Abstract:
A video sequence is to be transmitted at a selectable quality from a server over a network. A network control protocol operates in response to a control parameter to allocate to the server a share of available transmission capacity in proportion to the value of the control parameter. First, one determines a plurality of transmission rate values that are needed over successive time periods of the sequence for successful transmission of the sequence at a reference quality; then the control parameter is set, proportional to the transmission rate value and communicated to the control protocol at corresponding time instants. The sequences can then be transmitted, encoded at a quality from time to time selected in dependence upon the actual transmission capacity made available by the network to the server.

Description:
This application is the U.S. national phase of International Application No. PCT/GB2010/000217 filed 5 Feb. 2010, which designated the U.S. and claims priority to EP Application No. 09250346.5 filed 12 Feb. 2009, the entire contents of each of which are hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to the transmission of video signals over telecommunications networks. 
     BACKGROUND TO THE INVENTION 
     Transmission of video over data networks, such as the Internet, is commonplace today. To receive such signals, a user can use a suitably configured computer or other receiver such as a “set top box” (STB). STBs have become increasingly popular and many are provided with an IP connection allowing content such as video to be streamed or downloaded over the Internet. Television delivered over the Internet, commonly referred to as IPTV, is a good example of this growing service. 
     When streaming video data over an IP network, there are no guarantees that the data sent will reach its destination. When the network experiences congestion and other problems, delays will occur to the transmission of the data packets and some packets may even be lost. 
     To provide more reliable end-to-end delivery of data, the transmission control protocol (TCP) is often used as the transport protocol. Indeed, it is quite common to use TCP in video streaming systems for a number of reasons, but primarily because TCP provides mechanisms for ensuring reliable delivery, and managing network congestion. For example, one way in which TCP achieves reliability is by obliging the receiver to acknowledge to the sender all data received. If a packet of data remains unacknowledged after a predetermined period of time, TCP assumes the packet was not received and the same packet is retransmitted by the sender. One way that TCP manages congestion is by reducing the transmission rate of data as a function of congestion in the network. 
     Take the scenario where a number of video streams are being delivered using TCP and all share a contended piece of network. When congestion occurs, the TCP congestion control algorithm will force all the streams to back off their delivery rate to allow the congestion to clear. Each stream backs off by a fixed factor and eventually all streams will stabilise at approximately the same bandwidth (assuming a similar round trip time). 
     Use of such a method is not without problems. If the bandwidth becomes less than that required by the video content, play-out of the video could be stalled until sufficient data has been received to restart play-out. This situation can be mitigated by buffering data at the receiver having previously received it faster than necessary for play-out, and by switching the quality of the video transmitted, so that the required bandwidth is reduced to less than or equal to that now provided by the network. 
     Rate-adaptive, variable bit rate, video streams, where the transmitted video quality or bit rate is adapted over time, are also sometimes delivered over TCP. However, the above congestion scenario may still occur, and two streams each having a different average encoded bitrate for the same video quaity will still stabilise to roughly the same reduced transmission bitrate when the network is congested. This may result in some particularly undesirable results where, a first stream is initially encoded at a high bitrate, for example a video sequence with high frame activity such as a sports sequence, and a second sequence is encoded at a low bit rate, for example a video sequence with a low frame activity such as a news or drama sequence. 
     When congestion is experienced in the network, TCP will cut the available bandwidth for both streams to roughly the same level. This will affect the first stream, which was encoded at a higher bitrate and thus has a higher bandwidth requirement, more than the second stream, which was encoded at a lower bitrate and thus may still have enough bandwidth. Put another way, the first, high bitrate, stream will be more significantly affected than the second, low bitrate stream, as the first stream is given the same reduced bandwidth as the second stream. This will cause the quality of the video delivered to each user to vary over time, and the quality to vary from user to user depending on the type of video clip they are viewing. 
