Abstract:
There is disclosed a method and a device of method of dynamic ARQ windows management in a network for controlling data link transmission. The method comprises an ARQ management entity selecting a given operational control mode, and a transmitter sending one or more PDUs of a given data link to a receiver. The receiver stores received PDUs in a shared memory. A receiver ARQ instance sends back corresponding ARQ-ACK message including a specific information comprising selected operational control mode and/or a flow control data. It further removes acknowledged PDUs from the shared memory when delivering them to an upper layer. The transmitter ARQ instance continues or stops PDUs transmission upon reception of the ARQ-ACK message, based on the specific information comprised therein. The step of selecting the operational control mode is repeated dynamically on a per data link basis during PDUs transmission, while taking into account the available memory space within the shared memory, the respective associated QoS priority, as well as the sliding window memory requests received from the transmitters by the receiver.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Technical Field  
         [0002]     The present invention relates generally to communication networks, and more particularly to an ARQ windows management in a network based on an ARQ protocol.  
         [0003]     2. Related Art  
         [0004]     A communication network comprising several stations (STA) and nodes is considered herein. The data generated by the applications running on the STAs are exchanged over data links in respective data flows. Each data flow corresponds to a given transmitter, a given receiver and a given application. One or more data flows can be transported over a given data link. A data link is established from one transmitter STA to a receiver STA, passing through intermediate nodes or not. Referring to a network protocol layering model according to the ISO (“International Standardization Organization”), the data link is ruled by a Link Layer (LL) protocol, which includes a Medium Access Control sub-layer (MAC). At the LL level, the data are transported in LL Protocol Data Units (LL-PDUs) which are inserted in, or extracted from, the physical medium by the MAC sub-layer. In the following, a transmitter STA and a receiver STA will be respectively referred to as a transmitter and a receiver.  
         [0005]     In case of an error prone physical medium (e.g. radio, power line, etc . . . ), the LL layer includes an Error Control (EC) mechanism that is in charge of recovering the transmission errors experienced by the LL-PDUs. EC mechanisms are usually based on repetition of the lost or corrupted LL-PDUs as detected by the receiver. Protocols managing such repetitions schemes are also known as Automatic Repeat reQuest (ARQ) protocols. Generally, an ARQ instance is running both on the receiver and on the transmitter. A reverse data link is then provisioned so that the ARQ instance of the receiver can inform the ARQ instance of the transmitter about the reception status of the transmitted LL-PDUs. This ARQ protocol information is transported in specific signalling LL-PDUs also referred to as ARQ feedback messages or ARQ ACKnowledgement (ARQ-ACK) messages. In this type of protocols, the transmitter can send data to the receiver over a given data link only if an ARQ-ACK message relative to the previously sent LL-PDU has been correctly received. Consequently, when the transmitter is waiting for an ARQ-ACK message, the data transmission is blocked until the reception of the ARQ-ACK message. This blocking in a data transmission impacts the data link and then the transmission efficiency. In order to improve transmission efficiency by avoiding transmission blocking, a sliding window mechanism is generally introduced. The sliding window represents the number of LL-PDUs the transmitter can transmit without having received an ARQ-ACK message. The size of such a sliding window is defined by two boundaries, generally referred to as Bottom of Window (BoW) and Top of Window (ToW). Depending on the protocol, the size of the sliding window can be fixed, as for example with the HiperLAN/2 standard defined by the European Telecommunication Standard Institute (ETSI), or variable as for example with the Transmission Control Protocol (TCP). If the size of the sliding window is fixed, it is chosen at the time of the establishment of the data link. If the size of the sliding window is variable, it is advertised to the transmitter through the ARQ-ACK messages sent by the receiver. In this latter case, the reception of an ARQ-ACK updates the sliding window boundaries. During the data transmission, the transmitter controls the data flows over the corresponding data link based on these boundaries. Stated otherwise, the transmitter stops, resumes or continues LL-PDUs transmission by controlling the number of LL-PDUs sent without reception of ARQ-ACK. It results therefrom that the sliding window has a flow control effect that can disadvantageously affect the data link throughput even when no error occurs.  
         [0006]     ARQ protocols rely on the identification of the PDUs that is common to the transmitter and the receiver and used by the protocol signalling so that the receiver can indicate in an ARQ-ACK message sent to the transmitter which PDUs are not correctly received. The data unit identification is typically a sequence number specific to the data units handled by the considered layer only.  
         [0007]     On the other hand, the applications running on the STAs can have various Quality of Service (QoS) requirements. For example, these QoS requirements can be expressed in terms of bit rate (or bandwidth) or transmission delay constraints. The fulfilment of these constraints is possible if the data link offers the required bandwidth and/or transmission delay. In particular, bandwidth provisioning can be performed through resource reservation procedures. If successful, the resource reservation guaranties that the application data flow will be transported with the required bit rate. In the following section, for the sake of better understanding, only the bit rate QoS requirement is considered.  
         [0008]     A single data link can be used to transport several application data flows with similar QoS requirements. Thus, the bit rate of a data link can vary in time, depending on the activity of the different application data flows.  
         [0009]     This section exposes the basic principles and definitions for such ARQ protocols. As it has already been described, a sliding window is defined by its two boundaries and its size: respectively ToW, BoW and Window Size (WS). In the following, they are all expressed in the same unit, which can be a LL-PDU number, when LL-PDUs are of fixed size, as for example in HiperLAN/2, or a number of bytes as in TCP.  
         [0010]     This unit is also used to define the sequence number space used to identify each LL-PDU. The sequence number range is determined so that no ambiguity arises in LL-PDU numbering and each LL-PDU can be uniquely identified. BoW and ToW are then sequence numbers of the LL-PDUs respectively located at the bottom and at the top of the sliding window. In the following for the sake of better understanding, an LL-PDU sequence number and the corresponding LL-PDU are considered to be equivalent.  
         [0011]     BoW refers to the missing or corrupted LL-PDU of the lowest sequence number which is detected by the receiver, and ToW is determined from BoW by the following equation:
 
