METHOD FOR TRANSMISSION IN AUTONOMOUS PACKET RADIO NETWORKS

Abstract: The invention relates to a method of transmission of information in packet form in a channel between a number of transmitter/receiver stations. According to the method, information is transmitted between the stations using two access methods. The first access method listens to the channel and transmits on the channel after a certain time after the channel has been confirmed to be free. The second access method uses time division. According to the invention, these two access methods are combined in such a manner that a transmission (TC1) using the first access method with acknowledgement request (PF) is followed by acknowledgement transmissions (KV1-KVn) using the second, time division access method, and that acknowledgement information is always received in the transmitter station irrespective of the level of interference on the channel.

Description: METHOD FOR TRANSMISSION IN AUTONOMOUS PACKET RADIO NETWORKS The present invention relates to a method of transmission of information in packet form m a channel between a number of autonomous transmitter/receiver stations using a first access method which listens for whether the channel is free and, with a free channel and after a possible further delay, initiates transmission from a transmitter/receiver station requesting transmission, at the time of the transmission request the transmitter/receiver station (s) for which the transmission is intended being addressed by the transmitter/receiver station requesting transmission and the addressed stations which respond to the transmitter/receiver station requesting transmission transmitting acknowledgements relating to received transmissions . The method is suitable, for example, for communication between autonomous radio stations which communicate within narrow frequency bands with long propagation delays and with not inconsiderable interference levels, as well as a requirement for acknowledgement within a guaranteed period of time. The method can be used inter alia for communication within the defence sector where it is often desirable to maintain silence for long periods so as not to advertise one's presence at the same time as autonomous stations included in the system have to be able to exchange information rapidly and reliably without a central control . An example of the first access method is CSMA (Carrier Sense Multiple Access) . Confirmation of a free channel using the CSMA method is known per se through, for example, US 5 229 993. Known methods for autonomous stations have, however, proved to have limitations, in particular in the case of a requirement for immediate acknowledgement of individual transmissions. The number of collisions between different transmissions and associated retransmissions then threatens to increase to such a level that unacceptable transmission times are obtained which results n total blocking of the channel. .Another method of communicating between different transmitter stations is known through US Patent Specification 5 493 569 In this case, use is made of a time division access method, TDMA. In this case, it is not autonomous transmitter/receiver stations which are involved but the stations are connected in a star- type network to a common controller. Communication problems in the case of controlled star-type networks are entirely different to those m the case of networks with autonomous stations, and therefore the specification only confirms that the TDMA access method is known per se . The object of the present invention is to produce a transmission method which reduces the number of collisions, has improved control of the transmissions and also gives an acknowledgement within a guaranteed period of time. The object of the invention is achieved by a method characterized n that the acknowledgements from each addressed transmitter/receiver station are transmitted using a second, time division access method. By changing over to a time division access method during an acknowledgement period, effective utilization of the channel is obtained during the acknowledgement period. There is no risk of the acknowledgements colliding with one another. At the same time, the acknowledgements utilize a high proportion of the total capacity of the channel. Using the two different access methods in the manner indicated in a system with a number of autonomous transmitter/receiver stations allows long propagation delays at the same time as the number of collisions s reduced without the system becoming particularly complex as a result . Advantageously, the first access method can listen for any carrier wave on the channel using CSMA, and the second access method can work in single-cycle TDMA (Time Division Multiple Access) . An advantageous method according to the invention is characterized in that, at the end of every transmission using the first access method, a final packet is added, with an acknowledgement request containing a list of receiver/transmitter stations and their position during a subsequent acknowledgement cycle performed using the second, time division access method. By means of the method, receiving stations obtain easily accessible and adequate information so as to be capable of changing over to a time division access method during transmission of requested acknowledgements. When a receiving station has identified the packet with the acknowledgement request, it can conclude directly that the end of the transmission has been reached. According to another advantageous method, the second access method uses the final packet with the acknowledgement request transmitted using the first access method as a synchronization point for the transmitter/receiver stations which are to transmit acknowledgements. In this way, simple synchronization of the stations, which is sufficiently accurate for the purpose, is achieved. According to a further advantageous method according to the invention, every transmission cycle is introduced with an idle period t ix . By making t i > t T1 + 2* t τ2 - t T3 , preferably > t T1 + 2* t T2 , where t T1 designates the idle period from the end of a transmission using the first access method to the transmission of a first acknowledgement using the second access method, t T2 designates the length of a time interval during the second access method and t T3 