Energy-efficient WLAN with on-demand AP wake-up using IEEE 802.11 frame length modulation

Computer Communications 35 (2012) 1725–1735

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Energy-efficient WLAN with on-demand AP wake-up using IEEE 802.11 frame length modulation Yoshihisa Kondo a,⇑, Hiroyuki Yomo b, Suhua Tang a, Masahito Iwai c, Toshiyasu Tanaka c, Hideo Tsutsui a, Sadao Obana d a

ATR Adaptive Communications Research Laboratories, 2-2-2 Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-0288, Japan Faculty of Engineering Science, Kansai University, 3-3-35 Yamate-cho, Suita, Osaka 564-8680, Japan NEC Communication Systems, Ltd., NCOS Laboratory, 1753 Shimonumabe, Nakahara-ku, Kawasaki-shi, Kanagawa 211-8666, Japan d Graduate School of Informatics and Engineering, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan b c

a r t i c l e

i n f o

Article history: Available online 12 May 2012 Keywords: Energy-efficiency Wireless LAN Wake-up receiver Prototype design

a b s t r a c t This paper considers a radio-on-demand (ROD) wireless LAN (WLAN) in which access points (APs) are put into a sleep mode during idle periods and woken up by stations (STAs) upon communications demands. The on-demand wake-up is realized by a wake-up receiver which is equipped with each AP and is used to detect a wake-up signal transmitted by STA. In order to reduce the hardware installation cost at STA, we advocate to utilizing wireless LAN frames transmitted by each STA as a wake-up signal. We generate a wake-up signal based on frame length modulation (FLM) where each STA creates a series of WLAN frames with different length to which the information on wake-up ID is embedded. The simple and low-power wake-up receiver extracts the wake-up ID from the received frames. In this paper, we design and develop a prototype of the wake-up receiver and propose a wake-up protocol which defines a procedure to realize the on-demand AP wake-up in ROD WLAN. We evaluate system-level performance of ROD WLAN based on our prototype and our proposed wake-up protocol, and investigate appropriate settings of parameters for our proposed FLM to achieve the required system-level performance. Our numerical results confirm that the proposed wake-up protocol with FLM achieves smaller delay than a conventional AP employing passive scanning while maintaining small probability to be falsely woken up by continuous interference. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction IEEE 802.11-based wireless LAN (WLAN) has shown tremendous growth in its worldwide popularization over the last decade as a means to provide its users with ubiquitous access to the Internet. However, WLAN access points (APs), which are always kept powered-on and transmit beacon frames even while there is no associated station (STA), are not designed to be optimized in terms of energy consumptions. As indicated in [1], APs are actually idle (i.e., there is no communication demand using those APs) for most of the time and space. Consequently, a huge number of APs are consuming wasteful energy, which will be aggravated in the future as the number of deployed APs is increased at homes, offices, and public networks. Furthermore, recently, WLAN has attracted great attention as an entity of energy management systems such as smart grid aiming to improve efficiency of energy supply. Saving energy of WLAN contributes to realizing both green of communications and green by communications. A simple way to reduce consuming energy of APs is to apply a sleep mode to APs when there is no associated STA. In the sleep ⇑ Corresponding author. Tel.: +81 774 95 1462; fax: +81 774 95 1505. E-mail address: [email protected] (Y. Kondo). 0140-3664/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.comcom.2012.04.022

mode, almost all of the internal AP systems including a WLAN device must be turned off in order to save large amount of energy. The energy consumption of a WLAN device is relatively large compared to that of the whole internal AP systems which, in general, consist of embedded micro devices. However, if the WLAN device inside an AP is turned off, then users with communications demands within the AP coverage cannot initiate communications with its target AP over WLAN channels, which results in coverage loss for users. This problem can be solved by low-power wake-up transceivers which exchange wake-up signals for awaking AP independently of WLAN devices. The concept of this on-demand wakeup has originally emerged from research area of sensor networks which require low-power operations of nodes due to the limitation on battery [2]. The concept is also applicable to WLAN, and we have proposed in [3] a radio-on-demand (ROD) WLAN where a wake-up receiver is equipped with each AP to detect wake-up signals transmitted by STAs. In ROD WLAN, each STA is required to transmit a wake-up signal, however, the installation of new hardware into each STA for transmissions of wake-up signals is costly and prevents the wide and rapid spread of ROD-enabled devices. Therefore, in order to reduce the hardware installation cost at STA in ROD WLAN, we have also proposed in [3] a wake-up mechanism exploiting WLAN devices already installed at STAs. In our

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proposed scheme, STA transmits a wake-up signal which is a series of WLAN frames with different length. The length of each transmitted frame is decided according to a wake-up ID assigned to the target AP. Thus, IEEE 802.11 frame length is modulated according to the assigned wake-up ID. The wake-up receiver decodes the wake-up signal by detecting the length of WLAN frames with onoff-keying (OOK) demodulation, which is a simple operation with low energy consumption. The wake-up receiver decides whether it should wake up AP or not by identifying a unique wake-up ID assigned to each AP. Our proposed scheme does not require STAs to implement additional hardware for transmitting the wake-up signal and they can realize the on-demand wake-up only by updating their software. This feature enables conventional STAs to employ the on-demand wake-up of APs with small installation cost, and is considered to strongly support popularization of the ROD-based APs and green communication systems using these APs. In our previous work in [3], we have only provided a basic idea of the proposed wake-up scheme using IEEE 802.11 frame length modulation (FLM) and presented preliminary simulation results for its proof-of-concept. In this paper, we significantly extend the work presented in [3] with the following new contributions:  We design and develop a prototype of the wake-up receiver to extract the wake-up ID from the wake-up signal generated according to the proposed FLM. We present evaluation results obtained with experiments, which proves the practical feasibility of our proposed approach. The results are used to tune parameters of the proposed FLM and to evaluate the systemlevel performance of ROD WLAN.  We propose a detailed procedure of the on-demand wake-up in ROD WLAN, i.e., a wake-up protocol for ROD WLAN. We define several control messages to be exchanged between STA and AP and its transmission sequence to realize the on-demand wakeup, including a retransmission procedure.  We evaluate by computer simulation system-level performance of ROD WLAN based on the results obtained with our proto type and proposed wake-up protocol. We investigate the appropriate settings of parameters for our proposed FLM to achieve the required system-level performance and clarify the feasibility of our proposed on-demand wake-up scheme to realize energy-efficient WLAN. The remainder of the paper is organized as follows. In Section 2, we refer to some related work and clarify originality of our proposed on-demand wake-up scheme. The overview of proposed on-demand wake-up scheme is presented in Section 3. In order to make the paper self-contained, we also present the details of our proposed FLM with newly-proposed wake-up protocol in Section 3. The developed receiver prototype and its evaluations are presented in Section 4. The detailed design of wake-up signal based on the experimental evaluations is also given in Section 4. The system-level evaluations of the proposed on-demand wake-up scheme using computer simulations are presented in Section 5. In closing of this paper, we conclude our work in Section 6.

