Patent Publication Number: US-2010124189-A1

Title: Method for transmitting data packets and conrreponding reception method

Description:
1. FIELD OF THE INVENTION 
     The present invention relates to the wireless telecommunications field and more precisely the transmission of data, reliable by a base station via two subscriber stations (or remote stations). 
     2. TECHNOLOGICAL BACKGROUND 
     According to the prior art, several wireless network architectures are known. Some of them are based on a centralised architecture. Hence, the Wi-Fi system (based on the IEEE 802.11a standard) has a non-centralised task sequencing architecture (or scheduling) with a contention channel access. Such an architecture does not enable a sufficient Quality of Service (or QoS) to be managed effectively for some applications. The Wimax system (based on the IEEE 802.16 standard) possesses a centralised scheduling architecture which allows the implementation of a more appropriate quality of service for certain applications (maximum delivery time for a packet (typically 5 ms) and bandwidth guaranteed for each connection request). 
     Nevertheless, the techniques implemented in the Wimax networks do not enable a quality of service to be guaranteed for all the applications, for example for video type communications, data being transmitted by wireless cameras moving around in noisy radio-frequency environments, subject to interference or disturbed by obstacles creating signal losses or echoes. Hence, a communication with a wireless station can be cut off suddenly (for example, when the mobile station is moving around). Indeed, the support or the coverage of a wireless link cannot be guaranteed, which may cause transmission problems when a station or its environment moves. 
     The mobile networks such as those described in the patent document US 2006/0154603 entitled “Method and devices for efficient data transmission link control in mobile multicast communication systems” provide the systems with packet retransmissions as long as all the packets are not correctly acknowledged by all the receivers. However, the method described in this document is not optimised for the use of resources with a quality of service guarantee. 
     3. SUMMARY OF THE INVENTION 
     The purpose of the invention is to overcome the disadvantages of the prior art. 
     More particularly, the purpose of the invention is to enable the transmission and/or reception of data by at least one wireless station intended for relay stations, with a guaranteed quality of service and more specifically with an absence of cutting off of the communication (namely, with no loss of packets having to be received by the wireless station or stations) under normal conditions of use. 
     The invention concerns a transmission method of first data packets, the method being implemented in a first station, each first packet being transmitted by the first station intended for at least two second stations, the second stations belonging to a set comprising several second stations. In order to guarantee a quality of service, the method comprises:
         a reception of at least one acknowledgement of each first packet received correctly by at least one second station, said acknowledgement or acknowledgements being transmitted by the second station or stations having correctly received the first packet;   a retransmission of each first packet not acknowledged at least once, each first packet acknowledged at least once not being retransmitted.       

     Advantageously, the method comprises:
         A cut of a second data packet (SDU) in one or more first data packets (PDU);   a retransmission of each first packet not acknowledged at least once belonging to a second packet of which at least one first packet has not been acknowledged at least once, the first packets not being retransmitted when they belong to a second packet of which all the first packets have been acknowledged at least once.       

     Here, first packets or Packet data units or PDU corresponds to a current layer and second packets or service data units or SDU corresponds to packets that are in a layer over the current layer. 
     Advantageously, the method comprises: 
     a split of a second data packet (SDU) in one or more first data packets (PDU);
         a retransmission of each first packet not acknowledged at least once belonging to a second packet of which at least one first packet has not been acknowledged at least once, the first packets not being retransmitted when they belong to a second packet of which all the first packets have been acknowledged at least once by at least one second station, and advantageously by at least a same second station.       

     According to a particular characteristic, it comprises a discard request of at least one first packet to a second station when all the first packets of the second packet have been acknowledged at least once by at least one second station, and advantageously at least as same second station. 
     According to a particular characteristic, the packet or packets are in the medium access control communication layer called MAC layer. 
     Advantageously, the packet or packets are transmitted on a wireless channel, for example of the IEEE 802.16 type. According to a specific feature, connections between the first station and second stations are established according to a IEEE 802.16 protocol. 
     According to a specific feature, the method comprises the following steps:
         reception of data packets by first station, the data packets being built by second stations from a given service data unit packet and having a similar structure and the same numbering; and   transmission of an acknowledgement to each second station indicating that the first station has correctly received at least a data packet from at least a second station.       

