Abstract:
A radio communication system including a coordinator unit and at least one communications terminal configured for the radio communication system in which access to the radio resource is performed in accordance with time-division multiplexing, where the radio resource is divided into determinate time frames, the coordinator unit assigns to the at least one communications terminal, at least one useful data transmission time slot whose temporal position is determined in sequential time frames with respect to the beginning of the particular time frame, and the individual time frames are structured such that they contain a synchronization time slot, at least one useful data transmission time slot of equal length and one additional dynamic time slot.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a U.S. national stage of International Application No. PCT/EP2008/064313, filed on 22 Oct. 2008. Priority is claimed on German Application No. 10 2007 051 605.5, filed 23 Oct. 2007. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to radio communications and, more particularly, to a radio communication system, a coordinator unit and communications terminal. 
     2. Detailed Description of the Related Art 
     A large number of methods relating to when and how communications terminals obtain access to a data transmission medium are known to the person skilled in the art from communications technology, network technology and automation technology. In many cases, access to communication terminals, i.e., ‘slaves’, is allocated by a coordinator unit, i.e., a ‘master’. As the sole network user, the coordinator unit has the right to access the data transmission medium unasked. However, the communications terminals must wait for the allocation from the coordinator unit before they are allowed to access the data transmission medium. The solution known as the ‘master/slave method’ is also used in many bus systems in automation technology. 
     Wireless networks, i.e., Wireless Sensor Actor Networks, have recently been established in addition to “wired” bus systems for data communication between the coordinator unit and the communications terminals in automation technology. Access by the individual communication terminals to the radio resource, is again allocated by the coordinator unit. An important method with respect to access to the radio resource is time-division multiplexing in which the time is divided into individual time frames of uniform length, which are in turn divided into determinate time slots of equal length. Access to the radio resource is possible for a communications terminal only within the allocated time slot or slots within a time frame. The temporal position of the individual time slots in successive time frames is determined in relation to the start of the respective time frame. 
     The allocation of a time slot, which recurs in successive time frames, by a coordinator unit to a communications terminal allows deterministic transmission of data. Deterministic transmission should be taken to mean that the data are transported within a time known and determined in advance from a data source (the sender) to the destination, i.e. the data sink (the receiver). 
     Furthermore, the data should be transmitted within a low latency=delay time. This should, moreover, be the time within which, following the occurrence of an event which is registered by a communications terminal, this event is communicated to the coordinator unit. In the described time-division multiplexing with time frames and a time slot that is fixed for a communications terminal, the latency is substantially determined by the length of the fixed time slot and the length of the time frame. Conventional technical requirements demand latencies of a few milliseconds. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a radio communication system which allows access to the radio resource to be optimally controlled with a low latency and deterministic transmission. 
     This and other objects and advantages are achieved in accordance with the invention by a radio communication system comprising a coordinator unit and at least one communications terminal in which access to the radio resource is structured according to time-division multiplexing,
     a) the radio resource being divided into determinate time frames,   b) the coordinator unit allocating to the at least one communications terminal at least one useful data transmission time slot whose temporal position is determined in sequential time frames with respect to the beginning of the particular time frame,   c) wherein the individual time frames are structured such that they contain a synchronization time slot, at least one useful data transmission time slots of equal length and one additional dynamic time slot.   

     The object is also achieved in accordance with the invention by a coordinator unit designed for a radio communication system. 
     The object is also achieved in accordance with the invention by a communications terminal designed for a radio communication system. 
     In accordance with the invention, the radio communication system, the coordinator unit and the communications terminal allow access to the radio resources to be optimally controlled with a low latency and deterministic transmission of the data. 
     In an embodiment, the radio communication system is advantageously configured such that by means of a synchronization message transmitted in a synchronization time slot, the coordinator unit communicates with the at least one communications terminal in a useful data transmission operating mode in which direction radio communication occurs between the coordinator unit and the at least one communications terminal within this time frame. As a result, the coordinator unit can receive useful data from the at least one communications terminal, or the at least one communications terminal is prepared for direct parameterization by the coordinator unit. 
     In another embodiment, the radio communication system is advantageously configured in accordance with the above embodiments of the invention such that the synchronization message defines the downward direction from the coordinator unit to the at least one communications terminal as the direction of communication, and the at least one communications terminal and the coordinator unit then switch into a coordinator parameterization operating mode in which the at least one communications terminal is parameterized. Of the various possibilities for parameterization of the communications terminal, direct parameterization by the coordinator unit is performed in this case. 
