適用於IEEE 802.11無線區域網路電話系統中之省電傳送機制

國

立

交

通

大

學

網路工程研究所

碩

士

論

文

適用於 IEEE 802.11 無線區域網路電話系統

中之省電傳送機制

Energy-Efficient Transmission Mechanism for VoIP over

IEEE 802.11 WLAN

研 究 生：黃中暉

指導教授：曹孝櫟 教授

適用於 IEEE 802.11 無線區域網路電話系統中之省電傳送機制

Energy-Efficient Transmission Mechanism for VoIP over IEEE 802.11

WLAN

研 究 生：黃中暉 Student：Chung-Huei Huang

指導教授：曹孝櫟 Advisor：Shiao-Li Tsao

國 立 交 通 大 學

網 路 工 程 研 究 所

碩 士 論 文

A Thesis

Submitted to Institute of Network Engineering College of Computer Science

National Chiao Tung University in partial Fulfillment of the Requirements

for the Degree of Master

in

Computer Science

June 2007

Hsinchu, Taiwan, Republic of China

適用於 IEEE 802.11 無線區域網路電話系統中之省電傳送機制

學生：黃中暉

指導教授：曹孝櫟博士

國立交通大學網路工程研究所碩士班

Abstract (in Chinese)

無線區域網路電話(Voice over IP over WLAN, VoWLAN)為公眾與私人無線區域網路系 統中一項重要的應用服務。然而，這樣的服務面臨了手持裝置耗電與系統能支援的最大 使用者數目(VoWLAN Capacity)有限等技術挑戰，因此使得大規模的商業化佈建緩慢。 在本研究中，我們針對 IEEE 802.11 無線區域網路電話系統提出了一個跨層的省電傳送 機制，此機制考量語音封包可以忍受某種程度封包遺失(packet loss)的特性，並且根據目 前語音封包遺失率，動態的停用媒體存取控制層(MAC)對於語音封包的回應封包 (Acknowledgement)。在此機制下，可以縮短手持裝置傳送與接收封包所需的時間與對應 消耗的能量。模擬結果說明瞭本機制可以在通話品質被保證的情況下，大幅的改善無線 區域網路手持裝置的耗電情形，以及無線區域網路電話系統所能支援的最大使用者數 目。

Energy-Efficient Transmission Mechanism for VoIP over IEEE 802.11

WLAN

Student: Chung-Huei Huang

Advisors: Dr. Shiao-Li Tsao

Institute of Network Engineering

National Chiao Tung University

Abstract (in English)

Voice over IP (VoIP) over WLAN (VoWLAN) is an important application for public and

private WLANs. However, VoWLAN systems suffer from various technical challenges such

as power consumption of WLAN stations (STAs) and VoWLAN capacity issues, making the

commercial deployment of large-scale VoWLAN services problematic. This study presents a

cross-layer and energy-efficient mechanism for transmitting VoIP packets over IEEE 802.11

WLANs. The proposed mechanism considers the characteristics of VoIP that can tolerate

some packet loss, and dynamically disables the medium access control (MAC) layer

acknowledgement for voice packets according to the packet loss rate. In doing so, the time

reduced. Simulation results demonstrate that the mechanism significantly improves the energy

efficiency of a VoWLAN STA and WLAN utilization while the voice quality can be also

Table of Contents

Abstract (in Chinese) ...i

Abstract (in English)...ii

Table of Contents...iv

List of Figures... v

List of Tables ...vi

Acknowledgements ...vii

Chapter 1. Introduction ... 1

Chapter 2. Network Architecture and Transmission Schemes for VoWLAN Services... 4

Chapter 3. Energy-Efficient Transmission Mechanism ... 10

3.1. Design of the Proposed Mechanism ... 10

3.2. The Maximum Numbers of Transmission Attempts for Voice Packets... 15

3.3. Implementation Issues ... 17

Chapter 4. Simulation Results... 19

Chapter 5. Conclusions ... 28

List of Figures

Figure 1. A generic network architecture for a VoWLAN system... 4

Figure 2. PS-Poll VoWLAN transmission scheme ... 5

Figure 3. U-APSD VoWLAN transmission scheme... 8

Figure 4. The proposed mechanism applied to the PS-Poll scheme... 12

Figure 5. Aveage duty cycle and enegy consumption for a VoWLAN STA ... 22

Figure 6. Average loss rates for uplink and downlink voice packets ... 24

Figure 7. Average delay for uplink and downlink packets ... 25

Figure 8. The length reduction of a duty cycle per STA under different codec and BERs ... 26

