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Throughput Enhancement of IEEE 802.11 WLAN for Next Generation

Communications.

T.M Nazmul Huda , Md Nur Mostofa

September 2007 Master’s Thesis

Supervisor: Doru Constantenisco

Department of Telecommunication Systems School of Engineering

Blekinge Institute of Technology (BTH) SE-371 79 Karlskrona, Sweden.

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This thesis work submitted to the department of Telecommunication System, School of Engineering in pertial fullfillment of the requirments for the degree of

M.Sc in Electrical Engineering with emphasis on Telecommunication.

T.M Nazmul Huda Md. Nur Mostafa 741231-P131 781018-P938 School of Engineering Blekinge Institute of Technology (BTH) Karlskrona, Sweden. www.bth.se

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Acknowledgments First of all, we would like to express our heartiest gratitude to our supervisor Duro Constantenescu, for his good guidance, support, and valuable suggestions. He also taught us one important part of the course entitled Telesystem and provided us with very comprehensive understanding of wireless communications, networks and protocols. He impressed us both as a teacher and researcher, and inspired us to carry research on Wireless Local Area Networks (WLAN). He also taught us another course is named TCP/IP. We have learnt a lot from him, from course lectures and from our numerous discussions. We are very grateful to our friends who give us inspirations always to do this research. Finally but most importantly, we would like to thank our parents for their support, patience, blessings and understanding while we were writing this thesis. T.M Nazmul Huda , Md. Nur Mostafa Blekinge Institute of Technology (BTH). 371 79 Karlskrona, Sweden

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Research Proposal

Throughput Enhancement of IEEE 802.11 WLAN for Next Generation Communications.

Background: The standards of the forth generation consists a revolutionary development for mobile communications. The goal is not only the transmission of speech and transfer of data packets, as a co-sequence of the big expansion of the Internet, but also the support of real time audio and video streams. These goals in combination with the growing demand of the wireless network participants for higher bit rates, which allow the parallel operation of several channels, set new demands for the coming communication systems. Such networks, that were built Ad-hoc, consist of numerous mobile or fixed terminals, with the ability to communicate with other, over direct or multi-hop links. The network organization in different clusters and the operation succeed without any external help. Industries are seeking higher data rate for up coming multimedia applications. So enhancement of IEEE 802.11 MAC protocols is now current issue for both ad hoc based and infrastructure based wireless networks. Problem Statement:

The scope of this work is to derive the performance metrics of IEEE 802.11 MAC protocols (Achievable maximum throughput and achievable minimum delay) and proposed a new MAC

scheme, which improve the total throughput performance of IEEE 802.11. Expected result:

i) Mathematical derivation of maximum throughput and minimum delay of IEEE 802.11 MAC layer and important recommendation will be carried out from these performance matrices.

ii) The theoretical increasing throughput of IEEE 802.11 MAC layer will be independent on the performance matrices.

iii) Simulate the enhanced throughput of IEEE 802.11 MAC protocols.

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Supervisor and Examiner Doru Constantinescu Student Name: T. M Nazmul Huda Md. Nur Mostafa Student ID: 741231-P131 Student ID: 781018-P938 Email:tnhu05@student.bth.se Email: mnmo05@student.bth.se

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Table of Contents 1. Introduction

1.1 Background 12 1.2 Motivation 13 1.3 Task 13 1.4 Thesis Outline 13

2. Overview of IEEE 802.11

2.1 Introduction 15

2.1.1 Architecture of IEEE 802.11 2.1.2 IEEE 802.11 Services 2.1.3 Authentication and Privacy 2.2 Distributed Coordination Function (DCF) 19 2.2.1 Collision Avoidance and back off procedure 2.2.2 RTS/CTS mechanism 2.2.3 MAC frame 2.3 Point Coordination Function (PCF) 24 2.3.1 Access Procedure 2.4 Physical Layer (PHY) 26 2.5 Summary 31

3. Overhead Analysis and Throughput enhancement Schemes

3.1 Introduction 33 3.2 Overhead Analysis 33 3.3 Throughput Enhancement Schemes 39 3.3.1 Frame Aggregation

3.3.2 Burst Transmission and Acknowledgement (BTA) 3.4 Summary 47

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4. Proposed Throughput Enhancement Schemes

