Schedule-Aware Power Management for Energy-Efficiency Improvement in 802.11u WLAN

doi:10.4236/cn.2013.53B2084

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Communications and Network, 2013, 5, 455-460

http://dx.doi.org/10.4236/cn.2013.53B2084 Published Online September 2013 (http://www.scirp.org/journal/cn)

Schedule-Aware Power Management for Energy-Efficiency

Improvement in 802.11u WLAN*

Di Zhang1, Qinghe Du1,2, Pinyi Ren1, L i Sun1

1School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an, China

2National Mobile Communications Research Laboratory of Southeast University, Nanjing, China

Email: [email protected], [email protected], [email protected], lisun@mail.xjtu.edu.cn

Received May 2013

ABSTRACT

Mobile stations supporting the 802.11u standard can access WLAN automatically when they are within the coverage of

the network service provided by this WLAN. To achieve this goal, the stations need to keep “on” states including idle

and active all the time. However, studies have noted that the idleness of stations often lead to considerable power con-

sumption. Although the conventional power saving mode (PSM) can provide energy saving effect to some extent, its

own disadvantage leads to lower energy efficiency when the number of stations accessing the target WLAN. In this

paper, we propose a Schedule-Aware P SM (S -PSM), which can improve the energy efficiency in 802.11u WLAN. Par-

ticularly, we use the Generic advertisement service (GAS) defined in 802.11u standard to broadcast the transmission

schedule information and all stations switch off their radios based on this information accordingly. We introduce the

Respond Contention Window to reduce the collision probability of competition channel. When there is no packet in the

access point (AP), AP broadcasts the GAS frame and actives th e Idle Timer. All stations will turn in to sleep and AP will

not send GAS frame until Idle Timer expires. Simulations have shown that our proposed scheme can significantly re-

duce power consumption compared with the conventional PSM.

Keywords: 802.11u Standard; Power Saving; Transmission Schedule; Generic Advertisement Service (GAS)

1. Introduction

The 802.11u standard [1] is an amendment of the IEEE

802.11 medium access control (MAC), which can help

users discover and select the network automatica lly. They

defined the Generic advertisement service (GAS) to pro-

vide functionality that enables stations to discover the

availability of information related to desired network ser-

vices. When a mobile station moves into a WLAN area,

the station can query and exchange information with other

external networks through GAS, and access the network

after and information authentication. Consequently, the

precondition of access is that the station’s wireless inter-

face keeps “on ” all the time. But this automatica lly access

may result in that mobile stations stay in idle state for a

long time. As mobile station is power ed by th e batter y, in

order to maximize the battery lifetime, we need an effec-

tive power management scheme for the stations to im-

prove energy utilization efficiency. 802.11u employs the

power saving mode (PSM), which was defined by IEEE

802.11 standard [2]. The PSM lets stations can spend time

in the sleep state for energy saving and switch to idle or

active state periodically to listen the information or re-

ceive the packets. This mechanism may solve the energy

wasting problem above, but the limitation of the PSM

scheme is that it doesn’t consider the impact of stations

number on energy efficiency. Because in the PSM, when

one station occupies the channel, others need to be in idle

state u ntil the chan nel free for th e next compet ition. There-

fore, most energy of each station are wasted in idle state.

In the last few years much research were dedicated to

improving the performance of the Power Saving Mode

[3-6]. In [3], the authors proposed a new AP-centric PSM

to let the AP chooses the best Beacon Interval (BI) and

Listen Interval (LI) for all clients based on their traffic

patterns. The authors of [4] proposed a dynamic wake up

period in which each client chooses its LI according to

the current round trip time of its TCP connection. The [5]

presents an analysis of the effect of intermittent connec-

tivity on minimizing energy consumption in PSM. Al-

The research reported in this paper was supported by the National

Natural Science Foundation of China under Grant No.

61102078, the

National Science and Technology Major Project under Grant No.

2013ZX03002003-

004, the Specialized Res earch Fund for the Doctor al

Program of Higher Education under Grant No. 20110201120014, the

Open Research Fund of National Mobile Communicat

ions Research

Laboratory, Southeast University (No.2011D10), and the Fundamental

Research Funds for the Central University.

