SMACK - A SMart ACKnowledgment Scheme for Broadcast
Messages in Wireless Networks
Aveek Dutta
1
, Dola Saha
2
, Dirk Grunwald
1,2
, Douglas Sicker
2
1
Department of Electrical, Computer and Energy Engineering
2
Department of Computer Science
University of Colorado
Boulder, CO 80309-0430 USA
{Aveek.Dutta, Dola.Saha, Dirk.Grunwald, Douglas.Sicker}@colorado.edu
ABSTRACT
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 si-
multaneous 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.
The proposed method precisely fits into the domain of directly
reachable nodes where many group communication mechanisms
are commonly used in routing protocols and other physical-layer
mechanisms. We demonstrate how simultaneous transmission can
be used to implement a reliable broadcast for an infrastructure
and peer-to-peer network using a prototype reconfigurable hard-
ware. We also validate the notion of using simple spectrum sensing
techniques to distinguish multiple transmissions. We then describe
ho w the mechanism can be extended to solve group communication
problems and the challenges inherent to build innovative protocols
which are faster and reliable at the same time.
Categories and Subject Descriptors
C.2.2 [COMPUTER-COMMUNICATION NETWORKS]: Net-
work Protocols—Protocol Verification;C.3[SPECIAL-
PURPOSE AND APPLICATION-BASED SYSTEMS]: Signal
Processing Systems
General Terms
Design, Performance, Verification
Keywords
Software Defined Radio, Orthogonal Frequency Division Multi-
plexing
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SIGCOMM’09, August 17–21, 2009, Barcelona, Spain.
Copyright 2009 ACM 978-1-60558-594-9/09/08 ...$10.00.
1. INTRODUCTION
Noise and interference are fundamental aspects of communica-
tions, and are exceptionally important for wireless communications
because it’s more difficult to contain propagation without waveg-
uides such as wires and fibers. Avoiding interference or noise is a
fundamental design objective that limits the scope of simultaneous
multi-user communication. Conventional single carrier communi-
cation focuses on decoding the strongest signal while discarding
anything else as noise or interference.
Multi-user communication requires some form of orthogonal
channel for modulation that allows multiple parties to communi-
cate simultaneously. There are a number of ways to implement
orthogonal channels - code division multiple-access (CDMA) has
been adopted as a very reliable multiple access techniques by us-
ing specially designed codes with strong auto-correlation proper-
ties. With spatial frequency reuse, frequencies are allocated in a
way such that signals from far away communicating pairs will be
so strongly attenuated that they won’t interfere in local communi-
cation. Time division multiplexing, or taking turns using a channel,
is another method.
In this paper, 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. Variants of
the OFDM wav eform are used in a number of current wireless (and
wired) physical layers, including the 802.11a/g. Under OFDM,
different nodes can also communicate on different subcarriers,as
used in WiMax, which employs “scalable OFDMA” where users
use different subcarriers or set of subcarriers to transmit data over
the same medium and at the same carrier frequency.
The ability to distinguish multiple simultaneous transmissions
requires either the signal structure to be fairly simple or the de-
coding/detection mechanism to be complex. In this paper we fo-
cus on a set of network primitives that calls for a very simple an-
swer typically in binary; in the form of yes or no. Empowered
by subcarrier transmission using OFDM we can either transmit a
1 or a 0 to convey these binary answers. Not only is this form
of signaling simple, the detection of such a multiuser communica-
tion can be accomplished using spectrum sensing and energy detec-
tion. Simultaneous transmissions can be an advantage in a number
of network applications that call for multiple nodes to participate
and also use simple information. Examples include route requests,
leader election, network management and other operations involv-
ing broadcast or multicast messages. Not only does simultaneous
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