Abstract:
This invention relates to a method, a computer program product, apparatuses and a system for controlling a length of a frame buffer. The frame buffer is comprised in a receiver and buffers frames that are transmitted by a transmitter according to a frame transmission scheme and received at the receiver. The length of the frame buffer is controlled under consideration of a change in the frame transmission scheme.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a method, a computer program product, apparatuses and a system for controlling a length of a frame buffer. 
       BACKGROUND OF THE INVENTION 
       [0002]    Controlling a length of a frame buffer (corresponding to the control of the buffering time) is for instance of technical interest in the context of packet-switched transmission of speech data. 
         [0003]    International publication WO 2006/044696 A1 discloses systems and methods for such a controlling of the length of a frame buffer. Therein, a receiver-side de-jitter buffer, which adds delay to received packets, adaptively adjusts its size based upon the detected air link characteristic, such that the de-jitter buffer is appropriately sized for anticipated data packets before they are received at the subscriber station. 
         [0004]    A particular application area of packet-switched transmission of speech data are Voice over Internet Protocol (VoIP) systems. The packet-switched network transmission protocols typically employed for VoIP systems are the Real-time Transport Protocol (RTP, cf. IETF RFCs 3550 and 3551) on top of the User Datagram Protocol (UDP) on top of the Internet Protocol (IP). One or more speech frames, for instance obeying the Adaptive Multirate (AMR) codec or the AMR Wideband (AMR-WB) codec are encapsulated into RTP packets (IETF RFC 3267), which then form the payload of UDP packets, which in turn form the payload for IP packets. An example of such a VoIP application is the Internet Protocol Multimedia Subsystem (IMS) Multimedia Telephony standardized in the scope of the Third Generation Partnership Project (3GPP), cf. specification 3GPP TS 26.114. 
         [0005]    The checksums employed in the UDP and IP layers result in discarding all the packets in which the receiver detects bit errors. Since these packets contain the frames, the protocol stack does thus not convey any distorted frames to the application layer of the protocol stack. Hence, when the IP packets are transmitted over an error prone radio link or over any media introducing transmission errors, the application layer faces frame losses. On the other hand, none of the frames reaching the application layer contain any residual bit errors. Due to this phenomenon, the error concealment algorithm is not able to utilize partially correct frames, as can be done e.g. in the circuit-switched GSM telephone service, but the erroneous frame needs to be completely replaced. This is likely to make the error concealment less effective than the approach used in circuit-switched service. 
         [0006]    Various methods have been introduced to combat the packet loss conditions. There exist methods such as multiple description coding in which the information is distributed over several IP packets, and application level Forward Error Correction (FEC) in which the error correcting code is used to reconstruct the lost frames. 
         [0007]    In addition, two comparably simple approaches can be utilized to fight frame loss: redundant frame transmission (also denoted as frame redundancy transmission) and frame aggregation transmission. 
         [0008]    In redundant frame transmission, redundant copies of previously transmitted frames are transmitted together with the new frames, wherein the redundant frames are to be used in the receiver to replace frames carried in packets that were lost somewhere along the transmission path (note that this can be considered as a simple application level FEC). A packet thus contains one or more new frames and one or more redundant frames that have already been transmitted in previous frames. 
         [0009]    In contrast, in frame aggregation transmission, several new frames are combined into one packet. This lowers the relative RTP/UDP/IP packet overhead, and hence the overall bit rate, which may decrease the error rate on a loaded (radio) link. Furthermore, when several frames are aggregated into a single packet, the packets are transmitted less frequently, which is likely to reduce congestion in the routers of an IP-based network. 
         [0010]    Frame redundancy and frame aggregation can thus be understood to be defined by a frame transmission scheme that indicates which and how many frames are transmitted in an RTP packet. 
         [0011]    One of the advantages of redundant transmission is the low computation requirement. It is applied by simply attaching the current and one or more previously transmitted frames within the same RTP packet. Decoding the redundant stream is also very straightforward: when a packet is lost, the receiver only needs to wait for the packet containing the redundant copy of the frame to arrive to convey it further to the decoder. Therein, it should be noted that a redundant copy of a specific frame does not necessarily have to be transmitted in the packet immediately following the packet in which said specific frame was transmitted. For instance, if interleaving with a depth of N packets is performed, where N is an integer, a frame and its redundant copy may be contained in a first and an N-th packet in a sequence of packets, respectively. 