     The invention also relates to a reception method of first data packets, the method being implemented in at least two second stations, each first packet being transmitted by first station intended for said at least two second stations. In order to have a good quality of services, the method comprises:
         a reception of at least a first data packets by at least one second station;
           a transmission of at least one acknowledgement of each first packet received correctly by at least one second station, said acknowledgement or acknowledgements being transmitted to first station by the second station or stations having correctly received first packet from the first station,   a construction of a second data packet (SDU) from one or more first data packets (PDU), if all corresponding first data packets have been correctly received, and a sending of second data packet to a destination; and   a discard of at least one first packet upon a reception of a corresponding discard request ( 814 ,  840 ) from the first station.   
               

     Thus, when a second station receives a discard request, it discards the corresponding packet or packets, even if the first station has not received the corresponding packet or packets from this second station and has received the corresponding packet or packets from another second station. 
    
    
     
       4. LIST OF FIGURES 
       The invention will be better understood, and other specific features and advantages will emerge from reading the following description, the description making reference to the annexed drawings wherein: 
         FIG. 1  illustrates an example of a communication network architecture with elements implementing the invention, 
         FIGS. 2 and 3  diagrammatically show, respectively, a mobile wireless station and a relay station belonging to the network of  FIG. 1 , according to a particular embodiment of the invention, 
         FIGS. 4 and 5  show an implementation method in the wireless station of  FIG. 2 , according to the particular embodiments of the invention; 
         FIGS. 6 and 7  show an implementation method in the relay station of  FIG. 3 , according to the particular embodiments of the invention; 
         FIG. 8  illustrates an example of communication between different network elements of  FIG. 1 , and 
         FIG. 9  gives an example of frame exchanged by the network elements of  FIG. 1 . 
     
    
    
     5. DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a communication network comprising a wireless network  1  and a wired Ethernet network (or IEEE 802.3). 
     The wireless network  1  comprises one or more wireless stations, fixed or, advantageously mobile. A wireless station is for example a base station (or BS) BS  10 . If there are several base stations, they use different physical channels (for example, frequency channels or CDMA “Code Multiple Division Access” or “Multiple access by code division”, temporal allocations (TDMA “Time Division Multiple Access” or “Multiple access by time division”). 
     The Ethernet network comprises a destination  14  node. 
     Base station  10  can transmit or receive data (for example images) intended for or (for example control data) coming from node  14  via the relay stations (RS) or subscriber stations (SS) SS 1   11 , SS 2   12  and SS 3   13  (second wireless network stations). The subscriber stations  11  to  13  allow the interface between the wireless network  1  and the Ethernet network to be provided. Hence, the subscriber station  11  ( 12 ,  13  respectively) is connected via a bidirectional wireless link  110  ( 120 ,  130  respectively) to the base station  10 . The network architecture  1  is such that the network  1  comprises enough subscriber stations to cover the entire zone in which the base stations are likely to be found. Hence, at any time, each mobile station of the network  1  is connected to at least one base station of the network  1  by a wireless link enabling a wireless communication to be provided. The subscriber stations  11  to  13  are connected directly or via a hub by an Ethernet  15  link (or any other network enabling the transmission and the reception of data) to the node  14 . According to an embodiment variation, they are also connected to each other via an Ethernet link (or any other type of wired or wireless link). Hence, for example, if the base station  10  is connected to the subscriber station SS 1   11 , it can transmit data to the node  14  via the links  110  and  15 . Advantageously, the destination is capable of retransmitting the stream on another wired or wireless interface to another item of equipment. 
     The base station or stations are for example mobile cameras, equipped with wireless communication means and the node  14  is an image processing system (for example, a video recorder, a studio entry point etc.). Hence, the  FIG. 1  network enables a continued (i.e. without interruption) data transmission (for example images) to a processing system by cameras located on the interior or the exterior of buildings for retransmitting any event (for example a sporting event or a show) on an equally nondescript geographic zone. 
     Advantageously, the subscriber stations share the same radio frequency channel, the radio spectrum being a resource to be economised. The subscriber stations can possibly listen mutually to each other on the radio channel. According to an embodiment variation, the subscriber stations cannot listen mutually to each other on the radio channel. 
     Advantageously, the communications used between the nodes of the network of  FIG. 1  are of the IP type (Internet Protocol), the SS, the BS and the node  14  each having an IP address. IP is used to transport the flow in streaming mode, for example for transporting video and/or audio in unidirectional or bidirectional mode. 
       FIG. 2  diagrammatically illustrates a mobile station  2  of the network  1  corresponding to the base station  10 . 
     The mobile station  2  comprises, connected to each other by a bus  24  addresses and data, also transporting a clock signal:
         a microprocessor  21  (or CPU);   a non-volatile memory of the ROM (Read Only Memory) type  22 ;   a Random Access Memory or RAM  23 ;   a transmission module  25  of a signal on the wireless link;   a reception module  26  of a signal on the wireless link; and   an interface  27  to an application (for example the capture of images and/or sound).       

