A Novel Idle Mode Operation in IEEE 802.11 WLANs - MWNL

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The reason is rooted on IEEE 802.11 WLAN standard [2], which defines ... propose a novel Idle Mode (IM) operation. ... the IP paging, is not able to fall into idle state properly ..... [8] http://www.proxim.com/learn/library/datasheets/11bpccard.pdf.
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A Novel Idle Mode Operation in IEEE 802.11 WLANs Sunggeun Jin1 , Kwanghun Han2 and Sunghyun Choi3 School of Electrical Engineering and INMC Seoul National University, Seoul, Korea 1 [email protected], 2 [email protected] and 3 [email protected] Abstract—While the IEEE 802.11 Wireless Local Area Network (WLAN) became a prevailing technology for the broadband wireless Internet access, new applications such as Internet Protocol (IP) telephony are fast emerging today. For the battery-powered IP phone devices, the standby time extension is a key concern for the market acceptance while today’s 802.11 is not optimized for such an operation. In this paper, we propose a novel Idle Mode operation, which comprises paging, idle handoff, and delayed handoff. Under the idle mode operation, a Mobile Host (MH) does not need to perform a handoff within a predefined Paging Area (PA). Only when the MH enters a new PA, an idle handoff is performed with a minimum level of signaling. Due to the absence of such idle mode operation, both IP paging and Power Saving Mode (PSM) have been considered the alternatives so far even though they are not efficient. We develop a new analytical model to comparatively evaluate our proposed scheme. Our numerical results demonstrate that the proposed scheme outperforms the legacy alternatives with respect to power consumption, thus extending the standby time dramatically.

I. Introduction Recently, the IEEE 802.11 Wireless Local Area Network (WLAN) became a prevailing technology for the broadband wireless Internet access. Along with that, new types of applications such as Internet Protocol (IP) telephony are fast emerging today. The IP phones require a functionality to inform a user of sporadic incoming calls even if the user holding an idle Mobile Host (MH) moves around. For the battery-powered IP phone devices, the standby time extension is a key concern for the market acceptance while today’s 802.11 is not optimized for such an operation. The reason is rooted on IEEE 802.11 WLAN standard [2], which defines only two operational modes in which a MH can operate, namely, Active Mode (AM) and Power Saving Mode (PSM). In both modes, since an MH always has to stay connected with one of the APs even when there is no traffic to/from the MH, it has to perform a handoff at every AP cell boundary. That is, the IEEE 802.11 WLAN is naturally weak in supporting the mobility of MHs when there is no traffic to be served for the MHs. In this paper, in order to support the desired functionality properly, we propose a novel Idle Mode (IM) operation. Due to the absence of such an IM operation, both IP paging and PSM have been considered the alternatives to the IM operation so far. In [5], the authors introduce an IP paging to support IP-level mobility, when an MH operates in PSM. The IP paging proposed in [5], does not support efficient power consumption for an MH since both IP-level paging operation and PSM operate independently

so that IP paging has nothing to do with a PSM. For a longer standby time when adopting the IP paging, more suitable power saving scheme rather than PSM has to be considered. In [4], the authors propose efficient IP-paging schemes. If there is no Media Access Control (MAC)-level paging scheme, IP paging could be used as an alternative. However, because the original IP-paging concept is developed to support the mobility for IP layer regardless of MAC layer, the authors of [4] deals with only IP-paging algorithms ignoring MAC operations under the assumption that the MAC of an idle MH operates in an efficient power saving manner. Actually, in the case of the IEEE 802.11 WLAN standard, the MAC operates in the PSM when utilizing the IP paging. The MH in PSM, while running the IP paging, is not able to fall into idle state properly since the MH has to perform necessary operations in order to maintain the connection with an AP and perform IP paging-related operations. The authors of [6] show a good example of power saving by adopting the paging concept. The authors develop a practical device with the ability to receive paging signals, which can wake up an idle MH when a packet destined to the MH exists. The device for paging works so well that the MH with the device achieves longer standby time. However, the approach proposed in [6] is not standard compliant. In order to overcome the above explained problems caused by the inefficiency regarding power consumption when an IEEE 802.11 WLAN standard-based MH does not have traffic or on-going sessions, we propose an IM operation, comprising paging, idle handoff, delayed handoff, for IEEE 802.11 WLANs. Under this operation, the MH can stay longer time in the doze state and performs less operation than in the PSM. By utilizing our scheme, an MH does not perform any handoff within a predefined Paging Area (PA). The handoff with minimum operations, called idle handoff, is performed only when an MH leaves a PA. The paging provides a method to inform MHs in the IM of a new packet arrival resulting in an efficient power saving manner. The IP-level handoff is deferred until a paging success, and hence, it is called delayed handoff. The rest of the paper is organized as follows. In Section II, we discuss the limitations of PSM and IP paging as the alternatives to the IM. In Section III, we introduce the IM for the IEEE 802.11 WLAN. Additionally, we propose new protocols for paging, idle handoff, and delayed

