MACAW: A Media Access Protocol for Wireless LANs; Bharghavan, Demers, Shenker, & Zhang

This paper discusses MACAW, a media access protocol for wireless LANs based off of MACA meant for use at Xerox PARC. Xerox PARC had a setup consisting of "base stations" connected via Ethernet and personal mobile "pad" computing devices. The authors observe that collision occurs at receivers, not senders, and hence, CSMA is inappropriate for the wireless network because it only takes into account senders' transmitting.

A major building block of MACA is a link-layer equivalent of the 3 way handshake: RTS, CTS, and DATA. Nodes hearing these requests hold off from transmitting so as not to interfere. Symmetry between nodes also plays big role: if a node can hear an RTS but not a CTS, it's within range of the sender but not the receiver and is free to transmit as it pleases because the receiver can't hear it as noise. If a node can hear the CTS but not RTS, it needs to hold off on transmitting for the receiver's sake.

MACA employs binary exponential backoff, which the authors deem an unfair backoff strategy because the more a sender backs off, the less likely it'll gain access to the channel, since a non backed-off sender will probabilistically keep having control. The authors propose to remedy this by setting all senders' backoffs to the same value after a successful transmission. This "copy" strategy is accomplished by adding a field to the packet headers that list the backoff; when a station hears a packet, it copies the packet's backoff value as its own, so all stations within range of each other have the same backoff value. Additionally, the authors implement multiplicative (~1.5) increase, linear decrease for the backoff scheme to prevent large, sudden oscillations in the backoff values. Copying seems like it could be a problem in that one value will copy itself across multiple groupings of bases/pads in which drastically different backoff values are called for.

MACAW allocates bandwidth on a per-stream basis rather than per station, using a queue per stream and per-stream backoff. It also concludes its RTS-CTS-DATA sequence with an ACK packet so that senders don't have to waste time waiting for TCP timeouts before retransmitting data. Overhead from this ack seems to be made up for when loss rates are greater than 1 in 1000 packets. Also, when the receiver is transmitting data already, it doesn't matter if the sender can send without interfering elsewhere; plus, if the sender were to attempt to transmit to a busy receiver, it would have to increment backoff, so the sender might as well wait. The way for a sender to know its potential receiver is busy is through the addition of a data-sending packet (DS) that comes between CTS and DATA.

However, there are still more problems to take care of, to which the authors propose yet another control packet (the RRTS), but it's not enough to cover all the open problems including multicast. In comparison to MACA, MACAW gets better throughput when the layout of senders and receivers creates lots of potential collisions, but it almost feels like MACAW employs a lot of hacks to get around corner cases noted by use of MACA and that any solution the authors create just opens the door to more problems needing solved.