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  • 1. Power Saving in Wireless VoIP 6.829 Project Kwan Hong Lee Sung Hyuck Lee Wei Wu Media Lab Media Lab CSAIL Abstract This paper reviews existing power management tech- niques on wireless LAN and verifies the effects of different power management techniques applied to the case for VoIP over WiFi through experiments. Voice communication im- poses timely constraints such as short time interval between packets and the need to be always listening for potential in- coming connections. It also poses opportunities related to people being silent during voice communication to listen to the other party. As a result some of the existing power save techniques in the MAC layer do not apply to the VoWiFi scenario and one needs to take an integrated approach to reducing power consumption on the VoWiFi device by re- ducing the packet size during transmission through efficient Figure 1. VoWiFi Market Growth (gizmo- codecs, header compression, silence detection, and MAC power save mode that adjusts to the voice traffic pattern. One of the problems that the WiFi phones currently face 1. Introduction is that it is not as power efficient as the existing cellular phones. In a product by Cisco talk time is reported to be 3.5 VoIP is becoming a prominent way of replacing exist- hours and the standby time 30 hours. Current cell phones ing telecommunications mechanisms. It has been a growing may have up to 5 hours of talk time and standby time of market with mature commercial services provided by Von- over a week. age, Skype and 8x8.[5] It is currently taking over the mar- In this project we investigate where power is consumed ket for the landline telephone services. In the same man- in the WiFi devices and learned about many different re- ner, it is expected that as WiFi access points become more search to saving power on WiFi devices. We ran differ- widely deployed, VoWiFi is going to substitute existing cel- ent experiments to verify these different approaches and to lular communications in certain circumstances. Major cell measure its effects. Different approaches have been pro- phone manufacturers such as Nokia, Motorola and Samsung posed for physical layer, MAC layer, network layer and ap- are working on WiFi supporting cell phones and existing plication layer [14][9], but we have found that there is no in- network equipment manufacturers like Cisco are productiz- tegrated study of this multidimensional aspect of the power ing WiFi phones. Figure 1 shows potential growth of the saving mechanisms in VoWiFi. In a typical deployment of a market. ”The global market for WiFi phones rose 76 % in VoWiFi network as in Figure 2, the mobile phones and the 2005 to $102.5 million, and will reach $1.9 billion in 2009, access point interact to provide power savings to the end according to a report by Infonetics Research. The number user. We made several experiments to measure and confirm of units shipped rose 112 % last year, and will increase by the effects of power saving mechanisms proposed to pro- 158 % this year, the report adds.” [VOIP Magazine, Feb 1, vide design guidelines for VoWiFi power savings. 2006] In section 2 we review different approaches taken to save 1
  • 2. consecutive voice packets. Thus it affects the amount of power needed for each packet and how often the packets are sent and received. • Header Compression [3]. The size of voice packet is normally small due to the small payload generated by voice codec. However, a lot of packet encapsulation overhead is incurred for each voice packet. Apply- ing IP header compression between mobile device and Figure 2. Wireless VoIP layout [19] the access point can reduce the size (overhead) of each voice packet. power on WiFi devices that applies to VoIP and to general • Silence Detection. Silence detection and suppression applications. Section 3 discusses the unique characteristics [7] help avoid sending silent packets. In a normal con- of VoIP traffic over wireless compared to other IP traffic versation, mostly only one side is talkig at each any and analyze the constraints for power saving. Packet size is point in time. Therefore effective silence suppression one of the main concerns that determine power consump- can reduce about half of the voice packets. tion so header compression mechanism is discussed in sec- tion 4. The physical layer effect is described in section 5 It is also important to point out that although several in- comparing 802.11 a/b/g on power consumption. Section 6 teresting methods have been developed for saving power for describes WiFi power save mode and how it