2. Related works On-demand wake-up schemes for low-power operations of wireless networks have been studied mainly in research area of sensor networks [2]. In the on-demand wake-up scheme, two wireless interfaces are implemented at each node. One is a data transceiver and the other is a low-power wake-up transceiver. Thanks to the wake-up transceiver which exchanges wake-up signals and initiates data communications, the data transceiver can be kept power-off while there is no communication demand. Some of prior

work employed the wake-up transceiver having the same hardware configuration as that of data transceiver only operated in a low power mode [4]. The other work proposed and developed the low-cost and low-power transceivers specialized for wake-up operations. The simple modulation and detection schemes are applied to wake-up transceivers in order to realize low-power transceivers. For example, in [5–7], OOK modulation and non-coherent envelope detection are applied to wake-up transceivers. These transceivers realize both high reception sensitivity of signals and low-power operations of transceiver by fine-tuning amplifier and detection hardware. However, these works lack the design of wake-up signal and wake-up process. There have been also a lot of studies on reducing energy consumption of WLAN. The research focus has been mainly on mobile STA which has the limitation on battery. A power saving mode is defined in IEEE 802.11 [8], where APs manage sleep/wake-up of STA. This mode does not aim at reducing power consumption of AP. The work in [9] suggests a power saving scheme for PDA terminals using an additional wireless module which works on a separate control channel. Similarly to on-demand wake-up schemes mentioned above, WLAN communications are initiated by lowpower wireless communications. In [10], a low power sensor mote is used to monitor the WLAN channel and to detect energy of WLAN signals, which triggers the wake-up of data transceivers. This wake-up scheme does not require additional transmitter of wake-up signal and its basic idea is similar to our scheme. However, since the sensor mote uses only energy level to trigger the wake-up, the false wake-up probability becomes unacceptably large when it operates in a crowded ISM band. As for power saving of WLAN AP, a power saving protocol called SEAR is proposed for WLAN networks in [1]. SEAR groups neighboring APs which are densely deployed and puts redundant APs into a sleep mode. The redundant APs are selected so that the network coverage is not reduced. SEAR saves energy consumption of cooperating APs within a network, however, it does not save energy consumption of APs operating independently. Furthermore, SEAR does not have any mechanism for STA to wake up AP, which limits the achievable gain of energy saving. To the best of our knowledge, [3] is the first work which advocates to utilizing a simple and low-power wake-up receiver at WLAN AP and proposes the identity-based wake-up mechanism which does not require an extra hardware at each STA. The main focus in this paper is to prove the practicality and system-level robustness of the proposed on-demand wake-up scheme and protocol with an experimental test-bed and system-level evaluations. 3. On-demand AP wake-up in ROD WLAN 3.1. Overview of ROD WLAN and on-demand wake-up Fig. 1 shows an overview of ROD WLAN system. When an AP is in a sleep mode, most of its modules including the WLAN module are turned off except for the minimum area of internal CPU memory, which is required to realize the quick recovery of the Host CPU system. As a wireless interface, only wake-up receiver is kept powered-on and ready to receive wake-up signals. STA wakes up AP in the sleep mode by transmitting a wake-up signal when there is a need for communications. STA has no transceiver specialized for transmitting wake-up signals. The wake-up signal generator is a software module which generates wake-up signals to be transmitted via WLAN module. The detailed design for the wake-up signal is described in the next subsection. Note that our target is to reduce the energy consumption at AP and the modules and Host CPU system at STA are not the targets of sleep control. In fact, STAs attempting to wake up AP should have clear motivations to consume their energy since they have communications demands.

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Wake-up! WLAN Module

WLAN Module

Wake-up Receiver

Wake-up Signal Generator (Sof tware) Host CPU System

LAN Module

Host CPU System Power Supply

Fig. 1. Overview of ROD WLAN system.

In order to wake up an AP from the network side, LAN module at AP has to continue to be powered-on, however, if we apply Wake-ONLAN technologies [11], it can be also the target of sleep control. The wake-up signal carries wake-up ID which identifies the designated AP. ESSID of WLAN network is applicable to identify AP or WLAN network. However, ESSID has 32 bytes length at maximum and is too long to transmit by low-rate wake-up radio. Therefore, ESSID is compressed (e.g. by using a predefined hash function) to the smaller fixed length, e.g. 32 bits, which is registered by the Host CPU system into the wake-up receiver as the wake-up (WU) ID. The wake-up receiver turns on its host CPU system and the other modules when it detects its assigned wake-up ID. 3.2. Wake-up signal The wake-up signal transmitted by STA is generated based on our proposed FLM [3]. With FLM, the information on wake-up ID is embedded into the length of WLAN frames which are sequentially transmitted. The wake-up receiver detects the length of WLAN frames and decodes wake-up ID from the length. The wake-up signal generator in a STA can generate intended length of WLAN frames by controlling the size of MSDU payload and transmission rate. For example, 480 to 18848 ls (with 8 ls step) length of WLAN frames can be generated by using WLAN broadcast data frames which have 1 byte to 2304 bytes MSDU payload (with 1 byte step) when we use 802.11b long frame format and 1 Mbps transmission rate. If we try to directly control the length of MSDU payload at MAC level, we need to modify driver software of a WLAN chip. On the other hand, the length of frame to be transmitted by STA can be also controlled by modifying the payload length at higher layer, e.g. UDP packets. The different length of UDP packets can be generated by software applications at STAs without any change in WLAN chip devices and their drivers. Then, wake-up signals can be generated at STAs by simply installing these applications, which brings a large benefit to popularization of ROD WLAN. Therefore, we consider a case to generate UDP packets with different length of payload for creating wake-up signals at STA. If we employ UDP packets to transmit wake-up signals and 1500 bytes as MTU, then the length of WLAN frames are limited to 712 to 12480 ls (with 8 ls step) with 1 byte to 1472 bytes UDP payload (with 1 byte step). The wake-up signal generator has a table for mapping bit sequence to the size of payload. By using the table, bit sequence of wake-up ID is converted to wake-up signal. The wake-up signal generator generates MSDU payload or UDP payload whose size corresponds to bit sequence of wake-up ID and transmits it by using broadcast WLAN frame. If the bit length of a wake-up ID is larger than that represented by each WLAN frame, then the wake-up ID is represented by multiple WLAN frames and STA transmits multiple WLAN frames as a wake-up signal. Each WLAN frame which conveys different bit sequence is called a symbol. Table 1 shows an example of the mapping table when

