Scott@paradox.UUCP (Scott Beckstead) (03/08/90)
Ok I've seen a lot of discussion lately about modem compression and file compression. First of all most modems don't have time to run the fancy compression algorithms (though the Telebit modems use a modified COMPRESS I am told) However the most common (MNP 5) compression is done thusly First during an MNP connect both modems talk to each other synchronously so when compression is enabled they simply strip the start and stop bits from each byte transfered. This results in a 2 bit compression (8n1 is a 10 bit frame) meaning that only 8 bits are required to send 8 bit data it aint much but it helps (if you connected to the thing at 7 bits the 8th bit doesn't mean anything but it should always be sent anyway.). However in some of the high power modems they do have time to run a significant compression algorithm and some do quite nicely. It doesn't matter how decide to set the compression the key is to be able to leave it where you put it. I have found that transfering compressed files with compressing modems doesn't affect through put as much as trying to transfer text through non-compressing modems. So what happens when you tranfer a compressed file with a compressing modem? well worst case with compress (UNIX compress) I have found is a 5% increase in file size or at 2400 baud an extra 1.2 bytes per 100 bytes (check my math I've been known to misplace a decimal or two) or 12 bytes per K Not much to worry about at less than megabyte sizes. Leave compression on It can't hurt that much and you'll be a lot happier with the plain text/binary transfer rate Scott Beckstead Practical Peripherals -- A Hayes Company (yes THAT Hayes) We Do Modems Right!