Abstract:
A transmitting terminal transmits the data to a server and the server transmits the data to a receiving terminal. The server detects transmission errors in the data. If the server detects a transmission error, the server sends a request to the transmitting terminal for data correcting the effects of the detected transmission error on data affected by the detected transmission error. The server also sends the affected data to the receiving terminal prior to receiving the correction data. In other words, the server does not wait to receive the correction data before sending the affected data to the receiving terminal. Additionally, the server modifies the data received from the transmitting terminal to conceal the detected error from the receiving terminal.

Description:
BACKGROUND 
     This invention relates to transmitting data from a transmitting terminal to a receiving terminal through a third component, and more particularly to transmitting video data for videoconferencing and video telephony, and error correction and concealment during such transmission. 
     Transmitting data by a packet-switched network is one of the most common methods of transmitting data. As with any other type of data transmission, data transmitted by a packet-switched network can be affected by transmission errors such as loss of packets. 
     Loss of some packets in a data flow can significantly affect the rest of the data flow. For example, consider the situation where the transmitted data is digital video encoded by a prediction based compression technique. In that case, loss of packets will affect not only a particular frame to which the data in the lost packets belong, but also subsequent frames. In addition, if the compression technique uses motion compensation, then the lost packets will affect not only a particular region in the frames, but also surrounding regions in the subsequent frames, the extent of which depends on the value of the motion vectors. Similarly, if the transmitted data is digital video encoded using variable length coding (for example, Huffman coding), the packet loss can render the information contained in one or more of the subsequent packets unusable. 
     Various techniques have been developed to minimize, and even correct for, the effects of packet loss on transmitted video data. One set of techniques attempt to reduce the effect of packet loss by including redundant control data in all packets. For example, some packetization protocols require control data necessary for decoding a packet to be included in a packet&#39;s header, even though the same information is included in a preceding packet. 
     Another set of techniques attempt to reduce the effects of lost data on the video image by replacing the lost data with other data. For example, according to one such technique, the lost data is replaced with data from a preceding frame, thereby attempting to improve the image quality of the current frame and reduce errors in subsequently decoded frames. Yet another set of techniques provide methodologies for allowing a receiving terminal to determine whether a packet has been lost and, if so, send a request for a correction of the lost data to the transmitting terminal. The transmitting terminal then provides data which corrects the effects of the lost data. For a summary of various error concealment and correction techniques, see Y. Wang and Q. -F. Zhu, “Error Control and Concealment for Video Communication: A Review,”  Proc. IEEE,  vol. 86, no. 5, pp. 974-997, May 1998. In another technique, a network switch detects a packet loss and requests a retransmission of the lost packet. The switch does not forward subsequent packets until it receives the replacement packet. Of course, if the packets contain video data being transmitted for realtime display, the delay can have significant negative impact on the displayed video. 
     It should be noted some of the these techniques require that a receiving terminal detect the packet loss. However, error detection at the receiving terminal becomes difficult where the data transmission path includes two networks governed by different protocols because a gateway between the two networks typically removes packet headers from the packets before forwarding the information contained in the packets to the receiving terminal. Removal of the packet headers makes error detection more difficult, since the packet headers typically contain data which can be used for error detection. One type of error detection that a receiving terminal can perform is to use its video decoder to detect packet loss errors by checking whether the received video bitstream is in conformance with the bitstream syntax of the encoding algorithm. 
     SUMMARY 
     In a general aspect, the invention relates to transmitting data from a transmitting terminal to a receiving terminal through a server. The invention features detecting transmission errors in the data at the server and, if an error is detected at the server, sending a request from the server to the transmitting terminal for data correcting the effects of the transmission error on the data affected by the transmission error. The affected data is transmitted from the server to the receiving terminal prior to receiving the correction data. 
     In another general aspect, the invention relates to a server for receiving data transmitted by a transmitting terminal and transmitting the data to a receiving terminal. The server features a receiver receiving data transmitted from the transmitting terminal to the server; a detector detecting transmission errors in the data; a first transmitter sending, if an error is detected, a request to the transmitting terminal for data correcting the effects of the transmission error on the data affected by the transmission error; and a second transmitter transmitting the affected data from the server to the receiving terminal prior to receiving the correction data. 
