Patent Publication Number: US-2009225669-A1

Title: System and method for wireless communication network having round trip time test

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to wireless transmission in a wireless network, and in particular, to content protection for wirelessly transmitted video data based on proximity control. 
     2. Description of the Related Art 
     For copyrighted content transmission via wireless media, the content provider is typically subject to proximity control requirements so that the wirelessly transmitted content, e.g., video or music, is used within a limited range of area. An essential element to any proximity control scheme is the measurement of distance between two devices, e.g., a Source and a Destination. A Round Trip Time (RTT) test is one method used to measure the Source-to-Destination distance. Round trip time is the duration measured from when a message is transmitted by a Source until when a response from a Destination is received by the Source. Only when the RTT test is passed, authorization keys are exchanged and the content is transmitted between the devices. For example, in the Digital Transmission Content Protection (DTCP) protocol, which is widely used for point-to-point communication (e.g., between a DVD player and a digital television), the RTT needs to be less than 7 ms. 
     Certain existing proximity control protocols, such as the DTCP, are designed for point-to-point wired communication. In a point-to-point wired communication, the two communicating devices are largely free from interferences from other devices. However, for wireless communication networks, e.g., a WLAN, where multiple users are accessing the same medium, interferences from other devices in the network are commonplace; and, consequently, the RTT test between Source and Destination may fail because of random access delays caused by the interferences from other devices. The resulting retransmissions introduce delays, and in some instances, devices within an allowed proximity can be falsely refused. Accordingly, proximity control schemes that are designed for use in wireless communication networks are desirable. 
     SUMMARY OF THE INVENTION 
     The system, method, and devices of the invention each have several aspects, no single one of which is solely responsible for its desirable attributes. Without limiting the scope of this invention as expressed by the claims which follow, its more prominent features will now be discussed briefly. 
     In one embodiment, there is a method of providing a proximity control between devices in a wireless network, the method comprising transmitting a test message for a round trip time (RTT) test to a second device by a first device via a modified reservation function, wherein the modified reservation function is based on a distributed media access control scheme; receiving a response message for the RTT test from the second device via the modified reservation function; and computing a distance between the first device and the second device based at least partly on a round trip time delay between the test message and the response message. 
     In another embodiment, there is a system for providing a proximity control between devices in a wireless network, the system comprising a transmitter configured to transmit a test message for a Round Trip Time (RTT) test to a second device by a first device via a modified reservation function, wherein the modified reservation function is modified from a standard reservation function based on a distributed media access control scheme to utilize the RTT test; a receiver configured to receive a response message for the RTT test from the second device via the modified reservation function; and a processor configured to compute a distance between the first device and the second device based at least partly on a round trip time delay between the test message and the response message. 
     In another embodiment, there is a method of providing a proximity control between devices in a wireless network, the method comprising transmitting a test message for a Round Trip Time (RTT) test to a second device by a first device via a modified reservation function, wherein the modified reservation function is based on a polling-based centralized media access control scheme; receiving a response message for the RTT test from the second device via the modified reservation function; and computing a distance between the first device and the second device based at least partly on a round trip time delay between the test message and the response message. 
     In another embodiment, there is a system for providing a proximity control between devices in a wireless network for transmitting uncompressed video data, the system comprising a transmitter configured to transmit a test message for a Round Trip Time (RTT) test to a second device by a first device via a modified reservation function, wherein the modified reservation function is modified from a standard reservation function based on a centralized media access control scheme to utilize the RTT test; a receiver configured to receive a response message for the RTT test from the second device via the modified reservation function; and a processor configured to compute a distance between the first device and the second device based at least partly on a round trip time delay between the test message and the response message. 
     In another embodiment, there is a computer-usable medium in a system for providing a proximity control between devices in a wireless network having computer readable code comprising instructions for modifying a reservation function so as to support a Round (RTT) test between a first device and a second device in a wireless network, and computing a distance between the first device and the second device based at least partly on a round trip time delay between a RTT test message and a RTT response message transmitted and received via the modified reservation function. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating an example wireless local area network (WLAN) configuration using an access point. 
