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{{Short description|Slotted medium access control protocol widely used in ad hoc networks}}
== Principles of operation ==
[[
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"/>▼
▲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"/>
Any node overhearing an RTS frame (for example node F or node E in the illustration) refrains from sending anything until a CTS is received, or after waiting a certain time. If the captured RTS is not followed by a CTS, the maximum waiting time is the RTS propagation time and the destination node turnaround time.<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
If A has multiple data fragments to send, the only instant when node D successfully can initiate a data transfer is during small gaps in between that node A has completed data transfer and completion of node B next CTS (for node A next data transfer request). However, due to the node D backoff time period the probability to capture the medium during this small time interval is not high. To increase the per-node fairness, MACAW introduces a new control message called "Request for Request to Send" (RRTS).
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# “Data Sending” frame (DS) from D to C
# DATA fragment frame from D to C,
# Acknowledgement frame (ACK) from C to D
==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=978-1107039889|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 ===
MACAW does not generally solve the [[exposed terminal problem]]. Assume that node G has data to send to node F in our example. Node G has no information about the ongoing data transfer from A to B. It initiates the process by sending an RTS signal to node F. Node F is in the transmission range of node A and cannot hear the RTS from node G, since it is exposed to [[co-channel interference]]. Node G assumes that its RTS was not successful because of collision and hence backs off before it tries again. In this case, the solution provided by the RRTS mechanism will not improve the situation much since the DATA frames sent from B are rather long compared to the other frames. The probability that F is exposed to transmission from A is rather high. Node F has no idea about any node interested in initiating data transfer to it, until G happens to transmit an RTS in between transmissions from A.
Furthermore, MACAW might not behave normally in [[
== See also ==
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== References ==
{{Reflist}}
{{Channel access methods}}
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[[Category:Wireless sensor network]]
[[Category:Media access control]]
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