In wireless sensor networks (WSN), nodes have very limited power due to hardware constraints. Packet losses and retransmissions resulting from congestion cost precious energy and shorten the lifetime of sensor nodes. This problem motivates the need for congestion control mechanisms in WSN. In this thesis, we identify several network aspects, some of which are unique to sensor networks, that affect congestion, as for example: sensor reporting rate; number of MAC retransmissions; RTS/CTS mechanism; buffer size; average path length; ad-hoc routing protocols; and Minimum Contention Window. We also study congestion symptoms, such as delay and packet loss, for different Wireless Sensor Network Congestion Types. Moreover, since congestion affects on energy consumption and fairness, these are also examined. The RTS/CTS mechanism is widely used in ad hoc networks to avoid collisions caused by hidden nodes. In the current implementation, any node that receives an RTS or a CTS packet inhibits itself from transmitting without using any further information. In this thesis, we have shown that this approach of implementing the RTS/CTS mechanism leads to false blocking, where a node remains prohibited from transmitting even if no nearby node transmits. Moreover, false blocking may propagate through the network; as a result, a large number of nodes may get false blocked. We have also shown that propagation of false blocking can lead to pseudo-deadlocks. Due to false blocking, the throughput of the network goes to zero as the load of the network is increased beyond a certain value. Therefore, the RTS/CTS mechanism may congest a network instead of stabilizing it. We have proposed a simple solution to the false-blocking problem, called RTS Validation. A node that uses RTS Validation defers for an entire packet transmission period if DATA packet transfer begins, but defers only for a short time if no transmission takes place when DATA packet transmission is expected. By means of simulation, we have shown that the use of RTS Validation improves the network performance in three aspects: it eliminates congestion by stabilizing the network throughput at high load, it increases the peak throughput by 60%, and it significantly reduces the average delay. RTS Validation is a backward compatible solution and thus can be implemented incrementally. The resource control part of the scheme implements solutions that was previously tested with simulations in OMNeT++, where the results showed that such congestion control schemes can mitigate congestion and extend networks' lifetime when using network topology knowledge and turning off the extra resources as soon as the source traffic decreases, to save energy.
