Linux-Hams archive - June 1997: Oops...

Oops...

Mike Bilow ()
Sun, 08 Jun 97 02:37:00 -0000


John Ackermann AG9V wrote in a message to Mike Bilow:

JAA> I'm just trying to figure out the relationship between the
JAA> ax25 maximum-packet-length param and the interface MTU.

Oh. AX.25 "PACLEN" - AX.25 overhead = MTU. Overhead in the spec looks like:

First
Bit Sent

Flag Address Control FCS Flag
01111110 112/560 Bits 8 Bits 16 Bits 01111110

Fig. 1A -- U and S frame construction

First
Bit Sent

Flag Address Control PID Info. FCS Flag
01111110 112/560 Bits 8 Bits 8 Bits N*8 Bits 16 Bits 01111110

Fig. 1B -- Information frame construction

PACLEN excludes flag size, so the overhead in an non-digipeated U/S-frame is
14+1+2=17 bytes and in a non-digipeated I-frame is 14+1+1+2=18 bytes. Of
course, a digipeated I-frame can contain as much as 70+1+1+2=74 bytes overhead.

In general, you should allows for ((n+2)*7)+4 bytes of overhead, where "n" is
the number of digipeaters, where "n" can be as low as 0 and as high as 8.

-- Mike