Wireless Local Area Networks WLAN Design Network Computer Science Essay


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A Fewer last years Wireless Local Area Networks (WLAN) is become wide user in the world. For long times the computers are connected together by using wires or cables connection and through this connection the computer users can share any data between them. Wireless Local Area Networks providing to the users to connect the to the network by using wireless connection using Radio Frequency (RF),which carry data and other advance application. As we know this technology is working under protocols to control the service between users. In this project i study on the effect data fragmentation on the performance of IEEE 802.11 wireless LAN under PCF mode, I consider two type of protocol working under media access control (MAC ) . First one Distributed Coordination function (DCF) and second is Point Coordination function. Before i start for above study i have considered the IEEE802.11 different standards, than i have By providing the media access control (MAC) mechanism, it will very easy to understand effect data fragmentation . Fragmentation is the process of breaking apart a packet into smaller pieces. A source (wireless Network Interface Card or access point) uses fragmentation to divide 802.11 frames into smaller pieces (fragments) that are sent separately to the destination. The fragments are all marked in a way that allows the destination device to put them back together again after it gets all the fragments.

Before i start design of network i will be considering the requirement of WLAN network. I will be designing and simulating the WLAN Network, And i will be doing simulation of the WLAN through different media access application by using OPNET program. By using different parameters such as Throughput, Retransmission, RTS/CTS, media access delay. After that i will be evaluating and analyse the effect data fragmentation on the performance of IEEE802.11 wireless LAN under PCF and DCF.


Acknowledgments The patience and support of my family was the biggest help in the completion of this project. And would also like to acknowledge Mr.Manssor Ali and Mr Vallierage, for their guiding. Also, I would like to thank the project coordinator Ms. Pravenna S.M for encouraging me to work and my study.


Future wireless local area network (WLAN) enable people on the move to communicate with anyone and anywhere at anytime Caledonian college is one best example which is providing wireless service inside the college camps which allows the student to share data and web brows but increasing demand on portable computer for example Laptops and increasing communication equipment, its has responsibility of increasing in wireless network service. As we know this technology is working under protocols to control the service between users. Each type of wireless data network operates on a specific set of radio frequencies. For example, most Wi-Fi networks operate in a special band of radio frequencies around 2.4 GHz that have been reserved in most parts of the world for unlicensed point-to-point spread spectrum radio services. Other Wi-Fi systems use a different unlicensed band around 5 GHz.(3)

IEEE MAC 802.11physical layer are defined in three type [1]:

Direct sequence spread spectrum (DSSS):its operating in the 2.4 GHz band at data rate of 1Mbps and 2Mbps, the number of channels available depend on the bandwidth.

Frequency hopping spread spectrum(FHSS): operating in 2.4GHz at data rates of 1Mbps and 2Mbps, the available channel range is 23 in japan,70in USA.

Infrared : data rate is 1Mbps and 2Mbps operating at a wavelength between 850and 950 nm.

For IEEE 802.11 there are different standards as we can see ,but my study will be based on IEEE 802.11:[4]

• IEEE 802.11: Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) Medium Access Control (MAC), and 1 and 2 Mbps for DSSS, FHSS in 2.4 GHz band, and Infrared, ratified in 1997.

• IEEE 802.11a: Works at 6, 9, 12, 18, 24, 36, 48 and 54 Mbps in 5GHz band, ratified in 1999.

• IEEE 802.11b: Works at 5.5 and 11 Mbps in 2.4 GHz band, ratified in 1999.

•IEEE 802.11e: MAC enhancements for Quality of Service (QoS), work ongoing.

•IEEE 802.11f: Inter Access Point Protocol (IAPP), ratified in 2003.

Because the medium access control (MAC) for WLAN application comes under IEEE 802.11 standard and for that we will go in depth later.[4]

The WLAN can work on the AD-hoc network without using of the Access Point(AP) and also it can operate on an infrastructure network that which is using of the Access Point (AP).

There are four physical components standards for 802.11 network:

-Access point


- Wireless Medium

-Distribution System

The basic building block of the WLAN network is the 802.11 basic service set (BSS). A BSS defines a coverage area where all stations within the BSS connected.

There are two BSS network topologies:

Independent BSS (IBSS) Networks

Infrastructure BSS Networks

In the independent BSS network stations as it shown in figure(1.1) are communication directly with each other. Also called the IBSS as ad-hoc network, used to communicate all station in infrastructure network together . An ad -hoc network operates from one station to another station immediately, without any help from other device, that's mean no need for permission to access. This operation is working by using the independent basic service set(IBSS)[5].

