MAC Protocols for Wireless Sensor Networks

MAC layer is the layer immediately above the physical layer. In wireless networks many nodes contend use a single shared medium, the main function of the MAC layer is to regulate the access to this shared medium in such a way that all nodes are able to get their due. In other words the main duty of the MAC layer is to determine the time during which a node can send and receive, data, control and management packets. The MAC layer is a part of data link layer, other parts of data link layer are responsible for error and flow control.

The main challenge in designing MAC protocols for wireless sensor networks is to conserve as much energy as possible, as energy is present in the sensor nodes in very small quantity. So any MAC protocol designed should consider using energy present in the sensor nodes as less as possible.

The MAC protocols designed for wireless sensor networks have to necessarily consider all these problems, hidden station problem, exposed station problem, collisions because of multiple nodes accessing the medium together, noise added due to external interference, interference due to other systems working in similar frequency band, surge of data whenever event is detected, usage of as less energy as possible, sending control packets as less possible, keeping the overhead due to MAC headers and trailers as less as possible, keeping retransmission of packets as less as possible, low transmission delay, low access delay, and prioritize some critical packets.

The MAC protocols can be broadly divided into three categories, on demand assignment protocols, fixed assignment protocols and random access protocols. Examples for fixed assignment protocols are TDMA, FDMA, and CDMA. The protocols like CSMA, CSMA/CD, ALOHA, persistent and non persistent CSMA protocols are good examples for random access protocols. On demand assignment protocols assign resource to a node on demand and only for requested duration, after that the same resource may be given to some other requesting nodes. The on demand assignment protocols can be further classified into centralized and non centralized protocols. In centralized protocols a central server is responsible to assign all the requested resources in non centralized protocols all nodes by mutual understanding share the resources.
The most important issues to be considered to save energy are, over hearing, collisions, protocol overhead, idle listening. All these have to be as less as possible in all protocols designed for Wireless Sensor Networks and all protocols should be less complex in their operation and the foot print of the protocols should be small.

We shall understand some of the protocols designed for WSN in the next set of articles.