     In another embodiment, the radio communication system is advantageously configured such that by means of the synchronization message transmitted in the synchronization time slot, the coordinator unit communicates to the at least one communications terminal in the useful data transmission operating mode whether acknowledgement of received messages is necessary. As a result, communication security is increased and the latency reduced. 
     In yet another embodiment, the radio communication system is advantageously configured such that the dynamic time slot of a time frame is divided into a plurality of sub-time slots of equal length. As a result, a large number of additional units may communicate with the coordinator unit without disruption to data communication occurring between them. 
     In a further embodiment, the radio communication system is advantageously configured such that the dynamic time slot can be used by a communications terminal to which no useful data transmission time slot is allocated, and/or can be used by an accessory unit. Consequently, the dynamic time slot can be used for data communication with the coordinator unit in different ways. Here, it is used either by additional communications terminals for useful data transmission to the coordinator unit, or by an accessory unit with which additional communications terminals are parameterized by the accessory unit. 
     The radio communication system of the contemplated embodiment is advantageously configured such that by means of an accessory unit parameterization message the accessory unit indicates to the coordinator unit that it wants to parameterize the at least one communications terminal in an ad hoc parameterization operating mode, and by means of the synchronization message the coordinator unit then forces the at least one communications terminal to switch into the ad hoc parameterization operating mode. Parameterization of the communications terminals is thus performed by the accessory unit, radio channels being used which do not disrupt data communication between the coordinator unit and the communications terminals that are not parameterized by the accessory unit. 
     The radio communication system of the contemplated embodiment is advantageously configured such that, by means of an accessory unit parameterization message, the accessory unit indicates to the at least one communications terminal that it should leave the ad hoc parameterization operating mode, whereby the at least one communications terminal is forced to again synchronize with the synchronization message to communicate with the coordinator unit. As a result, the communications terminal parameterized re-participate by the accessory unit can quickly in data communication with the coordinator unit. 
     In a further embodiment, the radio communication system is advantageously configured such that the at least one communications terminal dwells in the power-saving sleep mode in the useful data transmission operating mode and only synchronizes with the synchronization message after the occurrence of an event, and sends a message to the coordinator unit in the allocated useful data transmission time slot in the time frame of this synchronization message. As a result, power-saving operation of the radio communication system is made possible with low latency and deterministic transmission. 
     In a still further embodiment, the radio communication system is advantageously configured such that, in the useful data transmission operating mode, the at least one communications terminal sends a status message to the coordinator unit in an allocated useful data transmission time slot in a predetermined time frame. As a result, the coordinator unit can quickly detect failure of a communications terminal and additional measures can be taken to restore data communication between this communications terminal and the coordinator unit. 
     In a still further embodiment, the radio communication system is advantageously configured to parameterize one or more of the following variables:
     duration of the synchronization time slots,   number of useful data transmission time slots allocated to the at least one communications terminal within one time frame,   number of communications terminals, of which there is at least one,   duration of the useful data transmission time slots,   duration of the dynamic time slot and the number and duration of sub-time slots, and   encoding requirement of the messages to be transmitted.   

     As a result, transmission between a large number of communications terminals and a coordinator unit is achieved in various ways and is adapted to specific requirements. 
     Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposed of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described therein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages of the invention emerge from the following description which, in conjunction with the accompanying drawings, describes the invention with reference to five exemplary embodiments, in which: 
         FIG. 1  is a schematic diagram showing a radio communication system comprising a coordinator unit, three communications terminals and an accessory unit; 
         FIG. 2   a  shows structuring of a time frame comprising a synchronization time slot, three useful data transmission time slots and a dynamic time slot which is structured as a monolithic area; 
         FIG. 2   b  shows  FIG. 2   a  expanded such that the dynamic time slot is divided into two sub-time slots; 
         FIG. 3  shows data communication between a coordinator unit and three communications terminals in a useful data transmission operating mode and a coordinator parameterization operating mode; 
         FIG. 4  shows data communication between a coordinator unit, a communications terminal and an accessory unit in a useful data transmission operating mode and an ad hoc parameterization operating mode; and 
         FIG. 5  shows structuring of a synchronization message transmitted in a synchronization time slot. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a radio communication system FKS comprising a coordinator unit KG, three communications terminals KEG 1 , KEG 2 , KEG 3  and an accessory unit ZG. The coordinator unit also establishes data communication with respect to the higher-order network infrastructure. The radio communication system FKS is, for example, a wireless sensor/actor network. The communications terminals KEG 1 , KEG 2 , KEG 3  can therefore be designed as sensors or actors. The person of ordinary skill will appreciate that a radio communication system FKS of this kind may also comprise additional communications terminals or accessory units (not shown for purposes of clarity). 