Table 1 An example of a bit-to-payload mapping table. Symbol index

Bit sequence

UDP payload

Length of frameframe frame

1 2 3 4 ... 64

000000 000001 000010 000011 ... 111111

1 byte 4 bytes 8 bytes 12 bytes ... 237 bytes

712 ls 736 ls 768 ls 800 ls ... 2600 ls

we employ UDP packets to transmit a wake-up signal with 6 bits as bit length transmitted by each symbol. The selection of the employed frame length will be discussed in detail in Section 4. 3.3. Wake-up receiver Fig. 2 shows a block diagram of our proposed wake-up receiver which detects the length of WLAN frames and makes a decision on the AP wake-up while Fig. 3 shows an example of the frame length detection using the proposed wake-up receiver. OOK bit detector (OBD) periodically detects signal power of a WLAN channel and outputs results of the detection to frame length detector (FLD). ‘1’ is the output when OBD detects wireless signal, and ‘0’ otherwise. The FLD detects frame length using the input bits. The FLD counts up series of ‘1’, and outputs the count to wake-up detector (WUD) as a frame length when the input bit changes from ‘1’ to ‘0’ as shown in Fig. 3. The WUD estimates a symbol from the frame length and checks if the received symbols represent its own wake-up ID. If the checking result is positive, then WUD outputs turn-on command to host AP system. In order to detect the length of each WLAN frame, it is necessary to accurately find the space between the succeeding frames. Each symbol is transmitted as an 802.11 broadcast data frame, therefore, as shown in Fig. 3, inter frame space between a previously transmitted frame and the transmitted symbol is more than 50 ls (DIFS). However, it can happen that an AP with 802.11e HCCA mode transmits a frame

Fig. 2. Block diagram of wake-up receiver.

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DIFS (50us)

PIFS(30us)

Back-off

1

1

1

WLAN Frame (CF-Poll)

WLAN Frame (Symbol of wake-up Sig.)

WLAN Frame 0

0

0

0

0

1

1

1

1

0 d 0

1

Detect sum of successive ‘1’ as frame length

Each bit detection timing

1

1

1

Detection interval must be less than PIFS

Fig. 3. Frame length detection using periodical bit detection.

with PIFS (30 ls) after the end of transmitted symbol. Therefore, in order to accurately detect ‘‘0’’ between succeeding frames, the interval of periodic bit detection must be less than 30 ls. The OBD consists of band pass filter (BPF), low-noise amplifier (LNA), envelope detector and analog to digital converter (ADC). The BPF passes signal on a specific WLAN channel, which is amplified by LNA. The ADC detects an energy level of envelope above certain threshold and outputs the corresponding bit. The other OOK receivers such as [5–7] are also applicable if they are tuned to operate over WLAN frequency channel. We can apply a duty-cycling to OBD in order to reduce power consumption, e. g. OBD is powered on with 10 ls interval for bit detection. The FLD and WUD can be implemented with low power microcontroller such as MSP430 [12]. 3.4. Wake-up detection With carrier sense multiple access with collision avoidance (CSMA/CA) employed in 802.11, any AP or STA may transmit WLAN frames while multiple frames constituting a wake-up signal are transmitted by a STA. The wake-up receiver is required to detect the wake-up signal robustly even with such interruptions. Furthermore, ID matching must be realized with a simple operation for which the table-based matching is inappropriate since the mapping between wake-up frames and bit sequence requires large computational load. In order to solve this problem, we introduce a state-machine based detector which is depicted in Fig. 4. Here, n represents the number of symbols constituting a wake-up signal. The host AP system converts, by using the mapping table, the wake-up ID to n indexes {x1, x2, . . . , xn} which represent corresponding symbols constituting the assigned wake-up ID. Each xi is an integer. n indexes are set to WUD while AP is in an active state. After transiting to the sleep mode, WUD waits to detect symbolx1 which is the first symbol of wake-up ID. If symbolx1 is detected, then WUD moves its state to ‘‘waiting symbolx2’’. If all the n symbols are detected, then WUD outputs the wake-up command to the host AP system. As stated above, it can happen that non-wake-up frames, such as data frames transmitted by the other APs, are detected between the wake-up frames. Such an interruption does not change the state of WUD unless the length of data frame matches with that of the waiting symbol. This makes the proposed WUD robust to the interference caused over the ISM band. However, the state is wrongly shifted if the interrupting WLAN data frame has the same length as the waiting symbol. Even if the frame transmitted by the other APs has different length from the waiting frame, the bit detection errors within non-desired frames can also cause the wrong shift of the