List of Tables

Acknowledgements

這篇論文可以順利完成，首先要感謝的人是我的指導教授，曹孝櫟老師。老師是我 在大學做專題期間一直到碩士班畢業的指導教授，也是影響我很深遠的人。在研究的指 導上，老師一直十分的栽培我，讓我有參與大型程式競賽、申請專利、出國參加研討會 這些其他碩士班學生可能沒有的經歷。而我在畢業後能到自己理想的公司工作，這一切 到要歸功於我的老師。而在待人處事上，老師也一直告訴我們要用積極進取的態度面對 生活，因為如此，讓我能有正確的價值觀來面對壓力與挑戰的到來。老師謝謝您，因為 您才有今天的我。 接著我要感謝實驗室夥伴們。謝謝建明學長、誌謙學長、一正學長、宥霖學長在我 研究過程中給予我的諸多建議，讓我在思考上可以趨於完備。謝謝彥筑、雅筑、政龍、 名杰在平日中對我的砥礪，這讓我有不斷努力下去的動力。謝謝建臻學弟、家祥學弟、 世永學弟常常陪我留在實驗室熬夜奮鬥。謝謝你們。 最後我要感謝一直支持我的家人。謝謝爸爸、媽媽、姊姊、哥哥願意當我的避風港， 讓我在遇到壓力時總是能得到紓解。謝謝女朋友乃瑛的默默陪伴，因為妳讓我不管作任 何事情都很有衝勁。

Chapter 1. Introduction

Extensively used in public and private networks, WLAN offers convenient and broadband

accesses to mobile Internet services. Having recently attracted significant interest from both

academia and industry, Voice over IP (VoIP) over WLAN (VoWLAN) is considered as one of

the most important applications for WLANs [1]. However, various technical challenges

inhabit VoWLAN systems such as power consumption, mobility management, WLAN

utilization and quality of services (QoS), making large-scale VoWLAN services difficult to

deploy. Two critical challenges for a VoWLAN system are the VoWLAN capacity, i.e. the

number of VoIP sessions that an access point (AP) can support, and the power consumption of

a VoWLAN station (STA), i.e. the standby and talk hours of a VoWLAN terminal.

The IEEE 802.11 WLAN adopts Carrier Sense Multiple Access/Collision Avoidance

(CSMA/CA) for its medium access control (MAC) [2]. The method performs inefficiently

when transmitting large numbers of small MAC frames. Unfortunately, voice packets are

small, and are generated periodically every 10 ms to 30 ms. Several reports have already

indicated that small voice packets significantly reduce the overall WLAN performance, thus

limiting a WLAN AP to only support a small number of VoIP sessions [3][4]. To improve the

WLAN utilization for transmitting voice packets, previous studies optimized WLAN

voice packets [5][6]. Wang et al. presented a multicast downlink transmission mechanism that

aggregates multiple downlink voice packets into one multicast packet, and transmits the

packet to a group of VoWLAN STAs [3]. The overheads for inter-frame spacing and

contentions between small voice packets are eliminated, and the WLAN utilization is thus

improved. However, this mechanism requires VoWLAN STAs to always stay awake to

receive downlink multicast packets, and it does not solve the power consumption problem.

Regarding the power consumption issue for a VoWLAN system, Chen et al. evaluated three

VoWLAN packet transmission schemes based on the IEEE 802.11 power saving mode (PSM)

and IEEE 802.11e automatic power saving delivery (APSD) mode [7][8]. Wang et al.

considered the characteristic of voice packets arrivals and suggested to periodically wake up

VoWLAN STAs to receive and transmit voice packets [9]. Therefore, a Time Division

Multiple Access (TDMA)-like accesses can be achieved for transmitting and receiving voice

packets, reducing the energy consumption of VoWLAN STAs. Another important

characteristic of voice packets that is not fully elaborated in the design of a VoWLAN

transmission scheme is the need for a reliable delivery [10]. Reliable transmission is essential

for data packets, but is not always necessary for voice packets, which could tolerate some loss.

In this study, a cross-layer and energy-efficient transmission mechanism that differentiates

WLAN MAC-layer schemes for voice and non-voice packets is thus proposed. The

attempt of a voice packet according to the current packet loss rate and the target voice quality.

