SMACK (SiMultaneous ACKnowledgment) and PAMAC (PHY-Aided MAC)

A Broadcast message followed by two SMACKs, using three prototype radio platforms

Network protocol designers, both at the physical and network level, have long considered interference and simultaneous transmission in wireless protocols as a problem to be avoided. This, coupled with a tendency to emulate wired network protocols in the wireless domain, has led to artificial limitations in wireless networks. In this paper, we argue that wireless protocols can exploit simultaneous transmission to reduce the cost of reliable multicast by orders of magnitude. With an appropriate application inter- face, simultaneous transmission can also greatly speed up common group communication primitives, such as anycast, broadcast, leader election and others.

In this research, we focus on using Orthogonal Frequency Division Modulation (OFDM) to provide distinct orthogonal signals. OFDM is a mechanism that splits the available spectrum into a number of orthogonal non-interfering subchannels. Being orthogonal, each of the subcarriers can be treated as an information carrying medium without significant interference with another subcarrier. In our protocol, since we only need to transmit a “yes” or “no” for an acknowledgment, we assign subcarriers to individual nodes. Using multicarrier modulation techniques, the AP receives ACKs from a greater number of clients in the shortest possible time, dramatically reducing the time to gather reliable acknowledgments for broadcasts.

Variation of Spectrum over Time, while transmitting SMACK

We exploit a programmable physical layer and SMACK to have clients signal whether they have packets to send. By detecting the high energy at the simultaneous transmission, the AP gets the following information: a) which stations have packets to send and b) whether the traffic load is high, medium or low. The AP schedules clients efficiently while wasting little of the spectrum on signaling overhead. The proposed protocol is a) fast, since no packet transmission is required for polling responses and all clients respond concurrently; b) reliable, as the poll response is contention free and c) scalable. We term this MAC protocol as PHY Aided MAC or PAMAC in short.