AX.25 Layer 2
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Example AX.25 Layer 2 Network

Network Diagram

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.