 ToW=BoW+WS− 1
 
         [0012]     The transmitter can transmit or retransmit any LL-PDU comprised between BoW and ToW included.  
         [0013]     BoW is updated by the receiver and advertised to the transmitter in the ARQ-ACK messages. When the size of the sliding window is variable, WS is also generally advertised in the ARQ-ACK messages, whereas when this size is fixed, WS is selected upon data link establishment.  
         [0014]     Commonly, the Round Trip Time (RTT) of a data link is defined as the time elapsed between the emission of an LL-PDU and the reception of the corresponding ARQ-ACK message, i.e. an ARQ-ACK message covering the emitted LL-PDU, by the transmitter. RTT includes transmission delays due to the physical medium and processing in the various intermediate nodes, as well as processing in the transmitter and in the receiver.  
         [0015]     This type of protocols is correctly adapted to data links where transmission errors occur. Conversely, a data link without any transmission errors may be impacted by such a protocol.  
         [0016]     Actually, when no transmission error occurs, the maximum data link throughput, referred to as B max , the corresponding RTT and WS are linked by the following equation:
 
 B   max   =WS/RTT 
 
         [0017]     This relation can be deduced from the following simplification assumptions.  
         [0018]     All transmitted LL-PDUs have the same size and the ARQ-ACK message transmission rate is not limited on the reverse data link, from the receiver to the transmitter. Stated otherwise, the reverse data link is transparent to the ARQ-ACKs.  
         [0019]     Assuming that the transmitter emits LL-PDUs at a constant throughput B, time gap between LL-PDUs can be referred to as T. The receiver emits one ARQ-ACK per received LL-PDU. If the first LL-PDU ever sent by the transmitter is considered as the BoW, the ARQ-ACK message acknowledging this LL-PDU is received after the RTT. The sliding window allows the transmitter to further transmit a given number n of other LL-PDUs until the first ARQ-ACK is received, n being an integer lower than WS. The following equation is deduced:
 
 n=B×RTT 
 
         [0020]     The received ARQ-ACK message gives the transmitter the opportunity to send one more LL-PDU. Stated otherwise, the BoW and consequently the sliding window progress. The subsequent ARQ-ACK messages are received every T, thus allowing a new LL-PDU to be sent every T, and therefore the transmitter continues emitting LL-PDUs at a throughput which equals B.  
         [0021]     Thus, in order to achieve the throughput B, a minimum value of WS has to be chosen so as to verify the following relation:
 