is achieved in Web access traffic they are not applicable to wireless VoIP. the MAC layer with introduction to UPSD mechanism that For example, the BSD (bounded slowdown)[8] technique is suited for VoIP traffic. Voice codec is a mature field in it- developed by R. Krashinsky and H. Balakrishna is an effi- self, but it has effects in power consumption, so we analyze cient mechanism for saving power when surfing the Inter- them in section 7. Finally we conclude with some insights net. However, the web traffic pattern (not continuous and and guidelines that we learned through the experiments and with at least seconds of interval between two accesses of previous research. web pages) is very different from VoIP traffic pattern (con- tinuous and with very small interval between two consec- 2. Approaches utive transmissions) that we are not able to use the power save mechanisms proposed. A wide range of technologies [10, 19, 8, 2, 13, 17, 11] Besides these technologies, transmission power manage- have been developed for saving power on mobile devices. ment, access point topologies, low power circuit and so on Some of them are more specific or applicable for wireless also have effect on power saving in wireless VoIP. However, VoIP. In this section we briefly introduce these technologies. they are beyond the scope of our project. The following sections present our experiments for evaluat- ing the effects of them on power-saving in wireless VoIP. 3. Background The essence of saving power in Wireless VoIP can be summarized as sending less and sleeping more. Sending 3.1. 802.11 Frame/Packet Format less helps save power because the wireless card consumes most power when it is transmitting. The wireless card also A closer look at the packet format shows what packet consumes a lot of power in idle mode, therefore it is im- sizes are encountered in a typical VoWiFi communication portant to put the network card into sleep mode when it is and what portion of the transmission is payload versus over- idle. head. The extra overhead incurred by packet encapsula- tion causes more power to be consumed during transmis- • 802.11 MAC [1], Power Save Mode (PSM) [12], Un- sion. For instance, the application could be configured to scheduled Power Save Delivery (UPSD) [19]. The encapsulate 20 ms of coded voice. As shown in Table 5, main idea of 802.11 Power Save Mode and UPSD different codecs have different payload size. For example, (which is developed specifically for wireless VoIP) is 160/40/20 bytes are generated for G.711/726/729, respec- to put the network card into sleep mode after send- tively. An RTP (Real-time Transport Protocol) header (12 ing and receiving the pending voice packets, and wake bytes) is first attached to the payload, then a UDP (8 bytes) up when needing to send and receive the next voice header is appended. Finally, the packet is encapsulated as packet. IP packet by attaching 20-byte header and a 4-byte CRC. • Voice Codec [6]. The type of voice codec determines During this course, a 44-byte overhead for IPv4 (64 bytes the size of voice packet as well as the interval between for IPv6) is added to the voice packet. In addition to the 2
  • 3. Figure 4. Power consumption of an WiFi mo- bile device when idle. [10] Figure 3. Wireless VoIP layout [16] each frame. This traffic pattern has an effect on Power Save IP overhead, 802.11 MAC overhead is added to the packet Mode (PSM) of 802.11 a/b/g. PSM enables 802.11 MAC to to form an MPDU. In this case, the latest 802.11e WME go to sleep mode with a timeout (before sleep) and a period MAC header and FCS have been appended to the packet, (for sleep) whenever transmission of voice packets is com- adding another 32 bytes to its length. Finally, the 802.11 pleted. However, since the codecs wake up a WLAN driver PHY header is concatenated to the MPDU. The whole pro- every 20ms or 30ms, 802.11 MAC can only have a snatch cess is depicted in the following figure. There is more than of doze and does not have enough time to go to sleep. As a 70 bytes overhead per voice packet. This overhead size result, existing implementations of Power Save Mode can- could not be further overlooked if payload of voice packets not work well with VoIP. The detailed description is shown is small: 20 bytes for G.729. Unless packet header over- in Section 6. head is reduced, benefits of low bit-rate codecs are not fully utilized and power saving is not optimized [16]. 