[ax25-layer2] Layer 2 Digipeating

[Gregg Wonderly]

pete at ae5pl.net wrote:
> The same is true for callsign-SSID.  It was intended to
> identify specific devices (up to 16 SSIDs) on a channel operated by the
> same control operator (callsign).  This is, in fact, the equivalent to a
> MAC address.  It is not generated by layer 3 but instead is resident in
> the layer 2 device.
> 
> Amateur radio communications many times requires repeaters to obtain
> reasonable coverage.  This is the purpose of the digipeater fields in
> AX.25.  As you point out, however, networking those digipeaters is _not_
> the purpose of the digipeater fields.

To say this another way...

Layer three routing requires that layer two be able to carry the traffic to the routed destination.  The AX.25 digi 
fields allow layer two to be configured to do so.  In a wired network, you know where things are plugged in.  You can 
determine that there is an ability of one device to "see" another for routing purposes.  In a wireless network, you want 
to create a link layer that gets between two endpoints just as connecting the wires do in a wired network.  Having an 
intermediate wireless node and having to specify the "wiring" in the AX.25 packet is part of the puzzle of putting 
together a wireless path.

As Pete said, that wireless skip might be better done on a different channel, or media.  But, the digi field in the 
AX.25 packet is needed in the case where there is an intermediate node which might have no layer three or higher knowledge.

Gregg Wonderly