     In yet another aspect, the invention relates to a system including a transmitting terminal, a receiving terminal, and a server, where the transmitting terminal transmits the data to the server and the server transmits the data to the receiving terminal. The invention features a server which detects transmission errors in the data, and, in response to certain of such errors, sends a request to the transmitting terminal for data correcting the effects of the transmission error on the data affected by the transmission error. The server sends the affected data to the receiving terminal prior to receiving the correction data. 
     Preferred embodiments of the invention may include one or more of the following features. 
     The server is connected between two networks transmitting data from one network to another. The data includes a plurality of packets. Data can be digital video or compressed digital video, such as video compressed by a compression technique which uses motion compensation. 
     The transmission error can be loss of a packet. Where the transmitted packets are consecutive packets of data identified by sequence numbers, the loss of the packet can be detected by identifying a missing sequence number. The server determines the pixels of at least one frame of video affected by the lost packet, and includes in the request sent to the transmitting terminal information identifying at least a portion of the pixels to the transmitting terminal. The pixels can be from a single frame or from more than one frame. 
     The data received at the server is processed to conceal the detected error from the receiving terminal or other downstream components. Where the data is transmitted by the server to the receiving terminal according to a predetermined syntax, the process of concealing the error includes modifying the packets such that the transmitted data conforms to that predetermined syntax. 
     In response to the request, data can be sent to the receiving terminal to correct the data affected by the transmission error. Where the data is digital video data compressed by a prediction based compression technique, and some of the data is lost, the digital video data can be compressed, without relying on video data corresponding to data lost during transmission, to provide the data to be sent to correct the data affected by the transmission error. 
     The invention may include one or more of the following advantages. 
     Embodiments of the invention reduce the time required to recover from transmission errors such as packet loss when transmitting data (for example, video) between two or more terminals. In addition, some embodiments improve end-to-end video quality when compressed video data is lost or damaged during transmission. 
     Where the transmitting terminal is multi-casting to multiple receiving terminals through a single server, by detecting the error at the server rather than at each of the receiving terminals, only a single error correction request is sent to the transmitting terminal as opposed to multiple requests from the receiving terminals. 
     In addition, by concealing the transmission error from the receiving terminals, picture quality at those terminals can be improved as those terminals need not wait for response to their error correction requests. Waiting for such a response can result in the terminal freezing the image on the monitor for a relatively long time. In addition, concealing the error from downstream components and receiving terminals reduces the possibility of receiving multiple error correction requests from those terminals. 
     The invention may be implemented in hardware or software, or a combination of both. Preferably, the technique is implemented in computer programs executing on programmable computers that each include a processor, and a storage medium readable by the processor (including volatile and non-volatile memory and/or storage elements). Program code is applied to data entered using the input device to perform the functions described above and to generate output information. The output information is applied to one or more output devices. 
     Each program is preferably implemented in a high level procedural or object oriented programming language to communicate with a computer system. However, the programs can be implemented in assembly or machine language, if desired. In any case, the language may be a compiled or interpreted language. 
     Each such computer program is preferably stored on a storage medium or device (e.g., ROM or magnetic diskette) that is readable by a general or special purpose programmable computer for configuring and operating the computer when the storage medium or device is read by the computer to perform the procedures described in this document. The system may also be considered to be implemented as a computer-readable storage medium configured with a computer program, where the storage medium so configured causes a computer to operate in a specific and predefined manner. 
    
    
     Other features and advantages of the invention will become apparent from the following description of preferred embodiments, including the drawings, and from the claims. 
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows an arrangement of components for transferring digital audio and video from a transmitting terminal to a receiving terminal through a server. 
     FIG. 2 shows a block diagram of parts of the server of FIG.  1 . 
     FIG. 3 is a schematic diagram of the manner in which a video frame is partitioned according to the ITU-T H.263 standard. 
     FIG. 4 schematically illustrates the relationship between positions of two related macroblocks in two consecutive frames and a motion vector characterizing that relationship. 
     FIG. 5 is a schematic diagram of the organization of data representing video frames compressed according to the ITU-T H.263 standard. 
     FIG. 6 shows a high-level architecture of an RTP packet. 
     FIG. 7 is a flow chart of the operation of the server of FIG.  2 . 
     FIG. 8 is a flow chart of the steps taken by an error handling program of the server in FIG. 2 to identify the area affected by a detected packet loss. 
     FIG. 9 illustrates areas or pixels in video frames affected by lost packets. 
