WO2007031924A2

WO2007031924A2 - Video telephone system, video telephone terminal, and method for video telephoning

Info

Publication number: WO2007031924A2
Application number: PCT/IB2006/053195
Authority: WO
Inventors: Laurent Petitjean
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-09-16
Filing date: 2006-09-11
Publication date: 2007-03-22
Also published as: WO2007031924A3

Abstract

A video telephone system for performing video telephony over a network (N) having at least one transmission channel (7) with associated network resources (TBW) and at least one control channel (10) is described. The video telephone system comprises a transmitter terminal (1) for transmitting at least one video data stream (s3) and first control data (ctrl msg) corresponding to the at least one video data stream (s3) over the at least one transmission channel (7), and a receiver terminal (2) for receiving at least one video stream (s4) and first control data (ctrl msg) transmitted over the transmission channel (7) and for transmitting second control data (ACK) relating to video characteristics of the transmission channel (7) to the transmitter terminal (1) in accordance with the received at least one video stream (s4) and/or first control data (ctrl msg). Furthermore, a video characteristic-adapting unit (6, 5) is provided for adapting the video characteristics of at least one video data stream and the first control data (ctrl msg) in the transmitter (1) in accordance with the second control data (ACK) received by the transmitter terminal (1) over the control channel (10), and for providing a bandwidth margin (BWM) by adapting the video characteristics of at least one video data stream and the first control data (ctrl msg) in such a way that the bandwidth (VD) of the at least one video data stream (s3), the bandwidth (TC) for the first control data (ctrl msg) and the bandwidth margin (BWM) correspond to the associated network resource (TBW) of the transmission channel (7).

Description

Video telephone system, video telephone terminal, and method for video telephoning

FIELD OF THE INVENTION

The invention relates to a video telephone system, a video telephone terminal, and a method for video telephoning.

Modern digital telephone networks allow transmission of digital audio and video data streams to provide bidirectional and real-time video communication for home video telephony terminals.

BACKGROUND OF THE INVENTION

Typically, a transmitter terminal transmits the audio and video data stream to a receiving terminal via a network. Degradation of the transmission conditions of a transmission channel within such a network has a direct impact on any visual aspects of images as transmitted in the video data stream, when these images are displayed on a video telephony terminal. Degradation of transmission conditions may also occur if an end user has selected a transmission rate for the video data stream which is too high to be transmitted over the available network bandwidth. Such degradation may occur because of congestion of the communication or transmission channel. Alternatively, such degradation may occur because of radio-link communication between a video phone terminal and its associated base or access point, wherein the degradation becomes more noticeable if the video phone terminal is used at the limit of the radio-link range. The impact of such a degradation of transmission conditions is typically observed as a lack of fluidity in a video sequence displayed on a videophone terminal. Such a lack of fluidity is expressed as a sequence of images each of which is frozen for a period of time. A further undesired effect of the degradation is that the lack of fluidity causes desynchronization between the audio and video data stream. It is therefore not acceptable from the end user's point of view. US 2005/0002453 Al discloses a video quality control system using a complex microprocessor system having a video processor for controlling an encoder, a video compression transcoder and a packet controller within a buffer memory. However, such a solution is not very cost-effective, as a great amount of resources is required. WO 2005/022845 Al shows a rate-based congestion control for packet networks. A rate controller is used to control the transmission of audio/video streams from the transmitter. The available bandwidth is estimated at the transmitter on the basis of the received acknowledgement packets in the receiving terminal. The flow of acknowledgement packets from the receiver to the transmitter is used to control the congestion. In particular, acknowledgement information from the receiver is forwarded to the transmitter n times to validate the available bandwidth. Here, no prediction concept is present. The scheme is based on a low-pass filter (post-processing) so as to react to a variable channel bandwidth according to the past information. However, this presents a major disadvantage on a skip variable bandwidth or any abrupt variable bandwidth. The main consequence is that the video stream must be frozen when this event happens on a reference picture.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the invention to provide a video phone terminal which is capable of providing an improved synchronization between the audio and video data stream, even if a disturbance occurs in a network.

