Abstract:
In one embodiment, a forward error correction code is applied to a group of data packets to create a coded group of packets by supplementing a set of parity packets for each group of data packets. The data packets of a first group of packets and a set of parity packets corresponding to the data packets are transmitted. If an acknowledgement is received, sending of additional packets is ceased. If an acknowledgment is not received, parity bits continue to be transmitted.

Description:
BACKGROUND 
   In order to distribute and store multimedia data, the multimedia data is transmitted over a communication channel. Multimedia data primarily refers to audio and visual data but may also include other types of data. The channel is often subject to noise and interference, as in the case of wireless channel, and to congestion, as in the case of wired Internet, both resulting in loss of data during transmission. 
   Two methods can be used to combat data losses during transmission. Forward error correction (FEC) is a method of transforming the data message, represented by a sequence of symbols from a finite alphabet, by supplementing a parity data, another sequence of symbols, to ensure that if components of a codeword are altered, below some designated threshold, the original data can be usually extracted intact. FEC therefore provides error resilience by increasing the amount of data to be sent. FEC does not require a return channel and is typically not adaptive to the current state of the channel. FEC does not guarantee that the data will arrive to the receiver without errors, however. A higher-level protocol implementing some form of repeat request for data that tolerates little errors is required for this to be addressed. Alternatively, in multimedia communications the delay requirements often dominate the error-free transmission requirements, making error-free transmission a lesser priority. 
   Basic automatic repeat request (ARQ) is an alternative approach to assist in robust data communications. ARQ operates by dividing the data into packets and appending a special error check sequence to each packet for error detection purpose. The data packets and error checks are communicated over a channel and the receiver decides whether a transmission error occurred by calculating the check sequence and comparing the calculated check sequence to the appended error check sequence. If a discrepancy is found the error is declared and the receiver requests the transmitter using the return channel to resend the packet by sending a negative acknowledgement signal. If no discrepancy is found the receiver sends a positive acknowledgement signal to the transmitter. To alert the transmitter of the error, ARQ requires two-way communication channel to be present. Often, the return channel uses the same physical medium as the forward channel, effectively expanding the data size because of retransmissions and communication of control information. The difference between the FEC and ARQ is that ARQ is inherently channel adaptive, since only lost packets are retransmitted, while FEC typically adds overhead to all packets. Yet, ARQ may introduce significant delays due to roundtrip propagation time and processing time. The last condition significantly limits the application of ARQ to multimedia communications. 
   What is needed is a way to combine the two error control methods to improve their performance for multimedia communications and to facilitate multimedia streaming services and user playback experience. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram illustrating the transmission of data packets according to one embodiment. 
       FIG. 2  is a block diagram of one embodiment of a group of packets. 
       FIG. 3  is a block diagram of a system to transmit data packets according to one embodiment. 
   

   DETAILED DESCRIPTION 
   The method and apparatus described herein may provide improved channel bandwidth utilization for multimedia communications. According to one embodiment, the method and apparatus described herein includes an FEC and ARQ component, (which may be referred to as a hybrid automatic repeat request (HARQ)). The FEC component is used to protect the user datagram protocol (UDP) transported multimedia data against channel fades and errors, and the ARQ component is used to ensure efficient channel utilization and robustness to errors in the return channel. As a result, an improved quality of multimedia can be obtained using the HARQ method compared to the conventional methods under limited channel bandwidth constraints. 
   The method and apparatus described herein can be used for robust multimedia communications over networks including wired (IP) networks, cellular packet data networks, wireless LAN&#39;s, power and telephone line networks, as well as many proprietary nonstandard packet-based networks. Incorporating a software and hardware support for the robust communication method and apparatus will facilitate multimedia communication applications including multimedia streaming, distant learning and mobile video communications. 
   In one embodiment, the HARQ system design is used on a packet erasure channel, specifically a channel that provides the locations of packets that had errors during transmission. A packet erasure channel is often implemented at the physical layer using cyclic redundancy check (CRC). 
