A power-saving scheduling for infrastructure-mode 802.11
wireless LANs
q
Hsiao-Po Lin, Shih-Chang Huang, Rong-Hong Jan
*Department of Computer and Information Science, National Chiao Tung University, Hsinchu 30050, Taiwan Available online 20 March 2006
Abstract
This paper presents a novel method to arrange wakeup schedule for sleeping stations such that the number of wakeup stations in each beacon interval is balanced in IEEE 802.11 wireless local area networks (WLANs). This method reduces the probability of collision and thus the station can save more power. Next, we consider how to poll the wakeup stations to send the PS-Poll frame to get back their buffered data so that the contention can be avoided. Three different access scheduling mechanisms are proposed for the contention avoid-ance. In the first mechanism, only one of wakeup stations is scheduled to access the buffered data. The second and third mechanisms based on the smallest association ID (AID) first and the smallest queue length first, respectively, arrange a subset of wakeup stations to get back their buffered data within a beacon interval. Simulation results show that the proposed methods are effective in the power-saving.
Ó 2006 Elsevier B.V. All rights reserved.
Keywords: Wireless LAN; Infrastructure mode; Power saving; Traffic scheduling
1. Introduction
Recently, due to the technology explosion in wireless communication (e.g., Bluetooth, IEEE 802.11, GSM/ GPRS, and WCDMA) and portable communication devic-es (e.g., notebook PCs, personal digital assistants, and smart phones), it has become possible for people to connect to the Internet anytime and anywhere, and remain on-line while roaming. Among these wireless communication
tech-niques, IEEE 802.11 wireless local area networks
(WLANs)[1–3]are the most widely-used local wireless
net-work system in schools, offices, airports, etc. Besides, almost portable devices can be equipped to access IEEE 802.11 WLANs. However, the energy sources of these
portable devices come from their equipped batteries. Once the battery has run down, it needs to be recharged and por-table device suffers a loss of network connectivity. Thus, the power-saving problem becomes an important issue
for prolonging the operation of a portable device[4].
For IEEE 802.11 WLANs, a common method for power-saving is to powering down the transceiver. When transceiver is off, we say the mobile station is in sleeping. A listen-interval is a period of time for which the mobile station may choose to sleep. Power conservation in IEEE 802.11 can be achieved by maximizing the listen-interval. However, longer listen-intervals increase the transmission
delay. In [5] and [6], they proposed listen-interval
adapta-tion mechanisms for power-saving in which the mobile sta-tion dynamically adapts the durasta-tion of listen-interval according to the traffic situation. Thus, if the traffic load is light, the mobile device can set a longer listen-interval to save more power. Another kind of listen interval
con-trolling is the quorum based scheme [7,8]. In the quorum
based scheme, each station in power-saving mode synchro-nizes its wakeup schedule with each other such that the 0140-3664/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.comcom.2006.01.029 q
This research was supported in part by the National Science Council, Taiwan, under Grant NSC 94-2219-E-009-005 and NSC94-2752-E-009-005-PAE, in part by the communication software technology of III, Taiwan, and in part by the Intel.
*
Corresponding author. Fax: +886 3 5721490. E-mail address:[email protected](R.-H. Jan).
station can deliver buffered frames to a power-saving sta-tion at right time when the radio of power-saving stasta-tion is turned on.
Controlling the transmission power is another way to
save mobile devices power [6]. The basic idea is using the
least power to transmit data. In[6], an adaptive
transmis-sion power mechanism is proposed in which the access point (AP) computes the optimal transmission power for each associated mobile station based on the received signal and informs the mobile devices their optimal transmission power. Then, the mobile station can adjust its transmission power to the optimal value to saving power.
Frame aggregation can also be used to save power. There are two kinds of frame aggregation. One is
combin-ing multiple relative frames into an aggregated frame [9].
The other is compressing the payload to reduce the amount of transmission data.
Another interesting way to saving power is using a low
power radio module, such as Bluetooth [10], or a special
signaling channel[11], to actuate the normal IEEE 802.11
radio circuit. In[10], the Bluetooth module is used for
sig-naling and IEEE 802.11 for data transmission. This method indeed can save mobile station power but the cost
is that it needs an extra hardware. The method in [11] is
similar to that in[10]except the low power radio module.