     Another way of streaming video that mitigates some of these problems experienced under TCP is to use a constant bitrate delivery system where the bitrate available to a video stream is fixed, for example by a reservation scheme, before the transmission of data starts. This method of delivery is easier to manage, but is not without its problems. 
     Again, take the example of the two video streams above, where we have a first stream that has very active frames such as a sports clip, and a second stream with less active frames such as a news clip. The bitrates reserved and used to deliver the two streams are fixed at a predetermined rate (that is considered to be sufficient for most applications and in this case for both streams). However, the second stream will not actually require that much bandwidth as the bitrate of the encoding can be much lower than that of the first sequence given that the activity in the second sequence is much less. The second stream transmitted using this fixed bandwidth is thus wasting much of its bandwidth. If the second stream increases the encoding rate so as to utilise the entire bandwidth reserved, the quality of the resulting video is likely to be of a lot higher quality than the first stream. However, this increase in quality may not necessarily be significant as perceived by the viewer and may thus be wasted. Moreover, having this redundant bandwidth is not an efficient use of network resources. 
     The problems above are heightened when one starts considering video sequences that vary in activity during the sequence itself. For example a relatively static news reading sequence might be interspersed with highlights of very active football clips. 
     International patent WO2008/119954 describes a method of delivering video streams over a contended network, where each stream delivered at a constant quality. 
     International patent WO2004/047455 describes a method of delivering a variable bit rate sequence over a network at a piecewise constant bit rate, with the rate of each piece decreasing monotonically. The resulting bit rate profile is referred to as a “downstairs” function. 
     U.S. Pat. No. B1-6,259,733 describes a method for statistical multiplexing, where multiple video sources are encoded at the same time and multiplexed into a single channel for transmission. The video sources are analysed for spatial and temporal complexity to get a relative need of bit rate, which is scaled according to an importance factor (high for movies, low for news for example), and which is then used to divide up the bandwidth. 
     US patent application 2006/224762 describes a method of estimating an encoding complexity for video sequences, and using that estimated encoding complexity to determine a bit rate for encoding. 
     SUMMARY OF THE INVENTION 
     According to the invention there is provided a method of transmitting a video sequence at a selectable quality from a server over a network, wherein the server operates a network control protocol operable in response to a control parameter to allocate to the server a share of available transmission capacity in proportion to the value of the control parameter, comprising 
     determining a plurality of transmission rate values needed over successive time periods of the sequence for successful transmission of the sequence at a reference quality; 
     setting the control parameter proportional to the transmission rate value; 
     sending the control parameter to the control protocol at corresponding time instants; 
     transmitting video, encoded at a quality from time to time selected in dependence upon the actual transmission capacity made available by the network to the server. 
     Other aspects of the invention are set out in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention reference will now be made by way of example only to the accompanying drawings, in which: 
         FIG. 1  is a graph showing the variation in bitrates used to encode a video sequence at a constant quality; 
         FIG. 2  is a network diagram of a system in an embodiment of the present invention; 
         FIG. 3  is a diagram showing two different video clips encoded at three quality levels; 
         FIG. 4  is a network diagram of a system in a second embodiment of the present invention; 
         FIG. 5  is a graph showing downstairs bit rate curves for a video sequence coded three times at the constant quality levels 2.6, 3.4, and 4.2. 
         FIG. 6  is a graph showing the ratio of the downstairs bit rates at qualities of 2.6 and 4.2 compared to the downstairs bit rates at quality 3.4. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It is proposed that video streams transmitted over a contended piece of network are encoded at a constant quality rather than at a constant bitrate. If a video sequence is encoded at constant quality, then the bitrate used is likely to vary dramatically.  FIG. 1  shows the bitrate over a 90 second video clip encoded at a constant quality level. In this example, the quality level has been determined by setting a quantiser parameter (qp) to 28. The quantiser parameter effectively controls the quality of the encoding—the smaller the quantiser, the better the quality. This clip is typical of many video sequences, with the bitrate varying depending on how complex the scene is at any given moment in time. For example, a sequence of frames where there is much movement or action usually requires a higher bitrate to encode at the same fixed quality. 
