Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a Continuation of U.S. application Ser. No. 09/940,305, filed Aug. 27, 2001 now U.S. Pat. No. 7,054,961. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to techniques of data transfer between two computers via a network, and in particular to a system and method of transferring streaming data from one computer to another and a repeater for use in such a data transfer system. 
   2. Description of the Related Art 
   There has been a streaming technique of distributing a real-time content such as audio or video, allowing a client computer to play the content almost concurrent with receipt of the content from a server computer. Since the client computer can play the content without downloading all the content data, a user can obtain fast response and it is effective especially in the case of small memory capacity in the client side. 
   In real-time content transmission in Internet, the following protocol may be employed: RTSP (Real-time streaming protocol, RFC 2326) and RTP (Real-time transport protocol RFC 1889). RTSP is a streaming control protocol allowing play, stop, and setup. RTP is a transfer protocol for real-time packet transfer. As application software for streaming transmission, RealSystem™ of RealNetworks and Windows Media Player™ of Microsoft Corporation are available. 
   In the case of a network such as Internet that does not sufficiently assure assignment of bandwidth or stability of delay, the quality of streaming data transmission is significantly affected by variations in delay time of packet. Since delays in packet delivery vary during transmission, it is necessary to adjust the content play timing at the client side even if the server has sent packets of the content at the same timing as played at the client side. 
   In order to compensate for momentary delays in packet delivery caused by delay variations to achieve uninterrupted sound or motion, the client side buffers received streaming packets before it starts playing. Such a buffering method is in general employed (see Japanese Patent Application Unexamined Publication No. 4-188929). 
   However, in the case of Internet access via mobile environment, a radio link causes a wider range of delay variation in packet delivery compared with fixed environment because the radio link itself exhibits a relatively long delay as well as loss of packet. Accordingly, there are cases where the above conventional delay variation control at the client side cannot compensate for delay variations in packet delivery. Since an amount of delay variation that can be compensated for is dependent on the capacity of a buffer, a wider range of delay variation cannot be compensated for without a larger capacity of a buffer. In mobile environment from which a high performance as in fixed environment cannot be expected, it is difficult to ensure a sufficient capacity of a buffer in a mobile device. 
   It is also considered that delay variation in packet delivery is controlled at some midpoint of a route from the server to the client to improve the quality of streaming data. However, since Internet traffic includes not only streaming data but ordinary data not requiring real-time transmission, it is necessary to discriminate between streaming data and ordinary data, resulting in complicated packet control. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a data transfer system, a data transfer method, and a repeater for use therein, allowing transfer of streaming data through a network without reducing the quality of played content at a receiving side having a relatively small capacity of a buffer, even if the network exhibits a relatively large delay variation in data delivery. 
   According to the present invention, a system for transferring streaming data in packets from a first computer to a second computer through a network, includes a repeater provided in the network, the repeater comprising a buffering controller for buffering a received packet for a set time period before forwarding it so that a receiving time interval of packets at the second computer is substantially equal to a sending time interval of packets at the first computer. 
   The set time period is preferably determined depending on a reception condition of the second computer, wherein the second computer notifies the repeater of the reception condition. 
   The second computer preferably has a buffering function of buffering received packets to absorb delay variations of the received packets. 
   The reception condition may include information of an available buffering capacity in the second computer. 
   According to an embodiment, a repeater through which streaming data in packets is transferred from a first computer to a second computer, includes: a flow registration table for storing time information of packets for each streaming flow; a packet memory for storing streaming packets for each streaming flow; a packet analyzer for analyzing a received packet to discriminate a packet related to a streaming flow by searching the flow registration table for address and port number information of the received packet and storing flow information identifying the streaming flow into the flow registration table; a header analyzer for analyzing a header of the packet related to the streaming flow to produce time information of a streaming packet of the streaming flow and store the streaming packet into the packet memory; a packet manager for storing the time information of the streaming packet into the flow registration table and determining the set time period from the time information to produce sending time of the streaming packet; and a packet sending controller for sending the streaming packet to the second computer when the sending time has come under control of the packet manager. 
   The packet analyzer preferably forwards packets other than a packet related to a streaming flow to the second computer. 
   According to another aspect of the present invention, a method for transferring streaming data in packets from a first computer to a second computer through a network, includes the steps of: the first computer sending a streaming packet to a repeater; the repeater buffering the streaming packet for a set time period before forwarding it to the second computer so that a receiving time interval of packets at the second computer is substantially equal to a sending time interval of packets at the first computer; and the second computer buffering the streaming packet received from the repeater before reproducing it. 
