A COMPARATIVE STUDY OF IEEE 802.11 MAC ACCESS

MECHANISMS FOR DIFFERENT TRAFFIC TYPES

Mohammad Saraireh, Reza Saatchi, Samir Al-khayatt, Rebecca Strachan

Sheffield Hallam University, Pond Street,Sheffield, UK

Keywords: Quality of Service (QoS), IEEE 802.11 Medium Access Control (MAC) Protocol, Network Performance.

Abstract: The fast growth and development of wireless computer networks and multimedia applications make the

Quality of Service (QoS) provided to their transmission an important issue. This paper aims to investigate

the impact of varying the number of active stations on the network performance. This was carried out using

different data rates. The investigations also considered both MAC protocol access mechanisms, i.e. the basic

access and the Request To Send / Clear To Send (RTS/CTS). The effect of traffic type i.e. Constant Bit Rate

(CBR) and Variable Bit Rate (VBR) traffics was also examined. The findings revealed that in large

networks (larger than 15 stations), the RTS/CTS access mechanism outperformed the basic access

mechanism since the performance of the latter was more sensitive to the increase and decrease of the

number of active stations. Increasing the data rate improved the network performance in term of delay and

jitter but it degraded the network performance in term of channel utilisation and packet loss ratio.

1 INTRODUCTION

Wireless systems are increasingly used for

transmitting different type of applications such as

voice, video and data. Wireless transmission

requires a controller to manage accessing the

medium in a fair and suitable manner and to share

the resources. Random transmission may lead to

incomprehensible or unpredictable results.

Therefore, a controller for accessing and sharing the

resources is an essential tool for achieving a

successful transmission process between the

communication parties.

The Medium Access Control (MAC) protocol in

wireless networks controls access to the shared

medium by applying rules and procedures that

permit the communication pairs to communicate

with each other in an efficient and fair manner.

The IEEE 802.11 standard defines two

coordination functions (IEEE, 1999). They are

Distributed Coordination Function (DCF) and Point

Coordination Function (PCF). The focus of this

study is the DCF that is part of the Carrier Sense

Multiple Access with Collision Avoidance

(CSMA/CA).

Under DCF protocol, data packets are transferred

using two mechanisms. The main mechanism is a

two-way handshaking process which is called basic

access mechanism. The optional or alternative

mechanism is called RTS/CTS access mechanism

that based on the exchange of RTS and CTS

messages before data packets transmission.

RTS/CTS access mechanism is used to reserve

the channel before data transmission. Under DCF,

all stations in the same Basic Service Set (BSS) have

to compete between each other to gain access to the

medium. The competition between stations is

controlled by different parameters of the physical

layer (PHY) and the MAC sub-layer. The

parameters include the Inter Frame Space (IFS) i.e.

time period between the transmission of frames,

Contention Window (CW), and backoff mechanism

that randomises instants at which stations are

attempting to access the channel.

All these parameters play important roles on the

network performance through their effect on the

degree of competition between the active stations

within the same BSS. Consequently, an increase in

the number of active stations in the BSS increases

the degree of competition which in turn increases the

probability of collisions. As a result of that, an

increase in the number of stations has an obvious

impact on the network performance.

Saraireh M., Saatchi R., Al-khayatt S. and Strachan R. (2005).

A COMPARATIVE STUDY OF IEEE 802.11 MAC ACCESS MECHANISMS FOR DIFFERENT TRAFFIC TYPES.

In Proceedings of the Second International Conference on e-Business and Telecommunication Networks, pages 28-35

DOI: 10.5220/0001420400280035

 SciTePress

2 RELATED WORK

The variation of the number of active stations in

IEEE 802.11 DCF protocol has been investigated in

several studies by both simulation tools and

mathematical models. An analytical model was

proposed to analyse DCF operation and compute the

saturated throughput performance through

employing Markov chain models (Bianchi, 2000).

This proposed model considered a finite number of

stations with ideal channel conditions. The results

obtained in this paper showed that the performance

of the basic access mechanism depends on the MAC

parameters mainly contention window minimum and

number of wireless stations in the wireless networks.

On the other hand, the results showed that the

RTS/CTS access mechanism is marginally

dependent on the system parameters. In another

study the capacity of the medium was investigated

by developing a mathematical model that calculates

the DCF throughput and the packet virtual

transmission time (Cali, 2000).

