PERFORMANCE OF VOIP OVER IEEE 802.11G DSSS-OFDM MODE

WITH IEEE 802.11E QOS SUPPORT

ainne Hanley, Se

an Murphy, Liam Murphy

Department of Computer Science

University College Dublin, Belﬁeld, Dublin 4

Keywords:

WLAN, IEEE 802.11e, QoS, Voice over IP, Medium Access Control.

Abstract:

This paper examines, via simulation, the performance of an 802.11e MAC over an 802.11g PHY operating in

DSSS-OFDM mode. The DSSS-OFDM scheme provides data rates of up to 54Mb/s as well as interoperabil-

ity with 802.11b nodes. Due to the widespread use of 802.11b nodes, such interoperability is an important

consideration. This paper involves a study of the number of simultaneous bidirectional G.711 VoIP calls that

can be supported by such a WLAN. The results show that this mode of operation introduces a very signiﬁcant

overhead. The actual number of calls that can be carried is limited to 12 when using the 24Mb/s data rate

and 13 when using either the 36Mb/s or 54Mb/s rates. These results demonstrate the well-known disparity

between uplink and downlink performance, with the downlink imposing the limit on the number of calls that

can be carried by the system in the cases studied. The results also show that when when a signiﬁcant amount

of lower priority trafﬁc is introduced into the system, it can have a signiﬁcant impact on VoIP call capacity

despite the use of 802.11e.

1 INTRODUCTION

WLAN support for QoS is one issue that is receiving

considerable interest at present. The IEEE 802.11e

standard, which provides standardised QoS support,

is in the ﬁnal stages of development. Frustrated by

the sluggish pace of development of the standard, the

Wi-Fi Alliance, an industry forum, is attempting to

promote development of WLAN QoS by offering cer-

tiﬁcation for a subset of the standard’s functionality

which it calls Wi-Fi Multimedia (WMM). A signiﬁ-

cant number of vendors are already certiﬁed.

While vendors are beginning to ship systems with

QoS support, there is still considerable debate within

the community regarding how best to operate these

systems. The standards have been written in such a

way as to offer much ﬂexibility to enable vendors to

differentiate their product offerings. This ﬂexibility,

coupled with the complexity of the system, means that

large differences in system performance are possible.

At present, it is not clearly known how best to conﬁg-

ure such systems. Hence, there is a need to understand

how these systems behave for different parameter sets

in different conﬁgurations.

One application driving the development of WLAN

QoS support is Voice over IP (VoIP). Many enter-

prises and WLAN operators are very interested in pro-

viding VoIP over WLAN (VoIPoW) and indeed some

vendors have product offerings which can address this

need. However, such product offerings are typically

proprietary. Standardised solutions offer many known

beneﬁts and for this reason, it is important to deter-

mine the performance of VoIPoW in a standardised

802.11e/802.11g setting.

As there are currently a very large number of

802.11b network interfaces in existence, it is inter-

esting to see how backwards compatibility issues af-

fect system performance for 802.11e/802.11g sys-

tems. 802.11g provides two modulation schemes with

backwards compatibility mechanisms. The ﬁrst is

called ERP-OFDM, while the other is called DSSS-

OFDM. ERP-OFDM is based on transmitting Request

To Send /Clear To Send (RTS/CTS) signalling at a low

rate such that legacy nodes know when the medium

is unavailable. DSSS-OFDM is based on transmit-

ting packet preamble and header information at an

802.11b compliant rate.

Some research has taken place to determine the per-

formance of the ERP-OFDM scheme in certain cir-

cumstances. This paper aims to determine the perfor-

Hanley G., Murphy S. and Murphy L. (2005).

PERFORMANCE OF VOIP OVER IEEE 802.11G DSSS-OFDM MODE WITH IEEE 802.11E QOS SUPPORT.

In Proceedings of the Second International Conference on e-Business and Telecommunication Networks, pages 20-27

DOI: 10.5220/0001416200200027

 SciTePress

mance of the latter – the DSSS-OFDM scheme – in a

VoIP context.