Figure (1.1) :Independent Basic Service Set(IBSS) [5]

An Infrastructure Basic Service Set(BSS) as we can see in figure (1.2) is a type of IEEE 802.11 network consists from stations and access point(AP) which are used to communicate all stations by (BSS).this will operate when first station is sending the information to AP and than the AP forward this information to station which let the information pass to a wired node and the AP passes to fixed network.

Figure (1.2): Infrastructure Basic Service Set [5]


1-understand the Theory of different standards of IEEE 802.11

2-Selection for suitable simulation tool.

3-design suitable WALN with appropriate settings.

4-evaluate and analyse the effect data fragmentation on the performance of IEEE 802.11 wireless LAN under PCF mode.

5-Study the critical evaluation of the result.

6-Write the final technical Report.

Literature Review:

The IEEE 802.11 [1]-[2] standards define the protocol and similar interconnections of data communication equipment in the "air" (radio frequency or infrared) in a local area network (LAN). Its contain the physical (PHY) and the media access control (MAC) layers of the ISO seven-layer network model. Within the MAC layer two different media access control (MAC) mechanisms: one is called distributed coordination function (DCF), which is based on carrier-sense multiple access with collision avoidance (CSMA/CA) the other is called point coordination function(PCF), which is based on polling. In this project we will be studying on the effect data fragmentation on the performance of IEEE 802.11 wireless LAN under PCF mode. Fragmentation is Used to improve the WLAN performance when the bit error rate is high.As we that PCF come as option at upper of DCF ,so we must under stand the access method of DCF ,Before we start the PCF access method which is based on polling .

DCF access method:

The DCF access method in IEEE 802.11 MAC protocol is based on CSMA/CA scheme [8], which has two important modules: carrier sense and binary exponential backoff. Stations waiting in the idle state sense the medium before any transmission attempt.[8]

Fragmentation mechanism :

It is the process of breaking apart a packet into smaller pieces. A source (wireless Network Interface Card or access point) uses fragmentation to divide 802.11 frames into smaller pieces (fragments) that are sent separately to the destination. The fragments are all marked in a way that allows the destination device to put them back together again after it gets all the fragments. The reason that fragmentation is used is because the bit rate errors in wireless is high and The probability of lost frame is much higher for wireless links, fragmentation can improve transmission reliability in the physical layer because the probability of successful transmission increase as the size of the frame will decrease [7], all fragmentation generate with equal size at same time, and all fragmentation are not change until they are transmitted . so for that the fragmentation is used. Now we must understand how the frame size can be decrease.

We understand that DCF include set of delays amount to priority scheme, inter frame space is one of these delays [8]

The DCF is based on CSMA/CA and it only provides asynchronous access for best effort data transmission. DCF consists of both a basic access method and an optional channel access method using RTS/CTS exchanges.

Inter frame spacing:

Inter frame spacing is important for coordinating access to medium transmission, as shows in figure (2.1), There are three they used to determine access medium. Collision avoidance which is built in 802.11 MAC, the stations delay transmission to give the medium time to get idle [9]. Inter frame spacing creates priority levels used for different traffic.

If the station has been given a high priority, it does not have to wait for the medium to become idle. This priority avoid grabs for network, before the stations with low priority has to try to access [10].

Figure (2.1) inter frame spacing

Types of inter frame space:

Short Inter Frame Space (SIFS):it is used for highest priority RTS/CTS frame with acknowledgement. Highest priority start to transmit after SIFS is over. Start transmission the highest priority mean traffic is transmitting and medium busy. microseconds and both are fixed per physical layer in direct sequence spread spectrum (DSSS) [9].

Point coordination functions Inter Frame Space (PIFS): it is used for medium priority, and used by PCF during contention free operation. Any station that has data for transmitting in the contention free time is able to transmit after PIFS.

Distributed coordination function Inters Frame Space (DIFS): it is used for lowest priority, and for non real time application ,this operation allowed stations to access the medium after minimum idle for contention services. The Station immediately has to access to the medium if it has been free for a period longer than DIFS. 9].

Carrier- sensing and NAV:

Carrier sensing is used for sensing the medium, If it's ready to use or not. There are two models of carrier sensing function used by 802.11 to controls this access:

Physical carrier model: which are provided by the physical layer, it is difficult to build this model for radio frequency by using hardware supporting media, [9] because the transceiver is able to transmit and to receive at same time only if they build costly electronics, also physical layer is not able to provide the necessary information [9].

Virtual carrier model: This is supported using Net Allocation Vector

(NAV). Any frame in 802.11 carries a period time, which is used for reserving the medium for a short time. NAV works as a clock used to indicate the time which is reserved for the medium [9].

IEEE Frame Exchange protocol :

The basic data transfer mechanism of IEEE 802.11 involves as an exchange of two frames or four frames:

When a station receives data frame from another station its send acknowledgment (ACK) frame to the source station. If the source dose not receive an (ACK ) within a short period of time ,maybe because its data frame was damage or because ACK was damage, so the source retransmits the frame.

Other optional one is a four-way handshaking mechanism, which uses request-to-send/clear-to-send(RTS/CTS) technique to reserve the channel before data transmission.[8]

Access method:

The RTS/CTS access method:

802.11 standards include the RTS/CTS to control the station to access to the medium.RTS/CTS is used to sense the medium and reply to the other station using a handshaking technique, its is used for long frame control. for example: access point A station sends an RTS frame to sense the access point if it's ready to receive data or not, when the frame reaches the access point, its reply by CTS frame which also, contain a time value to prevent the other station from accessing the medium while the others start to send RTS transmit the data.[11]

The CSMA/CA access method:

In 802.11, DCF also is used by RTS/CTS RTS first send RTS frame to check for transmission if it's idle the destination sent CTS frame to approve for sending after SIFS interval immediately following the reception of the RTS frame. The user source approves to transmit a packet only if the source receives the CTS. The others stations are able to update the NAVs based on RTS/CTS which helps to combat the hidden terminal problems. In fact, a station is able to receive the CTS frames correctly to avoid collisions to be able to sense the transmission from a sender. If send 2 or 3 frames of RTS/CTS and collisions occur, less bandwidth is wasted compared the stations with large data frame in the same station.[3]

Figure: (3.1) CSMA/CA and NAV

PCF access method:

PCF is working in combination with DCF as it shown in figure (5). PCF works satisfactorily under high traffic load, In PCF, the AP acts in the role of the point coordinator, and it controls the medium access in a poll-and-response manner. The period during which PCF operates is called the contention-free period (CFP). Before the CFP begins, the AP operates under DCF, but it makes use of the priority inter-frame space (PIFS) to take hold the medium, and then sends out a beacon packet containing the duration of the CFP.

Figure (4). DCF & PCF interaction


(1) http://www.qsl.net/n9zia/wireless/pdf/802.11.pdf

[2] W.Stalling Data and computer communications 8 ed published by Dorling Kindersly(India) Pvt,LTD 482, F.I.E Delhi, India 2008.

[3] Ross, John. Book of Wireless : A Painless Guide to Wi-Fi and Broadband Wireless (2nd Edition).

San Francisco, CA, USA: No Starch Press, Incorporated, 2008. p 15.


[4] Prasad, Neeli. .11 WLANs and IP Networking : Security, QoS, and Mobility.

Norwood, MA, USA: Artech House, Incorporated, 2005.

[5] http://msdn.microsoft.com/en-us/library/ff556962(VS.85).aspx

[6] Sarah Shaaban, Dr. Hesham M. El Badawy, Prof.Dr. Attallah Hashad -Proceedings of the World Congress on Engineering 2008 Vol I

WCE 2008, July 2 - 4, 2008, London, U.K.

[7] younggoo Kwon, Central R&D center,sumsung electronic .

[8] William Stallings .Wireless communication & Networks, second edition

[9] J. F Kurose and K. W. Rose (2006), Computer Networking 3 ed published by Dorling Kindersly(India) Pvt,LTD 482, F.I.E Delhi, India.

[10] W.Stalling Data and computer communications 8 ed published by Dorling Kindersly(India) Pvt,LTD 482, F.I.E Delhi, India 2008.

[11] Haitao Wu1, Yong Peng1, Keping Long1, Shiduan Cheng1, Jian Ma2

1 National Key Lab of Switching Technology and Telecommunication Networks,

P.O.Box 206, Beijing University of Posts & Telecommunications, Beijing 100876, P.R.China

[12] Tien-Shin Ho, Kwang-Cheng Chen ,Department of Electric a1 Engineering,

National Tsing Hua University, Hsinchu, Taiwan 30043, R.O.C.

[13] Sangho Shin, Andrea G. Forte Henning Schulzrinne Columbia University

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