Poster Abstract: Enabling Reliable and High-Fidelity Data
Chieh-Jan Mike Liang†, Jie Liu‡, Liqian Luo?, Andreas Terzis†
†Johns Hopkins University
RACNet is a sensor network that monitors a data cen-
ter’s environmental conditions at high temporal and
spatial resolutions. The data RACNet collects can im-
prove the energy efficiency of data centers, currently
one of the fastest growing energy consumers in the U.S.
RACNet overcomes the challenge of reliable and low-
latency data gathering from dense networks deployed in
harsh RF environments through rDCP, a novel network
protocol that decouples data collection from topology
control. Furthermore, rDCP automatically partitions
the network to multiple routing trees, each operating at
a different frequency channel. The combination of these
features enables rDCP to scale up while maintaining
high data yields. Preliminary results from a produc-
tion deployment of 694 sensors (including 174 wireless
nodes) show that rDCP achieves a data yield of 99%,
while delivering 90% of the measurements in less than
Data center energy consumption has attracted global
attention due to the fast growth of the IT industry
and increasing concerns about carbon footprints and
global climate change. Approximately 40% to 70% of
the total energy that a typical data center uses is lost in
the power distribution system or used by environmental
control systems such as Computer Room Air Condition-
ing (CRAC) units, water chillers, and (de)humidifiers [1,
6]. A root cause for this low energy efficiency is the lack
of visibility in the data center’s operating conditions.
For example, when servers issue thermal alarms, data
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center operators tend to decrease the CRAC’s tempera-
ture settings, which might not alleviate the problem .
Wireless sensor network (WSN) technology offers
many advantages for monitoring and control tasks. It is
low-cost, non-intrusive, and can be easily re-purposed.
Furthermore, WSNs require no additional network and
facility infrastructure, simplifying their deployment in
the already complex data center IT environment. At the
same time, data center monitoring introduces new chal-
lenges for WSNs. Temperature can vary by as much as
5◦C over a couple of meters inside a data center, which
means that thousands of sensors might be necessary to
cover a single colocation facility. Moreover, Liang et al.
showed that 83% of the motes in a colocation are within
one-hop distance from each other . The combination
of large scale and high density complicate the task of re-
liable and low-latency data collection. To make things
worse, the RF environment inside a data center is harsh
as it includes multiple metallic obstacles and interfer-
ence from WiFi networks.
RACNet uses custom-made motes, called Genomotes,
that form sensor chains using off-the-shelf USB cables.
These cables deliver measurements, and they also dis-
tribute power from a server’s USB port to all the sensors
on the same server rack. Each chain consists of multiple
sensing slaves and a master that is responsible for relay-
ing the measurements over its radio. An added benefit
of the chain configuration is that it reduces the num-
ber of nodes in the wireless network thereby reducing
contention. Both mote types use an MSP430 microcon-
troller, while the master also has a TI CC2420 802.15.4
radio, 1MB of flash for data buffering, and a battery for
continuous operation in case of server down time.
Figure 1 provides an architectural overview of RAC-
Net’s reliable Data Collection Protocol (rDCP). First,
rDCP uses a distributed protocol for maintaining bi-
directional routing trees (BiTrees) used for data col-
lection. Multiple BiTrees can coexist, each rooted at
a gateway that occupies a different frequency channel.
Membership in individual trees dynamically adapts to
the number of gateways in the network as well as link
Data Download Layer (DDL)
Topology Control Layer (TCL)
Figure 1: Reliable Data Collection Protocol (rDCP) ar-
chitecture.The Topology Control Layer (TCL) main-
tains the tree topology while the Data Download Layer
(DDL) reliably downloads data from individual master
qualities on each channel. Unlike centralized data collec-
tion protocols, such as Koala , the distributed topol-
ogy control nature of rDCP allows it to promptly adapt
to link quality changes and scale to large networks. Sec-
ond, data downloads in rDCP are centrally controlled
by the network’s gateways. This is different from dis-
tributed data collection protocols, such as CTP , in
that nodes wait for gateway requests before streaming
measurements. This centralized coordination minimizes
interference among network flows. At the same time,
the pull-based data download approach allows the gate-
ways to issue commands and negative acknowledgments
used to implement reliability.
To minimize data latency, rDCP attempts to balance
the load among all gateways, defined as the sum of hops
necessary to reach all nodes in the tree. Once a gate-
way’s load exceeds the network average by a predefined
threshold, it calculates two probabilities according to
its load relative to that of other gateways: A switch-
out probability that determines the number of nodes in
its own tree that should probe other channels, and a
switch-in probability that determines the channel that
those switched-out nodes should probe. The gateway
then disseminates these probabilities to its tree’s nodes.
Upon receiving the command, each node probabilisti-
cally probes other channels and switches to the channel
that provides the best parent link.
We report results from one of the production deploy-
ments. This deployment consists of 694 Genomotes (in-
cluding 174 wireless masters) in a 12,000 sq-ft Microsoft
colocation facility. Slave Genomotes sample their tem-
perature and humidity sensors every 30 seconds. The
network uses up to four wireless channels. The system
has been running for more than 7 months and collects
more than 2.5 million measurement records per day.
Figure 2 presents the data yield over a 72-hour period.
The percentage on the Y-axis is the ratio between the
0 12 24 36
48 60 72
Data Yield (%)
Figure 2: Average hourly data yield over a 72-hour pe-
riod from the 694 sensors in the production deployment.
< 10< 20 < 30
Data Latency (sec)
< 40< 50< 60 More
Figure 3: Data collection latency distribution of 10,000
data samples. The network had 3 gateways.
actual data received at the gateway during that hour
and the expected amount of data, based on the sampling
rate (i.e., 120 records per hour). One can see that the
network has a data yield close to 100%. The minor data
loss was the result of a misconfiguration where the base
stations immaturely gave up on a record after only a few
failed tries. Figure 3 shows that over 90% data have a
latency of less than 30 seconds.
Last but not least, a considerable amount of effort
was necessary to deploy thousands of sensors. One ma-
jor pre-deployment overhead we have observed is assign-
ing unique sensor addresses as this requires bookkeeping
and multiple versions of the software that differ in only
a single variable. To streamline the process, we include
a 64-bit serial ID chip and a matching barcode on each
Genomote. Since the collected measurements have spa-
tial significance, the barcode minimizes human error at
all stages of the deployment.
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