March ❘ April 2006 IT Pro 431520-9202/06/$20.00 © 2006 IEEE P u b l i s h e d b y t h e I E E E C o m p u t e r S o c i e t y

New 802.11hMechanisms CanReduce PowerConsumption

Daji Qiao and Sunghyun Choi

W ith the rise of portable computingand communication devices suchas laptops and PDAs, millions ofpeople worldwide have come to

rely on wireless networks for communication.Forindoor broadband wireless networking, the wire-less local area network,standardized as Std. IEEE802.11 WLAN, has become the dominant tech-nology. Known as hotspots, public WLANs havesprung up in conference venues, airport lounges,cafes, and coffee shops. Near the end of 2003, atotal of 31,580 hotspots served 1.53 million usersaround the world (“Public Wireless LANs:Hotspots—Finally Heating Up?” http://www.datamonitor.com, report no. DMTC0921, Sept.2003). By January 2006, the number had grown to100,355 hotspots in 115 countries (“Worldwide Wi-Fi Hotspots Hits the 100,000 Mark,” http://www.jiwire.com/press-100k-hotspots.htm).

The original 802.11 standard—which the IEEE802.11 working group began in1991, first published in 1997, andrevised in 1999—specifies the pro-tocols for a WLAN’s mediumaccess control (MAC) sublayer andphysical (PHY) layer. During thepast few years, the working grouphas focused on standardizing new

specifications to enhance WLAN performance.Figure 1 shows relationships among the existingand emerging specifications; the arrows indicateenhancements springing from established stan-dards (“Overview of Emerging IEEE 802.11Protocols for MAC and Above,” Sunghyun Choi,SK Telecom—Telecommunications Rev., Nov.2003). For example, 802.11e is the quality-of-service amendment to 802.11 MAC.The “HotspotStandard Rundown” sidebar lists descriptions(and publication details, when available) for thestandards in Figure 1.

The emerging standard 802.11h, which involvesamendments to both MAC and PHY, specifiesmechanisms for dynamic frequency selection(DFS) and transmit power control (TPC) forWLAN devices. Although the standard seeks tosatisfy European regulatory requirements, thetwo mechanisms have numerous other applica-tions that will be useful in all countries—auto-matic frequency planning, power consumptionreduction, range control, interference reduction,and quality-of-service enhancement.

IEEE 802.11H FOR SPECTRUM ANDTRANSMIT POWER MANAGEMENT

The original purpose of 802.11h was to extendWLAN operation in Europe to the 5-GHz band,

Originally developed to letWLANs share spectrum withradar and satellite systems,802.11h defines mechanismsthat can reduce both interference and power consumption.

Hotspot StandardRundown

Inside

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where WLAN devices need DFS and TPC to coexist withthe band’s primary users—radar and satellite systems.According to European regulations,when a WLAN devicein the 5-GHz band detects a radar signal on its currentoperational frequency channel, it must switch to anotherchannel. When a WLAN device detects a satellite signal,it must limit its transmit power to 3 dB below the regula-tory maximum (ordinarily, it could transmit at up to theregulatory maximum level). To meet these requirements,802.11h defines DFS and TPC mechanisms on top of 802.11MAC and 802.11a PHY.

Figure 2 diagrams the 802.11h layer management modelthat enables the DFS and TPC functions. Policy decisions,such as channel switching and TPC, reside in the stationmanagement entity (SME); the associated protocols residein the MAC sublayer management entity (MLME).Interactions between entities take place through serviceaccess points (SAPs), across which primitives are definedand exchanged.

Although 802.11h defines the mechanisms orprotocols for DFS and TPC, it does not cover theimplementations themselves. Both functionsinvolve implementation-dependent algorithms.So an 802.11h-compliant device needs a TPCalgorithm, for example, to correctly determine aframe transmission’s transmit power level.

Dynamic frequency selection

An 802.11 WLAN’s basic buildingblock is the basic service set, the BSS,which consists of a set of stations con-trolled by a single coordination func-tion and operating in the samefrequency channel. An 802.11a/hBSS occupies the 20-MHz channel:the US has 12 channels available for

802.11a, and Europe has 19. The WLAN usesDFS for dynamically switching a BSS from oneoperational frequency channel to another.Besides fulfilling regulatory requirements,a BSSmight have many other reasons to change itsoperational frequency channel. It might do so, forinstance, when the condition of its current chan-nel becomes poor because of interference fromneighboring devices. In this context, DFSenhances the WLAN’s quality of service.