This paper is structured as follows. Section II

brieﬂy outlines the operation of the 802.11e MAC and

the 802.11g PHY, and Section III outlines some of the

published work related to this study. In section IV,

the simulation setup is described in detail. This is fol-

lowed by a description of the simulations performed

and a discussion of the results obtained in section V.

The paper is concluded in section VI.

2 IEEE 802.11

The simulations studied here use an 802.11e MAC

layer in association with an 802.11g PHY layer; the

operation of these layers is brieﬂy outlined below.

2.1 IEEE 802.11e

IEEE 802.11e (IEEE, 2005) provides for centralised

and distributed QoS support at the Medium Access

Control(MAC) layer. As there is considerably more

complexity in the centralised scheme, the focus here

is on the distributed approach, the Enhanced Distrib-

uted Channel Access (EDCA).

EDCA provides for 4 different so-called Access

Categories (ACs). Different priorities for the 4 ACs

are realised through 4 coupled Carrier Sense Multiple

Access with Collision Avoidance (CSMA/CA) mech-

anisms. These mechanisms are coupled as they con-

tend for access to the same medium, but they differ in

that they are parameterised differently. More speciﬁ-

cally, the parameters for the higher priority trafﬁc are

chosen to enable it to obtain access to the medium

more quickly than the lower priority trafﬁc. Con-

tention between different priorities in a single station

is resolved such that the higher priority trafﬁc gains

access to the medium, while the lower priority enters

a backoff state.

Two of the key parameters which control how

the different access categories obtain access to the

medium are the Contention Window (CW) sizes and

the Arbitration Inter-Frame Space (AIFS). The for-

mer controls how much random waiting, or backoff,

delay should be introduced for each AC to avoid col-

lision and the latter controls how long each AC waits

after a transmission has terminated before attempting

to access the medium.

The 4 ACs have been labelled Voice (VO), Video

(VI), Best Effort (BE) and Background (BG). Para-

meter settings for each of the categories are shown in

Table 1.

The values of the 802.11g parameters are shown to

be relative to aCWmin and aCWmax. As stipulated

in the standard, an aCWmin of 31 and an aCWmax

Table 1: Default IEEE 802.11e EDCA Parameter Set

AC CWmin CWmax AIFSN

VO (aCWmin+1)/4-

(aCWmin+1)/2-

VI (aCWmin+1)/2-

aCWmin 2

BE aCWmin aCWmax 3

BG aCWmin aCWmax 7

of 1023 were used, to maintain compatibility with

802.11b systems carrying VoIP trafﬁc.

2.2 IEEE 802.11g

The 802.11g Physical layer (PHY) (IEEE, 2003)

enhancement outlines 4 modulation schemes. Two

of which are mandatory – ERP-OFDM and ERP-

CCK/DSSS – and two of which are optional – ERP-

PBSS and DSSS-OFDM. Of the four schemes, only

ERP-OFDM and DSSS-OFDM provide data rates

of up to 54Mb/s using OFDM modulation schemes,

while also providing explicit support for interoperat-

ing with 802.11b nodes. Such support is necessary

as 802.11b nodes are not able to detect or understand

OFDM modulated signals.

The ERP-OFDM scheme is a variant of the 802.11a

PHY scheme modiﬁed for use in the 2.4 GHz band.

The DSSS-OFDM scheme is a hybrid modulation

scheme that combines a DSSS preamble and header

with an OFDM payload transmission.

ERP-OFDM uses the RTS/CTS mechanism to pro-

vide 802.11b interoperability. This mechanism gives

the 802.11g nodes time to freely transmit at the higher

rates. In the DSSS-OFDM scheme, all packet headers

and preambles are transmitted at the lower 1Mb/s rate

using 802.11b compliant DSSS modulation. Thus,

802.11b nodes know how the medium is being used

even if they cannot detect OFDM payload transmis-

sion.

3 RELATED WORK

While WLAN performance analysis is currently a

very active research area, little has been published on

VoIP performance for 802.11e/802.11g systems.

Although Mangold et al. (Mangold et al.,

2003) analysed the performance of the 802.11e

standard, their study was performed in relation to

802.11e/802.11a. Therefore, backward compatibility

with 802.11b was not a concern.