     It is noted that the word “register” used in the description of memories  22  and  23  designates in each of the memories mentioned, a memory zone of low capacity (some binary data) as well as a memory zone of large capacity (enabling a whole programme to be stored or all or part of the data representing an audio/video service received). 
     The application is, for example, of the video type and constitutes respectively the source and destination of the data respectively transmitted and received by the mobile station  2  (the mobile station  2  is for example a camera or a radio system associated with the camera). 
     The ROM memory  22  notably comprises a programme “prog”  220 . 
     The algorithms implementing the steps of the method specific to the invention and described below are stored in the ROM  22  memory associated with the mobile station  2  implementing these steps. When powered up, the microprocessor  21  loads and runs the instructions of these algorithms. 
     The random access memory  23  notably comprises:
         in a register  230 , the operating programme of the microprocessor  21  responsible for switching on the mobile station  2 ;   The data or the PDU (“Packet Data Unit”) corresponding to the data packets of level  2  or MAC (“Medium Access Control”) containing this data in a register  231 ;   data packets of type SDU (“Service Data Unit”) being able to contain several PDU in a register  232 ;   an SID (“Stream ID”) flux identifier in a register  233 , the SID identifier enabling the classification to be made (“classifying” function according to the IEEE 802.16 standard);   One or more connection identifiers or CID in a register  234 ; and   an IP address of the mobile station  2  in a register  235 .       

       FIG. 3  diagrammatically illustrates a subscriber station  3  of the network  1  corresponding to SS 1 , SS 2  or SS 3 . 
     The subscriber station  3  comprises, connected to each other by an address and data bus  34 , also transporting a clock signal:
         a microprocessor  31  (or CPU);   a non-volatile memory of the ROM (Read Only Memory) type  32 ;   a Random Access Memory or RAM  33 ;   a transmission module  35  of a signal on the wireless link;   a reception module  36  of a signal on the wireless link; and   an interface  37  to an Ethernet network.       

     It is noted that the word “register” used in the description of memories  32  and  33  designates in each of the memories mentioned, a memory zone of low capacity (some binary data) as well as a memory zone of large capacity (enabling a whole programme to be stored or all or part of the data representing an audio/video service received). 
     The ROM memory  32  notably comprises a programme “prog”  320 . 
     The algorithms implementing the steps of the method specific to the invention and described below are stored in the ROM  32  memory associated with the subscriber station  3  implementing these steps. When powered up, the microprocessor  31  loads and runs the instructions of these algorithms. 
     The random access memory  33  notably comprises:
         in a register  330 , the operating programme of the microprocessor  31  responsible for switching on the subscriber station  3 ;   data or PDUs containing this data in a register  331 ;   data packets of type SDU (“Service Data Unit”) being able to contain several PDU in one register  332 ;   one flux identifier in a register  333 ;   one or more connection identifiers or CID in a register  334 , and   an IP address of the mobile station  2  in a register  335 .       

