Traditionally, protocols in wireless sensor networks focus on
low-power operation with low data-rates. In addition, a small set of protocols provides high throughput communication. With sensor networks developing into general propose networks, we argue that protocols need to provide both: low data-rates at high energy-efficiency and, additionally, a high throughput mode. This is essential, for example, to quickly collect large amounts of raw-data from a sensor. This paper presents a set of practical extensions to the low-power, low delay routing protocol ORW. We introduce the capability to handle multiple,
concurrent bulk-transfers in dynamic application scenarios. Overall, our extensions allow ORW to reach an almost 500% increase in the throughput with less than a 25% increase of the power consumption during a bulk transfer. Thus, we show that instead of developing a new protocol from scratch, we can carefully enhance an existing, energy-efficient protocol with high-throughput extensions. Both the energy-efficient low
data-rate mode and the high throughput extensions transparently coexist inside a single protocol

Landsiedel, Olaf

Nagy, Attila

Traditionally, protocols in wireless sensor networks focus onlow-power operation with low data-rates. In addition, a small set of protocols provides high throughput communication. With sensor networks developing into general propose networks, we argue that protocols need to provide both: low data-rates at high energy-efficiency and, additionally, a high throughput mode. This is essential, for example, to quickly collect large amounts of raw-data from a sensor. This paper presents a set of practical extensions to the low-power, low delay routing protocol ORW. We introduce the capability to handle multiple,concurrent bulk-transfers in dynamic application scenarios. Overall, our extensions allow ORW to reach an almost 500% increase in the throughput with less than a 25% increase of the power consumption during a bulk transfer. Thus, we show that instead of developing a new protocol from scratch, we can carefully enhance an existing, energy-efficient protocol with high-throughput extensions. Both the energy-efficient lowdata-rate mode and the high throughput extensions transparently coexist inside a single protocol