Choi and Pavon (Choi and Pavon, 2003) dis-

cussed backward compatibility of the 802.11g ERP-

OFDM scheme with regards to the 802.11b stan-

PERFORMANCE OF VOIP OVER IEEE 802.11G DSSS-OFDM MODE WITH IEEE 802.11E QOS SUPPORT

dard. Their results showed that the ERP-OFDM

RTS/CTS protection mechanism introduces a lot of

overhead, thus greatly reducing the performance of

the network when compared with a system contain-

ing only 802.11g nodes. Their results showed that

the transmission time of a packet on 802.11g with a

long preamble RTS/CTS exchange, was about dou-

ble that of 802.11g with RTS/CTS disabled. How-

ever, Bianchi (Bianchi, 2000) showed that without

the RTS/CTS mechanism, the performance of the

802.11b DCF scheme was highly dependent on the

number of nodes in the system and the size of the min-

imum CW. Given that the DSSS-OFDM scheme does

not require RTS/CTS signalling, it is interesting to see

the performance levels that can be obtained using this

scheme.

Garg and Kappes (Garg and Kappes, 2003) per-

formed an analytical analysis of VoIP capacity for the

802.11b and 802.11a schemes. They developed a for-

mula which can be used to calculate the VoIP call

capacity of a WLAN network under certain assump-

tions. However, their paper did not discuss the use of

the 802.11e QoS mechanism. In addition, although

quite useful, their formula does not hold, unless there

is only VoIP trafﬁc on the system. Also, their formula

is based on the assumption that there are only ever

two active senders in the network, whereby the AP

and one wireless node always have a packet to send.

For these reasons, it was unclear that their work could

accurately predict the levels of performance that can

be attained by VoIP trafﬁc on an 802.11e/g system.

An important phenomenon in these systems is the

disparity between the system performance in the up-

link and downlink; this has been reported in previ-

ous work by both Grilo and Nunes (Grilo and Nunes,

2002) and Casetti and Chiasserini (Casetti and Chi-

asserini, 2004). Both papers were however related to

the 802.11e MAC layer over the 802.11b PHY layer.

The same difference is apparent throughout the results

in this paper but is further examined and discussed in

relation to the 802.11g PHY scenario.

4 THE SIMULATION SCENARIO

In order to determine the performance of VoIP in

an 802.11e/802.11g system, a series of simulations

were performed. In these simulations, the wireless

nodes were arranged with an AP in the network which

formed a connection between every 802.11e node in

the wireless domain and a single node in the wired

domain (see Fig.1). This AP was connected to the

wired network by a high capacity link with negligible

delay, which was dimensioned such that it could eas-

ily carry all the trafﬁc and hence no loss occurred on

this link. All wireless nodes were within radio range

Figure 1: Network Topology

of each other, thus ensuring that no issues arose relat-

ing to hidden/exposed station problems. In addition,

all nodes were situated sufﬁciently close together so

that they were able to transmit at the highest data rate

supported by 802.11g. Static routing

was used, so

as to ensure realistic routing of the wireless trafﬁc.

The simulations were conﬁgured such that each

node used bidirectional CBR trafﬁc sources, so as to

model VoIP trafﬁc. This was represented as Constant

Bit-Rate (CBR) trafﬁc, transmitted using User Data-

gram Protocol (UDP). In a similar manner to that of

Yu, Choi and Lee (Yu et al., 2004), these sources were

parameterised to model G.711 voice at 64kb/s with

20ms payload. The G.711 scheme was chosen as it

is still commonly used, due to its simplicity, despite

the availability of schemes with better compression.

As in (Yu et al., 2004) a VoIP data payload size of

160 bytes was generated every 20ms, to which the

20 byte IP header, the 12 byte RTP header and the

8 byte UDP header were added. In order to avoid is-

sues with trafﬁc synchronisation, a low level of ran-

dom noise was introduced into the packet generation

process. This resulted in a source which generated a

200 byte packet approximately every 20ms, resulting

in 80kb/s of trafﬁc in total per node. This VoIP trafﬁc

was always transmitted at the highest priority level of

802.11e in accordance with the standard.