IEEE 802.11 CSMA/CA protocol over wireless

channel was investigated in (Kleinrock, 1975). They

provide an analysis for the channel performance

during the up-time of unstable channel. They

showed that CSMA theoretically exhibits behaviour

similar to ALOHA. In (Haitao, 2002) a scheme

named DCF+, which is compatible with DCF; to

enhance the performance of reliable transport

protocol over WLAN was proposed. Moreover, the

impact of increasing the number of stations on the

saturated throughput and delay in DCF and in the

proposed scheme DCF+ was investigated. Their

results revealed that increasing number of stations

has an obvious impact on the network performance.

In (Sweet, 1999), throughput performance

measures for varying number of stations in

CSMA/CA were presented. They showed that the

RTS/CTS access mechanism achieved higher

throughput for CBR traffic when the number of

stations increased above 10 stations. Their results

also showed that higher transmission speeds yielded

lower average throughput results.

Changing the number of active stations has an

obvious impact on achieving QoS over wireless ad-

hoc networks. This is due to the increase of collision

probability over the medium. Also varying the data

rate has a considerable impact on the average end-

to-end delay and jitter. These parameters have

critical impact on the transmission of multimedia

applications.

An aim of this study is to investigate the impact

of increasing the number of active stations and data

rate on the network parameters. In particular, on the

QoS parameters, throughput, end-to-end delay, jitter,

and data packets drop. The performance of MAC

protocol access mechanisms for CBR and VBR

traffics was analysed.

This paper is organised into five sections. In the

next section, the basics of the IEEE 802.11 MAC

protocol are introduced. The experimental procedure

is introduced in section 4. The findings and

discussions are presented in section 5. The

conclusion and future work is presented in section 6.

3 IEEE 802.11 MAC PROTOCOL

The IEEE 802.11 standard (IEEE, 1997) specifies a

CSMA/CA protocol. In CSMA/CA, when a station

has a packet to send, it first listens to the medium to

ensure no other transmission is currently taking

place. If the channel is idle, it then transmits the

packet. Otherwise, it picks a random "backoff

interval" which determines the period of time the

station has to wait until it is allowed to transmit its

packet. The selection of the random number of the

backoff time is based on a binary exponential

backoff algorithm. The competing stations select a

random number between 0 and CW-1 with equal

probability. If the data packet is successfully

transmitted, the backoff counter of the transmitted

station will reset and then the station starts to

compete with the other stations for accessing the

medium. During the idle period of the channel, the

transmitting station decrements its backoff counter.

When the backoff counter reaches zero, the station

transmits the packet as shown in Figure 1. During

the busy period the station suspends its backoff

counter. After successful receiving a packet, the

receiving station replies with a positive

acknowledgement (ACK) after waiting for a Short

Inter Frame Space (SIFS) period. If an ACK is not

detected within a SIFS period after the packet

transmission, the transmission is assumed to be

unsuccessful, and a retransmission is scheduled

according to the specified backoff rules. The

unsuccessful transmission is due to collision over the

link. If a collision occurs CW will be doubled until

reaching the maximum value CW

max

= 2

(CW

min

1) – 1, where m is the number of retransmission

attempts.

The RTS/CTS access mechanism is mainly used

to minimize the amount of time spent when a

collision occurs since collision occurs in these short

messages.

A COMPARATIVE STUDY OF IEEE 802.11 MAC ACCESS MECHANISMS FOR DIFFERENT TRAFFIC TYPES

Before commencing the transmission of a data

packet, the source station sends a short control

frame, called RTS, declaring the duration of the

forthcoming transmission. When the destination

station receives the RTS frame, it replies with a CTS

frame after SIFS interval, with the duration of the

future transmission. Upon hearing RTS and CTS, all

other stations in the vicinity of the sender and the

receiver update their Network Allocation Vectors

(NAV). This process reserves the medium for the

sending station. Thus, all stations in the

neighbourhood of the sender and receiver defer their

transmissions and receptions to avoid collisions.

After the successful RTS/CTS exchange, the source

station transmits the data packet then the receiver

responds with an ACK frame. Figure 2 depicts the

time line of the RTS/CTS access mechanism.

4 EXPERIMENTAL PROCEDURE

To analyse the impact of varying the number of

active stations and data rate on the network

performance for both the MAC protocol access

mechanisms and the two different traffic types a

number of simulation studies were carried out using

the network simulator package (ns2) version 2.27

(ns2). The studies were carried out under different

scenarios and they were based on the QoS

parameters; throughput, delay, delay variation, and

packet loss.