3.4. Experiment Method and Setup 3.2. Where Power is Consumed Initially, we started to work with the NOKIA Internet tablets (N770), however it had many limitations. The SDK If we look at the sources of power consumption in a typ- was difficult to setup and get to work. We wanted to try ical VoWiFi device, Figure 4 shows that WiFi dominates to recompile a SIP phone applicaiton to work with the power usage compared to CPU, SDRAM, Bluetooth, and tablets. There was only Google Talk available for VoIP ex- other devices. This is critical because this is the case when periements and there was no way of accurately reading the the device is in idle mode. It is critical to reduce power con- battery level. It was difficult to find the appropriate docu- sumption during idle mode to increase the standby life time. mentation for finding battery level and from the discussion In order to save power during idle mode, the power save forums it was concluded that one can only obtain a bat- mode is implemented in current WiFi devices. For WiFi tery level value from 4 step levels. As a result, we resorted phones the power save mode should operate without losing to working with the Linux laptops that had ACPI installed any incoming call requests to the phone. which allowed us to capture the battery consumption. The topologies for experimentsare as shown in Figures 3.3. Traffic Pattern 5 and 6. One was designed for experiments regarding IP header compression (ROHC) and 802.11 MAC protocols The popular codecs for VoIP generate a packet every a/b/g in Sections 4, 5, and 6. In this topology, three IBM 20ms or 30ms as shown in the Table 5. Thus, depending on laptops, T60p, T42, and X41 running Ubuntu Edgy Linux the type of the codec used, the packet generation rate would O/S are used as a wired sender, a measurement node, and a be 50 packets/sec or 33 packets/sec. The following graph wireless receiver, respectively. We used built in IntelWiFi- shows the MAC layer traffic content during a VoIP session. cards on the measurement node and the wireless receiver for The black line shows the UDP voice traffic, the red line power measurement of ROHC. MADWiFi driver and Net- shows all IP traffic including the voice traffic and the green gear Atheros card was used to measure power consumption line shows the sum of all frames. In addition to the over- of 802.11 a/b/g due to their support for a/b/g modulation head caused by assembling the packet, there is additional schemes. Formonitoringthetraffic,Kismet (version Kismet- traffic generated in the MAC layer for ACKs generated for 2006-04-R1)onanIBMT42laptopwasused. We set ’Chan- 3
  • 4. nelhopping = false’ to accurately measure one channel used. uses TCP error recovery mechanisms to recover from bit er- ACPI (Advanced Configuration and Power Interface) (ver- rors. In the case of VoWiFi, the traffic consists of only UDP sion 0.09) Battery daemon is used to measure power con- packets over lossy wireless links making the original header sumption. Also, a Phython program which read battery in- compression scheme inadequate. fomation from ACPI every 30 seconds during 10 minutes Robust Header Compression (ROHC) algorithm com- period was made to compute average power consumption. presses headers of UDP and RTP packets reducing header The second setup was used for codec experiments in Section size from 40 bytes down to 1 byte. It also keeps a con- 7. Two IBM T42 laptops were used for a wireless sender text between the compressor and decompressor, exchanging and a wireless receiver. In this case, we used a wireless feedback to dynamically adjust the compression mode de- sender instead of a wired sender to look for which laptop pending on the wireless link quality and bit error rate. [3] is the most appropriate to analyze traffic patterns of voice The algorithm works on a hop by hop basis and as a re- packets during the transmission. During the experiment, the sult it needs to be implemented on every hop. In practice sound of a news clip was used to emulate a person speaking it would be implemented on the wireless hops if there are on a VoIP phone during the 10 minutes (600 seconds) of the different Internet access gateways and routers that packets experiment. pass through. For example, a packet might start from a cell phone, to base station to Internet and to another access point and mobile device. In this case only the two end hops need to implement ROHC. If the packets pass through a satellite link, it would also use ROHC to reduce bandwidth and la- tency during that satellite hop. Currently the cell phone net- works use ROHC software to reduce packet header size be- tween mobile and base stations for VoIP transmissions. The current wireless access points do not implement ROHC, so we had to emulate the effects of ROHC on power consump- tion for our