 WS   min   =B×RTT 
 
         [0022]      FIG. 1  illustrates a transmitter  11 , a receiver  12  and a given data link  13 . The data link  13  transports a data flow at a bit rate referred to as B. A corresponding reverse data link  14  allows transmission of ARQ-ACK messages from the receiver  12  to the transmitter  11 . On the transmitter side, a queue of LL-PDUs pending for transmission, referenced  16 , comprises the LL-PDUs from LL-PDU# 11  to LL-PDU# 14 . The previous LL-PDUs, from LL-PDU# 1  to LL-PDU# 10 , have already been sent. Further, it assumes that the LL-PDUs from LL-PDU# 1  to LL-PDU# 7  have been received on the receiver side and that they are stored in a memory buffer referenced  15  until the receiver has sent their corresponding ARQ-ACK messages. As the receiver spends some time in LL-PDU and ARQ-ACK message processing, it is considering in the example shown that only the ARQ-ACK messages corresponding to the LL-PDUs from LL-PDU# 1  to LL-PDU# 3  have been processed and sent over the reverse data link  14 . An ARQ-ACK message, corresponding to a LL-PDU emitted at time t, is received at time t+RTT on the transmitter side. It results therefrom that the memory buffer  15  must be able to store the number of LL-PDUs received during a time of RTT at most. In the example illustrated in  FIG. 1 , when the data flow is transmitted at a given bit rate B, during the time of RTT, the receiver stores 7 LL-PDUs.  
         [0023]     In case of an error-prone transmission, a great part of the RTT can be spent in LL-PDU and ARQ-ACK processing in the receiver. It results therefrom that the receiver is provided with enough memory to store the LL-PDU transmitted during that time. Indeed, LL-PDUs are retransmitted upon reception of ARQ-ACKs indicating incorrectly received LL-PDUs. Apparent RTT becomes longer due to the time elapsed to obtain the retransmitted LL-PDU status through further ARQ-ACK messages. It is then preferable not to block the transmission of the transmitter while waiting for the ARQ-ACK messages. It results therefrom that a larger sliding window is preferably provided to compensate as much as possible for the latency due to retransmissions. Consequently, a large space of memory resource shall be reserved for some data links which exhibit stringent QoS constraints so that these constraints can be guarantied even when these data links experience errors.  
         [0024]     Regarding the previous description, either ARQ protocols are based on memory reservation and then a given throughput is guaranteed or they are based on a usage of memory without any reservation and then no throughput is guaranteed.  
         [0025]     It results therefrom that it is difficult to apply an efficient dynamic ARQ windows management on different data links having different QoS and having different sliding window size requirements, particularly since these sliding window size requests are variable as a function of transmission errors notably.  
       SUMMARY OF THE INVENTION  
       [0026]     Memory buffer used to implement a sliding window for a given data link is generally a limited resource taken in a shared memory resource between all the data links ending in the same receiver. Some applications have specific throughput requirements that have to be fulfilled by the underlying data link. It is then desirable to manage the sliding windows memory buffer to accommodate as well as possible all data links that share that common resource. In view of the foregoing, there is a need for a dynamic ARQ windows management adaptable to many types of applications having different QoS constraints.  
         [0027]     In a first aspect, the invention thus proposes method of dynamic ARQ windows management in a network comprising: 
        at least one transmitter managing a plurality of different data links, or a plurality of transmitters each managing at least one data link, the transmitter or transmitters sending sliding window memory requests, the data links transporting PDUs according to an ARQ protocol and being associated to a respective QoS priority belonging to a set of predetermined QoS priorities;     at least one receiver for receiving a plurality of the data links and acknowledging each transmitted PDU by sending an ARQ acknowledgement or ARQ-ACK message to the transmitter;     a memory shared among the data links, which is comprised in the receiver for storing received PDUs, at least until they are acknowledged;     an ARQ management entity comprising a transmitter ARQ instance and a receiver ARQ instance for receiving the sliding window memory requests from the transmitter or transmitters, and for controlling each of the data links according to an operational control mode selected out of a predetermined set of operational control modes; 
 
