## 4 Related Work  
Prior works on congestion control mechanisms in WSNs are mainly embedded in the end-to-end controls, such as CODA [15], ESRT [7], STCP [16], PORT [17], SenTCP [18] and [19]. The underlying method in these papers is the use of end-to-end rate adjustment to fulfil congestion control. These protocols detect and prevent congestion by reducing the number of packet retransmissions and energy used. We briefly summarize the main contributions of these papers. Congestion Detection and Avoidance(CODA) is one of the early papers discussing congestion control in wireless sensor networks. CODA is a energy efficient scheme which comprises of three mechanisms: (1) receiver-based congestion detection, (2) open-loop hop-by-hop backpressure, and (3) closed-loop multi-source regulation. CODA is evaluated by two metrics proposed by the authors, namely, energy tax and fidelity  
Figure 19: Comparison of value for random topology. Figure 18: Fairness of different classes of _Event_ packets for random topology. penalty. Event-to-Sink Reliable Transport (ESRT) is based on the observation that sensor networks are event-based systems. ESRT protocol operation is determined by the network state in terms of congestion condition in the network and path reliability. Simulation analysis of ESRT shows that proposed transport protocol achieves desired reliability with minimum energy consumption. Sensor Transmission control Protocol (STCP) is a scalable and reliable transport layer protocol for sensor networks. STCP is central control protocol since most of the functionalities are implemented at the base station. Simulations show that STCP can increase network lifetime and achieve high reliability. Price-Oriented reliable Transport (PORT) protocol is proposed to obtain reliability and minimize energy consumption. Price refers to the communication cost between sources and the sink. PORT uses price information to achieve reliability. Minimization of energy consumption is achieved by two schemes, upstream information optimization of the sink and downstream optimal routing scheme locally implemented in sensor nodes. Simulations show the effectiveness of PORT for reducing energy consumption comparing to existing schemes. SenTCP is an open-loop hop-by-hop congestion control protocol for wireless sensor networks to improve system throughput, reduce packet dropping, and minimize energy consumption. The work in [19] proposes a congestion control using the congestion degree calculated by the remaining buffer size and net flow. Rate-Controlled Reliable Transport (RCRT) protocol proposed in [20] ensures efficient and flexible rate control like previous protocols. However, RCRT has the improvement that combine reliable transmission and congestion control together. Congestion detection and rate adaptation functionality are performed by the sink. The author also evaluated RCRT on a 40-node wireless sensor network testbed and show that it achieves better performance compared with IFRC [8]. The studies reported in [21, 22, 23, 24] address the congestion problem using routing protocols. In [21], congestion control is achieved by dividing the monitoring areas into several subareas and adjust the local and forwarding traffic based on the transmission parameter. In [22], an interference-minimized multipath routing protocol is proposed for load balancing and a congestion control scheme to reduce the loading rate of the source. The main idea of [23] is to find a less congested node to forward packets to when congestion occurs. In [24] a routing protocol is proposed for congestion control in WSNs by selecting a route which use Network Allocation Vector (NAV)[25] information to determine the channel status. Our optimization algorithm is orthogonal with these protocols since they work in different layers. Other research based on priority fairness are [26], IFRC [8], Fusion [27], and [28, 29, 30].