Study on data throughput in 802.11a WLAN systems

Author. Manish Abraham, Student Number 11033698, Email- manish.abraham@northumbria.ac.uk

Abstract — This report is the consequence of the

study on data throughput in the 802.11a WLAN systems,

the study has been performed partially theoretically and

partially practically. The main aim of the study was to

1) Measure and apprise the system requirements. 2)

Designing, deploying and testing a wireless LAN system.

3) Analyze protocol information in a wireless LAN. This

paper mainly examines the 802.11 a protocol.

Index Terms — IEEE protocol, data through put in

802.11 a, wireless LAN system.

I. INTRODUCTION

The increase in mobility and flexibility in "the daily

routine" has led the development of wireless LANs.

Wireless LANs have become an indispensable due to

gliding data rates and reduction in costs. Throughput, in

comprehensible words, is the rate at which actual useful

data is received in the entire payload transfer rate. That

is to say if data transfer rate is 1Mbps, and in a payload

of 1Mb if 100Kb is useful data and 900Kb accounts for

overheads and losses, then throughput is 100Kbps.

802.11 is an IEEE WLAN standard. The IEEE 802.11,

802.11b and 802.11 a, g provides data rate of 2, 11, 54

Mbps [1]. The ever increasing need to develop and to

obtain unique bandwidth and faster throughput rates

IEEE projected 802.11n standards, which builds upon

the previous standards, but adds on MIMO (multiple in

and multiple out) thus offering an extremely high

through put transmission rate between 100 -200 Mbps

and claims to have throughput of 600 Mbps [2].

IEEE perpetually amends the data rates, but in reality,

the actual throughput is lower than the data rates owing

to MAC overheads, CSMA/CA, PHY overheads and

other overheads. In WLAN overheads are more than

wired LAN. This is for the simple reason that wireless

media is unsecure and prone to more errors than wired

media. To store the data secure and to have an error-free

recovery, "protocol developers" have put these

overheads, like the MAC layer. In this paper, a

theoretical and practical model is developed which

describes the data throughput in an 802.11a network. To

support the theoretical analysis, a practical series of

experiments were performed which calculated the actual

throughput rates for the system.

A propagation of 802.11 in a 5GHz band provides

speed up to 54 Mbps and is common in the application

of wireless LANs is 802.11a. Instead of FHSS or DSSS

in case of 802.11a for encoding we use something

known as orthogonal frequency division multiplexing

(OFDM). Practical application of 802.11a specification

includes access hubs as well as wireless ATM systems.

II. THEORETICAL THROUGHPUT DEVELOPMENT

An Optional Point Coordination Function and a

Distributed Coordination Function known as MAC

protocol with binary exponential back off with carrier

sense multiple access along with collision avoidance

(CSMA/CA) is employed in case of IEEE802.11 [3].

It works on two different ways;

1. Firstly the station needs to detect the channel before

it starts to transmit, if the channel is idle then the

station will wait for DIFS before transmitting.

Incase the channel is detected the station will use

the back off timer to delay the data. The process

continues till the back off time reaches zero, then

the station sends the data packet, then it passes the

data to ACK and followed by the SIFS.

2. In order to apply media occupancy before the

station sends the packet to the receiver it is send

to the RTS frame which is the request to send

frame.

The receiving station receives after it crosses the

SIFS, CTS, SIFS layers. Once the receiving station

receives the packets it transmits the ACK followed

by the time of SIFS. The two timing diagram has

differences, the accurate rates varies for various

data rates and spread spectrum technologies. Hence

we see so many sources of delay [4].

A. Theoretical maximum throughput (TMT)

In a unit time, the maximum number of MSDU s

ie.MAC layer service data units which can be

transmitted is defined as the TMT [5].

To develop a theoretical throughput few assumptions

need to be undertaken:

1. The theoretical maximum through put would be

the maximum throughput achievable (54Mbps

for 802.11a).

2. Main Interest lies in the real through put given by

the MAC layer.

