The TETRA Standard
TETRA is a Voice + Data radio communication protocol based on an open standard developed by the European Telecommunications Standards Institute (ETSI). The main purpose of the TETRA standard was to define a series of open interfaces, as well as services and facilities, in sufficient detail to enable independent manufacturers to develop infrastructure and terminal products that would fully interoperate with each other as well as meet the needs of traditional PMR users and organizations.
The TETRA standard is in practice a suite of standards covering different technology aspects, for example, air interfaces, network interfaces and its services and facilities. Because TETRA is an evolving (growing) standard, making advances and improvements in capabilities with no loss in interoperability, it has been developed in Releases (phases) known as TETRA Release 1 and TETRA Release 2.
Although the prime responsibility of ETSI is to develop standards for Europe, many of its standards are also adopted world-wide, as evidenced by the uptake of GSM, the first wireless technology standard to be developed by ETSI, & DMR, a two time slot TDM Protocol. Similarly, TETRA has already been deployed in many regions and nations outside Europe, resulting in TETRA becoming a truly global standard.
Both TETRA Releases have been completed and work continues within ETSI Technical Committee (TC) TETRA to further enhance the standard thus satisfying new user requirements as well as gleaning the benefits of new technology innovations. Outside of Europe the ETSI TETRA Standard has been formerly adopted in China and South Korea.
Why Open Standard & Interoperability (IOP)
- Economies of scale provided by a large harmonized market served by several independent manufacturers and suppliers competing for the same business resulting in competitively priced solutions
- Second source security if existing suppliers exit the market
- Evolution (instead of revolution) of the technology standard ensuring longevity and good return on investment for both users and suppliers, and no loss of operability continuity.
- Evolution (instead of revolution) of the technology standard ensuring longevity and good return on investment for both users and suppliers, and no loss of operability continuity.
- Choice of manufacturers for new products keeping prices down
- Greater choice of products for specialized applications
- Greater responsiveness to future needs by existing suppliers because of competition
- Interoperability between vendors is assured by a certification procedure(official procedure)
Evolution & Longevity
The ETSI TETRA standard will continue to evolve beyond Release 1 and Release 2 to provide additional enhancements as driven by user needs, technology innovations and other parallel standard developments. In summary, TETRA will evolve in a similar way as GSM has done, from providing a basic V+D “one to one” telephony service to supporting powerful multimedia applications and High Speed Data
Taking these previous factors into consideration and the fact that analogue MPT 1327 trunking networks are still being deployed across the world more than 28 years after the technology was first developed, TETRA networks are expected to be available well into the future also. Tetra thereby ensures a very good return on investment for users and organizations as well as manufacturers and suppliers.
The TETRA Association
Recognizing that important market requirements outside the responsibility of ETSI needed to be addressed to ensure the success of TETRA, a number of organizations formed the TETRA MoU (Memorandum of Understanding) Association in December 1994. This has evolved into the now named Tetra Critical Communications Association, TCCA.
Since its establishment, the TETRA Association has grown significantly and now provides a forum which acts on behalf of its members, being user organizations, manufacturers, application providers, integrators, operators, test houses, regulators, consultants, engineering firms, etc. The main objectives of the TETRA Association are to promote the TETRA standard and to ensure multi-vendor equipment interoperability. Recently the TETRA Association was renamed to the TETRA and Critical Communications Association (TCCA) and the objectives of the organization broaden to include future convergence of broadband and standardization for professional users.
The core technologies used in the TETRA standard, such as Digital, Trunking and Time Division Multiple Access (TDMA) also provide a number of inherent advantages and benefits. Nowadays, practically everything electronic uses digital technology and wireless communications are no exception. Even though analogue FM PMR communications will remain a viable option for several years, digital radio provides relative advantages in the important performance areas of:
- Voice Quality
- RF Coverage
- Non-Voice Services (digital)
- Spectrum efficiency
The main benefit of trunking is normally seen as spectrum efficiency, or service provided to more radio users per RF channel (carrier) compared with a conventional radio channel for a given Grade of Service (GoS). This is brought about by the automatic and dynamic assignment of the next available carrier resource, selected from a small number of communication channels, for use and shared amongst a relatively large number of users. This Trunking concept minimizes the ‘dead’ or ‘fallow’ time of a carrier, thereby maximizing the use of the RF spectrum, creating high spectral efficiency.
