# Distributed Routing and Channel Selection for Multi-Channel Wireless Sensor Networks +
## 1 Introduction  
Wireless sensor networks (WSNs) consist of small, inexpensive devices that comprise a low power microcontroller, one or more sensors, and a radio for communication. They are self-organized ad-hoc networks capable of sensing, data processing, and forwarding different physical parameters to a user using multi-hop communications. They offer a flexible, self-adaptable, low-cost solution for a number of distributed monitoring applications, especially in places with limited accessibility. Since batteries are difficult to replace, WSNs must use effective energy management schemes to conserve energy to extend their lifetime. An alternative approach for achieving long term operations in WSNs is to harvest energy from renewable resources, such as sunlight, vibration, heat, etc. However, renewable energy can have wide spatial and temporal variations due to natural (e.g., weather) and location specific factors (e.g., exposure to sunlight) that can be difficult to predict prior to deployment. Consequently, energy management in rechargeable wireless sensor networks must also address the variations of energy availability to avoid localized node outages in the network. This paper addresses the energy conservation problem as well as the issue of spatio-temporal variations of energy resources in rechargeable WSNs. It is well known that the radio transceiver typically dominates the energy consumption in wireless sensor nodes. The most effective strategy for conserving the energy consumed by the transceiver is by frequently setting it to an energy-conserving sleep mode, which can be achieved by duty-cycling between sleep and wake periods. The key challenge for applying duty-cycling is synchronization of the wake periods between a transmitter and a receiver. If the nodes are time synchronized, then network-wide or local scheduling policies can be applied that can enable nodes to synchronize their wake periods during transmission/reception for successful packet transmissions. However, challenges in achieving network-wide time synchronization and the latency in multi-hop transmissions caused by such synchronized scheduling principles are concerns with this approach. An alternative is to perform duty-cycling _asynchronously_, where all nodes wake up briefly at periodic intervals of time to check for transmissions and only remain awake if some activity is detected. Otherwise, the nodes return to their energy-conserving sleep states. Generally, a lengthy preamble is used for each transmitted packet so that the receiving node is able to detect it during its brief wake times. This provides an effective solution for energy conservation in asynchronous WSNs, especially under low data rates. Asynchronous duty cycling has been applied to a number of Low Power Listening (LPL) and preamble sampling MAC protocols [1,2]. Asynchronous duty cycling also has its problems, most notably that of energy wastage from _overhearing_, where a data packet transmitted to a specific neighboring node is also received (or overheard) by all other unintended neighbors of the transmitting node before being discarded. The long preamble used in LPL exacerbates the problem since the entire data packet must be received before the destination is known. Possible solutions to this overhearing problem include mechanisms for providing additional information in the preamble to enable neighbors to interrupt the reception of long preambles when not needed [3], adaptive duty-cycling (EA-ALPL, ASEEP) [4,5], and others. Despite these developments, overhearing remains to be a dominating factor in the energy consumption in asynchronous WSNs, especially under high node density and large network sizes. A number of efforts have been directed in the networking community to design routing protocols that address the energy conservation issue on single-channel sensor networks [6, 7, 8, 9]. Unfortunately, when only one channel is used, each node suffers from overhearing transmissions from all other nodes within its range, leading to high energy wastage.