experiment. Figure 5. An experiment topology for IP Header compression, 802.11 modulation schemes, and Power Save Mode 4.2. Experimental Result In order for ROHC to be used in current wireless LAN environment it has to be implemented on the accesspoints since ROHC operates hop by hop. The current access points that we use do not implement ROHC and as a result, in order to experiment the effects of ROHC on power consumption, we emulated by writing a program that adjusted the pay- load while keeping the header size constant. By adjusting Figure 6. An experiment topology for codecs the payload from 4 bytes to 40 bytes and measure the power consumed during two way operation, we were able to mea- sure power consumption on the packet size. There is also the power consumed due to the compression and decom- 4. IP Header Compression pression algorithms, however, we were not able to measure its effects due to lack of ROHC implementation available 4.1. ROHC on voice packet in the current wireless network. However, from the codec experiment we did we believe that the computational en- Original header compression algorithm by Van Jacob- ergy for compression and decompression is dominated by son is not adequate for VoIP traffic because it only supports the transmission power consumption. TCP packet compression while VoIP runs over UDP. Van The following table indicates the power savings that Jacobson’s algorithm is implemented in current TCP im- could be possible by using ROHC. From our experiment plementations and it uses delta compression where it only about 2/37 ≈ 5.4% of power is saved in the case for smaller sends differences in the header for subsequent packets. It header. The savings would be greater on a mobile device can compress 40 byte headers to 4 bytes. However, it is since the LCD monitor and other peripherals on the laptop not suitable for wireless links with real time traffic since it consume power in this experiment. 4
  • 5. Setting (payload size) ∆ power (mW) PHY ∆ power (mW/min) Header Size (µsec) 4 bytes 35 802.11a 40 20 40 bytes 37 802.11g 39 20 802.11b 45 96, 192 Table 1. ROHC Power Savings Table 3. a/b/g Difference with power con- sumed during 1 min VoIP session. MAC PHY Header Frequency Data Rate 802.11a 20 usec 5 GHz 54 Mbit/s 802.11g 20 usec 2.4 GHz 54 Mbit/s 802.11b 96/192 usec 2.4 GHz 11 Mbit/s 5.2. Experimental Result Table 2. Comparision with 802.11 modulation We performed experiments to compare power consump- schemes tion of 802.11 a/b/g based on the first toplogy. We use a Netgear Athero card driven by MAD-WiFi on a IBM X41 laptop to test the power consumption during a VoIP session using 802.11 a/b/g respectively. A 10 minutes conversation was performed for each 802.11 modulation scheme. The 5. IEEE 802.11 a/b/g experiment results given in Table 3 show that 802.11 a/b/g have different power consumption. 802.11 g has the lest power consumption because of its short transmission du- 5.1. Difference between 802.11 a/b/g ration and lower operation frequency among 802.11a/b/g. 802.11a has shorter transmission duration than 802.11b but higer operation frequency than 802.11g. The higher fre- In theory, lower throughput 802.11b modulation scheme quency of 802.11a is the reason why it consumes more would result in lower battery power consumption. If one power than 802.11g. As expected, 802.11b has the most only examines the power consumed to transmit or receive much power consumption due to its longer PHY header. a byte of data, then an 802.11b device would consume approximately 30 percent less power than an equivalent 802.11a/g device for a same amount of data (see Table 2). 6. PSM and UPSD But it can be very deceptive if the analysis is limited to an examination of solely the per bit or per byte power con- The most effective way of saving power for a mobile de- sumption when the device is in an active mode transmit- vice is to put the device into sleep mode when it is not work- ting or receiving data. Another critical factor in the overall ing actively. The wireless card consumes the largest portion power consumption of an 802.11 device is how long the de- of power of a mobile device and it drains a lot of power even vice must remain in an active mode to transmit or receive when it is idle, it is very important to put the wireless card a certain amount of data. Battery life is not just a function into sleep mode when it is not sending or receiving packet. of the power consumed per bit of data in an active mode In IEEE 802.11, there is a mechanism designed for mo- but also a cuntion of how long the device must remain in an bile device to save power. It is called the