Abstract:
Provided is a method for reducing a transmission delay on the basis of non-contention without performing backoff and polling when transmitting an audio frame of a radio frequency (RF) link section in a voice-over Internet protocol (VoIP) system having a wireless local area network (WLAN) function. The method includes the steps of: (a) obtaining audio information through at least one WLAN terminal; (b) performing one of setting, change and removal of a reservation number (RN) through an RN setting process between the WLAN terminal and an access point (AP); (c) transmitting an audio QoS frame including reservation information obtained by step (b) at every interval when an audio frame is generated in a radio frequency (RF) link section; and (d) setting up a reservation time on the basis of the reservation information included in the received audio QoS frame.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to and the benefit of Korean Patent Application No. 2005-119978, filed Dec. 8, 2005, the disclosure of which is incorporated herein by reference in its entirety.  
       BACKGROUND  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a media access control method in a wireless local area network (WLAN), and more particularly, to a WLAN media access control method for improving quality of service (QoS) of a voice over Internet protocol (VoIP) system using a WLAN.  
         [0004]     2. Discussion of Related Art  
         [0005]     In general, a wireless local area network (WLAN) operates within a range of 100 meters at a bit rate of 10 to 100 Mbps. When made up of a single cell, the WLAN can be appropriately used for a single-story office or a store.  
         [0006]     In addition, a WLAN terminal is connected to another terminal and an access point (AP) in a network through a radio frequency (RF) link using a wireless network interface card (NIC). Here, the AP allows the WLAN terminal to access a wired network through a backbone network.  
         [0007]     Approximately twenty five terminals can be connected in the single cell. A multiple cell can be made up using a plurality of APs connected to the wired network, and a WLAN environment can be constructed over an entire building using the multiple-cell.  
         [0008]     The Institute for Electrical and Electronic Engineers (IEEE) has developed a standard defining a protocol for data frame transmission between a WLAN terminal and an AP. As a result, a standard for WLAN medium access and physical layer was developed (IEEE Std. 802.11, IEEE Standard for Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY), 1999).  
         [0009]     IEEE 802.11 WLAN standard specifies two main components of WLAN: a mobile terminal and a stationary AP. In the IEEE 802.11 WLAN standard, the terminal and AP for a single cell are defined as a basic service set (BSS), and the terminals and APs for a multiple cell are defined as an extended service set (ESS).  
         [0010]     In addition, in the IEEE 802.11 WLAN, each terminal and an AP embody a MAC layer serving to exchange MAC frames. The MAC frames are used as a medium for control, management, and data transmission between the wireless terminal and the AP.  
         [0011]     The standard defines two different wireless media access modes for the MAC layer. Among them, one is a distributed coordination function (DCF), and the other is a point coordination function (PCF).  
         [0012]     According to the DCF, all stations can participate in contention for frame transmission. A basic access mode of IEEE 802.11 MAC is a carrier sense multiple access with collision avoidance (CSMA/CA) mode.  
         [0013]     In the CSMA mode, a station intended to transmit data through a WLAN wireless medium detects the medium in order to check whether or not data is being transmitted from another station. When the medium is not occupied, the data transmission is possible. Otherwise, the data transmission is delayed until the ongoing data transmission is completed.  
         [0014]     When the data transmission is enabled at the station immediately after the data transmission through the WLAN medium is completed, a plurality of stations can attempt to transmit the data. Hence, a probability of data collision increases. In order to solve this problem, a pause duration is given for a predetermined time after another data transmission is completed, binary random backoff for obtaining a transmission opportunity is performed to determine the size of a contention window (CW). The transmission opportunity is given to the station that has determined the minimum CW size. Such a process is a collision avoidance (CA) function.  
         [0015]     Further, in the PCF, a point coordinator (PC) controls the data transmission of the WLAN terminal. In other words, the PC serves as a polling master, and performs polling on all PCF pollable terminals in order to determine the terminal capable of transmitting the data. Meanwhile, the PC may be located in the AP. In this case, in the PCF, the terminal may be polled or not.  
