"Then we give brief definitions for the key medium-access control (MAC) protocols proposed forsensor networks, listing their advantages and disadvantages. Moreover, protocols that proposethe integration of MAC layer with other layers are also investigated. Finally, the survey of MACprotocols is concluded with a comparison of investigated protocols and future directions areprovided for researchers with regard to open issues that have not been studied thoroughly. 3 DEFINITIONPAIRWISE:PairWise, is a novel, low power, time division multiple access (TDMA) based protocolfor use in WSNs. PairWise is easily deployable in large scale WSNs as nodes are notsynchronized globally. Instead, they synchronize and establish a pair of channels with each oftheir neighbors independently. Each channel hops pseudo randomly in time according to a seedand maximum rendezvous period (MRP). Hence, nodes using PairWise experience very minimalto no collision during communications. Apart from that, higher layer protocols are able to controlthe MRP of each channel such that a node's duty cycle matches the observed traffic load. Wehave implemented PairWise in the ns-2 simulator, and compared it to Sensor Medium AccessControl (S-MAC) and a TDMA MAC. Our results show PairWise to have very low powerconsumption whilst ensuring packets have minimal delays. Moreover, PairWise has a highgoodput with increasing node density, where goodput is defined as the number of packets thatare transmitted by each node pair successfully over a given total number of packets.WIRELESS SENSOR NETWORK: WSNs are typically considered to be networks of hundreds to thousands of sensor nodes,randomly deployed over an area of interests. The fact that the networks are often converge- casting, i.e. many-to-one networks where the entire network communicates sensed data to one ora few data collector nodes called sinks, and that the sink cannot be reached with one hop createsan unparalleled MAC and routing challenge. This applies especially when the energy constraintof WSNs are taken into account and when the cost of deployment and maintenance, resulting insimple processors with limited resources, is minimised.Wireless sensor networks are constituted from clusters of devices using sensor technologiesdeployed over a specific area, wirelessly communicating data to a central system. Sensornetworks continually monitor physical properties, processes, chemical or magnetic properties,using viable and emerging communication infrastructures. 4 Wireless sensor networks rely on emerging technologies such as communication technologies(RF communication, ad hoc networking routing), semiconductor technologies (MEMS CMOSmicroprocessor), embedded systems and micro sensor technologies.A wireless sensor network consists of a large number of tiny sensor nodes, each of which isequipped with a radio transceiver, a small microprocessor and a number of sensors. Thesenodes are able to autonomously form a network through which sensor readings can bepropagated. Since the sensor nodes have some intelligence, data can be processed as it flowsthrough the network.MAC FOR WSN: The medium-access decision within a dense network composed of nodes with low duty- cycles is a challenging problem that must be solved in an energy-efficient manner.Maximizing the network lifetime is a common objective of sensor network research, since sensornodes are assumed to be dead when they are out of battery. Under these circumstances, theproposed MAC protocol must be energy efficient by reducing the potential energy wastes.To design a good MAC protocol for wireless sensor networks, the following attributes must beconsidered . The first attribute is energy efficiency. We have to define energy-efficient protocolsin order to prolong the network lifetime. Other important attributes are scalability andadaptability to changes. Changes in network size, node density, and topology should be handledrapidly and effectively for successful adaptation.Some of the reasons behind these network property changes are limited node lifetime, addition ofnew nodes to the network, and varying interference, which may alter the connectivity and hencethe network topology. A good MAC protocol should gracefully accommodate such networkchanges. Other important attributes such as latency, throughput, and bandwidth utilization maybe secondary in sensor networks. Contrary to other wireless networks, fairness among sensornodes is not usually a design goal, since all sensor nodes share a common task. 5 "