In some of the later experiments, an additional bidi-

rectional low priority, BE trafﬁc source was intro-

duced for each of the bidirectional VoIP sources in

the simulation. Like the VoIP trafﬁc, this was also pa-

rameterised as CBR trafﬁc over UDP but at a rate of

250kb/s and with a 1500 byte data payload. The aim

was to see how a VoIP application would perform if

there was a heavy trafﬁc load at the lower priority.

The No Ad-Hoc Routing (NOAH) patch for ns-2.26 was

used - http://icapeople.epﬂ.ch/widmer/uwb/ns-2/noah.

ICETE 2005 - WIRELESS COMMUNICATION SYSTEMS AND NETWORKS

Table 2: Extended Rate IEEE 802.11g PHY Characteristics

Characteristic Value

SlotTime 20µs(long), 9µs(short)

SIFSTime 10µs

CCATime <15µs(long),

<4µs(short)

aCWmin(0) 31

aCWmax 1023

Supported Rates 1, 2, 5.5, 6, 9, 11,12, 18,

24, 36, 48, and 54Mb/s

Mandatory Rates 1, 2, 5.5, 11, 6, 12, and

24Mb/s

The IEEE 802.11g parameters (see Table 2) for

the simulations using the DSSS-OFDM modulation

scheme were chosen to allow backward compatibility

with the IEEE 802.11b PHY. Hence, the long PLCP

preamble, long slot time, and long Clear Channel As-

sessment (CCA) time were used. In accordance with

the standard for DSSS-OFDM, the long PLCP pream-

ble and long PLCP header were transmitted at 1Mb/s.

For this study, a simulation model of the IEEE

802.11e with the EDCA mechanism, developed by

the TKN group in Berlin, for the Network Simulator

package NS was used (Wietholter and Hoene, 2003).

Since the primary focus of this study was the VoIP

trafﬁc, a 50 packet queue limit was chosen for every

node in the system, as there is no advantage in queu-

ing VoIP packets for extended periods because they

are delay limited. Furthermore, studies have indicated

(Yu et al., 2004) that there is a minimal performance

difference between a 50 and 100 packet queue.

Each simulation was run three times and the results

were averaged over these three runs. All simulations

were run for 250 seconds of simulation time, and the

maximum mandatory 802.11g data rate of 24Mb/s as

well as the optional higher 36Mb/s and 54Mb/s rates

were studied.

5 RESULTS

These results are an assessment of the performance of

the DSSS-OFDM scheme with regards to VoIP capac-

ity. This analysis is ﬁrstly performed at three differ-

ent data rates but in the absence of any other trafﬁc.

Then the effect of the addition of a large amount of

BE (see Table 1) trafﬁc is examined at the same three

data rates.

In these experiments, loss and delay measurements

were taken at the UDP layer of the protocol stack. The

downlink delays are the average packet delays from

the originating wired node, to the receiving wireless

node. Similarly, the uplink delays are the average

packet delays from the originating wireless node, to

the receiving wired node.

The loss examined here represents packets which

were sent by the UDP transport layer of the transmit-

ting node, but which were never received by the UDP

transport layer of the receiving node. These loss rates

therefore represent the percentage of packets which

are dropped due to collisions on the medium or Inter-

face Queue (IFQ) overﬂow.

There are MAC level retransmissions of all col-

lided packets, but those packets which exceed the re-

transmission threshold without being successfully re-

ceived are considered as lost. For these simulations,

the Short Retry Limit was set to 7 in accordance with

what is recommended by 802.11e.

VoIP requires certain quality levels: ETSI studies

(ETSI, 2002) indicate that a packet loss rate of 5% is

at the upper bound for acceptable voice quality. Also,

for this study, WLAN delays of greater than 50ms

were considered to be unacceptably high.

5.1 Analysis of VoIP Trafﬁc in an

802.11e/802.11g Network with

DSSS-OFDM Modulation

This set of simulations was performed using the

DSSS-OFDM parameters, and in the absence of any

trafﬁc other than the VoIP trafﬁc being analysed. The

simulations were run at the maximum mandatory data

rate of 24Mb/s, as well as at the optional higher rates

of 36Mb/s and 54Mb/s. The results show a compar-

ison of the uplink and downlink results for the end

to end delays and loss rates, the average contention

window sizes used for the backoff calculation and the

percentage occupancy of the IFQ.

