On Sun, Aug 16, 2009 at 1:13 PM, Lynn W. Deffenbaugh (Mr)<[email protected]> wrote:
> James Ewen wrote: >> AX.25 packet rules limit the value of "N" to a maximum of 7. >> > Just for clarity to those learning AX.25 by osmosis, the AX.25 packet > rules (http://www.tapr.org/pub_ax25.html) do NOT directly limit the > value of N to 7, but to 15 (4 bits as shown in 2.2.13.1.1 (A7) and > 2.2.13.2 (A21)). This is where the SSID limits of -0 (normally > suppressed) to -15 comes from. My suggestion that N be limited to a numeric value of 7 was not based on the SSID limit, but rather by the element limit you list below. > What AX.25 DOES directly limit is the number of elements in the PATH of > a packet. This is limited to 8 per 2.2.13.3 in the AX.25 spec. This > limit does not include the source and destination callsigns (the > Address-Field 2.2.13.1). This limit is indeed an AX.25 limit as you point out. > With callsign substitution occurring, a WIDE7-7 packet could become: > > DIGI-1,DIGI-2,DIGI-3,DIGI-4,DIGI-5,DIGI-6,DIGI-7,WIDE7* > > This consumes all 8 available path slots according to AX.25. By > extension, a WIDE8-8 or WIDE9-9 would run out of substitution spots in > the path before exhausting the path, thereby blaming the WIDE7-7 limit > on AX.25, but only because of the callsign substitution and traceability > of the WIDEn-N paradigm. While it is true that you could use an outgoing path of WIDE9-15 which is a perfectly acceptable path alias format according to one part of the AX.25 spec, it would not be acted upon by the Kantronics line, nor any other digipeater that I know of. I tried higher values of N when playing with the Kantronics line a number of years ago, and it limited the value it acted upon to ensure that the substitution spots weren't over run. One thing that I didn't try was to use something like WIDE7-7,WIDE7-7 to see what would happen if the substitution spots were full, yet the TNC was asked to insert yet another callsign. Would it overwrite the last callsign, not insert a callsign, or bomb out? This of course is getting a little deep, and a little off topic for the T2 reflector. James VE6SRV
