small
footprint. That worked in analogue
days with fairly simple radios.
“Now we are seeing larger
procurements driven by
interoperability requirements with
technically challenging network
infrastructure that is denser and
18 www.criticalcomms.com July 2019
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