The Online World resources handbook

Your first trip online. Typical pitfalls and simple solutions http://home.eunet.no/~presno/bok/v3.html
XMODEM's "packet size" (block length) is short. This has an impact on transfer speed,
and especially when downloading from timesharing systems, packet switched networks,
via satellites, and when using buffered (error correcting) modems.
The control method (8 bit check sum) and unprotected transactions give a low level of
safety against errors in the transmission. The transferred file may contain 127 bytes with
noise characters (at the end). The creation date of the file is lost in the transfer.
These weaknesses have given us better methods. Here are some of them:
XMODEM/CRC
CRC is an abbreviation for Cyclical Redundancy Check. The method guarantees
99.9969 percent free transfer. It still has the other weaknesses of ordinary XMODEM
transfers.
YMODEM Batch
is faster than XMODEM and gives a high level of safety in the transfers. When used
with some programs, YMODEM can transfer the files' creation time/date. You can
transfer updated documents. This will replace documents with an older creation date.
Only one party must enter the file name. YMODEM takes care of the rest.
Kermit
is used on many computer platforms, and especially where they use a terminal emulation
mode (like VT 100) which makes the use of XMODEM impossible. Kermit is one of the
few asynchronous error correction protocols that functions well exchanging files with
half duplex IBM front end machines.
Kermit can transfer more than one file at the time.
Super Kermit
is also called Kermit with Sliding Windows. It can transfer many packets before stopping
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ZMODEM
is the fastest known transfer protocol for many applications. All transactions are
protected with a 16 bit or 32 bit CRC. ZMODEM is immune against most error
conditions that prevent traditional protocols to achieve correct transfer.
ZMODEM transfers the creation date of the file and its exact contents. The file name is
read once, and all transfer commands may be given by the sending program.
Decompression of files
If a file has name extensions like ZIP, LZH, ARC, PAK, LQR, LBR, ZOO, ARJ, or
QQQ, you are facing a compressed file. We use such files to achieve faster transfers.
Files having the extension .EXE or .COM may be compressed files that have been
converted into a self extract format. To retrieve the files from a self extract compressed
file, just enter the file's name.
To decompress files that have not been made self extract, you need a utility program.
These programs have many names and are widely available through the Internet.
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