In an infrastructure BSS, which consists of anaccess point (AP) and multiple stations associ-ated with it, the AP determines when to switchand which channel to switch to. For this purpose,the AP monitors the status of the current channel,and it might also request that other stations meas-ure and report the status of the current channeland those at other frequencies. There are three

types of measurement:

• basic measurement determines whether another BSS,a non-802.11 OFDM (orthogonal frequency-divisionmultiplexing) signal, an unidentified signal, or a radarsignal is using the measured channel;

• clear-channel assessment (CCA) measures the fractionalduration of the channel’s busy period during the totalmeasurement interval; and

• received power indication (RPI) measures the histogramof the quantized measures of the received energy powerlevels as seen at the antenna connector during the meas-urement interval.

Based on its own measurement and reports from the asso-ciated stations, the AP continues to monitor the channel sta-tus so that the BSS can switch channels at the properinstance. Channel switching occurs immediately before atarget beacon transmission time (TBTT), which the AP

■ 802.11 Working Group Web site, http://www.ieee802. org/11■ Main standard, 802.11, MAC and PHY for WLANs: IEEE

Std. 802.11-1999 (R2003), Part 11: Wireless LAN MediumAccess Control (MAC) and Physical Layer (PHY)Specifications

■ 802.11a, Orthogonal Frequency-DivisionMultiplexing (OFDM) in the 5 GHz Band:IEEE Std. 802.11a-1999 (R2003), Supple-ment to IEEE Std. 802.11-1999: High-Speed Physical Layer in the 5 GHz Band

■ 802.11b, Complementary Code Keying(CCK) in the 2.4 GHz Band: IEEE Std.802.11b-1999 (R2003), Supplement toIEEE Std. 802.11-1999: Higher-SpeedPhysical Layer Extension in the 2.4 GHz Band

■ 802.11e, Quality of Service: IEEE 802.11e-2005, Amend-ment 8 to IEEE Std. 802.11-1999: Medium Access Control(MAC) Quality of Service (QoS) Enhancements

■ 802.11g, Orthogonal Frequency-Division Multiplexing(OFDM) in the 2.4 GHz Band: IEEE Std. 802.11g-2003,Amendment 4 to IEEE Std. 802.11-1999: Further HigherData Rate Extension in the 2.4 GHz Band

■ 802.11h, DFS and TPC: IEEE Std. 802.11h-2003,Amendment 5 to IEEE Std. 802.11-1999: Spectrum andTransmit Power Management Extensions in the 5 GHz Bandin Europe

■ 802.11i, Security: IEEE Std. 802.11i-2004, Amendment 6 toIEEE Std. 802.11-1999: Medium Access Control (MAC)Security Enhancements

Hotspot Standard Rundown

March ❘ April 2006 IT Pro 45

specifies, so that normalcommunication can takeplace for the followingbeacon interval at thenew operational fre-quency channel. The APtransmits the beaconframes periodically. DFSalso defines a channel-quieting operation, be-cause European regula-tions require the stationto become silent once itdetects a radar system inits operational frequencychannel. We explain anexample DFS algorithmin a later section.

Transmit power controlFor wide-area cellular systems, such as IS-95 CDMA

(code-division multiple access) and 3G W-CDMA (third-generation wide-band CDMA), TPC has three critical purposes:

• ameliorating the near-far problem (if all transmissionsuse the same power, those closer to the receiver drownout the more distant ones)—specifically, for CDMAuplink transmissions;

• minimizing interference to and from other cells (cochan-nel interference); and

• improving system performance on fading channels bycompensating for fading dips.

Until recently,TPC has not been considered as critical tosuccess for 802.11 WLANs; most of today’s 802.11 devicesuse fixed transmit power for frame transmissions. In recentyears,however,because 802.11a devices must support TPCto meet European regulatory requirements, researchershave zeroed in on finding other applications of TPC in802.11 WLANs. Because most WLAN devices are batterypowered, the idea of using TPC to preserve battery energyand thus extend battery life is especially popular. In themulticell WLANs common in offices and public-accessenvironments,TPC can also reduce intercell interference,because it improves error performance in a given area.

Standard 802.11h specifies two TPC-related functions.First, the AP in an infrastructure BSS or a wireless stationin an independent BSS advertises the regulatory and localmaximum-power level for the current frequency channelin the Beacon and Probe Response frames. They do sousing Country and Power Constraint elements. The localmaximum specifies the actual maximum power levelallowed in the BSS, which is less than or equal to the regulatory maximum. The stations in the BSS can use

any transmit power less than or equal to the local maxi-mum value.

