In Network Processing and Data Aggregation in

Idle listening: A network interface is possible to support several operation states, eachone consuming different amounts of energy. Usually, the hierarchy is the following (P xxrepresents the average consumed energy of state xx) :P sleep < P idle < P receive < P send .P idle is the energy that is spent in order to listen to an idle channel to receive possibletraffic. This amount is not negligible (50-100% of the energy required for receiving) andis larger than P sleep as it requires to be powered on more circuit elements of thetransceiver.Overmitting: Happens when a node transmits a packet but the receiver is not ready toreceive it (may be in the sleep state). The packet is dropped.It would be helpful to review some known MAC schemes for the case of a WSN (wehave already presented them at section 2.2.4).TDMA/FDMABoth of them are contention free mechanisms, so collisions never occur. Every onelistens in/to its own timeslot/frequency resulting in a zero probability of overhearing /overmitting. There is no need for control packets to be exchanged before thecommunication of two nodes. The problem with TDMA/FDMA is the difficulty in themanagement of the inter-cluster communications. Moreover, when topology changes(happens very frequently), the medium has to be statically reassigned between a new setof nodes. This allocation is not an easy process and in addition to the need for intraclustersynchronization, the solution of TDMA/FDMA is not appropriate for WSNs.CDMAIn section 2.2.2 we reviewed briefly how does CDMA work. We also made a smalldiscussion on the near far problem. The solution to this problem is to adjust dynamicallythe transmission power of each node so that for a given receiver, all signals reach thereceiver with the same power. In cellular telephony systems CDMA is used widelybecause in a cell, all mobile nodes interact only with the base station (BS). BS controlsthe transmitting power of all the nodes within the cell. In an ad-hoc communicationpattern like a WSN, this problem has not such an easy solution. Every one can send andreceive messages. The ideal transmission power for a particular receiver may induce a big