Abstract:
A WLAN compliant device which comprises a VCX), a MAC, a decoder which outputs video wherein said device compares time stamps to obtain a difference.

Description:
This application claims the benefit, under 35 U.S.C. §365 of International Application PCT/US04/00515, filed Jan. 9, 2004, which was published in accordance with PCT Article 21(2) on Jul. 29, 2004 in English and which claims the benefit of U.S. provisional patent application No. 60/439,092, filed Jan. 9, 2003. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a method and an apparatus for synchronizing digital transmission using beacon packets, and in particular, using beacon packets utilized in communication between a wireless access point and a wireless station operating in accordance with the IEEE 802.11 or derivative standards. 
     DESCRIPTION OF RELATED ART 
     The context of the present invention is the family of wireless local area networks or WLAN specifications developed by a working group at the Institute of Electrical and Electronic Engineers (IEEE). The IEEE 802.11 standards define conventional WLAN access points, which provide access for mobile devices to the WLAN and to other networks, such as hard wired local area networks and global networks, such as the Internet. One such wireless access point may receive input from a settop box as is utilized in conditional access broadcasting. 
       FIG. 1  illustrates an exemplary digital video and audio system suitable for implementing the present invention. At the head end a multiple video and audio content stream is converted into a digital format (typically in accordance with the MPEG-2 standard) and transmitted via satellite to a receiving dish, or other suitable means, which is attached to a broadcast receiver referred to as a cable set top box or other suitable receiving means such as a Transceiver/multiplexer/demultiplexer or TMD. U.S. Pat. No. 6,510,519, describes a representative system utilizing a headend and a set top box including tuners, de-modulators, decoders, transport de-multiplexers, microprocessors, program memories, video picture memories, MPEG video decoders, displays, and smart cards. Most digital broadcast system data streams are encoded or scrambled for security purposes and once decryption occurs, the system builds a video composite picture in memory, typically in accordance with the MPEG-2 standard, and displays the desired picture with its audio component on a monitor. In addition to descrambling the program, generally, further authorizations are provided to insure that the particular receiver has been enabled to receive a program or a set of programs. 
     As further illustrated in  FIG. 1 , the broadcast receiver or TMD or set top box (all hereinafter referred to as a set top box) may be designed and configured to further communicate with a wireless access point, which in the illustrative example provided, receives demultiplexed output including its timing signals so as to synchronize the transmission of the video and audio content. 
     The IEEE 802.11 standards define a WLAN architecture that is built around the notion of a Basic Service Set or BSS which is regarded as a basic building block. The BSS consists of a group of any number of access point stations that communicate with one another. In each independent BSS, the mobile stations communicate directly with each other. In an infrastructure BSS, all stations in the BSS communicate with the access point and no longer communicate directly with the independent BSS such that all frames are relayed between stations by the access point. The proposed invention may be used in the context of an Infrastructure BSS. 
     In order to effectively communicate in a network through a wireless connection to a wireless access point the various devices in the WLAN must be synchronized so as to particularly avoid pauses or jumps in the video presented to the display. The program content is typically stored in a forward and store buffer, where it awaits the clock signals that move it into subsystems for audio and video display. If the digital program content, typically a composite video signal, source leads, or transmits digital content signals before the destination has emptied its buffer and presented the video, the destination&#39;s forward and store buffer will overflow, causing a loss of video data. Conversely if the digital video destination store and forward buffer empties the buffer before additional data is received, or leads the digital video source, the destination&#39;s buffer can underflow leading to pauses, or freezes, in the video presentation. Synchronizing the rate at which the data is transmitted, stored, and consumed with respect to the rate at which it is produced reduces or eliminates these undesirable consequences. It is desirable to provide a method and apparatus for improving the tuning synchronization in a wireless system. 