         [0016]     When a pollable terminal receives a poll from the PC, only one MAC protocol data unit (MPDU) can transmit the data. When additional transmission is desired, the terminal should be on standby until the poll is received again. When specific data transmission is abnormally completed, the terminal cannot perform retransmission until the poll is received from the PC. Thus, the PCF provides a contention free mechanism (CFM) to give the terminal an opportunity to normally transmit the data.  
         [0017]     Therefore, Enhanced-DCF complying with IEEE 802.11e standard is intended to improve QoS by adjusting the CW. More specifically, a lot of stations contend for network access during the CW duration. In order to avoid collision, first, a MAC protocol requires each station to be on standby for the CW duration selected by the binary random backoff.  
         [0018]     In this manner, due to the CW duration selected by the binary random backoff, a probability of collision between the stations decreases.  
         [0019]     Here, the EDCF makes use of the CW to give a high priority to a specific station. More specifically, the EDCF gives the high priority to the specific station by providing the specific station with a short CW duration. Consequently, in most cases, the terminal having the high priority transmits the data earlier than a terminal having a low priority.  
         [0020]     Also, QoS is a means for measuring the quality of a service for users. Here, main criterions for measuring the QoS are message loss, message delay, network availability, and so forth.  
         [0021]     The transmission of time-sensitive (such as audio or video) data application traffic on a packet network requests requirements meeting delay, delay jitter, and error rates.  
         [0022]     Considering the above-mentioned wireless media access modes (DCF, PCF and EDCF) from the viewpoint of the QoS, the DCF mode performs the binary random backoff prior to frame transmission, so that a frame delay occurs.  
         [0023]     The EDCF mode, which makes complementary to the DCF mode, allocates a priority to an audio frame by providing the audio frame with the CW duration shorter than that of a data frame, but still performs the backoff, so that the frame delay occurs.  
         [0024]     Finally, the PCF mode allows the WLAN terminal to transmit a frame only by the polling in a contention free period (CFP), so that the frame delay occurs.  
       SUMMARY OF THE INVENTION  
       [0025]     It is an objective of the present invention to provide a method for reducing a transmission delay on the basis of non-contention without performing backoff and polling when transmitting an audio frame of a radio frequency (RF) link section in a voice over Internet protocol (VoIP) system having a wireless local area network (WLAN) function.  
         [0026]     An aspect of the present invention provides a periodic media reservation method for quality of service (QoS) data having a periodic transmission characteristic in a WLAN. The periodic media reservation method comprises the steps of: (a) obtaining audio information through a WLAN terminal; (b) performing one of setting, change and removal of a reservation number (RN) according to an RN setting process between the WLAN terminal and an access point (AP); (c) transmitting an audio QoS frame including reservation information obtained by step (b) at every interval when an audio frame is generated in an RF link section; and (d) setting a reservation time on the basis of a reservation interval included in the received audio QoS frame.  
         [0027]     Step (a) may comprise the step of receiving a generation interval and a playback section of the audio frame from an upper layer of WLAN media access control (MAC).  
         [0028]     The setting of the RN in step (b) may comprise the steps of: requesting, by the mobile terminal (MT), the AP to set the RN using an Association Request frame; checking, by the AP, a reservation list to assign the RN; and transmitting, by the AP, an Association Response frame, and receiving, by the MT, the assigned RN.  
         [0029]     The Association Request frame may include a generation interval and a playback interval, which are audio characteristic information.  
         [0030]     The AP may assign the RN so as to prevent reservation repetition using the reservation interval and a transmission duration registered with the reservation list.  
         [0031]     The changing of the RN in step (b) may comprise the steps of: requesting, by MT, the AP to change the RN using a Re-association Request frame; checking, by the AP, a reservation list to assign a new RN; and transmitting, by the AP, a Re-association Response frame, and receiving, by the MT, the assigned new RN.  
         [0032]     The Re-association Request frame may comprise the RN or the audio information or both.  
         [0033]     The removing of the RN in step (b) may comprise the steps of: requesting, by the MT, the AP to remove the RN using a Re-association Request frame; removing, by the AP, the RN from a reservation list; and transmitting, by the AP, a Re-association Response frame and checking, by the MT, the removed RN.  
         [0034]     The Re-association Response frame may comprise the RN having a value of 0.  
         [0035]     Step (c) may comprise the step of subtracting a reservation interval of a first frame by a time delayed through backoff.  
