The transmission of a real time video stream over a wide-area LAN requires consideration of several constraints. In particular it is necessary to face the fact that the available transmission bit rate is connection dependent and does, in general, varying over time. When the system is designed to serve more than one user receiving the images, it is necessary to modify the output bit rate of each connection depending on the network traffic statistics. A possible solution is to use several video coders running at the transmission end, one for each required bit rate. A different solution leads to a multi-layer video coder able to provide several video streams. This solution is obviously more efficient in terms of design simplicity and computational burden. In this paper we present a simple multi-layer video coder able to real time compress a QCIF video stream and to produce several outputs. The proposed video coder is based on an extensive use of the JPEG algorithm and it is fast enough to compress up to 25 frames per second running on an inexpensive personal computer.
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