Multiple Access with Collision Avoidance (MACA)

Multiple Access with Collision Avoidance (MACA) is a medium access control (MAC) layer protocol used in wireless networks to solve the hidden terminal problem and exposed terminal problem. The widely-used IEEE 802.11 RTS/CTS mechanism has been adopted from MACA.

Working Principle

The MACA protocol operates under the condition that stations are synchronized with identical frame sizes and data transmission speeds. It uses a four-way handshake involving RTS (Request to Send) and CTS (Clear to Send) frames before actual data transmission.

Consider a transmitting station STA sending data to receiving station STB:

• Station STA sends an RTS frame to station STB

• Station STB replies with a CTS frame upon receiving RTS

• Station STA begins data transmission after receiving CTS

• Station STB sends an ACK frame after successful data reception

How It Works

Any station that can hear the RTS is close to the transmitting station and remains silent long enough for the CTS frame transmission. If no CTS follows the RTS, stations wait for a maximum time equal to the RTS propagation delay.

Any station that can hear the CTS is close to the receiving station and remains silent during data transmission. These stations attempt transmission only after hearing the ACK frame.

Key Features

• Non-persistent slotted protocol − If the medium is busy, stations wait for a random time period before sending RTS

• Fair access mechanism − Random backoff ensures equal transmission opportunities

• Collision avoidance − RTS/CTS handshake prevents data frame collisions

Problems Solved

Conclusion

MACA provides an effective collision avoidance mechanism for wireless networks through its RTS/CTS handshake protocol. It successfully addresses both hidden and exposed terminal problems, forming the foundation for modern wireless MAC protocols like IEEE 802.11.