Dcf Distributed Coordination Function Computer Science Essay

Fragmentation It is the process used in MAC IEEE 8021.11 to break­ing apart a packet into smaller pieces. Fragmentation is process that dividing long frame to several shorter frame. Using this technology because the Wireless LANs have high bit error rates. The probability of losing frame is much higher for wireless links. The 802.11 uses fragmentation to reduce the frame error rate.

The PCF is supporting sensitivity traffic and support some high priority application. Therefore the real time application data need to transmitted without any delay. The probability of lost frame is much higher for wireless links. That’s why fragmentation is used to Reduces amount of time medium is in use and Reduces probability of collisions. And there some pervious work has been done which related to our title, will be shown in next sections.

Acknowledgment:

Acknowledgments the patience and support of my family was the biggest help in the completion of this project. And would also like to acknowledge for my supervisor Mr.Manssor Ali and for module tutor Prof. Vallavarj, and module leader Mr. Vijay Krishna, for their guiding. Also, I would like to thank the project coordinator Ms. Pravenna , for encouraging me to complete this work and my study.

Nomenclature

MAC: Medium Access Control.

DCF: Distributed Coordination Function.

PCF: Point Coordination Function.

AP: Access Point.

BSS: Basic Service Set.

IBSS: Independent Basic Service Set.

ESS: Extend Service Set.

RTS: Request To Send.

CTS: Clear To Send.

ACK: Acknowledgement.

OSI: Open System Interconnection.

LLC: Logical Link Control.

HTTP: Hyper Text Transfer Protocol

NAV: Network Allocation Vector.

DSSS: Direct Sequence Spread Spectrum.

Contents Page

Contents…………………………………………………………….. Page No

Abstract…………………………………………………………….……. i

Acknowledgement…………………………………………………………ii

Nomenclature……………………………………………………………..iii

Contents page……………………………………………………………..iv

List of figures………………………………………………………………v

Chapter 1: Introduction………………………………………………………………7

1.1) General Information…………………………………………………..7

1.2) Aim Of the project……………………………………………………7

1.3) Project Objective……………………………………………………..8

1.4) Honors challenges…………………………………………………….8

Chapter 2: literature Review…………………………………………………………..9

Background………………………………………………………………..9

IEEE MAC 802.11physical layer…………………………………………10

what is Fragmentation……………………………………………………………………….10

Why we are using this Technology…………………………………………………….12

The advantages and disadvantages of fragmentation……………………………..12

Distributed Coordination function (DCF)…………………………………………………13

Fragmentation Operation…………………………………………………………………….14

Point Coordination function (PCF)………………………………………………..15

Analysis The earliest work in data fragmentation IEEE802.11…………………16

Chapter 3 Literature Review analysis………………………………………………..19

solutions for all above problems…………………………………………………………….19

Future Work………………………………………………………………………………………..20

Chapter 4 Research Problem…………………………………………………………….21

Chapter 5 Design and Experiment Setup………………………………..22

Chapter 6 Conclusion…………………………………………………….23

Chapter 7 References…………………………………………………….25

List of figures

Figure name Page No.

Figure (1) Independent Basic Service Set(IBSS)……………………… 11

Figure (2) Infrastructure Basic Service Set …………………………… 11

Figure (3) Fragmentation &The RTS/CTS access method…………………. 13

Figure( 4) MSDU divided into several MPDU…………………………… 14

Figure (5). Combination DCF & PCF………………………………………………………. 15

Figure (6) The effect of (polling overhead )on network throughput…………17

Figure (7),Effect of fragmentation…………………………………………………. 18

Figure (8), WLAN Setup diagram in OPNET……………………………….. 22

Chapter 1 Introduction

1.1) Aim:

Study on the effect data fragmentation on the performance of IEEE 802.11 wireless LAN under PCF mode.fragmentation used to increase reliability transmit frame by divide 802.11 frames into smaller pieces (fragments) that are sent separately to the destination.

1.2) The objectives in this paper will be as follows:

1)The first objective in this paper will be understand the Theory and different

standards of IEEE 802.11.

2)To understand the what are structure of the wireless network

3)To evaluate and analyse the effect data fragmentation on the performance of IEEE 802.11 wireless LAN under PCF mode.

)Selection for suitable simulation tool4

5)To design suitable WALN with appropriate settings

6)Study the critical evaluation of the result.

1.3) Honours challenge:

Critical analysis and evaluation of the results.

