Berkeley Media Access Control
Berkeley Media Access Control ( B-MAC , " Berkeley Media Access Control ") is a network protocol for sensor networks . As Media Access Control (MAC) it controls access to the common communication medium of the network.
Network protocols define down to the smallest detail how the computers in a computer network exchange data with one another. Sensor networks consist of tiny, wirelessly communicating sensor nodes that are deployed in large numbers in an area, network independently and monitor their surroundings in group work with sensors until their energy reserves are exhausted. They thus form a special form of the mobile ad hoc network and place completely different requirements on a network protocol than, for example, the Internet.
B-MAC was introduced in 2004 by Joseph Polastre, Jason Hill and David Culler as the result of a collaboration between the University of California, Berkeley and JLH Labs . B-MAC is in the tradition of Sensor Media Access Control (S-MAC), but in terms of content it distances itself from this and similar protocols.
Protocol structure
The developers of B-MAC criticized the monolithic structure of earlier MAC protocols such as Sensor Media Access Control and Timeout Media Access Control and aimed at a modular protocol whose lean core is limited to media access control and functions such as network organization and synchronization in higher-level modules outsourced.
Clear Channel Assessment
Clear Channel Assessment is the name of the B-MAC developers for carrier testing . This ability to assess whether the communication medium is currently free and thus available for its own data transmissions is a key concern for B-MAC as a representative of the Carrier Sense Multiple Access approach .
B-MAC uses a noise suppression technique to make carrier verification and signal reception more reliable. If the radio receiver of a sensor node switches to receive, it will inevitably receive the omnipresent electromagnetic background radiation as noise . This noise is not the same everywhere, but varies with the environment and environmental factors such as nearby thunderstorms. B-MAC therefore uses dynamic noise suppression that constantly adapts to these factors. To do this, it measures the background noise and, taking previously measured values into account, determines a noise filter that cleans up subsequently received signals. To test the carrier, B-MAC listens briefly to the channel five times in succession and looks in these samples for outliers that clearly stand out from the signal energy of the background noise; if an outlier is found, the channel is considered occupied.
Low power listening
Data transmission is implemented in B-MAC using a preamble scheme, as can also be found in ALOHA variants. A sensor node that wants to send data first sends a preamble, an agreed signal that permanently occupies the channel. Nodes wake up briefly at regular intervals and check the channel with Clear Channel Assessment for occupancy. If the communication medium is free, the node immediately goes to sleep again, otherwise it prepares to receive the message announced by the preamble.
swell
- ↑ J. Polastre, J. Hill, D. Culler: Versatile Low Power Media Access for Wireless Sensor Networks , In: Proceedings of the Second ACM Conference on Embedded Networked Sensor Systems (SenSys'04) , November 3rd to 5th, 2004.