TDMA-based (time-division multiple-access) approaches
[12,13]can also assist mobile stations to conserve power by scheduling channel access in advance so that mobile stations can turn their transceivers off when it is not their turn to
transmit or receive. In [12], the access point periodically
broadcasts a schedule frame containing start time and duration of time slot for each mobile station. The mobile station may send or receive frames only during its time slot and outside its time slots, the mobile station can turn its
transceiver off to save power. In [13], they only focus on
the point coordination function (PCF). When the PCF is used, time on the medium is divided into the contention free period (CFP) and the contention period (CP). In a CP, the point coordinator learns what traffic needs to be transmitted
and then directs its transmissions in a CFP. In [14], they
propose a power-saving algorithm that incorporates a contention-free scheduling function for data transmission in 802.11 ad hoc networks.
This paper focuses on the power management problem for an infrastructure mode IEEE 802.11 WLAN. We pres-ent a novel method to arrange wakeup schedule for sleep-ing stations such that the number of wakeup stations in each beacon interval is balanced in IEEE 802.11 WLANs. This method can reduce the probability of collision and thus the station saves more power. Next, we consider how to poll the wakeup stations to send the PS-Poll frame to get back their buffered data so that the contention can be avoided. Three different access scheduling mechanisms are proposed for the contention avoidance. In the first mecha-nism, only one of wakeup stations is scheduled to access the buffered data. The second and third mechanisms sche-dule a subset of wakeup stations to retrieve their buffered data within a beacon interval. The access sequences of the second and third mechanisms are based on the smallest association ID (AID) first and the smallest queue length first, respectively.
The rest of this paper is organized as follows. Section 2
reviews the operation of power saving mode in IEEE 802.11 WLANs. A wakeup scheduling problem is
consid-ered in Section 3 and three contention avoidance
mecha-nisms for polling wakeup stations are presented in section
4. In Section5, we give the simulation results to show the
effectiveness of our proposed methods. Finally, the
conclu-sion and possible future research are given in Section6.
2. Power management in 802.11 WLANs
In infrastructure mode IEEE 802.11 WLANs, a mobile station can power down its transceiver and enter the power-saving mode (PS mode) for conserving power. The station can communicate its power management state to its AP. Thus, an AP knows the power management state of every station that has associated with it. When a frame arrives, the AP can determine whether the frame should be delivered to wireless network because the station is awake or buffered because the station is in PS mode.
After buffering frames, the next job for AP is to announce periodically which stations have frames waiting for them. That is, AP periodically broadcasts beacon frames with a traffic indication map (TIM) to its service stations. The TIM is a virtual bitmap in which each bit
Data TIM (in Beacon ) TIM (in Beacon )
TIM (in Beacon )
PS -Poll AP
STA
ACK Beacon interval
corresponds to a particular AID. When a station has asso-ciated to an AP, it receives an AID from the AP. The AP sets the bit in TIM if it has buffered frames for the station with AID corresponding to the bit position.
Mobile stations in power saving mode have to wake up to listen for beacon frames and check the TIM. By this way, a mobile station can determines whether the AP has buffered frames for it. If the AP seldom buffers frames for the station, the station does not require waking up to check every beacon frame. Instead, it wakes up every listen-interval to check the beacon frame. A listen-interval is a number of beacon inter-val for which the mobile station may choose to sleep. If the station finds that the AP has buffered data for it, it will send a PS-Poll control frame to retrieve the buffered frames. When multiple stations have buffered frames, all stations with buf-fered frame contend the medium for sending PS-Poll. After sending the PS-Poll, a station has to awake until the buffered frames are received or the bit in the TIM corresponding its AID is no longer set.
For example, as shown in Fig. 1, a station, denoted as
STA, is wakeup in the first beacon interval and receives the beacon frame in which the TIM indicates buffered data for it. Then, STA contends the medium for sending a PS-Poll frame to inform the AP that it is wakeup and ready to get back the buffered data. After AP receives the PS-Poll, it transmits a buffered frame to the STA. The STA returns an ACK frame to inform AP that the frame is received completely.
