Abstract:
A method and apparatus are described including receiving a coded packet of content associated with a layer, receiving feedback information regarding channel conditions and applying hybrid automatic repeat request to deliver the packet based on said feedback information. The method and apparatus wherein applying hybrid automatic repeat request layer-wise further includes determining if a resource is exhausted, scheduling delivery of layered coded content packets for a layer, if the resource has not been exhausted, determining if all layered coded content packets for the layer have been transmitted, proceeding to the first determining step, if all layered coded content packets for a layer have not been transmitted, determining if an acknowledgement message has been received from a user device, determining if the resource is exhausted, if no acknowledgement has not been received and scheduling delivery of layered coded forward error correction packets, if the resource has not been exhausted.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to streaming scalable multimedia data streams and, in particular, to streaming multimedia data streams in both unicast and multicast environments over lossy networks. 
       BACKGROUND OF THE INVENTION 
       [0002]    Multimedia streaming applications are typically among the real-time services offered by a wired or wireless network. Real-time services, such as multimedia streaming, are characterized by delay constraints. The multimedia streamed data (service data) that arrives late at the client are generally discarded by the client. Multimedia streaming data typically has a deadline associated with the data because it is being displayed to a user at the client device. Due to reasons such as congestion or channel impairment in the network, timely and correct delivery of multimedia streaming data to the client cannot always to be guaranteed. Therefore, at the client, there can be multimedia data missing during playback. 
         [0003]    Due to the large volume of multimedia data, multimedia streaming applications often require large network bandwidths. For those bandwidth-constrained networks, a data transfer rate limit is often imposed on such an application. Herein bandwidth, data transfer rate and network transfer rate are all used interchangeably. Bandwidth is typically measured in Hertz but in digital communications bandwidth is frequently specified in bits per second (bps), which is actually a transfer rate. Meanwhile the network bandwidth available to upper layer applications, for example, file downloading and web browsing, may also vary over time, depending on factors such as network congestion, physical layer channel outage, etc. When the available network bandwidth is less than the amount requested by the multimedia streaming application, the streaming server may be forced to discard multimedia data in order to reduce data rate according to some data dropping policy. Any of the above reasons can cause data loss to occur at the client, which can negatively impact client playback quality. 
         [0004]    It would be advantageous, therefore, to have a method and apparatus to improve multimedia playback quality when a delay and/or bandwidth constrained multimedia streaming application is delivering multimedia data over a lossy network. As used herein, “/” denotes alternative names for the same or similar acts or components. 
       SUMMARY OF THE INVENTION 
       [0005]    Multimedia streaming applications often have strict delay and bandwidth constraints. The present invention describes a method and apparatus for the server to stream multimedia content that are compressed by scalable codec(s) to the client(s) through a lossy network which provides a feedback channel. With the present invention, the multimedia playback quality at the client side can be improved. 
         [0006]    A method and apparatus are described including receiving a coded packet of content associated with a layer, receiving feedback information regarding channel conditions and applying hybrid automatic repeat request to deliver the packet based on said feedback information. The method and apparatus wherein applying hybrid automatic repeat request layer-wise further includes determining if a resource is exhausted, scheduling delivery of layered coded content packets for a layer, if the resource has not been exhausted, determining if all layered coded content packets for the layer have been transmitted, proceeding to the first determining step, if all layered coded content packets for a layer have not been transmitted, determining if an acknowledgement message has been received from a user device, determining if the resource is exhausted, if no acknowledgement has not been received and scheduling delivery of layered coded forward error correction packets, if the resource has not been exhausted. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The present invention is best understood from the following detailed description when read in conjunction with the accompanying drawings. The drawings include the following figures briefly described below: 
           [0008]      FIG. 1  is an overview of the environment within which the present invention operates in unicast. 
           [0009]      FIGS. 2A and 2B  taken together are a flowchart of the operation of the present invention. 
           [0010]      FIG. 3  is a block diagram of a packet scheduler in accordance with the principles of the present invention. 
           [0011]      FIG. 4  is an exemplary temporal scalability coding structure. 