   An exemplary diagram of transmitting packets, according to one embodiment, is presented in  FIG. 1 . The media data  110  is packetized into a Group of Packets (GOP)  120 . In one embodiment, the size of the GOP and the packet size are determined by the communication network being used and by the requirements of the application. For instance, a larger packet size may reduce the overhead due to headers of the transport protocols. On the other hand, the larger packet size may also lead to increased delays and inefficiency in high error-rate environments. The appropriate FEC code is applied to the GOP to generate the desired number of parity packets per GOP  130 . The GOP packets together with the parity packets constitute coded GOP (CGOP). In one embodiment, the number of parity packets is chosen depending on the tolerable delay, available bandwidth, and/or channel statistics. Additional considerations may also be considered. The parity packets are generated so that they can replace the lost data packets with little or no overhead. In one embodiment, the redundancy packets may include the original data. The data and redundancy packets may contain any additional information, possibly in the form of headers, required for the overall system control and operation. In one embodiment, a GOP number, a packet number, FEC parameters and/or the packet sizes may be included in the packet. 
   In one embodiment, the parity packets are generated using the systematic Reed-Solomon (RS) codes, wherein the number of parity packets replaces the same number of (any) data packets so that the data can be decoded intact. Any other suitable FEC channel code may be used to generate the parity packets, such as Tornado codes. 
   The data is packetized, FEC encoded and sent from the transmitter  140  to the receiver  150 . The receiver determines if the transmitting data can be decoded. If the data can be decoded, the receiver sends an acknowledgement to the transmitter, which terminates the transmission of any further redundancy for the current CGOP  170 . The transmission is then decoded  180  and sent to the user  190 . 
   The data and parity packets transmission order according to one embodiment is illustrated in  FIG. 2 . First the data packets of the current CGOP are sent to the receiver  210 . The data packets may be interleaved with the data, parity packets, or both from other CGOP&#39;s  220 . The parity packets corresponding to the current CGOP are then sent  230  until the acknowledgement from receiver arrives  240  or until the maximum predetermined amount of parity packets is reached or exceeded  250 . In one embodiment, the data packets of the current CGOP are sent before the parity packets of the same CGOP. As a result, the data transmission and processing overhead may be reduced when no packets from the current CGOP have been lost. In one embodiment, packets from different CGOP&#39;s can be interleaved to give the receiver enough time to process and send the acknowledgement to the transmitter. 
   In one embodiment, the receiver implements the GOP acknowledgement protocol, which sends an acknowledgement to the transmitter when the receiver can decode the GOP data. The receiver implicitly asks for more parity by not sending an acknowledgement to the receiver. The receiver may send multiple acknowledgements for the same GOP. Multiple acknowledgments can be used when the receiver suspects that the first acknowledgement was (or can be) lost on the return channel. 
   In an embodiment using RS coding, the acknowledgement can be sent when the number of correctly received packets exactly equals the number of original data packets. The acknowledgement can be sent before the actual decoding takes place to reduce the overall latency. If all the data packets arrive without errors no decoding is needed and the data can be passed directly to the user application. 
   In an embodiment using Tornado coding, the acknowledgement can be sent when the number of correctly received packets equals the number of original data packets times some predetermined constant greater than unity. The latter constant is determined to provide some desired probability of correct decoding and is determined for each Tornado code by a computer simulation. If all the data packets arrive without errors no decoding is needed and the data can be passed directly to the user application. 
   Several other acknowledgement mechanisms are compatible with this system. Acknowledgments packets include the CGOP number but may also contain additional information. The additional information may be in the form of control messages to the server, channel statistics and/or other information. In the case of errors on the return channel, such as packet erasures, the transmitter simply sends the maximum number of packets allowed by the algorithm and continues to the next GOP. If after all the parity is sent the data is still not decodable, the transmitter continues to the next GOP. In an embodiment using delay-sensitive multimedia information, the delivery time is upper-bounded so that the proposed solution can be used as is without adding an additional error resolution mechanism. One embodiment may define a higher-level error resolution protocol. The application can also be allowed to deal with the unrecoverable channel error situations. 
   In one embodiment, the proposed method and apparatus described herein is applicable to video streaming over IEEE 802.11 wireless LAN. At the UDP level, the IEEE 802.11 network acts as a packet erasure channel if the physical layer acknowledgements that are sent even for the UDP traffic are suppressed. In one embodiment, at the physical layer retransmits and acknowledgements from the mobile receiver are suppressed by a multicasting IP addresses in the video streaming application. UDP connections are maintained from the transmitter to the receiver for data traffic and from the receiver to the transmitter for acknowledgements. 
