How Does Energy Consumption Impact Performance in Bluetooth?1

Juan-Carlos Cano†, Jose-Manuel Cano‡, Eva Gonzalez‡, Carlos Calafate†, Pietro Manzoni†Department of Computer Engineering †

Polytechnic University of Valencia, SpainEmail: {jucano, calafate, pmanzoni}@disca.upv.es

Electronics Technology Department ‡University of Malaga, Spain

Email: {cano, eva}@dte.uma.es

Abstract

In this paper we investigate the power characteristics of theBluetooth technology when supporting low-power modes.We provide accurate power consumption measurements fordifferent Bluetooth operating modes. Such information couldbe used to drive technical decisions on battery type and designof Bluetooth-based end systems. Finally, we examine thetrade-off between power consumption and performance fora commercial off-the-shelf Bluetooth device. We find thatthe use of the sniff mode could be quite compatible with theuse of multi-slot data packets. However, when the channelconditions require selecting single slot data packets, the sniffmode highly impact performance, and so the power/delaytrade-off must be taken into consideration.

1 Introduction

The miniaturization of devices and low power wirelesscommunication standards have paved the path towardsa pervasive computing environment [1]. Bluetoothtechnology [2] is a major communication standard in thisnew arena. Bluetooth allows devices to communicateusing short range radio links, and it is characterized bylow complexity, low cost, and low power consumption.Energy-aware system design and the evaluation of networkprotocols for an ubiquitous networking environment requirepractical knowledge of the energy consumption behavior ofcommercial wireless devices.

From the user’s point of view there is little or no knowledgeabout Bluetooth power consumption. Data sheets usuallyshow power consumption values while transmitting andreceiving, which represent average values for static modesof operation, without relevant information for representativeoperating states such as startup, idle state and inquiry state,as well as for low-power modes, i.e., sniff, hold and park, asdefined by the Bluetooth standard.

In this paper we perform a full power characterization ofthe Bluetooth technology, providing accurate current andpower consumption measurements for different operatingmodes. The study has been experimentally validated forour platform prototype [3], a Bluetooth-based wireless nodedesigned to support spontaneous and ubiquitous computing.In particular, in this paper we focus on the power-delay andpower-throughput trade-off offered by a commercial off-the-shelf Bluetooth device using the sniff mode, and under a widerange of scenarios using both UDP and TCP traffic.

1This work was partially supported by the Ministerio de Educaciony Ciencia, Spain, under Grant TIN2005-07705-C02-01 and by Ajudescomplementaries per a projectes de I+D+I D.Gral d’Investigacio iTransferencia Tecnologica, Spain, under Grant ACOMP07/237

2 Bluetooth Power Consumption Characterization

In this section we perform a detailed analysis of thepower characteristics of the Mitsumi WML C11 [4] class 1Bluetooth module, which is based on the CSR BlueCore 2chipset, and is a fully qualified transceiver compliant with theBluetooth 1.1 Specification. We first investigate the behaviorof our Bluetooth module during startup and initialization.Figure 1 shows the current consumption during startup andinitialization. We observe that the Bluetooth module needsaround 2 seconds to finish initialization and then enter intothe standby state. During this time the average currentconsumption is about 20 mA which, using a system voltageof 3.3V, represents a power consumption of 66 mW.

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Figure 1: Current consumption (mA) during startup andinitialization.

After initialization the Bluetooth module enters into thestandby state, which represents the default state for theBluetooth technology. In this state the module is in a low-power mode where there can be no connections open and allcomponents, except the internal clock, are switched off. Thecurrent consumption during the standby state fluctuates from1.4 mA to 3 mA, and so the average consumption is about 2.2mA (7.3 mW).

A node that has been configured into the inquiry statecontinuously sends out inquiry messages to find other nearbynodes. We found that the power consumption is about231 mW (70 mA) and 139 mW (42 mA) for the inquiryand the inquiry scan states, respectively. So, the powerconsumption in either state is considerably high with respectto the standby state. Once the inquiring device discoversother close-by Bluetooth devices it can switch to the pagestate to setup a new connection. Nearby nodes in the pagescan state will answer to page messages in order to sharesome essential information and, finally, both enter into theconnection state. The consumption values are similar to thoseobtained previously, i.e., 208 mW (63 mA) and 149 mW(45 mA) for the page and the page scan states, respectively.

