On Sun, Jan 20, 2019 at 08:22:32AM -0800, Martin Cooper wrote:
> Hi Stuart,
> > A silly question, is the destination address in a UI frame case-sensitive?
> From the AX.25 2.2 spec, section 2.2.13, Address-Field Encoding:
> "Except for the Secondary Station Identifier (SSID), the address field
> should be made up of upper-case alpha and numeric ASCII characters
> only."

  The HDLC address field is extended beyond one octet by assigning the 
least-significant bit of each octet to be an "extension bit". The extension bit 
of each octet is set to zero, to indicate the next octet contains more address 
information, or one, to indicate this is the last octet of the HDLC address 
field. To make room for this extension bit, the amateur Radio call sign 
information is shifted one bit left.

=> Even if you try to use lowercase characters (what will break supposely every
existing packet-radio-software), due to the left shift, you won't be able to
encode letters p-z.

> > 1. Using the reserved bits.  There's two bits in the last octet of each
> >    SSID, which are normally set to '1'.  I could use one of those to
> >    indicate this is a "unicast" address; setting that to 0 would mean
> >    the message is being "multicasted" or "broadcasted".
> I'll note that the AX.25 'listen' source code uses these two bits in
> decoding. One is used to identify "EAX25" and the other "DAMA". It
> might be worth looking into what those are all about before reusing
> either or both of these bits.

Btw, Destination QST-0 (and in many implementations NODES-0 (for netrom
protocol) is defined as multicast address.

vy 73,
        - Thomas  dl9sau

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