3. Load-aware wakeup scheduling
Consider a wireless LAN having an AP and six sleeping stations, A, B, C, D, E, and F. The listen-intervals of sta-tions, A, B, C, D, E, and F are 1, 2, 3, 6, 6, and 6, respec-tively. Let wi(t) be an indication bit where wi(t) = 1 if the
station i wakes up at beacon interval t; wi(t) = 0, otherwise.
Let n(t) denote the total number of stations waking up at beacon interval t. Then, n (t) can be found by
Table 1 A sequence of wi(t) and n(t) t 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 wA(t) 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 wB(t) 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 wC(t) 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 wD(t) 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 wE(t) 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 wF(t) 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1 n(t) 3 2 1 3 2 3 3 2 1 3 2 3 3 2 1 3 2 3 Table 2
A sequence of wi(t) and n(t) after station J joins in power saving mode
t 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 wA(t) 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 wB(t) 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 wC(t) 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 wD(t) 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 wE(t) 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 wF(t) 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1 wJ(t) – – – 0 0 1 0 0 1 0 0 1 0 0 1 0 0 1 n(t) 3 2 1 3 2 4 3 2 2 3 2 4 3 2 2 3 2 4 Table 3
A sequence of wi(t) and n(t) for station J with first wakeup time t = 5
t 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 wA(t) 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 wB(t) 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 wC(t) 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 wD(t) 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 wE(t) 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 wF(t) 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1 wJ(t) – – – 0 1 0 0 1 0 0 1 0 0 1 0 0 1 0 n(t) 3 2 1 3 3 3 3 3 1 3 3 3 3 3 1 3 3 3
nðtÞ ¼X
i2S
wiðtÞ
where set S includes all sleeping stations.Table 1shows a
sequence of wi(t) for each station and total number of
wakeup stations n (t), t = 1,2, . . ., 18.
Assume that a station J enters the power saving mode at
beacon interval 3 and its listen-interval is 3.Table 2shows
a sequence of wi(t) and n(t), t = 1, 2, . . ., 18 after station J
joins in power saving mode. Note that there are 4 stations that will wake up at beacon intervals 6, 12, and 18. If these stations (i.e., stations, A, B, F, and J) find that the AP has buffered data for them at beacon intervals 6, 12, or 18, they may suffer the collisions for sending PS-Poll to retrieve the buffered data. Note that collision causes retransmission and thus the station consumes more power for retrieving the buffered data. If we can arrange the first wakeup time for station J, the maximum of n (t) can be reduced. This can save mobile stations’ power. For example, if we can schedule the first wakeup time for station J at beacon inter-val 5, the maximum number of contending stations will be
reduced to 3 (seeTable 3).
In order to trace the wakeup time of each station, AP
needs to maintain a wakeup counter, denoted as ci(t), for
each sleeping station i. The ci(t) indicates remaining beacon
intervals that station i will wake up. Initially, AP sets
ci(t) = ‘i 1 for station i whenever station i enters the
sleeping mode where ‘i is the listen-interval of station i.
The counter ci(t) will be decreased by one after beacon
frame is transmitted. A station i wakes up if its ci(t)
becomes zero. After that, the counter will reset to
ci(t) = ‘i 1 for further counting down. Thus, after
trans-mitting a beacon frame, AP sets counter ci(t + 1),i2 S, for
beacon interval t + 1 as follows: ciðt þ 1Þ ¼
ciðtÞ 1; if ciðtÞ 6¼ 0
‘i 1; if ciðtÞ ¼ 0
In general, if a mobile station i has wakeup counter ci(t)
and listen-interval ‘i at beacon interval t, AP can find
wi(t + k) for beacon interval t + k by
wiðt þ kÞ ¼
1 if k mod ‘i¼ ciðtÞ
0 otherwise
where k = 1, 2, . . .. Then, n(t + k) can be found by
comput-ing nðt þ kÞ ¼Pi2Swiðt þ kÞ for k = 1, 2, . . . where S
in-cludes all sleeping stations.
The wakeup scheduling problem (WSP) considered in this paper can be stated formally as follows: given a set of sleeping station S at beacon interval t consisting m stations and each of the station i having wakeup
counter ci(t) and ‘i, for a new sleeping station j assign
an initial value to its cj(t) such that the maximum value
of nðt þ kÞ ¼Pi2S[fjgwiðt þ kÞ; k ¼ 1; 2; . . . is minimized.