A Bluetooth device in the Connection state can be in theactive mode or in any of the low-power modes defined in thestandard. When an ACL link is established, the consumptionvalues fluctuate, leveling out at around 21 mA for the masternode and at 41 mA for the slave node. The master nodecan reduce the average consumption since it knows the exacttime when a packet transmission is going to occur, while aslave has no other option but keeping itself synchronized andactive at all times. In addition, when the master and the slavehave data to transmit, the current consumption increases byup to 34% and 30% for the slave and the master, respectively.We show that, in active mode, data transmission can startalmost instantaneously, but at the expense of increased powerconsumption.

Bluetooth nodes can reduce consumption by using the low-power modes, namely: sniff, hold and park. Figure 2 showsthe current consumption of a node entering the hold mode.After the hold time has been configured, a node can enter thismode, where consumption is reduced near to 3 mA.

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Figure 2: Current consumption (mA) during the Hold mode.The average current consumption during the park mode isabout 4 mA. When the parked slave listens to the channelto hear the beacon packet from the master, it increases thecurrent consumption to around 35 mA.

To enter the sniff mode, master and slave negotiate a sniffinterval, Tsniff and a sniff window, Twin. A slave nodewill thus listen to the piconet at regular intervals (Tsniff)for a short period (Twin). Figure 3 shows the currentconsumption using a Tsniff of 1.5 seconds and a Twin equalto 200 ms. The average consumption is around 9 mA. Theenergy consumption will remain low as long as Tsniff is largecompared to Twin.

The sniff mode, using the Ttimeout parameter, allows a nodewith enough data to be transmitted to be completely activeduring several consecutive Tsniff slots in order to reduce thedata latency. During this period the consumption will besimilar to the consumption during the active mode.

3 The Power-Performance Trade-Off in Bluetooth

We study the impact that the use of the sniff low power modehas over the performance of UDP data traffic and TCP datatraffic. Our experiments focused on evaluating the power-delay and power-throughput trade-off experimented by UDPand TCP traffic, respectively. We also performed a sensitivity

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Figure 3: Current consumption (mA) during the Sniff state.Tsniff = 1.5 s and Twin = 200 ms.

analysis to evaluate how distance and packet type affect powerconsumption, throughput and delay.

We first evaluate the effect that the use of the sniff modehas over the power consumption and the packet delay whenthe data traffic consists of UDP connections. Each UDPconnection generates 1 packet/second, with a packet size of1000 bytes. In our experiments we vary the packet type andthe distance between nodes.

The configuration of the sniff mode depends on theapplication’s requirements, and the definition of the bestTsniff represents a compromise between power consumptionand delay. The advantage of having a large Tsniff is lowconsumption and the inconvenience is high delay whensending data packets.

As expected, the Bluetooth interface has periodic peaks everyTsniff slot pairs, and the baseline value is similar to thestandby mode, which is lower that the active master and activeslave ones. If we compare the active mode operation betweenmaster and slave we confirm that the consumption at the slaveis significantly higher than the one at the master (just above20 mA). This effect is due to the continuous listening activitya slave is required to perform. When using the sniff mode, theslave can reduce the current consumption with respect to themaster.

We repeated all the tests performed by varying the packet typefrom DM (5, 3, 1) to DH (5, 3, 1). When selecting multi-slot packets, Bluetooth reduces the time we need to send eachdata packet, thereby reducing the current consumption. Wealso confirmed that the distance does not affect the powerconsumption of our Bluetooth devices (power regulation isoff).

We observe that, independently on the packet type chosen,Bluetooth offers a relatively stable packet delay up to 10meters. When surpassing the 10m limit Bluetooth still workswithout a sharp performance reduction, but when arriving atthe border of 15 meters, performance degradation starts to benoticeable.

Results confirm that the use of DHx packets increasesefficiency. Moreover, when using the multi-slot DH3 or DH5packets, the selected sniff mode, i.e., Tsniff = 20 slots,Twin = 1 slot, and Ttimeout = 1, allows to reduce

the energy consumption without significantly increasing thedelay. However, when we select the sniff mode and use eitherDH1 or DM1 data packets, the observed delay increasessignificantly (44% and 52%, respectively). According to theobtained results, we conclude that the use of the sniff modecould be quite compatible with the more efficient multi slotdata packets. However, when single-slot data packets mustbe used, the sniff mode does have an impact on performance,and so the power/delay trade-off should be considered.