    
    
     DESCRIPTION 
     FIG. 1 shows an arrangement of components  100  for transferring digital audio and video from a transmitting terminal  102  to a receiving terminal  104  through a server  110 . Terminals  102  and  104  can be videoconferencing units, each having an encoder/decoder  102 A and  104 A, respectively. Transmitting terminal  102  is connected to a network  106 , which is a packet switched network. Receiving terminal  104  is in turn connected to a network  108 , which can also be a packet switched network. Networks  106  and  108  are connected to one another through server  110  which can be a gateway, a multipoint control unit (MCU), or any other network component enabling transfer of data between networks  106  and  108 . Each of networks  106  and  108  operates according to a data transmission protocol which is different from the other network&#39;s data transmission protocol. For example, network  106  can be an Internet Protocol (IP) network, while network  108  can be an Integrated Services Digital Network (ISDN). 
     FIG. 2 shows a block diagram of components of server  110 . Server  110  includes a microprocessor  112 , a memory  114 , a direct memory access (DMA) controller  116 , and a peripheral bus  118 . Server  110  also includes two input/output (I/O) ports  120  and  122  connected to peripheral bus  118 . In addition, I/O ports  120  and  122  are respectively connected to networks  106  and  108 . Server  110  also stores a number of application programs in memory  114 , including an error handling program  130 , a data format convertor  128 , and a packet loss detector  126 . 
     Briefly, transmitting terminal  102  transmits data, such as compressed audio and video, to receiving terminal  104  through server  110 . During transmission, server  110  detects transmission errors, namely loss of data packet(s). Once server  110  detects such an error, server  110  performs two operations. First, server  110  sends an error correction request to transmitting terminal  102  so that transmitting terminal  102  can send data to receiving terminal  104  to correct for the lost data. Second, server  110  repairs its output bitstream so that receiver terminal  104  does not detect any errors in the bitstream it receives. In effect, server  110  conceals the packet loss error from receiver terminal  104 . Server  110 , without waiting for a response to its error correction request, forwards the repaired data to receiving terminal  104 . 
     To better understand the operation of server  110  and its components, it is important to first understand the manner in which video is encoded and packetized for transmission from transmitting terminal  102  to receiving terminal  104 . Although there are many ways to encode and packetize video data, the embodiments here will be described in reference to the H.323 standard established by International Telecommunications Union (ITU). This standard is a popular standard used for videoconferencing and video telephony and is widely deployed in Internet Protocol (IP) networks. 
     According to the H.323 standard, encoder/decoder  102 A of transmitting terminal  102  first compresses a video signal using an encoding algorithm implementing either the ITU Telecommunication Standardization Sector (ITU-T) H.261 standard or the ITU-T H.263 standard. (In the following description, we will describe the embodiments with reference to the H.263 standard.) Transmitting terminal  102  then packetizes the compressed video according to the ITU-T H.225.0 standard which uses the Real-Time Protocol (RTP) of the International Engineering Task Force (IETF). These packets are then transported by an underlying technology supported by network  106  such as User Datagram Protocol/Internet Protocol (UDP/IP). Hence, transmitting terminal  102  encapsulates the RTP packets into UDP/IP packets and transmits them to server  110 . I/O port  120  of server  110 , after receiving the UDP/IP packets, reconstructs the RTP packets before processing the RTP packets or retransmitting them. Format convertor  128  of server  110  transforms the RTP packets into a suitable format (for example, the ITU-T H.221 bit stream which is suitable for transmission across an ISDN network) for transmission over network  108  to receiving terminal  104 . There, receiving terminal  104  processes the received packets to recover the video data. 
     Referring to FIG. 3, according to the H.263 standard, a frame  300  of video is partitioned into rectangular regions called Groups Of Blocks (GOBs)  305 . Each GOB  305  is composed of multiple macroblocks  310 . A macroblock  310  contains 4 luminance blocks  315  and 2 chrominance blocks  320 , where each block is a square block of 8×8 pixels. 
     According to the H.263 standard, video frames can be compressed in two modes: intra mode where the frame is encoded without reliance on other encoded frames, and inter mode where the frame is encoded using inter-frame prediction such that only the prediction error, that is, the difference between the current frame and another encoded frame, is encoded. 
     The H.263 standard also uses motion compensation in encoding video data. Motion compensation is based on an observation an image in one video frame is often the same as an image in a preceding or a following video frame, except that the image may have moved. For example, two consecutive video frames showing a moving car would have the image of the car, except that the car has moved and its relative location has changed. 
