TETRA Network
TErrestrial Trunked RAdio is an open digital radio standard developed in conjunction with European Technical Standards Institute (ETSI) to provide voice and data services to Professional Mobile Radio users including mission critical applications. TETRA uses a 25Khz bandwidth channel producing 4 Time Division Multiple Access (TDMA) timeslots using DQPSK digital modulation.
The TETRA air interface standard incorporates a digital algorithm along with TDMA modulation to provide a secure communications channel from unwanted listeners and radio scanners. Additional Air Interface or End to End encryption standards can be utilised as part of the TETRA standard making it suitable for Government and Military organisations.
- Group communication
TETRA Group communication is most often called talkgroup communication. User that have the same talkgroup selected in the mobile radios are able to communicate with each other on a half duplex basis. Half duplex commununication means that one user is speaking (transmitting) while the others in the same group listen to the person that is transmitting. The listeners can only start to talk (transmit) when the other it finished (with different capabilities/permissions one can interupt which is called pre-empting, this is manufacturer depending). When a control room is available the control room dispatcher is also listening to the talkgroup. Most of the time the dispatcher is able to interupt the other talkgroup users (pre-empt).
In analogue networks users selected a frequency on the mobile radios. The users that selected the same frequency are able to communicate with each other. The number of available frequencies determined to number of groups to be formed. In TETRA a talkgroup has a number (GSSI) that is configured in the network and the mobile radios. Users select the alias on their radios that is associated to it (the talkgroup alias or talkgroup name). The network deals with assigning a temporary frequency for the time of the communication. When the transmission stops the frequency can be reused for another talkgroup. This means that the number of available frequencies doesn't affect the number of talkgroups. The number of possible talkgroups that can be used can be several thousands. Ofcourse care need to be taken that the number of simultaneous active talkgroups doesn't exceed the number of available frequencies/channels (since one frequency in TETRA consist of four channels).
- Individual calls
The TETRA individual call is a one-to-one call. It acts as 'telephone' call between two TETRA mobile radio's. The calling party selects the ISSI or alias in the mobile radio telephone book or by typing in the number using the keypad. On the receiving party the call can be answered and the user is usually notified by ring indication.
Once the call is established a full duplex or half duplex call between the two radio's can be made. When in TMO mode the network needs to support individual call and the user usually needs to have permission to make individual calls. The two users can be anywere in the network. In DMO mode only half duplex indivual call can be made.
- Short Data Services (SDS)
The TETRA Short Data Service (SDS) is a data service that is comparable with the Short Data Message (SMS) of GSM. A TETRA SDS message can carry up to 140 byte data per message. SDS message carrying different types of information are separated via a protocol identifier. A range of protocol identifiers are defined in the ETSI TETRA specification.
A variety of applications can use the SDS service to carry information. The most common usage is the sending an message that is entered via the keypad of the subscriber. But for example the LIP (Location Information Protocol) location information messages that carry the GPS location from the subscriber to the mapping application are usually also transported via SDS messages.
SDS message can be send between subscribers, between subscriber and dispatcher and subscriber and fixed host in the network. SDS messages can be send to individual subscribers or broadcasted on a number of basestations to all subscriber using that basestation. Short data messages between subscriber and fixed host makes it possible for an application to receive SDS message and based on their content do something with the received information. For example when the SDS message contained to GPS location of the subscriber the location can be displayed on a map. Or for example when the SDS message contains the water level the water level can be displayed and monitored. SDS messages are often used as transport mechanism for telemetry applications.
TETRA Short Data Message are transported via the Control Channel of a basestation. The control channel is also used for signalling of the voice traffic. To prevent overload of the control channel SDS message need to be used carefull. Contact TETRA-Consultancy to learn about the limitations.
- Full duplex Telephone Interconnect (similar functionality to mobile telephone)
- Emergency Priority of network resources and pre-emptive scheduling of emergency calls is an important safety feature exclusive to TETRA technology.
- Late Entry of a current group or emergency call is supported when a user registers on the network.
- Packet data applications include email, WAP, Internet Protocol (IP) and slow video transmission.
- Network management: Users are registered with the TETRA network so terminal activity, features and access can be managed via a Network Management Terminal.
- Dispatcher terminals can provide integrated command and control systems (ICCS) to be connected using PC's either via Internet protocol (IP) or wireless technologies.
- IP Based Infrastructure allows multiple TETRA sites to be connected anywhere around the globe with an IP network connection.
TOP 
TETRA vs Current Analogue PMR Systems:
Communication security, longer battery life, increased spectrum efficiency, superior audio quality, improved voice clarity in marginal RF coverage areas, simultaneous voice and data calls, full duplex communication.
Modes of operation
- TMO Trunked mode operation is where the terminal is registered to the main network infrastructure and communicates with other terminals via a TETRA network base station.
- DMO Direct mode operation is term used to describe terminals communicating directly with each other and not via a network (ie. out of coverage of the network). Typical DMO range is around 1km.
- DMO Repeater is a terminal that is used to store then forward a call or text from another terminal thus extending DMO range.
- A DMO Gateway stores DMO calls and data then retransmits them onto the TMO network and visa-versa.
An Analogue Gateway provides a method of connecting legacy analogue radio systems with a TETRA network to set-up voice calls simultaneously on both systems and support a managed migration path from analogue to TETRA technology.
Fallback provides additional resilience to the network when inter-site communication links go down. Individual sites can act as stand alone repeaters allowing group call communications governed by the local site database.
Mobile and network vendor Interoperability is a fundamental part of the TETRA Association which conducts continuous equipment compliance testing and issues interoperable functionality certificates for the majority of the TETRA equipment available on the market today allowing the network user to choose from a greater range of manufacturers with certified functionality.
TETRA & Evolutions