Power Save Mode active mode to transmit or receive a meaningful amount of (PSM). The basic idea behind the PSM is that when a mo- application data. bile device is idle it can transit to the sleep mode and let An 802.11b device may consume 30 percent less power the access point buffer the packets that are destined for it. than an 802.11a/g device to perform a single transmit or When the mobile device wakes up, it gets the packets from receive operation, but that same 802.11b device must re- the access point. main in an active state three to four times longer than an 802.11a/g device to transmit or receive the same amount of 6.1. Power-Save bit based Mechanism data. The reason is that the 802.11 PHY header must be concatenated to the MPDU. The length of the PHY header Power-Save bit based mechanism utilizes PS bit to no- of 802.11 b is much longer than 802.11 a/g as shown in tify the change of power state. A PS bit can be set to either Table 2. As a result, an analysis based on real-world us- active and sleep, and the PS bit is sent with a packet. The age patterns finds that on the average, the power needed to process of this legacy approach is shown in Figure 7. After support these longer transmit/receive times makes 802.11b waking up, the mobile device sends the up-link voice packet much less power efficient than 802.11a/g [16]. to the access point and within this packet it sets the PS bit to 5
  • 6. Figure 7. PS-bit [19] Figure 9. Unscheduled Power Save Delivery [19] designed a protocol called Unscheduled Power Save Deliv- ery (UPSD) for saving power during a wireless VoIP ses- Figure 8. PS-poll [19] sion. The basic idea of UPSD is that since we know that in each interval there will be one up-link packet and one down- link packet, these packets should be exchanged after the mo- active to tell the access point that it has waken up. After re- bile device wakes up, and the mobile device could transition ceiving the up-link packet and knowing the mobile device is to sleep mode once the bi-directional packet exchange is active, the access point sends the buffered down-link voice done. This protocol is more efficient than both PS-bit based packet to the mobile device. Note in the PS bit based mech- and PS-Poll based mechanisms because in UPSD only voice anism the mobile device must explicitly tell the access point packets and related MAC layer ACKs are sent and received. when it transfers to sleep mode. So after acknowledging the Figure 9 illustrates the idea of UPSD. down-link packet, the mobile device sends an empty (null) UPSD has been standardized by IEEE as part of the packet with PS bit set to sleep to the access point. Receiv- 802.11e standard, which is a QoS (quality of service) ex- ing this packet the access point will buffer the packets for tension for 802.11. this mobile device. This PS bit based mechanism is not ef- ficient because after exchanging a pair of voice packets the 6.4. Experimental Result mobile device has to send an extra (empty) packet and re- ceive the ACK for it before it can go to sleep mode. Since It will be interesting to evaluate how much power can the real voice packet is small (because the payload is small), be saved by legacy 802.11 PSM and UPSD. Unfortunately the overhead could be as high as nearly one third. there is no 802.11e enabled device and access point avail- able, therefore there is currently no way to investigate the 6.2. PS-Poll based Mechanism real effect of UPSD. The objective of this set of experiments is to see whether In the PS-Poll based method, the mobile device uses a the legacy PSM of 802.11 is useful in wireless VoIP session. PS-Poll frame to explicitly retrieve its down-link packet(s) The experimental results are shown in Table 4. To know buffered at the access point. After waking up, the mobile how much power is consumed by the network card (and device sends its up-link voice packet to the access point, VoIP application), we need to know how much power is and then solicits the down-link voice packet with a PS-Poll consumed by other components of the laptop. We did the packet. Since each down-link packet is retrieved explic- base experiment as follows (as shown in the second row of itly, the access point by default buffers the mobile device’s the table): we turn the wireless NIC off and leave the lap- down-link packets. This Mechanism is depicted in Figure 8. top idle, and we measure the amount of power consumed in This method incurs one PS-Poll packet overhead for each every minute. The next step of this experiment is to verify down-link voice packet. whether the power save mode is working correctly or not. We