 the method comprising the step of: 
    the ARQ management entity selecting a given operational control mode;     the transmitter or at least one of the transmitters sending one or more PDUs of a given data link to the receiver;     the receiver storing the received PDUs in the shared memory;     the receiver ARQ instance sending back the corresponding ARQ-ACK message including a specific information comprising the selected operational control mode and/or a flow control data,     the receiver ARQ instance removing acknowledged PDUs from the shared memory when delivering them to an upper layer;     upon reception of the ARQ-ACK message, the transmitter ARQ instance continuing or stopping PDUs transmission on the data link, based on the specific information comprised in the ARQ-ACK message;        
 
         [0038]     wherein the step of selecting the operational control mode is repeated dynamically on a per data link basis during PDUs transmission, while taking into account the available memory space within the shared memory, the respective associated QoS priority and the sliding window memory requests received from the transmitters by the receiver  
         [0039]     A second aspect of the invention relates to a device for controlling data flow transmission comprising means for carrying out the method according to the first aspect.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0040]     Further features and advantages of the present invention will become more apparent from the description below. The latter is given purely by way of illustration and should be read in conjunction with the appended drawings, of which:  
         [0041]      FIG. 1  illustrates a classical ARQ scheme according to the prior art;  
         [0042]      FIG. 2  illustrates a state machine between two operational control modes according to one embodiment of the invention;  
         [0043]      FIG. 3  illustrates exchanged messages between a transmitter and a receiver according to one embodiment of the invention. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0044]     One embodiment of the present invention proposes a method of ARQ windows management which defines a predetermined set of operational control modes and applies one of the operational control modes according to some specific characteristics. These characteristics can be the QoS constraints of a given application.  
         [0045]     A transmitter and a receiver are linked by a set of data links. Each data link has transmission characteristics corresponding to a given level of QoS offered to the served applications of which PDUs are transported over that data link. These transmission characteristics can correspond to classes of traffic. The IEEE 802.1D standard proposes a classification that is exposed herein below.  
                                       user_priority   Acronym   Traffic type                   1   BK   Background       2   —   Spare       0 (Default)   BE   Best Effort       3   EE   Excellent Effort       4   CL   Controlled Load       5   VI   “Video,” &lt; 100 ms latency and jitter       6   VO   “Voice,” &lt; 10 ms latency and jitter       7   NC   Network Control                  
 
