A Review of Protocol Implementations and Energy Efficient Cross-Layer Design for Wireless Body Area Networks <s> Transport and MAC, Physical Layer Schemes <s> Severe energy constraints of battery-powered sensor nodes necessitate energy-efficient communication protocols in order to fulfill application objectives of wireless sensor networks (WSN). However, the vast majority of the existing solutions are based on classical layered protocols approach. It is much more resource-efficient to have a unified scheme which melts common protocol layer functionalities into a cross-layer module for resource-constrained sensor nodes. To the best of our knowledge, to date, there is no unified cross-layer communication protocol for efficient and reliable event communication which considers transport, routing, medium access functionalities with physical layer (wireless channel) effects for WSNs. In this paper, a unified cross-layer protocol is developed, which replaces the entire traditional layered protocol architecture that has been used so far in WSNs. Our design principle is complete unified cross-layering such that both the information and the functionalities of traditional communication layers are melted in a single protocol. The objective of the proposed crosslayer protocol is highly reliable communication with minimal energy consumption, adaptive communication decisions and local congestion avoidance. To this end, the protocol operation is governed by the new concept of initiative determination. Based on this concept, the cross-layer protocol performs received based contention, local congestion control, and distributed duty cycle operation in order to realize efficient and reliable communication in WSN. Performance evaluation results show that the proposed cross-layer protocol significantly improves the communication efficiency and outperforms the traditional layered protocol architectures. <s> BIB001 </s> A Review of Protocol Implementations and Energy Efficient Cross-Layer Design for Wireless Body Area Networks <s> Transport and MAC, Physical Layer Schemes <s> Low-power transmissions of sensor nodes are easily corrupted by interference generated by co-located wireless terminals that leading to packet losses might increase energy consumption and result in unreliable communications. Dynamic spectrum access mechanisms can mitigate these problems allowing cognitive sensor devices to sense the spectrum and access the wireless medium in an opportunistic way. With this respect, energy efficient algorithms for spectrum sensing have to be designed in order to meet the power constraints of wireless sensor networks. In this paper we consider an energy constrained system comprising two sensor nodes that avoid interference by exploiting spectrum holes in the time domain. We design the algorithm used for spectrum sensing so as to minimize the average energy required for the successful delivery of a packet. While carrying our this task we adopt a cross layer approach that accounts for the average channel occupancy and the power of interfering transmissions at the physical layer as well as for the size of packets used by sensors at the transport layer. Our results show that accounting for the short length of packets commonly used in sensor networks can significantly improve energy efficiency leading to gains of up to 50% if compared to other spectrum sensing algorithms envisaged in the literature. <s> BIB002
Due to energy constraints and protocol simplicity, some low power standards, e.g., ZigBee that overlays 802.15.4, offers only best effort end-to-end delivery of individual packets. This means only limited transport layer functions (packet fragmentation and re-assembly) at the application layer as there is no fragmentation support at the network layer. Other standards that are designed to underpin the higher protocol layers require transport layer functions that overlay a low power MAC/physical to implement a full protocol stack. Typically for traditional wireless networks, the standard protocols operating at the transport layer are TCP-providing connection orientated services to the network, and UDP-providing connectionless services. Overlaying a transport layer protocol such as standard TCP or UDP over a WBAN/WSN structure can be problematic in that each has their own overheads that impact on the energy efficiency of low power networks. For example, transmitting IPv6 packets over 802.15.4 cannot take place without the inclusion of an additional transport layer between IPv6 and the 802.15.4 MAC layer that basically compresses the frame to meet the physical payload size required by the protocol. Standard UDP has no mechanism for reliable communication, therefore cannot be considered in the WBAN context, as there is no method to support QoS. Additionally, as UDP cannot control traffic rate at the transport layer. The only way the traffic can be managed is via connection admission control or end-to-end rate control. These schemes would need to be cross optimized with the physical layer to cater for the variable link capacity. Standard TCP can manage congestion avoidance, but as it was originally designed for wired networks, it suffers from performance issues related to interpretation of BER, limited bandwidth, and assumptions that packet loss is mainly caused by congestion. Performance gains can be made by optimizing a congestion control algorithm utilizing physical layer parameters as part of a cross layer scheme. As the wireless link is prone to varying channel quality, fading, and interference, transport layer protocols can deliver better performance when optimized to consider the varying link capacity BIB001 . Cross-layer design solutions operating at the transport and physical layers could, for example, report the channel condition from the physical layer as a new parameter to the transport layer that would enable a protocol to distinguish real congestion from packet loss to make better performance related decisions. There is a direct relationship between the physical/MAC layer and transport layer where adapted cross-layer protocols can deliver higher throughput and minimal end-to-end packet delay. Spectrum sensing mechanisms operating at the lower layers can mitigate the issues relating to neighbor node interference, allowing protocols to sense the spectrum and access the channel in an opportunistic way allowing the transport layer to make more informed decisions concerning congestion and collisions in delivering reliable communications BIB002 .