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Exploring Power Saving in 802.11 VoIP Wireless Links
Exploring Power Saving in 802.11 VoIP Wireless Links
Exploring Power Saving in 802.11 VoIP Wireless Links
Exploring Power Saving in 802.11 VoIP Wireless Links
Exploring Power Saving in 802.11 VoIP Wireless Links
Exploring Power Saving in 802.11 VoIP Wireless Links
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Exploring Power Saving in 802.11 VoIP Wireless Links


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  • 1. Exploring Power Saving in 802.11 VoIP Wireless Links * Barry Gleeson Dorel Picovici Ronan Skehill John Nelson Wireless Access Research Group University of Limerick Limerick, Ireland ABSTRACT which it consumes more energy. In this state, while a station does not transmit nor receive it is considered to be in listening state Batteries are a primary resource in wireless networks for (Figure 1). In the states transmit, receive and listening an many mobile devices. Minimizing energy consumption interface without affecting communication activities is crucial to prolong the lifetime and improve the robustness of the wireless connection. One effective way to reduce energy consumption is to set the entire, or part of the system in a low power mode whenever possible. This paper explores a power saving approach for VoIP and presents a new power saving scheme that does not require extra protocol overhead or modifications to operate. Adopting this scheme potentially allows considerable power savings without any adverse effect on VoIP quality. Evidence of this is in simulation results, showing that a device can enter sleep mode for over 75% of the time, with no decrease in throughput. Categories and Subject Descriptors See the ACM template, ACM classification scheme and add the Figure 1. Different Operating modes of a Wireless Interface appropriate descriptors here. consumes significantly more energy than in sleep state. Recent studies [3][8] have shown that nodes in the listening state General Terms: Limited to the 16 general terms, pick your consume only a little less energy than transmitting or receiving. choices and add here Hence, sending and receiving are not the dominant source of energy consumption, being awake and ready to send or receive Keywords: Voice over IP, wireless LAN, 802.11,power traffic is. As packets arrive at a constant rate for VoIP the wireless saving, sleep mode interface spends the intermediate time between successive incoming and outgoing packets in listening state 1. INTRODUCTION consuming unnecessary power. It is proposed that this time spent Voice over IP (VoIP) traffic is becoming more and more in listening mode could be spent in sleep mode leading to dominant and with the widespread existence of 802.11 networks considerable power savings. is in more demand. VoIP traffic requires a level of QoS to Section 2 describes the IEEE802.11 MAC medium access maintain a good quality voice conversation but consumes a mechanism and the power consumption during idle times for the relatively small amount of the total bandwidth offered by 802.11. case of VoIP. Section 3 explores the options in choosing transmit At any given time the energy consumption by a wireless interface and sleep times to reach the most optimal power saving approach. depends on its operating mode. A sleeping interface is one that Section 4 describes the power differences between sleep mode has its radio turned off and can neither transmit nor receive and wake mode. Section 5 describes a new Power Saving Protocol traffic. This therefore consumes little energy. For the station to be PS-WiVoIP. Section 6 introduces a test scenario, which is the able to transmit or receive it must transition to a wake state, in basis of simulation results. Section 7 concludes the paper. 