Dependency of Transport Functions on IEEE802.11 and IEEE802.15.4 MAC/PHY Layer Protocols for WSN: A Step towards Cross-layer Design

Introduction

The idea of low power, low cost, embedded wireless devices capable of gathering useful environmental information has led the development of Wireless Sensor Networks (WSNs) (Akyildiz et al., 2007; Potdar et al., 2009; Sharif et al., 2009). Today WSN technology has gained fundamental importance in a range of application scenarios like military, modern healthcare, environmental monitoring, surveillance etc. Fundamentally energy is considered as the biggest threat to WSN node design including both the hardware and the communication protocol design. WSN pays high energy cost in data retransmissions due to path loss, congestion and high bit error rate (BER) caused by interference or collisions at the receiving node. Either case will have a direct impact on the overall network throughput (Sharif et al., 2010), E-2-E packet latency and of course the longevity of the WSN. Reliable and high data transfer rate are the main concerns in event-critical applications e.g. Wireless Multimedia Sensor Network (WMSN). So facilitating congestion control and E-2-E reliability mechanism will be the primary objective of any transport layer protocol design.

In WSN, the transport layer protocol functionality is comprise of a congestion control component (Sharif et al., 2010) that handles minimum number of data packets (either scheduled or prioritized) in order to ensure the minimum latency and energy conservation (achieved by minimum retransmissions) under congested network conditions. Also the transport function must ensure the event and query reliability. In the WSN network dealing with multimedia information, (i.e., WMSN) the effect of congestion is even more pronounced. The nodes near the sink or those that are experiencing large amount of data above their handling capacity (bottleneck nodes) experience congested paths during data transmission that requires high data rate. This condition would result in performance degradation (due to collisions and retransmissions) and rapidly depletes the node’s energy. Thus the transport protocol should effectively detect the congestion and try to mitigate it immediately in order to avoid excessive collisions and retransmissions.

Second important functionality of the transport layer protocol is the data reliability feature (Sharif et al., 2010), which ensures how efficiently the transport protocol detects the data packet loss and notifies it to the nearby storage node for data packet retrieval (to epitome the correct information from the received data) in order to ensure the Quality of Service (QoS) objective of the application is not compromised e.g. event-critical applications (Sharif et al., 2009). Currently the WSN supports packet level and application level reliability. For packet level the transport protocol guarantees the reliable transmission of each and every data packet (packet level reliability) while certain applications (like Military) demand proportionally reliable transmission of all data packets (application reliability).