Transport Protocols for WSNs

Abstract

Transport layer protocols in WSNs should support multiple applications, variable reliability, packet-loss recovery, and congestion control. A transport layer protocol should be generic and independent of the application. Transport protocols are quite abundant, with varying design goals to match their intended use. Depending on their functions, WSN applications can tolerate different levels of packet loss. Packet loss may be due to bad radio communication, congestion, packet collision, full memory capacity, and node failures. Packet loss can result in wasted energy and degraded quality of service (QoS ) in data delivery. Detection of packet loss and correctly recovering missing packets can improve throughput and energy expenditure. There are two approaches for packet recovery: hop-by-hop and end-to-end . Hop-by-hop retransmission requires that an intermediate node cache the packet information in its memory. This method is more energy efficient since retransmission distance is shorter. For end-to-end retransmission, the source caches all the packet information and performs retransmission when there is a packet loss. End-to-end retransmission allows for variable reliability whereas hop-by-hop retransmission performs better when reliability requirements are high. A congestion control mechanism monitors and detects congestion, thereby preserving energy. Before congestion occurs, the source is notified to reduce its sending rate. Congestion control helps reduce retransmission and prevents sensor buffer overrun. As in packet-loss recovery, there are two approaches to congestion control: hop-by-hop and end-to-end . Hop-by-hop mechanism requires every node along the path to monitor buffer overflows. Hop-by-hop mechanism lessens congestion at a faster rate than the end-to-end mechanism. When a sensor node detects congestion, all nodes along the path change their behavior. End-to-end mechanism relies on the end nodes to detect congestion. Congestion is flagged when timeout or redundant acknowledgements are received. There are tradeoffs between hop-by-hop and end-to-end approaches for packet-loss recovery and congestion control mechanism. Depending on the type, reliability. , and time sensitivity of the application, one approach may be better than the other. As presented in details all over this chapter, transport layer protocols in WSNs addresses, with different interests, the above design issues.