Reliability in wireless sensor networks: A survey and challenges ahead

Ensuring energy efficient and reliable transport of data in resource constrained Wireless Sensor Networks (WSNs) is one of the primary concerns to achieve a high degree of efficiency in monitoring and control systems. The two techniques typically used in WSNs to achieve reliability are either retransmission or redundancy.Most of the existing research focuses on traditional retransmission-based reliability, where reliable transmission of data packets is ensured in terms of recovering the lost packets by retransmitting them. This might result in additional transmission overhead that not only wastes sensors’ limited energy resources but also makes the network congested and in turn affects the reliable transmission of data. On the other hand, employing redundancy to achieve reliability in WSNs has received comparatively lesser emphasis by the research community [35] and this area warrants further investigation. In redundancy-based reliability mechanisms, a bit loss within a packet can be recovered by utilizing some form of coding schemes. This ability to correct the lost or corrupted bits within a packet would significantly reduce the transmission overhead caused by the retransmission of the entire packet.Both retransmission and redundancy can either be performed on a hop-by-hop or an end-to-end basis. Hop-by-hop method allows the intermediate nodes to perform retransmission or redundancy. On the other hand, in the end-to-end approach, retransmission or redundancy is performed only at the source and the destination nodes. However, a hybrid mechanism with an efficient combination of these retransmission and redundancy techniques in order to achieve reliability has so far been neglected by the existing research. Depending on the nature of the application, it is also important to define the amount of data required to ensure reliability. This introduces the concept of packet or event level reliability. Packet reliability requires all the packets from all the relevant sensor nodes to reach the sink, whereas event reliability ensures that the sink only gets enough information about a certain event happening. Thus retransmission or redundancy techniques using hop-by-hop or end-to-end mechanisms aim to achieve either packet or event level reliability.This paper presents a survey on reliability protocols in WSNs. We review several reliability schemes based on retransmission and redundancy techniques using different combinations of packet or event reliability in terms of recovering the lost data using hop-by-hop or end-to-end mechanisms. We further analyze these schemes by investigating the most suitable combination of these techniques, methods and required reliability level in order to provide energy efficient reliability mechanism for resource constrained WSNs. This paper also proposes a 3D reference model for classifying research in WSN reliability, which will be used to perform in-depth analysis of the unexplored areas.