Ad Hoc & Sensor Wireless Networks Vol. 8, pp. 141–159
Reprints available directly from the publisher
Photocopying permitted by license only
©2009 Old City Publishing, Inc.
Published by license under the OCP Science imprint,
a member of the Old City Publishing Group
Trade-off Between Power Consumption
and Performance in Bluetooth
Juan-Carlos Cano1 , José-Manuel Cano2 , Carlos T. Calafate1 ,
Eva González2 and Pietro Manzoni1
1 Department
of Computer Engineering, Technical University of Valencia, Spain
Technology Department, University of Malaga, Spain
E-mail: jucano@disca.upv.es
2 Electronics
Received: December 15, 2007. Accepted: December 24, 2008.
Advancements in communication technologies, coupled with an increase
of computer’s performance, are allowing us to enter the age of ubiquitous computing. The Bluetooth communication standard appears as a
major solution in this new arena. Bluetooth was designed targeting lowpower systems from the beginning, and this design criteria should be
maintained for Bluetooth to be widely accepted in the marketplace. In
this paper we investigate the power characteristics of the Bluetooth technology when supporting low-power modes. We provide accurate power
consumption measurements for different Bluetooth operating modes. Such
information could be used to drive technical decisions on battery type and
design of Bluetooth-based end systems. Finally, we examine the trade-off
between power consumption and performance for a commercial off-theshelf Bluetooth device. We find that the use of the sniff mode could be
quite compatible with the use of multi-slot data packets. However, when the
channel conditions require selecting single slot data packets, the sniff mode
could have an impact on performance, and so the power/delay trade-off
must be taken into consideration.
Keywords: Bluetooth, power consumption, performance evaluation.
1 INTRODUCTION
The widespread advent of wireless networks and the rapid proliferation of
handheld computing devices have made mobile computing possible. In addition, the miniaturization of devices and low power wireless communication
standards have paved the path towards a pervasive computing environment [1].
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Bluetooth technology [2] is a major communication standard in this new
arena of advancement that offers seamless wireless connectivity between
Bluetooth-enabled computing devices. Bluetooth allows devices to communicate using short range radio links, and it is characterized by low complexity,
low cost, and low power consumption.
Bluetooth was designed for an inherently low-power operation, criteria
which should be maintained in order for Bluetooth to be widely accepted in
the marketplace. The emphasis on low power operation stems from portable
devices, which strongly depend on the efficient use of their battery. In fact,
Bluetooth allows making a trade-off between data transfer rates and power
consumption.
Energy-aware system design and evaluation of network protocols for an
ubiquitous networking environment require practical knowledge of the energy
consumption behavior of commercial wireless devices. Moreover, it may be
worthy investigating the impact that the use of energy efficient operation
modes will have over the overall system performance.
From the user’s point of view there is little or no knowledge about Bluetooth
power consumption. Even more, confusion exits at the technical level as well.
Data sheets usually show power consumption values while transmitting and
receiving, which represent average values for static modes of operation, missing relevant information for representative operating states such as startup,
idle state and inquiry state, as well as for low-power modes, i.e., sniff, hold
and park, as defined by the Bluetooth standard [2].
There are few published measurements of the power consumption of
Bluetooth network interfaces. In [3] Kasten and Langheinrich report some
experiments measuring the power consumption of a Bluetooth-based sensor
node. These authors claim that, in small devices such as autonomous sensor nodes, Bluetooth interfaces will consume most of the power. The study
focuses on power consumption for the transmission, reception, and inquiry
operating modes. However, since the selected Bluetooth modules lacked
many of the standard Bluetooth features, the authors do not cover low-power
modes. A mathematical framework for the analysis of energy efficiency in
Bluetooth systems is presented in [4]. This work presents some analytical
results of the system behavior using different packet types and under several
scenarios. The work is not based on experimental measurements, but rather
on theoretical assumptions. Finally, other Bluetooth-related works employ
rather old and inadequate power models that were derived from other wireless
technologies [5,6].