       FIG. 4  shows a method used in the wireless station  2  according to a particular implementation of the invention. 
     This method begins with an initialisation phase  40  during which the different parameters of the station  2  are updated. 
     Then, during a step  41 , the station  2  waits then receives at least one PDU from one application. 
     Then, during a step  42 , the station  2  numbers each PDU and transmits them in one or more bursts to the SS  11  to  13 . 
     Then, during a step  43 , the station  2  waits for the acknowledgements during a time limited by a timeout. The value of the timeout is for example comprised between 2 and 10 ms. 
     Then, during a test  44 , the station  2  verifies whether a positive acknowledgement (ACK) has been received for each PDU packet transmitted. In the affirmative case, the step  41  is repeated. 
     If for at least one packet transmitted, no positive acknowledgement has been received (no acknowledgement has been received or only negative acknowledgements or NACK have been received), then, during a step  45 , the packets not positively acknowledged are transmitted again to the SS  11  to  13 , each first packet acknowledged at least once not being retransmitted. The step  43  is then repeated. 
       FIG. 5  shows a method used in the station  2  according to a particularly advantageous implementation of the invention, in the context of a wireless network comprising notably subscriber stations  11  to  13  compatible with the IEEE 802.16 standard. The common steps with the method illustrated in  FIG. 4  have the same references and are not described in further detail. 
     After an initialisation step  40 , during a step  50 , a connection is open with the SSs with the advantageously identical parameters (bit-rate, latency, etc.). Advantageously, the same CID is associated with each downward connection between the SS and the BS. Hence, each SS functions according to the IEEE 802.16 standard. The BS transmits on the downward connection to all the stations following a same burst of the multicast type, or, according to a variant, a PDU is transmitted in several bursts, each burst being associated with a single SS. For the upward direction, each SS transmits the acknowledgements in the associated burst. The BS thus identifies the SS which transmitted an acknowledgement. According to a variant, the acknowledgement comprises an SS identifier (for example, a specific CID used for the acknowledgement transmission in the upward direction between the SS and the BS) which is used by the BS to identify the SS having transmitted the acknowledgement. 
     Then, during a step  51 , the station  2  waits then receives at least one SDU from the application  27  and intended for the node  14 . 
     Then during a preparation step of PDU  52 , the station  2  cuts out the SDU received in PDUs, memorises the PDUs and numbers them to identify them. An “untransmitted” status type is associated with each PDU. 
     Then, during a step  53 , the station  2  transmits the PDUs in one or more bursts to each of the SS according to a IEEE 802.16 type communication protocol. The status associated with each PDU is updated by becoming the “transmitted” type. 
     Then, during a step  54 , the station  2  waits for a cumulative and/or selective acknowledgement from the SSs recipients of the PDUs. This acknowledgement may be positive (ACK) or negative (NACK). An acknowledgement comprises a CID connection identifier used for the transmission of PDUs in the downward direction, an acknowledged PDU number (cumulative or selective ACK) or several acknowledged PDUs (cumulative selective ACK). The BS identifies the SS having transmitted the acknowledgement, for example, by the time slot used by the SS and/or by a specific field contained in the acknowledgement. 
     According to a variant of the invention, independently or in connection with reception of PDU transmitted by the BS, SSs are receiving SDU from the Ethernet network (e.g. from node  14 ) and transmit corresponding PDU to the BS, the building of PDU being the same for each SS (i.e. when a SDU is received by SS, then, the slicing of SDU into PDU made by each SS is the same). The numbering of the PDUs in the BS is also identical and unique for PDUs transmitted by all the SSs. The ACK received by the SSs correspond to the identical data in the BS. Then, according to this variant, each SSs receiving a positive ACK for a PDU discards it, even if this PDU transmitted by the given SS has not being received correctly by the BS, a corresponding (i.e. with same content and numbering) PDU transmitted by another SS being received correctly by the BS and acknowledged. 
     According to an advantageous variant, the station  2  launches a timeout during the transmission step and the flow of this timeout corresponds to the reception of a NACK. 
     Then, during a test  55 , the station  2  checks whether all the PDUs of the SDU or SDUs received are acknowledged by at least one SS (PDUs of a given SDU being acknowledged by one SS or different SSs) or, according to a variant, by at least one same SS (one SS or several SSs having acknowledged all the PDUs from a given SDU). 