         FIG. 2  is a functional block diagram of an example wireless local area network (WLAN) system, according to one embodiment of the system and method. 
         FIG. 3A  is a timing diagram illustrating an example of a Round Trip Time (RTT) test procedure between a Source and a Destination. 
         FIG. 3B  is a flow diagram illustrating an example process for conducting a RTT test for proximity control using a reservation function in a WLAN according to certain embodiments. 
         FIG. 4  is a flow diagram illustrating an example process for preparing the DCF function of IEEE 802.11 for a RTT test according to certain embodiments. 
         FIG. 5  is a timing diagram illustrating an example of the existing distributed coordinated function (DCF) defined within IEEE 802.11. 
         FIG. 6  is a timing diagram illustrating an example of RTT test support for the DCF function according to certain embodiments. 
         FIG. 7  is a timing diagram illustrating another example of RTT test support for the DCF function according to certain embodiments. 
         FIG. 8  is a flow diagram illustrating an example process for preparing the point coordinated function (PCF) of IEEE 802.11 for RTT test according to certain embodiments. 
         FIG. 9  is a timing diagram illustrating an example of the existing point coordinated function (PCF) defined within the IEEE 802.11. 
         FIG. 10  is a timing diagram illustrating an example of RTT test support for the PCF function according to certain embodiments. 
         FIG. 11  is a timing diagram illustrating another example of RTT test support for the PCF function according to certain embodiments. 
     
    
    
     DETAILED DESCRIPTION OF CERTAIN INVENTIVE EMBODIMENTS 
     Certain embodiments provide a method and system for proximity control in a wireless communications network. 
     The following detailed description is directed to certain sample embodiments of the invention. However, the invention can be embodied in a multitude of different ways as defined and covered by the claims. In this description, reference is made to the drawings wherein like parts are designated with like numerals throughout. 
     Embodiments include systems and methods of proximity control of transmission of copyright protected audio/video (A/V) contents in a wireless communications network will be described. In particular, certain embodiments representing proximity control schemes based on a Round Trip Time (RTT) test at the media access control (MAC) layer based on wireless protocols, e.g., IEEE 802.11, will be described. Also certain embodiments representing other proximity control schemes based on an authorization token will be described. 
     Exemplary implementations of the embodiments in a wireless local area network (WLAN) will now be described.  FIG. 1  is a block diagram illustrating an example wireless local area network (WLAN) configuration  100  using an access point  101 . The example WLAN configuration  100  includes one or more access points  101  and one or more wireless devices  103 . The access point is a device that connects a wireless device  103  to the WLAN. In an access point based topology, the access points are connected to a wired or wireless backbone and routes the traffic through the wired (e.g., Ethernet or token ring) or wireless backbone. In a WLAN, the access point enables a wireless device  103  to communicate with other wired or wireless devices on the WLAN. In certain embodiments, the access point supports the IEEE 802.11 Protocol. 
     In certain embodiments, the access point  101  includes a receiver of wireless signals, and the wireless client device  103  includes a sender of the wireless signals. In other embodiments, the access point  101  includes a sender of wireless signals, and the wireless client device  103  includes a receiver of the wireless signals. In some of such embodiments, the wireless signals include audio content. In other embodiments, the wireless signals include video content. In yet other embodiment, the wireless signals include text content such as a publication. For example, the access point  101  can be a sink of video and/or audio data implemented, such as, in an HDTV set in a home wireless network environment which is a type of WLAN. The wireless device  103  can be a source of compressed or uncompressed video or audio. Examples of the wireless device  103  include a set-top box, a DVD player or recorder, a digital camera, a camcorder, and so forth. In some embodiments, the content can be protected content. 