         [0012]    One drawback of frame redundancy is the increased bit rate. Basically, the bandwidth requirement is doubled when one redundant frame is attached to each transmitted packet. Furthermore, more importantly, the system delay is increased since the receiver needs to buffer the speech frames for the duration covered by the redundancy. 
         [0013]    A similar delay issue is emerging in frame aggregation as well. 
         [0014]    When changing the frame transmission scheme by switching on frame redundancy or frame aggregation in the transmitter, the receiver detects a delay step. For example, when switching from transmitting one frame per packet (which will be referred to as “normal transmission” in the following) into two frames per packet condition (either due to frame redundancy, where one of the frames is a new frame and one of the frames is a redundant frame, or due to frame aggregation, where both frames are new frames), an additional delay component of one frame length, typically an additional 20 ms, is introduced. Furthermore, if the redundant frame corresponds to a frame transmitted already N packets before (N&gt;1), the additional delay is even N×20 ms. 
         [0015]    This is due to the fact that, in case of switching from a normal transmission to a redundant transmission covering one frame, the receiver needs to increase the buffering time by the duration of a frame in order to allow the redundant frames to arrive in time for decoding. Consequently, when the redundant frame and its associated frame are not transmitted in subsequent packets, the buffering time increases even further. 
         [0016]    In case of switching from a normal transmission to frame aggregation transmission with for instance two frames per packet, the transmitter needs to wait for one additional frame duration to collect another frame for packetisation. 
         [0017]    Thus in both cases, the receiver needs to wait for a certain time period before decoding the first packet after the change in the transmission scheme. 
         [0018]    On the other hand, the decoder and playback devices being fed by the frame buffer require constant input, and therefore the gap introduced to the frame sequence by the change of the transmission scheme must be compensated. In the worst case, when the buffering time is set very short, the receiver may need to insert one or more error concealment frames for the decoder before the first aggregated or redundant frame packet arrives. This is likely to provide (short-term) speech quality degradation—although the actual purpose of the usage of frame redundancy of frame aggregation was to improve speech quality in challenging transmission conditions. 
       SUMMARY 
       [0019]    To combat the above-described speech quality degradations caused by changes in the transmission scheme, it may be considered to ensure that the length of the frame buffer at the receiver is large enough to cope with sudden delay increases. For example, a receiver buffer may require an additional headroom of one frame length (e.g. 20 ms) if change of the transmission scheme from normal transmission to frame redundancy transmission with 100% frame redundancy or from normal transmission to frame aggregation transmission with two frames per packet is expected to occur during a session. 
         [0020]    However, since particularly in real-time applications such as conversational or streaming applications, delays are experienced as annoying and may aggravate proper communication, it is desirable that the buffer length is kept to a minimum. 
         [0021]    A method is disclosed, comprising controlling a length of a frame buffer, which frame buffer is comprised in a receiver and buffers frames that are transmitted by a transmitter according to a frame transmission scheme and received at said receiver, under consideration of a change in said frame transmission scheme. 
         [0022]    Moreover, a computer-readable medium having a computer program stored thereon is disclosed, the computer program comprising instructions operable to cause a processor to control a length of a frame buffer, which frame buffer is comprised in a receiver and buffers frames that are transmitted by a transmitter according to a frame transmission scheme and received at said receiver, under consideration of a change in said frame transmission scheme. 
         [0023]    Therein, said computer-readable medium may be any medium that is capable of storing digital data in electric, magnetic, electromagnetic or optic form. Said medium may be a separate medium, or may be integrated in said receiver. Said processor may be particularly dedicated for controlling said length of said buffer, or may be a processor that also performs other tasks, such as for instance a central processor of said receiver. 
         [0024]    Therein, the invention is to be understood to cover such a computer program also independently from said computer-readable medium. 
         [0025]    Moreover, an apparatus is disclosed, comprising a control unit, configured to control a length of a frame buffer, which frame buffer is comprised in a receiver and buffers frames that are transmitted by a transmitter according to a frame transmission scheme and received at said receiver, under consideration of a change in said frame transmission scheme. 