     The two different traces in  FIG. 1  are for two different amounts of buffering. The more dynamic trace is where the bitrate is the average used over 3 frames, whereas the smoother trace is where a sliding window of 192 frames has been used. Thus, by increasing the amount of buffering, the bitrate is effectively smoothed. 
     The bitrate for any practical streaming system will vary considerably as a function of the difficulty of encoding the specific sequence of content as described above. This variation is even more apparent when comparing different genres of video clips. For example, sports clips might require a higher average bitrate and fluctuate more due to the high activity of typical scenes, whereas a news report clip might require a much lower bitrate and be relatively static. 
     As such, to deliver video streams at a constant quality to users sharing a contended network, a constant bandwidth method is not efficient to use across all the streams. The bandwidth allocated to each stream must be allowed to dynamically vary in time in accordance with the precise demands of the video being streamed at that time and also be within any network bandwidth constraints. 
       FIG. 2  shows an example of a system  200  comprising a video encoder  206  connected to a video store  208 , which is in turn connected to a server  210 . The server  210  can communicate with each of two receivers, receiver_A  216  and receiver_B  218 , over the IP network  214 . The receivers  216  and  218  can make requests to the server  210  for video clips. The IP network  214  operates under a modified TCP arrangement which is described below in an embodiment of the present invention. 
     The encoder  206  encodes video sequences for transmission to the receivers  216  and  218 . Here two video sequences are shown, sequence_A  202  and sequence_B  204 . The encoder  206  can encode at various bitrates and outputs the encoded video sequences to the data store  208  or directly to the server  210 . The data store  208  is used to store encoded video sequences until they are requested or needed by the server  210 . The server  210  retrieves the encoded video sequences from the data store  208  or directly from the encoder  206 , and transmits them as video streams over the IP network  214  to either of the two receivers  216  and  218 . Sequence_A  202  is requested and will be transmitted (after encoding) to receiver_A  216 , and sequence_B  204  is requested and will be transmitted to receiver_B  218 . The receivers may be suitably configured computers or set top boxes for example, and are adapted to decode the received video stream and decode the encoded sequences into the original video for viewing by a user. 
     When congestion occurs in the IP network  214 , it is handled using a modified network control mechanism. In this example, the standard TCP protocol is modified to handle the congestion. Specifically, the dynamics of the congestion control algorithm built into TCP is modified so that the fraction of bandwidth that is allocated to any video stream over the IP network  214  at a given point in time is a function of the bandwidth requirements of that content. Thus more complex video sequences, such as sports sequences, should be given more bandwidth than less complex sequences such as news report clips. At the same time, the video streams transmitted over the IP network  214  are modified so that they are transmitted at the maximum bitrate allowed by the congestion control mechanism. 
     Currently under TCP, congestion control is effected using a sliding window mechanism. The length of the sliding window determines how much data is sent before requiring an acknowledgement. The bitrate is a function of the size of this window as well as the round trip time (time between sending data and an acknowledgement being received). To ensure that more complex video sequences gets a larger share of the bandwidth, the dynamics of the congestion control algorithm are altered such that the more difficult or complex content has a larger sliding window. In effect, the ‘greediness’ of TCP is modified. 
     A number of methods can be used to alter the ‘greediness’ of TCP. One method is by modifying the backoff factor. Normally, TCP will halve the size of the sliding window (the backoff factor) associated with the stream in question when packet loss occurs (it is assumed that packet loss is caused by congestion). In one embodiment of the present invention, we can adjust this backoff factor, so that the window size is set to, for example, three quarters of its original size when packet loss is detected. The effect would be that the affected stream will be ‘greedier’ than normal and secure a larger bandwidth compared with the situation when the back-off factor was half. The stream being transmitted on this greedy TCP session can then be transmitted at a higher level of quality than would be possible if the TCP session were not greedy. 