         [0046]     The traffic classes exposed herein above and which are equivalent to QoS levels can be used, for example, to associate a QoS level to data flows managed by one embodiment of the present invention.  
         [0047]     A data flow corresponding to a given application transmitted from the transmitter to the receiver is mapped onto the data link whose class corresponds to the traffic requirements or QoS constraints of the given application. It is to be noted that an implementation of a proposed mechanism according to one embodiment of the present invention may rely on less or more traffic classes, which can be defined using other traffic characteristics or parameters. Classes will be identified as follows: C 1 , C 2 , . . . , C n , with C 1  having lower priority than C 2 , and so forth. A data link k belonging to class C n  will be referred to as DL k (C n ).  
         [0048]     The data links preferably offer a reliable data transfer thanks to an ARQ protocol. The ARQ protocol can be based on a sliding window mechanism which is well known by a person skilled in the art.  
         [0049]     As it has already been described above, an ARQ protocol requires a buffer memory on the receiver side to make the receiver able to manage the ARQ protocol. The memory used to implement such an ARQ protocol on each data link in the receiver is shared between all the data links.  
         [0050]     A dynamic ARQ window management according to one embodiment of the present invention is managed by an entity referred to as ARQ management entity. The ARQ entity comprises a receiver ARQ instance (ARQ-RX) on the receiver side and a transmitter ARQ instance (ARQ-TX) on the transmitter side. The ARQ-RX instance is located at the destination end of the data link which manages the ARQ window of the data link. A single ARQ Memory Buffer Manager (ARQ-MBM) is in charge of managing the available memory buffer on the receiver side. It is invoked by the ARQ-RX instance. The ARQ-TX instance is located at the source end of the data link and triggers the requests of memory reservation for management of a sliding window based on traffic requirements.  
         [0051]     In one embodiment of the present invention, for the sake of better understanding the predetermined set of operational control modes comprises two operational control modes. Of course, the invention encompasses an embodiment based on more operational control modes.  
         [0052]     The ARQ entity performs a selection of one of the operational control modes on a per data link basis. A first selection is performed at the time of a data link establishment. Such a selection can be preferably performed at any time during the data transmission of a given data link. A first operational control mode will be referred to as Binary Flow Control (BFC). The second operational control mode will be referred to as Window Flow Control (WFC).  
         [0053]     When the ARQ entity selects the BFC mode for a given data link, the ARQ-TX instance is preferably flow controlled using a flag in the ARQ-ACK messages. This flow control flag is referred to as S/R for Stop/Resume. Actually, this flow control flag comprises a flow control data to inform the transmitter about the fact that it has to stop or can continue (or Resume) the data transmission. In BFC mode, the receiver sets this flag based on the comparison between a threshold value and the available memory space, the threshold value being administratively set. When the available memory space is greater than the threshold value, the ARQ-RX instance set the flag to R for Resume and then the transmitter is allowed to continue the transmission corresponding to the given data link. Conversely, the ARQ-RX instance sets the flag to S for stop and the transmitter stops the respective data transmission.  
         [0054]     When the ARQ entity selects the WFC mode for a given data link, an ARQ Window Size (ARQ-WS) is fixed through a window reservation mechanism based on a memory reservation mechanism. ARQ-WS is a parameter used by both ARQ-TX instance and ARQ-RX instance. The Flow Control mechanism introduced in the BFC mode is reproduced in the WFC mode so that the transmitter can temporarily exceed the negotiated ARQ-WS.  
         [0055]     The memory reservation mechanism is preferably implemented through simple reservation messages that are added to the ARQ protocol message set. Of course, the present invention encompasses different protocol implementations. Two types of messages are defined: Window ReSerVation (ARQ-WRSV) message and Window GraNT (ARQ-WGNT) message. They can be transported in specific signalling LL-PDUs of the data link or integrated in already existing messages such as Requests for FeedBack (ARQ-RFB) and ARQ-ACK message. In that latter case, additional fields are preferably provided in the ARQ-RFB and ARQ-ACK messages. The ARQ-WRSV message includes information about the requested space of memory which is referred to as a Requested Window Size (ReqWS) field. The ARQ-WGNT message includes information about the reserved memory space which is really reserved in the shared memory of the receiver and which is referred to as a Granted Window Size (GntWS) field. Of course, the reserved memory space can be different from the requested memory space.  
         [0056]     Such a protocol also preferably uses a flow control flag that is piggy-backed in the ARQ-ACK messages. When the flow control flag is set to R, the ARQ-ACK message also preferably contains a sequence number of the last correctly received LL-PDU (LastRx). This information is provided so that the ARQ-TX can determine the LL-PDU from which it can resume the data transmission. In addition, the current selected operational control mode (BFC or WFC) is indicated by the ARQ-RX instance, preferably included in each ARQ-ACK message in order to use an existing ARQ protocol message, through a flag which is referred to as a mode flag.  
         [0057]     The ARQ Memory Buffer Manager (ARQ-MBM) is in charge of sharing the receiver shared memory between all ARQ-RX instances of data links ending in a given receiver. It maintains a table, referred to as ARQ-Table, of which each entry contains status information of one data link, as shown below. Such a table contains a Data Link Id field, a Class field, a Mode field and Window Size field.  
                                                               Data Link ID   Class   Mode   Window Size                      
 
