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PROTOCOL.INF
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1992-02-09
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File Transfer Protocols
Errors that occur during file transfer are automatically detected
and corrected by file transfer protocols. If a block of data is
received incorrectly, the receiving system sends a message to the
sending system and requests the re-transmission. This process is
automatic. When errors occur during file transfer, the communication
program shows an error in the file transfer status window.
ASCII
This is designed to work with ASCII text files only. Notice that
you do not have to use this protocol when transferring text files.
The ASCII protocol is useful for uploading a text file when you are
composing e-mail online.
Kermit
Kermit was developed at Columbia University. It was designed to
facilitate the exchange of data among very different types of
computers (mainly minicomputers and mainframes). You probably will
not need to use Kermit unless you are calling a minicomputer or
mainframe at an educational institution.
Sealink
Sealink is a variant of Xmodem. It was developed to overcome the
transmission delays caused by satellite relays or packet-switching
networks.
Xmodem
Xmodem is one of the most widely used file transfer protocols.
The original Xmodem protocol uses 128-byte packets and a simple
"checksum" method of error detection. A later enhancement, Xmodem-CRC,
uses a more secure Cyclic Redundancy Check (CRC) method for error
detection. Xmodem protocol always attempts to use CRC first. If the
sender does not acknowledge the requests for CRC, the receiver shifts
to the checksum mode and continues its request for transmission.
Xmodem does not maintain a file's original size (rounded up to the
nearest 128-byte increment). Xmodem sends only one file, and it
doesn't send the file's name, time or date.
Xmodem-1K
Xmodem 1K is essentially Xmodem CRC with 1K (1024 byte) packets.
Though Xmodem 1K uses 1K blocks, in all other respects, it is
identical to Xmodem-CRC, and it cannot send filenames, time, dates,
or multiple files. On some systems it may also be referred to as
Ymodem.
Ymodem (Ymodem-batch)
Ymodem is essentially Xmodem 1K that allows multiple batch file
transfer. On some systems it is listed as Ymodem Batch. While
transferring files, it keeps the correct name, size, and date, and
may use 128- or 1024-byte block sizes.
Ymodem-g
Ymodem-g is a variant of Ymodem. It is designed to be used with
modems that support error control. This protocol does not provide
software error correction or recovery, but expects the modem to
provide the service. It is a streaming protocol that sends and
receives 1K packets in a continuous stream until instructed to stop.
It does not wait for positive acknowledgement after each block is
sent, but rather sends blocks in rapid succession. If any block is
unsuccessfully transferred, the entire transfer is canceled.
Zmodem
This is generally the best protocol to use if the system you
are calling supports it. Zmodem has two significant features: it
is extremely efficient and it provides crash recovery. Zmodem can
transfer a group of files in one batch, keeping the exact file
sizes and dates.
Like Ymodem-g, Zmodem does not wait for positive acknowledgement
after each block is sent, but rather sends blocks in rapid succession.
If a Zmodem transfer is canceled or interrupted for any reason, the
transfer can be resurrected later and the previously transferred
information need not be resent.
Zmodem has two levels of implementation, with the most complete
being ZMODEM-90. The public domain Zmodem lacks the capability to
do data compression or operate over 7-bit data links.
Which file transfer protocol should you use?
In general, the answer is Zmodem. If Zmodem is not supported
by the system you are calling, use Ymodem batch. (If you are
connecting to a UNIX system in a university, you may have to use
Kermit or Xmodem to transfer files.)
PROTOCOL.INF Feb. 1992
Copyright 1992, Richard Martin - All rights reserved