A Survey of Low Duty Cycle MAC Protocols in Wireless Sensor Networks 69 A Survey of Low Duty Cycle MAC Protocols in Wireless Sensor Networks

This chapter examines various important low duty cycle MAC protocols and the two most important MAC protocols designed specifically for cooperative Multiple-Input MultipleOutput (MIMO) transmission. In most cases, the low duty cycle MAC protocols trade off latency for energy efficient operation. Also, we can observe later that asynchronous MAC protocols are more scalable than synchronous MAC protocols. On the one hand, when sensor nodes join or leave a group or a cluster, the MAC needs to resynchronise the network over and over in such protocols as LEACH and S-MAC. Frequent re-synchronisation can lead to higher energy consumption. The situation becomes more complex when global synchronisation is required instead of local synchronisation. Thus a balance must be made between frequent synchronisation and scalability in synchronous MAC protocol design. On the other hand, in some cases with asynchronous MAC, the higher scalability comes at the cost of higher transmission energy due to the implementation of a long preamble and overhearing in such protocols as RF Wake-up and B-MAC. However, the burden of long preamble transmission is reduced gradually by the introduction of short packet techniques such in SpeckMAC and X-MAC. Moreover, it is important to note that little attention has been paid to increasing the link reliability in SISO systems. The only mechanism used is the ACK packet feedback in protocols such IEEE 802.15.4 MAC and WiseMAC. The MIMO-LEACH and CMACON protocols provide measures to increase link reliability and at the same time reduce transmission power by exploiting spatial diversity gain. On the one hand, the MIMO-LEACH protocol employs a duty cycle mechanism through TDMA time slots assignments which reduces the total energy consumption. Furthermore, multihop communication between cluster heads is introduced to replace the direct communication which reduces further the total energy consumption. Also, collisions can be avoided with the distinct time slot assignment to each sensor node. The benefits come at the cost of higher latency (multi-hop communication). In addition, the scalability issue is not addressed at all. CMACON is more scalable and does not require pre-selection of cooperative nodes. CMACON does not suffer from tight synchronisation and overhead of cluster formation. Also, collision avoidance is provided through RTS-CTS signalling. Moreover, an ACK 5