Symbol x1 Symbol x1

state machine. In order to reset such an inconsistent state caused by the wrong interruptions, each state is initialized if the number of detection of non-desired symbol exceeds a predetermined parameter, m, as shown in Fig. 4. 3.5. Wake-up protocol The wake-up signal is transmitted over a fixed, predetermined WLAN frequency channel, called wake-up channel. The wake-up channel is not an exclusive channel and is one of WLAN channels where the other WLAN systems also operate. Each AP has the same or another WLAN frequency channel, called service channel, where the AP intends to provide its service. A STA with communications demands, which is not associated with any AP, first transmits a wake-up signal destined to a target AP. The wake-up sequence of the proposed wake-up protocol is shown in Fig. 5. Depending on the operating mode of the target AP, we have two cases: an AP in sleep mode receives a wake-up signal (Fig. 5a)) and an AP which is already in active mode receives a wake-up signal (Fig. 5b)). Here, we define the following three management packets for the wakeup procedure: 3.5.1. Wake-up notification (WN) APs notify STAs of their wake-up by using WN packets. Each AP transmits a WN packet by a broadcast WLAN data frame over the wake-up channel after detecting the wake-up signal intended for it and turning on its host system. The WN packet includes ESSID, BSSID and information on a service channel where the AP provides its service. 3.5.2. Reply to wake-up notification (RWN) After transmitting a wake-up signal, a STA waits for receiving a WN packet transmitted by a target AP. If the STA successfully receives the WN packet from the target AP and obtains the information on the BSSID and service channel, then it returns a RWN packet, addressed to the AP by using a unicast WLAN data frame over the wake-up channel. Due to the interference over the ISM band, each AP may be woken up without the actual transmissions of wake-up signal, however, APs can find such a false wake-up by checking whether the RWN packets are returned from a STA or not. 3.5.3. Active notification (AN) AP transmits an AN packet by a broadcast data frame instead of the WN packet when the intended wake-up signal is received during its active state. For transmitting the AN packet, the AP

Symbol x2

Symbol xn

Symbol x2

Symbolxn

m m Fig. 4. State machine of wake-up detection.

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STA

STA

Wake-upSignal

AP

Host WuRx Wake-up Signal

Wake-up Com.

Host WuRx

WN

Sleep to Active

AN

RWN Wake-up Ch. to Service Ch.

802.11 Association

Wake-up Ch. to Service Ch.

Data Communications

Wake-up Ch. to Service Ch.

a) A P is in a sleep mode

802.11 Association Data Communications

Wake-up Com. Service Ch. to Wake-up Ch. Wake-up Ch. to Service Ch.

b) AP is in an active mode Fig. 5. Association procedure.

changes the channel of WLAN device from service channel to wake-up channel. After transmitting the AN packet, the AP switches its channel to service channel immediately. Like the WN packet, the AN packet includes ESSID, BSSID and information on a service channel. We assume that these packets are transmitted by using WLAN data frames, however, they can be replaced by management frames of WLAN. When the wake-up signal is received by a sleeping AP as shown in Fig. 5a), AP and STA exchange WN and RWN packets over the wake-up channel. Then, both STA and AP change their operating channels to the service channel over which 802.11 association and data communications are carried out. On the other hand, when the intended wake-up signal is received by an active AP as shown in Fig. 5 b), STA obtains the information on service channel from AN packet, changes its channel to the given service channel, and proceed to the association and data communications. When STA does not receive a WN packet or an AN packet within a certain period T1 after transmitting a wake-up signal, it retransmits the wake-up signal. The STA scans the other channels by using passive/active scanning defined in IEEE802.11 standard if the number of retransmissions of wake-up signal exceeds a maximum value Rmax. If STA does not find the target AP with the scanning, then STA restarts the whole wake-up process with the retransmission parameters initialized or changes its target AP. When AP does not receive a RWN packet within a certain period T2 after detecting a wake-up signal and transmitting a WN packet, the AP changes its state back to sleep mode. Even if AP is falsely

woken up without the intended wake-up signal, active period of AP can be kept small if T2 is set to a small value, for example, 2 s. In an ESS which consists of multiple APs, multiple APs within a communication range of a STA transmitting a wake-up signal can be awaked simultaneously. The STA just associates with one of the awaked APs, therefore, the rest of APs consume energy wastefully. However, thanks to the wake-up protocol, the APs can be put into the sleep mode again promptly. The APs are turned on only during 2 s for example. Even if such wasteful wake-ups occur 100 times in a day, total time where the AP is in active mode wastefully is 200 s. 200 s is 0.2% of a day and energy consumed by the wasteful wake-up is negligible compared to energy saved by employing sleep mode which is considered to have length more than several hours in a day. 4. Evaluations using wake-up receiver prototype 4.1. Overview of evaluation We have developed a prototype of the proposed wake-up receiver in order to evaluate the practical feasibility of the proposed FLM. The appearance and the specifications of the prototype are shown in Fig. 6. The block diagram of the wake-up receiver is the same as Fig. 2. We employ 10 ls as bit detection interval and apply fixed threshold to ADC on the receiver prototype. We select the threshold so that the probability to erroneously detect ’1’ despite that there is no transmitted signal is nearly equal to 104.

Fig. 6. Developed receiver prototype.

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802.11b Signal

Coaxial Cable Receiver Prototype

ATT Laptop PC with WLAN Card (NEC WL54AG)

Variable Attenuator

Shied Box

Fig. 7. Configuration of our measurement system.

We evaluate our proposed wake-up scheme by using the receiver prototype. The configurations of our measurement system are shown in Fig. 7. WLAN packets are generated and transmitted by a laptop PC with a WLAN card using 802.11b 1 Mbps transmission rate. In order to precisely evaluate performance of proposed frame length detection with respect to received signal level, we need to finely tune the value of received signal strength at wake-up receiver. Therefore, the wake-up receiver prototype is connected with the WLAN card using a coaxial cable, and the received signal level is controlled by using a variable attenuator attached to the coaxial cable. The transmission power of WLAN frame is set to 5 dBm. The propagation loss of our measurement system is 4.2 dB when the attenuation level of the variable attenuator is set to 0 dB. 4.2. Evaluation of resolution for frame length detection Our proposed wake-up receiver detects WLAN frames without synchronization to the transmitter of the wake-up signal. Therefore, the detected counter value at FLD can slightly vary from the transmitted one due to the timing mismatch even if there is no bit error. We evaluate the variation of the counter value by using the developed receiver prototype. The WLAN frames are transmitted from the laptop PC with random timing and we obtain counter values detected by FLD. The length of the transmitted frames is controlled by varying payload of UDP packet. The employed sizes of payload are 1, 2, 3, 4, 5, 6, 7 bytes and the corresponding frame length are 712, 720, 728, 736, 744, 752, 760 ls respectively. The number of transmissions of each frame is 50000 and the attenua-

Detedtion probability

1

712µ 720µ 728µ 736µ 744µ 752µ 760µ

0.8 0.6 0.4 0.2 0 70

71

72

73

74

75

76

77

78

Number of count Fig. 8. Probability distribution of detected count.