5.1.1 End-to-End Delay and Loss Rates

At ﬁrst, the similarity in the results for each of the

three data rates, 24, 36 and 54Mb/s, seems surprising.

In fact, results show that increasing the rate at which

the actual data is sent has quite a small impact on

overall performance levels. This can be attributed to

the fact that in the DSSS-OFDM scheme, the longer

PLCP data is used and is sent at a slow 1Mb/s rate for

backwards compatibility. Therefore a large overhead

is introduced for each packet transmission. The neg-

ative performance effects of this large overhead dom-

inate the overall system performance and to a great

degree mask the positive impact of an increased data

rate.

Results show that increasing the maximum data

rate of the system from 24Mb/s, to 36Mb/s, and then

to 54Mb/s, does lower the delay experienced by pack-

ets (see Fig.2). However, if the average delay for 15

bidirectional VoIP calls is examined for each of the

PERFORMANCE OF VOIP OVER IEEE 802.11G DSSS-OFDM MODE WITH IEEE 802.11E QOS SUPPORT

three data rates, the average downlink end-to-end de-

lay at 24Mb/s is 111ms, at 36Mb/s is 96ms, and at

54Mb/s is 86ms. Although notable, these improve-

ments in delay at the higher data rates are quite low

given the greatly increased data rates.

The known disparity between uplink and downlink

performance is quite visible in these results: the delay

difference between uplink and downlink is especially

apparent when there are 13, 14 and 15 calls on the

network. For this region there are quite large down-

link delays in comparison to minimal uplink delays.

This can be attributed to the saturation of the down-

link occurring prior to that of the uplink. However, it

can be seen that after 15 calls the uplink delays also

begin to climb. By the 17 call point, there is more uni-

formity to the delays as both uplink and downlink are

saturated and hence delay levels begin to converge.

It is clear from the results that in the 24Mb/s sce-

nario a large increase in loss is experienced by the

downlink when there are only 13 bidirectional calls on

the system. In fact, at this stage loss rates are already

in excess of the 5% threshold (see Fig.3). Results

show that the 24Mb/s network can support only 12

bidirectional calls, and both the 36Mb/s and 54Mb/s

systems can support only 13 bidirectional calls before

loss rates on the downlink reach 6% or higher.

In this scenario, the majority of the downlink loss

was due to queue overﬂow, whereas the majority of

the uplink loss was caused by retransmission failure.

Loss at the AP can be explained by noting that the

AP has to handle downlink trafﬁc for all of the wire-

less nodes; if the medium is congested, then it may

not obtain sufﬁcient access to the medium for all this

downlink trafﬁc, its queue builds up and it suffers

packet loss. In contrast, the individual wireless sta-

tions have much lower trafﬁc levels and so the queues

at the wireless stations rarely contain many packets.

Hence, wireless stations do not encounter IFQ over-

ﬂow. However, it seems that the uplink does experi-

ence a higher probability of collision than the down-

link. These higher levels of collisions and retransmis-

sions on the uplink occasionally lead to a packet be-

200

400

600

800

1000

1200

1400

6 8 10 12 14 16 18

Delay [ms]

Bidirectional VoIP Calls

24 Mbps DL

24 Mbps UL

36 Mbps DL

36 Mbps UL

54 Mbps DL

54 Mbps UL

Figure 2: End-to-End Delays With Only VoIP Trafﬁc

100

6 8 10 12 14 16 18

% Loss

Bidirectional VoIP Calls

24 Mbps DL

24 Mbps UL

36 Mbps DL

36 Mbps UL

54 Mbps DL

54 Mbps UL

Figure 3: Loss Rates With Only VoIP Trafﬁc

ing dropped as it has exceeded its maximum number

of retransmission attempts without being successfully

received.

These results indicate that the use of parameters

which enable backward compatibility lead to a serious

reduction in the performance of VoIP applications.