By eliminating MAC-layer acknowledgment frames for voice packets, the time and energy

consumed by sending and receiving acknowledgement frames are minimized. Therefore, the

energy efficiency of a VoWLAN STA and the WLAN utilization are both improved. To realize

the proposed idea without modifying the IEEE 802.11 standard, this work adopts WLAN

MAC multicasting that does not require the acknowledgement to transmit voice packets. Then,

the proposed mechanism can be implemented on the existing WLAN infrastructures merely

through software upgrades.

The rest of this thesis is organized as follows. Chapter 2 describes a network architecture for a

VoWLAN system and VoWLAN transmission schemes. Next, Chapter 3 presents the design

of the proposed mechanism. Chapter 4 discusses the simulation results. Conclusions are

Chapter 2. Network Architecture and Transmission Schemes for VoWLAN

Services

Figure 1. A generic network architecture for a VoWLAN system

Figure 1 illustrates a generic network architecture for a VoWLAN system. A VoWLAN STA is

attached to WLAN via an AP, which is further connected to the Internet. A VoWLAN STA can

then use call setup protocols such as session initiation protocol (SIP) to establish a VoIP

session with a peer STA, which can be a fixed or a mobile node. Two transmission schemes

are used to reduce the power consumption of a VoWLAN STA during a VoIP session. The first

method, called the PS-Poll transmission scheme, adopts the PSM defined in the IEEE 802.11

to enter the PSM, and stays in the doze state, which consumes much less energy than the

WLAN receiving state. If a VoWLAN STA has an uplink voice packet to transmit, it wakes up

and sends the packet. After receiving the acknowledgement frame from the AP for the uplink

voice packet, the VoWLAN STA sends a PS-Poll frame to retrieve the downlink voice packet

buffered on the AP. Normally, an AP first responds with an acknowledgement frame to the

PS-Poll frame and, then, transmits a downlink voice packet to the STA. According to IEEE

802.11 specifications, acknowledging a PS-Poll frame is not mandatory, but most existing

APs implement the PS-Poll acknowledgement. Finally, the STA receives and acknowledges

the downlink voice packet. Figure 2 shows a timing diagram for uplink and downlink voice

packet exchange without any packet error and collision based on the PS-Poll transmission

scheme.

A duty cycle, , refers to the time period for transmitting one uplink and receiving one

downlink voice packet for the PS-Poll transmission scheme. Since downlink and uplink voice

packets are generated periodically, say every , the sleep period of a VoWLAN STA for

every is given by .This model assumes that the voice codec is a

constant bit rate (CBR), and the codecs for uplink and downlink voice packets are symmetric.

Poll PS− Poll PS Poll PS− − Poll

PS− PS−Poll PS−Poll PS−Poll PS−Poll PS−Poll

Poll PS− PS−Poll Poll PS Poll PS Poll PS Poll PS− − − − Poll PS Poll PS Poll PS Poll PS− − − − Poll PS− Poll PS− PS−Poll dc T i T i T T_sc =T_i −T_dc

Figure 2 also illustrates the power consumption of a VoWLAN STA. , and

denote the power consumption of a VoWLAN STA during the receiving, transmitting and

doze states, respectively. Clearly, a shorter length of a duty cycle a VoWLAN transmission

scheme introduces, the less energy a VoWLAN STA spends for transmitting and receiving

voice packets and also the less network resources a VoWLAN session consumes.

rx

P P_tx P_dz

The energy consumed by applying the PS-Poll transmission scheme during a duty cycle,

, is defined as , where and denote the

energy consumed when transmitting one uplink and one downlink voice packet, respectively.

According to Wang’s study

dc

E E_dc =E_ul +E_dl E_ul E_dl

[11], and can be further rewritten as:

, and

.

Above equations assume there is no packet loss due to packet collision or packet error during the transmission. denotes the energy consumption for a VoWLAN STA in the

WLAN MAC contention window. and denote the energy

ul E E_dl ul ul ul E BK E FR E SU E = _ + _ + _ dl dl dl E BK E FR E SU E = _ + _ + _ BK E _ ul FR E _ E _FR_dl

consumption for a VoWLAN STA that overhears other STAs’ transmissions.

and represent the energy consumption for a VoWLAN STA successfully

transmitting an uplink and receiving a downlink voice packet respectively.

Poll PS− Poll PS− Poll PS− ul SU E _ dl SU E _

Assuming that the average number of transmissions overheard by a VoWLAN STA during a

contention window period is given by , and that the VoWLAN STA consumes

during the overhearing period, the energy consumption due to the VoWLAN STA overhearing

the other VoWLAN STAs’ transmissions during a contention window period can be calculated

as t N P_rx ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ + × × = = − − −− −− − 2 _ _ Poll PS success dl Poll PS success ul rx Poll PS t Poll PS dl Poll PS ul T T P N FR E FR E .