This object is solved by a video telephone system as defined in claim 1, a video telephone terminal as defined in claim 9, and a method for video telephoning as defined in claim 11. To this end, a video telephone system for performing video telephony over a network having at least one transmission channel with associated network resources and at least one control channel is provided. The video telephone system comprises a transmitter terminal for transmitting at least one video data stream and first control data corresponding to the at least one video data stream over the at least one transmission channel, and a receiver terminal for receiving at least one video stream and first control data transmitted over the transmission channel and for transmitting second control data relating to video characteristics of the transmission channel to the transmitter terminal in accordance with the received at least one video stream and/or first control data. Furthermore, a video characteristic-adapting unit is provided for adapting the video characteristics of at least one video data stream and the first control data in the transmitter in accordance with the second control data received by the transmitter terminal over the control channel, and for providing a bandwidth margin by adapting the video characteristics of at least one video data stream and the first control data in such a way that the bandwidth of the at least one video data stream, the bandwidth for the first control data and the bandwidth margin correspond to the associated network resource of the transmission channel.

Accordingly, the telephone system comprises the encoder which does not need to be modified in such a way that existing coding systems can be re-used while providing a synchronization between a video and an audio stream, with the video stream still showing a certain level of fluidity.

According to an aspect of the invention, the second control data comprises at least one second control message being associated with at least one transmitted packet of the at least one video data stream. Hence, direct control of the received packets of the video data stream is possible.

According to another aspect of the invention, the transmitter comprises a camera unit for providing video data streams, a bit-stream controller for controlling the data streams generated by the camera unit, a video encoder for encoding the data stream and an encoder controller for controlling the output of the video encoder and for controlling the bit- stream controller in accordance with the second control data as received via the control channel. The receiver terminal comprises a video decoder for decoding the video data streams as received from the transmitter terminal, a display device for displaying the decoded video data stream and a quality-of-service control unit for determining the quality of service of the transmission channel and for forwarding quality-of-service information as second control data via the control channel to the encoder controller. A video telephone system can thus be implemented effectively.

According to a further aspect of the invention, the bit-stream controller is adapted to control the data stream generated by the camera unit, wherein the bit-stream controller performs a sub-sampling of the images generated by the camera unit if the video data stream generated by the camera unit is above the characteristics of the transmission channel and the video encoder, wherein the bit-stream controller is set to a transparent mode if the video characteristics of the video data stream generated by the camera unit correspond to the characteristics of the transmission channel, and wherein the bit-stream controller performs an over-sampling on the video data stream generated by the camera unit if the video characteristics of the video data stream are below the video characteristics of the transmission channel as requested by the video encoder. Hence, an optimal operation is provided, depending on the characteristics of the transmission channel.

According to a further aspect of the invention, the encoder controller is adapted to control the output of the video encoder in accordance with the bandwidth margin corresponding to the difference between the total available bandwidth and the bandwidth used for control information.

According to an aspect of the invention, dummy data are transmitted in the video data stream if the bit rate of the video data stream and/or the first control data is reduced. If no control data is sent, dummy data is sent, such that the network provider does not allocate any of the non-used resources during communication to other users. The freed bandwidth is thus used by dummy data.

The invention also relates to a video telephone terminal for a video telephone system having a network with at least one transmission channel with associated network resources and at least one control channel. The terminal transmits at least one video data stream and first control data corresponding to the at least one video data stream over a transmission channel. A video characteristic-adapting unit adapts the video characteristics of at least one video data stream and the first control data in accordance with the control data received from an external receiver terminal over the control channel, and provides a bandwidth margin in such a way that the bandwidth of the at least one video data stream , the bandwidth for the first control data and the bandwidth margin correspond to the associated network resource of the transmission channel.

The invention further relates to a method for performing video telephony over a network having at least one transmission channel with associated network resources and at least one control channel. At least one video data stream and first control data corresponding to the at least one video data stream are transmitted over the at least one transmission channel. At least one video stream and first control data transmitted over the transmission channel are received. Second control data relating to video characteristics of the transmission channel in accordance with the received at least one video stream and/or first control data is transmitted. The video characteristics of at least one video data stream and the first control data in accordance with the second control data received over the control channel are adapted. A bandwidth margin is provided by adapting the video characteristics of at least one video data stream and the first control data in such a way that the bandwidth of the at least one video data stream , the bandwidth for the first control data and the bandwidth margin correspond to the associated network resource of the transmission channel.

With such a video telephone system, there is no need to modify the network between the transmitter and the receiver. The above-mentioned video telephone system can be implemented by merely modifying the transmitter and receiver terminal settings. The invention relates to the idea of providing a video telephone system which allows control of a level of fluidity of a video sequence as displayed on a receiving video phone terminal. The video quality, which is required by the receiver, is adapted in accordance with the degradation level, i.e. the disturbances observed on the network. The video stream is then dynamically adapted in real time. The above-mentioned realization of the basic principles of the invention is advantageous, as it is only implemented in the terminals so that no action needs to be taken by the service provider and the network does not need to be modified. In particular, no specific hardware resources but merely a minimum of software resources are required to control the requested video stream. At least one acknowledgement ACK is assigned to one transmitted packet.