By observing the sequence of n (t) in Table 1, we find
that a pattern repeats every six beacon intervals, e.g., (n(1), n(2), . . ., n(6)) = (n(7), n(8), . . ., n(12)) = (n(13), n(14), . . ., n(18)) = (3, 2, 1, 3, 2, 3). The length of this repeating pat-tern, r, can be found by computing the least common
multi-ple (lcm) of listen-interval ‘i,i2 S. For example, the
AP Queue A Queue B Queue C
Suppose there are 1, 1, 1, 1 packets send to station A, B, C, D in each beacon interval
STA A LI = 2 STA B LI = 2 STA C LI = 3 STA D LI = 1 Queue D pA = 2+0 = 2 pC = 3+0 = 3 pD = 1+0 = 1 pB = 2+0 = 2 pD = 1+1 = 2 pD = 1+2 = 3 pA = 2+1 = 3 pB = 2+0 = 2 pB = 2+1 = 3 pA = 2+0 = 2 pD= 1+3 = 4 pD = 1+0 = 1 pD = 1+1 = 2 pC = 3+0 = 3 t t + 1 t + 2 t + 3 t + 4 t + 5
The time in awake state but cannot receive data
The new coming packets
The old packets buffered in the AP The station ramps up to listen
listen-intervals of stations, A, B, C, D, E, and F are 1, 2, 3, 6,
6, and 6, respectively, inTable 1. Then
r¼ lcmf1; 2; 3; 6; 6; 6g ¼ 6
Thus, n*= max{n(t + 1), n(t + 2),. . .,n(t + r)} = max{n(t + k)j k = 1, 2, . . .}. Now, we want to add a new sleeping station j with listen-interval ‘jto the sleeping set S with repeating pattern
size r and assign an initial value to cj(t). A stepwise solving
method for WSP problem is given as follows.
1. Find r = lcm{‘j, r} and a sequence of total number of
wakeup stations (n(t + 1), n(t + 2), . . ., n(t + r)) for the
first r intervals for sleeping station set S[ {j}.
2. For i = ‘j 1, . . ., 1, 0, perform the following operations:
(a) Set cj(t) = i and find (wj(t + 1),wj(t + 2), . . .,
wj(t + r));
(b) Set (n(t + 1),n(t + 2), . . ., n(t + r)) = (n(t + 1), n(t + 2), . . ., n(t + r)) + (wj(t + 1), wj(t + 2), . . ., wj(t + r));
(c) Find ni= max{n(t + 1), n(t + 2), . . ., n(t + r)}.
3. Find n*= min{niji = ‘j 1, ‘j 2, . . ., 0}, say n*= nk,
and thus set cj(t) = k.
For example, six stations with r = 6 as given inTable 1,
station J with ‘J= 3 enters the sleeping mode. The AP
applies the above solving method to determine the initial
value of counter cJ(t) for station J as follows.
1. r = lcm{3,6} = 6 and (n(t + 1), n(t + 2), . . ., n(t + 6)) = (3,2,1,3,2,3)
2. i = 2:
(a) Set cj(t) = 2 and find (wj(t + 1),
wj(t + 2), . . ., wj(t + 6)) = (0,0,1,0,0,1);
(b) Set (n(t + 1),n(t + 2), . . ., n(t + 6)) = (3,2,1,3,2,3) + (0,0,1,0,0,1) = (3,2,2,3,2,4);
(c) Find n2= max{3,2,2,3,2,4} = 4.
i = 1:
(a) Set cj(t) = 1 and find (wj(t + 1), wj(t + 2), . . .,
wj(t + 6)) = (0,1,0,0,1,0);
(b) Set (n(t + 1), n(t + 2), . . ., n(t + 6)) = (3,2,1,3,2,3) + (0,1,0,0,1,0) = (3,3,1,3,3,3);
(c) Find n1= max{3,3,1,3,3,3} = 3.
i = 0:
(a) Set cj(t) = 0 and find (wj(t + 1), wj(t + 2), . . .,
wj(t + 6)) = (1,0,0,1,0,0);
(b) Set (n(t + 1), n(t + 2), . . ., n(t + 6)) = (3,2,1,3,2,3) + (1,0,0,1,0,0) = (4,2,1,4,2,3);
(c) Find n0= max{4,2,1,4,2,3} = 4.