Because trunking systems support more radio users than conventional systems, national administrations actively support the deployment of trunking systems as this helps reduce pressure on meeting PMR spectrum demands. However, from a radio users operational point of view, spectrum efficiency may be a transparent conceptual factor.
What users want is to solve all the operational problems associated with conventional PMR (private mobile radio), yet still retain the simplicity of conventional open channel ‘all informed net’ operation or group communications. The fundamental element of trunking that solves these conventional PMR problems is the computer control of selection and assignment and the use of a single control channel.
Table 1 below lists some of the operational problems of conventional PMR and also lists how the use of trunking solves these problems.
Table 1: Conventional PMR problems solved by Trunking
It is important to note that the operational simplicity of conventional PMR ‘all informed net’ talk group communications is still retained by employing fast call set-up “Push To Talk” (PTT) operation on radio terminals.
Additional Services and Facilities
As the control channel acts as a signaling communications link between the Trunking Controller and all mobile radio terminals operating on the system, the Trunking Controller knows the status of the system at any moment in time as well as its historic usage, which is stored in its memory. For example, the Trunking Controller knows:
- The individual and group identity of all radio units registered on the system
- The individual identity and time radio units registered on the system
- The individual identity and time radio units de-registered from the system
- The individual and group identity, time and duration of all messages
With additional intelligence in both the radio terminals and the trunking controller the advantages and benefits of trunking can be increased. For example, the length of the control channel signaling messages can be increased by a set amount to accommodate a variety of new services and facilities. Also, the trunking controller can be programmed to handle calls in a variety of ways as required by the operator of the system.
Time Division Multiple Access (TDMA)
A four time slot TDMA technology was adopted in TETRA as it offered the optimum solution to balance the cost of equipment with that of supporting the services and facilities required by user organizations for a medium to high capacity network providing single site local RF coverage and/or multiple site wide area RF coverage.
RF Spectrum efficiency is a combination of three main factors being the occupied bandwidth per communication channel, the frequency re-use factor determined by the Carrier to Interference protection ratio C/I in dB’s and the trunking technology used. As previously mentioned TETRA utilizes the most advanced trunking technology.
Also, the TDMA technology used in TETRA provides 4 independent communications pathways “channels” within a 25
kHz RF bandwidth Channel, making it twice as efficient in occupied bandwidth terms as a traditional 12.5 kHz RF
bandwidth FDMA channel, a TETRA attribute. The overall spectrum efficiency advantage lies with TETRA, especially
for medium to high capacity networks.
Tetra 4 Time Slots per 25 KHz Bandwidth
A diagrammatic representation of the TDMA time slot structure used in TETERA is shown above..
From the base station equipment configuration in above figure it can be seen that the FDMA solution requires 4
separate transceivers where as the TDMA solution only requires 1 transceiver. As a consequence, the FDMA solution
requires a transmitter antenna combining and receiver splitting network to enable single transmit and receive antenna working.
Also, the RF power output of the FDMA transmitters will need to be higher in order to compensate for transmission losses in the transmit antenna combining network.
Because 4 slot TDMA already supports four independent communication paths, no antenna combining equipment is required to support the 4 time slots, thereby saving space as well as cost.
Because of using TDMA technology, the cost, equipment space, power consumption, and HVAC costs at base station sites can be significantly reduced compared with traditional FDMA technology trunking solutions. Another advantage of TDMA technology is that it enables new services and facilities to be supported with minimum cost. Some examples are:
Higher Data Rates The ‘laws of physics’ limits the maximum data rate in a given RF channel bandwidth, known as the Shannon Data Rate Limit. Assuming the use of the same modulation scheme, a wider channel bandwidth can pass a higher the data rate. Because TDMA uses wider channels than FDMA, the combined data rate on a single RF carrier is greater.