The performance of the IEEE 802.11 MAC

protocol was investigated when the number of active

stations in the same BSS was increased. Two

different channel data rates were chosen for data

packet transmission; low data rate equal to 2Mbps

and high data rate equal to 11Mbps. While the

control frames were transmitted at data rate equal to

1Mbps. IEEE 802.11b standard was used since it

offers multi data rates. The protocol parameter

settings were as shown in Table 1. A random

topology with 20 stations was adopted when all

stations were located in the same BSS. The network

was offered by 100% of offered load every time the

simulation run. Each connection was specified as a

source - destination pair in which the number of

connections was varied each time the network

simulation was run. The simulation was carried out

for CBR and VBR traffic at both MAC protocol

access mechanisms. The CBR traffic had fixed

packet size (1280-byte) while the VBR traffic had

variable packet size and variable interval (mean

packet size 3993 bytes and 2541 bytes standard

deviation).

All nodes were arranged in a random topology

with area of 200x200 metre with the help of random

way point model, and the same model was used for

all the simulations. Throughout the simulations, all

nodes were within range of each other and there

were no hidden terminals occurrences. Each scenario

was run for 10 times. The results were the mean

value for simulations. Each simulation was run for

duration of 100 seconds using Ad-hoc On-demand

Distance Vector (AODV) as the routing protocol

since it has proven to be efficient as opposed to

proactive protocol in Mobile Ad-hoc Networks

(MANET) (Broch, 1998).

Figure 1: Timeline of Basic access mechanism in DCF.

Figure 2: Timeline of RTS/CTS access mechanism in DCF.

ICETE 2005 - WIRELESS COMMUNICATION SYSTEMS AND NETWORKS

5 RESULTS AND DISCUSSIONS

This section outlines performance evaluation of the

Distributed Coordination Function (DCF) that is a

part of the IEEE 802.11 standard. It demonstrates

through simulations the performance of the IEEE

802.11 MAC protocol when the number of stations

is varied. Further, the impact of this variation on the

QoS parameters is analysed. A comparison of the

access methods provided by the IEEE 802.11 MAC

protocol is carried out and comments are made as to

when each should be employed.

5.1 Average Throughput

An increase in the number of contending stations in

the same BSS causes more collisions and as a result

more channel bandwidth is wasted. This wastage of

bandwidth causes a reduction in the achieved

throughput for both MAC protocol access

mechanisms

Figures 3a and 3b show the relationship between

the active stations and the channel utilisation

(channel utilisation is the ratio of the received bits to

the channel data rate). When the number of active

stations was increased, the channel utilisation

slightly declined when the RTS/CTS access

mechanism was used compared to the basic access

mechanism. In RTS/CTS mechanism, collisions only

involve control frames which are relatively small in

size compared to data packet sizes, hence the

bandwidth wasted in collisions is less than the basic

access mechanism. This explains the slight rate of

decrease in the channel utilisation curve when the

RTS/CTS access mechanism was used.

If there are few stations in the network, (i.e. less

than 10 stations), the RTS/CTS access mechanism

provided a lower channel utilisation and lower

average throughput. This was due to the overhead

introduced by the control frames RTS and CTS. The

impact of this overhead on the average throughput

became very small when data packet sizes was very

large (above 2000 bytes) as shown Figure 4. At

small packet sizes, the basic access mechanism

outperformed the RTS/CTS access mechanism due

to the impact of the overhead, while at large packet

sizes, the RTS/CTS access mechanism outperformed

the basic access mechanism since the size of RTS

and CTS is very small compared to data packet

sizes.

With regard to channel data rate, low data rate

(2Mbps) achieved better channel utilisation than

high data rate (11 Mbps). This is because in low data

rate the data packets were sent at 2 Mbps while the

headers and control frames were sent at basic rate (1

Mbps). The two data rates (low data rate and basic

rate) are relatively close to each other which resulted

in better channel utilisation. At high data rate, the

data packets were sent at 11 Mbps while the headers

and control frames were sent at 1 Mbps, the

difference here was relatively high compared to low

data rate which resulted in a high rate of reduction in

the channel utilisation. In this case, the transmission

of headers and control frames caused a bottleneck

when data packets were sent at high data rate.

