METHOD FOR CONTROLLING PARTIES IN
REAL-TIME DATA COMMUNICATION
FIELD OF THE INVENTION
 The invention relates to communications systems, and especially to real-time data (two-party or multi-party) communication in communications systems.
BACKGROUND OF THE INVENTION
 The most common call type is a call established between two parties for one-to-one communication. The standard way to set up a two-party call requires explicit control plane signalling that allows the call parties to establish a channel where the audio data can be transferred and to negotiate the communication capabilities; for example, the audio codec and the relative compression rate can be determined in this phase. Afterwards the actual voice communication can start and the audio data can be transmitted by the call parties.
 Voice over Internet Protocol (VoIP) enables a speech communication over an IP connection. The Session Initiation Protocol (SIP, RFC 2543) is conventionally used for call establishment in "VoIP" based communication systems.
 A mobile communications system refers generally to any telecommunications system which enables communication when users are moving within the service area of the system. A typical mobile communications system is a Public Land Mobile Network (PLMN). Often the mobile communications network is an access network providing a user with wireless access to external networks, hosts, or services offered by specific service providers.
 Professional mobile radio or private mobile radio (PMR) systems are dedicated radio systems developed primarily for professional and governmental users, such as the police, military forces, oil plants, etc. PMR services are offered via dedicated PMR networks built with dedicated PMR technologies. This market is divided between several technologies—analog, digital, conventional and trunked— none of which has a dominating role. TETRA (Terrestrial Trunked Radio) is a standard defined by ETSI (European Telecommunications Standards Institute) for digital PMR systems.
 One special feature offered by the PMR systems is group communication. The term "group", as used herein, refers to any logical group of three or more users intended to participate in the same group communication, e.g. a call. Group communication with a push-to-talk feature is one of the essential features of any PMR network. Generally, in group voice communication with a "push-to-talk, releaseto-listen" feature, a group call is based on the use of a pressel (PTT, push-to-talk switch) in a telephone as a switch: by pressing a PTT the user indicates his desire to speak, and the user equipment sends a service request to the network. The network either rejects the request or allocates the requested resources on the basis of predetermined criteria, such as the availability of resources, priority of the requesting user, etc. At the same time, a connection is established also to all other active users in the specific subscriber group. After the voice connection has been established, the requesting user can talk and the other users can listen on the channel. When the user
releases the PTT, the user equipment signals a release message to the network, and the resources are released. Thus, the resources are reserved only for the actual speech transaction or speech item, instead of reserving the resources for a "call". One interesting advantage of the push-to-talk communication, or more generally speech-item-by-speechitem communication, is a short call setup time, which also makes such speech communication attractive to several other types of users. U.S. Pat. No. 6,141,347 discloses a wireless communications system which uses multicast addressing and decentralized processing in group calls.
 A problem with such item-by-item communication is that a strict discipline or protocol is required from the parties in the speech communication, or other type of real-time data communication. Further, especially in group communication, it is difficult to know which the parties of the communication are at each specific moment, and therefore the communication must include spoken questions and acknowledgements.
SUMMARY OF THE INVENTION
 An object of the invention is to provide a new way to control parties in an item-by-item real-time data communication.
 This object of the invention is achieved by methods, systems and terminals as defined in the attached independent claims. Various embodiments of the invention are defined in the attached dependent claims.
 In the present invention, each receiving user terminal acknowledges the reception of a real-time data item by sending a real-time data item acknowledgement report after the end of the item. In an embodiment of the invention, the acknowledgement report may be sent after a successful reception of real-time data item, unsuccessful reception of real-time data item or in both cases, depending on the implementation and/or the user's selection. In an embodiment of the invention, the report may also contain information relating to the quality of the connection. In an embodiment of the invention, the receiving terminal sends an acknowledgement report only in response to a specific request sent by the sending user terminal from which the real-time data item originated, and no report is sent otherwise. This embodiment allows unnecessary reports to be avoided if the sending party is not interested in them. In another embodiment of the invention, the report is sent as a default. In an embodiment of the invention, each acknowledgement report is forwarded over a communication system to the sending user terminal from which the real-time data item originated. In this embodiment, no extra functionality is needed in the communication system infrastructure for the present invention. In another embodiment of the invention, a communication system infrastructure collects acknowledgement reports from a plurality of receiving user terminals and sends a combined acknowledgement report to the sending user terminal from which the real-time data item originated. This embodiment requires extra functionality in the communication system infrastructure for the invention but, on the other hand, requires less transmission capacity. When the sending user terminal receives the acknowledgement report(s) after the real-time data item has ended, it may display information which indicates to a sending user which terminal(s) or user(s) were receiving the previous real-time