Experimental Investigation of existing work.

1.4) General Information:

The motivation i select this project title , is because in my organization they are looking for wireless network technology ,which can provide such applications like for delivering traffic for real-time applications such as Voice and data . Therefore IEEE 802.11 under point coordinate function PCF can satisfy the requirement, we will see that later.

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wireless local area network (WLAN), make 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.

“The IEEE 802.11 WLAN has two different type of method channel accessing ,the distributed coordination function (DCF) and point coordination function (PCF). DCF is based on the carrier sense multiple access with collision avoidance (CSMA/CA) mechanism, while PCF is based on using the polling technique. The DCF mode of operation consists of two techniques for packet transmission. In the default its operate as a two-way handshaking technique where a positive acknowledgment is transmitted by the receiver station”. (Stallings, 2007) .

Fragmentation It is the process of break­ing apart a packet into smaller pieces. A source (wireless Network Interface Card or access point) uses for fragmentation is 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.

1.5)The some of earliest work in data fragmentation IEEE802.11:

Dynamic Fragmentation Scheme fro Rate -Adaptive WLAN.[7]

Improving the Aggregate Throughput of Access Points in IEEE802.11Wireless LANs[26].

Performance Analysis of Packetized Voice Transmission with PCF in IEEE802.11Wirelessnetwork.[27]

Delay Analysis of IEEE 802.11 PCF MAC based Wireless vetworks.[28]

Performance Evaluation of the IEEE 802.11 Wireless LAN Standards [6].

Performance Enhancement of Wireless Local Networks. [20]

Chapter 2 Literature Review

Before we will go in depth of our title on data fragmentation under PCF mode , we must understand The basic building block of the WLAN network in IEEE 802.11., And our first objective will be covered in this section.

2.1) Background:

In this section we will be discussing about wireless local area network (WLAN), which make 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. (Ross , 2008)

) IEEE MAC 802.11physical layer are defined in three type:2.2

“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.”( Stallings,2005)

2.3) There are four physical components standards for 802.11 network :

Access point-

Station-

-Wireless Medium

-Distribution System

The basic structure 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

The first one, In the independent BSS network stations, as it shown in figure(1) are communication directly with each other. Also its 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) .

Figure (1) :Independent Basic Service Set(IBSS)

Second an Infrastructure Basic Service Set(BSS) as we can see in figure, (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 work 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. This available at (http://msdn.microsoft.com/en-us/library/ff556962(VS.85).aspx)

Figure (2):Infrastructure Basic Service Set (http://msdn.microsoft.com/en-

us/library/ff556962(VS.85).aspx)

1] what is Fragmentation? and 2] Why we are using this Technology?

2.4) what is Fragmentation?

In this section we understand the Fragmentation It is the process of break­ing apart a packet into smaller pieces. Fragmentation is process that dividing large frame into several shorter frame. Each fragment consists of a MAC Layer header and frame check sequence (FCS), and a fragment number representing its ordered to position within the frame. Because the source station transmits each fragment independently, the receiving station reply with separate acknowledgement for each fragment (Yuguang and Kwon,2003 ) . All fragments are generate with equal size at same time, and all fragments will not change until they are transmitted in both PCF and DCF.

2.5) Why we are using this Technology?

Using this technology because the Wireless LANs have high bit error rates.

The probability of lasing frame is much higher for wireless links.

802.11 uses fragmentation to reduce the frame error rate.

2.6)The advantages and disadvantages of fragmentation:

Advantages : To reliability of data exchange between the stations. The uses fragmentation to reduce the frame error rate and fragmentation for larger sized packets improves .

Disadvantage: fragmentation will increase the overhead. (Tamer &Khatteb,2009)

2.7) Distributed Coordination function (DCF)

In 802.11, DCF also provides an optional way of transmitting data frames that involve transmission of special short RTS and CTS frames to the transmission of actual data frame. As shown in Fig(.3) an RTS frame is transmitted by a station, which needs to transmit a packet. When the destination receives the RTS frame, it will transmit a CTS frame after SIFS interval immediately following the reception of the RTS frame.