Second, 802.11h provides a transmit-power reportingmechanism. It defines a TPC Report element that containsTransmit Power and Link Margin fields. The TransmitPower field simply contains the transmit power used totransmit the frame containing the TPC Report element,while the Link Margin field contains the link margin, cal-culated as the ratio of the received signal strength (of thecorresponding TPC request frame) to the minimumdesired by the station.TPC Report elements are includedin TPC Report frames in response to TPC Request frames.In addition, the AP in an infrastructure BSS or a wirelessstation in an independent BSS autonomously includes aTPC Report element in any Beacon frame it transmits.

802.11 MAC

802.11 PHY

802.11e forquality of service

802.11b PHYCCK at 2.4 GHz

802.11g PHYOFDM at 2.4 GHz

802.11i forsecurity

802.11a PHYOFDM at 5 GHz

802.11h fordynamic frequency

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control

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Figure 1. IEEE 802.11 MAC/PHY and its enhancements.

Channel switchprotocol

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Figure 2. Layer management modelin 802.11h.

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Such a TPC Report element has a Link Margin field set tozero along with the frame’s transmit power information.

An immediate impact of such a transmit-power report-ing mechanism is that it makes energy-efficient frametransmissions feasible in 802.11h WLANs. In general, todetermine the proper transmit power level for a givenframe, a wireless station must estimate the link qualitybetween itself and the receiver.With 802.11h,a simple esti-mation scheme could work as follows. Whenever a wire-less station receives a frame including the TPC Reportelement, it can use the transmit power information in thatelement and the receiver signal strength informationobtained via RSSI (receiver signal strength indicator) toestimate link quality (in terms of path loss) from the send-ing station to itself by performing a simple subtraction.

AN EXAMPLE DFS ALGORITHMIn general, a DFS algorithm for an infrastructure-based

802.11h system consists of the following two phases: startupand regular.At startup, the AP performs a full DFS meas-urement on all frequency channels. Based on the meas-urement results, the AP selects a starting frequencychannel that is unoccupied by the primary users for its BSS.Moreover, the AP tries to avoid selecting the frequencychannels that are already used by other secondary users—other operating 802.11a/h systems.

Once the algorithm chooses the starting frequency, theAP begins broadcasting Beacon frames so that the wirelessstations can detect its presence and associate themselveswith the AP. The BSS then starts and enters the normal-operation state of the regular phase. In the regular phase,the DFS algorithm is better described with the finite-statemachine shown in Figure 3 (IEEE 802.15-01/072, “Liaison

Statement on the Compatibility between IEEE 802.11aand Radars in the Radio Location and Radio NavigationService in the 5250-5350 MHz and 5470-5725 MHz Bands,”IEEE 802.15 Working Group, Jan., 2001); it has four dif-ferent states:normal operation, channel DFS test, full DFStest, and frequency change.

The BSS remains in normal operation until either themeasurement timer expires or the link quality degrades,which suggests possible interference. If the timer expires,the state changes to channel DFS test, during which theAP reassesses the current operating-frequency channel. Ifthe channel status is good enough to continue operation,the state reverts to normal operation;otherwise, it changesto full DFS test, during which the AP performs a full DFSmeasurement on all frequency channels. During normaloperation, if the link quality degrades, state changesdirectly to full DFS test.

In full DFS test, the AP measures (or asks wireless sta-tions to measure) all the frequency channels.Then, basedon the measurement results, the AP makes the channelswitching decision. If the current frequency channel is thebest in terms of link quality, the BSS continues operatingon the current channel, and state reverts to normal oper-ation. Otherwise, the state changes to frequency change,in which the AP wakes up all the sleeping wireless stations,if any, announces what the new frequency channel is, andwhen operation in the new frequency channel will start.Finally, after the frequency channel switches successfully,the state returns to normal operation.

MISER: AN INTELLIGENT TPC MECHANISMBecause an 802.11h wireless station can use the transmit-

power reporting mechanism to estimate link qualitybetween itself and the receiver, it can also apply intelligentTPC to minimize communication energy consumption.So,based on the estimation scheme, we have developed anoptimal low-energy transmission strategy for 802.11hcalled MiSer (“MiSer: An Optimal Low-EnergyTransmission Strategy for IEEE 802.11a/h,”Daji Qiao andcolleagues, Proc. ACM MobiCom 03, ACM Press, 2003,pp. 161-175).

