[aprssig] What is "TNC Channel Switching"?

[Chris Moulding]

The tilde "~" character is used in APRS Messenger as a "wrapper" before 
sending the APRS packet over HF APRS over digimodes.

On transmit the CRC-16 checksum is calculated for the APRS packet and 
the three figures generated are added to the end of the packet. A tilde 
character is then added to each end of the packet and then it's 
transmitted over PSK-63, PSK-250, GMSK-250 or MFSK-16 with a short 
preamble of seven dots so that the AFC at the receive end has something 
to lock in to.

On receive the data is saved and the APRS packet plus checksum is 
extracted by looking for the text string between the tilde characters. 
The three figure checksum is removed and the APRS packet is checked 
against the checksum to confirm that it's been received correctly.

Without having the tilde character reserved in the APRS spec it wouldn't 
be possible to do that. It's not likely to affect any Kantronics modems 
as they can't decode the digimodes used.

Regards,

Chris, G4HYG

--
Chris Moulding
Cross Country Wireless
7 Thirlmere Grove, Bolton, Lancs, BL4 0QB, UK
Tel/fax:     +44(0)1204 410626
Mobile:      +44(0)7752 391908
Website:     http://www.crosscountrywireless.net
Yahoo group: http://uk.groups.yahoo.com/group/cross_country_wireless/

On 11/04/14 14:42, Stephen H. Smith wrote:
> On 4/11/2014 9:23 AM, Andrew Pavlin wrote:
>> Greetings, all.
>>
>> I was slogging through the APRS spec, and came upon a mention that the
>> ASCII characters '|' and '~' were reserved for TNC channel switching,
>> and couldn't appear in any messages or even as a symbol code.
>>
>> What does this mean, and is it still applicable? Will I break the
>> APRS-IS or some ancient TNC somewhere if I send those characters in an
>> APRS message?
>>
>> Just curious.
>>
>> Andrew Pavlin, KA2DDO
>> Sent from my Verizon Wireless BlackBerry
>> _______________________________________________
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>> aprssig at tapr.org
>> http://www.tapr.org/mailman/listinfo/aprssig
>>
>
> There are some classic serial-port-interfaced TNCs that actually have
> two separate modems in a single box connected to a single serial port.
>
> Notably the Kantronics "KAM" (Kantronics All Mode) which combines a
> 300-baud multimode HF modem and a 1200-baud VHF packet-only modem.   The
> Kantronics 9612 combined a 1200-baud packet modem similar to a KPC3 and
> a 9600-baud packet modem into a single box.    In either case, the two
> modems can be connected to two separate radios and operated
> simultaneously via a single serial line to/from the PC.
>
> Commands typed from the PC console were prefixed with either the "pipe"
> symbol or the tilde to indicated which of the two modems you wanted to
> send commands (or text to be transmitted) to.
>
> In the heyday of conventional packet, numerous third-party programs were
> developed that "covered up" this clumsy command-line interface by
> providing split-screen interfaces and/or "Radio1"/"Radio2" function key
> mappings to the user that handled the "stream" switches for you.
>
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