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handoff constituting Idle Mode. In Section IV, we develop an analytical model to evaluate the proposal. Through our mathematical model, we evaluate our proposal and demonstrate the superiority of our proposal compared to the legacy schemes. Finally, Section V concludes the paper with the summary of our efforts and results. II. Problems to Support Idle Mode in IEEE 802.11 WLANs The purpose of paging is to locate an idle MH when an incoming call destined to it arrives. However, if an IP-based wireless access network does not offer a paging scheme, IP paging can be used as the substitutional scheme of the original access network level paging. In the case of IEEE 802.11 WLANs, utilizing both PSM and IP paging has been considered. However, since the PSM is developed without considering IP paging, the use of IP paging along with the PSM is an inefficient approach. In this section, we discuss the reason why the combined PSM and IP paging are not suitable as an alternative to the IM for IEEE 802.11 WLAN. An MH consists of Wireless Network Interface Card (WNIC) and Handheld Device (HD) such as Personal Digital Assistance (PDA) or smart phone. The WNICs implemented based on the IEEE 802.11 WLAN standard have a MAC layer while the IP layer, as a part of Operating System (OS), is embedded in HDs. Therefore, in the perspective of functionality, WNIC is a representative of the MAC while HD is a representative of the IP. For this reason, in this paper, we use the term of WNIC and HD instead of station (STA) used in IEEE 802.11 WLAN standard and IP layer, respectively. A. Limitation of IEEE 802.11 PSM According to [2], a WNIC can be in either of awake and doze states at a given time. In awake state, a WNIC can transmit, receive or sense the physical channel, and it actually continues to sense the channel unless it either transmits or receives a frame. On the other hand, in doze state, a WNIC is not able to transmit or receive, and consumes very little power. How a WNIC switches between these two states is determined by the WNIC’s power management mode, i.e., AM and PSM. When a WNIC is in the AM, the WNIC always operates in the awake state. On the other hand, when a WNIC is in the PSM, the WNIC can change its state between the awake and doze states depending on the traffic pattern. However, the PSM as an alternative to the IM has limitations as follows. Since a WNIC running in the PSM should stay associated with an AP, the handoff should be performed at every AP cell boundary in order to maintain its association. During a handoff procedure, the WNIC has to stay in the AM since the handoff can be severely delayed otherwise. As to Layer-3 (L3) handoff, it takes several seconds to perform an L3 handoff due to the L3 operation features [14]. Note that mobility is harmful to power saving schemes since more power is consumed as the frequency of handoffs increases. Moreover, inevitable handoffs cause

