[Tinyos-help] cc2420 transmission delay ( backoff times)

[Philip Levis]

On May 28, 2008, at 2:06 AM, salvatore galati wrote:

> hi all.
> I'm a student and I'm working on WSN with telosb motes and I'm using  
> TinyOs 2.x.
> Telosb motes uses cc2420 Radio Stack , so i want to exstimate the  
> maximum transmission time for a packet of 28 bytes ( max payload  
> lenght)
>
> i think that the total transmission time depends on:
>
> time to transmit packet of 46 total bytes [28bytes MAC payload +  
> 2bytes FCF +10 bytes MAC Header + 5bytes SHR + 1 byte PHR] = 1,4  
> msec ( 46*8 / 250Kbit/sec)
>
>                            +
>
> initial backoff time bounded between 0,15--->4,785 msec ( based on  
> value in opt\tinyos-2.x\tos\chips\cc2420\CC2420.h and opt\tinyos-2.x 
> \tos\chips\cc2420\csma\CC2420csmaP.nc )
>
>                            +
> congestion backoff time bounded between 0,15---->1,185 msec ( based  
> on same files as over there )

I thought the initial backoff was bigger than that. The units of  
backoff are 10 jiffies (32kHz ticks), as defined in CC2420.h. The  
initial backoff is 1-32 backoff units; the congestion backoff is 1-8  
backoff units. Those correspond to 300us - 10ms and 300us - 2.5ms  
accordingly.

Phil