     Such a system, known as MuITCP, has been described by Crowcroft and Oechslin (J. Crowcroft and P. Oechslin, “Differentiated end-to-end Internet services using a weighted proportional fair sharing TCP”. In ACM SIGCOMM Computer Communication Review archive, volume 28, pages 53-69, July 1998). MuITCP differs from traditional TCP in that it takes a parameter N which allows a stream to obtain a fraction of the available bandwidth that is equal to N times that of a single stream of traditional TCP. The standard congestion control mechanism of traditional TCP is the Additive Increase Multiplicative Decrease (AIMD) algorithm which is modified in MuITCP to emulate the behaviour of N TCP flows. The value of N can be non-integer: a value of 1.5 would result in a flow that was 50% more aggressive than a single flow 
     An alternative system known as “ECN skipping” described in our co-pending European patent application (Agent&#39;s ref. A31751 filed on the same day as the present application and entitled “Data Transmission”) could be used as an alternative to such a system as MuITCP. Explicit Congestion Notification (ECN) (K. Ramakrishnan, S. Floyd, D. Black, “The Addition of Explicit Congestion Notification (ECN) to IP”, IETF RFC21368, September 2001) is a protocol that allows endpoints to be informed about congestion through the use of packet marking rather than by packet drops. Routers equipped with active queue management are able to mark IP headers signalling congestion to the endpoints prior to buffers overflowing and consequential packet loss. There is little need for retransmissions (under ideal conditions) which gives rise to an improved overall throughput. In a conventional implementation of ECN, some packets in the forward path would be marked when congestion occurs, and this marking would be echoed back to the transmitter in backward path packets. The transmitter would, on receipt of such echoes, adapt the TCP parameters in the same way as it would in the case of packet loss, and reduce its transmission rate. In the “ECN skipping” scheme, the receiver does not echo all congestion markings back to the transmitter, but echoes a fraction of them, in accordance with an aggression factor. The sender, by receiving less echoed markings, reduces its transmission rate less frequently than a conventional ECN sender, and consequently obtains a larger share of the network bandwidth. 
     MuITCP and “ECN skipping” are two examples of transport protocol modification that allow transmission paths to obtain unequal shares of the available bandwidth, allowing a bandwidth N times that of an unmodified transport protocol. Other techniques having similar control properties could be employed instead. Both techniques have been shown to be able to obtain up to about three or four times the bit rate of a conventional transport protocol stably and consistently; they can also be configured with N a little below 1 to get less bit rate than a conventional transport protocol. Each specific transport protocol has a different range of N over which it can operate effectively. We aim to operate the transport protocol within this range as much as possible. 
     In the following we describe how, by encoding a test set of video sequences at a fixed reference quality level and calculating the “downstairs” delivery schedule (to be explained below) required for timely delivery of the encoded data, we determine a suitable reference bit rate that can be used to normalise required bit rate values so that the bandwidth allocation factor N will be in the optimal range of the transport protocol for the majority of video content. For simplicity we assume in this description that the optimal range of N is between 1 and 4, although in practice this will vary with the transport protocol that is used. 
     A video sequence that has been encoded at variable bit rate can be delivered over a network at piecewise constant bit rate, with the rate of each piece decreasing monotonically. This is believed to have first been noted by Professor Mohammed Ghanbari. He referred to the resulting bit rate profile as a “downstairs” function. 
     According to our international patent application (Ghanbari and Sun), published as WO2004/047455, delivery of variable bit-rate video can usefully be analysed as follows: 
     Consider, at a receiver, some arbitrary time segment (but equal to a whole number of frame periods), extending from time t g  at which the decoder begins to decode frame g to time t h  at which the decoder begins to decode frame h. The duration of this segment is that of h-g frame periods, that is, t h -t g . Suppose, further, that the transmission rate during this segment is A bits/frame period. 
