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Multiple Access with Collision Avoidance for Wireless (MACAW)^[1] is a slotted medium access control (MAC) protocol widely used in ad hoc networks.^[2] Furthermore, it is the foundation of many other MAC protocols used in wireless sensor networks (WSN).^[2] The IEEE 802.11 RTS/CTS mechanism is adopted from this protocol.^[3][4] 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.^[1] 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.^[1]

Principles of operation[edit]

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.^[1]

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.^[1]

Any node (node C and node E) overhearing a CTS frame refrains from sending anything for the time until the data frame and ACK should have been received (solving the hidden terminal problem), plus a random time. Both the RTS and CTS frames contain information about the length of the DATA frame. Hence a node uses that information to estimate the time for the data transmission completion.^[1]

Before sending a long DATA frame, node A sends a short (RRTS).

Now, when node C, which cannot reply earlier due to ongoing transmission between node A and node B, sends an RRTS message to node D during next contention period, the recipient of the RRTS (node D) immediately responds with an RTS and the normal message exchange is commenced. Other nodes overhearing an RRTS defer for two time slots, long enough to hear if a successful RTS–CTS exchange occurs.

To summarize, a transfer may in this case consist of the following sequence of frames between node D and C:

1. “Request To Send” frame (RTS) from D to C
2. “Request for Request to send” frame (RRTS) from C to D (after a short delay)
3. “Request To Send” frame (RTS) from D to C
4. “Clear To Send” frame (CTS) from C to D
5. “Data Sending” frame (DS) from D to C
6. DATA fragment frame from D to C,
7. Acknowledgement frame (ACK) from C to D

Ongoing research[edit]

Additional back-off algorithms have been developed and researched to improve performance.^{[5][6][7][8][9]} 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.^[5] This reduces the number of collisions.

Unsolved problems[edit]

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 multicasting.

See also[edit]

• Multiple Access with Collision Avoidance (MACA)

References[edit]

1. ^ ^abcdefghVaduvur Bharghavan; et al. (1994-08-01). 'MACAW: A Medium Access Protocol for Wireless LAN's'(PDF). In the Proc. ACM SIGCOMM Conference (SIGCOMM '94), August 1994, pages 212-225. Retrieved 2007-01-18.Cite journal requires journal= (help)
2. ^ ^abWei Ye; et al. (2002-06-01). 'An Energy-Efficient MAC Protocol for Wireless Sensor Networks'(PDF). INFOCOM 2002. Archived from the original(PDF) on 2006-11-04. Retrieved 2006-11-26.Cite journal requires journal= (help)
3. ^Wei Ye; et al. (2004-06-01). 'Medium Access Control With Coordinated Adaptive Sleeping for Wireless Sensor Networks'(PDF). IEEE/ACM Transactions on Networking, Vol. 12, No. 3, pp. 493-506, June 2004. Archived from the original(PDF) on 2006-12-09. Retrieved 2006-12-27.Cite journal requires journal= (help)
4. ^Karl, Holger (2005). Protocols and Architectures for Wireless Sensor Networks. Wiley. p. 117. ISBN0-470-09510-5.
5. ^ ^abGuowang Miao; Guocong Song (2014). Energy and spectrum efficient wireless network design. Cambridge University Press. ISBN1107039886.
6. ^P. Venkata Krishna, Sudip Misra, 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
7. ^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
8. ^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.
9. ^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

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