     For each SDU received of which all the PDU are acknowledged by at least one SS, (a specific SS having sent the acknowledgment for all the PDU of the received SDU or, according to a variant, a specific SS or different SSs having sent the acknowledgment for all the PDU of the received SDU), during a step  57 , the station  2  transmits a discard type message to each SS having not acknowledged at least one PDU from the SDU considered. This message enables each SS to erase from its memory the PDUs from the SDU considered by supposing that they are received correctly, without sending them to the destination  14  node. Hence, the PDU are not retransmitted when they belong to a SDU of which all the PDU have been acknowledged at least once. It also enables that the PDUs from the considered and not received SDU are no longer awaited. The station  2  waits for a positive acknowledgement in response to the “discard” message and sends back the “discard” message as long as it is not acknowledged. According to a variant, after a determined number of acknowledgement absences from an SS, the BS considers the SS as desynchronised and will send it a resynchronisation order once the link with the SS is re-established with an adequate quality. This enables the corresponding SS to receive the PDUs again by having an ARQ reception window synchronised with the ARQ transmission window associated with the BS. The step  51  is then repeated. 
     If the test  55  result is negative, during a step  56 , the PDUs which are not part of the acknowledged SDU are retransmitted by the BS. 
     According to the variant of the test  55 , the BS considers that a SDU can be reconstructed as soon as all the PDU of the SDU have been received by at least one SS (and not necessarily by the same SS). In this case, the step  56  is also adapted so as to only transmit the PDU which have not been received by at least one SS. This variant can be implemented, for example, if the destination node, an intermediate node or one of the SS reconstructs the SDU from the PDU which can be received by the separate SS. According to a specific embodiment of this variant, SSs can forward to other SSs PDUs that are not received correctly by other SSs or to a specific SS, so that at least one SS can built a corresponding SDU, even if it has not received the PDUs of the SDU through the wireless network. 
     According to a variant implementing exchanges between the SS via any channel (for example, via the wired network  15 ), each SS transmits the acknowledged (positive) or not acknowledged status of the PDUs received previously and possibly the acknowledged status of a SDU to the other SS. The step  57  can then be deleted; each SS can then erase the PDUs corresponding to the acknowledged SDU from the memory. 
     The transmission of PDU acknowledgements by the SSs is advantageously done in a cumulative form. 
       FIG. 6  shows a method used in the subscriber station  3  according to a particular implementation of the invention. 
     This method begins with an initialisation phase  60  during which the different parameters of the station  3  are updated. 
     Then, during a step  61 , the station  3  waits then receives at least one PDU from the base station  10 , the base station transmitting a PDU with the same useful data to several SS. 
     Then, during a step  62 , the station  3  checks that the PDU transmitted by the base station  10  is received correctly (for example, by checking an error detection code present in the received PDU). 
     In the negative, the step  61  is repeated. 
     In the positive, during a step  63 , the station  3  transmits a positive acknowledgement or ACK of level  2  (MAC layer) indicating a correctly received PDU, at the base station  3 . The corresponding PDU is transmitted to the application and the step  61  is repeated. If a same PDU is transmitted several times to the station  3  while being received correctly, the station  3  discards the duplicates. 
     If for a given PDU, the station  2  does not correctly receive the corresponding PDU transmitted by the subscriber stations and if the station  2  identifies the number (for example, if a PDU with a more recent number is received correctly), according to a variant of the step  63 , the station  2  transmits a negative acknowledgement or NACK identifying the incorrectly received PDU, to the base station  10 . 
       FIG. 7  shows a method implemented in the station  3  according to a particularly advantageous implementation of the invention, within the context of a wireless link between the BS and the station  3 , compatible with the IEEE 802.16 standard. The common steps with the method illustrated in  FIG. 6  have the same references and are not described in further detail. 
     After an initialisation step  40 , during a step  70 , the station  3  opens a connection associated with the base station. Advantageously, the BS creates a connection in the downward direction which is identical for all the SS. The connection is associated with the same parameters (for example the bit-rate, the latency, ARQ parameters and classification parameters (SID)) and with the same CID identifier (“Connection Identifier”). This allows a simpler operation and a bandwidth saving on the network connecting the BS to the SS. 