         [0036]     Step (c) may comprise the step of giving up, by the MT transmitting a frame not including the reservation information, transmission when a remained transmission time is less than a desired transmission time of the frame to be transmitted by the MT.  
         [0037]     Step (c) may comprise the step of transmitting, by the MT transmitting a frame not including the reservation information, the frame according to a distributed coordination function (DCF) when a remained transmission time is 0.  
         [0038]     Step (c) may comprise the step of transmitting, by the MT transmitting frames including the reservation information, a final frame with the reservation time set to 0.  
         [0039]     Step (d) may comprise the step of setting, by the MT having received the frame including the reservation information, the reservation time. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0040]     The above and other features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail preferred exemplary embodiments thereof with reference to the attached drawings in which:  
         [0041]      FIG. 1  is a block diagram of a wireless local area network (WLAN) system performing a quality of service (QoS) function on an audio frame in a radio frequency (RF) link section, according to the present invention;  
         [0042]      FIGS. 2A  to  2 C illustrate reservation frame exchange processes for reservation number management, according to the present invention;  
         [0043]      FIG. 3  illustrates formats of frames used in a reservation frame exchange process, according to the present invention;  
         [0044]      FIG. 4  illustrates a method for reserving an RF link based on a generation interval of an audio frame, according to the present invention; and  
         [0045]      FIG. 5  illustrates a format of a frame used in an audio frame exchange process, according to the present invention. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0046]     Hereinafter, exemplary embodiments of the present invention will be described in detail. However, the present invention is not limited to the exemplary embodiments disclosed below and can be implemented in various forms. Therefore, the present exemplary embodiments are provided for complete disclosure of the present invention and to fully inform the scope of the present invention to those of ordinary skill in the art.  
         [0047]      FIG. 1  is a block diagram of a wireless local area network (WLAN) system performing a quality of service (QoS) function on an audio frame in a radio frequency (RF) link section, according to the present invention.  
         [0048]     Referring to  FIG. 1 , the WLAN system comprises mobile terminals (MT)  120  and  130 , and an access point (AP)  110 .  
         [0049]     Here, the MTs  120  and  130  make up a basic service set (BSS)  100  through RF links together with the AP  110 , and are connected to a WLAN. The AP  110  is connected to an Internet backbone (IB)  140 .  
         [0050]     Information transmission is possible in an up-stream direction from the MTs  120  and  130  to the IB  140  via the AP  110 , and in a down-stream direction from the IB  140  to the MTs  120  and  130  via the AP  110 .  
         [0051]     Here, the MTs  120  and  130  and the AP  110  use two media access management modes: distributed coordination functions (DCFs)  121 ,  131  and  111  complying with the Institute for Electrical and Electronic Engineers (IEEE) 802.11 standard developed in 1999, and periodic media reservation timers (PMRTs)  122 ,  132  and  112  according to the present invention.  
         [0052]     The MTs  120  and  130  of the BSS  100  are mounted with audio codecs, which have the identical frame generation interval.  
         [0053]     Here, the MTs  120  and  130  have reservation timers (RTimers)  123  and  133 , and manage information on the generation interval of a reservation frame. The AP  110  has an RTimer  114  and a reservation number list (RNList)  113 , and manages reservation states of the MTs connected thereto.  
         [0054]     Further, the MT  120 , which transmits an audio frame, exchanges frames including QoS information with the AP  110  before transmitting the audio QoS frame. Then, the MT  120  is assigned a reservation number (RN) for transmitting the audio QoS frame, and transmits the frame. In contrast, the MT  130 , which does not transmit the audio frame, activates the RTimer  133  when receiving the audio QoS frame.  
         [0055]     The AP  110  manages the RNList  113  alone, and is not involved in the audio QoS frame exchange and reservation processes.  
         [0056]     Methods for audio QoS frame transmission and its reservation state management will be described below in further detail with reference to  FIGS. 2, 4 ,  5  and  6 .  
         [0057]      FIGS. 2A  to  2 C illustrate reservation frame exchange processes for RN management according to the present invention. More specifically,  FIG. 2A  illustrates a frame exchange process for RN assignment in an RN management method,  FIG. 2B  illustrates a frame exchange process for RN change in an RN management method, and  FIG. 2C  illustrates a frame exchange process for RN removal in an RN management method.  