In this paper we perform a full power characterization of the Bluetooth
technology, providing accurate current and power consumption measurements
for different operating modes. The study has been experimentally validated
for our platform prototype [7], a Bluetooth-based wireless node designed to
support spontaneous and ubiquitous computing. Based on this power characterization, we proceed by studying the trade-off between power consumption
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143
and performance. In particular, in this paper we focus on the power-delay and
power-throughput trade-off offered by a commercial off-the-shelf Bluetooth
device using the sniff mode, and under a wide range of scenarios using both
UDP and TCP traffic. We evaluate the impact of the packet type and distance
between nodes on the observed network performance.
The rest of this paper is organized as follows: Section 2 offers a brief
overview of the Bluetooth standard. Section 3 investigates the power consumption of the Bluetooth technology, with a special focus on the low-power modes
defined by the standard. Based on this power characterization, Section 4 examines the trade-off that grants the lowest power consumption while meeting
some performance requirements. Finally Section 5 concludes this paper.
2 AN OVERVIEW OF BLUETOOTH
Recently, Bluetooth technology has emerged as a promising platform for short
range wireless networking. The Bluetooth standard defines a short range and
low cost wireless radio system; its purpose is offering an alternative to connect
portable devices without requiring drawing cables between them. It operates in
the 2.4 GHz ISM band, and is the baseline approach for the IEEE 802.15.1 [8]
Wireless Personal Area Network (WPAN) standard.
Bluetooth uses a polling scheme where a single master coordinates the
access to the medium of up to 7 active slaves, forming a piconet. It also
considers the possibility of interconnecting multiple piconets in the same area
to form a scatternet. Bluetooth is based on a connection-oriented scheme.
Nodes that are close-by can find their neighbors using the inquiry procedure.
After discovering close-by devices, a node can decide to page them to start a
connection. A dedicated protocol called Service Discovery Protocol (SDP) can
be used to interchange information about all the available services at each node.
The Bluetooth specification defines two different types of links: Synchronous Connection-Oriented (SCO) and Asynchronous Connection-Less
(ACL). The first one handles real time traffic, like voice, while the latter
is commonly used for data transmission.
An ACL link is parameterized by packet size and data encoding. Each
ACL link allows using 1, 3, or 5 slot data packets, where a slot is 625 µs
long. Additionally, it optionally allows using Forward Error Correction (FEC).
According to these parameters, Bluetooth offers 6 different data packets at the
baseband layer that can be classified in two main groups: Data Medium Rate
packets (DM ), which provide a 2/3 FEC Hamming code, and Data High Rate
packets (DH ), which provide no FEC coding at all. In general, DH packets
can be good candidates for more efficient transmissions at the cost of reducing
the probability of successful transmission under poor channel conditions.
A major challenge of Bluetooth is allowing for an inherently low power
design. The Bluetooth specification integrates various methods for achieving
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low power consumption. Also, Bluetooth devices can adjust the power depending on the range of communication. Devices of categories 1, 2 and 3 cover a
distance of 100, 10 and 1 meters, respectively. Although the standard specifies
up to 100 m outdoors, few companies include such products in their market
lines because such distances are rarely required/appropriate. A second method
of reducing power consumption deals with dynamically adjusting the transmitted power. According to the receiver signal strength indicator (RSSI) a
Bluetooth device can request an increase or a decrease of the other device’s
transmitted power. The higher the transmit power level selected, the greater
the communication range (though interference is increased as well). Finally,
Bluetooth also allows a device to regulate power consumption depending
on its activity. A Bluetooth node can be in the active mode when actively
participating on the channel; otherwise it can reduce consumption by eventually using the low-power modes (sniff, hold, and park). We now proceed to
study the power consumption of a class 1 Bluetooth module which offers all
power modes, and that has been configured for a maximum target distance of
10 meters.
3 BLUETOOTH POWER CONSUMPTION CHARACTERIZATION
The Bluetooth controller operates in two major states, i.e., standby and connection. The standby state is the default low power state in Bluetooth, where
only the native clock is running and there is no possible interaction with any
other device. A Bluetooth node uses the Inquiry and Page procedures to transit
from the standby to the connection state, where nodes can exchange packets.
A Bluetooth node in connection state can be in the active mode to actively
participate on the channel, or it can also reduce consumption by switching to
low-power modes (sniff, hold, and park), as defined by the Bluetooth standard.
In this section we perform a detailed analysis of the power characteristics
of the Mitsumi WML C11 [9] class 1 Bluetooth module, which is based on the
CSR BlueCore 2 chipset, and is a fully qualified transceiver compliant with
the Bluetooth 1.1 Specification [2].
