[Tinyos-host-mote-wg] [Tinyos-2.0wg] Network stack/AM proposal

[Philip Levis]

Background: last telecon, we briefly discussed what the HIL for a  
radio is. I raised the question of whether it is AM or something  
below AM (e.g., a raw data link packet). Vlado commented that he  
thought it should be the latter.

I noted that this separation means that, for AM to be hardware  
independent, it must have its own addresses and fields, independent  
of the underlying data link layer. E.g., it would preclude some of  
the 1.x 802.15.4 techniques, e.g., of using the 15.4 address as the  
AM address. The real kicker to this is that it can preclude software  
controlled synchronous acknowledgements (e.g., on the mica2), or, in  
order to allow synchronous acknowledgements, will require software to  
map between data link and AM addresses (e.g., on 802.15.4). Going  
down the ARP/RARP road seems like a bad idea.

I argued that Active Messages are the radio HIL. This means that  
there's a platform dependent implementation, allowing the kinds of  
optimizations that we've seen historically. This does not mean that  
there cannot be additional HILs, such as raw data link packets; it  
merely means that AM implementations are chip dependent.

Specifically:

Every radio chip has a component with the name of <chip>AM, which  
encapsulates AM-level communication, providing a parameterized  
AMSend, Receive, as well as Packet, SplitControl and AMPacket.  Every  
platform that supports AM has a component ActiveMessageC, which wires  
to the correct chip component.

This naming allows a platform to have two radios, support AM on both,  
but specify one as the "default" AM interface.

This leaves the AM packet structure completely hidden from anything  
above the HIL, which is good: only the HIL knows the format, and it  
can be different for different implementations. E.g., the 802.15.4  
can generate length() from the 15.4 length, while the mica2/CC1000  
has an AM length field.

Phil

-------

"We shall not cease from exploration
And the end of all our exploring
Will be to arrive where we started
And know the place for the first time."

- T. S. Eliot,  'Little Gidding'

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