Improved Data Throughput in Poor RF Signal Conditions The net data rate in TDMA is better than FDMA in poor RF propagation conditions. This is because Automatic Repeat Requests (ARQ’s) are required when received data is corrupted as a result of RF fading or other interference. This is due to the operation of TDMA terminal devices in full duplex mode, concurrently sending ARQ’s while receiving data they can be sent efficiently after each time slot transmission, instead of waiting until the end of each voice/data transmission, as is usually the case with FDMA.
Bandwidth on Demand
In TDMA any number of time slots up to the maximum limit of the technology being employed can be combined to increase data throughput as required for specific applications.
Concurrent Voice and Data
Because of the TDMA time slot structure it is possible to assign one time slot to support voice and the next time slot to support data in a two slot transmission from radio terminals. This capability effectively allows a single radio terminal to concurrently transmit or receive voice and data at the same time.
Full duplex Voice Communications
TDMA technology inherently supports full duplex communications. Although full duplex voice communications can be supported on FDMA systems, the continuous carrier aspect demands the need for RF screening between the transmitter and receiver and hence a duplexer to accomplish even simple single channel single antenna operation. Because of this, duplex FDMA radio terminals are usually less efficient, bulkier and more costly to produce and operate than TDMA terminals.
In developing the TETRA standard to meet the needs of, increase the efficiency of, and improve the capabilities of traditional PMR user organizations, numerous Tetra services and facilities have been provided.
Details of all the TETRA services and facilities can be found further in this document under Tetra Release 1 and Tetra Release 2
However, in this section it is considered appropriate to list some of the Key Services and Facilities, which clearly differentiate TETRA from other wireless technologies.
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 Authorized by Dispatcher
- Area Selection
- Late Entry
- Voice Encryption
This is probably the most basic voice service in TETRA but yet the most complex to support effectively and efficiently. This feature differentiates Tetra from cellular like technologies, such as iDen (Nextel), Harmony, GSM, cellular, and the flavors of LTE. 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 optimize 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 signaling protocol to ensure all users in a group are connected together when a call is first initiated (call acknowledgment signaling 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
- Providing traditional “Dispatch: radio operations
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 Network” calls, unlike TETRA which was primarily designed to support group calls “dispatch” 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 or other activation methodology located on the terminal.
Activating the emergency call automatically alerts the affiliated control room dispatcher and other terminal users in that persons talk group.
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 Call Retention.
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 service 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 addition, anyone so enabled, can initiate an Emergency Call, which usurps priority and activates additional features.
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 and request order.
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, commonly known as “talk Around” in conventional FDMA repeater systems. DMO is not new and has been a Tetra facility mandated and used by many traditional PMR user organizations for several decades.
The primary requirement for DMO has been brought about by the short range need to communicate between user terminals either out of range of the base station or situation not requiring the trunking features of Tetra. The features and functions of DMO cannot be supplied by public cellular networks or cellular like communications.
Dynamic Group Number Assignment (DGNA)
This service allows the creation of unique Groups of users to handle different communication needs and provide a controlled segregating of communications activities, it may also be used to group participants in an ongoing call. This service is considered by many public safety organizations 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 organizations 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. It also permits the dispatcher to talk to the terminal and user simultaneously while receiving audio. Additionally, this can be activated as a “hands free” operation for the terminal user.
This is an important service to utilize for those persons involved with intense instant emergency situations (hostage, shooting, fire fighter in building, EMS cardiac arrest / poisoning, etc.), 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 Authorized by Dispatcher
This services allows the dispatcher to verify call requests before calls are allowed to proceed. This is a useful service to utilize 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 authorizing calls can make this service unacceptable for some user organizations.
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
thereby segregating radio traffic to specific areas and relieving the remainder of the network of unnecessary activity.
This service also helps to improve network loading and overall spectrum efficiency by restricting the area of operation
for selected group calls.
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.
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 system operator or national security organizations.