       FIG. 2  illustrates a functional block diagram of an example wireless local area network (WLAN) system  200 . The example WLAN system  200  includes a wireless transmitter  202  and wireless receiver  204 . The transmitter  202  includes a physical (PHY) layer  206 , a media access control (MAC) layer  208  and an application layer  210 . Similarly, the receiver  204  includes a PHY layer  214 , a MAC layer  216 , and an application layer  218 . The PHY layers provide wireless communication between the transmitter  202  and the receiver  204  via one or more antennas through a wireless medium  201 . 
     The application layer  210  of the transmitter  202  includes a content pre-processing module  211  and a content control module  212 . The content pre-processing module  211  can perform pre-processing of the content such as partitioning of video content. The content control module  212  provides a standard way to exchange content handling information such as A/V capability information, For example, before a connection begins, the content control module negotiates the A/V formats to be used, and when the need for the connection is completed, content control commands are used to stop the connection. 
     In the transmitter  202 , the PHY layer  206  communicates with the MAC layer  208  and with a radio frequency (RF) module  207 . In certain embodiments, the MAC layer  208  can include a packetization module (not shown). The PHY/MAC layers of the transmitter  202  add PHY and MAC headers to packets and transmit the packets to the receiver  204  over the wireless channel  201 . 
     In the wireless receiver  204 , the PHY/MAC layers  214 ,  216  process the received packets. The PHY layer  214  communicates with the MAC layer  216  and with the RF module  213 . The application layer  218  of the receiver  204  includes a content post-processing module  219  and a content control module  220 . The module  219  can perform an inverse processing method of the module  211 , for example, to regenerate the uncompressed video, for example. In certain embodiments, the content control module  220  operates in a complementary way with the content control module  212  of the transmitter  202 . 
       FIG. 3A  is a timing diagram illustrating an example of a Round Trip Time (RTT) test procedure  300  between a Source  301  and a Destination  303 . The procedure  300  includes an authentication procedure and the RTT test itself. In certain proximity protocols, the RTT test needs to be authenticated so that the system will not be compromised by malicious users. During the authentication procedure, authentication messages—a RTT initialization message  310  and a RTT setup message  320 —are used to exchange authentication information between the Source  301  and the Destination  303 . These messages can be transmitted through a random access period or through reservation. In wireless communication networks, e.g., WLAN, where multiple users are accessing the same medium, interferences from other devices in the network are commonplace; and, consequently, the actual RTT test involving transmission and reception of the RTT test data  330  and the RTT response data  340 , respectively, may be delayed because of random access delays caused by the interferences from other devices. For example, if the authentication messages  310  and  320  are exchanged in the random access period, and the first transmission  330  failed because of interferences, second or third trials can be made. Because the RTT test is used to measure the round trip time, retransmissions introduce delays; and the devices in each other&#39;s proximity may be falsely refused. Certain embodiments of proximity control schemes designed to reduce interferences from other devices during RTT test are described below. 
     Some wireless protocols, e.g., IEEE 802.11, define and support reservation functions to avoid having stations transmit at the same time with the goal of reducing interferences and retransmissions in a wireless network, e.g., WLAN. In the IEEE 802.11 and 802.11e protocols, the defined reservation functions include Distributed Coordinated Function (DCF), Point Coordinated Function (PCF), and Hybrid Coordinated Function (HCF). Because these reservation functions are designed to avoid interferences from other devices during data transmission between two devices, they may be used as platforms for launching the RTT test for proximity control. Certain embodiments of RTT test supports for the 802.11/802.11e defined reservation functions, e.g., DCF, PCF, HCF, are described below. 