Chalmers Research

Towards Energy Efficient, High-speed Communication in WSNs

Chalmers Publication Library

Towards Energy Efficient, High-Speed
Communication in WSNs
Attila Nagy and Olaf Landsiedel
1 Gothenburg University, Sweden
2 Chalmers University of Technology, Sweden
{nagat@student.chalmers.se,olafl@chalmers.se}
Abstract. Traditionally, protocols in wireless sensor networks focus on
low-power operation with low data-rates. In addition, a small set of pro-
tocols provides high throughput communication. With sensor networks
developing into general propose networks, we argue that protocols need
to provide both: low data-rates at high energy-efficiency and, addition-
ally, a high throughput mode. This is essential, for example, to quickly
collect large amounts of raw-data from a sensor.
This paper presents a set of practical extensions to the low-power, low-
delay routing protocol ORW. We introduce the capability to handle mul-
tiple, concurrent bulk-transfers in dynamic application scenarios. Over-
all, our extensions allow ORW to reach an almost 500% increase in the
throughput with less than a 25% increase of the power consumption dur-
ing a bulk transfer. Thus, we show that instead of developing a new pro-
tocol from scratch, we can carefully enhance an existing, energy-efficient
protocol with high-throughput extensions. Both the energy-efficient low
data-rate mode and the high throughput extensions transparently co-
exist inside a single protocol.
Keywords: high-throughput, opportunistic routing, Wireless Sensor Net-
work
1 Introduction
In wireless sensor networks (WSNs), most protocol stacks are designed for low
data-rates. This is a widespread application scenario in WSNs and matches the
limited resources of sensor nodes in terms of bandwidth and energy. However,
there is a set of situations, in which we demand for high-speed bulk-transfers:
the energy efficient transport of large amounts of data through the resource
constrained WSNs. Such scenarios, for example, include the distribution of OS
updates and configurations, or the collection of raw measurement traces and logs
from individual nodes.
In this paper we argue that instead of developing a new protocol, it is suf-
ficient to extend an existing energy-efficient, low data-rate protocol with a set
of carefully designed high-throughput extensions. For this, we base our design
on the existing energy-efficient, opportunistic routing protocol, ORW [6]. Our
2 Attila Nagy, Olaf Landsiedel
B	  
C	  
A	  
Reliable	  Link	  
Intermediate	  Link	  
(a) Sample topol-
ogy: Node A
reaches C via B
on reliable links
or directly on an
unreliable link.
A
B
P	   P	   P	  
A	  
P	  
C
P	  
A	  
P	  
(b) Traditional unicast routing in
WSNs: Although C might over-
hear some transmission from A,
packets are addressed to B to en-
sure stable routing.
A
B
P	   P	  
C
A	  
P	   A	  
(c) Opportunistic Routing in
ORW: The first node that wakes
up, receives a packet, and pro-
vides sufficient routing progress
acknowledges and forwards it.
Fig. 1. Basic idea of ORW: Utilizing the first woken neighbor as forwarder, ORW
reduces energy consumption and delay. This exploiting of spatial and temporal link
diversity also increase resilience to link dynamics.
extensions cover a wide range of scenarios including intra-path interference, inter-
path interference and concurrent bulk transfers. We provide three key mecha-
nisms:
1. Our first extension to the ORW protocol initiates a novel collision avoidance
method for high-throughput scenarios. It is applied beside the already exist-
ing, well functioning collision detection technique for low data-rate settings.
2. The second extension’s purpose is to stabilize the EDC routing metric used
by the ORW protocol to estimate the latency, i.e., duty-cycled wake-ups,
required for a packet to reach the sink from a given node.
3. The third extension disables duty-cycling during a bulk transfer for the nodes
that are participating in an ongoing bulk transfer.
The remainder of this paper continues by briefly discussing the ORW protocol
to provide the required background in Section 2. We describe related work in
Section 3, and show the design of our high-throughput extensions to the ORW
protocol in Section 4. Section 5 evaluates these extensions and we conclude in
Section 6.
2 Background
ORW targets duty-cycled protocol stacks. For simplicity we here illustrate the
basic concept of ORW utilizing an asynchronous low-power-listening MAC, such
as in X-MAC [1]. In low-power-listening a sender transmits a stream of packets
until the intended receiver wakes up and acknowledges it (see Fig. 1b). To inte-
grate opportunistic routing into duty cycled environments, we depart from this
traditional unicast forwarding scheme in one key aspect: The first node that (a)
wakes up, (b) receives the packet, and (c) provides routing progress, acknowl-
edges and forwards the packet, see Fig. 1c. For example, in Figure 1a node A
Towards Energy Efficient, High-Speed Communication in WSNs 3
can reach node C either directly via an unreliable link or via B. Commonly, tra-
ditional routing ignores the unreliable link A→ C and relies on A→ B → C for
forwarding. ORW extends this, by also including A → C into the routing pro-
cess: If A→ C is temporary available and C wakes up before B, ORW will utilize
it for forwarding. This reduces the energy consumption and delay (see Fig. 1c).
To select forwarders, ORW introduces EDC (Expected Duty Cycled wakeups)
as routing metric. EDC is an adaptation of ETX [2] to energy-efficient, anycast
routing in duty-cycled WSNs.
Our design enables an efficient adaptation of opportunistic routing to the
specific demands of wireless sensor networks: (1) In contrast to opportunistic
routing in mesh networks, forwarder selection in ORW focuses on energy effi-
