1.1 TRADITIONAL MAC FAMILIESThere are two main approaches for controlling the access to a shared wireless medium: Contention-based and Reservation based approaches.1.1.1 Reservation Based ProtocolIt requires the knowledge of the network topology before to apply a schedule that allows each node to access the channel and communicate with other nodes in a WSN. The schedule have several goals such as ensuring fairness, or reducing collisions or more access to the channel and transmit at the same time among sensors nodes. TDMA (Time Division Multiple Access) is a representative example for such type reservation-based approach.

Figure: 1.1 The concept of time division multiple accessThe principle of TDMA shown in Fig. 1.1, TDMA-based protocols divide the bandwidth among all the nodes by dividing each logical channel into time slots, and further combining J of these slots into one frame. In every frame, every node has access to one dedicated time slot in which it can send and/or receive data on the entire bandwidth for that channel.

1.1.2 Contention-Based Protocols: This approach is simple compared to reservation-based protocols, it dont required neither global synchronization nor topology knowledge. ALOHA and CSMA (Carrier Sense Multiple Access) are canonical representative schemes of contention based approaches. In CSMA, for instance, a node having a packet to transmit first senses the channel before actually transmitting.

1.2 MAC PROTOCOL CLASSIFICATION BASED ON CONICAL SOLUTION1.2.1 Reducing Collisions: Following Protocol comes under this category: CSMA/CA MACA SIFT

1.2.1.1 CSMA/CAIt stands for Carrier Sensing Multiple Access/Collisions Avoidance, it is most widely used technique to reduce collisions in wireless networks. It is based on the exchange of RTS (Request To Send) and CTS (Clear To Send) mini packets prior to data transmission. In CSMA/CA, the transmission starts by sending a RTS packet to the receiver. RTS packets are small in size, the possibility of them colliding is reduce. When receiver receives a RTS, it sending a CTS packet. Although the RTS/CTS procedure effectively reduces collisions in traditional wireless networks.

Drawbacks Data packet sizes are also usually small in sensor networks so that their collision probability is in the same order as for RTS packets. Therefore, its does not improve performance as higher level. The use of RTS/CTS technique increases the energy consumption of the protocol. RTS/CTS packets can only be used for unicast transmissions.

1.2.1.2 MACAIt stands for Multiple Access Collision Avoidance, it improves CSMA/CA by adding a random backoff time before the transmission of RTS packet to avoid collisions resulting from synchronized forwarding by multiple neighbours.

Drawback The backoff time is picked according to a uniform distribution which is not the optimum choice.

1.2.1.3 SIFTIt is a MAC protocol for WSN and its designed based on the concept that nodes in sensor networks often encounter spatially-correlated contention, where multiple nodes in the same neighbourhood all sense an event they need to transmit information about. Furthermore, in many sensor network applications, it is sufficient if a subset of the nodes that observe the same event report it. Sift is a randomized CSMA protocol, but unlike previous protocols, does not use a time-varying contention window from which a node randomly picks a transmission slot. Rather, to reduce the latency for the delivery of event reports.

Drawback Sift uses a fixed-size contention window and a carefully-chosen, non-uniform probability distribution of transmitting in each slot within the window.

1.2.2 Reducing Overhead1.2.2.1 CSMA/ARCIt stands for Carrier sensing multiple access/Adaptive Rate Control, it avoids using RTS/CTS altogether to reduce the overhead while ensuring fairness among sensors nodes. CSMA/ARC modifies the basic CSMA/CA protocol by eliminating the RTS/CTS exchange and inserted a backoff that is shifted according to the application that reduces both backoff time and collisions in WSN. CSMA/ARC reduces the overhead further by avoiding explicit use of ACK packets: a data packet is considered successfully received when the upstream node sends it forward to its upstream node.

1.2.3 Reducing Overhearing1.2.3.1 PAMASIt stands for Power-Aware Multi-Access with Signalling, it is based on MACA but it used a separate channel for RTS/CTS exchange. It makes use of the RTS/CTS exchange to inform nodes that receive them about the source, the sink, and the time taken of the ongoing transmission. Hence a node that is not concerned with the transmission can switch off its radio to avoid overhearing unnecessary transmissions. It has been designed for ad hoc networks with high traffic loads and thus energy saving operations are increased. In sensor networks, the situation is different because traffic loads are fairly low and RTS/CTS are mostly eliminated.

Drawback It does not realize high energy savings in wireless sensor networks.

1.2.4 Reducing Idle Listening1.2.4.1 PSMIt also known as IEEE 802.11 PSM, it is energy saving in WLANs where all nodes are able to reach each other with a direct transmission. In PSM, nodes is in the sleep mode to avoid reducing energy in being in active mode. Based on the presence of an access point or not, two cases can be distinguished as follow: PSM with a BSS (Basic Service Set):- With an access point (BSS), each node that wants to save energy sends a packet to inform the access point (AP). When it receives a positive response from the AP, the node starts its PSM procedure. In PSM, a node spends most of the time in sleep mode. It wakes up periodically to receive beacons from the AP. PSM with an IBSS (Independent BSS):- Without an access point (IBSS), all nodes maintain synchronization in a distributed way through the periodic transmission of beacon packets.