Abstract:
There is disclosed a method and a device for managing a shared transmission medium, between a plurality of stations in a wireless local area network having an Access Point (AP). The AP transmits to all stations substantially periodic management frames, a SuperFrame being defined as the time period between two consecutives management frames. This SuperFrame is divided into a plurality of time periods and these time periods correspond to respective access schemes. This time division is performed according to a management information comprised in the periodic management frame. There is a first time period corresponding to a contention free access scheme which includes a Time Frame (TF) having at least one synchronous Frame Time Interval (FTI), and a second time period corresponding to a contention access scheme.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Technical Field  
         [0002]     The present invention relates generally to communication systems and in particular, to a method for managing a shared transmission medium between a plurality of stations in a wireless local area network having an Access Point (AP).  
         [0003]     2. Related Art  
         [0004]     In recent years, the IEEE 802.11 Standard, herein after referred to as “the Standard”, has emerged to propose specifications for the WLAN, with a Medium Access Control (MAC) part of the standard defining a MAC layer and a PHYsical (PHY) part of the standard defining a Physical layer. The IEEE 802.11 Standard is now frequently implemented, wherein MAC frames are created according to the MAC part of the Standard, and are encapsulated into Physical packets according to the PHY part of the Standard, which packets are emitted via the air interface. The MAC part of the Standard specifies a MAC layer which can relay on every Physical layer also defined by the standard and controls the transmission of user data into the air in order to share a common transmission medium between stations of a network.  
         [0005]     The MAC part of the Standard defines two medium access schemes: a Distributed Coordination Function (DCF) and a Point Coordination Function (PCF). Stations listen to the medium and decode all MAC frames during the DCF and the PCF. The DCF is based on a contention access scheme and it is implemented at the station level. The PCF, which is optional according to the Standard, provides a contention free access based on a centralised polling access scheme. It uses an Access Point (AP) somewhere in the network, which centralizes the managing of access to the medium by all stations in the network.  
         [0006]     The DCF uses the standard Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) access mechanism. Before attempting to transmit, each station checks whether the medium is idle. If the medium is not idle, the station defers transmission by handling an orderly exponential back-off algorithm to avoid collision. When DCF stations are not transmitting, they are listening to the medium to receive every data frames transmitted and they ignore these data frames which are not intended for them. This access scheme provides a best effort service and it cannot guarantee a transmission delay upper bounded for real time applications.  
         [0007]     In comparison, the PCF allows to better control the transmission delay, which is required by some applications. According to the MAC part of the Standard, the PCF is a function of the AP which ensures that the medium is provided without contention to all stations compatible with this type of medium access, through an “enforced” access to the medium. To separate the DCF and the PCF access schemes, the MAC part of the Standard introduces some distinct time periods reserved for one of those access schemes: a Contention Free Period (CFP) managed according to the PCF and a Contention Period (CP) managed according to the DCF.  
         [0008]     A Beacon frame, which is a specific management frame defined in the MAC part of the Standard, is periodically emitted by the AP. The time period between two consecutive Beacon frames is called a Superframe in what follows. Each occurrence of a Beacon frame indicates the beginning of a CFP. Each Beacon frame includes information on the respective length of the CFP and of the CP.  
         [0009]     The Beacon frames are listened to by all stations in the network, whatever the access medium scheme they use. Then, each station is informed about CFP and CP. The PCF is based on a polling list method. The AP manages a polling list of stations compatible with PCF, which are not allowed to transmit during the CFP, until the reception of a polling frame received from the AP. Each polling frame, which contains a MAC address of a given station, allows this station to transmit one frame. A station can transmit more than one frame only if it receives respective polling frames. The CFP is long enough for allowing the transfer of at least one frame of maximum size and of its associated acknowledgement. During the CFP, the medium access according to the DCF is locked out.  
         [0010]     In the IEEE 802.11 Standard extensions, other access methods are defined but are always derived from the PCF or the DCF. For instance, 802.11e defines the Hybrid Coordination Function (HCF) mode that implements an Enhancement Distributed Channel Access (EDCA) and a HCF Control Channel Access (HCCA) schemes. EDCA is based on a contention access scheme with different priority levels and is implemented during the CP. Consequently, it provides a best effort data transmission without any guarantee on transmission delay. HCCA shares the CFP with the PCF access scheme and is also based on a polling access scheme. This latter generates an important signalling overhead. As a polling frame can be received at a given station at any time during the CFP, it results from this the following disadvantages: a more complex Physical layer which has to perform synchronisation on each polling frame reception and a less efficient implementation of a power saving function.  
       SUMMARY OF THE INVENTION  
       [0011]     In view of the foregoing, there is a need for a medium access scheme allowing a WLAN network to support protocols providing data transmission with higher throughput and better real-time characteristics, without using a polling access scheme.  
         [0012]     A first aspect of the invention thus proposes a method for managing a shared transmission medium between a plurality of stations in a wireless local area network having an Access Point (AP), the method comprising the steps of: 
        the AP transmitting to all stations substantially periodic management frames, a SuperFrame being defined as the time period between two consecutives management frames,     splitting the SuperFrame into a plurality of time periods, which time periods correspond to respective access schemes, according to a management information comprised in the management frame, the SuperFrame comprising: 
            first time period corresponding to a contention free access scheme,     a second time period corresponding to a contention access scheme,    
            wherein the first time period comprises a Time Frame (TF) having at least one synchronous Frame Time Interval (FTI).        
 