     At time t g , the receiver must have already received the bits for all frames up to and including frame g, i.e. 
               ∑     j   =   0     g     ⁢       d   j     ⁢           ⁢   bits           
where d j  is the number of coded bits generated by the encoder for frame j.
 
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     Use of this rate means that the number of bits (h-g)A transmitted during the segment will exceed the number of bits generated for the segment, unless the maximum occurs for k=h, that is, at the end of the segment. On the premise that the continued use of the transmission rate thus calculated, after the maximum has passed, seems to represent the use of a rate higher than absolutely necessary, Ghanbari aimed to partition the data to be transmitted into segments in such a manner that these maxima always occurred at the end of a segment. In the patent application, this situation was visualised graphically by plotting, for each segment, average bit rate (over the relevant period g to k) against time. He also preferred to choose the length of each segment so that it extended up to the largest of the remaining maxima. In consequence, a graph of the needed bit rates against time appear as a decreasing staircase shape and is sometimes referred to as a “downstairs” function. 
     Alam, Khan and Ghanbari have observed that the positions in time at which changes to the downstairs bit rate occur are about the same when a video sequence is encoded multiple times at different levels of quality (“Multiple Bitstream Switching for Video Streaming in Monotonically Decreasing Rate Schedulers,” F. Alam, E. Khan, and M. Ghanbari, IEEE International Conference on Industrial Technology, 2006 (ICIT 2006), 15-17 Dec. 2006, pp. 973-978). 
     In order to set the value of N in an aggressive transport protocol, we hypothesize that the downstairs bit rate of a segment of a sequence of video encoded at a given perceptual quality can be approximated by a function that is separable into the product of a function of the content of the uncompressed video, and a function of the perceptual quality of the encoded video, thus,
 
 b   i ( q )= f   i   ·g ( q )  [8]
 
where b i (q) is the bit rate required to encode video stream i at perceived quality q, f i  is a function of the content of the uncompressed stream i, a measure of the encoding difficulty of the content within that stream, and g(q) is an invertible function of q, but which is independent of the content to be coded.
 
     We suppose that the bit rate on a contended network is now allocated in proportion to f i , the encoding difficulty of the i th  stream, as in 
                       b   i     ⁡     (   q   )       =       (       f     i   .           ∑   j     ⁢     f   j         )     ·     B   .               [   9   ]               
where B is the bit rate of the contended channel. Then by substituting [9] back into [8], we get equation [10] which shows us that such an allocation results in a quality q that is independent of the parameters of the stream i, and hence, if that allocation were used for all streams, then all streams would get the same quality.
 
     
       
         
           
             
               
                 
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     Importantly this shows the ratio between different f i  will be the same as the ratio between the bit rates at the reference quality b i (3.4). Hence, if we use an aggressive transport protocol for each video stream and set the respective values of N, N i , in proportion to f i , and therefore also in proportion to the encoded bit rate at the reference quality b i (3.4), the bandwidth each video stream would receive would be in proportion to this factor. And then from equation [10] we see that this will result in the same video quality being delivered for each video stream. 
       FIG. 5  shows the downstairs rate curve for a video sequence coded three times at the different fixed quality levels of 2.6, 3.4 and 4.2. As expected, it can be seen that the downstairs bit rate decreases with time. As encoding had been done with constant quality, from equation [8] it can be seen that f i  is also decreasing in time, in proportion to the downstairs bit rate. 
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     For a given quality q, this states that the ratio of the bit rate required at quality q to the bit rate required at the reference quality, 3.4 in this case, is constant, and hence independent of f i , that is, independent of the difficulty of encoding the video content. 
       FIG. 6  illustrates the validity of this. It shows the ratio of the downstairs bit rates at qualities of 2.6 and 4.2 compared with the downstairs bit rates at quality 3.4, showing that these ratios are mostly constant over the sequence. 