     Referring to FIG. 4, the H.263 standard motion compensation requires matching a macroblock  330  of a frame  335  to a macroblock  340  of a preceding frame  345 , and then determining a value for a motion vector  350 . This value is then included in the encoded video in association with the macroblock. 
     FIG. 5 is a representation of data representing a video frame  400  compressed according to the H.263 standard. Compressed frame  400  starts with a start code (SC)  405  (here, 16 0-bits followed by a single 1-bit). Start code  405  is followed by a frame header  410 . Frame header  410  includes such information as frame wide coding mode (for example, whether the frame is inter-frame or intra-frame coded, the H.263 annexes for encoding, etc.). Frame header  410  is followed by the GOBs in the frame. The first GOB (GOB 0 ) does not have a header field  420  while each of the other GOBs can optionally have a header field. Each header field  420  starts with a GOB start code SC  425  and a GOB header  430 . GOB header  430  will include such information as a GOB number and a quantization step value used in encoding the first macroblock in that GOB. 
     After each GOB header field  420 , the GOB data follows which includes the data from macroblocks corresponding to that GOB. An empty macroblock is stored as a 1 bit flag signaling that the block is empty. A non-empty macroblock is encoded and has variable number of bits of data. Each macroblock in a GOB will have a macroblock header which will include control data required for decoding that macroblock such as its coding mode, relevant motion vectors, and DCT coefficients. The macroblock header of the first macroblock in a GOB having a GOB header stores the actual value of the control data. The macroblock headers of the other macroblocks typically store prediction coded values of the control data, which in the described embodiment is the change in value of the control data from the value of the control data of the previous macroblock. However, each macroblock can optionally store the actual (i.e. non-prediction coded) value of these variables. 
     As mentioned, after compressing the frames of video, transmitting terminal  102  packetizes the compressed frames into RTP packets. FIG. 6 shows a high-level architecture of an RTP packet  500 . RTP packet  500  has a header section  505  which contains such information as a sequence number (SN), a time stamp (TS), and a marker bit. Sequence numbers (SNs) are consecutive numbers assigned to consecutive RTP packets in an RTP packet stream. If an RTP packet is lost, the sequence numbers of RTP packets arriving at server  110  will not be consecutive. The time stamp (TS) field in header section  505  stores the time when a frame was encoded. The time stamp of a video frame is unique to that frame. If a frame is packetized into multiple RTP packets, the time stamps (TS) for all the RTP packets corresponding to that frame will have the same value. Time stamps (TS) are typically used for achieving audio/video synchronization. The marker bit in header section  505  indicates whether the RTP packet contains the end of a frame. 
     The H.263 payload header  510  contains such information as the frame type (intra-or inter-frame coded), frame format (CIF, QCIF, 4CIF, etc.), and information on whether the RTP packet starts at a GOB boundary or a macroblock boundary. Payload header  510  can also have further information depending on whether the packet is a type ‘A’ or type ‘B’ RTP packet. Type ‘B’ RTP packets contain redundant control data from the preceding packet necessary for decoding. The redundant control data consists of control data from the GOB header of the last GOB and from the macroblock header of the last macroblock in the preceding RTP packet. The redundant control data includes the motion vector predictor, the quantization step size, and the address of the first macroblock in the current RTP packet. The values stored in payload header  510  are the actual values of these variables, as opposed to their prediction coded values. Hence, type ‘B’ RTP packets allow decoding the first macroblock without resorting to information contained in the previous RTP packet, in case that previous RTP packet is lost. Type ‘A’ RTP packets do not contain such redundant control data in their headers. 
     Encoder/decoder  102 A of transmitting terminal  102  determines whether an RTP packet will be of type ‘A’ or type ‘B’ depending on the video data to be included in the packet. There are three cases: video data starts at the beginning of a GOB having a header; video data starts at the beginning of a GOB not having a header; and video data starts at a macroblock boundary (that is, not at a GOB boundary). In the case of video data starting at a GOB having a header, RTP packet will be packetized as a type ‘A’ packet since information required for decoding the data will be in the GOB header. In the case of video data starting at a GOB which does not have a header, RTP packet can be packetized as either a type ‘A’ or type ‘B’ packet. Preferably, the packet would be packetized as a type ‘B’ packet, since the information for decoding the video data depends on information in the previous packet. In the case of video data starting at a macroblock boundary, RTP packet will be packetized as a type ‘B’ packet. 
