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Functions of mobile multimedia QoS control |
Extensions of session management protocol
Consider the situation where a user who is receiving the video stream with a mobile terminal moves to an out-of-range location such as a tunnel, and the video stream terminates. With previous multimedia session management protocols, the session is disconnected and the user must receive the stream from the beginning. Our extensions of the session management protocol handles temporary connection cut-off and re-establishment. The session remains connected and the user can play the stream beginning after the frames he has already seen. Here is an overview of our mechanism. A client (mobile terminal) preserves the receiving status when it loses the connection. The streaming server discovers that the client has moved out of range and stops distributing the stream. When the client reenters the service area, the server locates it and continues delivering the stream from the frame where the connection was lost. 
Sending bit-rate control mechanism
In wireless links, the effective forwarding rate is affected by user movements and changes in the surrounding environment. The base station may have to drastically change its bandwidth allocation control settings. If the required transmission rate of the video data remains over the limit imposed by the effective forwarding rate, packets will be lost and the quality of the video the user receives will be dramatically lowered. Our sending bit-rate control mechanism measures packet loss and delay time fluctuations caused by changes in reception conditions, estimates optimal values of the effective forwarding rate, and controls the amount of video data transmitted. 
Selection of QoS policy
The methods of controlling the amount of data can be changed in accord with user requests. For example, if the priority for the high quality of each frame is set to be higher than the priority for video smoothness, the frame rate should be reduced when the effective bandwidth is limited (news mode). If the priority for smooth video is higher than the priority for frame quality, the DCT coefficient can be reduced when the effective bandwidth is limited (sports mode). Of course the policy can be balanced between news mode and sports mode, so both the frame rate and the DCT coefficient are reduced. Using this method, a user can read the subtitles of a news video by using news mode, or enjoy the motion of a soccer video by using sports mode, even when the effective bandwidth is limited. The figure below compares how the sreamed video looks like when a user selects news mode and sports mode. The left client uses news mode, and the right client uses sports mode. You can see that you can read the subtitles of the left picture clearly. On the other hand, the right picture is somewhat rough because the DCT coefficient is reduced. But using the sports mode, you can enjoy the smooth motion of the video. |
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| Last modified 28 February 2002 |