3. Find n*= min{4,3,4} = 3, i.e., n*= n1, and thus set
cj(t) = 1.
Note that according to IEEE 802.11 standard, if mobile station j has no data to send, it can send a Null data frame with Power Management bit set to AP. The AP begins buffering frames and sends an ACK frame to the station after receiving the Null data frame. We can just modify this step to incorporate our wakeup scheduling in IEEE 802.11 standard as follows:
The AP begins buffering frames, determines cj(t) and
sends an ACK frame with cj(t) value to station j after
receiving the Null data frame. Then, the station j sets
AP Queue A Queue B Queue C
Suppose there are 2, 2, 1, 2 packets send to station A, B, C, D in each beacon interval
STA A(1) LI = 2 STA B(2) LI = 1 STA C(3) LI = 3 STA D(4) LI = 2 Queue D pA = 2+0 = 2 pA = 2+0 = 2 pA = 2+0 = 2 pB = 1+0 = 1 pB = 1+0 = 1 pB = 1+0 = 1 pB = 1+1 = 2 pB = 1+0 = 1 pB = 1+0 = 1 pC = 3+0 = 3 pC = 3+0 = 3 pD = 2+0 = 2 pD = 2+0 = 2 pD = 2+0 = 2 t t + 1 t + 2 t + 3 t + 4 t + 5
The time in awake state but cannot receive data
The new coming packets
The old packets buffered in the AP The station ramps up to listen
its wakeup counter to cj(t) and enters the sleeping
mode.
4. Contention avoidance traffic scheduling
In the previous section, we arrange stations’ wakeup times so that the number of wakeup stations is balanced. In this section, we consider how to inform stations that frames are buffered such that the contention is avoided. Three different access scheduling mechanisms are proposed for the contention avoidance problem. In the first mecha-nism, only one wakeup station is scheduled to access the buffered data in a beacon interval by marking one bit in TIM. The second and third mechanisms schedule multiple wakeup stations to get back their buffered data within a beacon interval. The access sequence within the beacon interval is according to their AIDs and the length of queu-ing data.
4.1. Multiple wakeups single access
One of simple ways to avoid contention is that we only choose a station to inform it that AP has its buffered frames at each beacon interval. So there is no contention problem of sending PS-Poll frame to get back its buffered
data. Let Sw(t) be the set including all stations waking up
at beacon interval t. That is, SwðtÞ ¼ fiji 2 S; ciðtÞ ¼ 0g
where S is the set including all sleeping stations. Let Sb(t)
be the set including all stations that frames are buffered in AP at beacon interval t. Thus, we can choose a station, say station v, from set Sw(t)\ Sb(t) with a largest
listen-in-terval ‘vto inform that the AP has buffered frames for it.
It is possible that there may exist some stations with small listen interval in set Sw(t)\ Sb(t) and they are never
chosen by AP. To avoid such a case, we associate each sta-tion v in Sw(t)\ Sb(t) with an age, denoted as av. Initially,
the age of each station is set to zero. For each beacon inter-val, if a station in set Sw(t)\ Sb(t) is not selected to inform,
AP increases its age by one; otherwise, AP sets its age to zero. Thus, AP can choose a station, say station v, from set Sw(t)\ Sb(t) with a largest value of ‘v+ avto inform.
Fig. 2 shows an example of this mechanism. Consider that there are four stations, A, B, C, and D, with listen-in-terval (‘A, ‘B, ‘C, ‘D) = (2, 2, 3, 1). Packet arrival rate of
each station is one frame per beacon interval. In beacon interval t, stations A, C, and D wake up in which station
C, has maximum pC = ‘C+ aC= 3 + 0, is indicated in
TIM to inform it that the AP has buffered its data. Stations A and D are deferred to their next wakeup beacon
intervals. The AP sets ages aA= aA+ 1 and aD= aD+ 1.
In the beacon interval t + 1, stations B and D wake up.