       FIG. 2   a  shows structuring of a time frame ZR comprising a dynamic time slot DYNZS, which is structured as a monolithic area. The time frame ZR begins with a synchronization time slot SYNZS, three successive useful data transmission time slots NDÜZS 1 , NDÜZS 2 , NDÜZS 3  which the dynamic time slot DYNZS follows. The following time frame, of which only the synchronization time slot SYNZS is shown, follows the illustrated complete time frame ZR. In the illustrated complete time frame ZR, the synchronization message SYNM is transmitted in the synchronization time slot SYNZS by the coordinator unit KG to the communications terminals KEG 1 , KEG 2 , KEG 3  which support data communication with the coordinator unit KG. These communications terminals KEG 1 , KEG 2 , KEG 3  should synchronize with the synchronization message SYNM contained in the synchronization time slot SYNZS of the time frame ZR, but should at all events implement the message content thereof. 
     The useful data transmission time slot NDÜZS 1  is allocated to communications terminal KEG 1 , useful data transmission slot NDÜZS 2  is allocated to communications terminal KEG 2  and useful data transmission slot NDÜZS 3  is allocated to communications terminal KEG 3 . The duration of the synchronization time slot SYNZS, the number of useful data transmission time slots NDÜZS 1 , NDÜZS 2 , NDÜZS 3  allocated to the communications terminals KEG 1 , KEG 2 , KEG 3  within one time frame, the number of communications terminals KEG 1 , KEG 2 , KEG participating in data communication, the duration of the useful data transmission time slots NDÜZS 1 , NDÜZS 2 , NDÜZS 3 , the duration of the dynamic time slot DYNZS and the number and duration of the sub-time slots UZS 1 , UZS 2  can be parameterized and is determined by software loaded onto the coordinator unit KG before the radio communication system FKS is put into service. The decision as to whether data communication from/to the communications terminals KEG 1 , KEG 2 , KEG 3  should occur in encoded or unencoded form can also be parameterized by the coordinator unit KG. 
     Parameterization is largely determined by the configuration of the radio communication system FKS and influenced, for example, by the following variables: number of communications terminals KEG 1 , KEG 2 , KEG 3  participating in data communication, the maximum latency that is to be maintained, size of the messages to be transmitted from to the communications terminals KEG 1 , KEG 2 , KEG 3  or number of accessory units ZG. 
     The dynamic time slot DYNZS can be used by additional communications terminals to which no useful data transmission time slot NDÜZS 1 , NDÜZS 2 , NDÜZS 3  has been allocated by the coordinator unit KG for cyclical data communication with the coordinator unit KG. Before a communications terminal uses the dynamic time slot DYNZS, the coordinator unit must synchronize with the synchronization message SYNM in the synchronization time slot SYNZS that preceded the dynamic time slot DYNZS. In particular, the dynamic time slot DYNZS can also be used if the data packets to be transmitted are too long to be transmitted within one useful data transmission time slot NDÜZS 1 , NDÜZS 2 , NDÜZS 3 . 
     The dynamic time slot DYNZS can also be used by an accessory unit ZG which is used to parameterize the communications terminals KEG 1 , KEG 2 , KEG 3 , as described in more detail subsequently. 
     Data collisions between different communications terminals or the accessory unit ZG in the dynamic time slot DYNZS are resolved by congestion-resolving mechanisms, i.e., backoff mechanisms, as known to a person skilled in the art in which a repeated sending of data packets takes place at subsequent, randomly selected times or data packets of certain communications terminals are prioritized. 
       FIG. 2   b  differs from  FIG. 2   a  only insofar as the dynamic time slot DYNZS in time frame ZR is divided into two sub-time slots UZS 1 , UZS 2 . As a result, two additional communications terminals or accessory units can perform a data communication with the coordinator unit KG without a disruption in data communication occurring between them. 
