The London Underground has 270 stations and 249 miles of
track, which makes it the longest metro system in the world. It
is also one of the busiest, carrying over one billion passengers
every year, according to Transport for London.
Around 45% of the network is underground, creating a unique and
highly challenging communications environment encompassing train,
station and control staff, as well as emergency services
personnel. This is now being covered by two separate but related
public radio systems using TETRA (Terrestrial Trunked Radio)
LUL's (London Underground Limited) commercial system is called
Connect. It began operation in 2006 and now serves around 12,000
users and 700 trains. The needs of the emergency services have
been met with an extension of the Airwave service, which is used
nationally by all police forces as well as ambulance and some
The task Airwave faced in creating a network for the Underground
was far from straightforward. It needed to be accessible by the
British Transport Police (BTP), Metropolitan Police, City of
London Police, London Ambulance Service and the London Fire
Brigade, and be able to support seamless handover from above to
below ground and vice versa.
The network comprises of Airwave-specific base stations in 125
deep underground stations, and connectivity in the underground
tunnels is achieved by 'piggybacking' on the Connect network.
Both systems operate in the 380-400MHz band, making this sharing
arrangement possible, with the leaky feeder cables covering the
entire spectrum band and the systems operating on different
frequencies within that band. Both Connect and Airwave also use
Motorola infrastructure, although this was a purely commercial
decision as TETRA is standard and therefore vendor equipment is
Deployment in the tunnels was incredibly complicated as the
system installation had to be conducted in the hours when the
Underground rail network was closed. With just a four hour
engineering window each day when work can be carried out, the
Connect network took years to deploy and a separate network for
Airwave was not feasible.
However, Airwave does have a separate transmission network and
network management centre, and it also installed its own base
stations in all 125 of the deep stations, which in itself was a
practical and technical challenge.
Simply finding appropriate locations for the base stations has
been difficult, as space is at a premium in the stations. Once
that had been done, Airwave also had to deploy the system during
engineering hours to avoid disturbing the running of the
Underground train network. Despite this, deployment started in
earnest after the completion of a pilot in July 2007 and was
finished by October 2008, five months ahead of schedule.
In terms of coverage and availability, the automated network
testing was supplemented by physical tests - Airwave staff used
handsets in every part of every station. It was a time consuming
process but one that ensures the emergency services are always
connected to the network. The handover optimisation was also
tested on every entry and exit to every station, and continues to
be tested to balance transmission power and receive sensitivity
to ensure effective handover. As Ray Mason, Airwave's programme
manager, explains: 'This is an ongoing process as any changes to
the above ground Airwave network affect the handover to
underground.' Airwave therefore has a full time
engineer dedicated to handover optimisation and the National
Policing Improvement Authority (NPIA) has its own engineers as
A final element of the network is provided by seven emergency
vehicles. These are sited at locations outside of London and can
be brought in to provide coverage in times of emergency, when
elements of the network are damaged or out of service. In total,
the LUL Airwave extension costs around Â£115m for the full life of
the system. This includes maintenance and operation of the
network until 2018.
However, as Airwave's Josh Berle explains, the standard service
level agreements associated with the Airwave network above do not
apply, as the piggybacking arrangement and access limitations
mean that Airwave cannot guarantee fault fixing in the way it can
above ground where it has sole responsibility for the network.
The complicated contract requires 'reasonable endeavour' in
Airwave co-operating with LUL Connect to fix faults and although
the terms may appear vague, the relationship is strong and
working well, according to Superintendent Phil Short from the
NPIA. He explains that a blame culture has been avoided:
'Availability on the network reflects the above ground targets
set by Airwave.'
The BTP did have its own system underground prior to the
introduction of Airwave but it meant they had to carry two
radios. Once underground they could not communicate with the
system above ground, other emergency services, or LUL staff. The
Airwave LU system provides all these services and also supplies
coverage in the tunnels and not just in the stations, a benefit
that is particularly useful for major events such as escorting
fans to football matches. This all makes for a safer policing
operation and has a direct effect on the experience of passengers
using the Underground and staff working there.
Including covert security forces, 3,500 talk groups have secure
access to services. Because of the security services'
involvement, even Airwave - as the operator - does not know all
the user groups. The BTP is the primary user with up to 1,500
Airwave is in the process of covering a further 15 tunnels on the
London Underground and providing additional capacity for 46
stations, which will help in the build up to 2012. Eight stations
already have uplifted capacity and Stratford is being worked on.
In addition, work is ongoing to mesh the BTP systems for all the
rail services involved including Network Rail, the Docklands
Light Railway, Eurostar and Javelin, with the Underground
personnel will use existing user groups as these do not put
additional strain on the thin layer of capacity, although some
new user groups will need to be added.
The Airwave deployment is working very well according to
Superintendent Short, who concludes that the only complaints
received from the BTP primary users are for above ground stations
that are covered by the national Airwave network. 'The coverage
is so good underground, the BTP in London have got used to that
level of service.'
What is TETRA?
TETRA (TErrestrial Trunked RAdio) is an ETSI developed standard
for Professional Mobile Radio systems. It is a TDMA-based
technology designed for mission critical communications and it is
used by emergency services, transport providers and government
agencies all over the world. Only in the US has it failed to gain
a significant foothold.
Its most significant characteristic is group calling mode, which
allows literally thousands of user groups to have access on the
same system, end-to-end encryption and instant call set up with
push-to-talk functionality. The set-up time for a single node
call is typically less than 250msec, while for a one-to-many
group call it is generally within 0.5 seconds. This compares with
the seven to 10 seconds that are typically required for a GSM