Figure(3) Fragmentation &The RTS/CTS access method (Anon.,2003)

The source station is allowed to transmit its packet only if it receives the CTS correctly. Note that all the other stations are capable of updating the NAVs based on the RTS from the source station and the CTS from the destination station, which helps to combat the hidden terminal problems In fact, a station able to receive the CTS frames correctly, can avoid collisions even when it is unable to sense the data transmissions from the source station. If a collision occurs with two or more RTS frames, much less bandwidth is wasted when compared with the situations where larger data frames in collision.(Haitao ,et al.,2002)

2.8) The Fragmentation operation:

A 802.11 data link layer is divided in two sub layers Logical Link Control (LLC) and Media Access Control(MAC). In 802.11, a MAC service data unit (MSDU) could be divided into a sequence of smaller MAC protocol data unit (MPDUs) as it (shown in figure 4). Fragmentation creates MPDUs smaller than the original MSDU length to increase the reliability, by increasing the probability of successful transmission of the MSDU in cases where channel characteristics limit reception reliability for longer frames. Fragmentation is able at each immediate transmitter. The process of combine MPDUs into a single MSDU is called as defragmentation . Defragmentation is adept at each immediate destination Only MPDUs with a unicast receiver address will be fragmented. Broadcast multicast frames shall not be fragmented even if their length exceeds a Fragmentation Threshold When a directed MSDU is received from the LLC with a length greater than a Fragmentation Threshold

the MSDU shall be fragmented. The MSDU is divided into MPDUs. Each

fragment is a frame no larger than a Fragmentation Threshold. (Yuguang and Kwon,2003 ).

Figure( 4) MSDU divided into several MPDU. (Yuguang and Kwon,2003 ).

) Point Coordination function (PCF):2.9

PCF is working in combination with DCF as it shown in figure (5). PCF works suitable for high traffic load, In PCF, the AP acts in the role of the point coordinator, and it controls the medium access, If point coordinator hears there is no traffic after PIFS time gap, it sends out beacon frame Field to indicate length of time that PCF (polling) will be used instead of DCF (contention) during this time.(Stalling,2008)

receiving stations must stop transmission for that amount of time

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.(Stalling,2008)

Figure (5). combination DCF & PCF .(Stalling,2008)

The fragmentation operation in PCF is same as DCF,but in PCF conation free period making different ,but because the DCF working with Contention period (CP) and PCF working with Contention free period(CFP) . (Yuguang & Kwon,2003 ).

There are some limitations on PCF which directly effetely the fragmentation, one of these limitation is the binary exponential back off , the frame need to wait even the medium is free ,because the binary exponential back off will double the random number from 15 to 1023 till the medium will be free, so this can effect the retransmit the frame in fragmentation .

Other limitation in PCF has been designed to support time-limited

for poor QoS performance. In particular the central pooling scheme is inefficient and complex which causes fall of the performance of PCF high-priority traffic under load, when a free station is allowed to send a frame of length between 0 and 2346 bytes, it introduce the difference of transmission time. (Dillip.2006).

2.1.0)Analysis The earliest work in data fragmentation IEEE802.11:

In this section we will analysis ,In this paper(Abhishek,2003) we can analysis that polling in PCF causes overhead . ”In PCF, point coordinator (PC )is a central coordinator that schedules channel access for all other polling able stations in CFP. PC maintains list of poll able nodes in BSS. At beginning of CFP it polls all stations in Round Robin fashion. Nodes receiving poll respond back, either by transmitting data or there is no data frame. If station has no pending data, then it sends no frame. If station fails to do either then its result in polling time out at PC and PC resumes polling. When most stations have pending data, in order polling provide to ordered channel access and this can reduces collisions. But when few stations have pending data and rest are silent, this polling operation becomes major overhead, It adds unnecessary delay for stations with data, due to unsuccessful poll attempts for stations, with no pending data. For this reason resulting in throughput decreasing. Form the figure(6) we compares the overall throughput of network with 32 and 64 nodes having 16 nodes that have data to transmit. We can analysis that Effect of polling overhead is clearly visible”(Abhishek,2003)

Figure 6: the effect of (polling overhead )on network throughput

From this paper, we can find the solution for pervious paper (Abhishek ,2003),by analysis that the overhead in PCF fragmentation is high ,so instead of allowing the transmission of multiple packets with a high data rate ,its possible to make MPDU large size to reduce the overheads caused by multiple packets transmission when channel condition is good. In Literature Review analysis we can describe it more clear(Yuguang and Kwon,2003 ).