       FIG. 3  shows data communication between a coordinator unit KG and three communications terminals KEG 1 , KEG 2 , KEG 3  in a useful data transmission operating mode and a coordinator parameterization operating mode in three successive time frames ZR 1 , ZR 2 , ZR 3  and the beginning of a fourth time frame which follows the third time frame ZR 3 . The construction of the time frames ZR 1 , ZR 2 , ZR 3  in each case matches that from  FIG. 2   a . Three useful data transmission time slots respectively, which to preserve clarity are not shown in  FIG. 3 , follow the synchronization time slots SYNZS, SYNZS 1 , SYNZS 2 , SYNZS 3  and SYNZS 4 . The coordinator unit KG allocates the three useful data transmission time slots to the three communication terminals KEG 1 , KEG 2 , KEG 3 , as described in  FIG. 2   a.    
     Data communication between the coordinator unit KG and the first communication terminal KEG 1  will firstly be described in more detail. 
     Before the first time frame ZR 1 , an event E 1  occurs that is registered by the first communications terminal KEG 1 . This event E 1  could, for example, be the exceeding of a temperature threshold, the detection of a fire, etc., in any case something which is detected, for example, by a communications terminal KEG 1  designed as a sensor and which must be communicated to the coordinator unit KG as a result of fixed processing guidelines in the sensor. 
     Thus, a message can be communicated to the coordinator unit KG that the first communications terminal KEG 1  must synchronize with the next synchronize message in the synchronization time slot SYNZS 1 . For this purpose, the first communications terminal KEG 1 , by evaluating the synchronization message, establishes that it is in a useful data transmission operating mode, i.e., can send data to the coordinator unit KG in the useful data transmission slot, following the synchronization time slot, allocated by the coordinator unit KG. This is shown in  FIG. 3  and later also in  FIG. 4  by a thick arrow being shown in the useful data transmission time slot NDÜZS 1 . Thick arrows in the useful data transmission time slots NDÜZS 1 , NDÜZS 2 , NDÜZS 3  represent sent messages, moreover. Thin arrows in the useful data transmission time slots NDÜZS 1 , NDÜZS 2 , NDÜZS 3  of communications terminals KEG 1 , KEG 2 , KEG 3  represent received messages, moreover. Thin arrows in the useful data transmission time slots NDÜZS 1 , NDÜZS 2 , NDÜZS 3  of the coordinator unit KG represent received messages, moreover, if messages have been sent by communications terminals KEG 1 , KEG 2 , KEG 3  in corresponding useful data transmission time slots NDÜZS 1 , NDÜZS 2 , NDÜZS 3 , or the readiness-to-receive of the coordinator unit KG if no messages have been sent by communications terminals KEG 1 , KEG 2 , KEG 3  in corresponding useful data transmission time slots NDÜZS 1 , NDÜZS 2 , NDÜZS 3 . If there are no arrows in useful data transmission time slots NDÜZS 1 , NDÜZS 2 , NDÜZS 3 , then this is intended to show that no messages are being sent from communications terminals KEG 1 , KEG 2 , KEG 3  to the coordinator unit KG in these useful data transmission time slots NDÜZS 1 , NDÜZS 2 , NDÜZS 3 . 
     In useful data transmission, time slot NDÜZS 1  of time frame ZR 1  communications terminal KEG 1  therefore sends a message to the coordinator unit KG. Due to synchronization of the communications terminal KEG 1  with the synchronization message in the second synchronization time slot SYNZS 2  of time frame ZR 2 , communications terminal KEG 1  receives the communication that it should switch from the useful data transmission operating mode into the coordinator parameterization operating mode in which communications terminal KEG 1  receives a parameterization message from the coordinator unit KG in the useful data transmission time slot NDÜZS 1  of the second time frame ZR 2 . Due to the amount of data, transmission of the parameterization message cannot be completed in a single useful data transmission time slot, however. If the communication terminal KEG 1  now synchronizes with the synchronization message in the third synchronization time slot SYNZS 3  of the third time frame ZR 3 , it receives the communication that the coordinator parameterization operating mode should be maintained. From the coordinator unit KG in the useful data transmission time slot NDÜZS 1  of the third time frame ZR 3 , the communication terminal KEG 1  now receives the second part of the parameterization message which could now be transmitted completely in the useful data transmission time slot NDÜZS 1  of the third time frame ZR 3 . 