4.1 Introduction 49 4.2 Overview of Extended DCF (EDCF) 49 4.3 Block Acknowledgement schemes in EDCA. 52

4.3.1 Assumptions and Analytical frame works.

4.4 Frame Aggregation and Block Acknowledgement (FABA) Schemes.54 4.4.1 Introduction 4.4.2 Mechanism of Frame Aggregation and Block Acknowledgement

(FABA) Schemes. 4.4.3 Algorithms for Frame Aggregation and Block Acknowledgement

(FABA) Schemes 4.5 Summary 58

5. Performance Evaluation using simulation.

5.1 Introduction 59 5.2 Necessary Assumptions. 59 5.3 Simulating Result 60 5.4 Summary. 64

6. Conclusions and future works. 65

References

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Glossary of Acronyms

WLAN Wireless Local Area Network IEEE Institute of Electrical and Electronics Engineers LAN Local Area Network MAC Medium Access Control DCF Distributed Coordination Function CSMA Carrier Sense Multiple Access PCF Point Coordination Function QoS Quality of Service HRD Higher Data Rates ACK Acknowledgement PHY Physical Layer IFS Inter-Frame Space DLL Data Link Layer OSI Open System Interconnection LLC Logical Link Control FHSS Frequency Hopping Spread Spectrum DSSS Direct Sequence Spread Spectrum OFDM Orthogonal Frequency Division Multiplexing BSS Basic Service Set STA Station DS Distribution System ESS Extended Service Set IBSS Independent BSS AP Access Point SS Station Service DSS Distributed System Service WEP Wired Equivalence Privacy DIFS DCF Inter Frame Space SIFS Short Inter Frame Space PIFS PCF Inter Frame Space NAV Network Allocation Vector RTS Request To Send CTS Clear To Send CSMA/CA Carrier Sense Multiple Access with Collision Avoidance CSMA/CD Carrier Sense Multiple Access with Collision Detection ELAN Ethernet LAN DA Destination Address SA Source Address TA Transmitter Address RA Receiver Address BSSID BSS Identifier CRC Cyclic Redundancy Code CFP Contention Free Period CF-Poll Contention-Free Poll MPDU MAC Protocols Data Unit

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IR Infrared BPSK Binary Phase Shift Keying DBPSK Dou BPSK PLCP Physical Layer Convergence Protocol PPDU PLCP Protocol Data Unit CDMA Code Division Multiple Access AC Access Categories FABA Frame Aggregation and Block Acknowledgement

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Chapter 1 Introduction 1.1 Background Today Wireless Local Area Network (WLAN) is widely used in home, office everywhere instead of wired network. Cost benefit, mobility, high data rate, flexibility of use, standardization, and interoperability makes it more popular. Now there are lots of versions of IEEE 802.11 are available in market due to lower cost and easy deployment. Such as 802.11b and 802.11a, it supports data transmission rates of up to 11 and 54 Mbps; respectively. There are basically two types of mode of operations in wireless LAN. Firstly infrastructure based Nomadic access and another peer-to-peer connection based Ad Hoc Networking. Infrastructure based nomadic access is mainly a wireless hub which provide a wireless link between mobile stations and infrastructure based core network. Another mode of operation is ad hoc based. There is no centralized server system, which provides immediate services if needed by peer-to-peer wireless connections. The basic Medium Access Control (MAC) mechanism of 802.11 is called Distributed Coordination Function (DCF). DCF is based on distributed channel access and employs Carrier Sense Multiple Access (CSMA) protocol for the medium access. Another Optional IEEE 802.11 access mechanism, called Point Coordination Function (PCF), based on centrally controlled access. Now a day many wireless devices manufacturer of the 802.11 use DCF. In very little area of PCF is hardly implemented this complex design access mechanism. Some standards are working with security like IEEE 802.11i, some are working with Quality of Service (QoS) like IEEE 802.11e and many of them are working with data rate, throughput and other parameters of wireless network.

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1.2 Motivation In multimedia applications such as audio/video streaming, teleconferencing, Internet telephony and interactive games demands more data rates. Industry is seeking Higher Data Rates (HRD’s) over 100 Mbps for IEEE 802.11a extension [3], which is undergoing discussions in IEEE 802.11 meetings. Throughput enhancement of the legacy IEEE 802.11 is the important issue for future wireless devices. Thus, a lot of research works have been carried out to enhance the throughput of IEEE 802.11 MAC protocols for wireless network. MAC protocol is not sufficient enough though the physical layer (PHY) data rate is significantly high. This is due mainly to the large overheads composed of medium access control (MAC) header, PHY preamble/header, backoff time, acknowledgement (ACK) transmission and some inter-frame spaces (IFSs) [4]. So we have to reduce MAC overhead of the legacy IEEE 802.11 for better throughput. 1.3 Task The thesis focus on some parameters of infrastructure based network which are very much involved its MAC performance and the scope of this work is to derive the performance metrics of IEEE 802.11 (Achievable maximum throughput and achievable minimum delay) and increasing the total throughput of IEEE 802.11. The main task of the thesis is pointed out below.