Furthermore, a margin bandwidth is required in order to significantly improve the video fluidity upon an unexpected drop of the bandwidth. The margin bandwidth is the part of a predictive scheme for determining the available network resources at "t+1" time, i.e. just before sending a new video packet, wherein resources are defined as the bandwidth, delays, latencies, etc.

Other aspects of the invention are defined in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the invention will now be described with reference to the Figures.

Fig. 1 is a block diagram of the overall system architecture according to the invention,

Fig. 2 is a block diagram of the video encoder of the transmitter terminal according to Fig.1, Fig. 3 shows the general format of the video data frames in accordance with an embodiment,

Fig. 4 shows a representation of the use of the base-band in accordance with an embodiment,

Fig. 5 shows a representation of the general format of the message types as transmitted between the receiver and transmitter terminals, and

Fig. 6 shows a representation of the system behavior in the presence of a disturbance.

DESCRIPTION OF EMBODIMENTS Fig. 1 is a block diagram of the overall system architecture in accordance with a first preferred embodiment. In particular, a transmitter terminal 1 is connected to a receiver terminal 2 via a network N having several channels 7, 10. The transmitter terminal 1 comprises a camera 3, a bit-stream controller 4, a video encoder 5 and an encoder controller 6. The receiver terminal 2 comprises a display device 12, a video decoder 11, a network layer unit 8 and a quality-of-service control unit 9. The transmitter terminal 1 transmits a video and audio data stream to the receiver terminal 2 via the channel 7. Furthermore, a feedback loop from the receiver terminal 2 to the transmitter terminal 1 is present via the quality-of-service control unit 9, the channel 10 and the encoder controller 6. The video camera 3 of the transmitter terminal 1 provides a digital video stream si representing the image captured by the camera 3. Alternatively, the camera may be implemented outside the transmitter terminal 1 or as a stand-alone camera. The bit-stream controller 4 is adapted to control the video stream as generated by the camera 3 and forward the video data stream s2 to an input of the video encoder 5. The function of the bit-stream controller 4 is based on three operating modes. In a first operating mode, a sub-sampling of the image is performed if the video stream from the camera 3 is above the conditions of the network N and the video encoder 5. In a second operating mode, the bit-stream controller 4 performs a transparent operation, i.e. it does not intervene if the video stream from the camera is adapted to the network conditions. In a third operating mode, an over-sampling is performed when the stream at the output of the camera 3 is below the conditions of the network as requested by the encoder 5. The video encoder 5 transmits the video data stream s3 to the transmission channel in packets.

The encoder controller 6 is adapted to monitor the video encoder 5 and the bit- stream controller 4. This can be performed by adapting the video coding layer VCL in the video encoder 5 and the data partitioning, i.e. the transmission of the macro block MB, the group of blocks GOB and others. The macro block MB and the group of blocks GOB are defined in ITU-T H.263 and later releases.

The network layer unit receives packets s4 from the network and re-assembles and re-arranges the packets in order to forward a suitable video sequence slO to the video decoder 11. The video decoder 11 decodes the stream si 0 to a stream si 1. The network layer also performs some information regarding QoS statistics through the stream s5.

By means of the feedback loop, i.e. quality-of-service control unit 9, the channel 10, and the encoder controller 6, the video stream as outputted by the transmitter terminal 1 is controlled in accordance with QoS data s7 from the receiver terminal 2. In particular, the information s6 of the quality-of- service data as provided by the quality-of- service control unit 9 is used to control the bit-stream controller 4 and the video encoder 5 to perform a dynamic and real-time adoption of the video data stream as outputted by the transmitter terminal 1 on the basis of the actual characteristics of the transmission channel 7. The predictive scheme is implemented inside the feedback loop according to Fig.l with the network layer unit 8, the QoS unit 9, the channel 10 and the encoder controller 6.

The display device 12 of the receiver terminal 2 displays the video data stream received over the channel 7 and decoded by the video decoder 11.

Fig. 2 is a block diagram of the video encoder 5 of the transmitter terminal 1 in accordance with a first embodiment of Fig.l. The video encoder 5 comprises a data control unit 51, a video-coding layer 52 and a data-partitioning unit 53. The video-coding layer 52 receives a data stream s2 from the bit-stream controller 6, and the data control unit 51 is coupled to the encoder control unit 6 via a connection s8.