3.1 Measurements setup
To obtain the power-consumption measurements we used the test circuit shown
in Figure 1. The circuit is based upon the INA193 [10], a high side current
monitor integrated circuit from Texas Instruments.
In our test circuit, a shunt resistor (Rs) is used to cause a small voltage
drop proportional to the current flowing into the Bluetooth module (Ibias ).
This voltage is amplified 20 times by the INA193 circuitry, providing an
output voltage that represents a measure of the bias current. The output of
the INA193 is connected to a PC based data acquisition system that digitizes
the signal. The circuit also incorporates a first-order low pass filter with cutoff
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Performance in Bluetooth
RS 1
Vcc=3.3V
RF
C
Ibias
Ibias
RF
MITSUMI
WML-C11
Bluetooth
Module
RS<<RF<<5k
GND
5k
5k
Vout
RL=100k
Rs RL
I bias
5k
Cutoff Frequency
1
fc
2 ·2 RF C
PC based data
acquisition and
representation
system
+
TI INA193
FIGURE 1
Test circuit for Bluetooth power measurements.
frequency fc = 1/2RC, equal to 200 Hz, which allows an accurate sampling
process while avoiding aliasing. To improve the accuracy, the measurement
circuit has been calibrated using a 6.5 digits digital multimeter.
The data acquisition system was built using an AduC812 microcontroller [11] from Analog Device, attached to a Personal Computer thought
a serial port. We select a sampling rate of 1 kHz (one thousand samples per
second), as it provides both accuracy and legible representation. Since each
sample measures the instantaneous
input current, we estimate the average cur
rent consumption by: I = N
i[n]/N
, where N represents the total number
n=1
of samples and i[n] represents the value of sample n.
3.2 Experimental power measurements
We now proceed by studying the power characteristics of our Bluetooth
module. Below we describe the power measurements for the following representative operating states: Bluetooth startup, standby, inquiry, page, active,
hold, park, and sniff.
For each state we trace the fluctuations in terms of current consumption
during a period that roughly encompasses the given state. The bottom and left
axes represent time and the corresponding instantaneous current consumption.
3.2.1 Startup and standby
We first investigate the behavior of our Bluetooth module during startup and
initialization. We are specially interested in knowing how much time and
energy are required to switch from power down to the active state. Figure 2
shows the current consumption during startup and initialization. We observe
that the Bluetooth module needs around 2 seconds to finish initialization and
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Consumption (mA)
Average
Consumption (mA)
Startup
Standby
Time (s)
FIGURE 2
Current consumption (mA) during startup and initialization.
then enter into the standby state. During this time the average current consumption is about 20 mA which, using a system voltage of 3.3V, represents a
power consumption of 66 mW.
After initialization the Bluetooth module enters into the standby state,
which represents the default state for the Bluetooth technology. In this state
the module is in a low-power mode where there can be no connections open
and all components, except the internal clock, are switched off. The current
consumption during the standby state fluctuates from 1.4 mA to 3 mA, and so
the average consumption is about 2.2 mA (7.3 mW).
Even though the average power consumption during the standby state is
relatively low, it is expected that power consumption will be significant since
nodes must maintain their Bluetooth interface in this mode for longer periods
of time. Applications can periodically switch off the Bluetooth module to
reduce the power consumption in the standby state; however, they should take
into account the time and energy required for the startup process.
3.2.2 Inquiry and page
The inquiry is an asymmetric procedure during which the inquiring node and
the inquired one use the complementary inquiry and inquiry scan states. A
node that has been configured into the inquiry state continuously sends out
inquiry messages to find other nearby nodes. Similarly, Bluetooth nodes that
have been put in the inquiry scan state reply to the inquirer as soon as they
detect an inquiry message.
We found that the power consumption is about 231 mW (70 mA) and
139 mW (42 mA) for the inquiry and the inquiry scan states, respectively. So,
the power consumption in either state is considerably high with respect to the
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standby state. A node in the inquiry state continuously sends inquiry messages
during the entire interval. However, a node in the inquiry scan state does not
need to be continuously listening to inquiry messages, and so it can alternate
between the standby and the inquiry scan states in order to save power.