     According to a variant, the station  2  controls the connections associated with several flows, these flows being able to correspond to the different data types and/or the separate sources. Each of the flows is identified by a SID which is specific to it and transmitted on specific connections (two connections on two separate flows are differentiated). 
     Then, during a step  71 , a feedback timeout is launched. 
     Then, PDU reception steps  61 , a correctly received PDU verification test  72  associated with a test step  62  are carried out. 
     If this is the case, the PDU or PDUs being correctly received, during a step  75 , each correctly received PDU comprising the data is memorised and the corresponding PDU status in the PDU descriptor is updated (the status switches from “not received” to “received”). If the PDU is of the “discard” type, the station  3  updates the corresponding PDU statuses by indicating a “received” status. If the “discard” is selective, the acknowledgement is advantageously selective. If the “discard” is cumulative, the acknowledgement is advantageously cumulative. According to a variant, when a “discount” type PDU is received, the station  3  forces the end of the “feedback” timeout. If the same PDU is received several times in the correct manner, the duplicates are discarded. Advantageously, the PDU are transmitted to the recipient node after reconstruction of an entire SDU comprising the corresponding PDU. According to a variant of the invention, an SDU can be constructed with the PDUs from the different SSs, for example, by the recipient node or an intermediate node and advantageously by an SS. The SS then transmits the correctly received PDUs of the BS, in the entity which reconstructs the SDU or keeps them for itself if it reconstructs the SDU itself from the PDUs that it receives from the BS or from other SS. 
     According to a particular embodiment, synchronisation timeouts are implemented for each connection and for each connection, an ARQ reception window which defines the first incorrectly received PDU. For a given connection, if the first incorrectly received PDU changes, then the synchronisation timeout associated with this connection is reset. 
     Following a negative result of the test  62  (timeout limit value reached or incorrectly received PDU detected) or of the step  75 , during a test  76 , the station  3  checks that a synchronisation timeout has reached a determined value limit (for example 100 ms) for each connection. 
     If this is the case (timeout limit value reached), during a step  77 , the synchronisation is lost and the station  3  waits for a resynchronisation order with the current PDU number from the BS. 
     According to a variant, the station  32  does not implement the synchronisation timeouts and therefore the test  76  and the step  77 . 
     Following a negative result of the test  76  (synchronisation timeout limit value not reached) or of the step  77 , during a test  78 , the CPU  31  checks whether the feedback timeout launched in the step  71  is passed. The maximum value is for example between 2 and 10 ms. In the negative, the step  72  is repeated. 
     If the result of the test  78  is positive, the timeout is passed and an acknowledgement (ACK) procedure and/or Automatic Retransmission Request (ARQ) is implemented. According to the invention, during a step  79 , if a PDU transmitted by the BS is correctly received, then the station  3  transmits a positive acknowledgement ACK to the BS. According to the invention, during a step  79 , if a PDU transmitted by the BS is correctly received, then the station  3  transmits a positive acknowledgement ACK to the BS. An ACK acknowledgement associated with a PDU identified by its number corresponds to a logical “OR” of the reception status of each PDU carrying the same identification number and transmitted by the BS. The acknowledgement feedbacks are sent to the BS by all the SS. According to the embodiment mode described here, the acknowledgement is selective. According to a variant, the acknowledgement is accumulative: Several acknowledgements corresponding to consecutive PDUs are accumulated; an acknowledgement corresponding to the last of the correctly received consecutive PDUs is transmitted to the BS. According to another variant, the acknowledgements are both accumulative and selective. The SS indicates the last correctly received PDU from a correctly received PDU sequence and the correctly received isolated PDUs after the last cumulative correctly received PDU. According to a variant, the test  78  and the step  79  are carried out in parallel with the step  41  (for example in a multitask environment). 
       FIG. 8  illustrates an example of communication between the base station  10 , the subscriber stations  11  and  12  and the destination  14  (these elements are represented by vertical lines; the actions, events and/or successive transmissions are chronologically illustrated). In order to facilitate the reading of the example, only two subscriber stations  11  and  12  are mentioned. The example can be extrapolated to any number of base stations and subscriber stations. 