designates the length of an acknowledgement, an ongoing acknowledgement cycle is prevented from being interrupted in the case of acknowledgement responses which have failed to appear as long as the station receives acknowledgements during at least every other time interval. The introduction of this idle period considerably increases the possibilities of completing an entire acknowledgement cycle even in difficult communication situations. According to another advantageous method according to the invention, the second access method is introduced with an idle period t T1 . By introducing the initial idle period, even the station which is the first one to transmit acknowledgements is given time to generate these. The second, time division access method does not have to be used exclusively for acknowledgement transmission, but can advantageously also be used for transmission of other information, such as urgent alarms or other access information. According to an e.xpedient method, in which the transmitter/receiver stations include at a higher level than a media access layer and a data link layer a respective transmitter/receiver unit, initiation of acknowledgement generation and identification of end of acknowledgement are carried out by a logic circuit included in the media access layer and acknowledgement packets are generated and transmitted between data link layers. Advantageously, all unacknowledged packets which should have been acknowledged positively or negatively during a transmission cycle are then considered egatively acknowledged. By means of the method, the time between retransmissions can be reduced and cases of acknowledgements failing to appear as a result of interference levels on the channel being too high are avoided completely. A guaranteed time for final acknowledgement is also obtained. The invention will be described in greater detail below with reference to the attached drawings, in which: Figure 1 shows an example of a number of transmitter/receiver stations which can form part of a communication system which uses the transmission method according to the invention; Figure 2 illustrates in a time diagram the transmission method according to the invention; Figure 3 illustrates the CSMA method in a time diagram; Figure 4 illustrates in greater detail in a time diagram a proposed time division access method which can form part of the transmission method; Figure 5 shows an example of how two communicating transmitter/receiver stations can be constructed; Figure 6 shows transaction handling of link acknowledgements, and Figure 7 illustrates the acknowledgement operation for transmitted packets in a diagrammatic manner for a state machine . In the communication system shown in Fig. 1, eight transmitter/receiver stations S/M1-S/M8 are shown. Communication between the various stations cannot always be guaranteed. In the figure, a possible range for the station S/Ml has been exemplified in particular by means of a circle of dashes 1. In the exemplified communication system, information can therefore not be exchanged between the stations S/Ml and S/M8. The total number of stations included in the communication system may be either smaller or considerably greater than the e.ight which are shown. In the system, one or more stations can be specifically addressed and asked for acknowledgements. An example of a transmission TC using the CSMA method is described first below with reference to Figure 3. In this case, the use of the CSMA method is proposed as the first access method. A number of other methods are possible, however, but are not dealt with in greater detail here. A transmitter/receiver station, e.g. S/Ml, which wishes to transmit, listens for whether the channel for transmission is free. The criterion for a free channel is that there is no carrier wave/signal on the channel . In the event that the channel was busy and is subsequently found to be free, a random delay is introduced within a time interval t i2 between the times T 2 and T 4 before the station is allowed to transmit. A condition for transmission at time T 4 is that the channel has been free from time T α . Transmission is then permitted for a time interval t i3 until time T s . In the event that a transmission according to the above paragraph requires acknowledgements, a final packet PF with an acknowledgement request is added to the transmission TCI, as shown in Figure 2. The acknowledgement request contains a list of addressed transmitter/receiver stations and their position during a subsequent acknowledgement cycle using the time division access method. A transmitter/receiver station, for example S/M2 , which listens to the channel and possibly waits until it can transmit on the channel, and as addressed receiver receives the packet PF, generates internally an acknowledgement KV2. The transmitter/receiver station S/M2 in this connection uses the final packet PF as a synchronization point SP and transmits its acknowledgement on the channel in its assigned position during the acknowledgement cycle KVl-KVn. Every acknowledgement is assigned a length t T3 and is accommodated within a time interval t T2 . As can best be seen in Figure 4, the acknowledgement cycle is introduced with an idle period t T1 . This idle period ensures that the acknowledgement in the transmitter/ receiver station which is to transmit first can be generated in time for transmission. After the acknowledgement cycle has been completed and all addressed stations have been given the opportunity to transmit acknowledgements, the stations revert to the CSMA method. A new transmission TC2 using CSMA is subsequently allowed to follow, transmitted from any one of the stations included in the system. In the event that retransmissions have to be made, this is effected by every retransmission being assigned a new transmission cycle of its own. Possible retransmission sequences are discussed in greater detail below. In order to prevent any of the stations reverting to the CSMA method during the acknowledgement cycle, every changeover to the CSMA method is introduced with an idle period t ix . By ensuring that the idle period is so long that an acknowledgement which has failed to appear between two received acknowledgements is not sufficient for changing over to the CSMA method, the acknowledgement cycle is guaranteed as long as two adjacent