D. ZHANG ET AL.

456

though these dynamic PSM compute the best system pa-

rameters for the clients in reducing energy consumption,

they do not consider that the time-driven scheme will re-

sult in that stations be in active when they know that they

have packets. In [6], the authors proposed a power-con-

serving algorithm, which dynamically switches off the

Network Interface Card of nodes when they are neither

transmitting nor receiving a packet in Mobile Ad-Hoc

Network. In [7], the authors proposed a protocol that using

a time-packet hybrid-driven method to dynamic switch-

off the wireless interface in ad-hoc network. These meas-

ures only focus on saving energy by enlarging the sleep

time and thus ignored the ineffective of the collision proba-

bility when completing the RTS/CTS dialogues. The higher

collision probability means more time used in retransmis-

sion which will surely waste energy and decrease the

system throughput. In this paper, under the 802.11u stan-

dard [2], we propose a new scheme called Schedule-

Aware Power S aving Mod e (S-PSM), which gets a better

energy utilization efficiency and reduces the collision

probability to some extent and gets an improvement in

system throughput.

The rest of this paper is organized as follows. Section

II presents the system model and the PSM in 802.11

standard. The design and implementation details of pro-

posed S-PSM scheme are given in Section III. Section IV

makes a performance analysis between these two schemes.

In Section V, we verify the two schemes’ energy saving

ability. Finally conclusions and future works are remarked

in Section VI.

2. System Model

We suppose that there are N wireless stations and an

access point (AP) in an infrastructure WLAN. Stations

communicate with the AP. We consider the downlink

traffic that AP buffers the incoming traffic packets. The

implementation of Power Saving Mode [8-10] allows the

station to be in one of the three power states: active, idle

or sleep. In the active or idle state, the station is fully

powered and is ready to receive the packets or listen to

the information at any time. During the sleep state, the

station is not able to transmit or receive for energy saving.

When a station works in PSM, it will listen to th e beacon

frame periodically. AP buffers the packets for the station

and announces the corresponding station every beacon

beginning via Traffic Indication Map (TIM) carried in

beacon frame, then the station sends PS-Poll frame to

retrieve the packets. Until all the packets are received, it

goes back to sleep.

Figure 1 illustrates an example of the power saving

mode in an IEEE 802.11 infrastructure network. In an

infrastructure WLAN, AP sends beacon frames to mes-

sage stations which have buffered packets at the begin-

ning of a beacon period, at th e same time, all the stations

Figure 1. Procedure of Conventional PSM scheme.

wake up and listen to the buffer information which is in

the TIM field. If the bitmap for one station is not set ( i.e.,

no buffered packe ts) in the TIM, the station goes back to

sleep immediately. Otherwise, the station sends a PS-Poll

frame to the AP by means of the standard distributed

coordinate function (DCF) procedure. Upon receiving a

PS-Poll, AP sends the destined packets and receives a

corresponding acknowledgement (ACK) frame from the

station. After this transmission, the station checks the

MORE DATA bit field in the beacon frame header to

know whether the received frame is the last one in this

beacon. If the field is set to zero, the frame is the last one

and therefore the station goes to sleep mode after receiv-

ing it. If the field is not zero, station sends another PS-

Poll frame to request the next packet until there is no

buffer packets during this beacon interval.

3. Design and Implementation of S-PSM

In this section, we proposed our Schedule-Aware Power

Saving Mode ( S-PSM). Compared with c onve ntional PSM,

the main difference of S-PSM is the message-driven me-

chanism and AP determines the transmission schedule for

stations. In the following, we will describe the S-PSM

scheme in detail.

3.1. Principle of the S-PSM Scheme

In our proposed S-PSM scheme, we let the stations switch

off their interface dynamically according to the schedule

information from AP. Stations may be in two operating

states: active state and sleep state. In the active state, they

receive the packets or listen to the information. During

the sleep state, the station is powered down for energy

saving. AP buffers the incoming packets for all stations

and informs them of buffer information. In our scheme,

we define one transmission process, during which every

destination station receives one packet successfully, called

Once Transmission Cycle. AP attaches the destination

station ID, transmission schedule and packets length to

the GAS frame, and broadcasts GAS frame to all stations.

If the information of station ID is not set for this station,

the station turns to sleep state immediately. The destination

stations stay in active state and AP determines the

trans m issi on s c hed ule by t he Fir st Com e F ir st Ser ve (FCFS)

D. ZHANG ET AL.

457

algorithm. Then AP announces a GAS frame to the active

stations making clock synchronization and notifies all

stations the tran smission schedule. These sta tions according

with the schedule switch off their interfaces when current

transmission is irrelevant to them. When there is no packet

for all the stations, AP attaches the sleep time in the

packet length field and sends to all the stations and opens

the Idle Timer. All stations will wake up and AP checks

the buffer when the Idle Timer expires.

