Re: [Contiki-developers] 868 MHz radios in contiki and Cooja

[Graeme B. <G....@ec...>]

Hi Dominic,

> Can WOR be used to do the periodic medium checks?  Is it a useful feature to implement ContrikiMAC?

Basically, ContikiMac works by turning on the radio to look for activity and turning off again if nothing is found or keeping the radio on if activity is found (until the packet is received or for a set period). These are CCAs and by default, a set of two CCAs are carried out 8 times per second (so there are 16 checks each second).  The amount of time that the radio remains on for in a check where no activity is detected is determined by how long it takes for the radio to give a valid channel clear indication.  This is in the order of a few ms  so the radio is off for the majority of the time.  However, ContikiMac needs to retransmit any packet sent several times as it is highly unlikely (without phase locking) that the radio will wake up at just the right moment to receive a single packet.  For low-power radios like the CC2420 this is not a huge issue as the energy difference between TX and RX is quite low.  For 868MHz, the radios tend to be higher power and the difference between TX and RX is greater.  I haven't done the calculations yet, but it could be that using ContikiMac with radios with a high transmit power actually uses more energy than NullRDC due to the retransmissions.

With the TI radios, EWoR does much the same as ContikiMac in that it turns on to look for activity and turns off again except it does not retransmit every packet.  EWoR will have the radio on for more time in each second.  EWoR would be used with NullRDC and NOT ContikiMac as the two working together would cause problems and node using NullRDC/EWoR would NOT be able to talk to a node running ContikiMac however a node using NullRDC should be able to talk to a node running NullRDC/EWoR.  As such, it is not a useful feature to implement with ContikiMac but it may be useful instead of ContikiMac.

> As much as I know only the TI radios support WOR. I think one can implement the same behaviour by software but with a higher current consumption as the cpu must wake up for the muedium check.

Implementing EWOR in Software is likely not easy/may not be doable as EWoR can be set to use the first few bits of the Sync word for AGC settling (so the RX does not use the preamble) so the radio would need to support this..

Thanks,
Graeme