additional problems such as signaling cost increase and session dropping. B. Limitation of IP Paging The original IP paging is targeted at the MHs which do not have on-going IP session. With the aid of IP paging, the network load and signaling cost to manage the mobility for the idle MHs are reduced. However, most of the algorithms [4], [5] are designed without considering the underlying MAC operations. This means that the IP paging and MAC-specific paging operate independently. We can assume that a particular wireless access network has its own paging algorithm and IP paging is also adopted by the wireless access network. In such a case, the IP paging protocol actually generates redundant overheads since both MAC and IP provide the same functionality, i.e., paging. Due to the absence of support for the paging in IEEE 802.11 WLANs, IP paging would be a useful alternative to the MAC-level paging. However, the IP paging scheme as an alternative to the MAC-level paging in IEEE 802.11 WLANs has the following limitations. Since the IM is not defined in [2], a MH should be in the PSM instead of the IM, and has to get associated with one of the APs all the time. In order to support IP paging, the MH, being associated with an AP, periodically listens to the IP packets for signaling, and performs IP pagingrelated operations. Despite the fact that the power consumption should have been a major concern, the research efforts related to IP paging made so far have been limited only to the signaling cost reduction. Additionally, the IP layer, located right above the 802.11 WLAN MAC, receives all types of IP packets through the MAC. Such tightly coupled relationship causes unexpected side effect to the PSM operation. For instance, if we assume that Paging Mobile IP (P-MIP) is adopted to IEEE 802.11 WLAN, the routers or Mobile IP (MIP) agents broadcast advertisement messages periodically when some of MHs are in the IM defined by [4]. Upon receiving the advertisement messages, the MHs in the IM are forced to wake up and receive the advertisement messages to perform P-MIP-related operations. It is impossible for a WNIC to differentiate P-MIP control packets from other broadcast packets. Therefore, the WNIC has to receive all broadcast packets, and after that, it is possible for the P-MIP layer to pick P-MIP control packets out of the received packets. From such a mechanism, one can easily imagine that an MH with both WNIC and HD having P-MIP consumes more power due to the unnecessary broadcast packets. In order to verify the reasoning, we measure the interarrival time of broadcast and multicast packets for 24 hours in a large-scale commercial WLAN operated by Korea Telecom. As shown in Table I, surprisingly the inter-arrival times under 10ms represent the major portion. The average packet inter-arrival time is 22.7845 ms. This means that there will be 4 to 5 broadcast/multicast packets every beacon interval (assuming 100 ms beacon interval) in average, and hence the WNIC has to wake up very often, e.g., every beacon interval, to receive these packets. Therefore,

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TABLE I Multicast and broadcast packet inter-arrival time Statistics 0∼10 ms 67 %

10∼100 ms 27 %

100ms∼1 s 6%

≥1 s 0%

Fig. 1. Paging area

Fig. 2. Procedure for the idle mode operation

any MHs adopting IP paging are compelled to consume their power in vain in order to receive unnecessary packets.

defined Paging Area Identifier (PAID) field. Each WNIC in the IM can differentiate a PA via its PAID. We define a new procedure, which is compatible to IEEE 802.11 WLAN standard, in order to support the IM. Fig. 2 shows the procedure when a WNIC enters and leaves the IM. After a session (e.g., a VoIP session) completion, the WNIC transmits a Disassociation-Request frame with Power saving Mode (PM) bit (in the frame control field) set to ‘1’ in order to enter the IM. After receiving the corresponding Disassociation-Response from its AP, the WNIC in the IM can move around within the same PA while the AP, which transmitted the Disassociation-Response, keeps the information for the WNIC to perform a handoff procedure in the future. This AP is referred to as Home-AP. After entering the Idle Mode, the WNIC starts listening to the beacons periodically (e.g., every 1 s). Even when a WNIC recognizes the change of AP cell through the beacon information, the WNIC keeps listening to the beacons only as long as the WNIC stays in the same PA. This continuous beacon listening operation is called AP-Reselection. For an efficient AP-Reselection, there could be many optimization issues as addressed in [1], [3]. However, we do not consider the AP-Reselection issues since they are beyond the scope of this paper. For simplicity, AP-Reselection is assumed to be performed without overhead, e.g., scanning, via optimization.1 When a packet destined to a particular WNIC in the IM arrives at the Home-AP, the Home-AP broadcasts a PageNotify message to all the APs, belonging to the same PA, which in turn start paging the destination WNIC. That is, the APs convey the paging information via their beacon frames. If a WNIC recognizes that it is paged by receiving such beacon(s) from an AP, it attempts to associate with the AP by transmitting a Reassociation-Request frame. After finishing all the preparations for serving the WNIC,