     The H.263 payload  515  of RTP packet  500  contains the encoded video data, such as compressed video frame  400  shown in FIG.  5 . 
     Referring to FIG. 7, the operation of server  110  will now be described in reference to flow chart  200 . During operation, I/O port  120  receives from network  106  UDP/IP packets from which I/O port  120  reconstructs RTP packets. When I/O port  120  has reconstructed an entire RTP packet (step  205 ), I/O port  120  indicates the receipt of the RTP packet to DMA Controller  116 . DMA controller  116  stores the RTP packet in a current packet buffer, replacing a previously stored frame in that buffer. DMA controller  116  also transfers the previously stored RTP packet to a preceding packet buffer (step  210 ). 
     Packet loss detector  126  determines whether any RTP packet has been lost since the receipt of the preceding RTP packet (step  215 ). To do so, packet loss detector  126  compares the sequence number of the RTP packet stored in the current packet buffer with the sequence number of the RTP packet stored in the preceding packet buffer. If the sequence numbers are not consecutive, packet loss detector  126  determines that one or more RTP packets have been lost. 
     If no RTP packet has been lost since the receipt of the preceding packet, packet loss detector  126  indicates to format convertor  128  to prepare the RTP packet for transmission over network  108  to receiving terminal  104 . Format convertor  128  converts RTP packets received from network  106  into a format acceptable to network  108  (step  220 ). For example, if network  108  is an ISDN network, format convertor  128  depacketizes the video data into an ITU-T H.221 bit stream packets for network  108 . Format conversion often entails removing the RTP Header and payload header from the received data. Upon conversion, format convertor  128  transfers the packets to I/O port  122  for transmission (step  225 ). 
     If packet loss detector  126  determines that an RTP packet has been lost during transmission, microprocessor  112  executes error handling program  130 . Error handling program  130  determines the area (that is, pixels) which were represented by the data in the lost RTP packet(s) (step  230 ). Error handling program  130  then sends an error correction request to transmitting terminal  102  (step  235 ). Error handling program  130  also conceals the packet loss error from downstream components, namely receiving terminal  104  (step  240 ). To do so, error handling program  130  repairs the bit stream transmitted downstream so that it conforms to the H.263 syntax for encoded video data. Since, in the described embodiment, receiving terminal  104  detects packet loss errors in the bitstream by determining whether the received data conforms to the H.263 syntax for encoded video data, receiving terminal  104  does not detect any transmission errors. After repairing the bitstream, error handling program  130  transfers the repaired bitstream to format convertor  128  for conversion and then transmission over network  108  (step  220 ). The operation of error handling program  130  will now be described in further detail below. 
     As mentioned, error handling program  130  as discussed in reference to step  230  in FIG. 7, first determines the pixels in the current frame, and any preceding frame(s) frame, whose data is contained in the lost RTP packet(s) (hereinafter, also referred to as the “affected area”). 
     FIG. 8 is a flow chart of the steps taken by error handling program  130  to determine the affected area. These steps can be divided into two functional categories. First, in steps  605 - 610 , error handling program  130  determines whether the affected area is confined to one frame (hereinafter, “intra-frame damage”) or extends across two or more frames (hereinafter, “inter-frame damage”). Referring also to FIG. 9, if the damage is an intra-frame damage, then a starting point  750  and an ending point  755  of an affected area  760  will be in the same frame  765 . If the damage is an inter-frame damage, then starting point  750  and ending point  755  of affected area  760  will be in different frames  770  and  775 . Note that, whether the damage is intra-frame or inter-frame, the starting point will be immediately after the end of the preceding packet. Similarly, whether the damage is intra-frame or inter-frame, the ending point will be immediately before the start of current packet. 
     Second, in step  615 , error handling program  130  determines the starting and ending points of the affected area, so that the area can be identified to transmitting terminal  102  in an error correction request (FIG. 6, step  235 ). The steps taken by error handling program  130  to perform these two functions will now be described in detail. 