   As described above, according to the present invention, even when a delay variation of streaming packets is unacceptably increased at a receiving side, the repeater buffers a streaming packet for a set time period before forwarding it to suppress such a great delay variation, resulting in improved quality of application of the receiving side. Since delay variation in packet delivery is suppressed, high-quality play of the delivered content at the receiving side can be obtained even if the receiving side has a relatively small capacity of a buffer provided therein. 
   When the receiving side sends a reception condition thereof back to the sending side, the repeater can adjust the packet buffering time depending on the reception condition of the receiving side. Accordingly, an absolute delay over the network can be minimized, resulting in reduced capacity of a buffer at the receiving side. 
   Further, when the repeater discriminates a packet related to a streaming flow, only streaming flow related packets are buffered at the repeater and other packets are directly transferred. Accordingly, the data transfer system and the repeater can be used in environment transferring various kinds of packets, for example, the Internet. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram showing an outline of a data transfer system according to the present invention; 
       FIG. 2  is a block diagram showing an internal circuit of a repeater according to an embodiment of the present invention; 
       FIG. 3A  is a block diagram showing an internal circuit of a server computer in the data transfer system of  FIG. 1 ; 
       FIG. 3B  is a block diagram showing an internal circuit of a client computer in the data transfer system of  FIG. 1 ; 
       FIG. 4  is a diagram showing a header format of a RTP packet used in the present embodiment; and 
       FIG. 5  is a diagram showing an example of a flow registration table in the repeater. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   As shown in  FIG. 1 , it is assumed for simplicity that a network including a repeater  1  connects a server device  2  and a client device  3 . 
   At the request of the client device  3 , the server device  2  starts sending streaming packets of a requested content to the client device  3  through the repeater  1  in the network. The network is typically the Internet. A streaming packet flowing from the server device  2  downstream to the client device  3  is buffered in the repeater  1  so as to adjust sending timing of the received streaming packet. Although only one flow of streaming packets for the content is shown in  FIG. 1  for simplicity, the repeater  1  according to the present invention is allowed to individually buffer a plurality of flows. The details of the repeater  1  will be described later. 
   The streaming packets buffered in the repeater  1  are sent to the client device  3  in adjusted timing. The client device  3  buffers streaming packet before it starts reproducing it to absorb delay variation in packet transmission. 
   In this manner, the requested real-time content is played by a predetermined application in the client device  3  without interrupted sound or motion. Since the repeater  1  temporarily buffers streaming packets received from the server device  2 , high-quality play of the delivered content at the client device  3  can be obtained even if the client device  3  has a relatively small capacity of a buffer provided therein. 
   In addition, the client device  3  may send a reception condition notification packet back to the server side. When receiving the notification packet, the repeater  1  adjusts the packet buffering time depending on the reception condition of the client device  3 . 
   Repeater 
   Referring to  FIG. 2 , the repeater  1  is provided with a packet analyzer  101 , a header analyzer  102 , a packet memory  103 , a flow registration table  104 , a packet manager, a packet sending controller, a server-to-client (downstream) signal reception terminal  107 , a client-to-server (upstream) signal reception terminal  108 , a server-to-client signal sending terminal  109  and a client-to-server signal sending terminal  110 . 
   The packet analyzer  101 , when receiving a packet from the reception terminal  107  or  108 , determines whether the received packet is a streaming packet or a feedback information packet related to a flow registered in the flow registration table  104 . When it is a streaming packet or a feedback information packet, the packet analyzer  101  transfers it to the header analyzer  102 . When it is neither a streaming packet nor a feedback information packet, the packet analyzer  101  forwards it to a selected one of the server-to-client sending terminal  109  and the client-to-server sending terminal  110  depending on its destination address. Further, as described in detail later, the packet analyzer  101  registers the address and port number of a streaming flow in the flow registration table  104  depending on whether the received packet uses a port number that is used by streaming transmission control protocol or is conventionally used by a streaming transmission application. 
   The header analyzer  102  analyzes the header of the received streaming packet and notifies the packet manager  105  of information of the received packet such as a sequential number and a time stamp indicating a time instant of the server device  2  sending it. 
   The packet memory  103  buffers streaming packets input from the header analyzer  102  and queues streaming packets for each flow until a sending instruction for a flow has been received from the packet sending controller  106 . A flow is defined as a set of packets related to a certain content transferring between a server device  2  and a client device  3 . 
   The flow registration table  104  contains information related to each streaming flow in table form as shown in  FIG. 4 , which will be described in detail later. 