       FIG. 3B  is a general flow diagram illustrating a process  350  for conducting a RTT test for proximity control using a reservation function in WLAN. Specific examples of systems and methods for conducting the RTT test are described in detail below. The process  350  starts at state  351 , where a reservation function is prepared for RTT test between an RTT Source  301  and an RTT Destination  303 , such as the ones shown in  FIG. 3A . The RTT test preparation can be different for different reservation functions and will be described in detail below with respect to individual reservation functions. The process moves to state  353 , where RTT test data is transmitted by the RTT Source during a contention-free period (CFP). The contention-free period refers to a time period in which transmissions by one or more devices in the WLAN other than the RTT Source and the RTT Destination are suppressed or disabled for reducing interferences. The process then moves to state  355 , where RTT response data is transmitted by the RTT Destination also during a CFP. The process then moves to state  357 , where the RTT response data is received by the RTT Source also during a CFP. The process then moves to state  359 , where distance between the RTT Source and the RTT Destination is computed for the purpose of proximity control of copyrighted content. 
     RTT Test Based on DCF 
     DCF is a reservation function that is fundamental to the IEEE 802.11 WLAN standard and is a form of a carrier sense multiple access with collision avoidance (CSMA/CA) protocol. CSMA/CA protocol requires stations in a wireless network to first sense the medium before transmitting. CSMA/CA, in turn, is a form of distributed media access control scheme for a distributed/ad hoc topology. Here, the distributed media access control scheme refers to techniques for accessing the media by individual devices without involving a central control device, e.g., a coordinator.  FIG. 4  is a flow diagram illustrating an example process  400  for preparing the DCF function of IEEE 802.11 for a RTT test according to certain embodiments. The process  400  begins at state  410 , where an RTT Source transmits a Request-To-Send (RTS) message. The process moves to state  420 , where an RTT Destination, after receiving the RTS, transmits a Clear-To-Send (CTS) message. The process then moves to state  430 , where after receiving either the RTS from the RTT Source or the CTS from the RTT Destination, one or more devices other than the RTT Source and the RTT Destination set a Network Allocation Vector (NAV) and stop transmissions. The NAV is used within a WLAN to prevent devices accessing the wireless medium and causing contention. In certain embodiments, the NAV is an indicator, maintained by each device, of time periods when transmission will not be initiated even though there is no traffic on the medium. 
       FIG. 5  is a timing diagram  500  illustrating an example of the existing distributed coordinated function (DCF) defined within the IEEE 802.11 and included for the purpose of illustrating certain embodiments of the RTT test support for the DCF function described below with reference to  FIG. 6  and  FIG. 7 . The 802.11 standard makes it mandatory that all stations implement the DCF, a form of carrier sense multiple access with collision avoidance (CSMA/CA). Schemes to support RTT test using the DCF function are described below. In DCF, a data communication  516  from a Source to a Destination takes place during a contention free period (CFP)  525 . The CFP condition is created when one or more devices other than the Source and the Destination receive either Request-To-Send (RTS)  512  transmitted by the Source or Clear-To-Send (CTS)  514  transmitted by the Destination and maintain a NAY  521 ,  523  and stop contending for data transmissions. The CFP expires after the Destination transmits an acknowledgement (ACK)  518 , and a contention period (CP)  527  and a corresponding contention window  530  follows. 
       FIG. 6  is a timing diagram  500  illustrating an example of RTT test support for the DCF function according to certain embodiments. To support a RTT test, modifications need to be made to existing DCF functions such as the one illustrated in  FIG. 5 . For example, in IEEE 802.11, in RTS and CTS frame formats, there can be two fields: type (control) 2 bits and subtype 4 bits. The fields are combined to indicate which type of control these RTS and CTS are for. To support RTT test, these bits are used to indicate that these RTS and CTS are for RTT control. In certain embodiments, a duration field of the RTS or CTS is also set accordingly for RTT control, so that the corresponding NAV is set specially for RTT test. In certain embodiments, the duration field includes a suggested time duration for the NAV. The reservation of DCF for RTT test is used for both directions of transmissions. Returning to  FIG. 6 , the RTT Source  301  transmits the RTS  512  after the wireless medium has been idle for a first DCF inter-frame space (DIFS) period  511 . After receiving the RTS  512 , the RTT Destination  303  transmits the CTS  514  following a first short inter-frame space (SIFS)  513 . After receiving the CTS, the RTT Source transmits a RTT test data  616  following a second SIFS  515 . After receiving the RTT test data  616 , the RTT Destination transmits a RTT response data  618  following a third SIFS  517 . 