         [0026]    Therein, said apparatus may for instance be a module for said receiver, or may be said receiver itself. Said apparatus may for instance be an electronic device with telephone functionality, such as for instance a mobile phone, a personal digital assistant or a computer. 
         [0027]    Finally, a system is disclosed, comprising a transmitter, configured to transmit frames according to a frame transmission scheme; and a receiver, comprising a frame buffer configured to buffer said frames received from said transmitter; and a control unit configured to control a length of said frame buffer under consideration of a change in said frame transmission scheme. Said system may for instance be a communication system that implements VoIP, for instance via the IMS Multimedia Telephony service as standardized by 3GPP. 
         [0028]    According to the present invention, a frame buffer is comprised in a receiver and buffers frames that are received from a transmitter, which transmits said frames according to said frame transmission scheme. A change in said frame transmission scheme may affect a required length of said frame buffer. To keep the length of the frame buffer at a minimum, and thus to reduce the delay encountered when extracting the frames from the frame buffer for further processing (e.g. decoding or rendering of the frames at a constant frame rate), it is advantageous that the length of the frame buffer is controlled under consideration of said change in said frame transmission scheme. Therein, the control of the length of said frame buffer does not only have to be based on said change of said transmission scheme. Said control may be based on additional parameters, such as for instance characteristics of a transmission network or link over which said frames are transmitted, as for instance the average or maximum transmission delay. Said control of said length of said frame buffer (corresponding to the buffering time) may for instance be handled by a buffer control unit by performing a frame insertion using error concealment or by causing a decoder that decodes said frames to perform time scaling to slow down the decoding and playback of the sequence of frames extracted from the frame buffer, and hence, increase the buffering time. 
         [0029]    According to an exemplary embodiment of the present invention, said change in said frame transmission scheme is a future change. A length of said frame buffer is thus controlled under consideration of a future change in said frame transmission scheme, i.e. a change in said frame transmission scheme is anticipated at the receiver proactively. Being able to control the length of said frame buffer in advance, i.e. before an actual change in said transmission scheme occurs, has the advantage that the receiver may prepare for the consequences of said change in said transmission scheme and may compensate said consequences in due time. Furthermore, the receiver may have more degrees of freedom left in deciding when to compensate said consequences. For instance, if said change in said frame transmission scheme causes a temporal gap in the sequence of frames extracted from the frame buffer for decoding and/or playback, a buffer control algorithm at the receiver may be able to perform frame insertion or to cause a decoder to perform time scaling in a controlled manner during a specific period selected to minimize the subjective quality distortion, for instance a period of low signal or high noise. 
         [0030]    Equally well, said change in said frame transmission may be a current or past change in said frame transmission scheme. Even if said change in said frame transmission scheme has already occurred, there may still be enough time to control the length of said frame buffer accordingly, for instance if said frames encounter large transmission delays. 
         [0031]    According to an exemplary embodiment of the present invention, said change in said frame transmission scheme is commanded by said receiver. Said receiver may for instance send, to the transmitter, a command for a change in said frame transmission scheme (e.g. a media adaptation command). Therein, said command may for instance comprise information on the desired frame transmission scheme, the level of redundancy or the level of frame aggregation. Said transmitter may be obliged to obey the receiver&#39;s command and to change the transmission scheme accordingly. Said receiver thus has full knowledge that a change in the transmission scheme will occur. 
         [0032]    According to an exemplary embodiment of the present invention, said change in said frame transmission scheme is requested by said receiver. Said receiver may for instance send, to the transmitter, a request for a change in said frame transmission scheme (e.g. a media adaptation request). Therein, said request may for instance comprise information on the desired frame transmission scheme, the level of redundancy or the level of frame aggregation. Said transmitter may not be obliged to fully satisfy said request, i.e. it may not perform any changes in said frame transmission scheme at all, may only partially change said frame transmission scheme, or may change said frame transmission scheme to an extent that exceeds the receiver&#39;s request. According to this embodiment, said receiver thus has at least implicit knowledge that a change in the transmission scheme might occur, so that, to avoid loss of frames or unexpected gaps in the frame sequence extracted from the frame buffer, it is advantageous to control the length of the buffer as if said change in said frame transmission scheme was for sure. 