         [0058]     Data Link ID is the identifier attached to the Data Link i (DL i ).  
         [0059]     Class is the priority class of DL i .  
         [0060]     Mode is the current operational mode of DL i  (Mode i )  
         [0061]     Window Size if the currently reserved window size of DL i  (WS i ).  
         [0062]     To perform sharing of the memory according to the memory reservation requests corresponding to all data links, the ARQ-MBM takes as input the ReqWS field as passed by the ARQ-TX instance through the ARQ-WRSV message and the contents of the different table entries.  
         [0063]     The following algorithm for sharing memory can be implemented in the ARQ-MBM:  
         [0000]     BMSize: Total buffer memory size  
         [0000]     BMRsv: Space of buffer memory occupied by all reserved windows  
         [0000]     BMRsv(CL i ): Portion of buffer memory occupied by the reserved windows of class i data links  
         [0000]     DL i : data link i  
         [0000]     Mode i : Current data link operation mode of data link i  
         [0000]     WS i : currently reserved window size of data link i (equals zero when in BFC mode)  
         [0000]     ReqWS i : Window size requested for data link i  
         [0000]     GntWS i : Window size granted by the receiver for data link i  
         [0064]     Upon reception of an ARQ-WRSV message on DL i :  
                                                                                                                                                                                                                                     BMRsv   =       ∑   i     ⁢   Wsi                         if (ReqWS i  ≦ WS i )                if (ReqWS i  = 0)                Mode i  = BFC                “reservation accepted”           GntWS i  = ReqWS i             WS i  = GntWS i              else                if (ReqWS i  &lt; (BMSize − BMRsv))                if (WS i  = 0)                Mode i  = WFC                endif           “reservation accepted”           GntWS i  = Req WS i             WS i  = GntWS i                  else                                                  if   ⁢           ⁢       ∑     j   &lt;   i       ⁢     BMRsv   ⁡     (   CLj   )           ≥     ReqWS   i                                      if (WS i  = 0)                Mode i  = WFC                endif           “reservation accepted”           GntWS i  = ReqWS i             WS i  = GntWS i             force BEG mode for all DL n  of class j &lt; i such as                                              ∑   n     ⁢     WS   n       ≥     ReqWS   i       ,     that   ⁢           ⁢   is     ,                       set the Mode field to BEG in the table for the       corresponding Data Link.                else                “reservation denied”           GntWS i  = WS                endif                endif            endif                  
 
         [0065]     At any time, the ARQ-MBM monitors the space of available buffer memory which can be calculated according to the following equation:
 
(BMSize−BMRsv).
 
         [0066]      FIG. 2  illustrates a state machine presenting the transitions between the BFC and the WFC modes. These transitions are triggered upon transmission (for the ARQ-RX instance), respectively reception (for the ARQ-TX instance), of an ARQ-WGNT or ARQ-ACK messages. Actually, the ARQ entity manages a transition referenced  22  from the BFC operational control mode to the WFC operational control mode when the receiver sends back, in response to an ARQ-WRSV comprising a requested memory space whose size is not equal zero, an ARQ-WGNT message to inform the transmitter that the memory reservation has been correctly performed. The ARQ entity manages a transition referenced  23  from the WFC operational control mode to the BFC operational control mode either upon reception of an ARQ-WRSV message comprising a requested memory space whose size is equal zero, or when the receiver is no longer able to guarantee the previously accepted memory reservation. In that case, the transition is triggered upon emission/reception of an ARQ-ACK message with the Mode field set to BFC. The transition referenced  24  illustrates the case where the transmitter requires a greater sliding window than the current one. In this case, the ARQ entity keeps the WFC operational control mode and only the reserved memory size is changed. On the other hand, the transition referenced  21  illustrates the case where the transmitter sends a ARQ-WRSV message to reserve a given memory space whose size is not equal zero and where the ARQ-RX instance is not able to accept this memory reservation. It results therefrom that the ARQ entity keeps the BFC mode.  
         [0067]     The following section describes the operations performed on the receiver side by the ARQ-RX instance according to one embodiment of the invention.  
         [0068]     Considering a given data link and whatever its operational mode is, upon reception of an LL-PDU, the ARQ-RX instance checks the available memory space in the shared memory buffer, which is calculated according to the following equation:
 