2. IEEE 802.11 MAC AND VOIP Permission to make digital or hard copies of all or part of this work for In order to save power, the challenge presented is for the radio to personal or classroom use is granted without fee provided that copies are be able to transfer from listening mode to sleep mode whenever not made or distributed for profit or commercial advantage and that possible, without degrading the VoIP link. Power saving schemes copies bear this notice and the full citation on the first page. To copy otherwise, or republish, to post on servers or to redistribute to lists, such as PAMAS [10] and STEM [9] use a separate channel for requires prior specific permission and/or a fee. control messages thus allowing the main radio stay in sleep mode IWCMC’06, July 3-6, 2006, Vancouver, British Columbia, Canada. until actual data is to be sent. In PSM [5] and LISP [4] stations Copyright 2006 ACM 1-59593-306-9/06/0007…$5.00. * This work was supported by Science Foundation of Ireland grant 03/IN3/I396 779
  • 2. Figure 2. IEEE802.11 MAC backoff process [6]. sleep for specific times but wake up at every beacon interval to packets. This suggests a lot of time is spent in listening mode, check for possible incoming data. The beacon interval is generally wasting power. set to 100ms, which makes this approach unsuitable for VoIP, which requires data packets to be sent and received every 20ms. 3. POWER SAVING APPROACHES The Wi-Fi Alliance has introduced the Wireless Multimedia By using appropriate values for 802.11b the percentage time a (WMM) Power Save Mode [12] for VoIP based on the approach station will be busy sending or receiving can easily be calculated proposed in [2]. In this approach a station enters sleep mode until as the time taken for two successful VoIP packet transmissions it has an uplink voice packet to send, at which point it transmits divided by the period of the packets being sent/received. In the as normal. This transmission establishes an unscheduled service case of VoIP the period is 20ms. Allowing for all header period in which the station receives all buffered downlink packets information a packet of 160bytes being transmitted at 11Mbps from the access point. The problem with this approach is that it requires changes to the access point operation and also with will take 701μS to be successfully transmitted, i.e. TVoIP = 701μS numerous VoIP connections these unscheduled service periods (see Table 1). The Wireless VoIP Channel Utilization is therefore: may overlap. Here a different approach is proposed, whereby a wireless station synchronizes its outgoing and incoming VoIP packets to arrive within a short interval of each other, thus ( 2 701 × 10 −6 ) × 100 = 7.1% (1) 20 × 10 − 3 allowing the station to sleep for the remaining time before the next packet exchange. The approach is independent of the access It is proposed that for a VoIP application, by predicting the arrival point and therefore requires no changes to its operation. time of packets a station can enter sleep mode until the next expected arrival after it has sent its outgoing packet. Two observations are made about the optimal approach to take. In IEEE802.11 [6], when a channel is idle, time is measured in units called system slot times σ. The backoff integer is a number 3.1 Optimal Wake Time for Packet Reception of system slot times chosen in the interval (0, CW − 1) , Firstly the reception of packets is solely considered. A channel with contending traffic resulting in the MAC backoff process where CW is the current contention window size of the station. before transmissions is considered. By assuming that over one The CW parameter is an indication of the number of unsuccessful wireless VoIP link, the variation in packet arrival time is caused transmissions attempted by the station. CW is doubled every by the backoff process it can be concluded that the earliest a time a transmission fails up until it reaches the maximum CW , packet can arrive is when it is sent after a backoff stage of zero CWmax . After every successful transmission CW is reset to backoff slots and the latest it can arrive is when it is sent with a CWmin . While a station observes the medium to be idle the backoff of W0 (or later if a collision occurs). When reference is backoff counter is decremented. When a transmission is detected made to a backoff slot α , it is referred to in the same manner as counter-decrementing is stopped and is reactivated again discussed in [1]1. If a station