hi all. I'm a student and i'm developing a transmission protocol to exchange data between two or more motes in a WSN. I'm working on cygwin an TinyOs 2.x and TmoteSky motes. In this application there is one mote ( mote A ) that send a static window of 12 packets via Radio and wait for aknoledgment sent via Radio by other motes (mote B, mote C ...) I must syncronize the sending mote ( mote A ) with other motes ( mote B, mote C ...) cause aknoledgment sent by mote B or mote C must arrive after that mote A has sent all the static windows ( 12 packets):
example of behaviour of the protocol over time: mote A ------> send packet 1 mote A ------> send packet 2 mote A ------> send packet 3 ..... ..... mote A ------> send packet 12 mote B ------> send ACK (mote A receive ACK) mote C ------> send ACK (mote A receive ACK) .... .... mote A ------> send packet 1 mote A ------> send packet 2 .... ... I used Tossim to simulate this behaviour , in my simulation mote A have WINDOW_SIZE ( =12 ) * 15 milliseconds time to transmitt all 12 packets, and than wait for x milliseconds the arrival of ACK sent by other motes. With command "make micaz sim" I load cc2420 radio stack but cause i suppose to send a packet in 15 ms ( the timing for syncronization in based on this value)---->this behaviour simulate the cc1000 radio stack . I want to simulate this behaviour for cc2420 radio stack for telosb motes , but telosb motes has 115200 baud rate ---> so , theorically, a packet can be sent in about 4 milliseconds--->40 bytes (Radio header + payload) / 115200 = about 3 ms to send a message , but when i see debug messages of my simulation in tossim i see that a message is sent in a variable time bounded between 3--->12 milliseconds ----->there isn't constant time to send a packet so i can't syncronize mote A with other motes. my question are: 1) This variable time of 3--->12 ms to send a packet is caused by backoff time in cc2420 or is caused by task scheduling in Tossim? 2) is there a way to have a constant time to send a packet in Tossim simulation? 3) If I install my application in a telosb mote with "make telosb install" there will be the same problem caused by backoff time or i can set a time to send a packet via Radio = 4ms (for example) and be sure that a packet will be sent via Radio in less than 4ms? 4) is there a way to totally delete backoff time or set backoff time to a const value bounded between 0,1---->0,5 milliseconds? ( i see that Backoff periods are set in /opt/tinyos-2.x/tos/chips/cc2420/CC2420.h , i try to set them to a const value =1 but in Tossim simulation there is the same problem (message sent in variable time of 3--->12 ms)) I hope in yours answers , even to answer 1 of 4 of my question Thanks a lot and best regards Bye , salvatore
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