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    The TETRA System

    For decades, radio communication has been the solution for flexible and efficient communication in the field. Radio enables instant communication between two or more people simply by pressing a button. This push-to-talk feature and the ability of people to communicate in groups is fundamental for authority communication.

    In most Public Safety organisations around the world, communication is a hot item;- in many countries old equipment must be renewed during the next few years. The change from conventional analogue to intelligent digital radio networks is a big one; involving both technical and financial complexity, especially for the users.

    One of the leading standards in digital radio is TETRA, which is an open standard for digital trunked radio systems. TETRA stands for TErrestrial TRunked RAdio. It is a set of open standards developed by the European Telecommunications Standards Institute (ETSI). TETRA is the first truly open international standard for the digital professional mobile radio environment. TETRA specifies a set of open interfaces and services for new Professional Mobile Radio Systems (PMR).

    Several European authorities and the leading radio terminal and system vendors in the world have participated in the specification work. As a result, both the high-level demands of the authorities and the possibilities of the newest technology have been combined. The European Union has reached an agreement to reserve the 380 - 400 MHz frequency band for public safety and security. This means that authorities in different countries can operate in each other's networks when the networks comply with the same standard and use the same frequency band. The 410-430 MHz frequency has been reserved for the professional cellular sector. In their every day activities, public safety organisations need various kinds of communications services.

    TETRA offers both the usual cellular services as well as professional radio services such as group communication, field workforce management services (dispatching) and efficient data services. TETRA is a unique combination of group voice communications, mobile telephony and mobile data services specifically designed for authority use. TETRA is a purpose built technology that offers public safety and security organisations major advantages over conventional radio systems. It was developed to meet the needs of the most demanding professional radio users who need fast one-to-one and one-to-many radio communication using voice and data in their daily work. Users are typically public safety and security organisations such as police, fire and rescue forces. TETRA fulfills the needs of professional users and replaces old analogue and proprietary radio communication systems that no longer meet professional radio-communication needs.

    Special Features of TETRA Network:

    Typical reasons to adopt PMR services are related to the need for special functionality such as group calls; instant communications with a sub-second call set-up delay, security and specialized dispatching services. The latter means managing the organization's field operations and related communications. For public safety organizations, security is a fundamental issue and includes authentication of the users in the network as well as encryption of the voice and data communication itself.

    For many organizations, having control of their own network resources is crucial and in many cases PMR services also offer the lowest overall communication costs.

    Push-to-Talk Operation:

    Instant call connection and push-to-talk operation to set up a call is probably one of the most important features in TETRA. In cellular networks such as GSM, calls are mostly one-to-one i.e. individual calls, which are initiated either by dialing the called-party number using the phone's keypad or using the speed dial key. In both cases the call set-up time i.e. the delay from dialing until the called-party's phone starts ringing is typically in the range of a couple of seconds. The B-party has to press the answer button on his/her phone to answer the call. This kind of communication is too slow to match the quick pace of communication needed by professional users such as police or firemen.

    In TETRA, a call is initiated with a push-to-talk button and connection is established in less than half a second. In addition, there is no need to answer the call - the calling party can start talking right away.

    Group Communications:

    Another differentiating feature between TETRA and cellular services is group communication. In addition to individual calls, TETRA also supports group calls. The members of the groups are predefined and a group may consist of tens or even hundreds of members. A TETRA handset typically has an easy way to select the group, typically by using a rotary knob or easy-to-use menu. The selected group is then the default communication group for the user. The call is initiated by pressing the push-to-talk button and all users who have selected the same group can hear the communication.

    Cellular services cannot provide group communication functionality. Conference calls in cellular systems are not suitable for the fast-paced group communication needed in field operations by professional users. It is all too clumsy to set-up and the number of members in the conference call is limited. In addition to predefined groups, a group can be created dynamically over the air during field operations. This is especially important in rescue operations where users from different organizations need to communicate together. The coordinator of the field operations, often called a dispatcher, can place the members into the dynamic group and the new group is created instantly over the air. The members of the newly created group get an indication that they belong to a new dynamic group and by selecting this group; they can start group communication immediately.

    Fleet Management:

    A dispatcher is a person who manages the field operations in a PMR network. Typically the dispatcher has a special graphical workstation i.e. dispatcher workstation, which helps to control the communication activities in the field. For example, in a police organization the dispatcher manages the radio communication with the mobile units in the field, gives task orders and instructions and receives information from the field force either as voice or data messages. The graphical dispatcher workstation helps him to communicate and get an overview of the status of units in the field.

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