           [0012]      FIG. 5  is an overview of the environment in which the present invention operates in multicast. 
           [0013]      FIG. 6  is a block diagram of a packet scheduler in accordance with the principles of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    Real-time multimedia services require high data reliability and a low bounded time delay. However, many networks over which such services are offered are error-prone and bandwidth limited. Thus, proper error control techniques are necessary to obtain acceptable service quality. 
         [0015]    As used herein “client” includes any user device, end device, mobile terminal, computer, processor, laptop, personal digital assistant, dual mode mobile phone, set top box or any other device that could be used to display or playback content including audio, video and multimedia content. As used herein source or service data includes any form of uncompressed/compressed contents, including audio, video and multimedia data. 
         [0016]    Forward error correction (FEC) coding is a commonly used error control technique to improve the throughput of a lossy network. FEC adds controlled redundancy to the service data and the redundancy data (also called FEC data herein) are also sent to the client over the network. Whenever there are data losses, the client can utilize the redundancy provided by the FEC data to detect and recover losses. However, in order to recover data losses effectively, the amount of FEC needs to be adjusted according to the network loss condition. But such accurate information is hard to come by due to time-varying characteristics of many network channels, for example, wireless networks. Therefore, for such networks, under provisioning FEC can cause FEC failure, while over provisioning FEC can unnecessarily decrease network throughput, both of which can cause multimedia playback quality degradation. 
         [0017]    Retransmission is another technique commonly used in a lossy network when the network provides a feedback channel and the client is capable of generating feedback information. A typical protocol designed for retransmission works as follows. At the transmitter side, when the service data is sent over the network, the server waits for an acknowledgment (ACK) from the client through the network feedback channel. At the receiver side, an ACK is only sent by the client when the client receives all the data correctly. After a certain time period (called time-out period), if the server has not received an ACK, it assumes the data is lost and transmits again the same data to the client. This process is repeated until the server receives the ACK from the client for the data, and then the same process begins for the following data if any. The retransmission method generally only transmits the data again that are lost during transmission, so it is bandwidth efficient. However, the server has to wait for the ACK to act on, which usually introduces a long delay that is not suitable for real-time multimedia services. Also the method does not scale well. When the number of clients is large, the number of ACKs may grow unbounded and, thus, can seriously impact throughput. The problem of unbounded ACKs is referred to as the ACK explosion problem. 
         [0018]    A third method is hybrid Automatic Repeat-reQuest (ARQ), which combines the benefits of both retransmission and FEC. With hybrid-ARQ, at the transmitter side, the server first sends out user data and then FEC data to the client. During this process, when the client has received all the user data or enough FEC data to recover losses, the client sends an ACK back to the server through a feedback channel provided by the network. Once the server receives an ACK, it stops sending any more data. Since the amount of FEC data sent to the client can be adjusted properly according to the current network conditions through ACKs, hybrid-ARQ is able to maintain high bandwidth efficiency. However, when the network experiences excessive losses, the server may have to send a large amount of FEC data before it can receive an ACK. When the amount of the FEC data exceed the data transfer rate limit, or the duration of the FEC delivery exceeds the delay constraint, content data losses can still occur with the hybrid-ARQ method, which can degrade multimedia playback quality. 
         [0019]    A scalable source coder compress a source content and generates a scalable coded bitstream so that parts of a bitstream can be removed in a way that the resulting sub-bitstream forms another valid bitstream for a given decoder, which represents the source content with a reduced reconstruction quality compared to the original bitstream. There are many scalable source coders, for example H.264/AVC (temporal scalability), SVC (temporal, SNR and spatial scalabilities) for video coding, and JPEG2000, SPIHT for image coding, etc. 
         [0020]    A scalable source bitstream generated by the aforementioned source coders can be divided into a number of layers. The first layer contains a representation of the source and is independently decodable. Each following layer contains additional information about the source and has to rely on all the previous layers for the correct decoding. Hence, in general, among all the layers of a scalable bitstream, the first layer has the highest importance in terms of reconstructing the original source, and the importance of the other layers decreases as its layer index increases. 