When communication between the transmitter terminal 1 and the receiver terminal 2 is initiated, a negotiation regarding the capabilities of the transmitter terminal 1 and the receiver terminal 2 is performed. Typical capabilities that need to be negotiated may include the total available channel bandwidth, the application bit rate, the video frame rate, the IP packet size, the preferred video codec list as well as a maximum number N of control messages ctrl msg which are tolerable by the transmitter terminal 1 and the receiver terminal 2.

Fig. 3 shows a format of the video data frames VDF transmitted over the transmission channel in accordance with the first embodiment. A video data frame VDF comprises a header HD and video-compressed data packets VCDP.

Fig. 4 shows a representation of the use of the base-band, i.e. the bandwidth BW, in accordance with a first embodiment. The encoder controller 6 controls the video encoder 5 in such a way that the video encoder 5 merely uses 100-X-Y % of the total available bandwidth TBW, i.e. a bandwidth VD. 100-X-Y % relates to a representation of the bandwidth which is used by packetized video data frames VDF as shown in Fig. 3. Y relates to the bandwidth percentage used by control frames for transport control TC, and X relates to the bandwidth margin BWM percentage which is responsible for the fluidity of transfer of a video data stream upon a variation of the resources of the transmission channel. For example, for a weakly animated video data stream, only a small part of the bandwidth is used most of the time. Fig. 5 shows a representation of the general format of the message types as transmitted between the receiver and the transmitter terminal. This format comprises a video data frame VDF and a number n of control messages ctrl msg. Here, the control message ctrl msg is duplicated several times n in order to create a redundancy. Thereafter, the duplicated control messages are sent in each video data frame as shown in Fig. 5. In other words, the video data frames VDF and control messages ctrl msg are transmitted from the transmitter terminal 1 over the channel 7.

Within a normal operating condition, e.g. without disturbances in the transmission channel, the video data stream traffic is delivered in accordance with the negotiated capabilities during initialization of the communication between the transmitter terminal 1 and the receiver terminal 2, such that the available video quality corresponds to the video quality as negotiated during initialization. This video quality will correspond to the best video quality which is supported by both terminals.

Fig. 6 shows a representation of the system behavior in the presence of a disturbance of the transmission channel 7. The predictive function is performed in such a way that a bandwidth margin gain is available all the time but is not used for the video stream. When there is a network congestion, a number of transmitted control messages are lost e.g. in the network N, i.e. the control messages ctrl msg are not received by the receiver 2. Therefore, the amount of feedback acknowledgement messages ACK, which corresponds to the transmitted control messages, is reduced so that the encoder output stream of the transmitter 1 is also immediately reduced. The number of ACK messages is continuously monitored by the transmitter 1 (in particular by the encoder controller 6) in order to detect an end of the network congestion, and when the end is detected, the system returns to the initial control messages amount associated with the initial bandwidth of the video stream. At tl, a network disturbance ND occurs and lasts until t2. Before tl and after t2, i.e. without a network disturbance ND, control messages ctrl msg are communicated between the transmitter and the receiver terminal 1, 2. During the disturbance, i.e. tl < t < t2, a number n of control messages ctrl msg sent by the transmitter 1 are lost so that merely N-n control messages ctrl msg are received by the receiver 2. However, if n(n>l and n<=N), control messages ctrl msg are lost at the time when the last frame has been received and no corresponding feedback control information ACK has been received by the encoder 5 regarding the coding of a current frame. The encoder controller 6 detects that control messages ctrl msg are lost. This is forwarded to the video encoder 5 which then automatically reduces the video output rate. Furthermore, the video encoder 5 suppresses n control messages ctrl msg in accordance with the reduced video output rate. Some additional information may be added to the remaining control messages ctrl msg. Monitoring information can be optionally transmitted. During the disturbance ND, a bit rate reduction BRR of the video data takes place to ensure the fluidity of display of the received video data stream.

The monitoring ACK messages are transmitted by the receiver terminal 2 to the transmitter terminal 1 and represent the quality of service. Preferably, at least one feedback control message ctrl msg ACK is transmitted from the receiver terminal 2 to the transmitter terminal 1. However, if none of the control messages ctrl msg is transmitted, a complete loss of feedback information is the result, so that the encoder controller 6 does not have any further control information. In other words, the feedback loop is opened. After that, the receiver as well as the transmitter terminal is automatically reconfigured in accordance with the previously defined quality criteria.