     First, as mentioned, error handling program  130  determines whether the damage is an intra-frame damage or inter-frame damage. In doing so, error handling program  130  uses the marker bits and time stamps of the preceding and current RTP packets. Error handling program  130  first, in step  605 , determines whether the time stamps of the preceding and current RTP packets are the same (i.e. both RTP packets belong to the same frame). If so, error handling program  130  determines that the damage is an intra-frame damage. Otherwise, error handling program  130  checks the marker bit of the preceding RTP packet (step  610 ). If the marker bit is set to 1, then the preceding RTP packet contains the end of a frame. Since an RTP packet contains data from only one frame and error handling program  130  program assumes that only one frame at a time can be lost, error handling program  130  determines that the damage is an intra-frame damage. If the marker bit of the preceding RTP packet is set to 0 (i.e., the RTP packet does not contain the end of a frame), then error handling program  130  determines that the damage is an inter-frame damage. Note that it should never be the case that the marker bit of the preceding RTP packet is set to 1 and that the time stamps of the preceding and the current RTP packet are the same, since that would indicate that time stamps of two different frames are the same. That would be an error. 
     Error handling program  130  next determines the starting point and the ending point of the affected area in each video frame affected by the packet loss (step  615 ). 
     Error handling program  130  identifies the starting point and the ending point of the affected area by identifying the lost and damaged macroblocks. Error handling program  130  identifies the lost and damaged macroblocks by their Macroblock Addresses (MBA). An MBA is a number assigned to each macroblock in a frame according to the position of that macroblock in that frame. The MBA numbers proceed from MBA 0  to MBA max  in each frame, with macroblock MBA 0  being located in the upper, left corner of the frame and macroblock MBA max  being located in the lower, right corner of the frame. MBA value numbers consecutively increase in the standard video scan order—left to right, top to bottom. In the H.263 standard, the macroblock address of a macroblock is determined by multiplying the GOB number of an immediately preceding GOB header by the standard number of macroblocks in each GOB, then counting the number of macroblocks between the GOB header and the macroblock of interest, and finally adding the two numbers together to obtain the desired MBA. 
     To identify the addresses of the lost or damaged macroblocks, error handling program  130  determines the starting point&#39;s MBA (MBA start ) and the ending point&#39;s MBA (MBA end ) for the damaged area. The process for determining the MBA start  and MBA end  in the case of intra-frame damages and inter-frame damages are the same except for one thing: for an inter-frame damage, MBA start  and MBA end  are in two different frames. 
     In the case of MBA start , error handling program  130  determines whether the marker bit for the preceding RTP packet is set to 1 or 0. If set to 1, then the preceding RTP packet represents the end of the previous frame. Since each RTP packet contains data only from one video frame, error handling program  130  determines MBA start  to be MBA 0  of the subsequent frame. In the described embodiment, it is assumed that only one frame can be lost and therefore, it is assumed that MBA start  is MBA 0  of the current frame. 
     If, however, the market bit is set to 0, error handling program  130  finds the address of the last complete macroblock contained in the preceding RTP packet. To do so, error handling program  130  processes the RTP packet beginning from the end of the preceding RTP packet and proceeding to the start of the preceding RTP packet. Error handling program  130  finds the position of the last received GOB start code. Error handling program  130  then uses the GOB number to determine the MBA for the first macroblock after the GOB header from the GOB number contained in the GOB header. Error handling program  130  then decodes the GOB according to the H.263 encoding syntax to parse the bits between the GOB header and the end of the preceding RTP packet so as to determine the MBA of the last macroblock in the RTP packet. MBA start  is set to be this determined MBA plus one. 
     Having determined MBA start , error handling program  130  then determines MBA end  by processing the current RTP packet. If the current RTP packet is aligned with a GOB header, then MBA end  is easily derived from the GOB number in the GOB header. If the current RTP packet is not aligned with a GOB header and the RTP packet is a type ‘B’ RTP packet, then the MBA end  is obtained from the H.263 payload header which stores the MBA of the first macroblock of the current RTP packet. If the current RTP packet is aligned with a GOB header and the RTP packet is a type ‘A’ RTP packet, then the MBA end  is set to be the MBA of the first macroblock following the first GOB start code in the current RTP packet. 
     As mentioned above, referring back to FIG. 7, having determined the area affected by the packet loss, error handling program  130  then performs two steps (steps  235  and  240 ): send an error correction request to transmitting terminal  102  and repair the outgoing bit stream so that it appears to receiving terminal  104  (or other downstream components) as not being affected by packet loss. Each of these steps will now be described in detail. 
