Load Migrating for the Hot Spots in Wireless Sensor Networks Using CTP

To suit the needs of data collection, routing protocols in WSN are normally required to form a collection tree where data flows from the source nodes to the sink nodes. These protocols, such as CTP and Multihop LQI, generally target at reducing the packet delivery cost without considering load balancing issues. We argue that load balancing is crucial for WSNs because load imbalance may cause certain nodes, which we call hot spots, to run out of energy faster than expected. The load imbalance may lead to holes and prominently degrade the performance of the network. In this paper, we propose BCTP (Balanced Collection Tree Protocol), which enhances CTP by enabling the network to migrate the load of the node under heavy traffic. BCTP uses the average transmission rate as the metric to measure a node's long term traffic load. Once a node is found heavily loaded, BCTP adopts a stochastic routing strategy to balance the load. BCTP is evaluated by test bed experiments with 9 Telosb motes. Experiment result shows that BCTP can reduce the load of the hot spot by up to 61.9% in a densely deployed network.

I. Introduction

Wireless sensor networks (WSNs) prove to be a powerful tool for prolonged surveillance of large scale phenomena [1]. A wireless sensor network is typically composed of large quantities of densely deployed low-cost wireless sensor nodes which are constrained both in computational ability and energy. Due to the limited transmission range of the senor nodes, WSNs usually adopt a multi-hop many-to-one routing pattern to transmit the data collected from the phenomenon area to the sink node. To suit the needs of WSNs, a number of many-to-one routing protocols are proposed and of these protocols, CTP (Collection Tree Protocol) [2] is a commonly used and thoroughly tested one. CTP uses ETX [3] as its routing metric to avoid lossy links. ETX stands for the expected total number of transmissions required to successfully deliver a packet. The greater the ETX of a path is, the lossier the path is. However, ETX doesn‘t address the problem of load balancing. Therefore, CTP is prone to the hot spot problem: certain nodes carry much heavier transmission load than other nodes. These nodes are likely to run out of battery faster than the ordinary nodes, which may create holes and undermine the performance of the whole system.