[Japanese | Thesis | Researches in Minoh Lab | Minoh Lab]
We propose a method for dynamic bandwidth control based on the priority of data and the state of network to reduce the influence on video and audio caused by packet loss and jitter through the transmission over the Internet.
The broader network bandwidth becomes, the more opportunities we have to transmit video and audio streams over the Internet. MPEG-2 is one example of stream format to be used for video and audio transmission. If MPEG-2 data stream at several or scores Mbps can be transmitted over the Internet, TV quality broadcasting or distance leaning environment can be made using the Internet. In Kyoto Univeisity, a series of distance lectures started with a university in the US over the Internet using MPEG-2 data stream last year. In the lecture, students in both universities can interactively communicate with each other.
To communicate interactively with transmission of video and audio stream over a network, it is important that the data stream should be transmitted with low delay.
To keep quality of transferred video and audio, certain bandwidth should be used constantly in the Internet. However, network bandwidth is shared by various communications, sometimes the available bandwidth may be less than required bandwidth for the MPEG-2 data stream. If available bandwidth is less than required bandwidth, some packets will be delayed or lost, and the jitter and packet loss cause some noises or corruption of video and audio. If the video and audio have much noise, participants are displeased and important information is lost.
MPEG-2 data stream consists of some types of segments which can be prioritized. To reduce such noises or corruption of video and audio in the Internet, it is considered that MPEG-2 data stream is split into segments and that each segment should be prioritized. If routers treat segments with higher priority not to be dropped or delayed, noises will be rather reduced. However, most of the routers in the Internet do not take priority of data into consideration.
In this paper, we propose a method for dynamic bandwidth control by both ends, a sender and a receiver, based on the priority to reduce corruption of video and audio.
To transmit MPEG-2 data stream over the Internet, it should be divided into some packets. Each packet contains a sequence of MPEG-2 segments. We classify each packet into some prioritized groups according to importance of included segments. For example, a segment contains synchronization information, sequence independent image called I picture, or audio information are considered to have higher importance. And the sender sends redundant packet containing only copies of segments classified into a group of high priority. This redundancy will reduce loss of important data and reduce noises.
On the other hand, sending redundant data requires more available bandwidth, and the increase of required bandwidth sometimes causes network congestion. To avoid this type of congestion, if receiver observes packet loss during redundant packet are sent by the sender, the receiver send a feedback to the sender not to send out redundant packet. Then if the receiver still observes packet loss, the receiver also sends feedback to the sender to decrease bit rate by suppressing some packets which do not include important segments.
In addition, large jitter should not be arise in transmission. Not to be so, the receiver tries to decrease influence of jitter by accumulating received data into a buffer, and adjusts bit rate of sending data according to the feedback by the receiver to prevent network congestion.
We evaluated our method by transmitting MPEG-2 data stream with changing rate of packet loss. As a result, loss rate of packet including important data decreased, and the noise of video and audio also decreased.