     With regards to the other devices  305  in the WLAN, they can maintain the NAV and stop contending channels for data transmissions in one of two ways. The first way is where the other devices maintain a RTS-triggered NAV  631  after receiving the RTS  512  transmitted by the RTT Source. The maintenance of the RTS-triggered NAV  631  can mark the beginning of a contention free period (CFP)  635 . The second way is where the other devices maintain a CTS-triggered NAV  633  after receiving the CTS message  514  transmitted by the RTT Destination. In the example embodiment shown, the RTS-triggered NAV  631  is set as 3*SIFS+CTS+2*DATA, while the CTS-triggered NAV  633  is set for a time period as 2*SIFS+2*DATA. After the period indicated by the NAV is past, the defer access period  635  finally expires following a second DIFS period  519 . After the expiration of the CFP  635 , a contention period (CP)  637  returns during which all devices in the WLAN are allowed to contend a channel to initiate transmissions. 
       FIG. 7  is a timing diagram  700  illustrating another example of RTT test support for the DCF function according to certain embodiments. The RTT Source  301  transmits the RTS  512  after the wireless medium has been idle for a first DCF inter-frame space (DIFS) period  511 . After receiving the RTS message  512 , the RTT Destination  302  transmits the CTS  514  following a first short inter-frame space (SIFS)  513 . After receiving the CTS, the RTS Source transmits a RTT test data  716  following a second SIFS  515 . After receiving the RTT test data  716 , the RTT Destination transmits an Acknowledgement (ACK)  518  following a third SIFS  517 . Immediately following the ACK  518 , the RTT Destination transmits a RTT response data  721 . After receiving the ACK message and the RTT response data, the RTT Source transmits its own ACK message  725  following a fourth SIFS  723 . In the example embodiment shown, the RTS-triggered NAV  731  is set as 4*SIFS+CTS+2*DATA+2*ACK, while the CTS-triggered NAV  733  is set as 3*SIFS+2*DATA+2*ACK. 
     RTT Test Based on PCF 
       FIG. 8  is a flow diagram illustrating an example process  800  for preparing the PCF function of IEEE 802.11 for RTT test according to certain embodiments. The process  800  begins at state  810 , where an access point (AP) prepares a poll packet. The poll packet includes a MAC address for the polled station. In certain embodiments, the polled packet includes a MAC address for the RTT Source. In certain other embodiments, the poll packet includes MAC addresses for both the RTT Source and the RTT Destination. The process moves to state  820 , where the AP transmits the poll packet. The process then moves to state  830 , where the poll packet is received by the polled station. The process  800  completes at an end state. 
       FIG. 9  is a timing diagram illustrating an example of the existing point coordinated function (PCF) defined within IEEE 802.11 and included for the purpose of illustrating certain embodiments of the RTT test support for the PCF function described below with reference to  FIG. 10  and  FIG. 11 . The original 802.11 protocol defines another reservation function called the Point Coordinated Function (PCF). The PCF is available only in “infrastructure” mode, where stations are connected to the network through an Access Point (AP). In the example, the AP is the coordinator in the WLAN. Unlike the DCF, which is mandatory in IEEE 802.11, the PCF mode is optional. Also unlike the DCF, which is a CSMS/CA-based reservation function based on a distributed media access control scheme, the PCF is a centralized media access control scheme involving a central control device, e.g. an access point (AP). In particular, the PCF is based on a polling-based centralized media access scheme in which the central control device, e.g., AP, polls devices to initiate a contention-free data communication between the polled devices. In the PCF mode, the AP sends beacon frames  905  at regular intervals. Between these beacon frames, the PCF defines two periods: the Contention Free Period (CFP)  901  and the Contention Period (CP)  903 . The NAV  807  is maintained throughout CFP  901 , suppressing data transmissions. In CP  903 , the DCF is simply used. In CFP  901 , the AP sends Contention Free-Poll (CF-Poll) packets  911 ,  913 ,  915 , and  917  to each station, one at a time, to give them the right to send data packets  921 ,  923 , and  927 . In the example given, no response to the third CF-Poll packet  925  was received. 
