Key Voice Services and Facilities:
* Group Call (commonly called ‘all in formed net’ and ‘talk group call’)
* Pre-Emptive Priority Call (Emergency Call)
* Call Retention
* Priority Call
* Busy Queuing
* Direct Mode Operation (DMO)
* Dynamic Group Number Assignment (DGNA)
* Ambience Listening
* Call Authorised by Dispatcher
* Area Selection
* Late Entry
* Voice Encryption
Group Call
This is probably the most basic voice service in TETRA but
yet the most complex to support effectively and efficiently.
This is because group calls need to:
* Use simple “Push To Talk” operation
to provide fast call set-up group communications
* Be operated and managed in particular ways to optimise network loading, some
examples being:
* Operate in simplex
* Operate on a "preferred" site
for optimum network loading
* Have a defined area of operation (Area selection)
* Have a very reliable call-set up signalling protocol to ensure all users
in a group are connected together when a call is first initiated (call acknowledgment
signalling is impractical for group calls)
* Have priority mechanisms to ensure that specified users in a wide area group
call (spanning multiple base station sites) are connected together when a network
is busy
It is this complexity needed to support
group calls that makes public cellular networks unsuitable,
simply because they were originally designed to support “One
to One” calls, unlike TETRA which was primarily designed
to support group calls at the outset.
Pre-emptive Priority Call
This call service, of which the highest priority is the emergency
call, provides the highest uplink priority and highest
priority access to network resources. If a network is busy,
the lowest priority communication is dropped to handle
the emergency call. Unlike 911, 112 or 999 initiated public
network emergency calls (which can also be supported on
TETRA) the TETRA emergency call can be initiated by using
a dedicated switch located on the terminal. Activating
the emergency call automatically alerts the affiliated
control room dispatcher and other terminal users in that
persons talk group.
Call Retention
This service protects selected radio terminal users from
being forced off the network as a result of pre-emptive
calls (emergency calls) during busy periods. When emergency
calls are supported in a network, it is essential that
only a small number of radio terminal users are provided
with this facility as the objective of retaining important
calls during busy periods could be lost.
Priority Call
During network busy periods, that service allows access to
network resources in order of user terminals call priority
status. As there are 16 levels of priority in TETRA, this
service is very useful in providing different Grade of
Service (GoS) levels (and tariff structures) during busy
periods. For example, front line officers would be provided
with the highest priority levels in a Public Safety network
to maintain the highest level of service access whilst
routine users would be provided with lower priority levels.
Busy Queuing
In TETRA a queue is provided in the trunking controller during
network busy periods to store and handle calls on a First
In First Out (FIFO) basis in order of user priority level.
The advantage is that a user only has to initiate a call
request once, knowing that even in busy periods the call
will be automatically established once a traffic channel
becomes free, thus reducing user stress and frustration
when contending with other users on a busy network.
Direct Mode Operation (DMO)
Direct Mode Operation (DMO) provides the ability for TETRA
radio terminals to communicate directly with each independent
of the TETRA network infrastructure. DMO is not new and
has been a facility mandated and used by many traditional
PMR user organisations for several decades. The primary
requirement for DMO has been brought about by the need
to balance the RF Coverage, Grade of Service (GoS) and
Reliability of a network with that of the network’s overall
cost. The requirement for DMO makes the use of public cellular
networks unsuitable.
Dynamic Group Number Assignment (DGNA)
This service allows the creation of unique Groups of users
to handle different communication needs and may also be
used to group participants in an ongoing call. This service
is considered by many public safety organisations to be
extremely useful in setting up a common talk group for
incident communications. For example, selected users from
the Police, Fire and Ambulance could be brought together
to manage a major emergency where close co-ordination between
the three emergency services is required. Similarly, DGNA
is also considered useful for managing incidents by other
user organisations such as Utilities and Transportation.
Ambience Listening
A Dispatcher may place a radio terminal into Ambience Listening
mode without any indication being provided to the radio
terminal user. This remote controlled action allows the
dispatcher to listen to background noises and conversations
within range of the radio terminal’s microphone. This is
an important service to utilise for those persons transporting
important, valuable and/or sensitive material that could
be ‘hijack’ targets. Similarly, this is a useful service
to have implemented in public service vehicles where a
driver’s health and safety could be at risk.
The number of user applications for
the Ambience Listening service are numerous and in many cases
application specific. However, it is important to note that
many users feel that this service invades a person’s privacy
and for this reason only those users who need Ambience Listening
as part of their work duties should be provided with this
service.
Call Authorised by Dispatcher
This services allows the dispatcher to verify call requests
before calls are allowed to proceed. This is a useful service
to utilise when radio user discipline needs to be maintained.
This service also reduces the amount of radio traffic on
a network as only essential work related calls are permitted.
However, the frequent need for all informed net group communications
between terminal users and the time delay experienced in
authorising calls can make this service unacceptable for
some user organisations.
Area Selection
Area Selection defines areas of operation for users and can
be chosen on a ‘call by call’ basis. This service basically
simulates the ability for a dispatcher to select different
base stations to make a call as was possible in conventional
networks. This service also helps to improve network loading
and overall spectrum efficiency by restricting the area
of operation for selected group calls.
Late Entry
This service provides continuous call in progress updates
to allow latecomers to join a communication channel. This
is not a service but an air interface feature that allows
a trunked radio terminal to behave in a similar way to
conventional PMR terminals. For example, if a user turns
on their TETRA terminal the control channel will automatically
divert the user’s terminal to a talk group call, if a call
is already in progress. Similarly, if the user’s terminal
has been outside radio coverage, for example in a tunnel,
the control channel will also divert the user’s terminal
to a talk group call assuming a call is already in progress.
Voice Encryption
The TETRA standard supports a number of over the air TETRA
Encryption Algorithms (TEA’s), the differences being the
types of users who are permitted to use them. The main
benefit of over the air encryption is that it can be implemented
as software within radio terminals and base station equipment,
instead of using encryption modules, which consume space
and increase cost. The TETRA standard also supports ‘end
to end’ encryption using a variety of other encryption
algorithms as deemed necessary by national security organisations.