   In one embodiment, the profile of the communicating channel is taken into account to the FEC parameters (the number of data packets and the parity packets in a CGOP) and other characteristics of the method and apparatus described herein. In one embodiment a CGOP size and the number of parity packets may be chosen so that the integral number of packet erasures over the length of CGOP with a high probability is less than the number of parity packets (for RS coding) or is less than the number of parity packets times some predetermined constant greater than unity (for Tornado coding). 
   In one embodiment, the method and apparatus described herein could be used for streaming of multimedia data over wireless IP network, from a streaming server to a receiving device. For example, one embodiment could provide the IP network with error resiliency while reducing temporal latency to improve proper playback of data in the streaming setup. 
   One embodiment could also be used to interface with media playback mechanisms. For example one embodiment may use the Intel® Media Processing Library framework, such that the robust streaming is integrated seamlessly with the playback mechanisms. 
     FIG. 3  shows the block diagram of one embodiment for streaming multimedia data over an IP network with UDP transport protocol. The multimedia data  310  consists of audio and/or video data, and is stored within a server  320  in the compressed or uncompressed form. The Application Program Interface (API)  321  is used to encode or transcode the media data and store it in the internal encoder buffer  322 . In one embodiment the encoder may be compliant with a Moving Picture Experts Group (MPEG) or other video and audio coding standard. In one embodiment, the API provides the packetizing and FEC encoding block  323  with location of the compressed stream headers. In one embodiment the packetizer and FEC create data packets and parity packets. The multimedia data may be packetized in a non-sequential order. Alternatively, different FECs may be used for different multimedia data segments. Conversely, some multimedia data may not be included in data packets. The packetized data and parity are stored in the internal packet buffer  324 . The API provides management functionality similar to the encoder buffer. Specifically, the input/output (I/O) block  325  is able to randomly access the data in the packet buffer on a packet basis. The API also provides other additional information about the content of the packets that is required by the I/O. The function of the I/O block is to perform the packet delivery over the IP network and to provide the control link between the server and the client for the ACK transmission. The I/O may send packets several times, drop packets from the transmission buffer or arbitrarily schedule the packet transmission to the Socket API that represents the IP network  330 . All three major blocks representing the server are controlled by a central higher-level process  326 , which sets the variable parameters of these three components using their API&#39;s and also manages the data flow between the blocks and the data buffers. 
   At the client side  340  the data from the IP network is received by the I/O block  341  and is placed into the packet buffer  342 . The I/O block is also responsible for sending the ACK&#39;s back to the server side at the direction of the client control process  343 . The I/O may also be used to send other control information to the server side. The depacketizing and FEC decoding block  344  processes the data from the packet buffer  342 . The depacketizing and FEC decoding block is responsible for correcting data packet erasures and presenting the multimedia encoded data in a form that can be processed by the following decoding block. The compressed multimedia data is passed to the API  345  for the decoding process through the decoding buffer  346 . The API decompresses the multimedia data and outputs it to the display  350 . The client control  343  manages the data flow between the three blocks described, controls ACK&#39;s and other communication to the receiver. 
   The methods described above can be stored in the memory of a computer system (e.g., set top box, video recorders, etc.) as a set of instructions to be executed. In addition, the instructions to perform the method described above could alternatively be stored on other forms of machine-readable media, including magnetic and optical disks. For example, the method of the present invention could be stored on machine-readable media, such as magnetic disks or optical disks, which are accessible via a disk drive (or computer-readable medium drive). Further, the instructions can be downloaded into a computing device over a data network in a form of compiled and linked version. 
   Alternatively, the logic to perform the methods as discussed above, could be implemented in additional computer and/or machine readable media, such as discrete hardware components as large-scale integrated circuits (LSI&#39;s), application-specific integrated circuits (ASIC&#39;s), firmware such as electrically erasable programmable read-only memory (EEPROM&#39;s); and electrical, optical, acoustical and other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.); etc. 
   Although the present invention has been described with reference to specific exemplary embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader spirit and scope of the invention. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.