         [0021]    In the following, a multimedia streaming application unicasting a scalable bitstream over a lossy network with feedback channel to a client is considered. The application has data rate transfer rate and delay constraint, which will be described below. 
         [0022]    Suppose a multimedia content is encoded by a scalable source coder into M layers (M∈N), such that the importance of layer m decreases as m grows (1≦m≦M, m∈N). Assume further that the bitstream from source layer m is packetized into K m  source packets for network delivery. In addition, L m  FEC packets are encoded for layer m, using certain FEC codes such as Reed-Solomon (RS) codes. In particular, assume L m  is large enough to recover possible data losses for layer m, without any data transfer rate or delay constraint. In the following, “data packets” include both source and FEC packets in general. 
         [0023]    In computer networks and as used herein, bandwidth is often used to indicate a data transfer rate and is, thus, usually denoted as bits per second. In most networks a communications path includes a series of links between individual nodes along the path. The bandwidth of the path is limited by the lowest bandwidth between any two links. 
         [0024]    Let B be the network transfer bit rate (in bps) assigned to the application, and T be the duration (in seconds) of a time slot during which the application is allowed to transmit data. For each time slot, BT equals to the total bit budget (total number of bits that are or can be transmitted over the link in a given time slot) allocated for the application and denote [d 1 , d 2 ] as the playback deadline incrementally for the video data inside. 
         [0025]    The present invention combines the hybrid-ARQ method with the scalable source coding property for multimedia data streaming. At the beginning of each time slot during which the server is allowed to transmit data, the server obtains the information about the number of packets for each layer of the scalable bitstream that have playback deadline fall in [d 1 , d 2 ]. Such information can usually be obtained at the content server, which encodes/stores the content data. The network server requests and receives such information from the content server. The network server then informs the client about the information, by means such as dedicated information packets, or as side information delivered through packets from the previous time slots, for example in the packet header. 
         [0026]    At the transmitter side, once the network server is guaranteed the client&#39;s receipt of the information regarding the number of source packets in the layer, it starts sending the data packets that belong to the time slot. In particular, the network server first sends the data packets that come from the layer with the lowest layer index (highest priority) and then proceeds to the next layer with an incrementally higher layer index. Within each layer, the source packets are sent first followed by the FEC packets. 
         [0027]    At the receiver side, the client receives data packets for a layer and constantly checks if it has received all the source packets for the layer, or it can use the received FEC packets to recover any missing source packets. This is possible because the client is aware of the exact number of source packets for each layer it expects for the time slot. Once all the source packets are available for the current layer, it sends an ACK through the feedback channel to the server. The client repeats the above process for each data packet it receives for the current layer. 
         [0028]    During the transmission, one of the following three events can happen to the server:
       1. An ACK from the client arrives.   2. The bit budget BT for the time slot is exhausted.   3. The time duration T has passed for the time slot.       
 
         [0032]    Event 1 indicates that the client has received or is able to recover all video packets from layer m. In response to this event, the server stops sending data packets for layer m and proceeds to sending data packets from layer m+1 (when m+1≦M). In case when the current layer is the highest available layer, the server simply stays idle and waits for the next time slot. When event 2 or 3 occurs, it indicates the application has reached its data transfer rate limit. The server then has to wait for the next time slot and repeats the above operations. 
         [0033]    The present invention can be further extended to multicasting multimedia data to groups of clients. For each layer m, only one ACK is sent back to the server each time, when every client in the group has received or is able to recover the source layer. In one embodiment, there are classes of clients/receivers/user devices in a wired/wireless network. Groups of clients are categorized/clustered based on their channel loss conditions. So the multicast group a client belongs to may change over time as the channel condition for the individual client changes. That is, an individual client may join or leave one or more groups over time based on its channel conditions. In such a scenario, the clients in a group communicate among themselves and a single ACK corresponding to a content layer may be transmitted for the group when the individual client that is last able to receive/recover the content layer has done so. In the case of a request for retransmission the individual client with the most packets needed to be retransmitted makes the request. That is, in all cases described above, the individual client with the most need/worst conditions is the only client in the group of clients to respond for each content layer. This addresses the ACK explosion problem by using feedback suppression. 