In previous experiments that were performed which

focussed on the ERP-OFDM scheme without back-

ward compatibility mechanisms, it was found that

at 24Mb/s an ERP-OFDM 802.11g WLAN system

could support approximately 48 bidirectional G.711

VoIP calls. In fact, in this scenario, the 802.11g

DSSS-OFDM modulation scheme performs little bet-

ter than a basic 802.11b in terms of VoIP call capac-

ity: Garg and Kappes (Garg and Kappes, 2003) have

shown that the 20ms G.711 capacity for an 11Mb/s

802.11b system is 12 calls.

5.1.2 Contention Window Size

For each of these simulations the Contention Window

(CW) size that was used for any transmission or re-

transmission was recorded. The following graphs (see

Fig.4) show the average size of the high priority CW

for the scenarios which involved between 7 and 17

bidirectional VoIP calls. Surprisingly, it was noted

that the average size of CW used by the AP was al-

ways smaller than that of the corresponding wireless

node.

This difference is as a result of the frequency with

which the AP attempts to access the medium as op-

posed to the frequency with which the wireless nodes

attempt to gain access.

Due to the greater level of trafﬁc at the AP, its IFQ

ﬁlls much more quickly than that at any individual

wireless node. This means that frequently when the

AP is attempting to gain access it is not in direct con-

tention with any of the wireless nodes. Due to this

lower contention level, the AP has a lower probabil-

ity of colliding with another packet and thus a lower

probability of having to retransmit with an increased

CW.

On the other hand, the wireless node will almost

ICETE 2005 - WIRELESS COMMUNICATION SYSTEMS AND NETWORKS

always be in direct contention with the AP when it

is attempting to transmit, hence, it will have a higher

potential for colliding with a packet sent by the AP.

5.1.3 IFQ Occupancy

The results show a breakdown of the IFQ occupancy

for high priority trafﬁc as a percentage of the simula-

tion time. These statistics are very informative as to

the point at which high priority trafﬁc begins to en-

counter notable delays in accessing the medium (see

Fig.5). If queue occupancy is at a high level for a

large amount of the simulation time, it is an indica-

tion that there is a lot of congestion on the network,

and hence packets are encountering difﬁculty in ac-

cessing the medium.

The results from the 24Mb/s simulations (see Fig.5

show that the queue occupancy rates of the wireless

nodes are lower than that of the AP. This behaviour is

not unexpected as the amount of trafﬁc that a single

wireless node has to handle is a lot lower than the

trafﬁc levels at the AP.

5.2 Analysis of VoIP Trafﬁc in the

presence of Best Effort trafﬁc

This section describes experiments that were per-

formed to determine the impact of high levels of Best

Effort CBR trafﬁc on the system, while using DSSS-

OFDM modulation.

5.2.1 End-to-End Delay and Loss Rates

If the end-to-end delays for these bidirectional VoIP

calls are compared with the results obtained in the ab-

sence of BE trafﬁc (see Fig.2 and 6), a signiﬁcant

increase in delay is observed. In addition, for this

network setup, results show a deﬁnite increase in the

losses at all three data rates, compared to the voice

only trafﬁc scenario (see Fig.3 and 7). It was found

that in the absence of BE trafﬁc, the 24Mb/s network

could support 12 bidirectional calls, and that both the

6 8 10 12 14 16 18

Average CW Size

Bidirectional VoIP Calls

24 Mbps DL

24 Mbps UL

36 Mbps DL

36 Mbps UL

54 Mbps DL

54 Mbps UL

Figure 4: Contention Window Sizes With Only VoIP Trafﬁc

36Mb/s and the 54Mb/s networks could support 13

bidirectional calls before delays and loss rates on the

downlink reach unacceptable levels.

Based on the loss and delay results in the pres-

ence of BE trafﬁc, it can be seen that the 24Mb/s net-

work can support only 9 bidirectional calls, and both

the 36Mb/s and 54Mb/s systems can support only 10

bidirectional calls. After this point, loss rates on the

downlink become excessive and the VoIP trafﬁc en-

counters a large increase in end-to-end delay. This

indicates that with this amount of additional trafﬁc

in this network scenario, the number of supportable

bidirectional VoIP calls is reduced by 3, which is a

signiﬁcant decrease in terms of VoIP call capacity.