         [0058]     First, the frame exchange process for RN assignment  210  will be described with reference to  FIG. 2A .  
         [0059]     When there is an audio QoS frame to be transmitted, an MT  211  encapsulates a Periodic Reservation Request (PRR) field  213  into an Association Request frame, and transmits the Association Request frame to an AP  212 . The AP  212  receiving the frame assigns a proper RN with reference to its own RNList  113 , and then encapsulates the assigned RN  214  into an Association Response frame and transmits the Association Response frame to the MT  211 .  
         [0060]     Next, the frame exchange process for RN change  220  will be described with reference to  FIG. 2B .  
         [0061]     An MT  221 , which needs to be assigned a new RN due to RN change or audio characteristic change, encapsulates a Periodic Reservation Change Request (PRCR) field  223  into a Re-association Request frame, and transmits the Re-association Request frame to an AP  222 . The AP receiving the frame assigns a new RN with reference to its own RNList  113 , and then encapsulates the RN  224  into a Re-association Response frame and transmits the Re-association Response frame to the MT  211 .  
         [0062]     Next, the frame exchange process for RN removal  230  will be described with reference to  FIG. 2C .  
         [0063]     When there is an audio frame to be transmitted, an MT  231  encapsulates a Periodic Reservation Remove Request (PRRR) field  233  into a Re-association Request frame, and transmits the Re-association Request frame to an AP  232 . The AP  232  receiving the frame removes a corresponding RN with reference to its own RNList  113 , and then sets an RN field  234  of a Re-association Response frame to 0 (null) and transmits the Re-association Response frame  234 .  
         [0064]      FIG. 3  illustrates formats of frames used in the reservation frame exchange processes, according to the present invention.  
         [0065]     Referring to  FIG. 3 , the AP and MT make use of Association and Re-association frames according to the IEEE 802.11 standard in order to set a RN value, and add a PRR field  310 , an RN field  320 , a PRCR field  330 , and a PRRR field  340  in order to set up an RN.  
         [0066]     In order to be assigned the RN, the MT encapsulates the PRR field  310  into an Association Request frame and transmits the Association Request frame. The PRR field has a format of an RN Request field  360 .  
         [0067]     Here, an Element identifier (ID)  361  is set to 32, and the RN Request field  360  includes a Reservation Interval field  362  denoting a transmission interval of the audio frame, and a Transmission Duration field  363  denoting a time required to transmit a single audio frame.  
         [0068]     Then, the AP requested to assign the RN encapsulate the RN field into the Association Response frame, and transmits the Association Response frame. The RN field has the format of an RN Response field  370 . Here, an Element ID field  371  is set to 33, and the assigned RN is denoted in a Reservation Number field  372 . However, in the case of a failure in assignment, the RN has a value of 0 (null).  
         [0069]     Meanwhile, when the RN is changed, or when a reservation interval or a transmission time is changed, the MT encapsulates the PRCR field  330  into a Re-association Request frame and transmits the Re-association Request frame. The PRCR field has a combination format of the RN Request field  360  and the RN Response field  370 .  
         [0070]     First, when only the RN is to be changed, the Element ID field  371  and the Reservation Number field  372  are set to 34 and the previously assigned RN value respectively, by using the format of the RN Response field  370 , and then transmitted.  
         [0071]     Next, when the reservation interval and the transmission time are to be changed according to characteristic change of the audio frame, both the RN Response field  370  and the RN Request field  360  are encapsulated. Here, the RN Response field  370  is encapsulated prior to the RN Request field  360 .  
         [0072]     More specifically, in the RN Response field  370 , the Element ID field  371  and the Reservation Number field  372  are set to 35 and the previously assigned RN value, respectively. In the RN Request field  360 , the Element ID  361  is set to 32, and the changed Reservation Interval field  362  and the changed Transmission Duration field  363  are set. Then, the encapsulated frames are transmitted.  
         [0073]     Then, the AP receiving an RN change request encapsulates an RN field into the Re-association Response frame, and transmits the Re-association Response frame. The RN field has the format of the RN Response field  370 . Here, the Element ID field  371  is set to 33, and an assigned RN is denoted in the Reservation Number field  372 . However, in the case of a failure in assignment, the RN has a value of 0.  
