# Distributed Routing and Channel Selection for Multi-Channel Wireless Sensor Networks +
## 3 Motivation Behind This Work  
Typical low-powered wireless sensor platforms such as MICAz nodes draw about 20 mA of current while transmitting and receiving (To be precise, the current consumption of a MICAz more is 17.4 mA and 19.7 mA in transmit and receive modes respectively [10]. However, for our discussions and performance evaluations we assume an approximate value to 20 mA for these two modes. ), whereas they draw about 20 \(\upmu\)A in idle mode and 1 \(\upmu\)A in sleep mode. This explains the need for minimizing radio active periods for achieving energy efficiency. As stated earlier, popular energy efficient wireless sensor networking protocols such as _XMesh_[15] employs low-power (LP) operation by letting nodes duty cycle in their sleep modes for brief periods of time to detect possible radio activity and wake up when needed. While this principle extends the battery life (lifetime) of the nodes considerably, a key factor that leads to energy wastage is _overhearing_, i.e., receiving packets that are intended for other nodes in the neighborhood. The effect of overhearing is illustrated in Figure 1, which depicts an experiment using six MICAz motes and a sink. The network is programmed with the _collection tree protocol_ (_CTP_) [17] application where each node transmits periodic data packets comprising of sensor observations with an interval of 10 s and routing packets (beacons) with an interval that varies between 0.128 and 512 s, similar to _Trickle_ algorithm used in CTP. The network uses the beacons to build link quality based least-cost routes from all nodes to the sink. All nodes use an extremely low transmit power of \(-\)28.5 dBm and apply the _LowPowerListening_ scheme [41] with a wake-up interval of 125 milliseconds. We run this experiment for 10 min and record the total number of beacons and data packets sent/received throughout the network as well as the network wide overhearing. The results, shown in Figure 1b, indicate that even with sleep cycles, overhearing is a dominating factor in the energy consumption in the nodes. Consequently, a mechanism to optimally distribute the network traffic over multiple channels would lead to reduction in overhearing and significant improvement in the lifetime of the network. In addition to reducing overhearing, a second consideration for improving the network lifetime is to address the effect of _differential battery drainage_ among the nodes. This is motivated by experimental observations from a real-life WSN that was developed by the authors for monitoring the health of equipment in a power substation. The project, sponsored by EPRI, was initiated in 2006, which led to the deployment of a 122-node WSN known as _ParadiseNet_ in a YVA-operated power substation in Kentucky [42]. The location site and an illustration of a deployed wireless sensor node is depicted in Figure 2. The sensor nodes were deployed in a \(1000\times 400\) feet area, which use a link-quality based routing protocol. Figure 3a depicts the location of nodes in _ParadiseNet_ and Figure 3b depicts the average drops in the battery levels in the four regions of the network over a period of five months of operation. It can be observed that while nodes closer to the base station generally have higher voltage drops, Zone C has the highest drop. This is basically due to the fact that sensor nodes in Zone C are responsible for forwarding most of the packets from Zone A and Zone B. In addition, nodes from Zone C also experience higher amount of overhearing traffic. This type of energy imbalance ultimately results Zone C nodes to deplete their batteries earlier than the ones in other zones which collectively result in network partitioning and decrease in the lifetime of the network. Consequently, it is important that in addition to addressing the overhearing problem, the routing and channel selection scheme should balance the energy consumption of the nodes so that the network lifetime is maximized.