The function of the video encoder 5 in accordance with the first embodiment will now be described in more detail. The encoder 5 may increase or decrease its output rate as controlled by the encoder controller 6. If the video encoder 5 is to decrease its output rate, the transmitter terminal 2 reduces the number of control channels N to N-M to temporarily assign the now available resources for video data to the present frame if all control channels of the preceding frame were used. Here, M is related to the resource gap as required for the video encoder 5. The intrinsic characteristics of the video encoder 5 can then be used. In other words, a system is re-converged to a situation with N control messages ctrl msg.

However, if a source image stream from the camera 3 equals the preceding rate, or is below or above the preceding rate, the video encoder 5 is adapted to increase the compression factor in order to decrease its output rate. Accordingly, the N control messages ctrl msg are retained. If the number of control messages ctrl msg is less than N, the system is re-converged to the N control messages. If there is a network disturbance ND, usage of the bandwidth is reduced and the bit rate of the video data stream is also reduced.

The routing priority level of the video data packets and the transmission control packets are indicated. The bandwidth Y as used by the control frames can be decreased in order to increase the number of N or the quantity 100-X-Y. The video data may be in a H.263 format as defined in the ITU recommendations ITU-H.263 and its annexes.

All intrinsic possibilities of the video encoder 5 are used, such as in the video- coding layer VCL, enhancement layer coding and motion compensation. Furthermore, all intrinsic possibilities of the transport layers are also used, such as in data partitioning, i.e. the transmission of macro blocks MB, group of blocks, etc. One advantage of the principles of the invention is that the coding system comprising the encoder and the decoder does not need to be modified, so that existing coding systems can be re-used. The monitoring data are only sent punctually. If no control data is sent, dummy data is sent, such that the network provider does not allocate any of the non-used resources during communication to other users. The freed bandwidth is thus used by dummy data. In addition, synchronization is required between the transmission channel 7 and the control channel 10. Furthermore, the control messages ctrl msg may be added with additional information such as re-synchronization or the like, such that frames are shifted in time between the receiver and the transmitter terminal.

The transmitter and the receiver terminal described above may be implemented as a single video telephone terminal for communication with other such single video telephone terminals.

According to the principle of the invention, no specific video processing is performed on the content of the received frames of the video data stream. Furthermore, no modification needs to be performed with regard to the real-time transmission characteristics. Such real-time characteristics may include full duplex transmission and reception, lip- synchronism (synchronization between the audio and video stream and low power consumption). The video phone system according to the invention allows real-time video communication and may use a multiplayer compression capability of the video codes, the quality of service of the channel, an encoder controller and a control of the rates of the input of the coder. This system is mainly based on a statistical analysis of the packets transported over the network. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. Use of the article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. A video telephone system for performing video telephony over a network (N) having at least one transmission channel (7) with associated network resources (TBW) and at least one control channel (10), the system comprising: a transmitter terminal (1) for transmitting at least one video data stream (s3) and first control data (ctrl msg) corresponding to the at least one video data stream (s3) over the at least one transmission channel (7), and a receiver terminal (2) for receiving at least one video stream (s4) and first control data (ctrl msg) transmitted over the transmission channel (7) and for transmitting second control data (ACK) relating to video characteristics of the transmission channel (7) to the transmitter terminal (1) in accordance with the received at least one video stream (s4) and/or first control data (ctrl msg), and a video characteristic-adapting unit (6, 5) for adapting the video characteristics of at least one video data stream and the first control data (ctrl msg) in the transmitter (1) in accordance with the second control data (ACK) received by the transmitter terminal (1) over the control channel (10), and for providing a bandwidth margin (BWM) by adapting the video characteristics of at least one video data stream and the first control data (ctrl msg) in such a way that the bandwidth (VD) of the at least one video data stream (s3), the bandwidth (TC) for the first control data (ctrl msg) and the bandwidth margin (BWM) correspond to the associated network resource (TBW) of the transmission channel (7).

2. A video telephone system according to claim 1, wherein the second control data (ACK) comprises at least one second control message (ACK) being associated with at least one transmitted packet of the at least one video data stream (s3).

3. A video telephone system according to claim 1 or 2, wherein the video characteristic-adapting unit (6, 5) is adapted to change a bit rate of the at least one video data stream (s3) in order to guarantee the best video stream match between the transmitter terminal (1) and the receiver terminal (2) on the basis of the available network resources (TBW).