Due to the nature of the 802.11e mechanism such a

large increase in delay seems somewhat surprising as

802.11e was designed to facilitate medium access to

higher priority trafﬁc largely at the expense of lower

priority trafﬁc. However, here it is demonstrated that

this is not always the case.

The inﬂuence of background trafﬁc is dependent

upon many factors. In general, extra trafﬁc will in-

crease the risk of collision, which will lead to an in-

creased number of retransmissions as well as a greater

number of lost packets.

In this case, the lower priority, BE trafﬁc packets

have a data payload of 1500 bytes as opposed to the

160 bytes in the VoIP packets, and hence will occupy

the medium for longer periods. Such longer trans-

mission times will further increase the risk of colli-

sion with VoIP trafﬁc. In fact, the transmission of a

BE packet at the 24Mb/s data rate will occupy the

medium for approximately 1ms as opposed to only

590µs for a VoIP packet. Therefore, during this ad-

ditional 410µs delay more VoIP packets will have ar-

rived in the AP queue. This will cause the queue at

the AP to build much more quickly than when no BE

trafﬁc is present. This will lead to increased queu-

ing delays, as well as an increase in back-off delays,

for the VoIP trafﬁc. Plus, as queue occupancy lev-

els increase, it could eventually lead to packets being

dropped due to queue overﬂow.

100

0 5 10 15 20 25 30 35 40 45 50

% Simulation Time

Size of IFQ

8 UL VoIP calls

8 DL VoIP calls

12 ULVoIP calls

12 DL VoIP calls

14 UL VoIP calls

14 DLVoIP calls

17 UL VoIP calls

17 DL VoIP calls

Figure 5: 24Mb/s AP and Wireless Node IFQ Sizes With

Only VoIP Trafﬁc

PERFORMANCE OF VOIP OVER IEEE 802.11G DSSS-OFDM MODE WITH IEEE 802.11E QOS SUPPORT

5.2.2 Packets Received After Retransmission

In order to further understand the increases in loss and

delay, the levels of packets received after retransmis-

sion was examined.

In this scenario, results show that the additional BE

trafﬁc leads to a large increase in retransmissions at

both the AP and at the wireless nodes. This can be

clearly seen in Fig.8.

The medium capacity used when packets collide

and are retransmitted is a highly inefﬁcient use of

resources. This inefﬁcient use of resources leads to

less available resources in the system, which results

in lower system throughput and lower service rates on

each queue in the system. This, in turn, causes queue

occupancy to increase.

Due to the high trafﬁc levels at the AP, an increase

in retransmission levels has a large impact on the

downlink delay in particular. Combined with its high

packet arrival rate this can often also lead to queue

overﬂow at the AP, which results in increased loss lev-

els on the downlink.

5.2.3 Average Contention Window Sizes

The results again show that the AP has a consistently

lower CW than the wireless nodes (see Fig.9). How-

ever, the results can be seen to show three phases in

the relationship between the contention window sizes

of the AP and the individual wireless nodes. Firstly,

from 7 to 10 VoIP calls, then from the 10 to 15 VoIP

calls and ﬁnally as the level of voice trafﬁc reaches 16

to 17 VoIP calls, a third distinct region can be seen.

If these results are considered in association with

the IFQ occupancy rates then an association between

both sets of results can be seen. In the ﬁrst phase,

both the AP and wireless nodes have an empty IFQ

for the majority of the time, that is, at least more than

50% of the time the queues are empty. However in the

second phase, it can be noted from the results that the

AP IFQ has reached a point whereby the IFQ contains

packets for most of the simulation time. Although at

200

400

600

800

1000

1200

1400

6 8 10 12 14 16 18

Delay [ms]

