Transcript of "Power Saving in Wireless VoIP 6.829 Project"
Power Saving in Wireless VoIP
Kwan Hong Lee Sung Hyuck Lee Wei Wu
Media Lab Media Lab CSAIL
email@example.com firstname.lastname@example.org email@example.com
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 mag.com)
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. 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 , 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
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 . 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  the access point can reduce the size (overhead) of each
power on WiFi devices that applies to VoIP and to general • Silence Detection. Silence detection and suppression
applications. Section 3 discusses the unique characteristics  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) 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 , Power Save Mode (PSM) , Un-
sion. For instance, the application could be conﬁgured to
scheduled Power Save Delivery (UPSD) . 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
IP packet by attaching 20-byte header and a 4-byte CRC.
• Voice Codec . 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
Figure 4. Power consumption of an WiFi mo-
bile device when idle. 
Figure 3. Wireless VoIP layout 
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 . 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-
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. 
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.
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 . to the access point and within this packet it sets the PS bit to
Figure 7. PS-bit 
Figure 9. Unscheduled Power Save Delivery
designed a protocol called Unscheduled Power Save Deliv-
ery (UPSD) for saving power during a wireless VoIP ses-
Figure 8. PS-poll  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
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 
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.  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.
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.
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  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.
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
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