Perception presents:
------------ Understanding The X-Modem File Transfer Protocol ---------------

			      by Em Decay

This has to be one of the most internationally accepted protocols for upload-
 ing and downloading binary and text files.  It is fairly straight-forward as
 to how it is set up and there are some error checking capablities.


--- Before you begin ---

Things you need to know beforehand...

The following terms are simply ascii codes:
    SOH = chr(1)  = CTRL-A =
    EOT = chr(4)  = CTRL-D = End of Transmission
    ACK = chr(6)  = CTRL-F = Positive Acknowledgement
    NAK = chr(21) = CTRL-U = Negative Acknowledgement
    CAN = chr(24) = CTRL-X = Cancel

In order to send the file, you must first divide it into 128 byte sections
 (packets).  Bytes 0-127 of the file make up the first packet, bytes 128-255
 make up the second packet, etc.

The packet number sent is simply the number of the packet.  If the packet
 number is greater than 255, then subtract 256 repeatly until the number is
 between 0 and 255.  For example, if you were sending packet 731, then you
 would send 731 - 256 - 256 = 219.

The 1's complement of a byte (to make life easy) is simply 255 minus the
 byte.  For example, if you had to take the 1's complement of 142, the answer
 would be 255 - 142 = 113.

The checksum is the value of all the bytes in the packet added together.  For
 example, if the first five bytes were 45, 12, 64, 236, 173 and the other 123
 bytes were zeroes, the checksum would be 45+12+64+236+173+0+0+...+0 = 530.
 However, to make each block one byte smaller, they repeatly subtract 256
 from the checksum until it is between 0 and 255.  In this case, the checksum
 would be 530 - 256 - 256 = 18.

The first byte the downloader sends is referred to as the NCGbyte.

Provided that you aren't lost already, here is what happens next.  The steps
 below describe who sends what when :)


--- The Actual Transfer ---

The uploader waits until the downloader sends a NAK byte.  The NAK byte
 is the signal that the downloader is ready to start.  This byte is referred
 to as the NCGbyte.  If the downloader takes too long or an error occurs then 
 the uploader will stop waiting or "Time Out".  If this happens, then the 
 file transfer must restart.

With each packet sent...

    The uploader sends:

    1. an SOH byte                             {1 byte}
    2. the packet number                       {1 byte}
    3. the 1's complement of the packet number {1 byte}
    4. the packet                            {128 bytes}
    5. the checksum                            {1 byte}
    The above five things are called the block.

    The downloader:

    1. ensures that the packet number sent matches the actual packet number
	 that it is (If the third block send has a '4' as the second byte,
	 something is wrong --> CANCEL TRANSFER (send CAN byte))
    2. adds the packet number and the 1's complement of it together to make
	 sure that they add up to 255.  if they don't --> CANCEL TRANSFER
    3. adds up all the bytes in the packet together --> THE SUM
    4. compares the last two significant digits of THE SUM with the checksum
    5. if everything looks ok (sum=checksum), then the downloader appends the
	 bytes in the packet to the file being created (sent).  The down-
	 loader then sends an ACK byte which tells the uploader to send the
	 next block.
       if the sums do not match then the downloader sends an NAK byte which
	 tells the uploader to send the same block it just sent over again.

When the uploader sends an EOT byte instead of an SOH byte, the downloader
 sends a NAK byte.  If the uploader sends another EOT immediately after that,
 the downloader sends an ACK byte and the transfer is complete.

Another thing, the downloader can cancel the transfer at any time by sending
 a CAN byte.  The uploader can only cancel between blocks by sending a CAN
 byte.  It is recommended that you send anywhere between 2 and 8 consecutive
 CAN bytes when you wish to cancel as some programs will not let you abort if
 only 1 CAN byte is sent.


--- Wrap Up ---

Hopefully, you were able to follow along. :)  If not, you can e-mail me at
 em_decay@norlink.net  and I will try to clarify it for you.  Have fun :)

Perception:    Em Decay -- Mark Korhonen
	       Cmf ------- Chris Fillion

Written on Dec.28/95
