Multiple Access with Collision Avoidance for Wireless: Difference between revisions

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{{Short description|Slotted medium access control protocol widely used in ad hoc networks}}

'''Multiple Access with Collision Avoidance for Wireless''' ('''MACAW''')<ref name="MACAW">{{cite journal|author=Vaduvur Bharghavan|title=MACAW: A Medium Access Protocol for Wireless LAN's|url=http://pdos.csail.mit.edu/decouto/papers/bharghavan94.pdf|version=In the Proc. ACM SIGCOMM Conference (SIGCOMM '94), August 1994, pages 212-225|date=1994-08-01|accessdate=2007-01-18|display-authors=etal}}</ref> is a slotted [[medium access control]] (MAC) protocol widely used in [[ad hoc network]]s.<ref name="SMAC_1">{{cite journal |author=Wei Ye|title=An Energy-Efficient MAC Protocol for Wireless Sensor Networks |url=http://www.isi.edu/~weiye/pub/smac_infocom.pdf|version=INFOCOM 2002|date=2002-06-01|accessdate=2006-11-26 |archiveurl = https://web.archive.org/web/20061104045110/http://www.isi.edu/~weiye/pub/smac_infocom.pdf |archivedate = 2006-11-04|display-authors=etal}}</ref> Furthermore, it is the foundation of many other [[Medium Access Control|MAC]] protocols used in [[wireless sensor networks]] (WSN).<ref name="SMAC_1"/> The [[IEEE 802.11 RTS/CTS]] mechanism is adopted from this protocol.<ref name="SMAC_2">{{cite journal |author=Wei Ye|title=Medium Access Control With Coordinated Adaptive Sleeping for Wireless Sensor Networks |url=http://www.isi.edu/~weiye/pub/smac_ton.pdf|version=IEEE/ACM Transactions on Networking, Vol. 12, No. 3, pp. 493-506, June 2004|date=2004-06-01|accessdate=2006-12-27 |archiveurl = https://web.archive.org/web/20061209195620/http://www.isi.edu/~weiye/pub/smac_ton.pdf |archivedate = 2006-12-09|display-authors=etal}}</ref><ref name=holger>{{cite book | last = Karl| first = Holger | author-link = Holger Karl | year = 2005| title =Protocols and Architectures for Wireless Sensor Networks | url = https://archive.org/details/protocolsarchite00karl| url-access = limited| publisher = Wiley | isbn = 0-470-09510-5 | page = [https://archive.org/details/protocolsarchite00karl/page/n144 117]}}</ref> It uses ''RTS-CTS-DS-DATA-ACK'' frame sequence for transferring data, sometimes preceded by an ''RTS-RRTS'' frame sequence, in view to provide solution to the [[hidden node problem]].<ref name="MACAW"/> Although protocols based on MACAW, such as [[S-MAC]], use [[carrier sense]] in addition to the RTS/CTS mechanism, MACAW does not make use of carrier sense.<ref name="MACAW"/>

== Principles of operation ==

[[ImageFile:MACAW protocol.JPGjpg|thumb|450px|An example to illustrate the principle of MACAW. It is assumed that only adjacent nodes are in transmission range of each other.]]

Assume that node A has data to transfer to node B. Node A initiates the process by sending a ''Request to Send'' frame (RTS) to node B. The destination node (node B) replies with a ''Clear To Send'' frame (CTS). After receiving CTS, node A sends data. After successful reception, node B replies with an acknowledgement frame (ACK). If node A has to send more than one data fragment, it has to wait a random time after each successful data transfer and compete with adjacent nodes for the medium using the RTS/CTS mechanism.<ref name="MACAW"/>

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MACAW is a [[non-persistent]] [[slotted]] protocol, meaning that after the medium has been busy, for example after a CTS message, the station waits a random time after the start of a time slot before sending an RTS. This results in fair access to the medium. If for example nodes A, B and C have data fragments to send after a busy period, they will have the same chance to access the medium since they are in transmission range of each other.

=== RRTS ===
Source:<ref name="MACAW"/>===

Node D is unaware of the ongoing data transfer between node A and node B. Node D has data to send to node C, which is in the transmission range of node B. D initiates the process by sending an RTS frame to node C. Node C has already deferred its transmission until the completion of the current data transfer between node A and node B (to avoid [[co-channel interference]] at node B). Hence, even though it receives RTS from node D, it does not reply back with CTS. Node D assumes that its RTS was not successful because of collision and hence proceeds to ''back off'' (using an [[exponential backoff]] algorithm).

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==Ongoing research==

Additional back-off algorithms have been developed and researched to improve performance.<ref name=Miao>{{cite book|author1=Guowang Miao|author-link=Guowang Miao|author2=Guocong Song|title=Energy and spectrum efficient wireless network design|publisher=[[Cambridge University Press]]|isbn=1107039886978-1107039889|year=2014}}</ref><ref>P. Venkata Krishna, Sudip Misra, [[Mohammad S. Obaidat|Mohhamed S. Obaidat]] and V. Saritha, “Virtual Backoff Algorithm: An Enhancement to 802.11 Medium Access Control to Improve the Performance of Wireless Networks” in IEEE Trans. on Vehicular Technology (VTS), 2010</ref><ref>Sudip Misra, P. Venkata Krishna and Kiran Issac Abraham, “Learning Automata Solution for Medium Access with Channel Reservation in Wireless Networks” accepted in Wireless Personal Communications (WPS), Springer</ref><ref>P. Venkata Krishna & N.Ch.S.N. Iyengar “Design of Sequencing Medium Access Control to improve the performance of Wireless Networks” Journal of Computing and Information Technology (CIT Journal), Vol. 16, No. 2, pp. 81-89, June 2008.</ref><ref>P.Venkata Krishna & N.Ch.S.N.Iyengar, 'Sequencing Technique – An Enhancement to 802.11 Medium Access Control to improve the performance of Wireless Networks', Int. J. Communication Networks and Distributed Systems, Vol.1, No.1, pp 52-70, 2008</ref> The basic principle is based on the use of sequencing techniques where each node in the wireless network maintains a counter which limits the number attempts to less than or equal to the sequence number or use wireless channel states to control the access probabilities so that a node with a good channel state has a higher probability of contention success.<ref name="Miao"/> This reduces the number of collisions.

=== Unsolved problems ===