acknowledgements do not fail to appear. The idle period can in this connection be dimensioned to comply with the relationship t ±1 > t T1 + 2*t τ2 - t T3 . Every transmitter/receiver station S/M1-S/M2 can, as shown in Figure 5, include a data link layer DL1, DL2 and a media access layer MAI, MA2 at a higher level than a respective radio unit RU1, RU2. Figure 6 shows how link acknowledgements can be handled in transmitter/receiver stations which include closely connected data link and media access layers. Outgoing packets from the station S/Ml are recorded in a list of transmitted packets in the data link layer DLL Incoming packets at the station S/M2 are recorded in a list of received packets in the data link layer DL2. The media access layer MA2 , which recorded the request for acknowledgements, orders generation of acknowledgements in the data link layer DL2. Acknowledgement packets are transmitted from the data link layer DL2 to DL1 during the assigned time interval. The media access layer MAI informs the data link layer DL1 of the end of acknowledgement . .Any request for retransmission is handled at higher level in the data link layer or in higher-level layers. Acknowledgements are conveyed upwards in the data link layer and higher- level layers. Figure 7 shows the acknowledgement operation for transmitted packets in the form of a diagram for a state machine. In the starting position, we are dealing with untransmitted packets symbolized by the state SI . Outgoing packets are recorded (A) and can be divided into three types : Al, packets which will be acknowledged during this transmission, A2 , packets which will not be acknowledged during this transmission, A3, packets which will not be acknowledged. For the A3 type, this means that the procedure is completed and with that the state S6 is reached directly. The Al type waits for acknowledgement during this transmission symbolized by the state S2. Acknowledgement received is acknowledged positively or negatively, see states S4 and S5. For a positive acknowledgement, the procedure is completed and the state Sβ is reached. For negative acknowledgement, retransmission is considered. The number of retransmissions is limited. Packets with later acknowledgement (A2 type) wait in state S3 for conversion to Al type m order then to follow the same operation as previously described for the Al type. Any retransmission request from the state S4 and also completed acknowledgements from the state S6 are transmitted upwards in the layer structure of the station.

Claims: Patent Claims 1. Method of transmission of information in packet form in a channel between a number of autonomous transmitter/receiver stations using a first access method which listens for whether the channel is free and, with a free channel and after a possible further delay, initiates transmission from a transmitter/ receiver station requesting transmission, at the time of the transmission request the transmitter/receiver station (s) for which the transmission is intended being addressed by the transmitter/receiver station requesting transmission and the addressed stations which respond to the transmitter/receiver station requesting transmission transmitting acknowledgements relating to received transmissions, characterized in that the acknowledgements from each addrsssed transmitter/receiver station are transmitted using a second, time division access method, TDMA. 2. Method according to Patent Claim 1, characterized in that the second access method works in single-cycle TDMA. 3. Method according to one of the preceding patent claims, characterized in that the first access method listens for any carrier wave on the channel using CSMA. 4. Method according to any one of the preceding patent claims, characterized in that, at the end of every transmission using the first access method, a Sinai packet is added, with an acknowledgement request containing a list of receiver/transmitter stations and their position during a subsequent acknowledgement cycle performed using the second, time division access method. 5. Method according to Patent Claim 4, characterized in that the second access method uses the final packet with the acknowledgement request transmitted using the first access method as a synchronization point for the transmitter/receiver stations which are to transmit acknowledgements. 6. Method according to any one of the preceding patent claims, characterized in that the changeover from the second access method to the first access method is introduced with an idle period t l > t T1 + 2* t T2 - t T3 , preferably > t T1 + 2* t T2 , where t T1 designates the idle period from the end of a transmission using the first access method to the transmission of a first acknowledgement using the second access method, t T2 designates the length of a time interval during the second access method and t T3 designates the length of an acknowledgement . 7. Method according to any one of the preceding patent claims, characterized in that the second access method is introduced with an idle period t T1 . 8. Method according to any one of the preceding patent claims, characterized in that the second, time division access method is also used for transmission of information other than acknowledgements, such as for example urgent alarm signals or access information. 9. Method according to any one of the preceding patent claims, in which the transmitter/receiver stations include a media access layer and a data link layer at a higher level than a respective transmitter/ receiver unit, characterized in that initiation of acknowledgement generation and identification of end of acknowledgement are carried out by a logic circuit included in the media access layer and in that acknowledgement packets are generated and transmitted between data link layers. 10. Method according to any one of the preceding patent claims, characterized in that all unacknowledged packets which should have been acknowledged positively or negatively during a transmission cycle are considered negatively acknowledged.

Inventors: JOHANSSON JAN, LIND HENRIK, LUNDBERG BJOERN, JOHANSSON, JAN, LIND, HENRIK, LUNDBERG, BJOERN
Applicants: CELSIUSTECH ELECTRONICS AB, JOHANSSON JAN, LIND HENRIK, LUNDBERG BJOERN, JOHANSSON, JAN, LIND, HENRIK, LUNDBERG, BJOERN
Application year: 1997
Main classification: H04J15
IPC codes: H04J99, H04L12