[aprssig] What is "TNC Channel Switching"?

Stephen H. Smith wa8lmf2 at aol.com
Fri Apr 11 09:16:08 CDT 2014

On 4/11/2014 9:42 AM, Andrew Pavlin wrote:
> Could one then presume that such TNC's could not participate in any binary radio network (such as TCP/IP over AX.25 or OpenTRAC), since the binary payload might contain those characters?
> Seems kind of archaic, especially with KISS having a means to send such commands without reserving byte values in the data stream.

Long before KISS, AX.25 packet communications meant keyboard-to-keyboard 
conversations using TNCs  operated from dumb ASCII devices like Teletype 
machines, ADM3 "glass TTYs" and VT-100 terminals.  Then with dumb-terminal 
emulating software like ASCII Express, ProComm, Hyperterminal, etc.   [The 
reason TNCs have considerable firmware "intelligence" is that, in the 
beginning, the devices connected to them were completely dumb keyboards; i.e. 
not computers.)

The user typed  either commands to control the functions of the TNC, or text to 
be transmitted over-the-air, all from the same keyboard.  You typed the command 
"CONV" (converse) before typing text over the air, and "Ctrl-C" before typing a 
command to the TNC's internal firmware.

There WAS NO binary data to be transmitted -- only what could be typed from a 
basic keyboard; i.e. 7-bit classic ASCII.   Later, full 8-bit bytes with values 
above 127 decimal or $7F were transmitted by using various means of expressing 
them as printable ASCII characters; i.e. something like hex dumps.

[This is similar to how email works even today.   The actual transmission of 
email still uses the original 7-bit ASCII coding; binary file attachments and 
UniCode text are converted into strings of printable ASCII text for 
transmission, and then converted back to 8-bit bytes at the receiving end.  The 
effect of this is that an attached "1 megabyte" file actually causes typically 
about 1.5 megabytes to be sent over the TCP/IP link.]

When packet started being about connecting computers together, rather than dumb 
keyboards and displays, the KISS protocal was added.  KISS bypassed the command 
interpreters in the TNC and truly used the TNC as just a dumb modem.  KISS is 
fully 8-bit transparent, but does provide it's own means of selectiong "logical 
ports" on a single physical channel.

The Kantronics "stream switch" characters were a way of accommodating multiple 
logical channels in the 7-bit ASCII world that predated KISS, by using two 
typeable characters that were not used much (at least in American English and 
if you weren't a Unix programmer!).

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