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.
A group call can include radio users, dispatcher operators and even external telephone users (e.g. a PABX, a PSTN or a even public mobile network users).
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.
This service provides a point-to-point voice connection between two users and can be either in half-duplex (one user can talk at a time and the other listen) or full duplex (both users can talk and listen at the same time like in a classic phone call).
TETRA can support individual calls between radio users, between a radio user and a dispatcher or even between a TETRA user (either radio user or dispatcher operator) and an external telephone user (e.g. a PABX, a PSTN or a even public mobile network user).
Emergency call and 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.
Priority Call and busy queuing
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.
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.
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.
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.
A dispatcher may as well covertly listen to an ongoing communication in the system without the involved parties having any perception of it.
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 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.
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.