         [0033]    According to an exemplary embodiment of the present invention, a necessity of said change in said frame transmission scheme is determined by said transmitter at least partially based on measurement data provided by said receiver. Said measurement data may for instance be related to the packet loss rate or to jitter conditions. Said measurement data may for instance be transmitted via RTCP receiver reports. When the receiver applies the same algorithm as the transmitter to decide if, based on the measurement data, a change of the transmission scheme is required, the receiver is enabled to anticipate the change of the transmission scheme by the transmitter. Alternatively, said transmitter may determine a necessity for said change in said frame transmission scheme due to a handover to another access technology with different transmission characteristics and/or due to a change in the available bandwidth. 
         [0034]    According to an exemplary embodiment of the present invention, said controlling comprises at least one of frame insertion and time scaling. When changing from normal transmission to frame aggregation transmission or to frame redundancy transmission, the frame buffer length has to be increased by one or more frame durations. This means that extraction of frames for decoding and rendering (e.g. playback) from the frame buffer is also delayed by said one or more frame durations, so that, when considering the situation before the change in the frame transmission scheme and after the change in the frame transmission scheme, there exists a temporal gap in the sequence of frames extracted from the frame buffer. This gap may for instance be handled by inserting an error concealment frame into the sequence of frames extracted from the frame buffer to fill the gap, wherein said error concealment frame may for instance be a copy of the temporally preceding or following frame or a function thereof. Alternatively, or additionally, the decoder may be triggered to perform time scaling in order to control the length of the frame buffer. Therein, time scaling is performed with the signals contained in the frames extracted from the frame buffer. For instance, if the signals are stretched in the decoder, a subsequent rendering unit (e.g. a playback unit) gets more data and thus requires less frames within a given period of time, so that the decoder needs to extract frames from the frame buffer less frequently. Consequently, the buffer occupancy is increased since the output of frames is now slower than the input, i.e. the length of the frame buffer is increased. If the frames are shrunk in the decoder, the rendering unit gets less data and thus requires more frames within a given period of time, so that the decoder needs to extract frames from the frame buffer more frequently. Consequently, the buffer occupancy is decreased, i.e. the length of the frame buffer is reduced. 
         [0035]    Therein, time scaling may allow a continuous and smooth concealment of the gap, whereas frame insertion may only allow an abrupt concealment of the gap. It may thus be advantageous if said inserted error concealment frame is surrounded by frames that only contain background/comfort noise or only a low or no audio signal at all. By time scaling and frame insertion, thus the gap in the sequence of frames may be concealed without excessively compromising the quality of the further processing of said sequence of frames. 
         [0036]    According to an exemplary embodiment of the present invention, said frame transmission scheme defines which and how many frames are transmitted in a packet of a transport protocol. A change of said transmission scheme may then for instance occur when the number of frames transmitted in a packet is changed, and/or when a rule defining which frames are transmitted in a packet is changed. 
         [0037]    According to an exemplary embodiment of the present invention, said transmission scheme is suited to define at least one of frame aggregation transmission and frame redundancy transmission. In frame aggregation transmission, a plurality of frames that not yet have been transmitted are transmitted in a packet. In frame redundancy transmission, at least one frame that has not been transmitted before and at least one frame that has been transmitted before are transmitted in a packet. A change of said transmission scheme may then for instance occur when a normal transmission, in which one frame is transmitted in a packet, is changed to a frame aggregation transmission, or is changed to a frame redundancy transmission. Moreover, a change of said transmission scheme may for instance occur when in a frame aggregation transmission, the number of frames transmitted in a packet is changed. Furthermore, a change of said transmission scheme may for instance occur when in a frame redundancy transmission, at least one of the number of frames that has already been transmitted and the number of frames that has not yet been transmitted is changed. 
         [0038]    According to an exemplary embodiment of the present invention, said frames are speech frames. Said speech frames may for instance obey the AMR or AMR-WB codec. 
         [0039]    According to an exemplary embodiment of the present invention, said frames are encapsulated into real-time transport protocol frames. Said RTP frames may in turn be encapsulated into UDP frames, which in turn may be encapsulated into IP frames. 
         [0040]    According to an exemplary embodiment of the present invention, said frames are transmitted via an IP-based network. 