ciency and delay instead of network throughput: It minimizes the number of
probes until a packet is received by a potential forwarder. (2) It integrates well
into duty-cycled environments and ensures that many potential forwarders can
overhear a packet in a single wake-up period. Thereby, ORW exploits spatial
and temporal link-diversity to improve resilience to wireless link dynamics. (3)
The fact that only a small number of nodes receive a probe at a specific point in
time simplifies the design of a coordination scheme to select a single forwarder.
This limits overhead of control traffic.
However, in the design of ORW we focused on low data-rate traffic, as this
is the most common scenario in WSNs. In this paper, we now take the next
step and widen our application scenarios: We extend ORW to high-throughput
settings, i.e., to support bulk transfers.
3 Related Work
There exist several approaches to high-throughput communication in WSNs. For
instance, Packet in Pipe [7] (PIP) is a connection-oriented, multi-hop, multi-
channel, TDMA-based solution. Another approach is Flush [5], a CSMA-based
protocol applying a rate-control algorithm along with end-to-end acknowledg-
ments. Both of these protocols are not designed to handle multiple concurrent
bulk transfers. Moreover, they do not integrate well with other routing protocols.
Their design is tailored to being the only routing protocol in place at a specific
point in time. We argue that this assumption is not practical as low data-rate
applications are the common application scenario in WSN. Thus, we believe any
high-throughput protocol must co-exist efficiently with low-data rate protocols.
On the other hand, the Lossy Link, Low Power, High Throughput [4] protocol
(LLH) allows for several concurrent bulk transfers crossing each others paths.
This protocol uses duty-cycling with low-power listening, has a high resistance
against both intra-path and inter-path interference, and applies a CSMA based
MAC protocol. From these three high-throughput solutions the LLH resembles
the most to the extended ORW protocol due to its low-power property and
the capability to handle concurrent bulk transfers. However, in contrast to our
work, LLH assembles a new protocol to be deployed alongside with existing low-
power, low-rate protocols. This leads to an increased code base and potentially
4 Attila Nagy, Olaf Landsiedel
P1 P1 P2 P3 P4 P5 P6 P7 P8
T
ra
n
sm
is
si
o
n A
B
C
D
u
ty
C
y
cl
e
Time
A
B
C
100% 100% 100% 100% 100% 100%100% 100%
Without busy-flag: With busy-flag:
Fig. 2. Collision avoidance by the extensions: Node B forwards all the packets
from node A. When node C starts its duty cycle, it receives the packets with busy-flag,
and therefore does not acknowledge them.
additional energy consumption, as two protocols need to be operated in parallel.
As a result, the may both have own state and each map apply own network
maintenance such neighbor discovery or wireless link estimation.
4 Design
In this section, we identify the limitations of ORW in high-throughput scenarios
and next introduce three extensions to enable high-throughput communication
in ORW.
4.1 Limitations of ORW in High-Throughput Scenarios
We begin with a discussion of the limitations of ORW in presence of high data-
rates. The base ORW protocol performs poorly in the bulk transfer scenario due
to the following key problems: (1) contention and packet collisions, (2) unstable
routing metrics, and (3) early termination of duty cycles.
1. Problem: High Contention and Packet Collisions. Bulk transfers are
streams of packets leading to many, concurrent transmissions in the network.
This inherently increases contention and as a result the possibility of colli-
sions, especially when there are multiple possible paths for a packet between
the source node and the sink node.
2. Problem: Unstable EDC Routing Metric. EDC as routing metric in
ORW estimates the expected duty cycles that are required for a packet to
traverse the topology from a node to the sink. Our analysis indicates that
this metrics tends to fluctuate rapidly in case of high contention. This is
especially the case in topologies where the path of a bulk packet has a high
number of hops but just a few possible paths exist for the packet. The result
Towards Energy Efficient, High-Speed Communication in WSNs 5
of this fluctuation are loops in the packet’s path and, via these loops, packet
loss.
3. Problem: Nodes do not stay awake until burst is completed. Duty-
cycling during a bulk transfer can lead to situations where nodes that are
heavily used turn off their radio receiver, as their duty cycles have expired.
This forces ORW to find a new, alternative forwarder or to wait for that par-
ticular forwarder to wake-up again. As a result, it increases the transmission
time of packets and the power consumption of the whole network.
4.2 Extending ORW to High-Throughput Scenarios
In the following, we present our extensions to ORW to mitigate the challenges
discussed above and to enable energy-efficient and reliable high-throughput com-
munication.
Collision Avoidance: Our collision avoidance extension is divided into two
parts: a sender and a receiver part. In the sender part, the design contains a
special flag, the busy-flag. A sender sets it to indicate that it has locked onto a
specific forwarder to transfer a bulk of packets. A forwarder shall only forward
packets from senders that have locked onto it. Other potential forwarders shall,
upon receiving packets with the the flag set, not forward it and quickly go back
to sleep to safe energy. As a result, this extensions limits contentions between
nodes to be elected as forwarder. Figure 2 illustrates how this collision avoidance
extension works with one sender node A and two receiver nodes B and C.
Stabilizing the EDC routing metric: ORW, by default, updates its rout-