The channel utilisation was degraded for CBR

and VBR traffics. For CBR traffic, the reduction

over an increase from 1 to 20 stations was 4.3% at

11 Mbps and 10.5% at 2 Mbps when the basic

access mechanism was used. When the RTS/CTS

access mechanism was used the reduction was

slightly smaller, it was 2.4% at 11 Mbps and 3.5% at

2 Mbps. For VBR traffic, the channel utilisation

degraded by 4.8% at 11 Mbps data rate and by

11.5% at 2 Mbps data rate when the basic access

mechanism was used. When the RTS/CTS access

mechanism was used the reduction in the channel

utilisation was 1.8% at 11 Mbps and 3.5% at 2

Mbps. The results obtained indicate that the channel

utilisation was degraded for CBR and VBR traffic in

both; the RTS/CTS and the basic access

mechanisms, but the RTS/CTS access mechanism

provided a smaller rate of decrease in the channel

utilisation when the number of stations was

increased. Also high data rate (11 Mbps) provided a

slight decrease in the channel utilisation compared to

the low data rate (2 Mbps).

Parameter Value

Data Rate 2, 5.5, 11 Mbps

Basic Rate for broadcast 1 Mbps

DIFS 50 µsecs

SIFS 10 µsecs

CWmin 31

CW max 1023

Slot time 20 µsec

Short Retry Limit 7

Long Retry Limit 4

Table 1: IEEE 802.11b Parameter (ORiNOCO) settings.

A COMPARATIVE STUDY OF IEEE 802.11 MAC ACCESS MECHANISMS FOR DIFFERENT TRAFFIC TYPES

2 4 6 8 10 12 14 16 18 20

Number of Stations

Channel utilisation [percent]

Channel utilisation at [11Mbps data rate] / basic

Channel utilisation at [2 Mbps data rate] /basic

Channel utilisation at [11Mbps data rate] /RTS/CTS

Channel utilisation at [2 Mb data rate] /RTS/CTS

2 4 6 8 10 12 14 16 18 20

Number of stations

Channel utilisation [percent]

Channel utilisation at [11 Mb-data rate] /basic

Channel utilisation at [11 Mb-data rate] / RTS/CTS

Channel utilisation at [2 Mb-data rate] /basic

Channel utilisation at [2 Mb-data rate] /RTS/CTS

0.40

0.55

0.70

0.85

1.00

1.15

1.30

1.45

1.60

1.75

0 500 1000 1500 2000 2500 3000 3500 4000

Packet size [bytes]

Avearge throughput [Mbps

]

Average throughput [Mbps] /basic

Average throughput [Mbps] /RTS

5.2 Average Delay

The packet delay from and end-to-end should not

exceed 400 ms for time sensitive applications in

order to achieve the required QoS (Coverdate,

2000). As shown in Figures 5a, 5b, 5c and 5d, low

data rate (2Mbps) in both MAC access mechanisms

does not meet this QoS requirement if the number of

active station was increased to more than 4 stations.

A high data rate (11 Mbps) achieved better

performance (small values of average delay). The

average delay was slightly increased which met the

QoS requirements up to 10 stations and then started

to exceed the limit as the number of active stations

was increased

Because of the strict delay and jitter

requirements for multimedia applications (CBR and

VBR traffics), the time interval between the

packet transmissions has to be within a given period.

This can be obtained by assigning small values of

min

and CW

max

for these applications. In this

study The IEEE 802.11 and IEEE 802.11b protocols

were used, and their CW

min

and CW

max

were kept

at the default values (31 and 1023 respectively) for

the

pairs of communication, therefore, their delay

and jitter values were increased at low data rates and

slightly increased at high data rates

At high data rate (11 Mbps) with the basic access

mechanism, the average delay was reduced by 69%

and 66% compared with low data rate for CBR and

VBR traffics, respectively. When the RTS/CTS

access mechanism was used, the average delay at

high data rate was also reduced by 58% and 63% for

CBR and VBR traffics respectively.

The values of average delay in both MAC access

mechanisms were located outside the desired range

of QoS (150 ms for high QoS and 400 ms the

minimum limit) when low data rate (2 Mbps) was

used (Coverdate, 2000). Conversely, high data rate

(11 Mbps) can provide acceptable QoS requirements

in term of average delay.

5.3 Average Jitter

One of the major roles of QoS is to keep delay, jitter

and packet loss for the transmitted applications

within the acceptable range (Coverdate, 2000). For

instance, to achieve high QoS for multimedia

applications, the average jitter should not exceed 20

ms.