     Parameterization messages, which the communications terminal KEG 1  receives in coordinator parameterization operating mode from the coordinator unit KG, are optionally acknowledged by the communications terminal KEG 1  in a successively allocated useful data transmission time slot by an acknowledgement message BM. Here, it is possible to parameterize how many parameterization messages correctly received by the first communications terminal KEG 1  are acknowledged with an acknowledgement message BM. As a result, it is possible to acknowledge each parameterization message separately, as well as blocks of parameterization messages, with an acknowledgement message BM. With a good data connection, acknowledgement messages (BM), which each acknowledge a block of parameterization messages, only minimally reduce data throughput. 
     If the communications terminal KEG 1  now synchronizes with the synchronization message in the fourth synchronization time slot SYNZS 4  of the fourth time frame, the communications terminal KEG 1  receives the communication that it should switch from the coordinator parameterization operating mode back into the useful data transmission operating mode. The communications terminal KEG 1  now sends messages (useful data) to the coordinator unit KG in the useful data transmission time slot NDÜZS 1  of the fourth time frame. 
     Data communication between the coordinator unit KG and the second communications terminal KEG 2  will now be described in more detail. 
     As a result of an event E 2  before the first time frame ZR 1  and following synchronization of the second communications terminal KEG 2  with the synchronization message SYNM in the first synchronization time slot SYNZS 1  of the first time frame ZR 1 , the second communications terminal KEG 2  sends a message to the coordinator unit KG in the useful data transmission time slot NDÜZS 2  of the time frame ZR 1 , which message could be transmitted completely in this useful data transmission time slot NDÜZS 2  of the first time frame ZR 1 . 
     Consequently, the radio communication system FKS can be operated in a power-saving manner, and if no repeated event precedes the second synchronization time slot SYNZS 2  of the second time frame ZR 2 , the second communications terminal KEG 2  does not synchronize with the synchronization message SYNM of the second synchronization time slot SYNZS 2  of the second time frame ZR 2  but dwells in the power-saving sleep mode. However, if the message could not be completely transmitted to the coordinator unit KG in the useful data transmission time slot NDÜZS 2 , and this is not shown in  FIG. 3 , the second communications terminal KEG 2  would synchronize with the synchronization message SYNM of the second synchronization time slot SYNZS 2  of the second time frame ZR 2  and transmit the part of the message, which has not yet been transmitted, to the coordinator unit KG in the second useful data transmission time slot NDÜZS 2  of the second time frame. 
     Following the second synchronization time slot SYNZS 2  of the second time frame ZR 2  an event E 3  in turn occurs, however, which is registered by the second communications terminal KEG 2  and has to be indicated to the coordinator unit KG. To achieve this indication, the second communications terminal KEG 2  synchronizes with the synchronization message SYNM of the third synchronization time slot SYNZS 3  in the third time frame ZR 3 , and in the second useful data transmission time slot NDÜZS 2  of the third time frame ZR 3  transmits a message (useful data) to the coordinator unit KG, which message, in turn, could be transmitted completely in the second useful data transmission time slot NDÜZS 2  of the third time frame ZR 3 . 
     The second communications terminal KEG 2 , in turn, then dwells in the power-saving sleep mode, as described previously with respect to the second time frame ZR 2 , until an event re-occurs. 
     Data communication does not occur between the coordinator unit KG and the third communications terminal KEG 3 . The third communications terminal KEG 3  dwells in the power-saving sleep mode. 
       FIG. 4  shows data communication between a coordinator unit KG, a first communications terminal KEG 1  and an accessory unit ZG in a useful data transmission operating mode and an ad hoc parameterization operating mode. Structure and presentation match those in  FIG. 3  unless modifications are specified. 
     As a result of an event E 4  before the first time frame ZR 1  and following synchronization of the first communications terminal KEG 1  with the synchronization message SYNM in the first synchronization time slot SYNZS 1  of the first time frame ZR 1 , the first communications terminal KEG 1  sends a message to the coordinator unit KG in the useful data transmission time slot NDÜZS 1  of the time frame ZR 1 , which message could be transmitted completely in this useful data transmission time slot NDÜZS 1  of the first time frame ZR 1 . 
