The group conversation is one of the standard call forms and is characterized by a very fast conversation structure. Access to the conversation is insured by assigning a priority. All users within a group can listen to the conversations within the own group and initiate them.
You can make a call for multiple groups within a company. Beforehand everyone is divided in groups so that only the preprogrammed units are being called.
It is possible to selectively call a user. Such a call allows communication between individual people. A selective call can be made both in simplex as in full-duplex mode.
The Entropia network offers by means of so-called “push-to-talk” the possibility to direct voice communication between users. The speech is of excellent quality, is digitally coded and cannot be tapped. This push-to-talk facility is essential for company critical applications and a connection is guaranteed within 300 milliseconds.
Both half-duplex as full-duplex connections are possible. This can be set up through simple configuration. Besides one to one traffic (selective call) like with a mobile phone, the Entropia Digital network offers many other call and communication facilities.
Division in groups
Within TETRA we distinguish individual and group calls. A participant can be a member of various groups while it is possible to create dynamic groups within the system. Different priorities among which also preemptive insure a smooth working of the calls. It goes without saying that the maximum priorities per user are authorized by the central system.
This refers to a call meant for multiple groups within a company. Only those units that have been preprogrammed in the general call group are being called.
The group conversation is one of the standard call forms in a TETRA trunking system. This allows all users within a subgroup to listen to the conversations of the own subgroup and to initiate them.
Besides the group call there is also the possibility to selectively call a user. The selective call allows communication between individual people.
In the system is the emergency call the call with the highest priority. There are two alternatives that can be programmed:
1. On the first place in the queue
2. Interruption of a busy channel
If alternative 1 is chosen and all channels are busy, the emergency call is put on the first place in the queue. As soon as a channel becomes free, this one is taken by the user with the emergency call.
If alternative 2 is chosen and all channels are busy, the trunking controller looks for the phone call with the lowest priority. This channel will be interrupted and occupied by the emergency call.
Busy signal in case of busy system
Even though trunking systems make better use of the channel capacity than conventional communication systems and it rarely occurs in practice, it may happen that all channels are busy. The user is informed by a busy tone (almost like the busy signal of a phone) of the fact that the system is busy. A user that pushes his send button while all system channels are busy, gets to hear this busy tone until he is given a channel or until he lets go of the send button. Another acoustic signal alerts the user of the radiotelephone/walkie-talkie that he is outside the coverage area.
Queue and answering by the system
All users that try to get access to the system while all channels are busy, are put in a queue. They are given a channel according to a fixed priority level and in order of call (FIFO = First-In-First-Out).
As soon as a channel becomes free, the trunking controller presents itself at the first radiotelephone or walkie-talkie in the queue by means of a system-response in the form of a series of short acoustic signals. Thanks to this facility the user can put down his microphone when he has heard a busy signal and wait for the automatic answering.
Automatic repeat of the call
If a request for assignment of a channel because of bad signal circumstances or interruptions of the trunking controller cannot be received, the radiotelephone or walkie-talkie automatically sends out the call until the request is answered by the central. As soon as a call is initiated, this is automatically completed as described here above, even if the speech button has been let go in the meantime. To further guarantee access to the system, the automatic repeats occur with arbitrary intervals to prevent the possibility of conflicts on the control channel.
Security against incorrect connections
Because of the digital air interface there is a constant communication between the central controller and the walkie-talkie(s). Because of this it is impossible that an arbitrary user can listen in on a channel without being authorized beforehand by the system.
Because of this it will not occur in practice that conversations will run through each other. The digital air interface also insures an excellent protection against joint users on the traffic channels. Analogue (interruption) signals can simply not be heard. The audio quality of the digital connection stays excellent under all circumstances within the range of the system.
Transmission of channel assignments on the control channel
As soon as a group is assigned to a channel, this assignment is constantly transmissted through the control channel during the period that this subgroup uses the channel. That way the users are insured that the newly called in radiotelephones/walkie-talkies are transferred to the correct channel, this means the channel on which they have connection with the other users of their subgroup. This function is also called “Late Entry”.
Tetra also provides the so-called Direct Mode. This implies that radios can communicate between each other without there being a network. There is one condition: they need to be within each other’s radio range.
In case of a telephone connection between the radio network and the PABX of the client, the radio is brought in as a telephone extension with all functionalities that are allowed through the PSTN.