wakes up after the earliest possible whenever the station is sensed idle for more than DIFS. The arrival time by a delay of t it runs the risk of missing the packet – backoff procedure of a number of stations is illustrated in Figure the bigger the value of t, the greater the risk. If t is too large the 2. Each station decrements its backoff time only while the packet will be missed and the sending station will increase its medium is idle. When a station backoff counter reaches 0 it contention window and retry sending the packet up until a transmits immediately. maximum number of retries. The sending station is modeled as VoIP traffic is typically sent at a rate of 64Kbps with a frame size of 160 bytes. Frames must therefore be sent at a rate of one every 1 A backoff slot is the time difference between two backoff decrements 20ms. With 802.11 networks operating at data rates up to 11Mbps and can either be idle in which case the duration will be equal to the in 802.11b or 54Mbps in 802.11a/g a very small fraction of the system slot time σ or it can be a lot longer if the channel in sensed busy total time is actually spent sending and receiving the VoIP for a period of time. 780
  • 3. having chosen a random backoff integer of b and the receiving station waking up a time t after the earliest possible arrival of the W0 , T * and α must be positive, the minimum value of Since all transmitted packet, i.e. E{TWAITED } will occur when t = 0 . This corresponds to an b = rnd (0, W0 ) average wait time of ⎛ W0 ⎞α , as would be expected. This suggests ⎜ ⎟ t = (0, W0 ) (2) ⎝ 2 ⎠ that for optimal power savings over a long period of time, a where W0 is the minimum contention window. In investigating station would ideally wake up to receive a packet at the earliest the optimal scenario of reception only, it is assumed that the possible moment of arrival, i.e. when t=0; probability of collisions with other links on the channel is 3.2 Optimal Send/Receive Contention constant at all time and therefore is not considered in the As a station must also send a packet it must be investigated calculations. The probabilities of the station waking up early whether (a) it is optimal to hold off sending the outgoing packet enough (t<b), on time (t=b) and late (t>b) are shown. until after the successful reception of the incoming VoIP packet or W0 − t t 1 (b) it is optimal to contend to send the outgoing packet at the Pt < b ] = , Pt > b ] = , P t =b ] = [ (3) same time as the incoming packet. It is assumed that in both cases [ W0 + 1 [ W0 + 1 W0 + 1 the stations’ last transmissions were successful (i.e. both stations To derive the times waited before successfully receiving a packet start with minimum contention window W0 ). It is also assumed we note that if the station wakes up before the transmission it will that the probability of collisions with packets from other stations wait the remainder of the backoff until the transmission. If the on the network is constant at all time and therefore is not station wakes after the transmission begins it must wait for the considered in the calculations. The two possible approaches are; * remainder of the transmission T plus another new backoff that (1) Wait to receive the packet before attempting transmission in this case will be derived from a doubled contention window (This approach reduces collision probabilities between incoming size, as specified in [6]. If it wakes at the exact time the and outgoing packets), or (2) Contend for the channel at the same transmission begins then no waiting is done. The times waited time as expected incoming traffic. before successfully receiving the packet are defined in (4). In Figure 3 approach (1) is shown. The station waits for the ⎧ (rnd (1, W0 − t ) )α t <b reception of the downlink packet from the access point and then TWAITED ⎪ ( = ⎨ T * + rnd (0,2W0 + 1) α ) t >b (4) attempts the uplink transmission. In this case two complete ⎪ 0 t =b backoffs take place, one by the access point and the other by the ⎩ station. The expected time for this send/receive to take place where α is the average length of a backoff slot and T is the * using approach (1), E1{T } is given by the following: average time that the station must wait for the transmission of a ⎛W ⎞ missed packet to complete. Using probability distribution the E1{T } = 2⎜ o ⎟α + 2(Tx ) (7) ⎝ 2 ⎠ average total time a station will wait before receiving a packet can be calculated. where W0 is the contention window, α is the average length of a ⎛W − t ⎞ ⎛ t ⎞ * ⎛ 1 ⎞ backoff slot and Tx is the transmission time of the packets being ⎟ ⎜ (⎟ ) ⎜ W + 1 ⎟(rnd (t + 1, W0 ) − t )α + ⎜ W + 1 ⎟ T + rnd (0,2(W0 ) + 1)α + ⎜ W + 1 ⎟(0) TWAITED = ⎜ 0 ⎜ ⎟ ⎝ 0 ⎠ ⎝ 0 ⎠ ⎝ 0 ⎠ sent. (5) In Figure 4 approach (2) is shown. Both stations begin contention The first term represents the case when the station wakes up for the channel at the same time. Three possible outcomes are before the transmission, the second term represents the case when shown. For the first two cases, Figure 4(a) and Figure 4(b), the the station wakes up too late and the third term is the case when total backing off time is clearly the maximum of the two backoff the station wakes up at the exact time the packet is sent. To values. It is essential that this approach also consider the added estimate the average time waited over a number of transmissions probability of collision between incoming and outgoing packets in all random variables are replaced by their average values. The the event that both have the same backoff value as shown in estimated average time a station will wait before successfully Figure 4(c). If the probability of more than one consecutive receiving a packet can be described as: collision is assumed to be minimal, then the average time taken to ⎛ W − t ⎞⎛ W0 − (t + 1) ⎞ ⎛ t ⎞⎛ * 1⎞ ⎜ W + 1 ⎟⎜ t + 1 + E{TWAITED } = ⎜ 0 ⎟ − t ⎟α + ⎜ ⎜ W + 1 ⎟⎜ T + W0 + 2 ⎟α ⎟⎝ ⎝ 0 ⎠⎝ 2 ⎠ ⎝ 0 ⎠ ⎠ ⎛ W − t ⎞⎛ W − t + 1 ⎞ ⎛ t ⎞⎛ * 1⎞ = ⎜ 0 ⎟⎜ 0 ⎜ W + 1 ⎟⎝ ⎟α + ⎜ ⎜ W + 1 ⎟⎜ T + W0 + 2 ⎟α ⎟⎝ ⎝ 0 ⎠ 2 ⎠ ⎝ 0 ⎠ ⎠ ⎛⎡ 1 ⎤ 2 ⎡ T * ⎤ ⎡W0 ⎤ ⎞ (6) = α⎜⎢ ⎟ ⎜ 2(W ) + 2 ⎥t + ⎢W + 1⎥t + ⎢ 2 ⎥ ⎟ Figure 3. Using approach (i), the station does not contend with expected ⎝⎣ 0 ⎦ ⎣ 0 ⎦ ⎣ ⎦⎠ incoming data for the channel. Instead it waits to successfully receive and then attempts transmission. 781
  • 4. The first term represents the time required when no collision occurs multiplied by the probability of this case. The second term represents the time required if the two packets collide multiplied by its probability. To evaluate which of the two approaches (1) and (2) gives the minimum average time, the difference between the two equations is defined in (11): E1 {T } − E 2 {T } = ⎡ βα 2T x W 0α Tx 2 βα 2T x ⎤ W 0α + 2T x − ⎢ βα + 2T x − − + + + + ⎥ ⎣ W 0 + 1 W 0 + 1 2W 0 + 2 W 0 + 1 W 0 + 1 W 0 + 1⎦ βα W0α Tx = W0α − βα − _ − W0 + 1 2W0 + 2 W0 + 1 ⎡ β W0 Tx ⎤ (11) = α ⎢W0 − β − − − ⎥ ⎣ W0 + 1 2W0 + 2 (W0 + 1)α ⎦ Noting that by definition for positive non-zero W0 , β < W0 , this equation will remain greater than zero when the following conditions hold: W0 ≥ 31 , Tx ≤ 5mS , α ≥ 20 μS . In a system that maintains these conditions approach (1) yields a greater average time than approach (2). It can therefore be concluded that optimal power savings will be made when two stations contend with each other for the channel concurrently, i.e. approach (2) is used. Figure 4. Using approach (ii), by contending with incoming traffic there 4. POWER CONSUMPTION are three possible outcomes. (a) The station gets the channel first (b) The The considered stations for the implementation of power saving access point gets the channel first; (c) They both access the channel at the same time causing a collision. VoIP are the U.L. MANET testbed wireless stations [11], which are each equipped with a Cisco Aironet 350 PCMCIA WLAN complete the send-receive process can be evaluated where the card. In experiments done on the stations it was shown that by total amount of time spent in contention is the maximum of the turning the WLAN cards radio off, the total drain on the whole two backoff values. The average maximum value of two random wireless station reduced by over 1.3W. As this was done at an application level transition times proved to be too large for the numbers chosen between 0 and W0 , β is evaluated