         [0074]     Meanwhile, in order to remove the assigned RN, the MT encapsulates the PRRR field  340  into the Re-association Request frame, and transmits the Re-association Request frame. The PRRR field has a combination format of the RN Response field  370 . At this time, the RN Response field  370  has the element ID field  371  and the Reservation Number field  372  set to 36 and the previously assigned RN value, respectively, and then transmitted.  
         [0075]     The AP receiving an RN removal request encapsulates an RN field into the Re-association Response frame, and transmits the Re-association Response frame. The RN field has the format of the RN Response field  370 . Here, the Element ID field  371  is set to 33, and the removed RN is denoted in the Reservation Number field  372 . However, in the case of a failure in removal, the RN has a value of 0.  
         [0076]      FIG. 4  illustrates a method for reserving an RF link based on a generation interval of an audio frame according to the present invention.  
         [0077]     Referring to  FIG. 4 , a reserved MT (RMT)  410  transmitting audio QoS frames transmits a first audio QoS frame according to a general DCF transmission mode in which it is checked whether or not a wireless medium is used at a point of time  412  when the first audio QoS frame is desired to be transmitted, and then the first audio QoS frame is transmitted.  
         [0078]     A second point of time for transmission starts when a Reservation Interval  413  reserved in an AP has lapsed from the point of time when the first audio QoS frame is desired to be transmitted (i.e. when audio data is generated). Therefore, the Reservation Interval  413  to be set for the header of the first audio frame has a value of the Reservation Interval reserved in the AP.  
         [0079]     Here, when a transmission delay is caused by backoff during the first audio QoS frame transmission using the DCF, the set value of the Reservation Interval  413  has no less than a difference of the delayed time.  
         [0080]     The other audio QoS frames starting from a second audio QoS frame have the Reservation Interval  413  set to the same value as the Reservation Interval reserved in the AP, and can be immediately transmitted without a backoff process.  
         [0081]     When the final audio QoS frame is transmitted, the value of the Reservation Interval  413  is set to 0.  
         [0082]     When a general MT  420  that does not transmit the audio QoS frame receives the audio QoS frame, it sets the value of a Network Allocation Vector (NAV)  422  as in the DCF and waits for the next transmission. And, the MT  420  activates an RTimer  423  with reference to the value of a Reservation Interval  413  included in a header.  
         [0083]     When the NAV  422  is terminated, the MT  420  starts transmitting desired data according to the DCF (denoted by reference number  424 ).  
         [0084]     When a Desired Transmission Time  427  required for next data transmission is more than a Remained Transmission Time  426 , the MT  420  gives up transmission.  
         [0085]      FIG. 5  illustrates a format of a frame used in an audio frame exchange process according to the present invention.  
         [0086]     Referring to  FIG. 5 , the frame additionally has a Reservation Interval field  510  denoting a reservation interval in a data frame format of the IEEE 802.11 standard. The Reservation Interval field  510  denotes a remaining time until the next reservation frame is transmitted.  
         [0087]     As described above, the present invention makes use of the RF link based on the PMRT scheme. According to the PMRT scheme, a terminal requested to transmit an audio frame informs an AP of both a reservation interval and a section, and is assigned an RN, sets the Reservation Interval field in a frame header at a point of time when the audio frame needs to be transmitted, and transmits the audio frame. The audio frame including such a Reservation Interval field is called the “audio QoS frame.” 
         [0088]     When receiving such an audio QoS frame, all wireless terminals of the same BSS set an RTimer according to a value of the Reservation Interval field, and avoid transmission at a reserved time according to the RTimer.  
         [0089]     As described above, the periodic media reservation method for the QoS data having the periodic transmission characteristic in the WLAN according to the present invention can transmit the audio frame without interference at a determined point of time. Therefore, it is possible to improve the QoS in the wireless transmission section by reducing the frame delay and jitter.  
         [0090]     In addition, the periodic media reservation method reduces the transmission delay and jitter of the audio frame of the RF link section in the VoIP system having the WLAN function, so that it can improve the QoS of the RF link section.  
         [0091]     While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.