3. Hiding information between the MAC layer &

802.11a is the transmission control protocol

(TCP) or UDP (user datagram protocol) over the

IP, over the (LLC) logical link control. Due to

overhead accumulation in each layer hence the

maximum through put of a layer is lower as the

layer gets higher. Maximum throughput

observed when no fragmentation was

encumbered in the lower layer is under.

Where:

α = total overhead above MAC.

TMT (APP) = TMT of application layer.

β = Application datagram size.

TMT (802.11) is the TMT of 802.11a MAC layer under

following assumptions:

a. Bit error rate (BER) is zero.

b. There are no collision losses.

c. Point coordination function mode is not used.

d. None of packet loss occurs due to buffer

flooding at the receiving node.

e. There are adequate packets to send by the sending

node.

f. The MAC layer does not use fragmentation.

g. Management frames such as beacon &

association frames are not considered.

Classification of 802.11 a.

TMT is categorized on various MAC schemes,

fundamental data rates & spread spectrum technologies.

The fundamental need of this category arises due to

various values of minimum contention window size

(CWmin), & inter-frame spacing (IFS).

Two sets of TMTs are measured in MAC schemes,

one for RTS/CTS & one for CSMA/CA. There are

different spread spectrum technologies which enhance

the calculations like (OFDM) orthogonal frequency

division multiplexing, (DSSS) direct sequence spread

spectrum, (FHSS) frequency hopping spread spectrum.

In 802.11a we can use data rates of 6Mbps, 12Mbps,

24Mbps, & the maximum data rate used is 54 Mbps.

The other layers include (PMD) Physical medium

dependent sub layer & (PLCP) physical layer

convergence protocol sub layer.

A Protocol data unit is the extent of transmission unit at

layer which includes the overhead.

We can define (SDU) service data unit as the length of

payload that is provided to the upper layer by a layer

below it. As the MAC SDU (MSDU) overheads takes

place in every intermediate layer then the upper layer

pushes the packet data to MAC layer [5].

B. Calculations

To calculate the TMT, the overheads in each sub layer

needs to be converted in to familiar unit and time.

Hence

The data rate within the similar PLCP PDU is not

always similar. The data rate of one MAC PDU is

ascertained by its type. For backward compatibility

RTS, ACK & CTS are transmitted at 1 Mbps. Due

to DC-bias suppression scheme the number of PLCP

frame bits rises when we use FHSS.

The length of delay element is observed from the

IEEE standards [6].

The transmission time will be depending on the

MDPU size and the data rate. Since we considered

that there were no collisions hence the size of the

contention window (CW) does not increase.

The total delay is calculated

In the format of function

The table shows the values for a & b parameter.

Since used 802.11a we use OFDM as the scheme,

since we don’t consider any collision we only

consider the CSMA/CA scheme. Hence according to

the TMT parameters a= 0.14815 & b= 159.94 as we

test the maximum speed of 54MB/S.

Theoretical Maximum throughput for maximum speed

(54 Mbps) 802.11 a.

III. EXPERIMENT SETUP

In order to determine the maximum throughput and to

achieve success in the study, the analysis conditions

should be utmost interference free. So in order to avoid

external interface, and to obtain accurate readings, the

evaluation was carried out in the anechoic chamber.

Hardware Used,

Access Point (Linksys WRT610N)

USB WNIC (Linksys WUSB600N)

Probe WNIC.

3 Computers.

Through put was measured using Jperf and Iperf traffic

generation software. Wireless traffic was transferred

between two computers connected individually to AP

and to WNIC. Data transfer was monitored using a

laptop which had software Omni peek, which was

connected, to a probe WNIC. In order to calculate the

average and maximum speed more efficiently a network

protocol analyzer called Wire Shark was Installed and

used efficiently. Wire shark was installed in both the

computers. We used the maximum data rate for

IEEE802.11a i.e. 54MB/S the packet size was varied

between 300 & 2100 bytes, with a difference of 300

bytes between each reading. We also calculated

throughput for varying data rates i.e. 20 MB/S, 40MB/S,

60MB/S & 80MB/s and we compared it with the

maximum throughput rate.

IV. RESULTS

Practical Throughput rates observed for different packet

sizes :

The graph below shows the changes in throughput rates

with the change in packet size and for different varying

data rates of 20, 40, 60 & 80MB/S respectively.