Once the inquiring device discovers other close-by Bluetooth devices it
can switch to the page state to setup a new connection. Nearby nodes in the
page scan state will answer to page messages in order to share some essential
information and, finally, both enter into the connection state. The consumption
values are similar to those obtained previously, i.e., 208 mW (63 mA) and
149 mW (45 mA) for the page and the page scan states, respectively. The
main difference of the page procedure with respect to the inquiry procedure
is that the duration of the scan window is smaller than the inquiry one. After
the page procedure completes both nodes are in connect state, and so data
transmission can start almost instantaneously.
3.2.3 Connection state
A Bluetooth device in the Connection state can be in the active mode or using
any of the low-power modes defined in the standard. In the active mode both
master and slave are kept synchronized with each other to participate actively
on the channel by listening, transmitting or receiving packets.
We observe that, when the ACL link is established, the consumption values
fluctuate, leveling out at around 21 mA for the master node and at 41 mA for
the slave node. The master node can reduce the average consumption since it
knows the exact time when a packet transmission is going to occur, adjusting
its transceiver accordingly. On the other hand, the slave has no other option
but keeping itself synchronized and active at all times.
In addition, when the master and the slave have data to transmit, the current
consumption increases by up to 34% and 30% for the slave and the master,
respectively. We show that, in active mode, data transmission can start almost
instantaneously, but at the expense of increased power consumption.
3.2.4 Low-power modes: Hold, park, and sniff states
Bluetooth nodes can reduce consumption by recurring to the low-power modes
defined by the Bluetooth standard, i.e., sniff, hold and park. When low-power
operation is activated, all the nodes within a piconet can reduce their duty
cycle.
Hold mode In this mode a node temporarily shuts down its radio interface,
taking some time-off to save power. Prior to entering hold mode, master and
slave should agree on when to return to active mode again.
Figure 3 shows the current consumption of a node entering the hold mode.
After the hold time has been configured, a node can enter this mode, where
consumption is reduced near to 3 mA. However, the node can not exit from
this state before the hold time expires.
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Consumption (mA)
Consumption (mA)
Consumption
Average
Active
Active
Hold Time
(Con
urable)
Time (s)
FIGURE 3
Current consumption (mA) during the Hold state.
Park mode When a slave node does not need to participate on the piconet,
but still wants to remain synchronized to the master, it can select the park
mode. The average current consumption for the park mode is about 4 mA.
When in the park mode the node should periodically listen to the channel
to remain synchronized within the piconet. In our measurements, when the
parked slave listens to the channel to hear the beacon packet from the master,
it increases the current consumption to around 35 mA.
Sniff mode In this mode a slave device, rather than listening on every slot
for the master’s beacons, only listens to the channel at specified times, agreed
upon with the master.
To enter the sniff mode, master and slave negotiate a sniff interval, Tsniff
and a sniff window, Twin . A slave node will thus listen to the piconet at regular
intervals (Tsniff ) for a short period (Twin ).
Figure 4 shows the current consumption using a Tsniff of 1.5 seconds and
a Twin equal to 200 ms. The average consumption is around 9 mA. The power
consumption will remain low as long as Tsniff is large compared to Twin . In
fact, when we modify the Twin parameter to 20 ms, the average consumption
decreases to around 4 mA (similar to the park state).
In addition, we check that the sniff mode allows, using the Ttimeout parameter, that a node with enough data to be transmitted is completely active during
several consecutive Tsniff slots in order to reduce the data latency. Of course,
during this period the consumption will be similar to the consumption during
the active mode.
Summary Table 1 summarizes the power consumption of the Mitsumi
Bluetooth module [9] we have used.
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Performance in Bluetooth
Active
Consumption (mA)
Consumption (mA)
Consumption
Average
Sn
Time (s)
FIGURE 4
Current consumption (mA) during the Sniff state. Tsniff = 1.5 s and Twin = 200 ms.