     We selected a test set of 32 video sequences, with durations ranging from 30 minutes to two hours, and representing a wide range of video content genres from action and drama movies to television drama, news and sport to children&#39;s television and user generated content. We encoded this test set of video sequences using MPEG-4 AVC, but could have used any other suitably configured video codec. The encoder was configured to encode with a fixed group of pictures structure, with regular encoding of Intra frames, to support random access into the encoded bitstream. The encoder was configured to encode with fixed perceptual quality, as described in our co-pending European patent application no. 0825081.5 (Agent&#39;s ref. A31594), although any other method of achieving constant or near constant perceptual quality, such as coding with fixed quantisation parameters, could have been used. We set the fixed quality level to a reference level, equal to 3.4 on the scale defined in BT.500.11 (“Methodology for the subjective assessment of the quality of television pictures”, International Telecommunications Union (ITU-R) Recommendation BT.500-11, 2002). 
     Then we calculated the downstairs curves for each of these encoded video sequences, and from these determined the downstairs bit rates and the durations for which these applied, A i  and k i . We created a list, in which each entry consists of the downstairs bit rate of one video segment, A i , and the duration of that video segment, k i . All segments of all video sequences in the test set are included in this list. The list is then sorted from lowest rate to highest rate. Then for each rate, starting at the lowest, the total amount of time, calculated as the sum of the duration elements in the sorted list, is calculated for elements in the list whose downstairs bit rate is between that of the current element and four times that value. We determined the maximum of these sums, and recorded the downstairs bit rate associated with this maximum, which we term the reference quality reference downstairs bit rate, R ref . 
     In general, when downstairs rates are normalised by this reference rate to get values of N, an optimal number will occur within the effective operating range of the transport protocol, N=1 to 4 in this specific example, but some may be outside of this range. The best course of action may depend on the characteristics of the actual transport protocol in use: it make be best to clip values outside of this optimal range to the limiting values of the range (1 and 4 in this example), or it may be best to operate with the actual value of N. 
     The method will now be described in more detail with reference to the system  200  of  FIG. 2 . 
     The encoder  206  is provided with two different video sequences, sequence_A  202  and sequence_B  204 . Each video sequence represents a different video clip. In this example, sequence_A  202  is of a sports clip such as a football match, and sequence_A is a news report clip. Both sequence_A  202  and sequence_B  204  are fed into video encoder  206 . The video encoder  206  takes each video sequence and encodes it. The encoding used is MPEG-4 AVC, but could be any other suitably configured video codec. 
     Each video sequence is encoded at 3 different fixed quality levels, one of which is the reference quality as above. The downstairs curve for each video sequence is calculated from the encoding at the reference quality. The downstairs bit rate associated with each encoded segment is recorded. Although we prefer to encode all three quality levels in advance, this is not strictly necessary; it is however necessary to record the downstairs bit rates for the reference quality sequence and therefore this needs to be encoded in advance or at least analysed sufficiently ahead of the transmission that the downstairs rates can be determined. 
     Of course, each video clip may be encoded at fewer or more quality levels. In this example, each of the two encoded sequences, at each encoded quality level, comprise four encoded chunks. This is shown in more detail in  FIG. 3 . A chunk represents an independently encoded portion of video. Switching between the transmission of one quality level and another quality level is possible at chunk boundaries without degrading the received pictures. In the preferred embodiment, a chunk is a group of pictures (as defined in the MPEG standard) of duration about one second (e.g. 24 frames for a 25 Hz video source), starting with an Intra frame: in a video sequence of duration one hour, there would be about 3750 chunks. Note that  FIG. 3  is diagrammatic; the chunks may all be of the same length within a particular sequence A, or as between sequence A and sequence B. In general the intersegment boundaries of Sequence B will not be simultaneous with those of sequence A. 