     The BS  10  transmits signals  800  and  801  comprising a PDU to each of the subscriber stations  11  and  12 . As an example, it is the same destination  14  node which receives or transmits the data coming from or destined for the base station  10 . According to  FIG. 8 , the PDUs are transmitted to the subscriber stations in the form of separate frames with a recipient address corresponding to a unique SS (unicast). According to a variant, the signals  800  and  801  are advantageously combined into one single signal (signal broadcast to all the SS (multicast)). Likewise, according to the different embodiments, the PDUs are transmitted by the BS in the same connection to all the SSs or in the different connections. 
     Then, for the correctly received frames, the SSs  11  and  12  transmit the acknowledgements corresponding to the base station  10 . In order to facilitate the reading of the diagram, it is assumed that the PDUs corresponding to the SDU  800  or  801  are transmitted in a single burst. 
     According to a first scenario, it is assumed that the PDUs  800  and  801  are correctly received by the SSs  11  and  12 . They are therefore acknowledged by the SSs  11  and  12  which transmit a positive acknowledgement ( 802  and  803 ) to the BS. Each positive or negative acknowledgement transmitted comprises the CID and the number of the PDU or PDUs (with, if necessary, a selective or cumulative acknowledgement identification). Then, supposing that the SDU comprises only the acknowledged PDU or PDUs, the SSs  11  and  12  transmit the SDU ( 804 ,  805 ) to the recipient node. 
     According to a variant not shown, the SSs implement the acknowledgement exchanges between themselves. 
     According to a second scenario, the BS  10  transmits signals  810  and  811  comprising one or more PDUs forming a single SDU to each of the subscriber stations  11  and  12 , only the signal  810  is correctly received by an SS. The SS  11  then transmits an acknowledgement to the BS  10  and the entire SDU  813  to the destination node. The BS  10  transmits to the SS  12  a message  814  of the discard type associated with the PDUs acknowledged by the SS 1 . The SS  12  acknowledges ( 815 ) the “discard” message. 
     According to a third scenario, the BS  10  transmits signals  820  and  821  comprising a PDU (PDU 3 =SDU 3 ) to each of the subscriber stations  11  and  12 . It is assumed that none of the signals  820  and  821  are correctly received by the SSs. The BS retransmits the non acknowledged PDUs in the form of signals  822  and  823  which it is assumed are correctly received. A scenario corresponding to the first scenario is thus obtained. 
     According to a fourth scenario, it is assumed that an SDU transmission split into PDU transmits in distinct bursts (PDU 41  and PDU 42 ) by the BS. The BS firstly transmits the bursts  830  and  831  respectively comprising a first set of one or more PDUs (PDU 41 ) extracted from the SDU (SDU 4 ) received from an application and to transmit to a destination node through SS, then the bursts  833  and  838  respectively comprising a second set of one or more PDUs (PDU 32 ) extracted from the SDU (SDU 4 ). 
     As an example, it is firstly assumed that only the burst  830  is received correctly by the station  11 . It then transmits a positive acknowledgement for the first set of PDU (PDU 41 ) in a burst  831 . 
     Then, the BS transmits a burst  833  comprising the second set incorrectly received by the SS  11  and a burst  834  comprising the first set correctly received by the SS  12 . The latter acknowledges the first set of packets ( 835 ). 
     Then, the BS transmits the bursts  836  and  838  comprising the second set. Only the SS  11  correctly receives the burst  836  that it acknowledges. It therefore constructs the SDU comprising the two sets and transmits it to the node  14  ( 839 ). As summary, the SS constructs a SDU from one or more PDU packets, if all corresponding PDU have been correctly received; then, it sends the constructed SDU packets to the destination. According to a variant, a SS receives PDU not correctly received directly from the BS, from another SS that has correctly received this PDU. Thus, the SS can construct the corresponding SDU without waiting a missing PDU from the BS (on the wireless link). 
     The BS  10  transmits to the SS  12  a message  840  of the discard type associated with the second set of PDUs acknowledged by the SS 1 . The SS  12  acknowledges ( 841 ) the “discard” message and updates the statuses of the corresponding PDUs. 
       FIG. 9  chronologically illustrates the emission and the reception of successive frames  90  and  91 . The frame  90  ( 91  respectively) is divided into two intervals corresponding respectively to the “downlink” direction, base station to SS and in the “uplink” direction, SS to base station. 
     The slot  90  comprises:
         one part reserved for frame headers in the time slots assigned to each BS,   one part reserved for the transmission of data to the SS or SSs connected in the time slots assigned to each BS,   one part (not shown) enabling exchanges in contention mode (notably to enable the SS not associated or not connected to do it), and   one part reserved for the transmission of data to the BS from the SS connected, in the time slots assigned to each SS.       