         [0034]    Given a bandwidth or delay constraint, the present invention allocates the remaining resources to guarantee the correct delivery of the current source layer, starting from the data with the most important, to the data with the least importance. For each successfully delivered layer of multimedia data, because of the use of hybrid-ARQ, the invention can maintain high bandwidth efficiency for a lossy network. In the case when the given resource is exhausted before all the multimedia data belonging to a time slot are delivered to the client, the present invention guarantees minimal performance loss by exploiting the scalability property of the source bitstreams. Hence, the present invention can provide flexible bit rate adaptation according to the network lossy conditions and provide improved multimedia playback quality at the client. 
         [0035]      FIG. 1  is an overview of the environment within which the present invention operates in unicast. Server  105  schedules content for transmission to clients/user devices over a lossy network  110 . Server  105  as will be shown in  FIG. 3  can include two servers—a content server and a network server. The network provides for a feedback channel to inform server  105  of receipt by the user device/client  115   a ,  115   b  of the transmitted content or the receipt by the user device of sufficient packets (content and FEC) in order to recover the current layer of coded content. 
         [0036]      FIGS. 2A and 2B  taken together are a flowchart of the operation of the packet scheduler of the present invention. In this embodiment of the present invention, the packets are transmitted/dispatched in unicast to a plurality of clients/user devices/receivers. At  205 , the packet scheduler (shown in  FIG. 3 ) receives layered coded packets (content and FEC) from content server (shown in  FIG. 3 ). The packet scheduler also receives feedback at  210  from the clients/user devices over a feedback channel from the network server (shown in  FIG. 3 ). The packet scheduler then initializes a time slot index at  215 . At  220 , the packet scheduler initializes a coded data layer index. A test is performed at  225  to determine if any resources have been exhausted for this time slot. That is, a determination is made if the bit budget for the time slot or the deadline for packet delivery expired. If any resource has been exhausted then the time slot index is incremented at  260 . If no resources have been exhausted then at  230 , the packet scheduler instructs/schedules the network server to transmit/dispatch the next content packet that belongs to the current coded data layer. At  235 , a test is performed to determine if all of the content packets belonging to the current layer have been transmitted/dispatched. If all of the content packets belonging to the current layer have not been dispatched then the process proceeds to  225 . If all of the content packets belonging to the current layer have been dispatched/transmitted then a test is performed at  240  to determine if an ACK has been received from the client. If an ACK has been received from the client then a test is performed at  265  to determine if the current layer index is the highest layer index. If the current layer is not the highest layer then the process proceeds to  255  where the packet scheduler increments the coded data layer index. The process then proceeds to  225 . If the current layer index is the highest layer index then the time slot is incremented at  260 . If no ACK has been received from the client then a test is performed at  245  to determine if any resources have been exhausted for this time slot. That is, a determination is made if the bit budget for the time slot or the deadline for packet delivery expired. If any resource has been exhausted then the time slot index is incremented at  260 . If no resources have been exhausted then at  250  the packet scheduler instructs/schedule the network server to transmit/dispatch the FEC packets. 
         [0037]      FIG. 3  is a block diagram of a server in accordance with the principles of the present invention. Server  105  shown in  FIG. 1  may be implemented as content server  305   a  and network server  305   b . Content server  305   a  is responsible for providing layered-coded video bitstreams to network server  305   b  as well as source information to the packet scheduler module  310 . Packet scheduler module receives the layered coded packets from the content server and receives feedback from user devices/clients via network server  305   b . Packet scheduler module  310  forwards packet scheduling information to network server  305   b  as discussed above with reference to  FIGS. 2A and 2B . Packet scheduler module  310  can be implemented both in hardware or software or a combination. Network server  305   b  transmits packets (content and FEC) to and over lossy network  315  over downlink channel  320 . Lossy network transmits packets (content and FEC) over downlink channel  330  to clients/user devices  340 . As used herein and especially in a multicast environment clients/user devices  340  can be multiple classes of clients/receivers/user devices. Lossy network  315  receives feedback from clients/receivers/user devices over feedback channel  335 , which feedback lossy network  315  forwards over feedback channel  325  to network server  305   b .  FIG. 3  shows a single client/user device/receiver. In actual practice it is assumed that there are multiple clients. The point of  FIG. 3 , however, is that this embodiment of the present invention is unicast to a plurality of clients. It should also be noted that the functionality of the packet scheduler can be in a module (software or hardware) on its own as shown or the functionality can be split between the content server  305   a  and the network server  305   b  or all three units (the content server  305   a , the network server  305   b  and the packet scheduler  310 ) may be combined in a single device. 
