Delay and Energy Efficient TDMA Based MAC Protocols in Wireless Sensor Networks

Abstract

Efficient energy consumption in wireless sensors is one of the major constraints in Wireless Sensor Networks (WSNs). Multiple contention-based and contention free Medium Access Control (MAC) protocols are designed to make them energy efficient. Sensor nodes are generally deployed in large number where contention based MAC protocols do not perform well due to increased chances of collision. In such scenario, contention free MAC protocols are preferred over contention based MAC protocols. Performance in terms of energy, delay and throughput are not adequate in most of the WSN applications. In this work, we proposed a couple of bit map assisted TDMA based MAC protocols for hierarchical wireless networks named as BS-MAC and BESTMAC. In addition to this, we suggested a modification in IEEE 802.15.4 standard, which enhances its performance without compromising on existing parameters. Both BS-MAC and BEST-MAC are designed for adaptive traffic flow and the main contribution of both of these protocols is that: (a) it uses small size time slots. (b) the number of those time slots is more than the number of member nodes. (c) Short node address (1 Byte) to identify member nodes. These contributions help to handle adaptive traffic loads of all network members in an efficient manner. In BS-MAC, Shortest Job First algorithm is applied to minimize network delay and to enhance the link utilization. However, in BEST-MAC, Knapsack algorithm is used to schedule time slots in an efficient manner to minimize the network delay and better link utilization. In addition to this, scalability is included to adjust new nodes in the mid of a TDMA round. Simulation results show that both BS-MAC and BEST-MAC perform better than the existing TDMA based MAC protocols. An efficient superframe structure for IEEE 802.15.4 Medium Access Control (MAC) layer is also proposed in this work. In this superframe structure, Contention Free Period (CFP) precedes the Contention Access Period (CAP) and more number of slots are used in the same CFP as of original 802.15.4 standard. The standard operates in three different frequency bands as 868MHz, 915MHz and 2400Mhz. As CFP precedes the CAP, the communication delay for the CFP traffic is exceptionally reduced. The Beacon frame is fine-tuned to achieve the above said superframe structure and makes it backward compatible with the original standard. Due to large number of small slots in CFP, smaller amount of data requesting nodes can be assigned CFP space for communication. The analytical results show that our proposed superframe structure has nearly 50 % less delay, accommodates almost double the number of nodes in CFP and has better link utilization compared to the original 802.15.4 standard during all three frequency bands.