     In the first part of the slot  90 , the base station emits or receives firstly a frame header or FH  900 . The attribution of slots for the FH is unremarkable (for example determined according to the declaration order in the network). When an SS is associated to a BS, the BS receiving the association demand allocates in a unequivocal manner the time resources for the transmission and/or the reception of data packets. 
     Then, the base station sends a frame  901  comprising two PDU sets (PDU 51  and PDU 52 ) to the SSs associated with a connection (CID 1 ). 
     Then, each of the SS  11  and  13  successively transmits a burst  902  and  904  respectively containing respectively an ACK for a first set of PDUs (PDU 51 ) and a NACK for a first set of PDUs associated with an ACK for a second set of PDUs (PDU 52 ). The station  12  does not send anything, the reserved time slot is empty. 
     The following frame  91  also comprises a header  910  similar to the header  900  and a retransmission of the frame  901  in a time slot  911 . 
     Then, the frame  91  comprises the acknowledgements  912  to  914  of the PDU or PDUs correctly received by the SSs. 
     Naturally, the invention is not limited to the embodiments previously described. 
     In particular, the architecture of the mobile stations and base stations can be different from those illustrated in  FIGS. 2 and 3 , in the respective function and/or form of the elements (the functions of the electronic elements can notably be grouped into a restricted number of components or, on the contrary, expanded into several components) and their layout. 
     The invention is not limited to an architecture as described with respect to  FIG. 1  but involves any architecture implementing a wireless network with local (for example a few tens of metres) or remote (for example a few kilometres according notably to a standard IEEE 802.16) coverage with one or more SS, each SS being connected at any time to at least one BS. According to one variant, the link between the BSs and/or between the BSs and the destination and/or source node is a wireless link (local or remote link). 
     The invention can also be applied with different communication protocols to those previously described. Hence, the application and/or control data can be transmitted according to any protocol (for example with a contention access or in polling mode) on the wireless links. The communication channels between the SS and the BS can use the same frequency channels for the upward and downward directions (mode known as “half duplex”) or different frequency channels (mode known as “full duplex”). The network or the links connecting the source to the SSs can also be unremarkable and is not limited to an Ethernet network. This means, for example, a standardised or proprietary protocol, wired or wireless enabling the data transmission from the source to each of the SSs. 
     Moreover, the packets (SDU) transmitted by the BS to the subscriber stations are advantageously and not necessarily split into packets (PDU) of MAC level. In the examples given previously the boundaries between SDU and PDU coincide. According to the variants of the invention, they do not coincide. According to other variants, a PDU can correspond to one or more SDUs. 
     According to specific embodiments, the subscriber stations are advantageously unremarkable stations compatible with the IEEE 802.16 standard. According to the variants of the invention, they comprise one part linked to wireless exchanges compatible with the IEEE 802.16 standard and one dedicated part aiming to improve the quality of service (for example, one part allowing the acknowledgements exchanged between subscriber stations to be managed). 
     The architecture of the base station is also not limited to the examples previously described. In particular, according to different embodiments, the application part of the base station (for example, data processing unit (notably voice and/or images), a camera control unit, etc.) can be integrated in an item of equipment comprising the radio and communication management part on the wireless link with the subscriber stations, or, on the contrary, separated completely or partly from this item of equipment. According to a particular embodiment, the application part of the base station is in a device separated from the communication part with the SS by a wired or wireless link: for example, the BS receives a video flow transmitted on Ethernet (or on a different wired or wireless link, following a standard protocol or proprietor) to a digital recorder, a screen or a computer. 
     Likewise, the architecture of the subscriber stations is also not limited to the examples previously described. In particular, according to different embodiments, the data source (for example, data processing unit (notably voice and/or images), an application control unit associated with the base station or stations, etc.) can be integrated in an item of equipment comprising the radio and communication management part on the wireless link with the base stations, or, on the contrary, separated completely or partly from this item of equipment. 
     According to a variant of the invention, a same PDU is not transmitted to all the SSs but to a sub-set (for example, to one or more SSs of which there is a good quality link with the BS (typically the SSs whose PDUs are correctly received by the BS)). 
     According to an embodiment variant of the invention, an SS can be temporarily removed from the SS sub-set communicating with the BS (if, for example the wireless link is bad, the wired link being maintained) reintroduced subsequently (for example when the link becomes satisfactory) after a resynchronisation of the ARQ windows. 
     The invention can advantageously be combined with the invention covered by the French patent application FR0755233 filed on the 24 May 2007 by Thomson Licensing and entitled “Data packet reception method and corresponding transmission method” or by corresponding international patent application. In an architecture similar to the  FIG. 1  network, the latter provides the transmission of data packets of PDU by at least two SSs (second stations) to the BS (first station). More precisely, SDU are received by the SSs. Each SS then slices the received SDU into PDUs. For a given SDU, the corresponding PDUs built by different SSs have a similar structure (especially the size of payload is the same), the same numbering and the same content (payload). PDU sent by several SSs may have or not the same SS source address. Each data packet corresponding to a given payload (PDU) is thus transmitted on several links between the SSs and the BS. The BS transmits an acknowledgement (in a unicast or multicast mode) to each SS indicating that it has correctly received this PDU packet coming from at least one SS. When a PDU packet has not been acknowledged by the BS, the SSs transmit it to the BS again. In particular, the BS and SS of  FIG. 1  network can advantageously implement both a transmission as disclosed in  FIG. 6  and a reception.