BMSize−BMRsv.
 
         [0069]     If the available memory space is lower than a given low threshold, the ARQ-RX instance sends an ARQ-ACK message with the flow control flag set to S, causing the ARQ-TX instance to stop the PDU transmission for the corresponding data link. LL-PDUs received after the ARQ-RX instance has sent an ARQ-ACK message with the flow control flag set to S may be discarded if the receiver is out of memory buffer.  
         [0070]     If the available memory space in the memory buffer is greater than a given high threshold and the ARQ-TX instance has stopped the LL-PDUs transmission on the corresponding data link, i.e. after a last ARQ-ACK has been sent with the flow control flag set to S, the ARQ-RX instance sends an ARQ-ACK message with the flow control flag set to R including a valid LastRx field, to indicate to the transmitter the last valid received PDU.  
         [0071]     In every other case when the ARQ-RX sends an ARQ-ACK message, the flow control flag is set to R.  
         [0072]     In any case, the ARQ-RX instance indicates the current data link operational control mode through the mode flag located in the ARQ-ACK. The mode flag is determined from the Mode field of the entry associated to the data link in the ARQ-Table.  
         [0073]     Considering a Data Link operating in BFC mode, upon reception of an ARQ-WRSV message, the ARQ-RX instance invokes the ARQ-MBM with the requested window size indicated in the ReqWS field. If the ARQ-MBM returns a “reservation accepted” indication, the ARQ-RX instance sends an ARQ-WGNT message with GntWS field comprising the same value as the ReqWS field and the operational control mode of the corresponding data link switches from the BFC mode to the WFC mode. If the ARQ-MBM returns a “reservation denied” indication, the ARQ-RX instance sends an ARQ-WGNT message with GntWS field set to zero and the data link remains in the BFC mode.  
         [0074]     Considering a given data link operating in WFC mode, upon reception of an ARQ-WRSV message, the ARQ-RX instance checks if the newly requested window size indicated in the ReqWS field can be accommodated with the available memory buffer space by invoking the ARQ-MBM. If the ARQ-MBM returns a “reservation accepted” indication, the ARQ-RX instance sends an ARQ-WGNT message with GntWS field comprising the same value as the ReqWS field. If the requested window size modification is denied, the ARQ-RX instance sends an ARQ-WGNT message with GntWS field set to the value previously in use. If GntWS field is set to zero, the ARQ entity switches to the BFC mode.  
         [0075]     The following section describes the operations performed on the transmitter side by the ARQ-TX instance according to one embodiment of the invention.  
         [0076]     Whatever the operational mode is, upon reception of an ARQ-ACK message with the flow control flag set to S, the ARQ-TX instance stops emitting new LL-PDUs on the given data link but can still retransmit erroneous LL-PDUs.  
         [0077]     Upon reception of an ARQ-ACK message with the flow control flag set to R, the ARQ-TX can continue to emit new LL-PDUs on the given data link if not stopped beforehand. If this message is received when stopped, the ARQ-TX instance resumes new LL-PDU transmissions starting from the sequence number indicated in the LastRx field.  
         [0078]     In one embodiment of the invention, considering a given data link operating in the BFC mode, the ARQ-TX instance can preferably trigger a window memory reservation procedure at any time when in the BFC mode, by transmitting an ARQ-WRSV message containing the requested window size in the ReqWS field. A timer is started while waiting for the reception of the corresponding ARQ-WGNT message. If the timer expires before the ARQ-WGNT message is received, the same ARQ-WRSV message can be repeated. Upon reception of the ARQ-WGNT message, the ARQ-TX instance checks the GntWS field of the latter. If the returned window size equals the requested value, the memory reservation is successful. If the returned value is different from the requested value, notably if the returned value is zero, the reservation has failed and the data link remains in the BFC mode.  