The overhearing probabilities for downlink and uplink transmissions assume equal. The

parameters and denote the time periods for each successful transmission

of an uplink and downlink voice packet adopting the PS-Poll transmission scheme, given b , and

,

where , , , , , and represent the time periods for the

DCF Inter Frame Spacing (DIFS), Short Inter Frame Spacing (SIFS), the transmission of an

uplink voice packet, a downlink voice packet, an acknowledgement frame and a PS-Poll

frame, respectively. The energy consumption for a VoWLAN STA to transmit an uplink and to

Poll PS− PS−Poll y Poll PS− Poll PS− success ul T ₋ T_dl−success ack sifs voice ul difs success ul T T T T T ₋ = + ₋ + + ack sifs voice dl sifs Poll PS difs success dl T T T T T T T ₋ = + ₋ + + ₋ + + difs

receive a downlink voice packet for the PS-Poll transmission scheme, i.e. and , can be denoted as Poll PS− Poll PS− ul SU E _ dl SU E _

(

difs sifs ack

)

tx ul voice

rx

ul P T T T P T

SU

E_ PS−Poll = × + + + × ₋ , and

(

difs sifs dl voice sifs

)

tx

(

PS Poll ack

)

rx

dl P T T T T P T T

SU

E_ PS−Poll = × + + ₋ + + × ₋ + .

Figure 3. U-APSD VoWLAN transmission scheme

The second method adopts the APSD mechanism defined in the IEEE 802.11e [8]. The APSD

improves the IEEE 802.11 PSM by averting the PS-Poll procedure. An uplink voice packet

can be configured as a frame to trigger a service period, which is used to transmit downlink

packets. Figure 3 shows an example of an unscheduled-APSD (U-APSD) transmitting a

downlink and uplink voice packet. An STA is initially in the doze state. Once an STA has an

uplink voice packet to send, it wakes up and transmits the packet. The AP responds with an

then transmits a downlink voice packet to the STA. This approach avoids the PS-Poll

Chapter 3. Energy-Efficient Transmission Mechanism

3.1. Design of the Proposed Mechanism

The proposed transmission mechanism considers the characteristics of voice packets, and

differentiates voice and non-voice packets in the WLAN MAC transmission. The initial idea

behind the proposed mechanism is to disable the MAC layer acknowledgement for voice

packets, and to minimize the overheads for performing acknowledgements. However, to

disable the MAC acknowledgement for voice packets, lost packets cannot be detected and

retransmitted, and the voice quality may degrade. Therefore, the proposed mechanism is

improved herein by dynamically turning the MAC-layer acknowledgement on and off for

each transmission attempt of a voice packet, depending on the WLAN packet loss condition

and the target voice quality. If packet error and loss rarely occur, then the MAC-layer

acknowledgment for a voice packet is turned off, which improves the efficiency of network

and energy. Conversely, while the packet loss rate increases, the MAC-layer

acknowledgement for voice packets is turned on to ensure the voice quality.

By applying the PS-Poll transmission scheme shown in Figure 1, a VoWLAN STA transmits

an uplink voice packet, and then uses a PS-Poll frame to retrieve a downlink voice packet on

the AP. In addition to voice packets, an AP and STA may exchange non-voice packets such as

and session description protocol (SDP), so that an AP can distinguish voice packets and

non-voice packets. For an uplink voice packet transmission such as Step 4 in Figure 1, a

MAC-layer transmission counter, given by , is associated with each uplink voice packet,

where denotes the maximum number of MAC-layer transmission attempts for an uplink

voice packet. Similarly, is associated with downlink voice packets such as Step 6 in

ul r N ul r N dl r N

Figure 1. and are dynamically adjusted based on the current packet loss rate, and

the target voice quality that a VoIP session sets. and are negotiated by an AP and

STA, and are stored on both sides. For instance, if , then an uplink voice packet is

sent only once, and does not need to be acknowledged. Since is stored on both the STA

and AP, the STA knows that the packet will not be acknowledged, so the STA can return to

sleep or perform the next packet transmission without waiting for the acknowledgement frame.