III. Proposed Idle Mode Operation A. Definition of Idle Mode In order to overcome the problems discussed in Section II, we define a new mode, i.e., IM, for IEEE 802.11 WLAN. We attempt to minimize required operations for the IM in order to maximize the power saving. When a WNIC is in the IM, it performs only essential operations to wake up in the future. The necessary operations for the IM are defined as follows: 1. A handoff does not occur at every cell boundary unlike a WNIC in the PSM. A handoff, called idle handoff, is performed only when an MH leaves a PA to enter another PA. 2. When a WNIC is in the IM, the WNIC is not associated with any AP. The only thing that the WNIC in the IM has to do is to listen to the beacons periodically at every predefined interval in order to switch itself to Active Mode when a packet destined to itself arrives. The typical beacon listening interval for receiving beacons to wake up is set to be 1 s, while beacons are transmitted by APs every 100 ms typically. 3. Only a successful paging makes a WNIC in the IM enter Active Mode. The paging operation to make a WNIC in the IM to wake up is presented in the following. B. Protocols for Idle Mode Fig. 1 illustrates the PA structure for paging. A number of neighboring AP cells are grouped into a PA. The APs belonging to different routers can also be grouped into a single PA. The APs in the same PA have the same identifier, which is broadcast through the beacons with a newly-

1 For example, the Neighbor Report information from the emerging 802.11k [3] will make this possible in the near future.

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Fig. 4. State diagram for Markov chain modeling

Fig. 3. Idle handoff

the new AP replies to the WNIC with an ReassociationResponse frame and broadcasts Paging-Success to the APs in the same PA to stop paging operations of these APs. At the same time, after a successful paging for the WNIC, the MH with the WNIC starts to perform a delayed handoff operation as explained below. C. Idle and Delayed Handoffs Idle handoff is the handoff that is performed whenever a WNIC in the IM moves across a PA boundary. Fig. 3 shows the procedure for idle handoff. After a WNIC enters a new PA, which can be identified by a newly-received beacon, it transmits a Reassociation-Request frame with the BSSID of its Home-AP. In the Reassociation-Request frame, the PM bit is set to ‘1’. Upon receiving the Reassociation-Request frame, the new AP replies with a Reassociation-Response frame. Then, the WNIC transmits Disassociation-Request frame in the same manner as to initially enter the IM, i.e., with PM bit set to 1. After the completion of a successful 3-way management frame exchange, the WNIC resumes listening to the beacons periodically in order to receive the paging information. The AP, which is involved with the 3-way frame exchange, is referred to as Most Recently Associated AP (MRA-AP). Now, the MRA-AP initiates to obtain the authentication information about the WNIC from the Home-AP. Through the operations, the MRA-AP performs a user validation check using the MAC address of the WNIC. Note that the user validation check is performed after the completion of the frame exchange with the WNIC in order to reduce the WNIC’s awake time as well as power consumption. After a successful validation check, the Home-AP informs the old MRA-AP in the PA, which the WNIC previously visited immediately before entering the new PA, by transmitting a Remove-Context message, that the WNIC moves to the new MRA-AP. After receiving the Remove-Context, the