iv) Mathematical derivation of maximum throughput and minimum delay of IEEE 802.11 MAC layer and important recommendation will be carried out from these performance matrices.

v) The theoretical increasing throughput of IEEE 802.11 MAC layer will be independent on the performance matrices.

vi) A propose new MAC mechanism for the enhancement of throughput of IEEE 802.11.

vii) Simulate the system throughput of proposed new MAC schemes. 1.4 Thesis Outline The research work mainly focus on throughput enhancement of IEEE 802.11 MAC protocol. So the whole content of the thesis work emphasis on the throughput parameters. However we have also considered the QoS issue in different cases. A new MAC schemes is proposed in the thesis and also evaluate the performance using simulation of this new proposed MAC schemes. In chapter 2, we discuss the overview of IEEE 802.11. Here we present DCF and

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PCF access procedure in wireless medium and PHY layer parameter also. Chapter 3 presents the concepts of overhead and also some important throughput enhancement schemes. In this chapter, we specially focus on the mechanism of Frame Aggregation [4] and the mechanism of Burst Transmission and Acknowledgement (BTA)[3]. Here we try to evaluate the performance of current MAC protocol and also the performance evaluation of the above two proposed enhancement schemes using simulation. Chapter 4 introduces the mechanism and algorithm of a new MAC scheme, is called Frame Aggregation and Block Acknowledgement (FABA). To make it easier for understanding, we discuss the Block Acknowledgment component before FABA schemes. Here it is important to mention that the FABA scheme is implemented in EDCA of legacy IEEE 802.11e. In chapter 5, we try to evaluate the performance of above all schemes using simulation, which is coded in Matlab 7.1 simulation software. Chapter 6 contains the conclusion of this research work and also related future work mentioned here.

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Chapter 2 Overview of IEEE 802.11 2.1 Introduction Work on wireless LANs within the IEEE 802 committee began in 1987 within the IEEE 802.4 group. The initial interest was in developing an ISM-based wireless LAN using the equivalent of a token-passing bus MAC protocol. After some work, it was decided that token bus was not suitable for controlling a radio medium without causing inefficient use of the radio frequency spectrum. IEEE 802 then decided in 1990 to form a new working group, IEEE 802.11, specifically devoted to wireless LANs with a charter to develop a MAC protocol and physical medium specification. In 1997, IEEE (Institute of Electrical and Electronics Engineers) released the 802.11 Wireless Local Area Network (WLAN) standards [1]. Figure 2.1 shows IEEE 802.11 standards OSI reference model. Here MAC exists in Data Link Layer (DLL) i.e Open System Interconnection (OSI) layer 2.

Figure 2.1 - IEEE 802.11 standards mapped to the OSI reference model.

MAC is in between PHY layer and Logical Link Control (LLC). In 1999, IEEE introduced two enhanced PHY layer specifications 802.11b [5] and 802.11a [6] with data transmission rates of up to 11 and 54 Mbps, respectively.802.11b and 802.11a are based on Direct Sequence Spread Spectrum (DSSS) and Orthogonal Frequency Division Multiplexing (OFDM) with 2.4 GHz band and 5 GHz frequency band respectively. Architecture, services, authentication and privacy of IEEE 802.11 are discussed in the next section below. 2.1.1 Architecture of IEEE 802.11

The complete IEEE 802.11 architecture is shown in figure 2.2. The Basic Service Set (BSS) is the basic building block of an IEEE 802.11 LAN and this consists of devices referred to as Stations (STA). Basically, the set of STAs, which can talk to each other can form a BSS. Multiple BSSs are interconnected through an architectural component, called Distribution

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System (DS), to form an Extended Service Set (ESS). An access point (AP) is a STA that provides access to DS by providing DS services, which mention above. If the LAN just consists of single BSS, then that is called Independent BSS (IBSS). This IBSS is often referred as adhoc network for it does not require any pre-planning. Figure 2.3 shows the BSS in adhoc mode.

