Abstract:
The present invention provides a method of controlling Wireless LAN (WLAN) medium access using Pseudo-Time Division Multiplexing (PTDM) to improve the Quality of Service (QoS) of the WLAN. The method includes the first step of a Mobile Terminal (MT) acquiring QoS information, which relates to a voice frame to be transmitted, from an upper layer; the second step of the MT exchanging a frame, including the QoS information, with the AP and being allocated QoS slot (QSLOT) information by the AP; the third step of dividing an RF link section between the MT and the AP by the creation period of the voice frame; and the fourth step of transmitting the voice frame in the creation period of the voice frame using the allocated QSLOT information.

Description:
RELATED APPLICATIONS 
   The present application is based on, and claims priority from, Korean Application Number 2004-0094254, filed Nov. 17, 2004, the disclosure of which is incorporated by reference herein in its entirety. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to a method of controlling wireless local area network medium access and, more particularly, to a method of controlling wireless local area network medium access using pseudo-time division multiplexing to improve the quality of service of Voice over Internet Protocol. 
   2. Description of the Prior Art 
   In general, a Wireless Local Area Network (WLAN) operates within 100 m in a transmission rate range of 10 to 100 Mbps. A WLAN composed of a single cell can be used in a single floor office or store. A WLAN terminal is connected to another terminal and an Access Point (AP) on a network via a Radio Frequency (RF) link using a wireless Network Interface Card (NIC). 
   An AP enables a WLAN terminal to access a wired network via a backbone network. Approximately 25 terminals are connected to a single cell. A multiple cell can be constructed using a plurality of APs that are connected to a wired network, and a WLAN environment can be built throughout the entire building using the multiple cell. 
   WLAN-related Institute of Electrical and Electronics Engineers (IEEE) standards are described below. 
   The IEEE developed a standard that defines a protocol regarding the transmission of a data frame between a WLAN terminal and an AP and, as a result, establishes a standard regarding Medium Access Control (MAC) and a Physical (PHY) layer (IEEE Std. 802.11, IEEE standard for Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY), 1999). 
   The IEEE 802.11 WLAN standard describes a mobile terminal and a fixed AP that are the two principal elements of a WLAN. A single cell using the IEEE 802.11 WLAN standard is defined as a Basic Service Set (BSS), and a multiple cell is defined as an Extended Service Set (ESS). 
   In the IEEE 802.11 WLAN standard, each terminal and each AP implements a MAC layer having a function capable of a MAC frame. The MAC frame is used as a medium for transmitting control and management data. 
   The IEEE 802.11 WLAN standard defines two different wireless medium access methods in the MAC layer: a Distributed Coordination Function (DCF) and a Point Coordination Function (PCF). 
   In the DCF, all the stations can participate in contention for the transmission of a frame. The basic access method of the 802.11 MAC is Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). In CSMA/CA, a station that intends to transmit data to the wireless medium of a WLAN detects a medium to determine whether data transmission from some other station exists. If the medium is unoccupied, data transmission is performed; otherwise data transmission is delayed until ongoing data transmission is completed. 
   If data transmission from the station can be performed immediately after previous data transmission has been completed, transmission attempts from a plurality of stations may occur and, therefore, there is a high probability of data collision. In order to solve the problem, after a certain pause period is provided after the completion of data transmission, the size of a Contention Window (CW) is determined by performing binary random backoff, and a station that has the smallest determined CW size is provided with an opportunity to perform transmission. This process is called a Collision Avoidance (CA) function. 
   Meanwhile, in the PCF, a Point Coordinator (PC) controls transmission from WLAN terminals. The PC functions as a polling master, and polls all the PCF polling-capable terminals to determine the terminal that can perform data transmission. The PC may exist in the AP. In the PCF, a terminal may be capable of polling or not. 
   If a polling-capable terminal receives a poll from the PC, only a single MAC Protocol Data Unit (MPDU) can perform transmission. When additional transmission is required, waiting must be performed until a poll is received again. If specific data transmission is abnormally performed, a terminal may not retransmit until it receives a poll from the PC. Accordingly, the PCF provides a contention free mechanism to provide an opportunity for a terminal to normally transmit data. 