MiSer challengeAccording to the 802.11 standard, a WLAN device can

operate in four modes: transmit, receive, idle, and doze. Itconsumes the most power in transmit mode and very littleenergy in doze mode. In idle mode, a WLAN device mustsense the medium, so it consumes a similar amount ofpower as when it is in receive mode.Researchers have pro-posed numerous power management policies to send aWLAN device into doze mode adaptively to save batteryenergy (“Dynamic Power Management for PortableSystems,” Tajana Simunic and colleagues, Proc. ACMMobiCom 00,ACM Press, 2000, pp. 11-19).An alternativeway to conserve energy is to apply TPC, which lets a

ChannelDFS test

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Own channel not OK

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Figure 3. Finite-state machine for a DFS algorithm.

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WLAN device use the mini-mum-required power level intransmit mode. Thus TPC cancomplement other power-man-agement policies.

MiSer designBecause 802.11 DCF (distrib-

uted coordination function) sys-tems necessarily involve conten-tion, MiSer’s effectiveness relieson the condition that applyingTPC to data transmissions willnot aggravate the hidden-nodesproblem or interference in thenetwork.The hidden-nodes prob-lem occurs when a wireless nodecannot hear from one or more ofthe other nodes so that multiplenodes might attempt to transmitsimultaneously, causing framecollisions and signal interferencewith each other.To deal with suchproblems,MiSer exchanges RTS/CTS (request to send/clear tosend) frames before each datatransmission attempt. More im-portantly, it transmits the CTSframes at a stronger power level,which not only allows dataframes to be transmitted atappropriate transmission ratesand transmit power levels to saveenergy, but also ameliorates thepotentially aggravated interfer-ence that TPC might cause.

MiSer uses a simple, table-based approach to determinethe most energy-efficient combination of transmission rateand transmit power for each data frame.The basic idea isthat,offline,a wireless station computes a rate-power com-bination table indexed by the data transmission status;each table entry is the optimal rate-power combination inthe sense of maximizing energy efficiency under the cor-responding data transmission status. MiSer characterizesdata transmission status as a quadruplet of data payloadlength, path loss between the transmitter and the receiver,and two frame retries. It defines energy efficiency as theratio of the expected delivered data payload to theexpected total energy consumption.At runtime, the wire-less station estimates the path loss between itself and thereceiver, updates the data transmission status, and thenselects the proper transmit rate and power for the currentdata transmission attempt through a simple table lookup.

Figure 4 shows a snapshot of MiSer’s rate-power com-bination table for a certain data payload length and fixed

numbers of remaining transmission attempts.The two partsof the figure show the transmit rate and power that achievethe most energy-efficient data communications, under dif-ferent path loss conditions. For example, when the pathloss is 80 dB, the most energy-efficient strategy is to trans-mit at 54 Mbps with 9 dBm power.Other snapshots resem-ble shifted versions of Figure 4. For example, when a dataframe carries a larger payload or fewer transmissionattempts remain for a data frame, the graphs shift left anda more conservative combination—lower rate, higherpower, or both—becomes optimal under the same pathloss condition.

MiSer performanceSimulation results show MiSer’s clear superiority to

the two-way Data-Acknowledgment frame exchangemechanisms in the presence of hidden nodes. In addition,compared with other four-way RTS-CTS-Data-Acknowledgment frame exchange mechanisms, MiSer

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Figure 4. Snapshot of MiSer’s rate-power combination table.

delivers about 20 percent more data per unit of energyconsumption than a scheme using the transmission rateadaptation alone without TPC. It also significantly out-performs single-rate TPC schemes, thanks to the excellentenergy-saving capability of transmission rate adaptation.

A s happens with any popular technology, along withWLAN’s worldwide success has come a clamor forthe technology’s evolution. Among the past few

years’ remarkable enhancements to 802.11, the spectrumand transmit power management tools of 802.11h providea particularly fertile bed for innovation. MiSer, our simple

application example for saving significant communicationenergy for 802.11h devices, is but one of many possibleexciting new directions for 802.11 technologies. ■

Daji Qiao is an assistant professor at Iowa State Univer-sity. Contact him at daji@iastate.edu.

Sunghyun Choi is an assistant professor at Seoul NationalUniversity. Contact him at schoi@snu.ac.kr.

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