Bluetooth mode
Current
Power
Startup and initialization
Standby
Inquiry
Inquiry Scan
Page
Page Scan
Active (no transmission) (Master)
Active (no transmission) (Slave)
Active (transmission) (Master)
Active (transmission) (Slave)
Hold
Park
Sniff
20 mA
2 mA
70 mA
42 mA
63 mA
45 mA
21 mA
41 mA
27 mA
55 mA
3 mA
4 mA
9 mA
66 mW
6.6 mW
231 mW
139 mW
208 mW
149 mW
69 mW
135 mW
89 mW
181 mW
10 mW
13 mW
30 mW
TABLE 1
Average current and power consumption. System voltage is 3.3 V
4 THE POWER-PERFORMANCE TRADE-OFF IN BLUETOOTH
The previous experimental study showed that Bluetooth modules implementing low-power modes could significantly alleviate the power consumption in
Bluetooth. In particular, the hold mode could be extremely useful to reduce
the power consumption for those applications using a predictable scheduling.
Applications with a high number of peers within a piconet, and using traffic
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FIGURE 5
Experimental test-bed configuration.
patterns with a regular scheduling, could benefit from using parked slaves.
Of special interest is the sniff mode, which can offer a trade-off between low
power consumption and low transmission delay. Moreover, the sniff mode
is the only low-power mode allowing a slave node to transmit information
while still remaining in a low-power state. In this section we focus on the sniff
mode to study the trade-off between power consumption and performance for
a Bluetooth enabled device.
We built a small test-bed using a set of laptops and commercial off-the-shelf
Bluetooth devices to test Bluetooth connections. Figure 5 shows our test-bed
network configuration, where each laptop runs on an Intel Pentium IV 2000
Mhz TOSHIBA Satellite 1900-303 based on the Suse Linux 8.1 distribution
and with a Belkin Bluetooth USB Dongle card. We used the BlueZ [12] protocol stack to configure each Bluetooth device using the Bluetooth Personal
Area Network (PAN) profile. BlueZ is the official Bluetooth protocol stack for
Linux, providing a set of monitoring and traffic tools to deal with the Bluetooth
technology.
We study the impact that the use of the sniff low power mode has over
the performance of UDP data traffic and TCP data traffic. Our experiments
focused on evaluating the power-delay and power-throughput trade-off experimented by UDP and TCP traffic, respectively. We also performed a sensitivity
analysis to evaluate how distance and packet type affect power consumption,
throughput and delay.
Each node in our test-bed has been configured using two different network
interfaces: the Bluetooth interface and the standard Ethernet one. This configuration allows redirecting all the traffic between the two interfaces, allowing
each node to evaluate the average packet delay through the feedback channel
(Ethernet), avoiding having to synchronize the system clocks of the two hosts.
With this setting we achieve a high accuracy in our measurements since the
delay introduced by Bluetooth transmissions is several orders of magnitude
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Performance in Bluetooth
higher than the delay caused by host processing and loopback transmission
via Ethernet.
To achieve the desired functionality we used the iptables tool, part of the
netfilter framework, to redirect the input/output traffic according to the corresponding source and destination address of the test nodes. As an example, the
following command would configure Host B in Figure 5 so that all the UDP
packets received from Host A via the Bluetooth interface are sent back to it
through the Ethernet interface.
iptables – t nat – A PREROUTING – d 10.0.0.254 – p udp – j DNAT – to –
destination 10.0.3.1
4.1 Power-delay trade-off analysis
We first evaluate the effect that the use of the sniff mode has over the power
consumption and the packet delay when the data traffic consists of UDP connections. Each UDP connection generates 1 packet/second, with a packet size
of 1000 bytes. In our experiments we vary the packet type and the distance
between nodes.
4.1.1 Power consumption evaluation
Figures 6 and 7 allow comparing the current consumption in active and sniff
modes during a data packet transmission, and using a DM1 type of packet.
The configuration of the sniff mode depends on the application’s requirements,
and the definition of the best Tsniff represents a compromise between power
consumption and delay. The advantage of having a large Tsniff is low consumption (see Figure 4), and the inconvenience is high delay when sending data
packets. In our experiments we select a relatively small Tsniff to avoid increasing the packet delay, and we fixed Twin to only one slot in order to reduce the
power consumption.
35
35
Packet transmission
(DM1)
Packet transmission
(DM1)
30
Consumption (mA)
Consumption (mA)
30
25
20
15
10
5
25
20
15
10
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Time (s)
(a) Master in active mode
0.8
0.9
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1
Time (s)
(b) Master in sniff mode
FIGURE 6
Current consumption (mA) of a packet transmission in active (a) and sniff mode (b) on a master
device using a DM1 packet type. Distance between master and slave is of 6m. (Tsniff = 20 slots,
Twin = 1 slot, Ttimeout = 1).