         [0041]    According to an exemplary embodiment of the present invention, said receiver is a multimedia telephony service over internet protocol multimedia subsystem receiver. 
         [0042]    These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0043]    In the figures show: 
           [0044]      FIG. 1 : a schematic block diagram of an exemplary embodiment of a system according to the present invention ; 
           [0045]      FIG. 2   a:  a flowchart of an exemplary receiver processing according to the present invention; 
           [0046]      FIG. 2   b:  a flowchart of an exemplary transmitter processing according to the present invention complementing the receiver processing of  FIG. 2   a;    
           [0047]      FIG. 3   a:  a further flowchart of an exemplary receiver processing according to the present invention; 
           [0048]      FIG. 3   b:  a flowchart of an exemplary transmitter processing according to the present invention complementing the receiver processing of  FIG. 3   a;    
           [0049]      FIG. 4   a:  a further flowchart of an exemplary receiver processing according to the present invention; and 
           [0050]      FIG. 4   b:  a flowchart of an exemplary transmitter processing according to the present invention complementing the receiver processing of  FIG. 4   a;    
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0051]      FIG. 1  is a schematic block diagram of an exemplary embodiment of a system  1  according to the present invention. Said system  1  may for instance be a VoIP system, in particular a Multimedia Telephony Service over IMS (MTSI) system as standardized by 3GPP. System  1  comprises a transmitter  2 , a packet-switched network  3  and a receiver  4 . It is readily understood that system  1  may comprise more than one transmitter and more than one receiver. 
         [0052]    In transmitter  2 , speech from a speech source  20 , such as for instance a microphone, is encoded in speech encoder  21 . Speech encoder  21  may for instance implement the AMR or AMR-WB standard and outputs frames of digital data, which are transmitted over the packet-switched network  3  via the packet-switched network interface  22 . For instance, said packet-switched network may be an IP-based network. Then, in the packet-switched network interface  22 , the frames may be encapsulated into RTP packets, which in turn are encapsulated into UDP packets, which in turn are encapsulated into IP packets to be routed through the network  3 . 
         [0053]    The packetization of the frames into packets is, inter alia, defined by a frame transmission scheme defining which and how many frames are transmitted in a single packet. In a normal frame transmission, for instance one frame may be transmitted in one packet. However, to account for packet losses in the packet-switched network  3 , it may be advantageous to include more than one frame into a packet. For instance, in case of frame redundancy transmission, at least one frame that has not yet been transmitted and at least one redundant frame that has already been transmitted before are transmitted in one packet. In case of frame aggregation transmission, more than one frame are transmitted in a single packet (without redundant frames), in order to reduce protocol overhead and thus to decrease the overall bit rate, which may decrease the error rate on a loaded link and may reduce congestions in the routers of the packet-switched network  3 . 
         [0054]    To account for varying transmission characteristics of the packet-switched network, which may for instance be caused by radio links within the packet-based network  3 , the transmission scheme applied in packet-based network interface  3  can be changed by frame transmission scheme control unit  23 . For instance, the frame transmission scheme may be changed by changing the number of frames that are aggregated into a single packet, or by switching from normal transmission to redundant transmission, or by changing the number of redundant frames per packet, to name but a few possibilities. 
         [0055]    Receiver  4  comprises a packet-based network interface  42 , where packets received from network  3  are processed to recover the frames encapsulated therein. For instance, if packet-switched network interface  22  in transmitter  2  uses an RTP/UDP/IP protocol stack for encapsulation, also packet-based network interface  42  may use a corresponding RTP/UDP/IP protocol stack for recovering the frames. 
         [0056]    The frames as output by the packet-based network interface  42  generally do not arrive at a constant frame rate due to varying transmission delays of each frame in the packet-switched network  3 . This delay jitter is compensated for by a frame buffer, which, according to the present invention, is a variable-length frame buffer  44 . Frames buffered in frame buffer  44  may be read out from buffer  44  at a constant frame rate to be processed by speech decoder  41 , and the decoded speech is then forwarded to speech sink  40 , which may for instance be a loudspeaker. 