As we understand from this paper, that voice packets transmitted by use of PCF mode ,According to our analysis, PCF mode can well support packetized voice transmission using echo canceller fragmentation ,when the load are fragmented into small packets ,large fragment size such as 2000 or 2304 bits dose favour to voice traffic at the available bandwidth for data transmission.(Xiyan,al.,2003)

In this paper ,we can analysis that, they propose a new rate adaptive MAC protocol with adynamic fragmentation. The major advance is the use of multiple fragmentation thresholds, for different rates to generate a new fragment from (remaining) MSDU only after the rate for next transmission is selected. With this scheme, the nodes with good channels can transmit more data than the ones with bad channel.( Byung,al.,2005)

In this paper ,several methods been used to improve wireless network performance ,the result indicate as we can see in the figure (7),that the effect of fragmentation threshold can increase throughput and improve the wireless network performance, but how, the answer is when the bit error rate selected to be and the fragmentation threshold to be 256 bytes or 512 bytes, the result show that there is no any major effect on the network performance.(Walid &Ajlouni,2006)

figure (7),Effect of fragmentation, ( Walid & Naim,2006)

This paper ,proposes an adaptive control algorithm to tune the performance of IEEE802.11wireless LANs, so that traffic demands with different characteristics will be handled mostly by the MAC protocol that fits them. The algorithm aims is control the percentage of CFP duration within a super frame by measuring the throughput in PCF mode.(James,al.,2003)

Chapter 3 Literature Review analysis

T In this section we will be analysis ( 3.1 he out come from literature Review analysis:

-Fragmentation It is the process of break­ing apart a packet into smaller pieces. Fragmentation is process that dividing long frame to several shorter frame, Each fragment consists of a MAC Layer header, frame check sequence (FCS), and a fragment number indicating its ordered position within the frame. (Yuguang & Kwon,2003 ).

-The main reason we are using fragmentation is because the Wireless LANs have high bit error rates.

-The period during which PCF operates is called the contention-free period (CFP).

– When most stations have pending data, in order polling provide ordered channel access and reduces collisions. But when few stations have pending data and rest are silent, this polling mechanism becomes major overhead, It adds unnecessary delay for stations .

– There are some limitations on PCF which directly effetely the fragmentation, one of these limitation is the binary exponential back off , the frame need to wait even the medium is free ,because the binary exponential back off will double the random number from 15 to 1023 till the medium will be free, so this can effect the retransmit the frame in fragmentation .(Class lecture ,Simulation Network Multimedia)

3.2 ) What will be solutions for Some Of above problems:

1) When there will be high overhead in PCF fragmentation is, so instead of allowing the transmission of multiple packets with a high data rate ,its possible to make MPDU large size to reduce the overheads caused by multiple packets transmission when channel condition is good.

2) when the bit error rate selected to be or the fragmentation threshold to be 16 bytes or 256 bytes, the result show that there is no any major effect on the network performance, so this maybe will be one of in our design and experiment we will select the above bit rat error than simulated it and observe the result.( Walid & Naim,2006)

3) We go for IEEE 802.11e which introduces new coordination function called as a Hybrid Coordination Function (HCF) that is only useful for QoS network. The HCF supports channel access mechanisms, Enhanced Distributed Channel two additional Access (EDCA) for contention-based and HCF Coordination Channel Access (HCCA) for contention-free. And this stander will solve all problem with Polling and CW, which effect on fragmentation in PCF.

) Future Work:3.3

During my research i found that there is new standard been developed in 2007 , it know as IEEE 802.11e. To the best of our knowledge, there is very little literature , that deals with polling overhead in PCF .Other People have mentioned the problem earlier and there also come with some proposed solution to deal with it. also that people have mention the importance of polling for providing QoS service in IEEE 802.11 WLAN. New upcoming standard for QoS, IEEE 802.11e its called HCF (Byung,al.,2005)

Chapter 4 Research Problem

As i mention earliest the reason of selecting the study of data fragmentation effect in IEEE 802.11 under PCF mode is , is because in my organization they are looking for wireless network technology ,which can provide such applications like for delivering traffic for real-time applications such as Voice and data.

There are some limitations on PCF which directly effetely the fragmentation, one of these limitation is the binary exponential back off , the frame need to wait even the medium is free ,because the binary exponential back off will double the random number from 15 to 1023 till the medium will be free, so this can effect the retransmit the frame in fragmentation .