       FIG. 3  shows video sequence_A  202  encoded into three separate encoded video sequences: encoded video sequence_A1  300 , sequence_A2  310  and sequence_A3  320 . Encoded video sequence_A1  302  is encoded at a first and highest quality. Encoded video sequence_A2  310  is encoded at a second, medium quality. Encoded video sequences_A3  320  is encoded at a third and lowest quality. 
     Each of the encoded video sequences  300 ,  310  and  320  is divided into four individual chunks, where the start of each chunk corresponds to the same point in the un-encoded video sequence_A  202 . Thus, the start of chunk A1 — 2  304  corresponds to the start of chunk A2 — 2  314  and also A3 — 2  324 , but where the chunks are encoded at different quality levels. 
     Also shown in  FIG. 3  are the encoded sequences of video sequence_B  204  comprising encoded sequence_B1  330 , sequence_B2  340  and sequence_B3  350 . The three encoded sequences are each encoded at the same quality levels used for sequence_A  202 . Thus, sequence_B1  330  is encoded at the same highest quality level as sequence_A1  300 . Sequence_B2  340  is encoded at the same medium quality level as sequence_A2  310 . Sequence_B3  350  is encoded at the same low quality level as sequence_A3  320 . 
     Thus, the encoder generates encoded video sequences at three quality levels for both the video streams (to give six encoded streams), which are then sent to the data store  208 . The data store  208  stores these until they are needed. 
     The server  210  now receives a request from the receiver_A  216  for the video sequence_A  202 , and also a request from receiver_B  218  for the video sequence_B  204 . The server  210  retrieves the corresponding encoded video sequences from the data store  208 . Alternatively, the encoded sequences may be generated dynamically by the encoder  206  and sent directly to the server  210 . 
     The server  210  retrieves the downstairs bit rate, A i , associated with the first chunk of the video sequence_A  202 , as calculated from the encoding at the reference quality level. It then calculates a value of N, N i , as: 
                     N   i     =       A   i       R   ref               [   13   ]               
and configures the transport protocol for the transport of video sequence_A  202  with this value of N i . Similarly, the server  210  calculates and sets the value of N for the transport protocol for the transport of video sequence_B  204  using the downstairs data for the encoding of that sequence at the reference quality level. This value of N will be sent to the MuITCP control software which is be located inside the server  210 . In the case of MuITCP, the server N will need to know the value of N. In the case of ECN skipping, the receiver will need to know the value of N. It could be told this at the start of the streaming session or could be told the current value of N from time to time during the session, including for example, when it changes (being told all values at the start is our current preference).
 
     Some observations about the constant of proportionality 1/R ref  are in order here. Firstly, the constant used should preferably be the same or similar for both (or, in the general case, all) video sources on the same network. Secondly, if the network is used only for video managed in this manner, the absolute value of 1/R ref  is not very critical in the sense that variations will not change the picture quality provided that it keeps the value of N within the range (as discussed above) that the network control mechanisms can handle. If the network is shared with other traffic, on the other hand, it is desirable to avoid low values of N (e.g. by selecting a lower value for the reference rate) since values below unity will effectively give precedence to the other traffic in allocation of bandwidth. 
     In this embodiment of the invention, the same reference quality (q=3.4) is used for both (all) streams. Assuming that all streams are to have the same quality, this is the most convenient way to proceed. In principle, however, this is not essential. If two streams have their downstairs rates determined at different reference quality levels q1 and q2, then equal quality can be achieved if the difference (corresponding to a factor g(q1)/g(q1) is corrected for, for example by estimation of this ratio from training data or by separate determination, at the respective reference rate, of a respective value of R ref . Alternatively, it would be possible to deliberately use differing reference quality levels to provide for differing quality between streams in order to provide differing quality levels (standard, premium, etc.). If two streams use reference quality levels of 3.0 and 3.4 respectively one would expect the latter stream to have an allocation of bandwidth so that the quality achieved by the second stream was on average 0.4 units better. 