old MRA-AP removes the information of the WNIC. There could be several security issues about our scheme. However, more detailed security issues are beyond the scope of this paper. When there is at least one idle handoff, the Home-AP actually transmits a Page-Notify to the MRA-AP, which in turn forwards it to all the APs in the same AP. Since our proposed scheme enables IEEE 802.11 WLAN to keep track of the locations of the MHs in the IM, the IP layerrelated operations including IP paging becomes redundant. That is, our proposed protocol replaces IP paging. Therefore, in our approach, the handoff operations related to the IP layer are postponed until a successful completion of paging. For this reason, we call this handoff operation, which delays the activation of IP layer, a delayed handoff. During performing the delayed handoff, the operations to check the user validity are also performed. IV. Performance Evaluation A. Assumptions We consider a VoIP telephony as the target application for our analysis. When a user uses a VoIP phone, a session is initiated by an incoming or outgoing call. We assume that the PSM is not used while a VoIP session is on-going, i.e., the state transition from the awake state to the doze state does not occur during the whole on-going session time. We also assume that an MH is always powered on in order to receive incoming calls. Fig. 4 shows two operational states of an MH with a WNIC for a Markov chain modeling. 1. State 1 (AM): When an MH is in this state, WNIC is in the awake state and the HD is powered on. The MH has an on-going session for traffic. The MH performs a handoff whenever it moves across AP cell. The transition to the doze state occurs when the session is terminated. 2. State 2 (IM or PSM): When an MH is in this state, a WNIC switches between the awake and doze states every predefined time interval in order to receive the beacon including the paging information. If the IM is utilized, the WNIC performs an idle handoff whenever it leaves each PA. The only successful paging or outgoing call makes the MH enter State 1. Since an MH with WNIC in the IM does not need to perform the IP operation as explained in Section III, the HD transits its power-mode to standby-mode. On the other hand, when the legacy scheme, i.e., utilizing both IP paging and PSM, is performed [7], the MH must perform inter-AP handoff or both inter-AP handoff and the IP-related handoff even if IP paging scheme is adopted. By this reason, in this case, the HD is powered on.

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TABLE II Parameter Definitions Parameter Tbli P W N awk P W N slp P HD act P HD slp Tb Tp T IHO T DHO T ras T das T auth T IAP P T 1x T L3HO N P A cng Nb N L2HO N L3HO Dp

Definition beacon listening interval ave. power consumption of WNIC in awake state min. power consumption of WNIC in doze state ave. power consumption of HD being active min. power consumption of HD being idle beacon frame transmission duration ave. time for paging procedure ave. time for idle handoff procedure ave. time for delayed handoff procedure ave. time for reassociation procedure ave. time for disassociation procedure ave. time for authentication procedure ave. time for IAPP procedure ave. time for 802.1x procedure ave. time for L3-level handoff ave. number that an MH leaves PAs ave. number of beacon listening in IM ave. number of L2-level handoffs ave. number of L3-level handoffs ave. delay for paging message delivery

B. Numerical Analysis In our numerical analysis, we calculate the power consumed by an MH with WNIC adopting the IM. The steadystate analysis is based on the semi-Markov process because state changes occur with a Markov chain, but take a random amount of time between changes. The analyses based on semi-Markov process were previously presented in [11], [12]. In our numerical analysis model, we derive a new energy consumption model. In order to determine the steady state probabilities and the average energy consumption of an MH in each state, i.e., State 1 and State 2, we make the following assumptions. 1. Incoming and outgoing calls at an MH occur according to a Poisson process with rates λin and λout , respectively. 2. Session holding time is generally distributed with a density function fs (t) with the mean 1/λs . 3. The cell sojourn time and the PA sojourn time are i.i.d., and follow exponential distributions with average 1/λcs  and 1/λP A , respectively [13]. Moreover, 1/λP A = Nap /λcs , where Nap is the average number of APs in a PA. The parameters used for the analysis are listed in Table II. In Fig. 4, P12 and P21 are the state transition probabilities, representing a session completion in State 1 and a session arrival in State 2, respectively. Both P12 and P21 are simply 1, and hence we can easily obtain the stationary probabilities of this Embedded Markov Chain as π1 = 1/2 and π2 = 1/2, respectively. In addition, we can analyze the average time, which the MH stays in each state, as T 1 = 1/λs and T 2 = 1/(λin + λout ) + Dp , where Dp = Tbli /2. Then, we obtain the steady state probabili-

ties of the semi-Markov process as follows: πi T i , P i = 2 j=1 πj T j

i = 1, 2.