Bidirectional VoIP Calls

24 Mbps DL

24 Mbps UL

36 Mbps DL

36 Mbps UL

54 Mbps DL

54 Mbps UL

Figure 6: End-to-End Delays With Best Effort Trafﬁc

100

6 8 10 12 14 16 18

% Loss

Bidirectional VoIP Calls

24 Mbps DL

24 Mbps UL

36 Mbps DL

36 Mbps UL

54 Mbps DL

54 Mbps UL

Figure 7: Loss Rates With Best Effort Trafﬁc

100

6 8 10 12 14 16 18

% Packets Retransmitted

Bidirectional VoIP Calls

DL (BE Traffic)

DL (no BE Traffic)

UL (BE Traffic)

UL (no BE Traffic)

Figure 8: VoIP Packets Received After Retransmission

With Best Effort Trafﬁc at 24Mb/s

this stage the wireless node queues still remain empty

for most of the time. The ﬁnal section corresponds to

a stage when both the AP and wireless nodes queues

are full for the majority of the simulation time and the

CW size can be seen to increase more rapidly until

the CW size of the uplink and downlink ultimately

converge.

The difference in the uplink and downlink CW

sizes is a reﬂection of the differing levels of retrans-

missions on the uplink and downlink since retrans-

missions are sent with an increased CW; as the uplink

has more retransmissions, its CW is higher. Also, in

this scenario, there is correlation between the IFQ oc-

cupancy and the mean CW size. As the mean CW size

6 8 10 12 14 16 18

Average CW Size

Bidirectional VoIP Calls

24 Mbps DL

24 Mbps UL

36 Mbps DL

36 Mbps UL

54 Mbps DL

54 Mbps UL

Figure 9: Contention Window Sizes With Best Effort Trafﬁc

ICETE 2005 - WIRELESS COMMUNICATION SYSTEMS AND NETWORKS

increases (due to retransmissions on the medium),

there are increased delays and lower medium through-

put resulting in increasing occupancy levels at the

IFQ.

6 CONCLUSION

The VoIP capacity of the DSSS-OFDM modulation

scheme when using parameters which provided back-

ward compatibility, showed that the 24Mb/s network

can support only 12 bidirectional calls, and both the

36Mb/s and 54Mb/s systems can support only 13 bidi-

rectional calls. Above these capacities, loss rates on

the downlink reach an unacceptably high level. The

results also clearly show that, as expected, the down-

link quality begins to suffer long before the perfor-

mance of the uplink begins to deteriorate.

Interestingly, the average size of CW used by the

AP was generally smaller than that of the wireless

nodes. This was due to the different medium access

requirements of the AP and the individual wireless

nodes. The AP almost always had a packet waiting

in its IFQ for transmission and so due to the lower

levels of trafﬁc being sent by each wireless node of-

ten it was not in direct contention with another node.

In contrast, the wireless nodes were frequently in di-

rect contention with the AP, hence the wireless nodes

often had a higher probability of collision. Since col-

lided packets are retransmitted with an increased CW

size, this led to a difference between the average CW

size at the AP and at the wireless nodes.

Surprisingly, results also showed that the addition

of BE trafﬁc leads to an increase in end-to-end delay,

loss rates and IFQ occupancy for the VoIP trafﬁc. Ad-

ditional trafﬁc leads to an increased risk of collision

for the VoIP packet, which was further increased by

the large size of the BE load. It was found that the

delays resulting from the additional collisions and re-

transmissions cause a large increase in queuing delay

and so decreased the packet service rate, particularly

at the AP.

In fact, it was found that, under such conditions,

the 24Mb/s network can support only 9 bidirectional

calls, and both the 36 and 54Mb/s systems can sup-

port only 10 bidirectional calls before loss rates on

the downlink are in excess of 10%. These unantic-

ipated results indicate that for this network scenario

and with this amount of additional trafﬁc, the number

of supportable bidirectional VoIP calls is reduced by

3 calls.

Future work involves further investigation of back-

ward compatibility mechanisms for 802.11g, further

investigation into ways to equalise the division of

resources between the uplink and the downlink and

investigating the optimum transmission opportunity

sizes for this scenario.

ACKNOWLEDGEMENTS

The support of the Informatics Research initiative of

Enterprise Ireland is gratefully acknowledged.

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PERFORMANCE OF VOIP OVER IEEE 802.11G DSSS-OFDM MODE WITH IEEE 802.11E QOS SUPPORT