     No new event precedes the second synchronization time slot SYNZS 2  of the second time frame ZR 2 . The first communications terminal KEG 1  does not synchronize with the synchronization message SYNM of the second synchronization time slot SYNZS 2  of the second time frame ZR 2  therefore but dwells in the power-saving sleep mode. 
     In the dynamic time slot DYNZS of the first time frame ZR 1 , however, the accessory unit ZG, to which no useful data transmission time slot is allocated, indicates to the coordinator unit KG by an accessory unit parameterization message that it wants to parameterize the first communications terminal KEG 1  in an ad hoc parameterization operating mode. The coordinator unit KG then forces the first communications terminal KEG 1  to switch to the ad hoc parameterization operating mode by means of the synchronization message SYNM in the second synchronization time slot SYNZS 2  of the second time frame ZR 2 . The first useful data transmission time slot NDÜZS 1  of the second time frame ZR 2  in the coordinator unit KG is not used for receiving messages therefore, which is represented by 0 in  FIG. 4 . 
     In the second dynamic time slot DYNZS of the second time frame ZR 2 , the accessory unit ZG then sends a parameterization message to the first communications terminal KEG 1  over a radio channel which does not disrupt data communication between the coordinator unit KG and the communications terminals not parameterized by the accessory unit ZG. 
     In the second dynamic time slot DYNZS of the second time frame ZR 2 , the first communications terminal KEG 1  then sends a response message to the accessory unit ZG. The accessory unit ZG indicates to the first communications terminal KEG 1  by an accessory unit parameterization termination message that the first communications terminal KEG 1  should leave the ad hoc parameterization operating mode again. The first useful data transmission time slot NDÜZS 1  of the third time frame ZR 3  is not used in the coordinator unit KG for receiving messages, however. 
     As a result of an event E 5  before the fourth synchronization time slot SYNZS 4 , the first communications terminal KEG 1  again synchronizes with the synchronization message SYNM in the fourth synchronization time slot SYNZS 4  and the first communications terminal KEG 1  sends a message to the coordinator unit KG in the useful data transmission time slot NDÜZS 1  of the fourth time frame ZR 4  (not shown in full or denoted in  FIG. 4 ). 
     Naturally (and this is not shown in  FIG. 4 ), as a result of synchronization with the synchronization time slot SYNZS 3 , the communications terminal KEG 1  could also send a message to the coordinator unit KG as early as in the useful data transmission time slot NDÜZS 1  of the third time frame ZR 3 . Parameterization of the first communications terminal KEG 1  by the accessory unit ZG can also extend over a plurality of dynamic time slots DYNZS and the response of the first communications terminal KEG 1  to parameterization of the accessory unit ZG does not necessarily have to be made in the same dynamic time slot DYNZS. 
     In  FIGS. 3 and 4 , the communications terminals KEG 1 , KEG 2  send messages (useful data) as a result of events E 1 , E 2 , E 3 , E 4 , E 5  that have occurred. If no events occur over a relatively long period, however, the communications terminals dwell in the power-saving sleep mode. 
     For a relatively long period, the coordinator unit KG then does not know whether a communications terminal is still operating correctly or has already failed. It would therefore not be possible for parameterization or diagnostics to reach a communications terminal that has failed. The radio communication system FKS can therefore also be parameterized such that, irrespective of the occurrence of an event, the communications terminals have to send a status message to the coordinator unit KG at predetermined intervals and in allocated useful data transmission slots. The absence of a status message indicates the failure of the communications terminal. The status message may also contain diagnostics information. 
       FIG. 5  shows structuring of a synchronization message SYNM transmitted in a synchronization time slot. The synchronization message SYNM consists of a first section comprising a preamble bit pattern P for time synchronization, specifications relating to the frame length L, specifications relating to the frame type T and a network address NWA (a network address with a length of 8 bits means 256 networks may be distinguished, and this is sufficient in the practice of factory buildings or industrial plant) and a second section for controlling the time slots ZS 1 , ZS 2 , ZS 3  allocated to the three communications terminals KEG 1 , KEG 2 , KEG 3 . The control message (second section) relating to time slot ZS 1  is described by way of example and comprises a first part BM, with which the coordinator unit KG indicates to the first communications terminal KEG 1  whether a preceding message, or a plurality thereof, sent by the first communications terminal KEG 1  has been correctly received by the coordinator unit KG or not and/or whether an acknowledgement is required from the first communications terminal KEG, a second part RM which fixes the direction of data communication between coordinator unit KG and first communications terminal KEG 1  within this time frame, and a third part BAM which indicates to the first communications terminal KEG 1  in what operating mode the first communications terminal KEG 1  is being operated in this time frame. 