     Initially, the server  210  uses the lowest quality encoded sequences, sequence_A3  320  and sequence_B3  350 . The server  210  can use any of the other sequences as well, depending on what bandwidth the server  210  thinks the IP network  214  has available. If the network  214  handles these streams comfortably and indicates that there is further bandwidth available, perhaps by advertising a buffer overflow in the buffers of the receivers (which might also indicate that the receiver is not capable of consuming the data quickly enough), then the server  210  switches both encoded sequences over to the next higher quality sequence (at a chunk boundary to ensure continuity of the video sequence). 
     As the server  210  delivers data representing the video sequence  202 , it retrieves the downstairs bit rate calculated for the encoding of this video sequence at the reference quality for the chunk about to be delivered. When the downstairs bit rate for this chunk is different from that for the immediately previously transmitted chunk, it calculates a new value of N i  for the transmission of the video sequence according to [13] and configures the transport protocol with the new value of N. 
     Similarly, the server  210  performs the same operations as it delivers data representing the video sequence  204 , using downstairs bit rates calculated for the encoding of video sequence  204  at the reference quality. 
     Note that although the transmission of video sequences  202  and  204  may have started at the same time, the number of chunks of each sequence that have been transmitted at some subsequent time may not be equal. 
     When the IP network  214  becomes congested, the bandwidth made available to the server  210  for streaming the encoded video sequences must be reduced. The use of an aggressive transport protocol, configured with a value of N appropriate to the relative demands of the video sequences, enables the available network bandwidth to be shared, not necessarily equally, but such that nearly equal quality can be delivered for each video sequence. 
     The server  210 , or the receivers  216  and  218 , monitor the transmission rate that has been achieved through the network, and the amount of data that has been delivered and the amount that has been decoded and displayed, and select appropriate video quality levels to be transmitted, using a suitable selection mechanism such as that described in our co-pending European patent application no. 08253946.1 (Agent&#39;s ref. A31750), so that if the current network transmission bit rate were sustained, video data would be delivered in time for continuous decoding and display, without stalling. 
     The result is that the system provides equitable quality video streaming across multiple video sequences, so that each video sequence is delivered at an equal quality level to every other stream, even when experiencing congestion. 
     When the network is being fully utilised, the sum of the bitrates of the chunks being delivered at any point in time (e.g. A1 — 2 and B1 — 2 when both streams are at highest quality, perhaps when there is no congestion, or A2 — 4 and B2 — 4 when both streams are at medium quality, perhaps when slight congestion occurs) will be equal to the network capacity. By altering the TCP dynamics to ensure that each stream gets a proportion of the bandwidth that it needs for a given quality, then the chunks being delivered at any moment in time will all be of similar quality, even though they may require very different bandwidths. 
       FIG. 4  is a network diagram of a system in a second embodiment of the present invention; This is like that of  FIG. 2  but with two independent encoders and servers: each of the video sources  202  and  204  is connected to a respective encoder  206 A,  206 B, each connected to its own video store  208 A,  208 B which is in turn connected to its server  210 A,  210 B. Each server  210 A,  210 B can communicate with one of the two receivers, receiver_A  216  and receiver_B  218 , over the IP network  214 . 
     It will be understood that, on a dedicated network, all the traffic may be video streaming managed in the manner described above. Alternatively, the network may be one which also carries other types of traffic, as illustrated by the data source  220  and data receiver  222  in  FIG. 4 . 
     It will be seen that the methods we have described control the transmission of multiple video streams over a congested network so that each stream receives an equitable share of the bandwidth dependent on the quality of the encoding. Note that, although the above example uses the “downstairs” bit rate A i , other measures of picture complexity can be used instead, such as the “downstairs” bit rate evaluated over a limited time window, the instantaneous needed bit rate, or a smoothed version of the latter.