did the experiments as shown with the third and forth 6.3. Unscheduled Power Save Delivery rows in the table. We turn the wireless card on but do not (UPSD) generate any traffic. In two experiments we turn the power save mode (PSM) off and on respectively. The results show An important characteristic of voice traffic is that the that PSM works very well– with PSM off the card consumes packets are sent and received with fixed (and known) in- 34mW every minute while it consumes only 16mW every terval which depends on the voice codec. This means that minute when PSM is on. both up-link and down-link voice packets are predictable. In the last four experiments (shown with the last four Based on this observation, researchers from Motorola rows of the table) we study whether PSM is useful in VoIP 6
  • 7. Setting Total ∆ Codec Period Payload Size Data Rate (mW) (mW) (ms) (bytes) (kbps) NIC off (base) 202 G.711 20 160 80 NIC on, no traffic, PSM off 236 34 G.729 20 20 24 NIC on, no traffic, PSM on 218 16 G.723.1 30 24 17 NIC on, VoIP traffic, PSM off 244 42 GSM FR 20 33 29.2 NIC on, VoIP traffic, PSM on, level 1 243 41 GSM EFR 20 31 28.4 NIC on, VoIP traffic, PSM on, level 3 242 40 iLBC 20ms 20 38 31.2 NIC on, VoIP traffic, PSM on, level 5 238 36 iLBC 30ms 30 50 24 Table 4. Effect of PSM (mW) sampled every Table 5. Codec Comparison [6] minute. ∆ is the power consumed during 1 min VoIP session. lows. The mobile device wakes up with fixed interval time so that AP knows when the mobile device will wake up. Af- ter the mobile device wakes up (AP knows this time point), session. We initiated a wireless VoIP session between the the AP sends the down-link packet (or an empty down-link laptops and measure power consumption, and in each ex- packet) as a trigger. After receiving and acknowledging the periment we use different power save levels1 . The results down-link packet, if the mobile device has up-link packet, it show that there are some differences between PSM-off and sends the up-link packet; if it does not have up-link packet PSM-on with different PSM levels, but the percentage of (due to silence suppression), it directly go to sleep (and saving is not significant. We believe this is because the wake up at the next interval). In this protocol, the up-link voice packets are generated with very short interval period empty packet is removed3. (20ms), therefore the wireless card needs to wake up every 20ms to send and receive voice packets2 . Note that transi- tion between sleep and active mode also takes some energy. 7. Voice Codecs This frequent transition overhead could make the very short sleep not beneficial. 7.1. Voice Codecs 6.5. Interaction between silence suppres- As shown in the Table 5, different codecs generate voice sion and UPSD packets of different sizes and transmit at different rates. Therefore codec has an effect on both packet size and the UPSD makes use of the VoIP traffic characteristics that number of packets sent per second, and will affect the power the up-link packet and down-link packet are symmetric and used for sending and receiving the voice packets. Since both up-link and down-link are predictable. UPSD always the overhead of encapsulating a voice packet is fixed, the lets the mobile device initiate the packet exchange after smaller the payload is, the smaller the packet size will be. waking up. However, when silence detection (and suppres- From this point of view, the codec that generates smaller sion) is used, during the silence period, there will be no payload will help save transmission power. Encapsulation up-link packets. of voice packet incurs overhead, and sending and receiving One simple solution for this problem is that the mobile packets consume power, so a codec that generates packets device can send an empty up-link packet to the access point with longer period (30ms) will be preferable. However, dif- to trigger the packet exchange process when it has no up- ferent codecs have different coding and decoding complex- link packet. [19] takes this solution. However, sending an ity and will consume different amount of CPU time. Lower empty packet and receiving the corresponding ACK are a bit rate codecs require more computation as shown in Fig- wasting of power. ure 10. So there is a trade-off between transmission power We propose that rather than letting the mobile device and CPU power. (WiFi-Phone) be responsible for triggering the packet ex- To understand the overall effect of codec on power con- change process, we can make the access point be responsi- sumption, we designed the following experiments. We close ble for this task. A revised UPSD protocol can work as fol- all the application on a laptop (IBM T42, Ubuntu Edgy), and run a VoIP application (linphone and kphone) for 15 1 Intel wireless card. PSM level 1: timeout 350ms, period 400ms; PSM minutes. Each time we use a different codec for the VoIP level 3: timeout 75ms, period 1000ms; PSM level 5: timeout 25ms, period 1000ms. 3 We are not sure whether this protocol is feasible and interesting. If 2 Whenever there is an out-going packet the card will wake up. you think it is worth putting more time on it, please let us know. 7