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35
Packet transmission
(DM1)
35
Consumption (mA)
Consumption (mA)
25
20
15
10
25
20
15
10
5
5
0
Packet transmission
(DM1)
30
30
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Time (s)
(a) Slave in active mode
0.8
0.9
1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Time (s)
(b) Slave in sniff mode
FIGURE 7
Current consumption (mA) during a packet transmission in active (a) and sniff mode (b) on a slave
device using a DM1 packet type. Distance between master and slave is of 6 m. (Tsniff = 20 slots,
Twin = 1 slot, Ttimeout = 1).
Figures 6 and 7 highlight the bursty behavior of the sniff mode. As expected, the Bluetooth interface has periodic peaks every Tsniff slot pairs, and
the baseline value is similar to the standby mode, which is lower that the
active master and active slave ones. If we compare the active mode operation
between master and slave we confirm that the consumption at the slave is
significantly higher than the one at the master (just above 20 mA). This effect
is due to the continuous listening activity a slave is required to perform. When
using the sniff mode, the slave can reduce the current consumption with respect
to the master’s.
We repeated all the tests performed by varying the packet type from DM
(5, 3, 1) to DH (5, 3, 1). When selecting multi-slot packets, Bluetooth reduces
the time we need to send each data packet, thereby reducing the current
consumption. We also confirmed that distance does not affect the power consumption of our Bluetooth devices (power regulation is off). Figure 8 shows
the current consumption in sniff mode during a data packet transmission using
a more efficient DH5 multi-slot packet, fixing the distance between master
and slave to about 9 meters.
4.1.2 Packet delay evaluation
We now evaluate the impact on packet delay of varying the ACL packet type
and the distance between devices. For each test we obtain the average packet
delay of 100 independent data packets. Figure 9 shows the results obtained
when the master node sends data to the slave using either DHx or DM x packets,
using a similar scenario that the one showed in Figure 5.
We observe that, independently of the packet type chosen, Bluetooth offers
a relatively stable packet delay up to 10 meters. When surpassing the 10 m limit
Bluetooth still works without a sharp performance reduction, but when arriving
at the border of 15 meters, performance degradation starts to be noticeable.
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35
Consumption (mA)
Packet transmission
(DH5)
30
25
20
15
10
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Time (s)
FIGURE 8
Current consumption (mA) during a packet transmission in sniff mode on a master device using
a DH5 packet type. Spatial distance between master and slave is 9 m. (Tsniff = 20 slots, Twin =
1 slot, Ttimeout = 1).
200
Data Hi gh Rate (DH5 )
175
Data Hi gh Rate (DH3 )
Data Hi gh Rate (DH1 )
End to end delay (ms)
150
125
Data Hi g
(DH5 )
Data Hi g
(DH3 )
Data Hi g
(DH1 )
100
75
50
25
0
0m
3m
6m
9m
12 m
15 m
12 m
15 m
Distance (m)
200
Data Medium Rate (DM5)
Data Medium Rate (DM3)
Data Medium Rate (DM1)
(DM5)
(DM3)
(DM1)
175
End to end delay (ms)
150
125
100
75
50
25
0
0m
3m
6m
9m
Distance (m)
FIGURE 9
Comparison of the UDP packet delay in active and sniff mode under different DHx and DMx
packets with respect to distance.
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30
100%
25
Histo gram delay (
20
)
Cumulati ve delay (DH1 )
Cumulati ve delay (
)
60%
15
40%
10
Cumulati ve distribution function
Histogram frequency (# Tests)
80%
Histo gram delay (DH1 )
20%
5
0
0%
0
25
50
75
100
1 25
150
1 75
Time (ms)
FIGURE 10
Comparison of the cumulative UDP packet delay in active and sniff mode for 100 data packets.
Packet type is DH 1 . Distance between devices is of 6m.
As expected, results confirm that the use of DHx packets increases efficiency. Moreover, when using the multi-slot DH 3 or DH 5 packets, the
selected sniff mode, i.e., Tsniff = 20 slots, Twin = 1 slot, and Ttimeout = 1,
allows to reduce the power consumption without significantly increasing
the delay. However, when we select the sniff mode and use either DH 1 or
DM 1 data packets, the observed delay increases significantly (44% and 52%,
respectively). According to the obtained results we conclude that the use of
the sniff mode could be quite compatible with the more efficient multi slot
data packets. However, when single-slot data packets must be used, the sniff
mode does have a significant impact on performance, and so the power/delay
trade-off should be considered.
