On Jun 8, 2008, at 12:32 PM, salvatore galati wrote: > hi all. > I'm a student of Information Engeenering at University of Pisa. > I'm working on a thesis on data transfer protocol in WSN. > I'm using TmoteSky motes an TinyOs 2.x on cygwin. > My problem is that i can't be able to exstimate the average > transmission time of a packet of 40bytes payload. > > When I run Tossim simulations ( with "make micaz sim" ) i found that > the average transmission time is about 6,8 milliseconds ( time > between AMSend.send() and AMSend.sendDone() ) > but when I install my application on motes (with "make telosb > install" ) i found that the average transmission time is about 10 > milliseconds > ( telosb mote send packets back-to-back and is the only mote that > send packets ----> NO possible collisions ) > > It is possible that this time difference is caused by backoff times > or other delays? > Both Micaz and Telosb platforms uses CC2420 Radio stack, so i think > that backoff time should be the same. > In my python script i don't change the default values of backoff > times: > > from TOSSIM import * > t = Tossim(n.variables.variables()) > > mac = t.mac() > > mac.setInitHigh(640) ( should be initial backoff maximum value ) > mac.setInitLow(20) ( should be initial backoff minimum value ) > mac.setHigh(160) ( should be congestion backoff maximum value ) > mac.setLow(20) ( should be congestion backoff minimum value ) > > Otherwise from 'opt\tinyos-2.x\tos\chips\cc2420\CC2420.h' and 'opt > \tinyos-2.x\tos\chips\cc2420\csma\CC2420CsmaP.nc' i think that > initial backoff is random and bounded between 10-320 ticks ( 0,3 ---- > >9,8 milliseconds ) > congestion backoff is random and bounded between 10-80 ticks > ( 0,3----> 2,4 milliseconds ) > > Tossim values are double in regard to cc2420 values founded in 'opt > \tinyos-2.x\tos\chips\cc2420' directory. > > I don't understand why in telosb mote ( 250kbit/sec bit-rate) there > is this average transmission time of 10 milliseconds: > > theorically a 40 bytes payload packet should be sent in: > > about 2 msec ------------>to send 40bytes payload + 18 bytes > ( preamble + header + CRC ) at 250kbit/sec-----> (58*8)/ 250 000 > + > about 5 msec ( average initial backoff ( max backoff time - min > backoff time )/2 ) > = > about 7 milliseconds ( this value is similar to tossim average > transmission time) > > > Please anyone can explain to me why there is this difference between > tossim simulation and test on motes? > or anyone can send to me a datasheet , document, pubblication in > which I can found an explanation of backoff time values in cc2420 > radio stack on telosb motes? > It is very important to me.
It's possible your motes are sensing other 2.4 GHz traffic (e.g., 802.11) and backing off in response to it. There's also the ACK listening time. Phil _______________________________________________ Tinyos-help mailing list [email protected] https://www.millennium.berkeley.edu/cgi-bin/mailman/listinfo/tinyos-help
