This is an example snippet of two AX.25 single channel LANs
interconnected using Layer 3 gateways. In this example, all of the PCs are
running applications which are using layer 3 protocols. To show how the
same LAN can be used for different layer 3 protocols, this diagram shows Layer
3(A) and Layer 3(B) to designate two different layer 3 protocols. Each
AX.25 link (the two LANs and the backbone link) are 100% independent (AX.25
packets from one do not show up as AX.25 packets on the others). This is
an example only. In reality, there are many more stations and many more
combinations of layer 3 networking that are available. The key here is to
show how AX.25 can be used as a layer 2 protocol providing generic link layer
functionality to higher layers.
A routing example would be an application on the left PC wants to talk to an
application on the right PC. The application is aware of how to
communicate using the Layer 3(A) protocol. The application says "I want to
talk to the application on the PC on the right" using whatever addressing is
available at the different layers. Essentially, the left PC would try to
see if the right PC is on the AX.25 LAN using some method of discovery like ARP
for instance. It would not find it so it would then look for the Layer
3(A) gateway (router), again using a layer 3/2 discovery. It would find
the gateway that is directly attached to the center digipeater. It would
then ask that gateway to route its packets to the right PC. The gateway
would then use its capabilities (in this case, an RF backbone) to talk to the
gateway on the right and see if the right PC can be found. That gateway
will discover the right PC and establish a route between the two PCs using the
Layer 3(A) protocol. In this case, AX.25 is solely used as a layer 2
protocol and does not appear at the higher layers. The same could be done
using the Layer 3(B) protocol where, in this case, the gateway-gateway backbone
is supplied by the Internet.
Another example would be APRS, a protocol directly using AX.25, making use of
this architecture. In this case, consider the PCs on the left and right
running APRS clients and the PC in the middle is running IGate software (layer 2
to layer 3 translating gateway). This could also be done with gateways
linked by RF. In this case, all of the left and right PCs' packets make it
into the Internet. However, the left PC doesn't see the right PC's packets
because they are broadcast and the total packets on the Internet far exceed the
LAN capacity. The left PC can communicate with the right PC by simply
sending a message to it. The IGate software will then "gate" the ack
message and the left PC's position packet out to RF in a third-party format.
This provides true tactical local area operation for the APRS users, while still
maintaining communications interconnectivity with other LANs.