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TIPS-002.ASC
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1992-08-23
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TIP #: 002
KEYWORDS: MNP-5, V.42bis, Modem, Slow Download Speeds
UPDATED: August, 1992
PROBLEM: When using a 2400 baud MNP-5 modem, and downloading ZIP files from
a Bulletin Board System, transfer rates of 280 cps are possible
from some boards, while only 250 cps can be achieved on others.
SYMPTOMS: When downloading from some Bulletin Board Systems, the download
speed of ZIP files is about 280 cps. From other boards only about
250 cps is possible.
SOLUTION: MNP-5 compression is not effective on ZIP files. In fact, if you
try to compress a ZIP file, things actually slow down. As a
result, most boards disable MNP-5 compression. You can still
achieve an error correcting link, but there will be no data
compression. Under this scenario you will achieve about 280 cps
download speeds at the expense of screen update speed. The screens
full of menus are not compressed either! On my board, MNP-5
compression is enabled. If you call in with MNP-5 your screen
updates will be very fast, but your downloads of ZIP files will be
in the 250 cps area. If you download Text or ASCII files, you may
achieve rates as high as 500 cps. The solution is to defeat
compression on the callers end when you want to download ZIP files.
There is usually an "AT" command to defeat MNP-5 compression. Read
your modem manual. If you intend to do a lot of downloading,
defeat MNP-5. If you are just calling to browse, leave it on.
V.42bis handles this problem more elegantly. V.42bis is capable of
up to 4:1 data compression, but will not try to compress data that
has already been compressed. That means that text files and menu
screens benefit from the 4:1 compression, but ZIP files transfer
with no speed penalty. V.42bis is clearly the best of both worlds.