Because ‘B+ aB= ‘C+ aC= 2, the AP selects station B,
arbitrarily, to inform it has buffered frames. Similarly, sta-tion A is chosen to inform in beacon interval t + 2. At bea-con interval t + 3, ‘B+ aB< ‘C+ aC< ‘D+ aD and thus
station D is chosen to inform.
4.2. Multiple wakeups multiple accesses
Although the multiple wakeups single access mechanism avoids the contention among stations, it may lower the bandwidth utilization and increase the transmission delay. However, the AP knows how many frames it has buffered in queue, transmission rate and the length of beacon interval. Thus, the AP can determine how many frames it can transmit in a beacon interval and schedule the buffered
AP Queue A Queue B Queue C
Suppose there are 2, 2, 1 packets send to station A, B, C in each beacon interval
STA A LI = 2 STA B LI = 1 STA C LI = 2
The time in awake state but cannot receive data
The new coming packets
The old packets buffered in the AP p A= 2+0 = 2 pB = 1+0 = 1 pC = 2+0 = 2 p A = 2+0 = 2 p A = 2+0 = 2 pB = 1+0 = 1 pB = 1+0 = 1 pC = 2+0 = 2 pC = 2+0 = 2 t t + 1 t + 2 t + 3 t + 4 t + 5
The station ramps up to listen pB = 1+0 = 1 pB = 1+0 = 1 pB = 1+0 = 1
frame by means of announcing the TIM. In the following, we give two methods to arrange the access sequences of stations.
4.2.1. The smallest AID first
In order to control the traffic load in a beacon interval, AP selects a set of stations with an appropriate size from
Sw(t)\ Sb(t) to inform them to retrieve the data. That is
the total amount of buffered frames of selected stations should be less than the capacity of a listen interval. This can avoid a station that awakes within whole beacon inter-val but can not get back its buffered data. Next, we modify the power management scheme of 802.11 WLAN such that a station retrieves the buffered frame according to the sequence of AID marked in TIM. That is, the station with smallest AID among the selected stations sends PS-Poll frame to retrieve buffered data first.
Fig. 3shows an example of the AID sequence method. There are four stations, A, B, C, and D with listen interval (‘A,‘B, ‘C, ‘D) = (2, 1, 3, 2) with under the service of an AP.
Their corresponding AIDs are 1, 2, 3, and 4 for stations A,
B, C, and D, respectively. Suppose packet arrival rates of stations A, B, C, and D are 2, 2, 1, and 2 per beacon inter-val. The maximum size that AP can transfer to stations in a beacon interval is 8 frames. In beacon interval t, all of four stations wake up. Because the number of buffered frames is 2 + 2 + 1 + 2 = 7 (7 < 8), the AP marks AIDs 1, 2, 3, and 4 in the TIM. The stations check the TIM in beacon frame. They learn that 4 stations will send PS-Poll to retrieve their buffered frames and every station knows which station pre-cedes it in access sequence. For example, station C has to wait stations A and B finishing their access. In beacon
interval t + 2, Sw(t + 2)\ Sb(t + 2) = {A, B, D} and the
number of frames buffered for stations, A, B, and D is 10
(10>8). Thus, based on the values of pA and pD, the AP
selects stations A and D to inform them to retrieve the buf-fered data.
4.2.2. The smallest queue length first
Instead of the smallest AID first, the AP can arrange the access sequence for the selected stations according to their associated queue lengths. The station with smallest queue length receives a highest precedence and thus it can have a longer sleeping time. In this method, we need to add an information element, describes the access sequence, as a component of the beacon frame. The station checks this information element for the access sequence.
