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Encryption
A typical line is:
Data
Status
I
LTA
I
The PC file transfer program would read some amount of data to
be
transmitted to
the host from its diskette. Appended to this data would usually be a status field of
predetermined length to indicate the state of the transfer and a LTA character. The
status would typically contain some sort of error checking field such as a checksum
or CRC.
Upon receiving the PC's line of data, the host program would interpret the status
and send its own line of data requesting that the PC resend the information, transmit
the next line of data, or terminate the file transfer.
Because the 3708 normally uses LT A characters to control communication between
the PC and the host, L T A characters should appear only at the end of each line.
LTA characters should not occur in either the data or status portion of a line.
Files containing LT A characters or binary data, which can be interpreted as LT A
characters, must
be
encoded in some manner to
be
transferred successfully through
the 3708.
Typically this is done through a byte expansion technique. The simplest technique
(also the most expensive in terms of bandwidth) is a two-for-one expansion. In this
method, two alphanumeric characters encode the binary representation of each
character. For example, assume that an ESC (X' lB) is to
be
transmitted to the
host. Using a two-for-one expansion, the ESC would be expanded and transmitted
as two ASCII characters, X' 31' , X '42' (the number '1' and the letter 'B' in
ASCII).
Files that contain binary data or embedded LTA characters can either be entirely
encrypted or (to reduce the required bandwidth) may be encrypted using a "BSC
like" transparency feature. Encoded characters are preceded and followed by some
special delimiter sequences. For example, a DEL STX sequence may indicate the
beginning of an encoded sequence, and it DEL ETX the end of the encoded
sequence. Characters used in such delimiters cannot be defined as LT A characters
and may require to be transmitted in a unique way when they occur in the file itself.
For example in BSC transparency, a DEL DEL sequence is needed to transmit the
single DEL character.
Other encoding schemes may also be employed to reduce the amount of bandwidth.
For example, the 256 possible bit combinations can be divided up into quadrants.
Displayable code characters are chosen to identify each quadrant and the hex codes
within each quadrant. The algorithm to encode a file is:
• Find the code character for both the quadrant and the particular hex code.
• If the hex code was in the same quadrant as the previous hex code, transmit only
the code character for the hex code.
• If the hex code was not in the same quadrant as the previous hex code, transmit
code character for both the quadrant and the hex code.
Appendix D. Personal Computer File Transfer through the 3708
D-3
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