     The error correction request sent by error handling program  130  indicates to transmitting terminal  102  that a packet loss has occurred and also identifies lost pixels identifying the lost macroblocks. For an intra-frame damage, the request includes a frame index value which identifies the current frame to transmitting terminal  102 , MBA start , and MBA end . In the H.263 standard, frame index values are referred to as Temporal References (TR) and are the time stamps of the frames. For an inter-frame damage, two requests are sent: one for the preceding frame, identifying the damaged macroblocks therein (a frame index value for the preceding frame, MBA start , MBA max ); and one for the current frame (a frame index value for the current frame, MBA 0 , MBA end ). 
     Being supplied with this information, rather than encoding an entire frame in intra mode (that is, generate an I-frame), transmitting terminal  102  identifies the affected macroblocks and performs error correction techniques which do not require encoding an entire intra mode frame. Such error correction techniques require less data to be sent to receiving terminal  104  to enable receiving terminal  104  to correct for packet loss. Because less data need be transmitted to correct for packet loss errors, problems such as worse frame quality, reduced frame rate, and momentarily frozen images are at least reduced, and possibly avoided altogether. 
     In the described embodiment, to correct for the error, transmitting terminal  102  re-encodes the video data to be sent without using data representing the area (that is, pixels) identified in the error correction request message. Hence, the video data is encoded without being predicted based on the lost (or damaged) macroblocks. When this re-encoded video data is received by transmitting terminal  104  and decoded, the decoded video data will not have any errors associated with the data in the lost macroblocks, since the data is not encoded using the data in those macroblocks. 
     To encode the video data in this manner and then send the data, upon receiving the error correction request message from server  110 , transmitting terminal  102  stops encoding and packetizing the video data. Transmitting terminal  102  then uses the information in the error correction request to identify the lost macroblocks. Encoder/decoder  102 A then re-encodes the already sent video, starting at some point at or before the first lost macroblock, without using the lost data. Transmitting terminal  102  then recommences packetizing the re-encoded data for transmission, starting at the macroblock immediately after the last macroblock sent to receiving terminal  104 . For a more detailed description of this error correction technique, see W. Wada, “Selective recovery of video packet loss using error concealment,”  IEEE J. Select. Areas Commun.,  vol. 7, pp. 807-814, June 1989). An alternative error correction technique that can be used is the H.263 error tracking technique (see ITU-T Recommendation H.263,  Video Coding for Low Bitrate Communication,  1998). 
     As mentioned, error handling program  130  also repairs the outgoing bit stream so that the output bitstream is restored to a legal bitstream according to the H.263 encoding syntax. Thus, to receiving terminal  104 , the bitstream appears not to be affected by any transmission errors. To repair the bitstream, error handling program  130  performs two functions. First, error handling program  130  generates an ITU-T H.221 bit stream containing “empty” macroblocks corresponding to the lost macroblocks which were previously identified. For example, the error handling program  130  generates appropriate GOB headers together with data representing empty macroblocks. In the case of H.263, such macroblocks would have a coded/unencoded (COD) bit, which would be set to indicate that the blocks are unencoded. Second, error handling program  130  uses the information in the RTP header from the current packet to convert prediction coded control data, such as the first macroblock&#39;s motion vector, into their actual values. Hence, the first macroblock can be properly decoded by encoder/decoder  104 A of receiving terminal  104 , even though receiving terminal  104  does not detect a packet loss error and therefore fails to retrieve, from the RTP packet header, the relevant control data. 
     Other embodiments are within the scope of the following claims. 
     For example, more than one server may be located between the transmitting terminal  102  and receiving terminal  104 . In such a case, each of the servers can perform error correction and concealment in the manner described here. Note that, when a server conceals an error, another server downstream preferably does not detect the same error. The downstream server will detect errors which occur during transmission between the two servers. Additionally, one or more of these servers may form part of a network, such as network  106  or  108 . In that case, the entire data path between transmitting terminal  102  and receiving terminal  104  can be within the same network. 
     The techniques described here can be used for transmission and receipt of two-way, interactive videoconferencing and video telephony. The techniques can also be used for internet video streaming. 
     The embodiments and techniques described here are mainly described in reference to videoconferencing or video telephony, that is transfer of audio and video data. For example, terminals  102  and  104  are described as being videoconferencing or video telephone systems. However, these techniques can also be used for transfer of other types of data and for other applications. 
     It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and embodiments are within the scope of the following claims.