Fig. 4 shows an example of the smallest queue length first method. There are three stations, A, B, and C with lis-ten-interval (‘A, ‘B, ‘C) = (2, 1, 2) with under the service of
an AP. Suppose packet arrival rates of stations A, B, and C are 2, 2, and 1 frames per beacon interval. The maximum size that AP can transfer to stations in a beacon interval is assumed to be 8 frames. In beacon interval t, all of the three stations wake up. Because the number of buffered frames are 2 + 2 + 1 = 5 (5 < 8), the AP marks AIDs 1, 2, and 3 in the TIM and adds the access sequence C, A, Table 4
Detail simulation configurations
Data rates 11 Mbps
MAC header 28 bytes
IP header 20 bytes
UDP header 20 bytes
Beacon frame 28 bytes
ACK frame 14 bytes
PS-Poll frame 14 bytes
SIFS 0.00001 s DIFS 0.00005 s Slot time 0.00002 s Beacon interval 0.1 s 5 10 15 20 25 30 120 130 140 150 160 170 180 Number of Station(s) Sleep Time(second)
The Average Sleeping Time
802.11 PSM LAWS LAWS+MWSA LAWS+SAF LAWS+SQLF
and B in the beacon frame. In beacon interval t + 2, the access sequence is stations C, B, and A. Note that if two
stations have same queue length, AP uses their pv values
to break the tie.
5. Simulation and results
5.1. Performance metrics and environment setup
In this section, we show the performance analysis for the proposed schemes:
1. Load-aware wakeup scheduling (LAWS);
2. LAWS with multiple wakeups single access
(LAWS + MWSA);
3. LAWS with multiple wakeups multiple access and the smallest AID first (LAWS + SAF);
4. LAWS with multiple wakeups multiple access and the smallest queue length first (LAWS+SQLF).
Note that all four schemes are enhancements of the 802.11 PS mode. The LAWS arranges station’s wakeup time. The MWSA, SAF, and SQLF schemes can be used by AP to sche-dule the access sequence by marking the bits in TIM. We com-pare their performances again pure 802.11 PS mode by simulation. The performance metrics are given as follows:
5 10 15 20 25 30 1 2 3 4 5 6 7 8x 10 6 Number of Station(s) Throughput(bps)
The Throughput of Each Scheme
802.11 PSM LAWS LAWS+MWSA LAWS+SAF LAWS+SQLF
Fig. 6. The average throughput for each scheme.
5 10 15 20 25 30 0 2 4 6 8 10 12 Number of Station(s) Latency(second)
The Latency of Each Scheme
802.11 PSM LAWS LAWS+MWSA LAWS+SAF LAWS+SQLF
1. Average sleeping time of the station: This measure is the duration that a station stays in the sleeping mode. If a scheme can make stations stay more time in sleeping, then stations will save more power.
2. Average throughput: This value shows the total amount of data successfully transmitting per second. If AP can efficiently schedule and distribute the access of its serving stations, it will have higher data throughput.
3. Average latency of a successful transmission: The latency is defined as the time duration starting while a packet is issued and buffered at AP and ending when the target station returns the acknowledge. An AP with a good scheduling scheme will make the latency as small as pos-sible. Thus, the resources required for buffering data can be reduced.
Our simulation uses an IEEE 802.11b wireless com-munication module with 11 Mbps data rate. An AP can serve at most 30 stations. Each station will randomly set 1 to 5 beacon intervals as its listen-interval size and its packet arrival rate is 3 packets per beacon interval. Packet size in our simulation is fixed and set to 1 kbyte. Communication channel assumes to be clear and sym-metric. The total simulation time is 3 min. The details of other simulation configurations such as header length,
and inter-frame spaces (IFS) are listed inTable 4.
Simu-lation results will compare the IEEE 802.11 PS mode with the proposed LAWS, LAWS + MWSA, LAW-S + LAW-SAF, and LAWLAW-S + LAW-SQLF schemes.
5.2. Results and discussion
Fig. 5shows the relation between average sleeping time and number of stations. Considering contention-based schemes, LAWS can have more sleeping time than IEEE
802.11 PS mode in any size of stations. By using LAW-S + MWLAW-SA, LAWLAW-S + LAW-SAF, and LAWLAW-S + LAW-SQLF schemes to reduce the contention within a beacon interval, stations can have more time on staying in sleeping than LAWS and IEEE 802.11 PS mode. In this figure, it seems that S + MWSA has better sleeping time than both LAW-S + LAW-SAF and LAWLAW-S + LAW-SQLF. However, we will find in
Fig. 7that it trades the transmission latency with the sleep-ing time.