3.2. Protocol Description of the S -PSM Scheme

In our scheme, AP attaches the transmission schedule

information in the GAS frame to inform stations buffer

information [11]. These information include destination

stations ID, transmission schedule, and packets length. In

our implementation, we add one extra information ele-

ment (using one of the reserved Element IDs) of the GAS

frame for this purpose. The format of the information

element is shown in Figure 2. We can use a field, which

is reserved in the GAS Initial Request/Respond frame

body format, to carry the relative information. Hence we

use 8 bits to contain the destination station ID and queue

length information respectively. The Table 1 is the de-

scription of S-PSM process.

The overall scheme of proposed S-PSM is shown in

Figure 3. In Figure 3, station S4 receives the GAS frame

and ﬁnds that there is no its packet in the buffer, then it

turns to sleep state immediately. S1, S2, S3 will be in ac-

tive state and enter the Respond Contention Window

(RCW). They send the GAS-Respond frames to inform

AP that they are ready for receiving the data by means of

DCF procedure. This basic access procedure (DCF) has

clearly speciﬁed in 802.11u standard. When one destina-

tion station delivers the respond information su ccessfully,

it will not attend the competition anymore and get out

Respond Contention Window. Finally, after all the des-

tination stations GAS -Respond frame have been received,

the Respond Contention Window closes. Then the trans-

mission process starts. AP determines the transmission

schedule under the First Come First Serve (FCFS) algo

Figure 2. Format of the GAS frame in S-PSM.

Figure 3. Procedure of S-PSM sch em e

Table 1. The Process of S-PSM.

S-PSM process

01.

checks the buffer;

02. IF Current Buffer Size > 0

03. AP achieves the destination stations ID;

04. AP sends GAS frame;

05. Other stations turn into sleep state;

06. Respond Content ion Wi ndow opens;

07. IF Sj sends the frame successfully;

08. Sj quits the compete process;

09. END

10. AP receives all stations’ GAS -Respond;

11. Respond Contention Window closes;

12. AP determine the transmission sequence(FCFS);

13. AP send GAS fra me ;

14. Begin transmission;

15. After transmission, turn to 1;

16. ELSEIF Current Buffer Size = 0

17. AP broadcasts GAS frame;

18. Idle Timer opens;

19. All stations switch off the interfaces;

20. AP not check buffer anymore;

21. IF Idle Time expires

22. All the stations wake up at the set time;

23. AP checks the buffer

24. Turn to 1;

25. END

26. END

rithm. In Figure 3, the transmission schedule is (S1, S3,

and S2). Then AP sends GAS frame, and stations ac-

cording with the schedule and the packets length infor-

mation to estimate the sleeping time as well as to wake

up to receive the pack ets. For each transmiss ion , AP sends

a GAS frame to make the clock synchronization. When

this transmission cycle ends, all the stations be in active

state and waiting the next GAS frame.

4. Performance Analyses

4.1. Energy Saving Efficiency Analysis

The PSM provides that station who works in PSM must

observe some provisions. First, when one station has

packets buffered in AP, this station must be active until

the last one is received during the entire beacon cycle.

Second, the station sends frame to AP by means of the

standard distributed coordinate function (DCF) procedure

[12-14]. So, when one station occupies the channel, other

stations will be in idle state until the channel becomes

free. In order to analyze the idle time of stations conve-

niently, we fail to consider the back off time and colli-

sion situation and neglect some secondary factors lik e the

transmission delay time. Let Ti is the transmission time

(including the DCF inter-frame space (DIFS), frame

transmission time t, and ACK) for one station receiving

the packets. We can write Ti as .

We assume that there are N stations in the network. The

number of possible transmission schedules is . Let

Mk be the kth schedule (). represents

the total idle time of N station in schedule. Then,

D. ZHANG ET AL.

458

the expected idle time E[t] can be expressed as

[]( )

ETI M

∑

(1)

according to the analysis results of references [13], we

can get

1 11

1( 1)!

[]( )

( 1)

N NN

k ii

ETI MiT

−

== =

−

= =

−

∑ ∑∑

∑

(2)

So, we can clearly see that the total idle time of sta-

tions in one random schedule increase as the number of

accessed stations. While considering our S-PSM, the prob-

lem of energy wasting in idleness is solved by utilizing

the transmission schedule to driven stations’ states. All

the stations switch off their interfaces by the transmission

schedule and the packets length information. During one

transmission cycle, stations turn to sleep state when the

transmission is irrelative to them. Then, we can obtain

the total idle time of stations in the schedule, which is

determined by FCFS algorithm:

[] 0ET =

(3)

In addition, the sleep time of jth station is

()

sleep i

i ij

Tj T

= ≠

∑

(4)

Therefore, with the increase of number of stations,

each station has more time to sleep. Hence, we can con-

clude that our S-PSM scheme has a better energy saving

effect than PSM.