as follows: purpose of power saving voice over IP. In [8] measurements are presented for the Cisco 350 WLAN card low power doze/sleep r1 = rnd (0,W0 ) state in which at a hardware level the station can turn off most of its circuitry but still maintain transition times to and from the state r2 = rnd (0,W0 ) of the values of 0.1ms and 1 ms. These times are sufficiently small for the sleep times between consecutive packet exchanges β = max (r1 , r2 ) for VoIP data and the power consumption of doze/sleep mode is as little as half that of idle mode, reduced from 1.4W to 0.75W[8]. {} W0 2 Eβ = (W0 + 1)2 ∑ n(n + 1) (8) 5. PS WIVOIP ALGORITHM n =0 PS WiVoIP is targeted at reducing the power usage of a station in a managed infrastructure network, with a duplex VoIP link to the The probability of two stations’ packets colliding, P is given col Access Point (AP), contending with other traffic on the wireless by: channel. It is motivated by the observations of Section III and is developed whereby the power saving station will buffer any 1 (9) Pcol = outgoing VoIP packet until the earliest expected time at which it W0 + 1 expects to receive a VoIP packet. The station will then attempt to transmit any buffered data, contending with its own incoming The average time it will take for the station to send and packet and any other traffic on the channel. Once a packet is both receive a packet using approach (2) E2{T } is given by the sent and received, the station enters sleep mode for the remainder following: of the time before the next expected arrival of a packet. The ⎛ 1 ⎞ ⎛ 1 ⎞⎛ ⎡W0 ⎤ ⎞ algorithm is as follows where nominally the training threshold is E2 {T } = ⎜1 − ⎜ W + 1 ⎟(βα + 2Tx ) + ⎜ W + 1 ⎟⎜ ⎢ 2 ⎥α + Tx + 2 βα + 2Tx ⎟ ⎟ ⎜ ⎟⎜ ⎟ ⎝ 0 ⎠ ⎝ 0 ⎠⎝ ⎣ ⎦ ⎠ defined as ⎡W0 ⎤ . ⎢2⎥ ⎢ ⎥ (10) 782
  • 5. BEGIN Table 1. Parameter values for Model Analysis and Simulation training_count=0; 31 W0 exp_arrival_time=∞; GoTo TrainingPhase; σ 20μS Training Phase: SIFS 10μS When station receives a packet at time = t: δ 2μS Set exp_arrival_time := min(exp_arrival_time,t)+0.02s; MAC Header 25μS = 272 bits @11Mpbs training_count ++; Send packet using 802.11 DCF. PHY Header 192μS = 192bits @ 1 Mbps If (training_ count>training_threshold) ACK 304μS = PHY Header+ 112 bits@1Mbps GoTo SleepPhase; DIFS 50μS = 2σ+ SIFS ELSE 116μS = 160 bytes @ 11Mbps LVOIP GoTo TrainingPhase; SleepPhase: TVOIP 701μS = DIFS + PHY Header +MAC Header + Sleep Until exp_arrival_time. LVOIP + SIFS + δ + ACK + δ At exp_arrival_time send packet using 802.11 DCF. LSAT 727μS = 1000 bytes @ 11Mbps After outgoing and incoming data transmission is complete: If (outgoing and incoming transmissions complete) 1312μS = DIFS + PHY Header +MAC Header + TSAT exp_arrival_time= exp_arrival_time+0.02s LSAT + SIFS + δ + ACK + δ GoTo SleepPhase; ELSE GoTo Begin; Taking into consideration the average backoff slot length it can be estimated that a backoff slot will contain at least one saturated END traffic packet every 1 slots. 6. ANALYSIS AND SIMULATION ⎛ W0 ⎞ ⎜ ⎟ ⎝ 2 ⎠ 6.1 Test Scenario For the proposed algorithm a network with a VoIP duplex link The average backoff slot time α is therefore: contending with a saturated 1000 byte link to the A.P. is ⎛ ⎞ considered. The VoIP duplex link consists of two flows of ⎜ ⎟ 1 ⎜1 − 1 ⎟(σ ) = 103μs 160bytes sent every 20ms. Parameters of Table 1 are used, where α= (TSAT ) + ⎜ ⎛ W0 ⎞ ⎛ W0 ⎞ ⎟ the left hand column contains MAC parameters and the right hand ⎜ ⎟ ⎜ ⎜ ⎟⎟ ⎝ 2 ⎠ ⎝ ⎝ 2 ⎠⎠ column contains the values used in the calculations. Using the proposed algorithm taking the best-case scenario in which no (16) collisions occur, the total percentage time the power saving VoIP where σ is the system slot time. station will be able to sleep Persleep is: Tx = TVoIP = 701μs ⎛ βα + 2Tx ⎞ PerSLEEP = ⎜1 − −3 ⎟ × 100 (14) Taking contention time into account, without collision the ⎝ 20 × 10 s ⎠ maximum sleep time over the longer term, a PS-VoIP β = ∑ n(n + 1) ∑ n(n + 1) W0 31 (15) station in this scenario could achieve while maintaining n =0 W0 = n =0 31 = 21 throughput for 802.11b is: ∑ (n + 1) ∑ (n + 1) ⎛ βα + 2Tx ⎞ ⎛ 3565×10−6 s ⎞ PerSLEEP = ⎜1 − ⎟ ×100 = ⎜1 − ⎜ ⎟ ×100= 82.18% 20×10−3 s ⎟ n =0 n =0 −3 ⎝ 20×10 s ⎠ ⎝ ⎠ (17) Due to a greater transmit rate this figure is greater for 802.11g. VoIP Saturated 6.2 Simulation and Results Link Link Using the ns2 version 2.28 simulator [7] a network with a duplex 160kbps VoIP link to an access point as well as a saturated link to the AP Duplex is set up using parameters as in Table 1. The simulation time is 20 seconds for the analyzed station in both standard and PS WiVoIP mode. The results shown (see Table 2) illustrate that the VoIP Figure 5. Test Scenario representing one VoIP duplex link contending throughput is maintained with little impact on the throughout of with a saturated link 783
  • 6. the saturated traffic. A sleep percentage of over 77% is achieved. 8. REFERENCES This represents 94% of the maximum sleep percentage possible [1] Bianchi, G. Performance analysis of IEEE 802.11 distributed calculated in the previous section. The 6% difference can be coordination function, IEEE Journal on Selected Areas in explained by the fact that the optimal scenario assumes no Communications, vol. 18, no. 3, pp. 535-547, March 2000. collisions with the saturated traffic and allows for no training [2] Chen, Y., Smavatkul, N. and Emeott, S. Power management phase. for VoIP over IEEE 802.11 WLAN, WCNC 2004 - IEEE Wireless Communications and Networking Conference, vol. 7. CONCLUSIONS 5, no. 1, March 2004 It has been shown in this paper that by accurately determining the [3] Feeney, L.M. and Nilsson, M. Investigating the energy earliest possible arrival time of voice packets over a wireless link Consumption of a Wireless network interface in ad hoc in an infrastructure network, a station can potentially spend a networking environment, In Proceedings of IEEE significant amount of time in sleep mode by synchronising its INFOCOM, April 2001. own outgoing packets with this arrival time. On a 802.11b link a [4] Hu, C. and Hou J. LISP: A Link-Indexed Statistical Traffic VoIP wireless station can enter a sleep state for over 75% and Prediction Approach to Improving IEEE 802.11 PSM, In even more in 802.11g. It was analytically determined that a Proc. IEEE Int'l Conf. on Distributed Computing Systems station should be in wake mode at the earliest anticipated arrival (ICDCS'04), March 2004. time. It was also determined that under typical 802.11 MAC [5] IEEE standard for Wireless LAN-Medium Access Control settings the most optimal power saving approach is when the and Physical Layer Specification, 802.11, November 1997. VoIP station attempts to transmit at the same time as the access P802.11. point begins attempting transmission to it. The PS-WiVoIP [6] IEEE 802.11. Wireless LAN Medium Access Control (MAC) scheme uses no extra protocol overhead or modifications to and Physical Layer (PHY) Specifications, 1999. operate. In simulation the throughput achieved by using this [7] Network Simulator (ns), University of California at protocol is maintained. Berkeley, CA, 1997. Available at [8] Qadeer, W., Rosing, T.S., Ankcorn, J., Krishnan, V. and De Table 2. Simulation Results with VoIP station operating in Micheli, G. Heterogeneous wireless network management In both Standard and proposed PS-WiVoIP. Proceedings of PACS 2003, SanDiego, CA, USA, December 2003, Standard PS WiVoIP [9] Schurgers, C., Tsiatsis, V. and Srivastava, M. STEM: UpLink VoIP Topology Management for Energy Efficient Sensor 160× 103 160×103 Networks. In IEEE Aerospace Conference '02, Big Sky, MT, bytes bytes March 10-15, 2002. DownLink VoIP 3 160× 10 160× 103 [10] Singh, S. and Raghavendra, C.S. PAMAS: Power Aware bytes bytes Multi-Access protocol with signaling for ad hoc networks, ACM Sigcomm Computer Communication Review (CCR), Saturated Link 11.446 ×10 11.439 ×106 6 July 1998. bytes bytes [11] Skehill, R., Picovici, D. and Kent, W. Wireless MANET Testbed for reproducible Voice over IP Evaluation, In Percentage Time 0% 77.45% PROC. Of International Communication Sciences and spent in Sleep Mode Technology Association MeshNets, Budapest, July 2005 [12] WMM Power Save for Mobile and Portable Wi-Fi Certified Devices Wi-Fi Alliance Whitepaper December 2005. Available at 784