         [0018]     A second aspect of the invention relates to a device for managing a shared transmission medium between a plurality of stations in a wireless local area network, the device comprising: 
        means to transmit to all stations substantially periodic management frames, a SuperFrame being defined as the time period between two consecutives management frames,     means to split the SuperFrame into a plurality of time periods, which time periods correspond to respective access schemes, according to a management information comprised in the management frame, the SuperFrame comprising: 
            a first time period corresponding to a contention free access scheme,     a second time period corresponding to a contention access scheme,    
            wherein the first time period comprises a Time Frame (TF) having at least one synchronous Frame Time Interval (FTI).        
 
         [0024]     In a preferred embodiment, the TF comprises a given number N of FTIs of same duration D. Of course, the duration of the TF shall be shorter than the duration of the first time period. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]     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:  
         [0026]      FIG. 1  illustrates an exemplary SuperFrame structure with two different time periods being the CP and the CFP as defined in the Standard.  
         [0027]      FIG. 2  shows an exemplary of the TF structure according to the invention.  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0028]     An exemplary embodiment of the invention is described herein below as an enhancement of the IEEE 802.11 Standard. The invention is however applicable to any wireless local area network including an Access Point and a plurality of stations. Use of the invention is compliant with the current MAC part of the Standard, so that stations using PCF/DCF/HCF access schemes can inter work in the same network.  
         [0029]     As the invention is based on the MAC part of the Standard, the following section first remind basic concept useful to correctly understand the enhancement proposed herein.  
         [0030]     For the sake of better understanding,  FIG. 1  illustrates different time periods defined in the Standard. The Beacon frame  14  is periodically emitted by the AP to indicate the beginning of one SuperFrame, and the beginning of the CFP thereof. The time interval between two consecutive Beacon frames  14  defines a SuperFrame  11  and it contains one CFP  12  followed by one CP  13 .  
         [0031]     The Beacon frame  14  must be listened to by all the stations: this Beacon frame indicates to all stations the length of the CPF, the SuperFrame duration, already defined above, and the AP capabilities. All stations decode the Beacon frame. Stations using exclusively the DCF access medium scheme are not allowed to emit during the CFP. Consequently, they do not generate interference and errors during this period. The SuperFrame duration is a static parameter of network configuration, as described in the IEEE 802.11 standard. On the contrary, the CFP  12  duration is variable and is determined by the PCF for each SuperFrame  11 . A CFP is not mandatory in a SuperFrame, therefore the CFP duration may be nil. As the AP sends all Beacon frames at a periodic substantially fixed time, the SuperFrame and consequently the CFPs always start at a somehow expected time.  
         [0032]     The current MAC part of the Standard (see in particular Edition 1999, section 7.1.2 and 7.1.3.1 page 34) describes the general MAC frame format, which depicts a set of fields that occur in a fixed order in all MAC frames. It includes a frame control field comprising a Protocol Version subfield. The stations check this subfield in each received data frame in order to determine if they can interpret it. Therefore, different MAC protocols can be supported inside an IEEE 802.1 Standard network, when this Protocol Version subfield is correctly used.  
         [0033]     The invention suggests modifying the SuperFrame and the Beacon frame format defined in the MAC part of the Standard. These modifications guaranty the compatibility with the medium access schemes (DCF/PCF/HCF) defined by the Standard.  
         [0034]     According to the Standard (see in particular the 1999 Edition, section 7.2.3.1 page 46, table 5), a Beacon frame contains some fixed length fields which are mandatory and some variable length fields which are inserted only if the AP supports a particular function. Such fields are named Information Element (IE) in the MAC part of the Standard and exhibit the following common format: 
        Element Identifier (1 byte)     variable length of value L in bytes (1 byte)     information of variable length (L bytes).        
 