Figure: 2.2, Basic Service Set (BSS) with Access Point (AP) in Distributed System (DS)

Figure: 2.3, Basic Service Set (BSS) without Access Point (AP) in adhoc mode

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IEEE 802.11 specifies the services that are to be provided by STAs and DS. It does not specify how DS is to be implemented; it can be either distributed or central. A service provided by a station is called Station Service (SS) and a service by a DS is called Distributed System Service (DSS).

2.1.2 IEEE 802.11 Services

There are following four services which are provided by station:

Authentication: The authentication of devices is important to prevent access by unknown devices. This simulates the physical security provided by the wired network. This service is used by all stations to establish their identity to stations with which they will communicate. The standard supports several authentication processes but does not mandate any of them.

De authentication: This is invoked when an existing authentication is to be removed. Just do opposite function of authentication.

Privacy: The standard specifies optional Wired Equivalence Privacy (WEP), which provides a security level equivalent to a wired LAN.

MSDU Delivery: This is for the delivery of MAC service data unit.

Distribution system services include the following five services:

Distribution: This is the primary service invoked by a STA for all messages when operating an ESS. The onus of moving a message generated by a STA in a BSS to another STA in a different (or same) BSS is performed by this service call.

Integration: This service is used for the transmission of messages from IEEE 802.11 STA to a non-IEEE 802.11 network. The STA which connects non-IEEE 802.11 networks to ESS is called portal.

Association: A STA uses this service to associate itself with an AP with out which it can not send any messages on that ESS. Each STA can associate with no more than one AP.

Disassociation: This service is used when an existing association needs to be terminated.

Reassociation: This service is used by the mobile STAs when they move from one BSS to another and want to change their APs

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2.1.3 Authentication and Privacy

Authentication

Because wireless LANs has limited physical security to prevent unauthorized access, 802.11 define authentication services to control access to the WLAN. The goal of authentication service is to provide access control equal to a wired LAN. The authentication service provides a mechanism for one station to identify another station. Without this proof of identity, the station is not allowed to use the WLAN for data delivery. All 802.11 stations, whether they are part of an independent BSS or ESS network, must use the authentication service prior to communicating with another station. IEEE 802.11 defines two types of authentication services. Open system authentication: This is the default authentication method, which is a very simple, two-step process. First the station wanting to authenticate with another station sends an authentication management frame containing the sending station’s identity. The receiving station then sends back a frame alerting whether it recognizes the identity of the authenticating station. Shared key authentication: This type of authentication assumes that each station has received a secret shared key through a secure channel independent of the 802.11 network. Stations authenticate through shared knowledge of the secret key. Use of shared key authentication requires implementation of encryption via the Wired Equivalent Privacy or WEP algorithm. De-authentication The de-authentication service is used to eliminate a previously authorized user from any further use of the network. Once a station is de-authenticated, that station is no longer able to access the WLAN without performing the authentication function again. De-authentication is a notification and cannot be refused. For example, when a station wishes to be removed from a BSS, it can send a de-authentication management frame to the associated access point to notify the access point of the removal from the network. An access point could also de-authenticate a station by sending a de-authentication frame to the station. Privacy The privacy service of IEEE 802.11 is designed to provide an equivalent level of protection for data on the WLAN as that provided by a wired network with restricted physical access. This service protects that data only as it traverses the wireless medium. It is not designed to provide complete protection of data between applications running over a mixed network. With a wireless network, all stations and other devices can "hear" data traffic tacking place within range on the network, seriously impacting the security level of a wireless link. IEEE 802.11 counters this problem by offering a privacy service option that raises the security of the 802.11 networks to that of a wired network. The privacy service, applying to all data frames and some authentication management frames, is an encryption algorithm based on the 802.11 Wired Equivalent Privacy (WEP) algorithms.

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Data Delivery Data delivery service is similar to that provided by all other IEEE 802 LANs. The data delivery service provides reliable delivery of data frames from the MAC in one station to the MAC in one or more other stations, with minimal duplication and reordering of frames.

2.2 Distributed Coordination Function (DCF)

The 802.11 MAC layer provides functionality to allow reliable data delivery for the upper layers over the wireless PHY media. The data delivery itself is based on an asynchronous, best-effort, connectionless delivery of MAC layer data. There is no guarantee that the frames will be delivered successfully. Distributed Coordination Function (DCF) is the basic access mechanism of 802.11, which is based on Carrier Sense Multiple Access (CSMA). CSMA operates as like listen-before-talk, i.e, before transmitting a frame to wireless media STA must sense the media. If the media is free at least for short time called DCF Inter Frame Space (DIFS) time period then the STA can access the media and can transmits data. During this time period of transmission other STAs must wait until medium becomes idle again at least for DIFS time period. Figure 2.4 illustrate the simple flowchart of Media Access Control (MAC) logic.

