Frequency HoppingFrequency Hopping Spread Spectrum (FHSS) refers to a method of hopping across multiple frequencies, and is a proven way to sidestep interference and overcome RF challenges with agility rather than brute force. A complementary technique to address RF challenges is Direct Sequence Spread Spectrum (DSSS), which is used with all IEEE 802.15.4 2.4 GHz standard radios. This technique provides a few dB of coding gain and some improvement in multi-path fading. Combining FHSS with DSSS combines the best of both worlds – the interference rejection from FHSS and the coding gain from DSSS. The other technique for overcoming interference is increasing the radio power – effectively “turning up the volume.” With 802.15.4 radios turning up the volume, while often effective, kills battery life, so it is not an effective solution for low-power WSNs. As with most communications mechanisms, increasing the number of distinct channels proportionally increases the throughput of the system. In the case of TSMP, employing FHSS on top of the 802.15.4 radio effectively increases bandwidth by 16 times. For low data rate applications this means that even if the majority of the band is blocked by RF interference, the messages will still find a clear channel and get through. In either case the effect of FHSS is to greatly increase the reliability of the system. A frequency hopping TDMA protocol is a very efficient means to coordinate node communications. It has been demonstrated that over 1000 TSMP nodes can operate in the same radio space with each other without effecting end-to-end reliability. In contrast, dense networks of nodes using collision-based protocols like CSMA (Carrier Sense Multiple Access) can experience cascading collisions and network failure.
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