         [0038]     Thanks to this format, the Beacon frame is able to support and include new lEs in order to describe some future capabilities defined in the MAC Layer protocol extensions.  
         [0039]     In one embodiment of the invention, the Beacon frame format is changed in order to allow the PCF to inform the stations about TF characteristics on a Superframe basis.  
         [0040]      FIG. 2  illustrates one example of the SuperFrame structure according to the invention. In this example, CFP  21  and CP  22  are two consecutive time periods which form the SuperFrame, according to the MAC part of the Standard. Here, the CFP  21  includes a TF period  25  with at least one synchronous FTI  23 . The AP may thus manage the shared transmission medium according to a synchronous TF access scheme during the TF period  25 . Only stations which are able to use the TF access scheme are allowed to use the TF period  25  for data transmission. The following details explain how it is ensured. As TF is included in CFP, all stations using exclusively the DCF access scheme are not allowed to emit during the TF. In addition, since the AP does not send any polling frames during the TF  25 , all stations using PCF access, but not TF access, will not receive any polling frame, and therefore they will never emit during the TF. The TF is then not disturbed with DCF or PCF access scheme.  
         [0041]     In one preferred embodiment, the TF is composed exclusively of a set of a given number N of consecutive synchronous FTIs. The N FTIs in a given Superframe have the same duration D. The number N of FTIs in a given SuperFrame and the duration D of each FTI are fixed on a per SuperFrame basis. Stated otherwise, N and D may vary from one SuperFrame to another.  
         [0042]     Compared to current MAC part of the Standard (Edition 1999), the Beacon frame format is extended in order to include a new Information Element, namely the FTI Parameter Set  27 , which comprises the following fields: 
        TF Element Identifier (1 byte)     the length of the FTI Parameter Set field (1 byte)     the number N of FTIs in a TF (1 byte)     the FTI duration, in μs (2 bytes)     K bytes for additional protocol information used within the FTI, K being a variable value which depends on the protocol used in the FTI. It is important to note here that the protocol used in the FTIs can be different from protocol described in the MAC part of the standard. As it has been already described above, if the Protocol Version subfield included in all MAC frames is filled in with a specific value different from values which have been already reserved in the MAC part of the Standard, any MAC protocols can be used inside the FTIs. All stations which implement the MAC part of the Standard and its current extensions must ignore the frames with a Protocol Version subfield that they are not able to recognise as a valid value. Accordingly, such a format prevents stations not able to use the TF access scheme from handling and wrongly interpreting frames sent in a given FTI according to any protocols that they do not support. In the MAC part of the Standard the value 0 is the only reserved for Protocol Version subfield value.        
 
         [0048]     Specifying the additional protocol information would extend beyond the scope of the present description.  
         [0049]     Of course, the Beacon frame format includes also CF parameter Set  26  as it is defined in the MAC part of the Standard, when the AP includes a PCF. The CF parameter set contains the set of parameters necessary to support the PCF and notably the CFPMaxDuration which indicates the maximum duration of the CFP that may be generated by the PCF. The CF parameter set has the following format: 
        CF Element Identifier (1 byte)     the length L of the information field (1 byte)     the information field (L bytes)        
 
         [0053]     In one embodiment, the TF Element Identifier field contains a specific element identifier value reserved among the unused values defined in the Standard. This value is used to guaranty compatibility between all medium access schemes of the standard: all stations decode the Beacon frame and check the value of this specific element identifier. Either they support this value, meaning they are able to use the TF access scheme; or they do not support it and they ignore all data transmitted during the TF period. In any case, stations which are compliant with the MAC part of the Standard support the TF as it is explained in detail below.  
         [0054]     When a station of the network decodes a Beacon frame received from the AP, it checks the CF Element Identifier value. When the station is only able to use the DCF access scheme, the station switches to a listening only mode, until the end of the CFP. Else if the station does not recognise the TF Element Identifier value, the station can still listen during the CFP including the TF, also it will not receive any polling frame until the end of the TF. In addition, the Protocol Version subfield value used in the TF prevents it from handling and wrongly interpreting messages.  
         [0055]     According to the invention, the AP is in charge of determining for each SuperFrame the value N and the duration D to define the TF.  
         [0056]     In one preferred embodiment, stations can communicate their transmission requirements to the AP. The AP takes into account this information in order to determine N and D dynamically using a specific algorithm to achieve the station requirements.  
         [0057]     To summarize, the invention provides a method to introduce a more efficient MAC protocol based on time frame of fixed duration like TDMA/TDD service and provides respective advantages. Consequently, the TF access scheme can help to provide a transmission service more adapted to real time applications due to a substantially synchronous transmission. During the TF, the synchronisation at the PHY layer can be simplified. Most of the synchronisation is performed at the Beacon frame reception. Moreover, the invention may reduce signalling overhead compared to signalling overhead generated with a polling access scheme.  
         [0058]     In addition to this, the invention is compatible with the medium access functions defined by the current MAC part of the Standard.  
         [0059]     According to a further advantage, the invention provides a method to share dynamically the medium between different MAC protocols according to the requirements of the applications running on respective stations. The invention allows to implement any protocols during the TF: the flexibility of the MAC part of the Standard is hereby increased. The SuperFrame and the Beacon format according to the invention make the AP able to allocate transmission resource to the different implemented access schemes depending on the station capabilities and their applications, the TF transmission resource being preferably allocated to the real time applications