         [0079]     Considering a given data link operating in the WFC mode, the window size is fixed to the value returned in the most recently received ARQ-WGNT message containing a non-zero value in the respective GntWS field.  
         [0080]     The window size can be modified at any time upon request of the ARQ-TX instance by using the same procedure as in the BFC mode.  
         [0081]     Upon reception of a ARQ-WGNT message, the ARQ-TX instance updates the window size with the returned GntWS field value. If GntWS field equals zero, the data link mode switches to the BFC mode.  
         [0082]     Upon reception of an ARQ-ACK message with the operational control mode flag set to the BFC mode, the data link mode switches to the BFC mode.  
         [0083]     As mentioned before, the control flow flag set to S is interpreted as in the BFC mode. This last feature allows the ARQ-TX instance to overrun the negotiated window size when memory buffer resource is available in the receiver.  
         [0084]      FIG. 3  illustrates some cases of switching between the BFC mode and the WFC mode for a given data link on a given data link via exchanged messages in one embodiment of the present invention. Initially, the ARQ entity has selected the BFC mode  313 . It results therefrom that no memory space is reserved on the receiver side for the given data link. The transmitter  301  sends an ARQ-WRSV message  303  to the receiver  302  to reserve a memory space and simultaneously starts a timer  318 . The size of the requested memory space is set in the ReqWS field of the ARQ-WRSV message. The receiver  302  sends back to the transmitter  301  a ARQ-WGNT message  304 . The transmitter  301  does not receive the latter before the timer expiration. The transmitter  301  repeats the same ARQ-WRSV message  305 . As the available space of memory is greater than the memory space requested for reservation, the receiver  302  sends back to the transmitter a ARQ-WRSV message  306  accepting the memory reservation. The ARQ entity switches to the WFC mode  314  for the given data link. In order to release the reserved resource, the transmitter  301  sends an ARQ-WRSV message  307  comprising a requested memory size of zero. Upon reception of this message  307 , the receiver releases the corresponding memory space and sends back to the transmitter  301  an ARQ-WGNT message  308 . The ARQ entity switches to the BFC mode  315 . Latter on, the transmitter  301  requires a memory reservation procedure by sending an ARQ-WRSV message  309  comprising a memory space size in the ReqWS field. The receiver accepts this reservation by sending back an ARQ-WGNT message  310  comprising the reserved memory space size which is equal to the requested memory space size. The ARQ entity switches to the WFC mode  316 . Latter on, the receiver pre empts the memory space reserved to the given data link and then sends an ARQ-ACK message  311  comprising the mode flag set to BFC. Consequently, the ARQ entity switches to the BFC mode  317 .  
         [0085]     One embodiment of the present invention can be advantageously applied to a network where different application data flows presenting various QoS constraints are transmitted. Notably, a great flexibility is offered to application data flows without any particular QoS requirements. Actually, this type of application can advantageously use memory resource depending on their instant throughput, without having to use fixed size of reserved memory. Consequently, the memory buffer distribution between the data links can be dynamically performed. On the other hand, the application data flows with stringent QoS constraints are guarantied resources by using the reservation procedures. Moreover, every application data flow can instantly reach the maximum throughput offered by lower layer as soon as there is enough memory buffer in the receiver. It results therefrom that the transmission efficiency is improved by a dynamic flexible and efficient memory management which uses the memory resource as long as it is available and which avoids to waste memory resource.