   The packet manager  105  inputs the information of the received streaming packet from the header analyzer  102  and registers it for each flow into the flow registration table  104 . The packet manager  105  also outputs a packet sending instruction for each flow to the packet sending controller  106  when the time to send has come. 
   The packet sending controller  106 , when receiving a packet sending instruction for a certain flow from the packet manager  105 , reads a head packet of a queue for the flow from the packet memory  103  and forwards it to the server-to-client signal sending terminal  109 . 
   A packet sent from the server device  2  toward the client device  3  is received at the server-to-client signal reception terminal  107  and is forwarded to the packet analyzer  101 . A packet sent from the client device  3  toward the server device  2  is received at the client-to-server signal reception terminal  108  and is forwarded to the packet analyzer  101 . A packet output from one of the packet analyzer  101  and the packet sending controller  106  is sent from the server-to-client signal sending terminal  109  toward the client device  3 . A packet output from the packet analyzer  101  is sent from the client-to-server signal sending terminal  110  toward the server device  2 . 
   Server 
   Referring to  FIG. 3A , the server device  2  is provided with an application  21  and a communication terminal  22  through which packets are sent and received to and from the client device  3 . The application  21  allowing streaming packets to be sent to a client device corresponds to an application for processing streaming packets to play installed in the client device  3 . 
   Client 
   Referring to  FIG. 3B , the client device  3  is provided with a reception condition notifying section  31 , an application  32 , and a communication terminal  33 . The application  32  corresponds to the application  21  of the server device  2  and is designed to process streaming packets to play sound and video of the requested content. As described before, the application  32  is provided with a buffer for absorbing delay variations in packet transmission. 
   The reception condition notifying section  31  calculates a difference between packet arrival timing and play timing of streaming packets processed by the application  32 . Based on the calculated timing difference, the reception condition notifying section  31  produces a reception condition indicating the capacity of the buffer provided in the application  32  and the amount of available capacity thereof, and sends a reception condition notification packet carrying the reception condition as feedback information to the repeater  1  through the communication terminal  33 . 
   Operation 
   Assuming that the server device  2  sends streaming packets to the client device  3  through the repeater  1  in the network, an operation of the repeater  1  will be described hereinafter. 
   In the above-described repeater  1 , when a packet is received at the server-to-client signal reception terminal  107 , the received packet is forwarded to the packet analyzer  101 . 
   When receiving the packet, the packet analyzer  101  searches the flow registration table  104  for the destination address and port number (flow identifying information) of the received packet to determine whether the received packet is a streaming packet of a flow registered in the flow registration table  104 . 
   Further, in order to register the address and port number of a streaming flow in the flow registration table  104 , the packet analyzer  101  determines whether the received packet uses a port number that is used by the streaming transmission control protocol (e.g. RTSP, port number 554) or is conventionally used by a streaming transmission application (e.g. port number 6970 in a streaming transmission system RealSystem™ of RealNetworks). 
   When a packet conforming to the streaming transmission control protocol has been received, the packet analyzer  101  analyzes the contents of the received packet to obtain information of address and port number used in streaming transmission. When a packet using a port number for conventional streaming transmission has been received, the packet analyzer  101  obtains information of address and port number of the received packet. In this manner, the packet analyzer  101  registers a pair of address and port number for each of source and destination into the flow registration table  104  (see  FIG. 5 ). This method allows streaming data to be extracted from received various packets in the network such as an IP network transferring various kinds of packets. 
   The packet analyzer  101  searches the flow registration table  104  for the destination address and port number of the received packet and, when a match is found (that is, when it is a streaming packet), the packet analyzer  101  transfers it to the header analyzer  102 . When it is not a streaming packet, the packet analyzer  101  forwards it to a selected one of the server-to-client sending terminal  109  and the client-to-server sending terminal  110  depending on its destination address. In other words, the repeater  1  plays a role as an ordinary repeater. 
   In the case of a packet using a port number for conventional streaming transmission, it has not been registered in the flow registration table  104  but it is a streaming packet. Accordingly, it is forwarded to the header analyzer  102 . In the case of a packet conforming to the streaming transmission control protocol, the contents of the packet is analyzed and registered in the flow registration table  104  and then is forwarded to a selected one of the server-to-client sending terminal  109  and the client-to-server sending terminal  110  depending on its destination address. 
   When receiving a feedback information packet indicating the reception condition of the client device  3 , the packet analyzer  101  forwards it to the header analyzer  102  and the reception condition information is output to the packet manager  105 . 
