Wrongly calcutated packet size in CoojaRadio

[maxime-peim]

Hi all,

Recently, I have been experiencing a weird situation when a Cooja mote sends a packet to a z1 one. Upon reception, the z1 mote was always measuring a RSSI of -100 dBm, even if the motes were close (using the UDGM radio medium).

I found out that z1 mote defined in apps/mspsim/src/org/contikios/cooja/mspmote/Z1Mote.java uses the Msp802154Radio radio type, which simulates the reception of a packet by delivering it byte by byte in the method setReceivedPacket. However, before sending the bytes to the radio object, the packet received from a Cooja mote is converted using the class CC2420RadioPacketConverter, which adds 6 additional bytes of header (preamble + synch + length).

Now, if we take a look at the code for a new transmission in a java/org/contikios/cooja/contikimote/interfaces/CoojaRadio.java, we can see that 2 additional footer bytes (lines 373 & 374) are not taken into account in the size.

Hence, the variable size in CoojaRadio does not count 8 bytes received by the simulated mote. Since size is used to determine the time needed for a Cooja mote to sleep before advertising the end of a transmission, the transmission is announced as finished earlier. When it happens, the RSSI is resetted to -100 dBm by the UDGM.updateSignalStrengths method, while the z1 mote's radio is still receiving 8 bytes, which is sufficient enough to have a falsy computed value by Msp802154Radio.setCurrentSignalStrength.