Therefore, the STA can save the time and energy to receive acknowledgement frames from

the AP. Similarly, for a downlink packet transmission, the STA can conserve energy by not

sending an acknowledgement frame for a downlink voice packet. Hence, the overheads to

perform the MAC acknowledgement for voice packets are eliminated, and both the energy

efficiency and WLAN utilization can be improved. If , then the uplink voice packet

that is sent at the first time needs an acknowledgement from the AP. If the AP receives the

packet, it acknowledges the uplink voice packet, and the transmission is complete. Otherwise,

if the STA does not receive the acknowledgement from the AP, the STA detects the packet loss

ul r N N_rdl ul r N N_rdl 1 = ul r N ul r N 2 = ul r N

and resends the uplink voice packet again. Since , the resent voice packet does not

need the MAC acknowledgement. The STA proceeds for the next transmission or returns to

sleep after resending the voice packet. Restated, the STA sends the uplink voice packet at

most times that require the MAC-layer acknowledgement. The packet resent at the

time needs not be acknowledged. Clearly, increasing the values of and

generally improves the voice quality, but also increases the radio resource and energy

consumption for a VoWLAN session. The method to determine and is discussed

later in this section. Non-voice packets such as SIP messages and PS-Poll frames, which are

important packets and need to be reliably delivered, are transmitted by the standard WLAN

MAC mechanism. 2 = ul r N 1 − ul r N ul r N N_rul N_rdl ul r N N_rdl

Figure 4. The proposed mechanism applied to the PS-Poll scheme

Figure 4 shows a timing diagram of the proposed method applied to the PS-Poll transmission

retransmitted. Comparing Figure 2 and Figure 4 reveals that sending packets without

acknowledgement frames significantly reduces the length of a duty cycle. This is because the

length of an acknowledgement frame is nearly equal to that of a small voice packet. Then, the

energy consumption of a VoWLAN STA for an uplink and downlink transmission by applying

the proposed mechanism to the PS-Poll scheme, denoted as and

respectively, become

, and

.

, , , and are further defined

as M Poll PS− − PS−Poll−M M Poll PS M Poll PS M Poll PS M Poll PS− − − − − − − − M Poll PS M Poll PS M Poll PS M Poll PS− − − − − − − − M Poll

PS− − PS−Poll−M PS−Poll−M PS−Poll−M

ul E E_dl ul ul ul E BK E FR E SU E = _ + _ + _ dl dl dl E BK E FR E SU E = _ + _ + _ ul FR E _ E _FR_dl E _SU_ul E _SU_dl ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ + × × = = − − − − −− − −− − − − 2 _ _ M Poll PS success dl M Poll PS success ul rx M Poll PS t M Poll PS dl M Poll PS ul T T P N FR E FR E . , and . , and voice ul difs success ul T T TPS₋−Poll−M = + ₋ M Poll PS− − M Poll PS− − voice dl sifs Poll PS difs success dl T T T T T ₋ = + ₋ + + ₋ voice ul tx difs rx ul P T P T SU E_ = × + × ₋

(

difs sifs dl voice

)

tx PS Poll

rx dl P T T T P T SU E_ PS−Poll−M = × + + ₋ + × ₋ M Poll PS M Poll PS Poll PS Poll PS Poll PS− − − − − − − .

The decrease in energy consumption per duty cycle from adopting the proposed mechanism

for the PS-Poll scheme is thus obtained:

(A)

Assume that the PS-Poll scheme with and without the proposed mechanism suffers the same

dl ul dl ul dc E E E E E = + − − Δ

numbers of overhearing transmissions, i.e. . Additionally, the WLAN

MAC contention windows for the PS-Poll transmission scheme with and without the proposed

mechanism are assumed to be equal, i.e. . Then, Equation (A)

can be rewritten as M Poll PS Poll PS− − − M Poll PS Poll PS− − − t t N N = BK E BK E_ = _

(

− + −

)

+ =

ΔE_dcPS−Poll E_SU_ulPS−Poll E_SU_ulPS−Poll−M E_SU_dlPS−Poll E_SU_dlPS−Poll−M

(

E_FR_ulPS−Poll−E_FR_ulPS−Poll−M +E_FR_dlPS−Poll−E_FR_dlPS−Poll−M

)

=

[

P_rx×

(

T_sifs +T_ack +T_sifs

)

+P_tx×T_ack

]

+

[

N_tPS−Poll×P_rx×

(

T_sifs +T_ack

)

]

(B) Equation (B) reveals that the performance improvement by applying the proposed mechanism

comes not only from the elimination of acknowledgement frames for downlink and uplink

voice packets, but also from reducing the length of the period that a VoWLAN STA overhears

the other VoWLAN STAs’ transmissions.

This proposed mechanism can also be applied to the U-APSD transmission scheme,

eliminating the acknowledgement frame for downlink voice packets. Notably, the uplink

voice packet in the U-APSD scheme is used to trigger download packet transmissions.