We first determine the energy E 1 spent in State 1 for an arbitrary time, t, as follows: E1 = EW N

awk

+ E HD

act .

where E W N awk = P W N awk P1 t and E HD act = P HD act P1 t, respectively. Now, the energy E 2 spent in State 2 for an arbitrary time, t, is dependent on whether the IM is employed or not. When the IM is employed, E 2 = E IM , and E IM =E W N

slp

+ E HD

slp

+ E p λin /(λin + λout ) + E b N b

+ E IHO N P A cng + E DHO P r[N P A cng > 0], where the first two terms represent the energy consumed when WNIC and HD are in doze and idle states, respectively, and are determined by EW N

slp

+ E HD

slp

= (P W N

slp

+ P HD

slp )

· (P2 t − T p

− T IHO N P A cng − T DHO Pr[N P A cng > 0]). Here, we assume that P2 t  T p , which should be reasonable. Second, the energy required for paging E p = P W N awk T p and λin /(λin +λout ) represents the proportion of the incoming calls. Third, the energy for the periodic beacon listening E b N b = P W N awk T b N b , in which the number of the beacon listening in the IM during time t is N b = P2 t/Tbli . Fourth, the energy consumed for each idle handoff E IHO = P W N awk (T ras +T das ), and idle handoffs occur N P A cng times, where N P A cng = λP A P2 t. Finally, when the MH moves across a PA boundary at least once with probability Pr[N P A cng > 0], a delayed handoff is initiated once, and E DHO = (P W N awk + P HD act )(T DHO ). Since we assume that the PA sojourn time follows exponential distribution, Pr[N P A cng > 0] = 1 − e−λP A P2 t . On the other hand, if the IM is not employed, E 2 = E P SM , and E P SM =E W N

slp

+ E HD

act

+ EbN b

+ E L2HO N L2HO + E L3HO N L3HO .

(1)

where the energy required for an MS in the PSM E W N slp = P W N slp · (P2 t − T b N b − T L2HO N L2HO − T L3HO N L3HO ). The energy for an L2 handoff E L2HO = T L2HO P W N awk = (T ras + T auth + T IAP P + T 1x )P W N awk and the energy for an L3 handoff E L3HO = T L3HO P W N awk , in which P L3HO is the required power for IP handoff operation, respectively. N L2HO and N L3HO are λcs P2 t and λP A P2 t, respectively. Now, we can determine the average power consumption of an MH by P total = (E 1 + E 2 )/t.

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100

1400

Idle Mode, Tbli=1s PSM, Tbli=1s PSM, Tbli=100ms

TABLE III Parameter Values

Idle Mode, Tbli=1s PSM, Tbli=1s PSM, Tbli=100ms

1200 90

Parameter

80

Average Remaining Energy (mAh)

Average Power Consumption (mW)

1000

70

60

Tb T ras T L2HO

800

P W N awk P HD act

600

400

λin 1/λs 1/λP A Battery cap.

50 200

40

Value 5 ms 0.1 ms 100 ms/1 s 45 mW 86 mW 2 times/h 2s 115 meters 10

0 0

50

100 150 200 250 300 350 400 Average Cell Sojourn Time (second)

(a) Power consumption

450

500

0

2

4 6 Standby Time (hour)

8

(b) Remaining energy

Fig. 5. Comparison of proposed and legacy schemes

C. Numerical Results Table III lists the values of all the parameters used for the numerical evaluation including (1) the measured values (from Cisco AP) and (2) the values from the data sheets (related to the power consumption) [8], [9]2 , and finally (3) some assumed values. For simplicity, we ignore the state transition overhead of a WNIC. Fig. 5 (a) shows the average power consumption of a WNIC, when in the State 2, as the cell sojourn time varies. We observe that our proposed scheme consumes less power than the legacy scheme with different beacon listening interval Tbli . Even if the same value of Tbli is used for both the legacy scheme and our scheme, ours outperforms the legacy one thanks to the fact that ours does not require to perform handoff at every cell boundary. As to the power consumption required for the legacy scheme, as the cell sojourn time increases, the power consumption for handoff decreases, and eventually converges to a constant value. Fig. 5 (b) depicts the remaining energy of an MH as the “standby time” increases when the average sojourn time 1/λcs is set to 133 seconds under the assumption that an MH walks at a velocity of 6 km per hour. For numerical analysis, we assume that the HD is activated in active state because the HD must perform IP related operations embedded in the OS when the PSM is utilized. The power consumption under this condition can be easily derived from Eq. (1) by setting E HD act a constant value. As shown in Fig. 5 (b), it takes 9.04 hours to exhaust an MH’s energy if our scheme is employed while it takes 5.66 hours and 5.53 for the legacy scheme with Tbli set to 100 ms and 1 s respectively. The reason why the standby times are nearly the same for different Tbli values is because the power consumption for receiving beacons is negligible compared with the power consumption required for system-wide operations. From all the above observations, we conclude that our scheme makes an MH stay longer in the standby mode since it needs less energy consumption compared with the 2