     SUMMARY OF THE INVENTION 
     The invention provides an apparatus and a method for timing synchronization beyond the set top box through the wireless access point to the wireless station which is the final destination of the video data The IEEE 802.11 standards provide for a synchronization feature known as a beacon packet. This packet synchronizes the wireless access point with the receiving station it manages. By using the set top box&#39;s Voltage Controlled Crystal Oscillator, or VCXO, clock output as an input to the media access control, typically referred to as a MAC chip or subsystem, of the wireless access point, the timing information in the beacon packet can be adjusted to additionally compensate for variations already detected in the set top box. This beacon packet is then transmitted by the access point to all wireless stations within its basic service set or BSS. 
     The wireless station&#39;s CPU can then use this beacon information to control a local VCXO to adjust the rate at which its MAC chip consumes data from the wireless link. By adjusting the rate of the wireless station, its time base can match the rate at which the video data is produced at the Head End, thereby reducing the risk of buffer underflow/overflow. The result is smoother video stream with less risk of pausing and skipping. 
     The invention disclosed herein includes a receiver transmitter system comprising: a means for receiving a time stamp indicating the time of a video transmission; a means for determining a relative time difference between the time stamp and a previous time stamp; a means for communicating the relative time difference to a transmitter having as one feature of transmission a time base; and a means for the transmitter to adjust the time base according to the relative time difference. The transmitter in the receiver transmitter system further communicates, to one or more receivers, the adjusted the time base according to the relative time difference. Ultimately, the one or more receivers adjust a time base according to the relative time difference. 
     The present method utilizes a set top box configured to receive digital packets referred to as AUX from the Head End containing periodically inserted precise timestamps referred to as system clock reference or SCR into the video stream. At the exact time an AUX packet is detected in the set top box, a local VCXO clock is latched. By comparing the time difference between the current and last AUX packet&#39;s SCR to that of the current and last value latched in the local clock, adjustments can be made to the VCXO to synchronize the data rate with the Head End. The invention disclosed herein includes a method comprising the steps of: determining a relative time difference between the time stamp and a previous time stamp; communicating the relative time difference to a wireless access point transmitter having as one feature of transmission a time base; adjusting the time base according to the relative time difference received from the set top box; transmitting the adjustment to one or more wireless station receivers wherein each of the wireless station receivers adjust the time base according to the relative time difference. These changes in the rate of the wireless access point beacon and the affect such change has upon the wireless station VCXO have a corresponding effect on the rate at which the video data is consumed and reducing the risk of buffer underflow/overflow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is described with the following detailed description in connection with the accompanying drawings. 
         FIG. 1  is a block diagram of a conditional access system. 
         FIG. 2  is a block diagram of the invention for securing an adjusted time base. 
         FIG. 3   a  and  FIG. 3   b  is a flowchart showing the steps for securing an adjusted time base. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the figures to be discussed the circuits and associated blocks and arrows represent functions of the process according to the present invention which may be implemented as electrical circuits, and associated wires or data busses, which transport electrical signals, and/or software modules. Alternatively, one or more associated arrows may represent communication (e.g., data flow) between software routines, particularly when the present method or apparatus of the present invention is implemented as a digital process. 
     The arrangement shown in  FIG. 1  provides an overview of a service provider  100  system that supplies video programming, for example television programming according to a digital television standard. All digital broadcast system data streams contain video, audio, timing information and are encoded or scrambled for security purposes to insure only authorized subscribers can view the programs transmitted. 
     In a subscriber based digital broadcast system, the customer receives in addition to the composite video and audio information, various administrative and control messages such as entitlement control messages, which contains an exploitation key necessary to decrypt the encrypted control word necessary to decode a descrambling key so as to permit the decryption and assembling of the digital video and audio data. Once decryption occurs, the system builds a video composite picture in memory, typically in accordance with the MPEG-2 standard, and displays the desired picture on a display. 