Intended symbols

symbol1 (710 )

symbol2 (740 )

79

80

tion value of variable attenuator is set to 60 dB. With this condition, the bit error probability is so small that the variation of counter value is dominantly caused by the timing mismatch. Fig. 8 shows the probability distributions of detected counter values for each length of frames. When the length of transmitted WLAN frame is 712 ls, 72-count is detected with the probability of about 0.7 and 73-count, about 0.3. 73-count is also detected when the length is 720 ls and when the length is 728 ls. Therefore, a frame with the length of 712 ls cannot be distinguished from those with 720 ls and 728 ls by the wake-up receiver. However, the detected counts of a frame with 736 ls do not overlap with that of 712 ls. As for the other length of frames, the detected counts do not overlap with each other if the lengths are separated more than 24 ls. From this result, we can conclude that the proposed wake-up receiver can detect length of WLAN frames with the resolution of 24 ls. Since the wake-up receiver detects bit at the interval of 10 ls, simply, we employ the multiple of 10 ls as the length of symbol with 30 ls step. That is, we allocate 710 ls to symbol1 and {710 + 30 (i  1)} ls to symboli where i is an integer (i > 0). The maximum value of i is 2j when a symbol transfers j bits and the longest symbol is {710 + 30 (2j  1)}. The WUD on the wake-up receiver detects symboli when the input counter value from FLD is {71 + 3(i  1)}, {72 + 3(i  1)} or {73 + 3(i  1)}. When the wakeup signal generator in STA intends to transmit a symbol, it generates and transmits a WLAN frame which has the nearest length to the symbol. For example, if the intended symbol is 710 ls (symbol1), then the wake-up signal generator transmits 712 ls WLAN frame by using 1 byte payload. The relationship between the symbol, length of WLAN frame, and detected counter value at FLD is shown in Fig. 9. 4.3. Evaluation of symbol error ratio We evaluate symbol error ratio of our proposed wake-up signal with the developed prototype and the wake-up signal defined in the previous subsection. Each symbol has different error ratio because the length of symbols are different with each other. Assuming that each symbol is transferred by using UDP packet and conveys 6 bits, the shortest symbol is symbol1 (710 ls) and the longest symbol is symbol64 (2600 ls). We evaluate error ratios of the shortest symbol and the longest symbol. The length of WLAN frames corresponding to these symbols are 712 ls and 2600 ls which are transmitted from the laptop PC. The number of transmissions of each frame is 50000 and the attenuation value of the variable attenuator is varied from 81 dB to 87 dB with 1-dB step. Fig. 10 shows the obtained symbol error ratio of each symbol. When the received signal power is less than 84 dBm, the symbol error ratios are almost 1 and the proposed wake-up scheme is hard to operate. When the received signal power is around 83 dBm, the symbol error ratio of symbol1 is about 0.05 while that of symbol64 is about 0.25. In order to detect the length of WLAN frame correctly, all successive ‘1’s must be detected without any error. Therefore, the symbol error ratio becomes larger as the

symbol3 (770 )

symbol4 (800 )

symbol5 (830 )

Length of transmitted WLAN frames [ s] Detected counter values Detected symbols

symbol1

symbol2

symbol3

symbol4

Fig. 9. Relationship between symbol, frame length, and number of detected counts.

symbol5

1731

10

0

1

m=5, n=3 m=5, n=5 m=10, n=3 m=10, n=5

712µ 2600µ

-1

10

0.8 -2

10

-3

10

10-4

-86

-85

-84

-83

-82

-81

-80

Received signal power [dBm] Fig. 10. Symbol error ratio of each symbol.

length of symbol becomes longer. However, when the received signal power is around 82 dBm or more, its difference becomes negligible, and values of the symbol error ratio become constant. Around this region, the received signal power is considered to be large enough to detect successive ‘1’s correctly. However, another necessary condition for detecting the length of a WLAN frame correctly is that ‘0’s in both ends of successive ‘1’s are correctly detected. The symbol errors with large received signal power are dominated by the detection errors of ‘0’s, which is tuned to be a constant value by adjusting the ADC threshold. This is the reason why we observe constant symbol error ratio with the large received signal power. 5. System-level performance evaluations Two adverse events can degrade the quality of service (QoS) observed by WLAN users in our proposed system: false negative and false positive. The false negative is an event where the target AP does not wake up despite that a STA transmits a wake-up signal toward the AP. The false positive is the one where APs are woken up by non-intended frames such as wake-up frames transmitted by surrounding STAs to the other APs, WLAN data frames transmitted by the other APs or STAs, or the other interference signals observed in the ISM band. The false negative causes the increase of the delay for STA to connect to the desired AP while the false positive causes unnecessary APs to wake up, which is apparently the waste of energy. 5.1. Evaluation of false negative False negative probability (FNP) when there is no interrupting frame/signal for the wake-up signal can be denoted as follows:

FNP ¼ 1 

n Y

ð1  SERðxi ÞÞ

ð1Þ

i¼1

where n represents the number of symbols required for transmitting a wake-up signal and xi represents an index number of each symbol which constitutes the wake-up signal. SER represents symbol error ratio of each symbol, symbolxi. When the received signal power is 82 dBm or more, the symbol error ratio is about 3  104 as observed in the experimental results in the previous section. Then, FNP is about 9  104 when n = 3 and about 1.5  103 when n = 5. The proposed wake-up detection using state machine shown in Fig. 4 does not succeed when more than m interrupting frames are detected between the desired symbols. In order to evaluate FNP when there is interrupting traffic, we implement the proposed on-demand wake-up scheme in a network simulator QualNet 5.0 [13] and measure FNP with background traffic. We deploy STA and AP which carry out the wake-up process so that they achieve the symbol error ratio of 3  104. The interrupting nodes are located so that they are within the carrier-sensing range of each other

False negative probability

Symbol error ratio

Y. Kondo et al. / Computer Communications 35 (2012) 1725–1735

0.6

0.4

0.2

0

1

2

3

Number of interference nodes Fig. 11. False negative probability with interference traffic.

and of both STA and AP. The number of interrupting nodes is varied from 1 to 3. Each interrupting node generates UDP packets with 500 bytes payload and the generation rate of 1 Mbps, which are transmitted by 802.11b broadcast WLAN frames employing 1 Mbps transmission rate. Even when the number of interrupting nodes is 1, the wake-up channel is fully occupied by interrupting traffic. The STA contends to seize a transmission opportunity in each contention period. We employ 3 or 5 as the number of frames constituting a wake-up signal n, and 5 or 10 as m which represents the number of permissible interrupting frames detected between successive two symbols. The number of trials on each configuration is 1000. In each trial, n symbols are randomly selected from Table 1 as a wake-up signal and they are set to both STA and its target AP. Fig. 11 shows simulation results of FNP with interrupting nodes. With n = 5 and m = 5, FNP is larger than 0.18 in all the cases. FNP increases as the number of interrupting nodes increases due to larger probability of interruption inherent to CSMA/CA operations. However, by increasing m or decreasing n, the FNP can be decreased since the larger m prevents the frequent initialization of state machine and the smaller n requires less number of symbols required to detect a wake-up signal. With m = 10 and n = 3, we achieve FNP less than 0.2 for all the number of interrupting nodes. 5.2. Evaluation of association delay caused by false negative In order to investigate what is the permissible value of FNP, we evaluate delay for association with the proposed on-demand wake-up scheme. The large FNP causes large delay for STA to associate with the target AP and it damages the usability of users. We implement the proposed on-demand wake-up protocol defined in Section 3.4 in QualNet simulator. We assume 802.11b which has 13 operating channels. The number of maximum retransmissions Rmax is set to 5 and the maximum waiting time for WN packets, T1, is set to 1100 ms. The wake-up delay of AP, which is the duration between the detection of a wake-up signal and the start to transmit a WN packet is set to 1000 ms. The wake-up channel is set to 1ch and the service channel of AP is randomly selected in each trial. FNP of each wake-up signal is varied as a simulation parameter. In order to investigate the impact of number of wakeup and interrupting frames on the delay, we simulate the transmission of these frames. The number of symbols required for transmitting a wake-up signal is 5 with their length randomly selected in

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each trial. The number of interrupting nodes is set to 2 or 3, which generate interrupting traffic with the same parameter as Section 5.1. If the wake-up signal and management packets for the proposed on-demand wake-up are successfully exchanged, then authentication and association process between the STA and its intended AP is started. We evaluate delay time which is the duration from the beginning of transmissions of a wake-up signal to the start of association process. As a target for comparison, we evaluate the association delay when using passive scanning defined in IEEE802.11 standard. In this case, APs do not apply any sleep mode. We employ 200 ms as transmission intervals of beacon frames, which is a commonly-used value for example in [14]. The time required to scan a WLAN channel is set to twice of the beacon interval, 400 ms. The service channel of an AP for the passive scanning is also randomly selected in each trial. Fig. 12 shows simulation results of the delay time. The passive scanning requires 2900 ms for finding a beacon frame transmitted by the intended AP and starting the authentication process on average. If there is no interrupting traffic (a curve with ‘‘ROD’’ in the figure), the obtained delay time is 1300 ms when FNP is less than 101 for the proposed system. The delay time of the proposed system is less than that of the passive scanning if FNP is less than 0.4. The FNP value is easily achievable with our developed receiver prototype if the received signal power is –82 dBm or more. On the other hand, when there are 2 interrupting nodes (ROD W/ Traffic (N:2)), the delay time is less than that of the passive scanning if FNP is less than 0.3. When there are 3 interrupting nodes (ROD W/ Traffic (N:3)), the delay time is less than that of the passive scanning if FNP is less than 0.2. Here, we compare the results given in Fig. 11 and Fig. 12. In Fig. 11, all the cases except the case m = 5 and n = 5 achieve FNP less than 0.3 when the number of interrupting nodes is 2. Therefore, by employing m = 10 or n = 3, APs can robustly wake-up even if there are 2 interrupting nodes with delay less than that of passive scanning as shown in Fig. 12. When the number of interrupting node is 1, FNP achieving the same delay with passive scanning is even larger. In Fig. 11, FNP values are less than 0.3 in all the case when number of interrupting node is 1. Therefore, with all the pair of m and n, APs can wake-up even if there is 1 interrupting node with less delay time than passive scanning. When the number of interrupting nodes is 3, FNP must be less than 0.2 in order

5000

Delay [ms]

4000

ROD ROD W/ Traffic (N:2) ROD W/ Traffic (N:3) Passive Scanning

3000

2000

1000

0 10-3

10-2

10-1

False negative probability Fig. 12. Association delay of the proposed ROD system and passive scanning.