         [0057]    Frame buffer  44  is of variable length to allow that the buffer length may always be kept down to a minimum. Since the buffering in frame buffer  44  adds an additional delay to each frame, and since delays are considered annoying particularly in conversational or streaming applications, it is highly desirable to keep this additional delay as small as possible. On the other hand, in case of deteriorating transmission characteristics of the packet-switched network, it may become inevitable to change the transmission scheme at the transmitter to avoid packet loss. For instance, in case of packet loss in the packet-switched network, it may be inevitable to switch from normal transmission to frame redundancy transmission or to frame aggregation transmission. Both mentioned changes in the frame transmission scheme require an increased length of frame buffer  44 . For instance, if the transmission scheme is switched from normal transmission to frame aggregation transmission with e.g. two frames per packet, the packet-based network interface  22  at transmitter  22  has to wait for an additional frame length (of the second frame in the packet) before the packetization can be finished. Thus, compared to normal transmission, the length of frame buffer  42  has to be increased by an additional frame length. Similarly, if the transmission scheme is switched from normal transmission to frame redundancy transmission with e.g. one redundant frame and one non-redundant frame per packet, wherein the redundant frame is associated with a frame in the directly previous packet, the length of frame buffer  42 , compared to the case of normal transmission, has to be increased by one frame length to allow that also the redundant frame arrives in time for decoding (and can be considered for decoding). 
         [0058]    To this end, the length of frame buffer  42  is controlled by buffer length control unit  43  under consideration of a change in the frame transmission scheme at transmitter  2 . This control of the frame buffer length may be most effective if said change of said transmission scheme has not yet occurred, i.e. if said change is a future change. It is then possible to adapt the present buffer length to the desired new buffer length in a controlled manner, for instance in a way that minimizes the subjective quality distortion (e.g. of the media playback). This adaptation may for instance be performed by buffer length control unit  43  by means of insertion of an error concealment frame into the stream of frames extracted from frame buffer  44 . Alternatively, the buffer length control unit  43  may adapt the length of frame buffer  44  by sending a time scaling command to speech decoder  41 , as indicated by the arrow between buffer length control unit  43  and speech decoder  41 . 
         [0059]    Speech Decoder  41  then performs time scaling during or after decoding of the frames extracted from frame buffer  44 , wherein time scaling is performed with the speech signals represented by the frames in the following manner: If the signals are stretched in speech decoder  41 , speech sink  40  gets more data and thus requires less frames within a given period of time, so that speech decoder  41  needs to extract frames from frame buffer  44  less frequently. Consequently, the buffer occupancy is increased since the output of frames is now slower than the input, i.e. the length of frame buffer  44  is increased. If the frames are shrunk in speech decoder  41 , speech sink  40  gets less data and thus requires more frames within a given period of time, so that speech decoder  41  needs to extract frames from frame buffer  44  more frequently. Consequently, the buffer occupancy is decreased, i.e. the length of frame buffer  44  is reduced. 
         [0060]    Both frame insertion and time scaling thus allow to slow down decoding and playback of the sequence of frames extracted from the frame buffer and thus allow to increase the buffering time in case of additional delays. Therein, time scaling may allow to adapt the length of the frames from the decoder  41  in a continuous way, whereas frame insertion may be used for an abrupt adaptation of the frame buffer length. 
         [0061]    Receiver  4  can be enabled to anticipate changes of the frame transmission scheme by transmitter  2  in a variety of ways. In the following, three exemplary embodiments will be discussed, in which the transmission performance of the packet-switched network  3  is measured by means of the measurement unit  45  in receiver  4  and used as a basis for the decision if a change in the frame transmission scheme at transmitter  2  is required or not. 
