Network Resilience
becomes a resilience feature in that users can still be
connected when access to a base station is no longer
possible. In addition, the QCI (Quality of service Class
Identifier) features enable preferential access for critical
users when public LTE service is also used. Base stations
can also work in isolated mode if all links are down.
Stephane Daeuble, head of marketing – enterprise
solutions at Nokia, and Hansen Chan, its product
marketing manager, outline some of the resilience features
in LTE. Daeuble observes: “The normal resilience level for
a private LTE system is 99.9 per cent, but this can be easily
increased to four nines or higher, with dual connectivity
(using two different frequency layers) to base stations, and
reserve power. Also, by using two different frequencies,
users are connected to both frequencies at the same time
and this will bring availability up to 99.999 per cent.
Looking at base station availability, much lower power
consumption has made it easier to run on alternative
power sources such as batteries or solar panels.”
Nokia has moved to greater levels of silicon integration
and increased power amplifier efficiency, meaning that a
small cell with RF level equivalent to a macro base station
can be run on 90-200W, giving a radius of operation
of over 90km with the right antennas and deployment
height. Miniaturisation has also enabled the development
of portable base stations, which can be deployed in an
emergency or network failure.
A helicopter could transport an operator with a
rucksack-sized base transceiver station (BTS) to provide
coverage over several kilometres radius, making for a
completely standalone system for a major emergency.
The later addition of a satellite link allows this standalone
network to connect to the internet or a wider network to
enable communication between the team in the field and
people in response centres.
This size reduction also makes it possible to have a base
station on a drone, which could be tethered to a vehicle
to maintain power nearly indefinitely. Balloon-mounted
small-cell BTSs could work for several days typically, versus
hours with older generation macro BTS.
Asked whether LTE networks can backhaul themselves,
Daeuble responds: “With frequencies being a precious
resource, I would say that it is better to use other
technologies such as fibre or microwave or unlicensed
frequencies for reserve links for base stations.”
Expanding on the switching and core aspects, Daeuble
and Chan say: “Regarding the core, our default is two cores
or blades, but you can also enhance this with geographical
redundancy and even use the core of the public mobile
service when possible as the third level of resilience. It’s
important to have procedures in place to control when
switch back-up is initialised.
“There can be a constant monitoring of the network
end-to-end by heartbeat messages through the backhaul
network between the base station and the core, then
automatic switching if a failure occurs. But there is a case
for manual intervention if it is felt that approvals would be
needed before switching takes place. Last but not least, the
backhaul network also needs to be resilient.”
As regards 5G in public safety, Daeuble and Chan add:
“5G with new radio (NR) and core (SA) will offer very
Real-world MC LTE implementations
The UK and the USA are both actively moving to an LTE-based public safety
communications system, with some differences. In the UK, EE is the provider of
the network under contract to the government. It has described how additional
resilience is being built into both the core and radio networks.
An EE spokesperson says: “EE have already demonstrated the core network’s
high level of resiliency to the emergency services. This is delivered through
geographic separation and redundancy of switches and servers in each core
node. 4G sites are typically closer together than TETRA sites, primarily for
capacity, but this overlapping coverage adds to the network’s resilience. Many
more EE sites now have transmission resilience, with diverse fibre routing,
microwave back-up or satellite back-up.”
Power resilience is addressed through battery power supplies, permanent or
portable generators, ensuring continuity for key sites. “EE’s Business Continuity
& Disaster Recovery Plans have been designed, implemented and tested around
robust ITIL-based Service Management Frameworks to meet demanding
performance measures.
“Our mobile base stations and rapid response vehicles can be deployed
quickly to restore coverage in the case of a total site failure (such as road traffic
accidents or arson attacks) and portable satellite backhaul can quickly get sites
back online if transmission is damaged. The emergency services get priority
access to the EE 4G network as per GSMA standards, enabling connections
even in highly congested sites.”
EE claims that this range of measures has proven to be successful in
preventing outages during extreme weather events and power cuts.
In the case of the USA, a First Responder Network Authority (FirstNet)
representative states: “We worked hand-in-hand with first-responders across
the United States to understand their critical communications needs for a
public safety LTE network. They emphasised the importance of resilience and
redundancy to ensuring the availability of the network, and the FirstNet Authority
outlined them as objectives in our Request for Proposal for the network.
“Public safety also identified that having a dedicated set of deployable assets
was vital to emergency response, and we have seen much success to date with
the FirstNet deployable fleet (including blimp below). High Power User Equipment
(HPUE) is another potential tool to help increase the effective range of cell sites
at the edge of the network. AT&T plans to test and roll out HPUEs for FirstNet
subscribers which are only authorised for use in FirstNet’s Band 14 spectrum.”
March 2020 @CritCommsToday 33