Fig. 6 shows the average throughput for each scheme. From this figure, we can explicitly find that the throughput of IEEE 802.11 PS mode falls down when station number is greater than 20. However, our proposed schemes, LAWS, LAWS + MWSA, LAWS + SAF, and LAWS + SQLF, are not influenced as number of station increases. This is because our schemes can efficiently avoid the data collision among the stations.
In Fig. 7, we show the average latency of a successful transmission for each scheme. For LAWS, LAWS + SAF, and LAWS + SQLF, all of their latency is smaller than 0.3 s and increase slowly as number of stations grows. Because only one station is indicated within a beacon inter-val, the latency of LAWS + MWSA scheme is longer than the other proposed schemes. The pure IEEE 802.11 PS mode, however, will suffer the worst latency while number of stations increases.
Finally,Fig. 8shows the improving rate of sleeping time
for each proposed scheme (compared to pure IEEE 802.11
PS mode). The improving rate Riof scheme i is defined as
Ri¼SiSS00 100%, where S0and Siare the average sleeping
times for pure 802.11 PS mode and the proposed scheme i, respectively. By efficiently scheduling the wakeup time of sleeping stations, the sleeping duration of LAWS, LAW-S + MWLAW-SA, LAWLAW-S + LAW-SAF, and LAWLAW-S + LAW-SQLF schemes can be improved significantly.
5 10 15 20 25 30 0 5 10 15 20 25 30 35 40 Number of Station(s) Improving Rate(%)
The Sleeping TIme Improving Rate of Each Scheme (Compare to Original Power Saving Mode of IEEE 802.11)
LAWS LAWS+MWSA LAWS+SAF LAWS+SQLF
6. Conclusion
In this paper, we propose a load-aware wakeup schedule scheme for infrastructure mode of IEEE 802.11 WLANs. The LAWS scheme balances the number of wakeup sta-tions in each beacon interval to reduce the amount of con-tention stations. For avoiding the concon-tention, MWSA, SAF, and SQLF scheme are used to arrange the access sequence of the wakeup stations within a beacon interval. Simulation results show that comparing to 802.11 PS-mode, the proposed LAWS, MWSA, SAF, and SQLF schemes can efficiently improve the sleeping duration of each station, average throughput, and transmission delay. The following two issues should be considered in the implementation of the proposed schemes:
1. AnagingfunctionshouldbeimplementedintheAPtodeter-mine when buffered frames are old enough to be discarded. 2. If the mobile station misses the beacon, it should remain
awake until it receives the next beacon. The mobile sta-tion checks the beacon frame. If the bit corresponding to its AID is set to zero in the TIM, or else it has retrieved all buffered frames, the mobile station can resume the sleeping mode by asking AP for a new wakeup counter
cj(t). In the LAWS + SAF and LAWS + SQLF schemes,
the mobile station misses the beacon can not show up to retrieve the buffered data in its turn. The next station in the access sequence can send PS-Poll frames to get back its buffered data if it finds that the medium has been idle for longer than the distributed coordination function inter-frame space (DIFS).
Finally, finding a shorter repeating pattern for LAWS scheme or extending LAWS scheme to wireless ad hoc net-works might be interesting for possible future work. References
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Hsiao-Po Lin received the B.S. degree and M.S. degrees in Computer and Information Science from National Chiao Tung University, Taiwan, in 2003 and 2005, respectively. His research interests include wireless networks, mobile com-puting and internet protocol.
Shih-Chang Huang received the B.S. degree in Computer Science and Engineering from Tatung University, Taiwan, in 1997, and received M.S. degree in Computer Science from National Tsing Hua University, Taiwan, in 1999. Since 2002, he has been working toward the Ph.D. degree in Computer and Information Science at National Chiao Tung University, Taiwan. His research interests include wireless network and wireless sensor network.
Rong-Hong Jan received the B.S. and M.S. degrees in Industrial Engineering, and the Ph.D. degree in Computer Science from National Tsing Hua University, Taiwan, in 1979, 1983, and 1987, respectively. He joined the Department of Com-puter and Information Science, National Chiao Tung University, in 1987, where he is currently a Professor. During 1991–1992, he was a Visiting Associate Professor in the Department of Com-puter Science, University of Maryland, College Park, MD. His research interests include wireless networks, mobile computing, distributed systems, network reliability, and operations research.