Figure 2.4 IEEE 802.11 Medium Access Control (MAC) Logic

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As the destination STA successfully receives a frame then it acknowledges the sender by sending back immediate an ACK frame after Short Inter Frame Space (SIFS) time duration. Figure 2.5 shows the basic access mechanism of 802.11

Figure 2.5 Basic access mechanism of IEEE 802.11

There are three Inter Frame Space (IFS) defined in IEEE 802.11 to control the access to the wireless media. Two inter frame spaces, DIFS and SIFS we have seen in above figure 2.4. Another inter frame space called PCF Inter Frame Space (PIFS) is used by AP in PCF.SIFS is the shortest Inter Frame Space among of the three. Figure 2.6 illustrate the relationship of three Inter Frame Space (IFS).

Figure 2.6 Relationship among three Inter Frame Space (IFS)

SIFS is the inter frame space (IFS) between the data frame from sender and ACK frame from receiver. As we mention above SIFS is smaller than DIFS duration, so no data frame from other stations will access the medium before ACK frame.

The second shortest IFS is PIFS which is used by AP in Point Coordination Function (PCF) as we mention earlier. PCF is optional polling based access mechanism of IEEE 802.11 that’s poll stations individually which is centrally controlled. In PCF, PC/AP is given priority over ordinary stations such that PIFS is lower duration than DIFS time period.

The values of IFS are depending on the underlying Physical layer (PHY) condition and it is the multiple of slots time. Slots time is derived from propagation delay, transmitter delay, and other physical dependent parameters [12]. PIFS is equal to SIFS plus one slot time and DIFS

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is equal to PIFS plus one slot time. In mathematically, SIFS<PIFS<DIFS. If SIFS is equal to 7 slots time, then PFIS is equal to (7+1)=8 slots time and DIFS is equal to (8+1)=9 slots time.

Carrier sensing is done by the stations, which want to communicate with the other station. The IEEE 802.11 MAC specifies both physical and virtual carrier sensing as a means to avoid collisions. While physical carrier sensing is done through signal measurement, virtual carrier sensing is done using the Network Allocation Vector (NAV). In the next section 2.2.2 we will discuss about NAV with Request To Send (RTS)/Clear To Send (CTS) schemes.

2.2.1 Collision Avoidance and Back off procedure

The 802.11 standard includes a basic Distributed Coordination Function (DCF). The DCF is the fundamental access method used to support asynchronous data transfer on the best effort basis. As specified in standards, the DCF must be supported by all the stations in a basic service set (BSS). The DCF is based on Carrier Sense Multiple Access with Collision Avoidance CSMA/CA. There are two techniques used for packet transmitting in DCF. The default one is a two-way handshaking mechanism, also called basic access method. The destination station transmits a positive acknowledgement (ACK) message signal for a successful packet transmission. The other optional mechanism is a four-way handshaking access method, which uses the request-to-send/clear-to-send (RTS/CTS) technique to reserve the channel before data transmission, which is discussed in next section 2.2.2.

The fundamental access method of 802.11 is Carrier Sense Multiple Access with Collision Avoidance or CSMA/CA. CSMA/CA works by a "listen before talk scheme". This means that a station wants to transmit must first sense the radio channel to determine if the channel is busy or not. If the medium is not busy, the transmission may proceed. The CSMA/CA protocol avoids collisions among stations sharing the medium by utilizing a random back off time if the station’s physical or logical sensing mechanism indicates a busy medium. The period of time immediately following a busy medium is the highest probability of collisions occurring, especially under high utilization. The CSMA/CA scheme implements a minimum time gap between frames from a given user. Once a frame has been sent from a given transmitting station, that station must wait until the time gap is up to try to transmit again. Once the time has passed, the station selects a random amount of time (the back off interval) to wait before "listening" again to verify a clear channel on which to transmit. If the channel is still busy, another back off interval is selected that is less than the first. This process is repeated until the waiting time approaches zero and the station is allowed to transmit. This type of multiple accesses ensures judicious channel sharing while avoiding collisions.