     In accordance with  FIG. 1 , a head end  110  digitally formats video and audio content  116  in an encoder  112  which are modulated by modulator  114  so as to be transmitted from a transmitter  102  via satellite  104  to a receiving dish  106  located at a receiving end for television service to conditional access customers. 
     The receiving end typically is a digital set top box  120  which electronically connects to the receiving dish  106 . The set top box contains a demodulator  122  that demodulates the composite video and audio information, various administrative and control messages and outputs the demodulated signal to a central processing unit or CPU/demultiplexer  126  that processes the many packetized streams by routing select packets to various control, data and status subsystems. For example, typically the selected packetized video and audio stream is sent to a decoder  128  for translation into a format suitable for output to a television  150   a , such as NTSC, PAL or SECAM. 
     A VCXO  124  synchronizes the time base that provides a clock to the CPU/demultiplexer  126  which adjusts the time base for variations in timing between the video source at the head end  110  and the set top box  120 . At the head end digital packets containing precise timestamps  118  are periodically inserted into the content  116  prior to transmission  108 . These timestamps are called the System Clock Reference (SCR). 
     The present invention is directed to a broadcast  100  system that utilizes components to present a video and audio data stream to an IEEE 802.11 compliant architecture, that is a wireless local area access device as indicated by transmission  160  utilizing a wireless access point  130  and one or more wireless stations  140 . As such an IEEE 802.11 complaint system is comprised of several components, each of which contains a Medium Access Control or MAC  134 ,  142 ; and a Base Band Process or BBP  132 ,  143  responsible for at least Forward Error Correction, or FEC, to validate that the packet has not been corrupted by noise and to fix it if possible. BBP  132 ,  143 , and radio receiver/transmitters  138 ,  140  as well as services to provide station mobility transparent to the higher layers of the network stack. However, a station is any device that contains the functionality of the IEEE 802.11 protocols, that being MAC and Physical Layer or PHY, and a connection to the wireless media such as wireless station  140 . Typically, the IEEE 802.11 functions are implemented in the hardware and software of a network interface card (unshown). By way of example, the wireless station  130  connects to other wireless medium such as wireless station  140 . 
     A wireless complaint device may be representative of wireless station  140 , which may in turn depict a laptop personal computer, a handheld device, or may be representative of an access point  130  which manages wireless stations, such as wireless station  140 . Therefore, stations may be mobile, portable, or stationary and all stations that are IEEE 802.11 compliant provide for services of authentication, de-authentication, privacy, and data delivery. 
     The invention provides an apparatus and a method for timing synchronization beyond the set top box through the wireless access point  130  to the wireless station  140 , which is the destination of the video data which is decoded in decoder  147  and displayed on device  150  (b). The IEEE 802.11 standards incorporate a synchronization feature referred to as a beacon packet. This packet is used to synchronize the wireless access point  130  data streams with one or more stations it is managing, such as wireless station  140 . By using the set top box&#39;s  120  VCXO  124  clock output as an input to the wireless access control  130  input, shown as a MAC chip  134  of the wireless access point  130 , the timing information in the beacon packet can be adjusted to additionally compensate for variations already detected in the set top box  120 . This beacon packet is then transmitted via radio transmitter  138  of wireless access point  130  to all wireless stations capable and configured to receive, such as wireless station  140  and corresponding radio receiver  148 , within its basic service set. 
     The wireless station&#39;s  140  CPU  146  can then use this beacon information to control a local VCXO  144  to adjust the rate at which its MAC chip  142  consumes data from the wireless link  160 . By adjusting the beacon information by the relative time difference determined by the set top box  120 , the wireless station  140  can match its time base to the rate at which the video data is produced at the head end, thereby reducing the risk of buffer underflow/overflow. The result is smoother video with less risk of pausing and skipping. 