         [0062]    According to a first embodiment, which is depicted in the flowcharts of  FIGS. 2   a  and  2   b,  relating to the processing at receiver  4  and transmitter  2  (see  FIG. 1 ), respectively, receiver  4  measures the transmission performance of the packet-switched network  3  in a step  200 , for instance in terms of packet loss rate and jitter. Based on these measurements, receiver  4  decides, in a step  201 , if a change in the frame transmission scheme at transmitter  2  is required. If this is the case, a command for a change in the frame transmission scheme is sent to transmitter  2  in a step  202 . This command may for instance contain control information related for example to the desired frame transmission scheme, the number of redundant frames in frame redundancy transmission or the number of frames in frame aggregation transmission, to name but a few. Said command may for instance be a media adaptation command that further includes control information related to the codec mode (e.g. AMR/AMR-WB). The transmission of this command, which is illustrated in  FIG. 1  as a dashed line between buffer length control unit  43  and frame transmission scheme control unit  23 , may be in-band (e.g. included in the media stream in the reverse direction) or out-of-band (e.g. as separate signaling packets). Since receiver  4  knows that transmitter  2  will obey the command for change in the frame transmission scheme transmitted in step  202 , the buffer length can now be controlled by buffer length control unit  43  of receiver  4  in a step  203  to anticipate the forthcoming change in the frame transmission scheme. The flowchart of  FIG. 2   a  then jumps back to step  200  and performs an anew measurement of the transmission performance of network  3 , thus to be able to proactively react to further variations in transmission performance that may require a change in the frame transmission scheme. In this way, it is ensured that the length of frame buffer  44  is always optimal. If the transmission performance of network  3  deteriorates, receiver  4  triggers a change in the frame transmission scheme, for instance by commanding that frame redundancy transmission is used, and accordingly increases the length of frame buffer  44 . If, after a while, the transmission performance of network  33  improves, receiver  4  triggers a further change in the frame transmission scheme, for instance be commanding that the frame transmission scheme is changed from frame redundancy transmission to normal transmission, and reduces the length of frame buffer  44  accordingly. 
         [0063]    The corresponding processing at transmitter  2  is illustrated in the flowchart of  FIG. 2   b.  As soon as a command from receiver  4  is received, which is continuously checked in step  204 , the frame transmission scheme is changed by frame transmission scheme control unit  23  of transmitter  2  in a step  205 . The flowchart then jumps back to step  204  to wait for further commands. 
         [0064]    According to a second embodiment, which is depicted in the flowcharts of  FIGS. 3   a  and  3   b,  relating to the processing at receiver  4  and transmitter  2  (see  FIG. 1 ), respectively, receiver  4  once again measures the transmission performance of the packet-switched network  3  in a step  300 . Based on these measurements, receiver  4  decides, in a step  301 , if a change in the frame transmission scheme at transmitter  2  is required. However, instead of sending a command to transmitter  2 , as it was the case in step  202  of the flowchart in  FIG. 2   a,  buffer length control unit  43  now sends a request to change the frame transmission scheme to frame transmission scheme control unit  23  of transmitter  2  in a step  302 . This request may contain control information as already described above with respect to the previous embodiment. Said request may for instance be a media adaptation request that further includes control information related to the codec mode (e.g. AMR/AMR-WB). In contrast to the command, the request leaves transmitter  2  some flexibility in deciding if such a change in the frame transmission scheme is feasible or not. Subsequently, in a step  303 , the length of frame buffer  44  is controlled by buffer length control unit  43  according to the request sent to transmitter  2 , and the flowchart jumps back to step  300 . 
         [0065]    The flowchart of  FIG. 3   b  illustrates the corresponding processing in frame transmission control unit  23  of transmitter  2 . Upon reception of the request, which is checked for in a step  304 , it is determined if the request for a change in the frame transmission scheme is feasible. For instance, the number of redundant frames per packet requested by receiver  4  in its request may be too large, so that the transmitter  2  decides to change the frame transmission scheme only in a way that frame redundancy transmission with less redundant frames is performed. The flowchart of  FIG. 3   b  then returns to step  304  to check for further requests. 
         [0066]    It should be noted that the effect of the fact that transmitter  2  did not completely satisfy the request of receiver  4  with respect to the requested number of redundant frames is that receiver  4 , expecting that its request will possibly be fully met, sets out from a larger required buffer length than it is actually required, so that the delay of the frame buffer  44  will be controlled to a too large value. However, this situation may be quickly cured by prescribing that transmitter  2 , when not fully satisfying the receiver&#39;s request for a change of the transmission scheme, informs receiver  4  accordingly, so that the length of frame buffer  44  can be reduced accordingly. However, it is apparent that this embodiment, despite the possible overshooting of the frame buffer length, nevertheless allows to avoid that frames are lost due to a change in the frame transmission scheme and to adapt the length of the frame buffer to the required length in a controlled manner. 