   Another wireless medium access method, that is, an Enhanced-DCF (EDCF) that conforms to IEEE 802.11e, intends to improve Quality of Service (QoS) by adjusting a CW. During a CW period, a plurality of stations contends for network access. In order to avoid a collision, a MAC protocol requests individual stations to wait for the CW period that is determined by binary random backoff. A probability of collision between stations is reduced due to the CW period that is determined by the binary random backoff. In this case, the EDCF employs the CW to grant higher priority to a specific station. Higher priority is granted to the specific station by providing a short CW to the specific station. As a result, in most cases, a higher priority station transmits data earlier than a lower priority station. 
   QoS is a measurement of service quality for a user. Principal measurements of QoS include message loss, message delay and network availability. The transmission of time-sensitive data application traffic (such as voice or video) on a packet network requires conditions that satisfy delay, delay jitter and error rate requirements. 
   When the wireless medium access methods DCF, PCF and EDCF are examined in terms of QoS, the DCF causes frame delay because it performs binary random backoff before the transmission of a frame, the EDCA supplementing the DCF can grant priority by providing a voice frame with a CW shorter than that which is provided to a data frame but causes frame delay because it still performs backoff, and the PCF causes frame delay because a WLAN terminal can transmit a frame only through polling during a contention free period. 
   In the meantime, U.S. Pat. No. 6,747,968 B1 entitled “Method and system for weighted PCK polling lists for WLAN QoS support” discloses a method and system for providing weights when an AP polls terminals on a WLAN. U.S. Pat Publ. No. 2004/0081133 A1 entitled “Method of communication device initiated frame exchange” discloses a method for accessing a WLAN channel and providing QoS for voice in a system supporting both voice and data services. However, the preceding patents are problematic in that they have limitations in ensuring a sufficient bandwidth for an RF link due to the performance of polling and transmission delay time is excessively long due to channel contention and polling. 
   SUMMARY OF THE INVENTION 
   The present invention provides a method of controlling WLAN medium access using PTDM, which is capable of improving QoS by reducing transmission delay without performing backoff and polling when the voice frame of an RF link section is transmitted in a VoIP system having a WLAN function. 
   The present invention provides a method of controlling WLAN medium access using PTDM to improve the QoS of a VoIP system to which a WLAN, which includes a BSS having a plurality of Mobile Terminals (MTs) and an Access Point (AP) that are connected via an RF link, is applied, including the first step of a Mobile Terminal (MT) acquiring QoS information, which relates to a voice frame to be transmitted, from an upper layer; the second step of the MT exchanging a frame, including the QoS information, with the AP and being allocated QoS slot (QSLOT) information by the AP; the third step of dividing an RF link section between the MT and the AP by the creation period of the voice frame; and the fourth step of transmitting the voice frame in the creation period of the voice frame using the allocated QSLOT information. 
   The method may further include the steps of the MT waiting for reception of an ACK from the AP after an SIFS; and terminating the transmission of the voice frame if the MT receives the ACK from the AP, and retransmitting the voice frame through contention in a CP after an CFP if the MT does not receive the ACK from the AP. 
   The present invention provides a method of controlling WLAN medium access using PTDM to improve the QoS of Voice over Internet Protocol. In a PTDM scheme, an RF link is divided to correspond to the creation period of a voice frame, and a QoS slot (QSLOT) is allocated to an MT in each period, thus allowing the MT to transmit a voice frame. The QSLOT includes a voice frame, an acknowledgement (ACK), and an Inter-Frame-Space (IFS). In the QSLOT, only a corresponding MT can transmit a voice frame without contention. As described above, in accordance with the present invention, a voice frame can be transmitted at the scheduled time, so that QoS can be improved in an RF link section by reducing frame delay and jitter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a configuration diagram of a WLAN system that performs a QoS function on a voice frame in an RF link section in accordance with an embodiment of the present invention; 
       FIGS. 2   a  to  2   c  are diagrams showing processes of exchanging QoS frames to manage a QSLOT in accordance with the present invention; 
       FIG. 3  is a diagram showing the formats of frames that are used to perform the QoS frame exchange processes according to an embodiment of the present invention; 
       FIG. 4  is a diagram showing the division of an RF link  160  according to the creating period of a voice frame in accordance with an embodiment of the present invention; 
       FIG. 5  is a diagram showing a process of transmitting a voice frame using a QSLOT in the creation period of a voice frame based on PTDM in accordance with an embodiment of the present invention; and 
       FIG. 6  is a diagram showing a process in which an AP transmits a voice frame between the WLAN terminals of the same BSS. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference now should be made to the drawings, in which the same reference numerals are used throughout the different drawings to designate the same or similar components. 
     FIG. 1  is a configuration diagram of a WLAN system that performs a QoS function on a voice frame in an RF link section in accordance with an embodiment of the present invention.  FIG. 1  schematically illustrates an example of the WLAN system according to the present invention. Those skilled in the art can understand that other elements are included in the WLAN system. 
   Referring to  FIG. 1 , a BSS  100  includes a plurality of Mobile Terminals (MTs)  120  and  130  and a single AP  110 , and constitutes a WLAN. The MTs  120  and  130  are connected to the AP  100  via an RF link. The WLAN is connected to an Internet Backbone (IB)  150  via an Internet Protocol (IP) router  140 . In this case, data is transmitted via an upstream path extending from the MTs  120  and  130  through the AP  110  to the IB  150  and via a downstream path extending from the IB  150  through the AP  110  to the MTs  120  and  130 . The MTs  120  and  130  and the AP  110  employ DCF modules  111 ,  121  and  131  conforming to the IEEE 802.11-1999 standard and PTDM modules  112 ,  122  and  131  conforming to the present invention. The MTs  120  and  130  existing in the BSS  100  are equipped with voice codecs having the same creation period of a frame. 
   The MTs  120  and  130  possess QSLOT managers  123  and  133 , respectively, and manage information on the creation period of a voice frame and QSLOT information. The AP  110  possesses a QSLOT manager  114  and a QSLOT list  113 , and manages the QSLOTs of the MTs  120  and  130  that are connected to the AP  110 . 
   In  FIG. 1 , the MTs  120  and  130  exchange frames, including QoS information, with the AP  110  before the transmission of a voice frame, are assigned QSLOTs through which voice frames can be transmitted, and then transmits the voice frames. The frame exchange process and the transmission of a voice frame using a QSLOT are described with reference to  FIG. 2  and  FIGS. 4 ,  5  and  6 , respectively. 
     FIGS. 2   a  to  2   c  are diagrams showing processes of exchanging QoS frames to manage a QSLOT in accordance with the present invention. As shown in  FIGS. 2   a  to  2   c , a QSLOT management mode according to the present invention includes QSLOT setting  210  shown in  FIG. 2   a , QSLOT change  220  shown in  FIG. 2   b , and QSLOT removal shown in  FIG. 2   c.    
   With reference to  FIG. 2   a , a QoS frame exchange process for the QSLOT setting  210  according to the present invention is described. Referring to  FIG. 2   a , when there is a voice frame to be transmitted, the MT  211  transmits a QSLOT Request To Send (QRTS) frame to the AP  212  at step  213 . The AP  212 , having received the QRTS frame, transmits a QSLOT Grant To Send (QGTS) frame, including a QSLOT Number (QN)  332  (shown in  FIG. 3 ), to the MT  211  according to a set QSLOT list  113  at step  214 . In this case, the QN is the unique number of a QSLOT that is assigned to the MT  211 . The MT  211 , having been assigned the QN  316  by the AP  212 , waits for the reception of a QoS beacon (Q-beacon) from the AP  212 , and extracts QSLOT information from the Q-beacon and transmits the voice frame to the AP  212  in a corresponding QSLOT when the Q-beacon  215  is received from the AP  212  at step  215 , at step  216 . The AP  212 , having received the voice frame from the MT  211 , terminates the frame exchange by transmitting an ACK frame at step  217 . The transmission of the voice frame in the QSLOT is described in detail with reference to  FIG. 4 . 
   With reference to  FIG. 2   b , a QoS frame exchange process for the QSLOT change  220  according to the present invention is described. Referring to  FIG. 2   b , when the AP  222  intends to change the QN of the MT  221  to keep the QSLOT list  113  minimized, the AP  222  transmits a QSLOT Change Request To Send (QCRTS) frame to the MT  221  at step  223 . The MT  221 , having received the QCRTS frame, updates the QN and transmits an ACK to the AP  222  at step  224 . 
   With reference to  FIG. 2   c , a QoS frame exchange process for the QSLOT removal  230  according to the present invention is described. Referring to  FIG. 2   c , after completing the transmission of the voice frame, the MT  231  transmits a QSLOT Remove Request To Send (QRRTS) frame, which is used to remove the QN from the QSLOT list  113  that is located in the AP  232 , to the AP  232  at step  233 . The AP  233 , having received the QRRTS frame, removes the QSLOT list of the MT from the QSLOT list  113  and then transmits an ACK to the MT  231  at step  234 . 
     FIG. 3  is a diagram showing the formats of frames that are used to perform the QoS frame exchange processes according to an embodiment of the present invention. Referring to  FIG. 3 , a QoS beacon (Q-beacon)  310  includes a beacon frame, which is used for IEEE 802.11, and an additional information field  311 . The information field  311  includes an identifier (ID), a data length (LENGTH)  313 , a QSLOT Network Allocation Vector (QNAV)  314 , a QoS Frame Transmission Interval (QTX-INT)  315 , a QSLOT number (QSLOT NUMBER: QN)  316  and a QSLOT time (QSLOT TIME: QT)  317 . In this case, the ID  312  is allocated to TDM parameter setting (TDM PARAMETER SET). The LENGTH  313  indicates the length of the information field  311  on a bit basis. The QNAV  314  indicates the length of a Contention Free Period (CFP). The QTX-INT  315  indicates the creation period of a voice frame. The QN  316  is an identification number that is allocated to an MT. The QT  317  starts from one and increases in proportion to the number of MTs that have requested QSLOTs. The QT  317  is the length of QSLOTs that corresponds to the QN  316 . When a plurality of MTs have requested QSLOTs from the AP, the QN  316  and the QT  317  exist, the number of each of which corresponds to the number of MTs. 
   A QSLOT REQUEST TO SEND (QRTS) frame  320  is used when the MT requests a QSLOT. In this case, a receiver address  321  is the AP, and a transmitter address  322  is the MT that requests the QSLOT. A QVOICE RATE  323  is transmission rate information that is used when the MT transmits a voice frame. The AP calculates the QT  317  using the QVOICE RATE. The QTX-INT  324  indicates the creation period of the voice frame of the MT. 
   A QSLOT GRANT TO SEND (QGTS) frame  330  is used when the AP allocates the QN to the MT. The receiver address  331  is the MT that transmits the QRTS frame. The QN  332  is a Q slot ID number that the MT, having requested the QSLOT, can use. 
   A QSLOT CHANGE REQUEST TO SEND (QGRTS) frame  340  is used when the AP changes the QN of the MT that is connected thereto. The AP updates the QSLOT list to keep the length of the QNAV minimized. In order to update the QSLOT list, the change of the QN of the MT may be requested. In this case, the QN  342  to be changed is inserted into the QCRTS frame  340  and transmitted to a corresponding MT. The MT, having received the QCRTS frame  340 , updates the QN. 
   A QSLOT REMOVE REQUEST TO SEND (QRRTS) frame  350  is used when the MT requests that the QN of the MT should be removed from the QSLOT list of the AP after completing the transmission of a voice frame. The QN  351  is the QSLOT ID number that the MT is currently using. 
     FIG. 4  is a diagram showing the division of an RF link  160  according to the creation period of a voice frame in accordance with an embodiment of the present invention. Referring to  FIG. 4 , the QTX-INT  410  is the time that divides two Q-beacons  420  and  430  according to the creation period of a QoS voice (QVOICE) frame (e.g., 160 bytes per 20 ms). Each QTX-INT  410  includes a CFP  411  and a Contention Period (CP)  413 , and the CFP  411  includes one or more QSLOTs  411 A,  411 B and  411 C. After the CFP  411 , a DCF Inter Frame Space (DIFS)  412 , that is, the time interval between frames that is used in the DCF, is waited for, and the CP  413  is then entered. Every MT suppresses the transmission of a frame by setting a QNAV  414  for a CFP. However, the MT, having been allocated a QSLOT, can transmit a voice frame in the QSLOT. 