Because of the massage-driven method, if there is no

packet in the buffer, AP will check the buffer frequently,

and stations will receive empty GAS frame and have

nothing to do. After we introduce the Idle Timer, AP will

not detect the buffer and all stations can turn to sleep

state for energy sa ving. Let Tc be the time of once trans-

mission cycle, then we can get

( 2)

cii

TT BOGASGAS

= +++

∑

(5)

where BOi is the back off time of ith stations, GAS is the

GAS frame space. The inter packet arrival time Tinter

takes on exponential distribu tions. We can get the proba-

bility of Tinter > Tc

( )1

1( |)

inter

tT T

p TTedt

λλ

−

−−

>=−

= −−=

∫

(6)

We consider the situation that there is no packets ar-

rive during this transmission time Tc. So, the probability

of this situation can be regarded as

() c

N cNT

zero interzero

PPpTT Pe

−

= >=

(7)

Where the Pzero represents the probability of that AP

transits all the buffered packets in a transmission. In or-

der to observe the effect of stations number N on the

probability of this situation, we consider the average

transmission time and back off time

() (21)

cavg avg

TN TBONGAS= + ++

(8)

Then, we can wri t e

[ ()(2 1)]

avg avg

N TBONNGAS

zero

PP e

−+ ++

(9)

We can see that with the increase of N, the value of P

will have a great decrease. So, this situation will not hap-

pen when more stations access the WLAN. Therefore, it

is necessary to introduce the Idle Timer for energy saving

when there are little stations in the WLAN. In the con-

sequently simulations, we can see the impact of Idle Ti-

mer on the S-PSM scheme.

4.2. Collision Probability Analysis

We know that the basic 802.11 MAC protocol Distri-

buted Coordination Function (DCF) is based on the Car-

rier Sense Multiple Access (CSMA). When more than

two stations whose back off time is zero detect the chan-

nel as free at the same time, a collision occurs. The initial

congestion window for all stations is CWmin, and the

back off time is BT (

min

0BT CW<<

). The probability of

BT is

min

1/p CW=

. In the conventional PSM, the prob-

ability of ith station access the channel successfully is

psmj j

PCpN MNi

=−≥≥ −+

∑

(10)

where M is the number of stations who involve in the

channel competition.

In our scheme, we introduce the respond contention

window (RCW) for stations competing the channel. Dur-

ing the window, when a station delivers the respond frame

successfully, it will not attend the competition any more.

The probability of ith station access the channel success-

fully is

Ni jj

i Ni

P Cp

−+

=− ∑

(11)

Because of the 0 < p < 1, we know that

() 1

M Ni

psmjjjj

i iMNi

M Ni

jjj jj

MM Ni

jNi j

P PCpCp

CpCCp MNi

M Ni

−+

= =

−+

= −+=

−= −

+−> −+



=

= −+



∑∑

(12)

D. ZHANG ET AL.

459

Therefore, we can get

psm

PP≥

(13)

From the Equation (13), we can see the respond Con-

tention window can reduce the probability of collision.

5. Simulation Evaluation

In this part, we have implemented the proposed S-PSM

scheme and make a comparison with the PSM. The si-

mulation environment and parameters are in Table 2. In

addition, stations need to switch-off interface frequently,

therefore we must consider the energy consumption of

switch off which is twice of active state. In the PSM si-

mulation process, we set the beacon interval (BI) and the

listen interval (LI) is 100 milliseconds. Besides that, the

minimal congestion window for all stations is 32 slots.

We design and implement the simulator based on MAT-

LAB. The set of energy consumption for each state is

reference d [5 ,15].

Figures 4 and 5 show the comparison results between

S-PSM and PSM. We can clearly find that our scheme

can overcome the defect of PSM that stations waste ener-

gy in idle state. As shown in the Figure 4, the time being

in idle state under PSM has a conspicuous increase as the

number of stations increase, and the resident time of

Table 2. Table type styles (Table capt ion is indispensable).

