Another approach in wireless access is a schedule-based MAC protocol. In this scheme, a wireless node accesses the shared medium in a deterministic fashion. Under a proper scheduling policy, this scheme shows good performance with high traffic loads. TDMA (Time Division Multiple Access) or FDMA (Frequency Division Multiple Access) and CDMA (Code Division Multiple Access) techniques are a good example. However, in most cases, these techniques are used with contention-based techniques for efficient use of the shared medium.2.2. Energy Efficient Medium Access Control Protocol for the Sensor NetworksTo reduce the energy of a sensor network, it is essential to manage the radio activity to conserve the energy of a node.

This scheme puts the radios into sleep mode either periodically or whenever possible when a node is neither receiving nor transmitting.

This scheme can be categorized into two classes: asynchronous and synchronous. In an asynchronous scheme, a node wakes up and checks another nodes�� activity to send or receive data. In a synchronized scheme, a node is activated in a synchronized manner with a neighbor node and starts to send data immediately after activation.Asynchronous wakeup solutions do not suffer from synchronization overhead, and each node consumes lower power than a synchronous solution. El-Hoiydi [9] introduced a mechanism for waking up sleeping radios asynchronously in ALOHA and named it Preamble Sampling.

Hill and Culler [10] used it in CSMA and named it Low Power Listening (LPL). In these schemes, the sender transmits a long preamble and the receiver periodically Carfilzomib wakes GSK-3 up and senses the channel.

If no activity is found, it goes back to sleep again. If activity is sensed, the receiver will change to its fully active receiving mode.LPL with preamble sampling has one serious shortcoming. The long preamble can cause throughput reduction and energy waste for both sender and receiver. The WiseMAC protocol [11] builds upon preamble sampling to overcome it. By using additional contents of an ACK frame, each node can learn the periodic sampling times of its neighbor nodes. It uses this information to send a shorter wakeup preamble at just the right time.

The frames in WiseMAC also contain a bit which the transmitter uses to signal to the receiver if it needs to awake a little longer in order to receive additional frames intended for it. In Receiver-Initiated MAC (RI-MAC) [12], the receiver sends out invitation beacons at regular intervals. The sender must wait until it sees one and respond by sending the message. Collisions are detected at the receiver, who then sends out a new beacon specifying a contention window, increasing its length on consecutive collisions.