17. MACAW: A Media Access Protocol for Wireless LAN's

Wireless network is a shared environment and resource is limited. WLAN are either token-based or multiple access. The protocols discussed in this paper focus on multiple access, including Carrier Sense Media Access (CSMA), Multiple Access Collision Avoidance (MACA), and their own modification of MACA, called MACAW.

CSMA:

• avoid collisions by testing the signal strength nearby
• collisions occur at receiver side, not sender

MACA:

• A Request-to-Send RTS) packet is sent before the transmission starts, and receiver respondes with Clear-to-Send (CTS) packet.
• Upon receiving a(n) RTS/CTS, all stations defer according to the proposed data transmission length included in the packet.
• avoids collisions at receiver, not sender
• Binary exponential backoff (BEB): backoff time is doubled after every collision, and reduced to min after every successful RTS-CTS exchange (unfair)

MACAW:

• add a backoff counter in the packet header to achieve fairness
• gentle adjustment when there's a collision (1.5x and -1, Multiplicative Increase and Linear Decrease, MILD)
• multiple stream model to be fair among uplinks and downlinks
• basic message exchange (802.11)

Comment: The idea of adding an ACK in RTS-CTS is great, and their high throughput / fair allocation lives up to the trend of today's network. I just wished they had more real world experiments to proove that MACAW is, indeed, better than MACA.