Finally, when the slave node acts as the sender and the masternode acts as the receiver, packet delay increases around 10%for all the different data packet types. When the master nodeacts as the sender, it can reduce the average packet delay sinceit knows the exact time when a packet transmission is goingto take place.

We now evaluate the impact that the use of the sniff mode hason throughput and power consumption when the data trafficconsists of TCP connections. We configured the system tocontinuously download a data block of 512 kbytes varyingthe packet type and the spatial distance among the master andthe slaves. We configured each TCP connection to generatedata packet of 1500 bytes including the TCP header.

Figure 4 compares the current consumption in active andsniff modes for a slave Bluetooth device sending data usingdifferent packet types.

Firstly, we observe that by using multi-slot Bluetooth packetsthere is a slight increase of the average current consumptionwith respect to the use of the DM1 or DH1 data packets.On the other hand their use allows to drastically reducethe transfer time, thereby reducing the average currentconsumption. We also observe that by using the sniff modethe reduction on average current consumption is limited to 5%with respect to the active mode.

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Figure 4: Current consumption (mA) for 6 consecutive TCPconnections in sniff mode.

We can further reduce current consumption by increasing theTsniff parameter of the sniff mode; however, the observedthroughput will also decrease.

We now evaluate the impact of the Ttimeout parameter usingeither Ttimeout = 0 and Ttimeout = 1. We fixed the(Tsniff = 100slots, and the Twin = 20slot. We observe that,when we configure Ttimeout = 1, a node with enough data tobe transmitted can be active during several consecutive Tsniff

slots. As observed, the current consumption increases but theobserved performance will be highly improved.

We now evaluate the impact that the use of the sniff mode hason TCP throughput with varying packet types. We evaluatedthe differences of having the master or the slave node actingas the data source. When we select the more efficient DHdata packets, throughput always increases with respect tousing DM packets. At 6 m in active mode, DH packetsoutperform DM ones by 60%, 45% and 35% for 1, 3, and5 slot packets, respectively. When the use of the sniff mode isselected the behavior is quite similar, though the throughputdecreases by about 20%. With respect to throughput, thereare no noticeable differences between the two selected casestudies, i.e., the master or the slave acting as the data source.

Finally, we observed that all the results obtained are below themaximum throughput stated in Bluetooth specification. TheBluetooth standard provides the following reference values:723.2kbit/s for DH5, 585.6kbit/s, for DH3 and 172.8kbit/sfor DH1; 477.8kbit/s for DM5, 387.2kbit/s, for DM3 and108.8kbit/s for DM1. A master in active mode gets a 78%,79%, and 93% of the maximum throughput provided in thestandard for the DH5,3,1 packets respectively. When usingthe more conservative DM5,3,1 packets, these percentagesare of 85%, 86%, and 90%. These results confirm that idealconditions can not always be achieved due to distance andnoise; hence, bandwidth limitations and fluctuation should beconsidered.

4 Conclusions

In this paper we investigate the power characteristics of theBluetooth technology when supporting low-power modes.We provide accurate current consumption measurements fordifferent operating modes of the Bluetooth technology, andprovide useful information to drive technical decisions onbattery type for Bluetooth-based end systems. We findthat the low-power modes stated in the standard couldsignificantly alleviate the power consumption of Bluetoothnodes in active mode. We observed that Bluetooth offersa relatively steady packet delay and throughput up to 10meters, independently of the selected packet type. We alsoobserved that the use of the sniff mode could be quitecompatible with the more efficient multi slot data packets.With respect to throughput, we observed that a master nodein active mode gets a 78%, 79%, and 93% of the maximumthroughput provided in the standard for DH5,3,1 packets,respectively. When activating the sniff mode, the behavioris rather quite similar with respect to power consumption,though throughput decreases by about 20%.

References[1] Mark Weiser. The computer for the 21st century.Scientific American, (256):94–104, July 1991.[2] Promoter Members of Bluetooth SIG. Specification ofthe Bluetooth System - Core. Version 1.1. Bluetooth SIG, Inc.,February 2001.[3] J.C. Cano, J.M. Cano, E. Gonzalez, C. Calafate, andP. Manzoni. Power characterization of a bluetooth-basedwireless node for ubiquitous computing. In InternationalConference on Wireless and Mobile Communications, July2006.[4] Mitsumi Electric. Bluetooth Module WML-C11 Class1. http://www.mitsumi.co.jp/english/.