  • 8. yet (due to time constraint), and it will be much more inter- esting if we can do the same experiments using a handheld device rather than a modern laptop. 8. Conclusion/Guidelines The objective of this paper is to provide guidelines on the deployment of VoWiFi for power saving based on real Figure 10. Codec complexity measure in DSP experiments we have performed. Our experiments on power processing requirement [16] consumption of VoWiFi consist of three parts. The first is about whether overhead of voice packets has effect on power consumption. The second is to verify effect of Power Codec Total VoIP Save Mode (PSM) as a mechanism for saving power of (mW) (mW) WLAN. The last one is about which codec for voice packets PCMU (20 ms, 64 Kbps) 3810 420 is the most appropriate. GSM (20 ms, 13.5 Kbps) 3790 400 Firstly, the experiments show that IP header compres- Speex (20 ms, 20 Kbps) 3580 190 sion is useful for power saving because we can reduce the iLBC (20 ms, 31.2 Kbps) 3790 400 IP header size dramatically. It is especially helpful when iLBC (30 ms, 24 Kbps) 3710 320 the payload is small. The results of second set of experi- ments (PSM) show that the Power Save Modes of 802.11 Table 6. Codec Power Consumption: Total are not much useful for voice packets even though it led to power consumed is during 15 minute inter- less power consumption. The effect of 802.11e UPSD may vals and VoIP power consumed is obtained be more obvious since it is more optimal and a lot of litera- by subtracting 3390 mW that gets consumed tures claimed that UPSD help save power in wireless VoIP. when laptop is left idle. The third set of experiments (codecs) shows that the com- putational complexities of codecs has less effect on power consumption than packet sizes generated by them. A choice of proper codecs is important for power saving of VoWiFi. application. During the experiment, the sound of a news clip is used to mimic a conversation. We measure the bat- 8.1. Guidelines on deployment of VoWiFi tery level every 5 seconds in the VoIP conversation. From for power saving the power measurement, we calculate how much power is consumed during the VoIP session. The main ideas of power saving in wireless VoIP can be To know how much power is incurred by the VoIP ap- summarized as “Talk Less and Be Lazy.” plication (including CPU and transmission power), we need Talk Less means that we should reduce the size and num- to know how much power the laptop will consume when ber of voice packets. IP header commpression (ROHC) can it is in idle state– no application running, no network traf- be used for reducing size of packets. Silence detection can fic (network is turned on, but no application is sending or be used to reduce the number of packets. Different voice receiving any data). We leave the laptop in idle state for codecs generate voice packets with different sizes. Furth- 15 minutes, and measured its power consumption. The lap- more, 802.11g modulation scheme should be considered for top consumes 3390 mW in 15 minutes. We simply take the fast transmission. They lead to less power consumption. additional power the whole system consumes as the power Be Lazy means that the wireless card should be in sleep incurred by the VoIP application. mode as long as possible. It is interesting to point out that From the experiments, the VoIP application’s power con- Talk Less (Send Less) has a positive effect on Be Lazy sumptions with different codecs are shown in Table 6. (Sleep More). The UPSD protocol of 802.11e should be used if proper devices are available to reduce the overhead 7.2. Effects (experiments) of Codecs of synchronization of state (active or sleep) between mobile device and access point. Silence detection should be one of From this set of experiments, we see that codec has an requirements for efficient power saving mechanmism, and obvious impact on the amount of power the VoIP consumes. then machanisms on power save mode (PSM) need to be Picking the right codec will help save power. It is important used to manage unnecessary power consumption during the to note that this set of experiments has not been repeated voice call. 8
