AX.25 simulation on NS-2

I think that there is no such protocol readily available for ns2.

class LL/Sat is currently the only one available in ns2. Refer Chapter 17 of ns2 manual for more information.

You have to extend/modify any available Link-Layer Protocol of ns2 and with AX.25 Amateur Packet-Radio Link-Layer Protocol specifications
http://www.tapr.org/pub_ax25.html

But it is not a simple task – It may take months/year or more to complete it successfully.