Figure 10 shows the histogram distribution of the packet delay as a function
of time for the 100 tests, at a distance of 6 m. The results show the impact that
the use of the sniff mode has on the packet delay when using single slot DH 1
packets. Besides we find that the median is of 62 ms when the sniff mode is
not used, and it grows to 85 ms when the sniff mode is activated. Moreover,
we achieve the 95% percentile with 70 ms and 120 ms when using or not the
sniff mode, respectively.
Finally, when the slave node acts as the sender and the master node acts
as the receiver, packet delay increases around 10% for all the different data
packet types. When the master node acts as the sender, it can reduce the average
packet delay since it knows the exact time when a packet transmission is going
to take place.
4.2 Power-throughput trade-off analysis
We now evaluate the impact that the use of the sniff mode has on throughput
and power consumption when the data traffic consists of TCP connections. We
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configured the system to continuously download a data block of 512 kbytes
varying the packet type and the spatial distance among the master and
the slaves. We configured each TCP connection to generate data packet of
1500 bytes including the TCP and IP headers.
4.2.1 Power consumption evaluation
Figure 11 compares the current consumption in active and sniff modes for a
slave Bluetooth device sending data using different packet types.
6 TCP connection
varying data typ e
DM1
40
DM3
DM5
DH3
DH1
DH5
35
Active,
no transmission
Consumption (mA)
30
25
20
15
10
5
0
0
20
40
60
80
100
120
140
Time (s)
(a) Slave in active mode
DM3
6 TCP connection
varying data typ e
40
DM5
DH3 DH5
DH1
DM1
Consumption (mA)
35
30
25
20
Active,
no transmission
15
10
5
0
0
20
40
60
80
100
120
140
160
Time (s)
(b) Slave in sniff mode
FIGURE 11
Current consumption (mA) for 6 consecutive TCP connections in active (a) and sniff mode (b) on
a slave device varying the packet type. The data block is of 512 kbytes and the distance between
master and slave is of 6 m. (Tsniff = 20 slots, Twin = 1 slot, Ttimeout = 1).
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Firstly, we observe that by using multi-slot Bluetooth packets there is a
slight increase of the average current consumption with respect to the use of
the DM 1 or DH 1 data packets. On the other hand, their use allows to drastically
reduce the transfer time, thereby reducing the average current consumption.
We also observe that by using the sniff mode the reduction on average current
consumption is limited to 5% with respect to the active mode.
We can further reduce current consumption by increasing the Tsniff
parameter of the sniff mode; however, the observed throughput will also
decrease.
We now evaluate the impact of the Ttimeout parameter. Figure 12 shows a
portion of a TCP connection when using either Ttimeout = 0 or Ttimeout = 1. To
40
35
Consumption (mA)
30
TCP connection in sni
Timeout = 0
de,
25
20
15
10
5
0
0
0.5
1
1.5
2
Time (s)
40
35
Consumption(mA)
30
TCP connection in sni
Timeout = 1
25
de,
20
15
10
5
0
0
0.5
1
1.5
2
Time (s)
FIGURE 12
Comparison of the current consumption (mA) when varying the Ttimeout parameter. (Tsniff =
100 slots, Twin = 20 slot.
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Master to slave,
active
Master to slave,
sniff
Slave to Master,
active
Slave to master,
sniff
DM 1
DM 3
DM 5
DH 1
DH 3
DH 5
98.4 kb/s
333.6 kb/s
410.4 kb/s
160.8 kb/s
460.0 kb/s
564.0 kb/s
80.8 kb/s
278.4 kb/s
283.2 kb/s
129.6 kb/s
401.6 kb/s
424.0 kb/s
100.0 kb/s
336.0 kb/s
416.8 kb/s
154.4 kb/s
486.4 kb/s
577.6 kb/s
80.0 kb/s
272.0 kb/s
382.4 kb/s
128.0 kb/s
392.0 kb/s
520.0 kb/s
TABLE 2
Comparison of the TCP throughput in active and sniff modes under different DHx and
DMx . Spatial distance between master and slave is 6m and Ttimeout = 1
better appreciate the difference, in this test we have fixed the Tsniff to 100 slots,
and the Twin to 20 slots. We observe that, when setting Ttimeout = 1, a node
with enough data to be transmitted can be active during several consecutive
Tsniff slots. As observed, the current consumption increases but the observed
performance will be highly improved.