to achieve less delay than passive scanning. In Fig. 11, only the case with m = 10 and n = 3 achieves FNP less than 0.2. Therefore, when there are 3 interrupting nodes, m = 10 and n = 3 must be employed in order to overcome passive scanning. 5.3. Evaluation of false positive Next, we evaluate false positive probability (FPP) by our custommade simulator. There are two types of false positive to consider. One is the false wake-up caused by a series of WLAN data frames which have the same set of length and order as the wake-up signal. The other is the false wake-up caused by a continuous interfering signal in which several bits are erroneously detected at OBD. First, we evaluate the latter case. In the ISM band, there are many devices which generate interfering signals such as codeless phones and microwave ovens. Our proposed wake-up receiver does not distinguish WLAN frame and interfering signal, therefore these interfering signals can cause false wake-up. We assume a continuous signal which is detected by OBD with certain bit detection error ratio. We evaluate FPP within 1 minute when there is the continuous signal. In order to investigate the impact of the employed length of frames on FPP, we consider 4 groups with different set of symbol length, group1: 710–1160 ls, group2: 1190–1640 ls, group3: 1670–2120 ls and group4: 2150–2600 ls. We evaluate FPP of each group. Like the simulation in Section 5.1, we employ 3 or 5 as n, and 5 or 10 as m. The bit error ratio to detect ‘0’ within continuous signal is varied as a simulation parameter. These bit errors are assumed to occur randomly. Fig. 13 shows simulation results of FPP. From this figure, we can see that FPP becomes smaller as the length of the employed symbol becomes larger. This means that the use of smaller symbol is more susceptible to the continuous interference with bit errors. On the other hand, the increase of n can decrease FPP significantly. When n = 3 and group1 is employed, FPP shows large increase where the bit error ratio is around 102. When n = 5 and m = 10, the maximum FPP is about 0.1. When n = 5 and m = 5, the maximum FPP is about 0.02 and FPP of 0 is observed in almost all the bit error ratio. The large m causes the increase of FPP since the state machine keeps the given state without initialization and persistently waits for the desired frames, however, by combining with larger value of n, FPP can be suppressed largely. Next, we investigate the false wake-up caused by a series of WLAN frames which have the same set of length and order as the wake-up signal. To this end, we analyze reference trace data of WLAN frames captured in public spaces with nano-second time resolution [15]. The trace data was obtained in 6 deferent places around Portland, Oregon in 2007 and has 19 h length as a total. Fig. 14 shows number of different symbols of wake-up signal observed within the trace data. The relationship between each symbol and frame length is shown in Table 1 and Fig. 9. Here, we assume that no bit error occurs for OBD detection. The different numbers of symbols are detected over wide range. The most frequently observed symbol is the 28th symbol (symbol28) which is detected when FLD outputs 152, 153 or 154-counts, followed by the 23rd symbol (symbol23) and the 4th symbol (symbol4). Here, in order to evaluate the upper-bound of FPP, we evaluate number of false positives when these frequently observed symbols are employed for a wake-up signal. We employ 3 and 5 as n, and 5 and 10 as m like the simulations of FNP and FPP. Table 2 shows numbers of the false positives observed per 1 h when the wake-up receiver continuously detects WLAN frame given in the trace file. {a, b, c} represents a case where a wake-up signal consists of symbola symbolb, and symbolc. When we employ n = 5, no false positive is observed even if the wake-up signal consists of only frequently observed symbols. On the other hand, if we employ n = 3, false positives are observed: when m = 10 and wake-up signal is {4, 4, 4}, 5.8

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1

m=5, n=3 m=5, n=5 m=10, n=3 m=10, n=5

0.8

False positive probability

0.8

False positive probability

1

m=5, n=3 m=5, n=5 m=10, n=3 m=10, n=5

0.6

0.4

0.2

0.6

0.4

0.2

0

0

10-3

10-2

10-1

100

10-3

Bit error ratio

1

False positive probability

0.6

0.4

m=5, n=3 m=5, n=5 m=10, n=3 m=10, n=5

0.8

0.6

0.4

0.2

0.2

0

0 10-3

10-2

10-1

100

10-3

10-2

10-1

Bit error ratio

Bit error ratio

c) Symbol group3 (1670–2120 µs)

d) Symbol group4 (2150–2600 µs)

Fig. 13. False positive probability of the proposed ROD system.

6

Number of detection

False positive probability

100

b) Symbol group2 (1190–1640 µs)

m=5, n=3 m=5, n=5 m=10, n=3 m=10, n=5

0.8

10-1

Bit error ratio

a) Symbol group1 (710–1160 µs) 1

10-2

10

5

10

4

10

3

10

2

10

1

10

0

10

5

10

15

20

25

30

35

40

45

Symbol index Fig. 14. Number of each symbol detected in the trace data.

50

55

60

65

100

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Y. Kondo et al. / Computer Communications 35 (2012) 1725–1735

Table 2 Average number of false positive per 1 h. Wake-up Sig. (n = 3)

m=5

m = 10

Wake-up Sig. (n = 5)

m=5

m = 10

{28, 28, 28} {23, 23, 23} {4, 4, 4} {28, 23, 4}

0.0 0.1 2.2 0.0

0.8 0.6 5.8 0.1

{28, 28, 28, 28, 28} {23, 23, 23, 23, 23} {4, 4, 4, 4, 4}

0.0 0.0 0.0

0.0 0.0 0.0

Table 4 Examples of amount of saved energy per year (power consumption and its crude oil equivalent). RActive 0.1 106