The Practical throughput rate and TMT for different

Packet sizes for the data rate of 54MB/S:

The graph shows the TMT and the practical gained

value for 54 MB/S.

From the graph above we can see that:

The throughput rate is always lower than the data

rate.

The highest throughput rate was observed when

the packet size was 1472 bytes.

Till 1472 bytes the throughput rises steadily with

rise in packet size, but at 1472 bytes we

observed that the throughput suddenly degrades.

The overall throughput remains less than the

maximum throughput that was measured. The

actual throughput rate in practise is always less

than the maximum throughput rate irrespective

of the packet size.for e.g. at 600 bytes the

maximum throughput was measured as

19.29MB/S but on ground we got the throughput

as 16.89MB/S.

V. DISCUSSION

In a unit time, the number of MSDU s ie.MAC layer

service data units which can be transmitted is defined as

the throughput.

The highest value of throughput was observed as

23.32MB/S at the packet size of 1472 bytes, when

compared to lower data rates for e.g. 600bytes the

throughput rate was 16.89MB/S this is very much

evident from the basic formula and because the

overhead ratio is small when we transfer small packet

sizes.

We observed that the throughput rates are always less

than the actual data rates. A Mac protocol consists of an

Optional Point Coordination Function and a Distributed

Coordination Function. The DCF consists of SIFS &

DIFS deferral, data transmission ACK frame, and back

off, RTC, CTS frame. The distance between the sending

station, AP (access point) and receiving station as well

as environmental factors and noises are the reason why

the throughput rate is always less than the actual data

rate. Experimental value of throughput is far less than

the data rates, and the practice throughput is less than

the TMT (theoretic maximum throughput).

The peak of the experimental throughput was

calculated when the packet size was 1472 bytes then the

value of throughput degrades, hence we can say that the

efficient packet size is 1472bytes, this can be explained

by a study which says that maximum payload data rate

can be 1500 bytes for an Ethernet frame. 20 bytes gets

reduces as the IP frame and 8 bytes as the UDP frame.

When the data packet size becomes larger than 1472

bytes fragmentation occurs and the packet data gets

broken in to frames hence reduces the throughput rate

[7].

We also observed that the value of throughput rate

observed is less than the TMT this is for a reason that

when we calculated the TMT we took certain

assumptions like:

a. Bit error rate (BER) is zero.

b. There are no collision losses.

c. Point coordination function mode is not used.

d. None of packet loss occurs due to buffer

flooding at the receiving node.

e. There are adequate packets to send by the sending

node.

f. The MAC layer does not use fragmentation.

Management frames such as beacon & association

frames are not considered

But when it comes to experiment we tried to reduce

the external interference of noise by having an anechoic

chamber but we can not over rule all these assumptions.

The knowledge of maximum through has various

applications, which include;

LAN data access, by maximum throughput we

can provide sufficient range of coverage.

Optimal network provision which can be used for

both data, as well as multimedia applications.

In adhoc networks, it is the primary factor which

influences the topological distribution of nodes.

VI. CONCLUSION

We measured the actual throughput rates for different

packet sizes, calculation, of the theoretical maximum

throughput of 802.11a network was presented. To

extend the pertinence of the results, different packet

sizes, physical layer and MAC layer variations were

considered. To represent the practical implication of

maximum throughput, series of experiments were

carried out & it was noticed that practically the

throughput rates are remarkably less than the TMT. A

wireless LAN system was designed deployed and tested.

We observe that the throughput rates rise till 1472 bytes

and then decreases due to fragmentation and also

reasons why throughput rate is less than the packet size

and experimental through put rate is less then theoretical

maximum throughput was discussed.

The various application of knowledge of maximum

throughput was discussed.

The Maximum payload data rate which is also known as

the efficient packet size was determined as 1472bytes.

For further development consideration would be taken

into the 802.11ac standard which takes into

consideration the improvement of total network instead

of a single link and by providing larger band width along

with multi user multiple input multiple output which in

5GHz band gives a data throughput rate of 1 GB/S [8].

.

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