   When receiving a streaming packet, the header analyzer  102  analyzes the header of the received streaming packet to extract a sequential number of the relevant flow and a time stamp indicating a time instant of the server device  2  sending it. The address, port number, time stamp, and sequential number for the received streaming packet are output to the packet manager  105  and the streaming packet is output to the packet memory  103 . In many instances, the header format employs RTP (Real-time transport protocol RFC 1889) as shown in  FIG. 4 . 
   The packet manager  105  registers the information of address, port number, time stamp, and sequential number for each flow identified by a pair of address and port number, into the flow registration table  104 . An example of the flow registration table  104  is shown in  FIG. 5 . 
   Some streaming delivery application does not employ the RTP or cannot analyze it. In this case, the header analyzer  102  cannot obtain the above information. Accordingly, the header analyzer  102  notifies the packet manager  105  that necessary information cannot be obtained from this packet and forwards it to the packet memory  103 . 
   The packet memory  103  buffers streaming packets input from the header analyzer  102  for each flow under the sending control of the packet sending controller  106 . This buffering absorbs packet delay variations due to transmission from the server device  2  to the repeater  1  through the network. 
   Scheduling of Packet Sending Time 
   The packet manager  105  performs the scheduling of time of issuing a packet sending instruction for each flow to the packet sending controller  106  by referring to the packet registration table  104 . 
   In a certain flow, the first-arrival packet is scheduled to be sent at the time set by adding a predetermined time period to its arrival time. The set time may be stored in a next sending time field of the flow registration table  104  (see  FIG. 5 ). When the set time has come, the packet manager  105  issues a packet sending instruction for that flow to the packet sending controller  106 . The second or later packet is scheduled to be sent at the time set by adding a sending time difference to the set time of the previous packet. The sending time difference is calculated from a difference between the time stamp of the second or later packet and the time stamp of the previous packet. 
   When the time interval of packets received from the server device  2  is always constant, the second or later packet may be scheduled to be sent at the time set by adding the constant time interval to the set time of the previous packet. The constant time interval is calculated from a difference between the time stamp of the second packet and the time stamp of the first packet. 
   In the case where information related to sending time cannot be extracted from a received packet, the playing rate informed from the client device  3  or an average reception rate of packets that have been received so far in the relevant flow can be used to determine the sending time interval for scheduling. 
   Feedback Control of Packet Sending Time 
   When receiving a feedback information packet indicating the reception condition of the client device  3 , the packet analyzer  101  forwards it to the header analyzer  102  and the reception condition information is output to the packet manager  105 . The packet manager  105  can adjust the packet sending time interval depending on the reception condition of the client device  3 . 
   For example, when a variation in the arrival time interval of packets in a certain flow becomes greater, the packet manager  105  temporarily makes the packet sending time interval longer to absorb a greater delay variation in packet delivery. Contrarily, when a variation in the arrival time interval of packets in a certain flow becomes smaller, the packet manager  105  temporarily makes the packet sending time interval shorter to reduce the buffering time at the repeater  1 . This reduces absolute delay time occurring between the server device  2  and the client device  3 . 
   The delay variation reduction by the repeater  1  buffering packets causes the absolute delay occurring between the server device  2  and the client device  3  to be increased. However, the control of delay variation by the repeater  1  using the feedback information from the client device  3  suppresses the absolute delay occurring between the server device  2  and the client device  3 . 
   When receiving a packet sending instruction for a certain flow from the packet manager  105 , the packet sending controller  106  reads a head packet of a queue for the relevant flow from the packet memory  103  and forwards it to the server-to-client signal sending terminal  109 . A packet to be sent has the source address and port number of the packet originating device (here, the server device  2 ) included in its header, not those of the repeater  1 . Accordingly, for the client device  3 , the packet appears received directly from the server device  2 . 
   When the client device  3  receives a streaming packet from the server device  2  through the repeater  1 , the streaming packet is forwarded to the application  32  through the communication terminal  33  (see  FIG. 3B ). After buffering the streaming packet to absorb delay variations in packet transmission, the application  32  plays the content based on the streaming packet. 
   As described before, in the application  32  performing packet reception and play, the amount of available buffer capacity varies depending on a difference between playing rate and reception rate. The reception condition notifying section  31  produces a reception condition indicating the amount of available capacity of the buffer provided in the application  32  and a packet reception rate and sends the reception condition notification packet carrying the reception condition as feedback information to the repeater  1  through the communication terminal  33 . At the repeater  1 , the reception condition is used to adjust the buffering time of the buffer to allow adaptive delay variation absorption, suppressing the absolute delay occurring between the server device  2  and the client device  3 .

Technology Category: h