N

10

7

108

of false positive is observed on average during 1 h. However, as shown in Table 2, we can reduce number of false positives by employing smaller m. Another way to reduce number of false positives is to avoid simple repetition of frequently observed symbols for the wake-up signal. As shown in Table 2, when we employ a combination of the frequently observed symbols, {28, 23, 4}, as the wake-up signal, number of false positives is significantly reduced for each m. Note that, as stated above, these results are upper-bound of false positive since we intentionally employed only symbols which are frequently observed within the trace. If we employ less-observed symbols for the wake-up signal, we can certainly achieve much less number of false positives. 5.4. Discussions of settings of m and n As discussed in Section 5.1, FNP is decreased by increasing m or by decreasing n. On the other hand, as in Section 5.3, the larger m and smaller n cause false positive with higher probability. Thus, there is a clear trade-off between the achieved delay and energyefficiency. Table 3 shows summary of simulation evaluations. As observed from the results in Section 5.1 and Section 5.2, it is necessary to employ n = 3 or n = 5 and m = 10 in order to achieve delay time less than that of passive scanning when there are 2 interrupting nodes. If we define permissible FPP caused by a continuous interfering signal is 0.2, both of FPP and delay requirements can be satisfied with n = 5 and m = 10. With this setting, false positive caused by WLAN data frame transmitted by other user is not observed in the simulation results shown in Table 2. As a result, only a pair of n = 5 and m = 10 achieves assumed requirements for both false negative and false positive. In this case, the number of symbols for wake-up signals is 64 and each symbol conveys 6 bits per symbol. Each wake-up ID can consist of 30 bit length. Furthermore, by using only symbols which are included in group3 and group4, FPP less than 0.2 can be achieved with n = 3 and m = 5. The delay in Fig. 12 is obtained with n = 5, therefore, by employing n = 3, we have certainly smaller delay, which enables us to satisfy the delay requirement even with these parameters. In addition, by avoiding employing frequently observed symbols within WLAN trace for wake-up signals, the number of false positives can be reduced adequately. If the 4 most frequently-observed frames are eliminated from symbols for wake-up signals, then the number of symbols for wake-up signals is 28 and each symbol conveys about 4.8 bits per symbol. In this case, each wake-up ID can consist of 14 bits length. In order to optimize m and n more precisely, requirements for false positive and false negative based on analysis of real-world traffic in 2.4 GHz band are necessary. Furthermore, in the simulation

Table 3 Summary of simulation evaluations.

n=3 n=5 n=3 n=5

m=5 m=5 m = 10 m = 10

FN

FP (Cont.)

FP (WLAN)

U  U U

U(conditional) U  U

U(conditional) U  U

3.8  104 8.1  103 3.8  105 8.1  104 3.8  106 8.1  105

[MWh] [t] [MWh] [t] [MWh] [t]

0.3

0.5

2.9  104 [MWh] 6.3  103 [t] 2.9  105 [MWh] 6.3  104 [t] 2.9  106 [MWh] 6.3  105[t]

2.1  104 4.5  103 2.1  105 4.5  104 2.1  106 4.5  105

[MWh] [t] [MWh] [t] [MWh] [t]

tests of number of false positive caused by WLAN data frames, we analyzed only 19 h of WLAN trace data. For precise evaluations of number of the false positive, analysis on WLAN traffic in diverse places and time is necessary. Measurements of real-world traffic and optimization of parameters of ROD WLAN based on analysis of the measurements are kept for our future works. 5.5. Evaluations of total energy saving ROD WLAN enables APs to be put into a sleep mode when there is no associated STA. In this subsection, we discuss the amount of energy saving brought by the proposed ROD WLAN. We define power consumption of an active AP as PActive [W] and ratio of time where the AP is used (one or more STAs associated with the AP) as RActive. Then, ratio of time where the AP can be in the sleep mode is denoted as (1  RActive). Power consumption of the AP when it is in the sleep mode is denoted as PSleep [W]. In this case, power consumption of AP in ROD WLAN (PROD_AP) is denoted as follows:

PROD

AP

¼ PActiv e RActiv e þ PSleep ð1  RActiv e Þ

ð2Þ

APs without employing ROD technologies are always powered on regardless of AP states, therefore, its power consumption is PActive. Then, power consumption saved by ROD WLAN for each AP (PSaved) is represented as follows:

PSav ed ¼ ðP Activ e  PROD

AP Þ

¼ ð1  RActiv e ÞðP Activ e  PSleep Þ

ð3Þ

As RActive decreases, PSaved becomes larger. ROD WLAN is more effective when APs have a larger amount of idle periods. The amount of saved energy per year is represented as 24365NPSaved [Wh], where N is the number of APs which newly employs ROD technologies instead of being always powered on. Here, we try to estimate the amount of saved energy per year on a national scale. We employ 4.9 W as PActive based on Top Runner Program [16] defined in Japan, which lists values of power consumption to be achieved by energy-efficient consumer products. Whole energy consumption of receiver prototype shown in Section 4 is 110 mW including energy consumption of MCU. Therefore, we assume PSleep as 110 mW. Note that the receiver prototype is not optimized in terms of energy consumption and the energy consumption is considered to represent an upper limit. The numerical results with this calculation are shown in Table 4. We employ 106, 107 and 108 as N, and 0.1, 0.3, and 0.5 as RActive. We also calculate crude oil equivalent of saved energy. We assume 40% as oil-toelectric conversion efficiency and 41.9 GJ as energy released by burning 1 ton of crude oil [17]. If we assume that more than 107 of APs are operated in a country and their average ratio of active time is 0.3. Then, in the country, 2.9  105 MWh of energy (6.3  104 ton of crude oil) can be saved by our ROD WLAN. 6. Conclusions In this paper, we introduced radio-on-demand (ROD) WLAN, and provided the detailed design of identity based wake-up

Y. Kondo et al. / Computer Communications 35 (2012) 1725–1735

mechanism using IEEE 802.11 frame length modulation (FLM). We designed and developed a prototype of wake-up receiver which extracts the wake-up ID from the wake-up signal generated according to the proposed FLM. We also proposed a detailed procedure of the on-demand wake-up in ROD WLAN, and evaluated its impact on the system-level performance such as connection delay observed by users. We discussed results of the system-level evaluations based on the prototype evaluations and clarified the appropriate parameter settings for the wake-up signal and the wake-up receiver. We employed bit detection interval of 10 ls for on-off keying bit detector in the wake-up receiver and 30 ls step for FLM symbol. Evaluations using the prototype confirmed that we can achieve symbol error ratio of 3  104 when the received signal power is –82 dBm or more. Furthermore, simulation results, which were conducted based on experimental results of the prototype, confirmed that delay time less than that of passive scanning is achievable when the permissible false positive probability (FPP) is 0.2. All these results confirm the practicality and potential for our proposed wake-up protocol with FLM to realize energy-efficient WLAN without sacrificing QoS observed by users. Our future work includes the design of sleep scheduling, system-level evaluations with testbed which is currently under development, and measurements of real-world traffic over 2.4 GHz band for optimization of parameters of ROD WLAN.

Acknowledgement This work is supported by the Promotion program for Reducing Environmental loaD through ICT innovation (PREDICT) funded by Ministry of Internal Affairs and Communications, Japan.

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