We turn to compression to reduce network bandwidth. If each frame is compressed before sending, the network bandwidth will be reduced.
We assume JPEG compression for flying. The Joint Photographic Experts Group (JPEG) have been working to establish the first international compression standard for continuous-tone still images, both grayscale and color [20]. Although JPEG is intended as a still picture standard, it has greater flexibility for video editing, and is likely to become a ``de facto'' intraframe motion standard as well.
The quality factor in JPEG lets you trade off compressed image size
against quality of the reconstructed image: the higher the quality
setting, the closer the output image will be to the original image and
the larger the JPEG file.
A quality of 100 will generate a
quantization table of all 1's, eliminating loss in the quantization
step (but there is still information loss in subsampling, as well as
roundoff error).
Since the data is to be as close to the original data as possible, we
assume a quality of 100 must be used for flying compression. Pilot
tests show that JPEG with a quality of 100 reduces the size of
PPM images by about 70%. In all
subsequent predictions, we assume a compression ratio of 70%.
Figure 3 shows our predictions of the effects of compression versus the number of simultaneous users. Figure 4 shows our predictions of the effects of compression versus the number of servers.
Figure: Bandwidth versus Simultaneous Users. The
top curve depicts the total bandwidth without compression. The lower
curve depicts the total bandwidth with 70% compression. The
horizontal lines are OC network bandwidths.
Figure: Bandwidth versus Servers. This graph
depicts the average bandwidth per server for a variable number of
servers. The top curving line represents bandwidth without
compression. The lower curving line represents bandwidth with
compression. The horizontal lines are OC network bandwidths.
With compression and 75 servers, we can satisfy the user-level network bandwidth requirements for 200 simultaneous users.
