The Politecast Communication Primitive for Low-Power Wireless

By: 
Marcus Lundén and Adam Dunkels
Appears in: 
CCR April 2011

In low-power wireless networks, nodes need to duty cycle their radio transceivers to achieve a long system lifetime. Counter-intuitively, in such networks broadcast becomes expensive in terms of energy and bandwidth since all neighbors must be woken up to receive broadcast messages. We argue that there is a class of traffic for which broadcast is overkill: periodic redundant transmissions of semi-static information that is already known to all neighbors, such as neighbor and router advertisements. Our experiments show that such traffic can account for as much as 20% of the network power consumption. We argue that this calls for a new communication primitive and present politecast, a communication primitive that allows messages to be sent without explicitly waking neighbors up. We have built two systems based on politecast: a low-power wireless mobile toy and a full-scale low-power wireless network deployment in an art gallery and our experimental results show that politecast can provide up to a four-fold lifetime improvement over broadcast.

Public Review By: 
P. Levis

Low power wireless networks differ greatly from their high performance siblings. Rather than optimize for throughput (data bits per second), low power wireless networks seek to minimize the energy to transmit a bit (joules per bit). This simple change turns many of our assumptions of how protocols behave on their head. Receiver-initiated MAC protocols begin look very attractive; application-level information can improve performance without significantly breaking layered abstractions; explicit bursts are preferable to steady, constant bitrate traffic. Furthermore, the vagaries and challenges of long-term wireless behavior exert a strong pressure towards very simple mechanisms which are easy to understand.
This paper presents Politecast, a deceptively simple API for low-power communication. Politecast skates the difficult tension between the energy cost of transmission and the energy cost of reception by leveraging application-level information. Politecast nodes periodically advertise information; a receiver wakes up to hear advertisements when application-level knowledge leads it to believe it needs them. This mechanism might seem so simple as to be obvious, but sometimes it is these subtle twists on what are believed to be completely understood areas that have the greatest insight and contribution. For the past decade, low-power research has increasingly focused on unicast. This paper takes a step back, “rediscovering” a primitive which we can now better reconsider with a much deeper understanding of the associated problems and challenges. Best of all, the paper does more than just present the mechanism and evaluate its implementation. The paper also presents details on how two applications use it, including an operational low-power wireless application for art gallery patrons.