Simulation Parameters Values

Number of clients

Data Transmission rate

Packet size

Beacon interval

PS-Poll/ACK frame size

average inter-frame arrival time

Slot Time

SIFS

DIFS

Sleeping power

Active/Idle power

AP buffer size

2 to 14

11 Mbps

1500 bytes

100 ms

14 bytes

10 ms

20*10−6 s

10*10−6 s

50*10−6 s

50 mW

750 mW

2 GB

Figure 4. Comparison of Average Resident Time of each

state between PSM an d S-PSM.

sleep state has a significant drop. On the other hand, the

S-PSM scheme gets longer resident time of sleep state. In

our scheme, stations utilize the transmission sequence to

dynamically switch-off their interface when they have no

data to receive, consequently, stations will be in sleep

state to save energy. For the competition time, S-PSM is

less than PSM. In S-PSM, all stations only take part in

competition during the respond contention window, which

will effectively reduce the competition time. Figure 5

plots the throughput and energy consumption for one sta-

tion during this simulation process. We can see the S-

PSM has a preferable performance in throug hput and the

energy consumption has a significant downward. With

increase of station number, the average throughput is big-

ger than PSM. This is because that the collision probabil-

ity goes down by introduction the Respond Contention

Window. So, more time will be utilized to transfer the

packets. The energy consumption of S-PSM will be around

3Joule, which is about 80% lower than that of PSM. This

is in agreement with theoretical analysis that station in

S-PSM will has more time to sleep for energy saving.

Figure 6 shows the influence of Idle Timer on the

Figure 5. Comparison of Throughput and Energy Con-

sumption between PSM and S-PSM.

Figure 6. Impact of Idle Timer for S-PSM.

2 4 6810 1214

The Number of St ations

The average time for station at each state (s)

S-PSM sleep state

PSM sleep state

S -PSM com petition state

PSM competition state

PSM idle state

05 10 15

0.5

0.6

0.7

0.8

0.9

1.1

1.2

1.3

T he Number of Stati ons

Average Throughout for Each Station (Mbps)

S-PSM scheme

PS M scheme

05 10 15

T he Number of Stati ons

Average Energy Consumption for Each Station(J)

S-PSM scheme

PSM scheme

05 1015

The Number of St ations

Average Time for Station at Each State (s)

S-PSM sle e p time

S-PSM(without Idle

Timer) idle tim e

S-PSM(without Idle

Timer) slee p time

0510 15

The Number of St ations

Average Energy Consumption for one Station (J)

S-PSM

(without Idle Timer)

S-PSM

D. ZHANG ET AL.

460

performance of S-PSM. First, let’s observe the perfor-

mance of S-PSM without Idle Timer. From the simula-

tion result, we can see the station will spend some time in

idleness when M < 8. And the corresponding energy

consumption is very large. When AP has no buffered

data, AP will check the buffer frequently, and stations

will receive empty GAS frame and have nothing to do.

When the number of stations is little or network load is

low, this issue will appear frequently and cause the most

energy wasting, which is consistent with the performance

analysis of Equation (9). We introduce the Idle Timer

whose value is set as 50 slots to solve this problem.

When there are no packets buffered in AP, AP sends

GAS frame to tell all the stations turn to sleep state and

opens the idle timer. After the timer expires, all stations

switch to active state and AP checks buffer information

to begin the next transmission. We can see the energy

consumption remaining about 3Joule, which is a huge

energy saving improvement.

We have verified the two mechanism’s validity and

analyze their performances from three aspects (resident

time, system throughput, and energy consumption). We

observed that our proposed scheme has a better perfor-

mance in energy saving. By introducing the Idle Timer

and using message-driven scheme to let stations know

the transmission schedule, S-PSM overcomes the short-

age of the PSM and obtains a significant improvement in

terms of energy saving.

6. Conclusions

We have proposed the S-PSM that increases energy effi-

ciency of all wireless clients in an infrastructure network.

The AP in S-PSM determines the transmission sequence

and stations change their own states by the information

attached in GAS frames. AP plays a centralized role that

controls the transmission process. The stations convert

the state according to the transmission sequence which

can reduce unnecessary wake ups and maximize energy

saving. The Respond Contention Window (RCW) could

reduce collision probability effectively, which is helpful

to improve the system throughput, and the transmission

cycle assures fairness among stations. However, how the

AP communicates with sleep stations through broadcast

needs to be paid more attentions. We utilize the Idle Ti-

mer to solve this problem in our model, but cause unne-

cessary packets delay. Our needs to switch the interface

frequently, and that does harm to the interfaces. So, in

future work, we can improve S-PSM performance via

these aspects.

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