     FIG. 5  is a diagram showing a process of transmitting a voice frame using a QSLOT in a QTX-INT based on the PTDM in accordance with an embodiment of the present invention. Referring to  FIG. 5 , MT 1  and MT 2  intending to transmit voice frames request QNs by transmitting QRTS frames to the AP. The AP, having received the QRTS frames  1  and  2 , allocates QSLOT  1   511  and QSLOT  2   512  to the MT 1  and the MT 2 , respectively. The MT 1  and the MT 2 , having been allocated the QNs, wait for the reception of Q-beacons  540 . The information field of each Q-beacon  540  includes a QN and QT that correspond to the MT 1  or MT 2 . QSLOTs are created in the order of QSLOT  1 , QSLOT  2 , QSLOT  3 , . . . , with the QN increasing from one by one on the basis of the Q-beacon. 
   When the Q-beacon  540  is received, every MT extracts a QNAV  550  and sets the length of a CFP  510 , the MT 1  and the MT 2  prepare for the transmission of voice frames in corresponding QSLOTs  511  and  512 , and the remaining MTs defer the transmission of frames. 
   Since the MT 1  has a QN of one, the MT 1  waits for a PIFS  511 A, which is the time interval between frames that is used in the PCF, after completing the reception of the Q-beacon. If the RF link  160  is idle after the PIFS, the MT 1  transmits a voice frame QVOICE  1   511 B. The AP, having normally received the QVOICE  1 , transmits an ACK 1   511 D and terminates the QSLOT  1  after the SIFS  512 C. 
   After the QSLOTs are all terminated (that is, the CFPs are terminated) and the DIFS  520  is waited for, an AP and MTs within the same BBS can transmit frames through contention in the CP  530  under the DCF. In the CP, all the MTs and the AP check the remaining time of the QTX-INT before the transmission of frames, and defer the transmission of frames to the next CP if the time required for the exchange of frames is longer than the remaining time of the QTX-INT. 
     FIG. 6  is a diagram showing a process in which an AP transmits a voice frame from MT 1  to MT 3  (the MT 1  and the MT 3  pertain to the same BSS). Referring to  FIG. 6 , when the QVOICE frame is transmitted from the MT 1  to the MT 2  (the MT 1  and MT 2  pertain to the same BSS), the AP stores the QVOICE  1  received from the QSLOT 1   611 , waits for the PIFS  613 A after the CFP  610  has been terminated, and then transmits the QVOICE 3   613 B (QVOICE 1  frame transferred from the MT 1  to the MT 2 ) to the MT 3 . The MT 3  normally receives the QVOICE 3  and then transmits ACK 3   613 D after the SIFS  613 C, thus completing the frame exchange process. If the AP does not transfer a QVOICE frame any further, a DIFS is waited for and a delayed CP then begins. For the MT 1  and the MT 2 , frames can be transmitted after a CFP and a DIFS, so that the AP that waits only for a PIFS acquires the right to use the RF link. 
   In the meantime, the above-described embodiments may be written in the form of a computer program that can be executed on a computer, and can be implemented in a general-purpose digital computer using a computer-readable storage medium. 
   The methods according to the present invention may be implemented on computer-readable storage media in the form of computer-readable code. The computer-readable storage media include all types of storage media in which computer system-readable data are stored. Although examples of the computer readable storage media are Read-Only Memory (ROM), Random Access Memory (RAM), Compact Disk (CD)-ROM, a magnetic tape, a floppy disk, and an optical data storage, the methods according to the present invention can be implemented in the form of transmission via the Internet. Furthermore, the computer-readable storage media are distributed throughout a computer system connected via a network, and code, which can be read by a computer in a distributed manner, can be stored and executed. 
   In accordance with the present invention, an RF link is divided according to the creation period of a voice frame using PTDM, a QSLOT is provided to each MT in each period and a voice frame is transmitted via the QSLOT, so that the voice frame can be transmitted without contention in the QSLOT, and the QoS of an RF link section can be improved due to the decrease in frame transmission time and jitter because the voice frame is transmitted in a scheduled time. 
   Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.