There for i found that IEEE 802.11 under PCF mode ,will not satisfy my

Chapter 5 Design and Experiment Setup

In This section i will be doing simulation , by using OPNET software as we can see in figure (8) to design and evaluate the performance of the IEEE 802.11 MAC protocol for wireless LANs. The aim of the project is to study the effect of data fragmentation in IEEE802.11under PCF mode. The main objectives of this experiment is to

Study the effect of data fragmentation in PCF by choosing different bit rate error and observe the major effect on the network, as we analysis that using low bit error rate ,and give fragmentation value 256 bytes or 512bytes ,the result must show that there will be will no effect on the data transmission or on network performance. We can also do measuring response time (seconds) for PCF with 256 fragmentation and fixed data rate 5.5Mbps, and for page response time (seconds) for PCF with 256 fragmentations with fixed data rate 11Mbps.Finaly Compare the result obtain.

Figure 8: WLAN Setup diagram in OPNET

Chapter 6 Conclusion

During my working in this project I have identify the effect of data fragmentation in IEEE802.11 under PCF in wireless network. I come out that this standard will help me in my organization to be used in library office only.

The probability of lost frame is much higher for wireless links. That’s why fragmentation is used to Reduces amount of time medium is in use and Reduces probability of collisions .The IEEE 802.11 WLAN has two different channel accessing mechanisms, namely, the distributed coordination function (DCF) and point coordination function (PCF).

We understand that When most stations have pending data, sequential polling provide ordered channel access and reduces collisions. But when few stations have pending data and rest are silent, this polling mechanism becomes major overhead, It adds unnecessary delay for stations with data.

Also we understand that when there will be high overhead in PCF fragmentation is, so instead of allowing the transmission of multiple packets with a high data rate ,its possible to make MPDU large size to reduce the overheads caused by multiple packets transmission when channel condition is good.

Anew standard has been developed in 2007 , it know as IEEE 802.11e. which support QoS. and can solve polling overhead in PCF.

The References

Books :

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

Stalling, W,Willim., 2008, Wirless communications & Network second ed published by Asoke K. Ghosh (India) Delhi, India.

E-Books:

Ross, John.,2008. Book of Wireless : A Painless Guide to Wi-Fi and Broadband Wireless (2nd Edition).

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

Available at:

http://site.ebrary.com/lib/caledonian/docDetail.action?docID=10218384&p00

=a%20painless%20guide%20wi-fi%20broadband%20wireless%20(2nd%20edition).

Journal Papers:

Tamer, M & Samir ,K .,2009.Performance Analysis of Wireless Local Area Networks ,WLANs A thesis submitted to the Faculty of Engineering at Cairo University in partial fulfillment of the requirement for the degree ofMASTER OF SCIENCE.

Yoon & Hougwon Kim & Jeonge. ,2003. Data fragmentation scheme in IEEE 802.15.4 wilerlss sensor network, Department of computer Scinse and Engieenring , Korea Unifersity

Dillip, K., 2006.Quality of Service Provisioning with modified

IEEE 802.11 MAC Protocol, Department of Computer Science and

Engineering National Institute of Technology Rourkela Rourkela-769

008, Orissa, India

Byung,S,& Younggoo, K. ,2003.Throughput Enhancement Through Dynamic Fragmentation in Wireless LANs ,Student Member, IEEE, Yuguang Fang, Senior Member, IEEE, Tan F. Wong, Senior Member, IEEE.

Abhishek ,G. , 2003.Dynamic Adaption of DCF and PCF mode of IEEE 802.11 WLAN, School of Information Technology,

Indian Institute of Technology, Bombay .

Xiyan, M ,Tamio, S.,2003.,Performance Analysis of Packetized Voice Transmission with PCF in IEEE802.11Wirelessnetwork, Dept. of electronic Engineering .Tsinghu University ,Beijing ,China.2003.

– Shakil ,A.,2007. Effect of Fragmentation in WLAN Systems with

Interference Problems. College of Information Technology

University. s.akhtar@uaeu.ac.ae .UAE

James X.Dong,S.,2004 Mustafa Ergen Improving the Aggregate Throughput of Access Points in IEEE 802.11

Wireless LANs, and Anuj Puri Department of Electrical Engineering &

Computer Science University of California at Berkeley.

Ziouva E. and Antonakopoulos T.2003., Efficient Voice Communications over IEEE802.11 WLANs Using Improved PCF Procedures.

Walid ,H&Naim ,A.,2006 Performance Enhancement of Wireless Local Area Networks,Dept. of Computer Science,Amman Arab University for Graduate Studies,P 0 Box 2925, Amman 11821, Jordan.

Web Page:URL

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