Value Parameter Measured values 500 µs Tp 1.3 ms T das 178 ms Tbli From the data sheet (925+2565)/2 mW P W N slp 625 mW P HD slp Assumed values 2 times/h λout 5 min T L3HO √ AP s per P A/λcs Cell radius 1250 mAh APs per PA

PWN

awk

is the average of the reception and transmission powers.

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legacy scheme. V. Conclusion In this paper, we propose a new protocol to support the Idle Mode operation in the 802.11 WLAN. The proposed protocol can be easily applied to already-deployed products by just updating their firmwares or device drivers. In order to evaluate our proposal, we develop an analytical model. The numerical results demonstrate that our proposed IM operation outperforms legacy schemes with respect to the power consumption. As a result, it enables a longer standby time of the 802.11-equipped MHs. References [1] A. Mishra, M. Shin, and W. A. Arbaugh, “Context Caching using Neighbor Graphs for Fast Handoffs in a Wireless Network,” in Proc. IEEE INFOCOM’04, 2004. [2] IEEE Computer Society LAN MAN Standards Committee, IEEE Standard for Information Technology: Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, 1999. [3] IEEE 802.11k/D2.0 Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: Radio Resource Measurement: Draft Amendment 7 to IEEE 802.11 Standard-1999 Edition, Draft 2.0, 2004. [4] X. Zhang, J. Gomez, G. Castellanos, and A. T. Campbell, “PMIP: Paging Extensions for Mobile IP,” ACM Mobile Networks and Applications, July 2002. [5] R. Ramjee, L. Li, T. La Porta, and Sneha Kasera, “IP Paging Service for Mobile Hosts,” in Proc. ACM MobiCom’01, 2001. [6] E. Shih, P. Bahl, and M. J. Sinclair, “Wake on Wireless: An Event Driven Energy Saving Strategy for Battery Operated Devices,” in Proc. ACM MobiCom’02, 2002. [7] M. Liebsh and X. P´ erez-Costa, “Utilization of the IEEE 802.11 Power Save Mode with IP Paging,” in Proc. IEEE ICC’05, 2005. [8] http://www.proxim.com/learn/library/datasheets/11bpccard.pdf. [9] http://h18000.www1.hp.com/products/quickspecs/11646 na/ 11646 na.HTML#TechSpecs(IPAQ h5500). [10] http://download.intel.com/design/pca/applicationsprocessors/ datashts/28000304.pdf. [11] S. Kwon, S. Nam, H. Hwang, and D. Sung, “Analysis of a Mobility Management Scheme Considering Battery Power Conservation in IP-Based Mobile Networks,” IEEE Transactions on Vehicular Technology, Nov. 2004. [12] Y. Chung, D. Sung, and A. H. Aghvami, “Steady State Analysis of Mobile Station State Transition for General Packet Radio Service,” in Proc. IEEE PIMRC’02, Sep. 2002. [13] Y. Fang, I. Chlamtac, and Y. Lin, “Call Performance for a PCS Network,” IEEE Journal on Selected Area in Communications, Oct. 1997. [14] H. Yokota, A. Idoue, T. Hasegawa, and T. Kato, “Link Layer Assisted Mobile IP Fast Handoff Method over Wireless LAN Networks,” in Proc. ACM MobiCom’02, 2002.