     In the simplest case, one bit is provided for each part BM, RM, BAM of the control message, i.e., 3 bits in total. The bit value 0 of the first part BM indicates to the first communications terminal KEG  1  that a preceding message, or a plurality thereof, sent by the first communications terminal KEG 1  has been correctly received by the coordinator unit KG. The bit value 0 of the second part requests a transmit mode by the first communications terminal KEG 1  and a receive mode by the coordinator unit KG, and the bit value 0 of the third part requests operation of the first communications terminal KEG in the useful data transmission operating mode or coordinator parameterization operating mode. Conversely, the bit value 1 of the first part BM of the first communications terminal KEG 1  indicates that no preceding message, or a plurality thereof, sent by the first communications terminal KEG 1  has been correctly received by the coordinator unit KG. The bit value 1 of the second part requests a receive mode by the first communications terminal KEG 1  and a transmit mode by the coordinator unit KG, and the bit value 1 of the third part requests switching of the first communications terminal KEG 1  into an ad hoc parameterization operating mode. 
     It should be determined in advance in the coordinator parameterization operating mode whether the coordinator unit KG has to acknowledge correct receipt of messages from the first communications terminal KEG 1 . To minimize the latency in the useful data transmission operating mode, each message sent by the first communications terminal KEG 1  in the preceding useful data transmission time slot and correctly received by the coordinator unit KG is acknowledged by the coordinator unit KG in the synchronization message SYNM in the subsequent synchronization time slot. If this message has not been correctly received by the coordinator unit KG, however, there is no acknowledgement message BM from coordinator unit KG. 
     If the coordinator unit KG does not send an acknowledgement message BM in this case, the first communications terminal KEG 1  must re-send the last-sent message to the coordinator unit KG in the following useful data transmission time slot. 
     As described previously, individual messages can be encoded in the radio communication system FKS by instructions to the communications terminals KEG 1 , KEG 2 , KEG 3  in the synchronization messages. Alternatively, all messages may be encoded. Each radio communication system FKS receives its own code. As a result, it is not necessary for a separate network address to be used. If a message can be decoded, the message originates from a communications terminal KEG 1 , KEG 2 , KEG 3  in the radio communication system FKS. 
     In accordance with the disclosed embodiments, the radio communication system FKS has the following advantages
     1. Separate operating modes a) for transmitting messages(process values) in the useful data transmission operating mode, b) for parameterizing communications terminals by the coordinator unit in the coordinator parameterization operating mode, c) for parameterizing communications terminals by the accessory unit in the ad hoc parameterization mode, whereby the following advantages are achieved:   d) low latency for the transmission of process values   e) parameterization of communications terminal in the coordinator parameterization operating mode and the ad hoc parameterization operating mode without adverse effects on the latency.   2. No parameterization of communications terminals by the accessory unit in the useful data transmission operating mode, whereby the following advantages are achieved:   a) parameterization by the accessory unit without adverse effects on the latency,   b) the entire bandwidth of a radio channel is available for parameterization by the accessory unit.   3. Information on the operating mode of the communications terminals in the synchronization message, whereby the following advantages are achieved:   a) the communications terminals can be put into different operating modes,   b) it is possible to switch between operating modes with minimal latency.   4. Inclusion of the requirement for the communications terminals to send an acknowledgement message in the synchronization message, whereby the following advantage is achieved:   length of the times slots, and therefore the latency, can be minimized.   5. Inclusion of direction information in the synchronization message, whereby the following advantages are achieved:   a) the coordinator unit can separately switch the direction of data communication to the communications terminals,   b) no separate radio channel, which would increase the latency, is required for the direction from coordinator unit to communications terminal.   6. The number and length of the time slot can be freely parameterized, such that the radio communication system can be advantageously optimized for minimal latency in the case of a few users (communications terminals) and for a large number of users with an increased latency.   

     The invention is not limited to the specific exemplary embodiments but includes further modifications that are not explicitly disclosed, provided use is made of the essence of the invention. 
     Thus, while there have been shown, described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. Moreover, it should be recognized that structures shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.