     Referring to  FIG. 1  and  FIG. 2  the invention disclosed herein includes one or more devices  220 ,  230  each comprising: a means for receiving a time stamp indicating the time of a video transmission; a means  224 ,  226  for determining a relative time difference between the time stamp  118  at time t=t 0  and a previous time stamp  118  at time t=t −1 ; a means  224  for communicating the relative time difference to a transmitter  230  having as one feature of transmission a time base. The transmitter  230  includes means to adjust the received time base in according to the relative time difference. The transmitter  230  further communicates to one or more receivers  240 ( 1 ),  240 ( 2 ), through  240 ( n ) the adjusted time base according to the relative time difference. Ultimately, the one or more receivers adjust each corresponding time base according to the relative time difference in order to synchronize the data rate with the transmitted program  208  transmission rate. 
     Referring to  FIG. 2 , the invention disclosed herein includes a receiver  220  comprising: a means for receiving a transmitted program  208 ; a means for receiving a transmitted time base  224 ,  226 ; a means for comparing the time difference between the current and last AUX packet&#39;s SCR to that of the current and last value latched in the local clock  224 , a means to adjust the time base  225  and thereby synchronize the data rate with the transmitted program  208 ; and a means to communicate the adjusted time base  225  to a means  230  in order to synchronize the data rate with the transmitted program  208 . 
     Referring again to  FIG. 2 , the invention disclosed herein includes a receiver  230  comprising: a means for receiving an adjusted time base  225  and thereby synchronizing the data rate with the transmitted program  208 ; and a means to communicate the adjusted time base  225  to one or more receiving a means  240 ( 1 ),  240 ( 2 ), through  240 ( n ), in order to synchronize the data rate with the transmitted program  208 . 
     A method of the present invention is best understood with reference to  FIG. 2  and  FIG. 3   a  where—in the set top box  220  receives digital packets imbedded in the transmitted program  208  referred to as AUX that are periodically inserted  312  into the video stream. The local VCXO  224  counter having been previously started  310 , is latched  316  at the exact time the AUX packet is detected  314  in the set top box  220 . By comparing the time difference between the current and last AUX packet&#39;s SCR to that of the current and last value latched in the VCXO  224  local clock, adjustments to the VCXO  224  time base produces a signal  225  that synchronizes the transmitted program  208  transmission rates. 
     Referring to  FIG. 3   a  and  FIG. 3   b  the invention disclosed herein includes a method comprising the steps of: determining  318  a relative time difference between the time stamp and a previous time stamp; communicating  330  the relative time difference to a wireless access point transmitter having a time base as one feature of transmission; transmitting  333  the relative time difference to one or more wireless station receivers, wherein each of the one or more wireless station receivers adjusts the time base according to the relative time difference. 
     Referring again to  FIG. 3   a    FIG. 3   b  the invention disclosed herein includes a method comprising the steps of: determining a relative time difference between the current and previous AUX SCR to the current and last value latched in the local VCXO clock  318 ; adjusting the clock in the VCXO to synchronize data rata in the set top to the head end  320 ; outputting the VCXO as adjusted, to the BSS MAC Chip  330 ; adjusting the beacon packet in accordance with the VCXO adjusted clock  332 ; transmitting the adjusted beacon packet to a wireless station in the BSS  333 ; and utilizing the adjusted beacon to adjust the rate of the local VCXO rate  334 . Accordingly using this method, each of the one or more wireless station receivers adjusts the time base according to the relative time difference. 
     These changes in the rate of the wireless access point beacon and the affect upon the wireless station VCXO have a corresponding effect on the rate at which the video data is consumed and therefore reduce the risk of buffer underflow/overflow. 
     It is to be understood that the form of this invention as shown is merely a preferred embodiment. Various changes may be made in the function and arrangement of parts; equivalent means may be substituted for those illustrated and described; and certain features may be used independently from others without departing from the spirit and scope of the invention as defined in the following claims. For example, although the invention is described in the context of IEEE 802.11 based WLANs, it is to be understood that the invention may be applied to structures based on other wireless LAN standards wherein the synchronization is to be maintained.