The average jitter increased as the number of

active stations was increased. In other words, as the

number of stations was increased; the probability of

collisions increased due to a high degree of

competition between stations. This in turn forced the

MAC protocol to retransmit the collided packets. If

the collided packets were successfully received at

the destinations, they experienced delay variation,

and this variation depended on the number of packet

retransmissions.

Figure 4: Average throughput at Basic and RTS/CTS

access mechanisms when the packet size increased

(a) (b)

Figure 3: Channel utilisation for CBR and VBR traffic at two different data rates and at two MAC protocol

access mechanisms. (a) Channel utilisation for CBR. (b) Channel utilisation for VBR.

ICETE 2005 - WIRELESS COMMUNICATION SYSTEMS AND NETWORKS

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

2.20

2 4 6 8 10 12 14 16 18 20

Number of stations

Average end-to-end delay [second]

Average End-to-End Delay

at [11 Mbps-data rate]

Average End-to-End Delay

at [2 Mb-data rate]

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

2.20

2.40

2 4 6 8 101214161820

Number of stations

Average end-to-end delay [second]

Average End-to-End Delay

at [11 Mbps-data rate]

Average End-to-End Delay

at [2 Mb-data rate]

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2 4 6 8 10 12 14 16 18 20

Number of stations

Average end-to-end delay [second]

Average End-to-End Delay

at [11 Mbps-data rate]

Average End-to-End Delay

[

2 Mb-data rate

]

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2468101214161820

Number of stations

Average end-to-end delay [second]

Average End-to-End

Delay [11Mb-data rate ]

Average End-to-End

Delay [2 Mb-data rate]

0.00

0.01

0.02

0.03

0.04

0.05

0.06

2 4 6 8 10 12 14 16 18 20

Number of stations

Average jitter [second]

Average Jitter at

[11 Mbps-data rate]

Average Jitter at [2

Mb-data rate]

0.00

0.01

0.02

0.03

0.04

0.05

0.06

2 4 6 8 10 12 14 16 18 20

Number of stations

Average jitter [second]

Average Jitter at [11

Mbps-data rate]

Average Jitter at [2

Mb-data rate]

0.00

0.01

0.02

0.03

0.04

0.05

0.06

2 4 6 8 10 12 14 16 18 20

Number of stations

Average jitter [second]

Average Jitter at [11

Mbps-data rate]

Average Jitter at [2

Mb-data rate]

0.00

0.01

0.02

0.03

0.04

0.05

0.06

2 4 6 8 10 12 14 16 18 20

Number of tations

Average jitter [second]

Average Jitter

[11 Mb-data rate]

Average Jitter [2

Mb-data rate]

As shown in Figures 6a, 6b, 6c and 6d, the

transmission of data packets with high data rate

(11Mbps) had a noticeable positive impact on the

achieved value of average jitter. High data rate

resulted in small values of average jitter. This was

because the transmission time of data packets at high

data rate was smaller.

The results obtained at 11 Mbps indicated that

the values of average jitter for CBR and VBR traffic

in both MAC access mechanisms were kept within

the acceptable range of QoS (less than 20 ms),

where as low data rate resulted in large values of

average jitter (more than 20 ms).

5.4 Packet Loss

In this study the packet loss was due to collisions,

especially when the MAC retry limit exceeded and

buffer overflow. Figures 7 and 8 show that

the performance was downgraded with the increase

in the number of stations in the same BSS.

It is well-known that high transmission rates

have a lower Signal to Noise Ratio (SNR) than low

data rates (Thruong, 2003). Therefore, at high data

rates, the probability that a packet can not be

received correctly by the destination is high.

2 4 6 8 10 12 14 16 18 20

Number of stations

Data packets drop due

to collisions [percent]

Percentage of data packet drop due t o

collision at [11 Mbps-data rate]

Percentage of data packet drop due t o

collision at [2 Mb-data rate]

2 4 6 8 10 12 14 16 18 20

Number of stations

Data packets drop due to collisions

[percent]

Percentage of dat a packet drop due to

collision at [11 Mbps-data rate]

Percentage of dat a packet drop due to

collision at [2 Mb-data rate]

2 4 6 8 10 12 14 16 18 20

Number of stations

Control frames drop due to collision

[percent]

Percentage of data packet drop due

to collision at [11 Mbps-data rate]

Percentage of data packet drop due

to collision at [2 Mb-data rate]

2 4 6 8 10 12 14 16 18 20

umber of stations

Control frames drop due to

collisions [percent]

Percentage of data packet drop due

to collision [11 Mb-data rate]

Percentage of data packets drop

due to collision [2 Mb-data rate]

(a): Average end-to-end

delay for CBR / basic.