Therefore, the proposed design categorizes the important uplink voice packets as non-voice

packets in the U-APSD scheme, and processes them by using the standard MAC transmission

3.2. The Maximum Numbers of Transmission Attempts for Voice Packets

Disabling MAC-layer acknowledgement may introduce packet loss which degrades the voice

qualities. First, the contention window of each voice packet transmission does not increase

when sending packets over WLANs without the MAC-layer acknowledgement. The

probability of collisions rises when the number of VoWLAN sessions increases. Second, the

WLAN channel error also introduces packet loss. To reduce packet loss and maintain voice

quality, the proposed transmission mechanism defines the maximum numbers of transmission

attempts for each uplink and downlink voice packet, which are denoted as and

respectively. controls the uplink packet loss caused by uplink packet collision and

packet error, while controls the loss rate for downlink voice packets. Notably,

, since collision never occurs for transmitting downlink voice packets guarded by a

PS-Poll frame for the PS-Poll scheme, or by an uplink trigger frame for the U-APSD scheme.

Hence, is only used to reduce packet loss caused by WLAN channel error. A VoWLAN

system must set a target voice quality in terms of the packet loss rate. If the loss rates of the

downlink and uplink voice packets reach that predefined values, then and increase

to improve the downlink and uplink voice qualities. Conversely, and are

decreased to reduce the energy consumption when the packet loss rate is low. Typically, the

target packet loss rate is set to 1–2% for a VoWLAN application

ul r N N_rdl ul r N dl r N ul r dl r N N ≠ dl r N ul r N N_rdl ul r N N_rdl [12]. Another issue is

receiver-report mechanism like real-time control protocol (RTCP) between an AP and

VoWLAN STAs [13]. The sender and receiver reports containing the number of voice packet

sent and received by an AP and STA are periodically exchanged to obtain the current

3.3. Implementation Issues

To apply the proposed mechanism to the IEEE 802.11 standard, the WLAN MAC

multicasting that does not require acknowledgement from peers can be adopted to transmit

voice packets without acknowledgement frames. A VoWLAN STA, say STA i, is configured

with two MAC addresses, one unicast address, MAC STAi, and one multicast address, MAC

Mi. The WLAN MAC frame with a unicast address as the destination MAC address is used to

send non-voice packets, and the WLAN MAC frame with a multicast address as the

destination MAC address is used to send voice packets between an AP and STA. First, a

VoWLAN STA associates itself with an AP using its unicast MAC address. The AP and STA

then set up a shared multicast MAC address, by either implicitly producing MAC multicast

address from the STA’s unicast address, or explicitly exchanging a message between the STA

and AP to configure a new multicast MAC address. The multicast address is only shared by

the AP and STA, and not with other STAs. Restated, each STA has its own unicast MAC

address, and its own multicast address which is shared by the AP for uplink and downlink

voice packet transmission. A multicast address is used as the destination MAC address of

voice packets only because the MAC multicast frame does not need an acknowledgement

frame from the receiver, but the multicast frame has no intent to be sent to a group of STAs. If

or , then the first or transmission attempts for a voice

packet use the unicast MAC address, implying that the packet needs to be acknowledged. The 1

>

ul r

multicast MAC address is used at the and time transmission to avoid the MAC

acknowledgement. This design enables the proposed mechanism to be implemented in the

existing WLAN infrastructure through only software upgrades.

ul r

Chapter 4. Simulation Results

Simulations were conducted to evaluate the WLAN performance and the energy consumption

of a VoWLAN STA for different packet transmission schemes. The VoWLAN simulation

environment comprised an AP and several VoWLAN STAs, which established VoIP sessions

and exchanged voice packets with wired-line nodes in the same subnet. Table 1 summarizes

the simulation parameters.

Table 1. Simulation parameters Parameters of Voice codec

GSM G.711

Bit rate 13.2 kbps 64 kbps

Framing interval 20 ms 20 ms

Payload 33 bytes 160 bytes

Parameters of the WLAN MAC accesses

802.11b 802.11g

SIFS/DIFS/Slot time 10 μs/50 μs/20 μs 10 μs/28 μs/9 μs

PHY preamble + header 192 μs 20 μs

PS-Poll/ACK 80 μs/56 μs 27 μs/19 μs

CWmin/CWmax 32/1024 16/1024

Parameters of the power consumption of a WLAN interface (ORiNOCO PC Gold, IEEE 802.11b) [14]

tx

P /P_rx 1400 mW/950 mW

dz

P 60 mW

Bit Error Rates (BERs)

