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are compressed again using run-length compression (described later). Compression
ratios of 50:1 can be achieved. It is not simple to write (from scratch) a program to per-
form this type of compression. Here are some web sites and a PDF discussing image
compression:
http://it.wce.wwu.edu/jongejan/461/Video.html
http://arachnid.pepperdine.edu/grosenkrans/compression.htm
http://poseidon.csd.auth.gr/LAB_PUBLICATIONS/Books/dip_material/
chapter_4/main.htm

Run-Length Compression
One of the oldest, and most intuitive, techniques of compression is simple run-length
compression. Instead of sending, for example, a series of 2,415 zero byes, we can sim-
ply send a block of data that is about 4 bytes long, explaining it represents 2,415 zero
bytes. The protocol is simple and can be written from scratch if need be. The following
URLs explain a few different types of run-length encoding:
www.rasip.fer.hr/research/compress/algorithms/fund/rl/
http://datacompression.info/RLE.shtml

Huffman Compression

Huffman compression can be used if the data can be broken up into symbols (like text
bytes). Then all the symbols are reassigned a different code before transmission. The
most often transmitted symbols are assigned short codes. Symbols that are rarely trans-
mitted are assigned longer codes. Run-length coding is also applied. The following web
pages are interesting lectures on data compression in general:
www.eee.bham.ac.uk/WoolleySI/All7/body0.htm
www.cc.gatech.edu/ kingd/comp_links.html
www.eee.bham.ac.uk/WoolleySI/All7/links.htm

Encryption and Security

Instances occur when data must be encrypted before it is transmitted. The physical links
of most communication channels move through areas that are in public. Certainly, all
radio frequency (RF) communications that move through free space can be intercepted,
and phone communications move through common wiring and facilities. We’ve all