       FIG. 10  is a timing diagram  1000  illustrating an example of RTT test support for the PCF function according to certain embodiments. In the example, the AP, e.g., the coordinator, after transmitting the beacon  905 , polls a pair of stations, e.g., Device A and Device B, at one time by transmitting a CF-Poll packet  1011 . In the first CF-Poll packet  1011 , MAC addresses for the both polled stations are included. In response to the CF-Poll packet  1011 , one station transmits a first data packet  1021 . The first data packet  1021  includes RTT test data and ACK. In response to the first data packet  1021 , the other station transmits a second data packet  1023 . The second data packet includes RTT response data and ACK. After the AP receives the ACK from the RTT Destination, it can poll another pair of stations by sending another CF-Poll packet  1031 . 
       FIG. 11  is a timing diagram  1100  illustrating another example of RTT test support for the PCF function according to certain embodiments In the example, MAC addresses for a pair of stations being polled are put into different CF polling packets  1111  and  1113 . However, in some embodiments, the MAC addresses of this pair of stations can be put into the polling queue adjacent to each other. In certain embodiments, the stations are polled one after another to reduce any access delays. In the example, the CF polling packets include a first CF polling packet  1111  and a second CF polling packet. In response to the first CF polling packet  1111 , one station acting as the RTT Source transmits a first data packet  1121 . The first data packet  1121  includes RTT test data and ACK. In response to the second CF polling packet  1113 , the other station acting as the RTT Destination transmits a second data packet  1123 . The second data packet includes RTT response data and ACK. The total delay for the RTT test in this example, however, can be longer than that in the example given in  FIG. 10 . Time-out is used to detect whether the packet is lost or not. 
     RTT Test Based on HCF 
     IEEE 802.11e is an approved amendment to the IEEE 802.11 standard that defines a set of Quality of Service (QoS) enhancements for WLAN applications through modifications to the MAC layer. The 802.11e standard includes a new reservation function, the Hybrid Coordinated Function (HCF), that represents an enhancement over a polling-based reservation protocol such as the PCF. For the AP and stations which have the HCF and the QoS support, the AP can allocate Transmission Opportunity (TXOP) period to support data sequence transmissions between the AP and the station. The supported sequences are specified in IEEE 802.11e. However, for the RTT test, transmission opportunities for two devices need to be reserved through the AP in certain embodiments. In order to support RTT test between device A and device B, a bidirectional RTT TXOP can be defined and used. The RTT TXOP can include a RTT test data frame (from device A) and a subsequent RTT response data (from device B). Within the HCF framework, a flexible period of time can also be reserved to accommodate retransmissions, for example, 7 ms or less in certain embodiments. 
     Various advantages of performing RTT test during one of the 802.11 defined functions—DCF, PCF, and HCF—described above will be apparent to the one skilled in the art. For example, the defined functions, having been designed to reduce data collisions and interferences in a wireless network, provide appropriate platforms to launch the RTT test for proximity control in WLAN. In particular, the RTT test can be implemented in the DCF and the PCF with only minor modifications to the existing defined features. In contrast, existing RTT schemes cannot support accurate RTT in a wireless network without incurring long random access delays. 
     Although the systems and methods for RTT test supports for proximity control are described above with regards to specific IEEE 802.11/802.11e functions, it will be apparent to the one skilled in the art that the systems and methods can be applied to other wireless protocols, such as Bluetooth. 
     CONCLUSION 
     While the above detailed description has shown, described, and pointed out the fundamental novel features of the invention as applied to various embodiments, it will be understood that various omissions and substitutions and changes in the form and details of the system illustrated may be made by those skilled in the art, without departing from the intent of the invention.