WLAN channel conditions 1.0e-5/1.0e-4

VoWLAN STA by adopting the PS-Poll, U-APSD, PS-Poll with the proposed mechanism

(denoted as PS-Poll-M) and U-ASD with the proposed mechanism (denoted as U-APSD-M)

were first evaluated. The first simulation used the IEEE 802.11b and GSM voice codec, and

assumed a good WLAN channel condition, i.e. BER = 1.0e-5. Figure 5(a) shows the average

duty cycle per VoWLAN STA under different numbers of concurrent VoWLAN STAs in an

AP. The y-axis of Figure 5(a) shows the length of a duty cycle over a voice framing interval

which indicates the percentage of time that a VoWLAN STA must stay awake to transmit one

uplink and one downlink voice packet every voice framing interval. A larger percentage of a

duty cycle per voice framing interval implies that more energy is consumed for a VoWLAN

STA. Figure 5(a) indicates increasing the number of concurrent VoWLAN STAs in an AP also

increases the average duty cycle per VoWLAN STA, because a VoWLAN STA needs to spend

more time contending the WLAN channel when more concurrent VoWLAN STAs are served

by an AP. The figure illustrates that while the number of VoWLAN STAs in an AP is 1, the

PS-Poll and U-APSD transmission schemes with the proposed mechanism decrease by about

10% length of a duty cycle than the transmission schemes without the proposed mechanism.

This performance improvement is gained only by eliminating acknowledgement frames. The

performance improvement from applying the proposed mechanism grows as the number of

concurrent VoWLAN STAs in an AP increases. This is because that a VoWLAN STA usually

STAs. The proposed mechanism reduces the transmission time of voice packets, and hence,

significantly reduces the average length of an overhearing period while the WLAN load

becomes heavy. For instance, the average duty cycle per VoWLAN STA for the PS-Poll and

PS-Poll-M schemes when supporting 9 concurrent VoWLAN STAs are 4.48 ms and 3.16 ms

respectively. The proposed mechanism reduces the duty cycle by about 29%. The average

duty cycles for the U-APSD and U-APSD-M schemes are 1.96 ms and 1.48 ms respectively,

indicating a reduction of about 24% when adopting the proposed mechanism for the case that

9 VoWLAN STAs are served by an AP. The simulation results reveal that the proposed

method removes the acknowledgement frames for voice packets, thus shortening each duty

cycle, which also reduces the WLAN channel waiting time while a VoWLAN STA tries to

transmit a voice packet. To evaluate the improvement of energy consumption, the energy

consumption of a VoWLAN STA using various transmission schemes was investigated.

ORiNOCO PC Gold Card is used for this simulation [14]. Figure 5(b) shows the

corresponding energy consumption of Figure 5(a). The proposed mechanism reduced the

average energy consumption of PS-Poll and U-APSD by about 23% and 17%, respectively,

0 20 40 60 80 100 1 3 5 7 9 11 13 15 17 19 21 23 25 Number of VoWLAN STAs in an AP

A v er age d ut y c y c le pe r V o W LA N S T A ( % ) _PS-Poll U-APSD PS-Poll-M U-APSD-M (a) 0 150 300 450 600 750 900 1 3 5 7 9 11 13 15 17 19 21 23 25 Number of VoWLAN STAs in an AP

A v e ra ge pow er c on s um pt ion pe r Vo W L AN ST A ( m W ) PS-Poll U-APSD PS-Poll-M U-APSD-M (b)

Voice packets for which acknowledgement frames are not sent might be lost due to packet

error or collision, degrading the voice quality. Therefore, the proposed mechanism

dynamically adjusts the maximum number of transmission attempts, i.e. and , for

each voice packet based on a target packet loss rate. In the simulations, the maximum

permissible packet loss rate was set to 2%.

ul dl

t

N N_t

Figure 6 shows the average loss rates for uplink

and downlink voice packets by applying different transmission schemes. The simulation

results shows that although the packet loss rates of PS-Poll-M and U-APSD-M were slightly

higher than those of PS-Poll and U-APSD, the packet loss rates for the proposed transmission

mechanism are still less than 1%. The packet delays of different transmission schemes were

then investigated. Figure 7 shows the average delay for uplink and downlink voice packets.

