Mac protocols

About MAC Protocols in Wireless Sensor Networks

1. MAC PROTOCOLS PRESENTED BY, M. JUNO ISABEL SUSINTHRA

2. Medium Access Control (MAC) protocols is the first protocol layer above the Physical Layer (PHY) . The fundamental task of any MAC protocol is to regulate the access of a number of nodes to a shared medium in such a way that certain application-dependent performance requirements are satisfied. INTRODUCTION

3. Requirements and design constraints for wireless MAC protocols Important classes of MAC protocols MAC protocols for wireless sensor networks FUNDAMENTALS OF MAC PROTOCOLS

4. The most important performance requirements for MAC protocols are throughput efficiency, stability, fairness, low access delay, and low transmission delay, as well as a low overhead. Overhead of MAC protocols: Per packet overhead Collision- Multiple sender wants to talk. Collisions can happen if the MAC protocol allows two or more nodes to send packets at the same time. Requirements and design constraints for wireless MAC protocols

5. Hidden-terminal problem The hidden-terminal problem occurs specifically for the class of Carrier Sense Multiple Access (CSMA) protocols, where a node senses the medium before starting to transmit a packet. Exposed-terminal scenario In wireless networks, when a node is prevented from sending packets to other nodes because of a neighbouring transmitter is known as the exposed node problem .

6. The exposed terminal analogy is described as follows: B sends to A, C wants to send to another terminal D not A or B C senses the carrier and detects that the carrier is busy. C postpones its transmission until it detects the medium as being idle again But A is outside radio range of C, waiting is not necessary C is “exposed” to B Tip: Hidden terminals cause collisions, where as Exposed terminals causes unnecessary delay.

7. A huge number of (wireless) MAC protocols have been devised during the last thirty years. They can be roughly classified into the following classes. fixed assignment protocols Demand assignment protocols Random access protocols. IMPORTANT CLASSES OF MAC PROTOCOLS

8. fixed assignment protocols - In fixed-assignment schemes, each communicating node is assigned  a frequency band in FDMA systems or a time slot in TDMA systems . Demand assignment protocols - Demand Assigned Multiple Access (DAMA) is a protocol used in satellite communications, particularly Very Small Aperture Terminal (VSAT) systems.

9. Random access protocols -  In random access or contention methods, no station is superior to another station and none is assigned the control over another. -Transmission is random among the stations. -That is why these methods are called random access.

10. Balance of requirements Energy problems on the MAC layer MAC PROTOCOLS FOR WIRELESS SENSOR NETWORKS

11. Balance of requirements -The balance of requirements is different from traditional (wireless) networks. Additional requirements come up, first and foremost, the need to conserve energy. Important requirements for MAC protocols -scalability -The need for scalability is evident when considering very dense sensor networks with dozens or hundreds of nodes in mutual range.

12. Energy problems on the MAC layer Collisions -Multiple sender wants to talk. Overhearing -The wireless medium is a broadcast medium and all the source’s neighbours that are in receive state hear a packet and drop it when it is not destined to them; these nodes overhear the packet. Protocol overhead -RTS and CTS packets or request packets in demand assignment protocols, and furthermore by per-packet overhead like packet headers and trailers. Idle listening – active listening to an redundant channel. A node being in idle state is ready to receive a packet but is not currently receiving anything.

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