  • 9. In summary, our choices on deployment of VoWiFi can intelligently be adjusted dependent on the AP-to-node power saving are distance. Access points in our surroudings have been de- ployed without a blue print for VoIP. If we can change ac- • Select an appropriate codec for generation of voice cess point toplogies to a cellular-like one, contention-less packets environments will be provided, and thus power consump- tion caused by the contention of RTS/CTS may be reduced. • Use 802.11g modulation scheme for fast transmission, but use 802.11e for quality of services if it is deployed References • Compress header of IP/UDP/RTP packets (e.g., ROHC) [1] V. Bharghavan, A. Demers, S. Shenker, and L. Zhang. • Enable silence detection mechanisms during the Macaw: a media access protocol for wireless lan’s. In SIG- COMM ’94: Proceedings of the conference on Communica- packet transmission tions architectures, protocols and applications, pages 212– • Apply Power Save Mode (PSM) and set proper time- 225, New York, NY, USA, 1994. ACM Press. out/period parameters of PSM. Use the UPSD (Un- [2] S. Chandra and A. Vahdat. Application-specific network management for energy-aware streaming of popular multi- scheduled Power Save Delivery) of 802.11e if possible media formats. In USENIX, 2002. • Apply mechanisms for reducing unnecessary power [3] Effnet. The concept of robust header compression, rohc. Feb consumption of idle mode 2004. [4] L. M. Feeney and M. Nilsson. Investigating the energy con- sumption of a wireless network interface in an ad hoc net- 8.2. Tips on experiments of VoWiFi power working environment. In INFOCOM, 2001. consumption [5] D. Frommer. Vonage dominates home voip market. March 2006. Through many experiments on power consumption, we [6] M. Gast. How many voice callers fit on the head of an access also discovered Myth and Reality on power save mode com- point? Technical report, O’Reilly, 2005. mands in linux. “iwconfig” command can turn on PSM, [7] W. Jiang and H. Schulzrinne. Analysis of on-off patterns in but does not change specific parameters on timeout and pe- voip and their effect on voice traffic aggregation, 2000. riod. However, “iwpriv” command correctly works within [8] R. Krashinsky and H. Balakrishnan. Minimizing energy for a bound previously determined by vendors, and “iwlist” wireless web access with bounded slowdown. In MobiCom, 2002. command shows correct PSM state infomation for madwifi [9] R. Kravets and P. Krishnan. Application driven power man- driver even though it can not turn on or modify Power Save agement for mobile communication. Wireless Networks, Mode. pages 263–277, September 2000. [10] T. Pering, Y. Agarwal, R. Gupta, and R. Want. Coolspots: 8.3. Further Study reducing the power consumption of wireless mobile devices with multiple radio interfaces. In MobiSys 2006: Proceed- To take full advantage of the power efficiencies of VoW- ings of the 4th international conference on Mobile systems, iFi, the following researches may further be required. applications and services, pages 220–232, New York, NY, USA, 2006. ACM Press. • Very low power consumption under idle conditions [11] V. Raghunathan, T. Pering, R. Want, A. Nguyen, and P. Jensen. Experience with a low power wireless mobile • Rapid wake-up cycle from idle to active state computing platform. In ISLPED ’04: Proceedings of the 2004 international symposium on Low power electronics • Ability to intelligently process 802.11 beacons and design, pages 363–368, New York, NY, USA, 2004. ACM Press. • Transmit power control [12] C. Rhl, H. Woesner, and A. Wolisz. A short look on power saving mechanisms in the wireless lan standard draft ieee • Cellular-like Access Point topology 802.11. [13] B. P. Shih, E. and M. J. Sinclair. Wake on wireless: An Especially, the power consumption in an idle mode is event-driven energy saving strategy for battery operated de- expensive. The number of standby hours is a more seri- vices. In MobiCom, 2002. ous problem than the number of talking hours. Very low [14] T. Simunic. Power saving techniques for wireless lans. power idle mode is essential. The current WLAN dirvers pages 96–97, March 2005. just set maximum transmission power (e.g. 16dBm) regard- [15] R. H. J. L. SV Andersen, WB Kleijn. ilbc-a linear predictive less of the distance between the access point and the WLAN coder with robustness to packet losses. In Speech Coding, node. It will be very interesting if the transmission power 2002, IEEE Workshop Proceedings., 2002. 9
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