The figure indicates that the voice packets encounter longer delays as the number of

VoWLAN STAs served by an AP increases. This is because that increasing the WLAN load

causes more packets to be queued on the AP, increasing the packet delay. Since the proposed

mechanism reduces the length of a duty cycle, it increases the maximal number of VoWLAN

STAs that can be supported by a WLAN. Simulation results indicate that PS-Poll-M can

support two more VoWLAN STAs than the PS-Poll scheme under a 50 ms delay and 2% loss

rate constraints for voice packets. U-APSD-M was found to support four more VoWLAN

STAs than the U-APSD scheme. Simulation results depicted in Figure 6 and Figure 7 reveal

transmission schemes by about 18% and 24%, respectively and also provides a similar voice quality. 0 2 4 6 8 10 1 3 5 7 9 11 13 15 17 19 21 23 25 Number of VoWLAN STAs in an AP

A v er age upl ink pac k et l os s r at e (% ) PS-Poll U-APSD PS-Poll-M U-APSD-M (a) 0 1 2 3 4 5 1 3 5 7 9 11 13 15 17 19 21 23 25 Number of VoWLAN STAs in an AP

A v er ag e d ow nl in k p ac k et l os t r at e ( % ) PS-Poll U-APSD PS-Poll-M U-APSD-M (b)

0 40 80 120 160 1 3 5 7 9 11 13 15 17 19 21 23 25

Number of VoWLAN STAs in an AP

A v er ag e upl in k p a cket d e la y (m s 200 ) PS-Poll U-APSD PS-Poll-M U-APSD-M (a) 0 40 80 120 160 200 1 3 5 7 9 11 13 15 17 19 21 23 25

Number of VoWLAN STAs in an AP

A v er age dow nl ink pac k et del ay ( m s ) PS-Poll U-APSD PS-Poll-M U-APSD-M (b)

Figure 7. Average delay for uplink and downlink packets

different voice codecs and BERs was further evaluated. Figure 8 illustrates the length

reduction of a duty cycle per VoWLAN STA from applying the proposed mechanism to the

PS-Poll and U-APSD schemes. Simulation results indicate that the PS-Poll-M scheme using

GSM codec performed better than that using G.711 codec, because the acknowledgement

frame introduces relatively more overhead for low bitrate codecs such as GSM than high

bit-rate codecs such as G.711. Additionally, the simulation results show that PS-Poll-M

performs better at a BER of 1.0e-5 than 10e-4. This is because a poor WLAN channel quality

leads to a high packet loss, implying that the proposed methods must increase the maximum

number of transmission for voice packets to maintain an acceptable voice quality.

0 5 10 15 20 25 30 1 2 3 4 5 6 7 8

Number of VoWLAN STAs in an AP

T h e l e n g th r e du ct io n of d u ty cycl e p e r V o WLA N ST A ( % ) PS-Poll-M (GSM, 1.0e-5) PS-Poll-M (G.711, 1.0e-5) PS-Poll-M (GSM, 1.0e-4)

Finally, the performance improvement under different WLAN standards was investigated.

Figure 9 shows the average duty cycle per VoWLAN STA when using GSM codec in the

IEEE 802.11g with BER = 1.0e-5. While the number of concurrent VoWLAN STAs that can

be served by an AP increases, the probability to overhear other STAs’ transmission also

increases. Therefore, the proposed mechanism that reduces the length of overhearing period

efficiently reduces the length of a duty cycle for a VoWLAN STA for high speed WLAN

standards. For instance, while 51 concurrent VoWLAN STAs are supported by an IEEE

802.11g AP, the proposed mechanism reduces the average duty cycle of PS-Poll and U-APSD

by about 40% and 28%, respectively.

0 20 40 60 80 100 1 11 21 31 41 51 61 71 81 91 101 111 121 131 Number of VoWLAN STAs in an AP

A v e ra g e du ty cycl e p e r V o WLA N S T A ( % ) PS-Poll_U-APSD PS-Poll-M U-APSD-M

Chapter 5. Conclusions

This study presents a cross-layer and energy-efficient transmission mechanism for a

VoWLAN system. The mechanism dynamically disables the MAC acknowledgement for each

transmission of a voice packet according to the current packet loss rate and the target voice

quality, thus reducing the time and the energy consumed by a VoWLAN STA in transmitting

and receiving voice packets. Simulation results indicate that the proposed mechanism

significantly reduces the energy consumption of a VoWLAN STA, improves the WLAN

utilization for transmitting voice packets, and can also guarantee the voice quality.

Furthermore, WLAN multicasting is used to transmit voice packets without the MAC

acknowledgement, thus enabling the proposed mechanism to be implemented on the existing

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