         [0038]      FIG. 4  is an exemplary temporal scalability coding structure using H.264 main profile to perform the encoding. H.264 is one of several possible coding schemes. The set of pictures in  FIG. 4  is called a “group of Pictures” or GOP, which can be divided into a base layer (BL) and multiple enhancement layers (ELs).  FIG. 4  shows 3 ELs in the temporal direction. The BL includes slices of instantaneous decoder refresh (IDR) and P/B types. The ELs include slices of reference and non-reference B-types (what are B-types?) denoted as Bs and B respectively. The arrows in  FIG. 4  indicate the reference dependencies during encoding. The reference dependencies form a hierarchical motion prediction structure. From  FIG. 4  it can be seen that the BL is independently decodable and El-n has to rely on BL and EL-m (where m&lt;n) for correct decoding. Furthermore because of the prediction dependencies, a missing picture from EL-m only affects pictures from higher layers such as EL-n. A lost picture, however, from the BL can affect all of the following pictures until the next IDR is received. Consequently, in such a coding structure, the BL is generally the most important (and thus layer having the lowest index) followed by EL- 1 , EL- 2  and EL- 3  in order of decreasing importance. Such a coding structure satisfies layered content coding. It is specifically noted that the layered hybrid-ARQ can also work with other content coding scalability dimensions. 
         [0039]      FIG. 5  is an overview of the environment in which the present invention operates in multicast. Server  105  and lossy network  110  are as described above with reference to  FIG. 1 . Multicast group  1   505  is a class of clients/user devices/receivers which have joined multicast group  1  based on their channel conditions, where channel conditions are often a function of distance from the source. Multicast group  2   510  is another class of clients/user devices/receivers which have joined multicast group  2  based on their channel conditions. There may be additional multicast groups in the environment. 
         [0040]      FIG. 6  is a block diagram of a packet scheduler in accordance with the principles of the present invention. In this embodiment of the present invention the packet scheduler performs scheduling for groups of clients as opposed to individual clients in accordance with the description of the extension of the principles of the present invention from unicast to multicast.  305   a ,  305   b ,  310 — 335  are as described above with reference to  FIG. 3 . Multicast client groups  605   a ,  605   b  and  605   c  each can have a single client or a plurality of clients. 
         [0041]    It is to be understood that the present invention may be implemented in various forms of hardware, software, firmware, special purpose processors, or a combination thereof. Preferably, the present invention is implemented as a combination of hardware and software. Moreover, the software is preferably implemented as an application program tangibly embodied on a program storage device. The application program may be uploaded to, and executed by, a machine comprising any suitable architecture. Preferably, the machine is implemented on a computer platform having hardware such as one or more central processing units (CPU), a random access memory (RAM), and input/output (I/O) interface(s). The computer platform also includes an operating system and microinstruction code. The various processes and functions described herein may either be part of the microinstruction code or part of the application program (or a combination thereof), which is executed via the operating system. In addition, various other peripheral devices may be connected to the computer platform such as an additional data storage device and a printing device. 
         [0042]    It is to be further understood that, because some of the constituent system components and method steps depicted in the accompanying figures are preferably implemented in software, the actual connections between the system components (or the process steps) may differ depending upon the manner in which the present invention is programmed. Given the teachings herein, one of ordinary skill in the related art will be able to contemplate these and similar implementations or configurations of the present invention.