         [0067]    According to a third embodiment, which is depicted in the flowcharts of  FIGS. 4   a  and  4   b,  relating to the processing at receiver  4  and transmitter  2  (see  FIG. 1 ), respectively, receiver  4  once again measures the transmission performance of the packet-switched network  3  in a step  400 . In a step  401 , the measured data is directly transmitted to transmitter  2 , for instance in the form of RTCP receiver reports. This may be performed by the buffer length control unit  43  (as illustrated by the dashed line in  FIG. 1 ), or by the measurement unit  45  itself. In a step  402 , the buffer length control unit  43  nevertheless determines if a change in the frame transmission scheme is required or not, and, if this should be the case, controls the length of frame buffer  44  accordingly in a step  403 . The flowchart then returns to step  400 . The rationale behind this approach is that it is assumed that both transmitter  2  and receiver  4  use the same algorithm or a similar algorithm for determining whether a change in the frame transmission scheme is required, wherein said algorithm is used by receiver  4  in step  401  and by transmitter  2  in step  406  (see description below). Thus based on the same set of measurement data, both transmitter  2  and receiver  4  will come to the same decision, so that the receiver  4  can anticipate the change in the frame transmission scheme when performing the control of the length of the frame buffer in step  403 . 
         [0068]    The flowchart of  FIG. 4   b  illustrates the corresponding processing in frame transmission control unit  23  of transmitter  2 . Upon reception of a receiver report, which is checked in a step  405 , it is determined if a change in the frame transmission scheme is required in a step  406 . Therein, for determining whether said change in said frame transmission scheme is required, transmitter  2  deploys e.g. the same or a similar algorithm than the algorithm that is used by receiver  4  in step  402  in the flowchart of  FIG. 4   a.  If it is determined that a change in the frame transmission scheme is required, the frame transmission scheme is changed in a step  407 . The flowchart then jumps back to step  405 . 
         [0069]    In the three embodiments described above, the change in the frame transmission scheme at the transmitter was at least partially based on measurements of the transmission performance of the packet-switched network  3 , wherein the measurements were performed by measurement unit  45  of receiver  4 . It should however be noted that transmitter  2  may equally well come to the decision to change the transmission scheme without requiring measurement data from receiver  4 . For instance, said frame transmission scheme may be changed by transmitter  2  due to a handover to another access technology with different transmission characteristics, or due to a change in the available channel bandwidth. In such cases, receiver  4  may nevertheless gain knowledge of the change in the frame transmission scheme and control the length of the frame buffer accordingly. Furthermore, said change may still be a future change, i.e. receiver  4  may gain this knowledge before the change occurs, so that there is enough time to take care that the adaptation of the buffer length is performed in a controlled manner. For instance, transmitter  2  may inform receiver  4  of a forthcoming change in the transmission scheme by in-band or out-of-band signaling, or receiver  4  and transmitter  2  may be configured to analyze the same parameters that trigger a change in the frame transmission scheme, so that, when transmitter  2  comes to the conclusion that a change in the frame transmission scheme has to be performed, also receiver  4  comes to this conclusion. 
         [0070]    It is readily clear for a skilled person that the logical blocks in the schematic block diagrams as well as the flowchart and algorithm steps presented in the above description may at least partially be implemented in electronic hardware and/or computer software, wherein it depends on the functionality of the logical block, flowchart step and algorithm step and on design constraints imposed on the respective devices to which degree a logical block, a flowchart step or algorithm step is implemented in hardware or software. The presented logical blocks, flowchart steps and algorithm steps may for instance be implemented in one or more digital signal processors, application specific integrated circuits, field programmable gate arrays or other programmable devices. Said computer software may be stored in a variety of storage media of electric, magnetic, electro-magnetic or optic type and may be read and executed by a processor, such as for instance a microprocessor. To this end, said processor and said storage medium may be coupled to interchange information, or the storage medium may be included in the processor. 
         [0071]    The invention has been described above by means of exemplary embodiments. It should be noted that there are alternative ways and variations which are obvious to a skilled person in the art and can be implemented without deviating from the scope and spirit of the appended claims. In particular, the present invention is not limited to application in VoIP systems and/or to transmission of speech or audio frames. 
         [0072]    While there have been shown and described and pointed out fundamental novel features of the invention as applied to preferred embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices and methods described may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto. Furthermore, in the claims means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures.