[Tinyos-help] Message Formats for Basestation

[Philip Levis]

On Jan 1, 2007, at 4:15 PM, Mr. Jonathan A. Gomez wrote:

> (Sorry for sending it to you again, and not to the list.  I wasn't  
> attending to the return
> address!)
>
> Thank you for the prompt response!
>
> I'd like to make sure I understood properly, and I have some  
> further questions.
>
>
>> There are two separate forms of dispatch at play here, which is
>> probably the source of the confusion. AM ids specify the format of
>> the AM packet payload, while serial ids specify the format of the
>> AM packet.
>
>
> Suppose this were the packet being sent over serial:
>
> 7e 44 01 00 ff ff 04 22 06 00 00 01 00 95 91 7e
>
> Then the 44 is the serial protocol type, indicating a request for  
> acknowledgment,

Yes.


> the leftmost
> instance of 00 is the id of the format of the AM packet,

Yes. 00 is the platform independent serial format.

> and the 06 is the am_id_t payload
> format identifier?

Yes.


> This payload format identifier is also the "AM packet type" you  
> mention
> below?

Yes. But don't confuse format identifier with am_id_t; that's  
generally what it means, but it doesn't always. The active message  
type or AM packet type is the AM id. This specifies a protocol.  
Different protocols can have different packet formats, but do not  
always do so.

>
>> Applications define AM handlers. That is, they define which AM
>> packet types they receive and send by wiring to particular AM ids.
>> The AM id therefore specifies what the format of the AM payload is.
>
> Makes sense.  I'd like to know more about how BaseStation does  
> this, since that is the
> application in which I vest a particular interest?
>
> Does BaseStation ever filter out packets based on am_id_t, that is,  
> the payload format, or does
> it merely pass them all on, no matter which ids are employed?  This  
> latter seems to make the
> most intuitive sense.

It doesn't filter; that would defeat its purpose. I'd suggest reading  
the code.

>
> Also, I'm looking into the code, but I'd like to ask too, if I  
> may:  Does the
> SerialActiveMessage mechanism always send platform independent  
> messages?  And it is always
> capable of receiving them?
>

Yes. I'm not sure what "always capable of receiving them" means. If  
you instantiate a SerialAMReceiverC, then it will be signaled when  
the node receives AM packets that:

1) follow the framing and format of the serial layer
2) follow the serial AM packet format
3) have the right AM id


> [SNIP]
>
>> This additional layer of dispatch exists to remove the TinyOS 1.x
>> problem of having platform-specific serial packet formats. The
>> default packet format, which is platform independent, is a basic AM
>> packet. But if you want to send/receive platform-specific packets
>> (e.g., see all the fields of a CC2420-specific AM packet), you can
>> still do so.
>
> Hmmm... Depending again on the apps, or is it more transparent?

If you wanted to send/receive packet formats besides the platform  
independent serial one, then you need to write components which do  
so. You would instantiate different sender/receiver components. I  
think there's some degree of support for 802.15.4, but as far as I  
know no-one has ever used it, so it might not work very well.

Phil