4.2.2 Throughput evaluation
We now evaluate the impact that the use of the sniff mode has on TCP
throughput with varying packet types. We evaluated the differences of having the master or the slave node acting as the data source. Table 2 shows the
obtained results when the distance between the master and the slaves is of
6 meters.
When we select the more efficient DH data packets, throughput always
increases with respect to using DM packets. In active mode, DH packets
outperform DM ones by 60%, 45% and 35% for 1, 3, and 5 slot packets,
respectively. When the use of the sniff mode is selected the behavior is
quite similar, though the throughput decreases by about 20%. With respect
to throughput, there are no noticeable differences between the two selected
case studies, i.e., the master or the slave acting as the data source.
Finally, we observed that all the results obtained at this distance are below
the maximum throughput stated in Bluetooth specification. The Bluetooth
standard provides the following reference values: 723.2 kbit/s for DH5,
585.6 kbit/s, for DH3 and 172.8 kbit/s for DH1; 477.8 kbit/s for DM5,
387.2 kbit/s, for DM3 and 108.8 kbit/s for DM1. A master in active mode
gets a 78%, 79%, and 93% of the maximum throughput provided in the
standard for the DH 5,3,1 packets respectively. When using the more conservative DM 5,3,1 packets, these percentages are of 85%, 86%, and 90%.
Such results confirm that ideal conditions can not always be achieved due to
distance and noise; hence, bandwidth limitations and fluctuation should be
considered.
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5 CONCLUSIONS
In this paper we investigate the power characteristics of the Bluetooth technology when supporting low-power modes. We provide accurate current
consumption measurements for different operating modes of the Bluetooth
technology, and provide useful information to drive technical decisions on
battery type for Bluetooth-based end systems. We find that the low-power
modes stated in the standard could significantly alleviate the power consumption of Bluetooth nodes in active mode. We found that the hold mode could
be extremely useful to reduce the power consumption for those applications
using a predictable scheduling. Applications with a high number of peers and
using data traffic patterns with a regular scheduling could benefit from using
parked slaves. Of special interest is the sniff mode, which can offer a trade-off
between low power consumption and low transmission delay.
We have shown that the definition of the best Tsniff parameter for the sniff
mode represents a compromise between power consumption and delay. The
advantage of having a large Tsniff is low consumption, and the inconvenience
is high delay and low throughput when sending data packets.
Our study includes a performance evaluation of the Bluetooth technology
to characterize the trade-off between power consumption and performance.
We evaluated the impact that the use of the sniff mode has on network performance. Our experiments focused on evaluating the power-delay and the
power-throughput trade-off experimented by UDP and TCP traffic, respectively. We also performed a sensitivity analysis to evaluate how distance and
packet type affect power consumption, throughput and delay.
We observed that Bluetooth offers a relatively steady packet delay and
throughput up to 10 meters, independently of the selected packet type. We
also observed that the use of the sniff mode could be quite compatible with
the more efficient multi-slot data packets. However, when the channel conditions require selecting single-slot data packet, the sniff mode could impact
performance and so the power/delay trade-off should be considered.
With respect to throughput, we observed that a master node in active mode
gets a 78%, 79%, and 93% of the maximum throughput provided in the standard for DH 5,3,1 packets, respectively. When activating the sniff mode, the
behavior is rather quite similar with respect to power consumption, though
throughput decreases by about 20%. Finally, our experiments also confirm
that delay and throughput do not increase significantly with distance.
Overall, the results obtained provide useful insight to determine whether
or not to activate the sniff mode according to the selected packet type and the
application’s requirements.
As future work we would like to obtain some clear relationships among
performance and the usage of Bluetooth low power modes, evaluating the
impact that the size of the piconet has over the trade-off between performance
and power consumption.
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159
ACKNOWLEDGMENTS
This work was partially supported by the Ministerio de Educación y Ciencia,
Spain, under Grants TIN2008-06441-C02-01, and TEC2006-12211-C02-01.
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