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Multiple Access with Collision Avoidance (MACA) is a protocol for slotted media access control used in wireless LAN data transmission. MACA is used to avoid data collisions caused by hidden station problems as well as simplifying known station problems.
In MACA, a wireless network node announces that it is going to send the data frame, informing the other nodes to remain silent. When a node intends to transmit the data frame, it communicates using a signal known as Request-To-Send (RTS) that includes the length of the data frame to transmit. If the recipient permits the transmission, it responds back to the sender with a signal known as Clear-To-Send (CTS), which includes the length of the data frame that it is about to receive.
In the meantime, the nodes that listen to the RTS signal must remain silent until the data is fully transmitted in order to avoid conflict with CTS. Collisions among RTS packets may still occur in MACA, but they are minimized using a randomized exponential back-off strategy, much like the one that is used in regular Carrier Sense Multiple Access (CSMA).
Although collisions can occur between RTS packets, MACA still has an edge over CSMA, provided that the RTS packets are substantially smaller compared to the data packets. If the RTS packets are significantly smaller, the collisions between RTS packets create less impact.
WLAN data transmission collisions can still happen, and MACA for Wireless (MACAW) is brought to extend the functionality of MACA. It demands nodes to send acknowledgments after every successful frame transmission. MACAW is commonly used in ad hoc networks. Moreover, it is the basis of various other MAC protocols found in wireless sensor networks (WSN).