Versatile Low Power Media Access for Wireless Sensor Networks
(Polastre et al., Sensys 2004)
Taming the Underlying Challenges of Reliable Multihop Routing in Sensor Networks
(Woo et al., Sensys 2003)
Today we had another round of interesting discussions on another two low level issues in sensor networks: radio channel characteristics and low power MAC design.
We started by talking about what clear channel assessment (CCA) means when used to detect transmissions. CCA tells you when the channel is 'clear' instead of when someone is actually transmitting a packet. The B-MAC showed that using a simple threshold is a poor approach but applying EWMA filter to the CCA result performs reasonably well. Then we discussed hidden terminal problem and benefit/overhead of RTS-CTS scheme as we talked about the comparison of B-MAC and S-MAC. Many of us thought the main difference is configurability. B-MAC exposes more knobs to the application, such as LPL preamble length, check interval and Acks, while S-MAC parameters are not controllable by other layers of the system. The result is higher performance from B-MAC because with application-specific knowledge one can turn on and off these low level features to minimize the overhead of the MAC protocol. On the flip side, this also means that the application programmer needs to know how to turns these knobs that are related to each other. It might seem that each node can tune these parameters dynamically and optimize based on the local conditions. However, it is often not so easy because neighboring nodes need to agree on many of the parameters. For example, you would almost always want the preamble length to be longer than check intervals in order to guarantee waking up of the receiver. Turning on and off Acks could be very confusing too. So, one might ask whether B-MAC is providing too much flexibility and forcing the application to be more complicated.
Another interesting question that was brought up today was: why not use power control, a knob that's there provided by the radio chip? Cell phones do it all the time to save power. Motes should do it too. Lowering transmission power can reduce the size of your neighborhood and therefore possibly increase overall throughput. I think this is an interesting option that hasn't been fully exploited by existing sensor network systems. Or, perhaps it makes higher layers, such as routing protocol, too difficult so people discarded the idea?
In the B-MAC paper, S-MAC was beaten up in almost every metric considered in the paper. With such observation, students asked about how the WSN community usually evaluates MAC protocols. The answer seems to be that every MAC paper chooses different application domains and metrics to show the protocol proposed in the paper performs better than other alternatives but there really hasn't been a complete study to compare them in a standard set of representative environments objectively. Given that so many MAC protocols have been proposed, perhaps it is a good time to do such a study.
One more interesting point that was brought up in class today was: what makes sensor networks a different research area than mobile ad hoc networks (MANET) that has been around for a long time. This is similar to the question we asked on Tuesday on why motes are so different than other computers that it needs new OSes. The papers we read so far mention very little about MANET work while WSN shares some of the challenges with MANET, such as multi-hop routing in wireless networks. I think the distinction still comes from the focus on resource constraints of sensor nodes. MANET community has mostly assumed that the nodes in the network are at least PDA or laptop class of devices. Many of them are even constantly powered (e.g. in military vehicles) so the processing power and energy available to the nodes are plenty. So, MANET research tends to focus on infrastructureless network architecture (no cell tower, no base station), node mobility and dealing with rapid topology changes of the network instead of efficiently managing system resources on the moble nodes. Another distinction is research approach. For example, a lot of MANET research results were derived from simulation based on simple unit-disc model. WSN research community, instead, puts more effort into real experiments to evaluate protocols running on top of real radio.