4. A video telephone system according to claim 1, wherein - the transmitter (1) comprises a camera unit (3) for providing a video data stream (si), a bit-stream controller (4) for controlling the data stream (si) generated by the camera unit (3), a video encoder (5) for encoding the data stream (s2) and an encoder controller (6) for controlling the output of the video encoder (5) and for controlling the bit- stream controller (4) in accordance with the second control data (ACK) as received via the control channel (10), and wherein the receiver terminal (2) comprises a video decoder (11) for decoding the video data stream (slO) received from the transmitter terminal (1), a display device (12) for displaying the decoded video data stream (si 1) and a quality-of-service control unit (9) for determining the quality of service of the transmission channel (7) and for forwarding the quality-of-service information as second control data (ACK) via the control channel (10) to the encoder controller (6) in the transmitter (1).

5. A video telephone system according to claim 4, wherein the bit-stream controller (4) is adapted to control the data stream (si) generated by the camera unit (3), wherein the bit-stream controller (4) performs a sub- sampling of the images generated by the camera unit (3) if the video data stream (si) generated by the camera unit (3) is above the characteristics (TBW) of the transmission channel (7) and the video encoder (6), wherein the bit-stream controller (4) is set to a transparent mode if the video characteristics of the video data stream (si) generated by the camera unit (3) correspond to the characteristics (TBW) of the transmission channel (7), and wherein the bit-stream controller (4) performs an over-sampling on the video data stream (si) generated by the camera unit (3) if the video characteristics of the video data stream (si) are below the video characteristics (TBW) of the transmission channel (7) as requested by the video encoder (5).

6. A video telephone system according to claim 4 or 5, wherein the encoder controller (6) is adapted to control the output of the video encoder (5) in accordance with the bandwidth margin (BWM; X) corresponding to the difference between the total available bandwidth (TBW; 100) and the bandwidth (TC ;Y) used for control information.

7. A video telephone system according to claim 6, wherein - the video data stream (s3) is transmitted over the transmission channel (7) in video data frames (VDF), and wherein the video data frame (VDF) comprises a plurality of control messages (ctrl msg).

8. A video telephone system according to claim 1, wherein dummy data are transmitted in the video data stream (s3) if the bit rate of the video data stream (s3) and/or the first control data (ctrl msg) is reduced.

9. A video telephone terminal (1) for a video telephone system having a network (N) with at least one transmission channel (7) with associated network resources (TBW) and at least one control channel (10); said terminal (1) being adapted to transmit at least one video data stream (s3) and first control data (ctrl msg) corresponding to the at least one video data stream (s3) over a transmission channel (7), said terminal comprising: - a video characteristic-adapting unit (6, 5) for adapting the video characteristics of the at least one video data stream (s3) and the first control data (ctrl msg) in accordance with the control data (ACK) received from an external receiver terminal (2) over the control channel (10), and for providing a bandwidth margin (BWM), such that the bandwidth (VD) of the at least one video data stream (s3), the bandwidth (TC) for the first control data (ctrl msg) and the bandwidth margin (BWM) correspond to the associated network resource (TBW) of the transmission channel (7).

10. A video telephone terminal (1, 2) according to claim 9, wherein the terminal (2) is adapted to receive at least one video stream (s4) and first control data (ctrl msg) transmitted over the transmission channel (7) and for transmitting second control data (ACK) relating to video characteristics of the transmission channel (7) to an external transmitter terminal (1) in accordance with the received at least one video stream.

11. A method for performing video telephony over a network (N) having at least one transmission channel (7) with associated network resources (TBW) and at least one control channel (10); the method comprising the steps of: transmitting at least one video data stream (s3) and first control data (ctrl msg) corresponding to the at least one video data stream (s3) over the at least one transmission channel (7), receiving at least one video stream (s4) and first control data (ctrl msg) transmitted over the transmission channel (7) and transmitting second control data (ACK) relating to video characteristics of the transmission channel (7) in accordance with the received at least one video stream (s4) and/or first control data (ctrl msg), adapting the video characteristics of at least one video data stream and the first control data (ctrl msg) in accordance with the second control data (ACK) received over the control channel (10), and providing a bandwidth margin (BWM) by adapting the video characteristics of at least one video data stream and the first control data (ctrl msg) in such a way that the bandwidth (VD) of the at least one video data stream (s3), the bandwidth (TC) for the first control data (ctrl msg) and the bandwidth margin (BWM) correspond to the associated network resource (TBW) of the transmission channel (7).