(b): Average end-to-end

delay for VBR / basic.

(c): Average end-to-end

delay for CBR / RTS.

(d): Average end-to-end

delay for VBR / RTS.

Figure 5: Average end-to-end delay vs. number of stations.

(a): Average jitter for

CBR / basic.

(b): Average jitter for

VBR / basic.

(c): Average jitter for

CBR / RTS.

(d): Average jitter for

VBR / RTS.

Figure 6: Average jitter vs. number of stations.

(a): Data packet drop due to

collision for CBR/basic.

(b): Data packet drop due

to collision for VBR/basic.

(c): Control frame drop due

to collision for CBR/RTS.

(d): Control frame drop due

to collision for VBR/RTS.

Figure 7: Percentage of Collision drop vs. Number of stations.

A COMPARATIVE STUDY OF IEEE 802.11 MAC ACCESS MECHANISMS FOR DIFFERENT TRAFFIC TYPES

2 4 6 8 10 12 14 16 18 20

Number of stations

Percentage of data packets drop due

to buffer overflow [percent]

Percentage of data packet drop due to

buffer overflow at [11 Mbps-data rate]

Percentage of data packet drop due to

buffer overflow at [2 Mb-data rate]

2 4 6 8 10 12 14 16 18 20

Number of stations

Percentage of data packets drop due

to buffer overflow [percent]

Percentage of dat a packet drop due to

buffer overflow at [11 Mbps-data rate]

Percentage of dat a packet drop due to

buffer overflow at [2 Mb-data rate]

2 4 6 8 10 12 14 16 18 20

Number of stations

Percentage of data packets drop due to

buffer overflow [percent]

Percentage of data packet drop due to

buffer overflow at [11 Mbps-data rate]

Percentage of data packet drop due to

buffer overflow at [2 Mb-data rate]

2 4 6 8 10 12 14 16 18 20

Number of stations

Percentage of data packets drop due to

buffer overflow [percent]

Percentage of data packet drop due to

buffer overflow [11 Mb-data rate]

Percentage of dat a packet s drop due

to buffer overflows [2 Mb-data rate]

The results obtained showed that the drop due to

buffer overflow is relevant in the total loss ratio only

with low data rate if the number of active stations

was increased, while with high data rate the main

cause of packet loss was the collisions (MAC retry

limit) as shown in Figures 7 and 8

As shown in Figures 8a, 8b, 8c, and 8d, at high

data rates the packet loss ratio was larger by 30%

and 23% than the obtained values at low data rates

for CBR and VBR traffics, respectively when the

basic access mechanism was used. When the

RTS/CTS access mechanism was used, the packet

loss ratio was larger by 36% and 35% than the

obtained values at low data rates for CBR and VBR

traffic, respectively.

In this scenario, the basic mechanism

outperformed the RTS/CTS mechanism in term of

packet loss when the number of stations was small.

For large networks, the RTS/CTS access mechanism

outperformed the basic access mechanism because

of collisions. In RTS/CTS access mechanism,

collisions occurred for control frames while in the

basic access collisions occurred for data packets as

well as control frames.

6 CONCLUSION

In this study extensive experiments were carried out

using ns2 simulation software to investigate the

performance of the IEEE 802.11 MAC protocol by

varying the number of active stations and varying

the channel data rate.

Both MAC protocol mechanisms, i.e. the basic

access and RTS/CTS access mechanisms were

employed. The effect of traffic types (i.e. CBR and

VBR) on the performance of the access mechanisms

was also analysed.

The study indicated that increasing the number

of active stations had an impact on the average

throughput when the basic access mechanism was

used.

High data rates improved the average

throughput, but degraded the channel utilisation.

This was because the control frames were sent at

low data rate (1 Mbps).

The basic access mechanism outperformed the

RTS/CTS access mechanism when the number of

active stations was small. For a large network size,

greater than 15 stations, the RTS/CTS access

mechanism outperformed the basic access

mechanism.

In the future a detailed evaluation of QoS

parameters for various applications such as audio,

video, file transfer and data will be carried out for

small and large networks. Furthermore, the network

parameters will be used for predicting the QoS and

other network conditions as an approach for

improving the protocol performance.

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