ing estimates after each transmission. Thus, based on its success or failure the
quality estimation of the wireless link to the neighboring nodes is updated. In
high-throughput scenarios, with many current transmissions this leads to erro-
neous estimates. Our solution is to simply prohibit the EDC update of a node if
it is involved in a bulk transfer. Thus, we update the routing metric at the end
of each bulk transfer, i.e., after 10 to 20 packets, and not during it.
Keeping nodes awake: Finally, we prohibit nodes that are involved in a
bulk transfer from going back to sleep before the bulk transfer has completed.
5 Evaluation
Our evaluation presents two types of benchmarks: (1) micro-benchmarks and
(2) macro-benchmarks. For the micro-benchmarks we use a WSN simulation
environment, Cooja, and for the macro-benchmark we utilize Indriya [3], a three-
dimensional wireless sensor network deployed across three floors of the National
University of Singapore.
The micro-benchmarks use two types of metrics: (1) reliability, (2) power
consumption of the whole bulk transfer. We evaluate each of our three extensions
separately and we show their combination:
1. ORWE-BF: busy-flag extension to avoid contention and collisions.
2. ORWE-EDC: stabilization of the EDC routing metric.
6 Attila Nagy, Olaf Landsiedel
0.95
0.96
0.97
0.98
0.99
1
1.01
1 2 3 4 5 6 7 8 9
R
el
ia
b
ili
ty
[%
]
Number of Parallel Forwarders
ORW
ORWE
ORWE-BF
ORWE-EDC
ORWE-DC
(a) Reliability: Our extensions have the
same level of reliability as the base ORW
design.
0
1
2
3
4
5
6
7
8
1 2 3 4 5 6 7 8 9
P
ow
er
C
on
su
m
p
ti
on
[s
]
Number of Parallel Forwarders
ORW
ORWE
ORWE-BF
ORWE-EDC
ORWE-DC
(b) Power consumption: Drastic in-
crease in the ORW design when the topol-
ogy increases. In contrast, our extensions
show only a minimal increase.
Fig. 3. Parallel forwarder nodes.
0.5
0.55
0.6
0.65
0.7
0.75
0.8
0.85
0.9
0.95
1
1.05
1 2 3 4 5 6 7 8 9
R
el
ia
b
ili
ty
[%
]
Number of Hops
ORW
ORWE
ORWE-BF
ORWE-EDC
ORWE-DC
(a) Reliability: ORWE has a higher re-
liability than the base ORW protocol.
0
5
10
15
20
25
30
1 2 3 4 5 6 7 8 9
P
ow
er
C
on
su
m
p
ti
on
[s
]
Number of Hops
ORW
ORWE
ORWE-BF
ORWE-EDC
ORWE-DC
(b) Power consumption: ORWE has
the lowest power consumption.
Fig. 4. Intra-path interference.
3. ORWE-DC: stabilization of the duty-cycle.
4. ORWE: the combination of all three extensions.
Our micro-benchmarks evaluate three types of scenarios in a controlled envi-
ronment. The first scenario, inter-path interference, shows the best performance
improvement. The topology we use for this type contains a set of parallel for-
warder nodes between the source and sink nodes. The more potential forwarders
we deploy, the higher the chance for collisions of acknowledgments is. While the
base ORW protocol resolves collisions per packet, our extensions allow to lower
this intensity to one collision per bulk transfer. Figure 3 shows the reliability
and power consumption of this scenario: The power consumption stays on a low
level without any performance degradation in the reliability.
The second type of scenario, intra-path interference, aims to show the per-
formance of our extensions in a topology where there is only one path between
Towards Energy Efficient, High-Speed Communication in WSNs 7
0.5
0.6
0.7
0.8
0.9
1
1.1
1 2 3 4 5 6
R
el
ia
b
ili
ty
Number of Concurrent Bulk-Transfers
ORW
ORWE
ORWE-BF
ORWE-EDC
ORWE-DC
(a) Reliability: The bottleneck scenario
highlights the limitations of all design: the
contention leads to packet losses in all de-
signs. Nonetheless, ORWE improves over
default ORW.
-1
0
1
2
3
4
5
6
7
8
1 2 3 4 5 6
P
ow
er
C
on
su
m
p
ti
on
[s
]
Number of Concurrent Bulk-Transfers
ORW
ORWE
ORWE-BF
ORWE-EDC
ORWE-DC
(b) Power consumption: ORWE has
the lowest power consumption.
Fig. 5. Bottleneck scenario: Multiple sources with 4 potential forwarders.
the source and the sink, but that path contains a high number of hops. In this
scenario, the extensions perform significantly better than the base protocol in all
the metrics. For instance, ORWE uses the 10% of the power that is used by the
base ORW protocol for the same bulk transfer. Moreover, we increase the relia-
bility in the meantime. Figure 4 shows the reliability and power consumption for
this scenario. The third type of scenario shows that our extensions are capable
of handling multiple concurrent bulk transfers with only a minor performance
degradation.
The third type of topology evaluates a case when a number of source nodes
concurrently sending bulk packets exceeds the number of forwarders, see Fig-
ure 5. This topology formulates a bottleneck scenario in the sense that the num-
ber of forwarder nodes are not able to serve the performance demand generated
by the source nodes creating a special collision critical situation.
The macro-benchmark serves as a platform for the evaluation on a larger
scale. Overall, our results show that our extensions use 25% of the power that
are used by the base ORW protocol with a slightly higher reliability.
6 Conclusion
In this paper we show, that by adding three carefully designed extensions to the
ORW protocols, we can extend it from a low data-rate protocol to also support
high-throughput scenarios at high energy-efficiency. Our future work includes a
detailed testbed evaluation and an experimental comparison to the state of the
art.
Conversely, by applying our extensions we obtain a significant performance
improvement in all the metrics that we presented in this paper. Lastly, these
extensions have no effect on the non-bulk packet transfer, since they are only
8 Attila Nagy, Olaf Landsiedel
activated when the transmission rate exceeds a certain level. Thus, these exten-
sions transparently integrate into ORW and are backwards compatible with the
base protocol.
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