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2.2.2 RTS/CTS mechanism

DCF also provides an optional access method that introduces an additional operation on top of the basic access method before a packet transmission is taken place. When the back-off timer reaches zero, instead of transmitting the packet, the user first transmits an RTS frame to request for a transmission right. Upon receiving the RTS frame, the receiver replies with a CTS frame after a SIFS period. Once the RTS/CTS is exchanged successfully, the user transmits a data packet. Figure 2.7 illustrates the RTS/CTS packet transmission method.The frames RTS and CTS carry the information of the length of the packet to be transmitted. Therefore, all the other users are capable of updating the NAVs based on the RTS from the source user and the CTS from the destination user. This helps to solve the hidden terminal problems. If a user can detect the transmission of at least one RTS or CTS frame, it can avoid collision even to sense the channel busy. If a collision occurs with two RTS frames far less bandwidth is wasted compared a collision with larger data frames. Collisions between RTS packets can still occur in CSMA/CA. These are minimized with a randomized exponential back-off strategy similar to that used in regular CSMA.

Figure 2.7 RTS/CTS mechanism for IEEE 802.11

NAV (RTS) NAV (CTS)

receiver

sender

other

DIFS

SIFS

ACK

SIFS SIFS payload packet

RTS SIFS

CTS

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2.2.3 MAC frame

The 802.11 MAC provides a controlled access method to the shared wireless media called Carrier-Sense Multiple Access with Collision Avoidance (CSMA/CA). CSMA/CA is similar to the collision detection access method deployed by 802.3 Ethernet LANs (ELAN). Here we will see how the MAC frame format looks like. Figure 2.8 shows the details of MAC frame format. The fields of this frame are as follows:

2 2 6 6 6 2 6 4

Frame Control

Duration/ ID

Address1

Adderss2 Address3 SequenceControl

Address4 Frame body

CRC

MAC Header 2 2 4 1 1 1 1 1 1 1 1

Protocol Version

Type Subtype To DS

From DS

More Frag

Retry Pwr mgt

More Data

WEP Order

Figure 2.8 IEEE 802.11 MAC frame Format

The frame control field contains total 16 bits, which is shown in above figure. Protocol version, 2 bits indicate its version. Currently it is zero. The idea behind is that major protocol change will change its version number. Type field (2 bits, B3B2) 00 indicate management frame, 01 indicate control frame, 10 indicate data frame, and 11 is reserved for future. Subtype field indicates more specifically of different types of field. To DS and from DS fields combine with address fields makes valid address format that indicate different functions. Below table 2.1 shows the different valid address format. In ad-hoc mode, To DS and From DS fields are zeros i.e To DS and From DS fields are only valid for infrastructure based operations. More frag field indicates that the large frame fragmented into more small frames so that lost rate of ACKs keeps low. Retry field indicate that this frame is re-transmission due to no ACK frame received by sender. Power management frame indicates that the station weather it is in power mode or not.

B0 B15

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Functions To DS From DS Address1 Address2 Address3 Address4

Ad-Hoc 0 0 DA SA BSSID -

From AP 0 1 DA BSSID SA -

To AP 1 0 BSSID SA DA -

Within DS 1 1 RA TA DA SA

Table: 2.1 Valid MAC Address format.

More data means more frames is coming and keep power on (to avoid sleep mode of stations). Wired Equivalent Privacy (WEP) set to 1 if the optional wired equivalent protocol is implemented. WEP is used in the exchange of encryption keys for secure data exchange.[7] Order means received frame must be processed in order at the receiving end. Duration or Connection ID field indicates the time of a successful transmission of MAC frame. This is actually the channel allocated time for successful MAC frame transmission in microseconds. Connection ID is used as an identifier in some control frame for association or connection. The duration value is also used for the Network Allocation Vector (NAV) calculation. There are four address fields in MAC frame. Each address field is six (6) byte in length. Table 2.1 illustrates the valid address format which shows that the combination of four addresses field with frame control address field. Those combinations indicate the different functions. Two byte sequence control field consists of fragment number and sequence number. It is used to represent the order of different fragments belonging to the same frame and to recognize packet duplications.

Data field contain original data which to be transmitted from sender to receiver. It is variable in between zero to 9196 byte in length.

Another field is 32 bits Cyclic Redundancy Code (CRC) is type of hash function which is used to produce a small, fixed-size checksum of a larger block of data.

2.3 Point Coordination Function (PCF)

The point coordination function (PCF) is a centralized MAC algorithm used to provide contention-free service. Contention-free services are polling based access mechanism usable for infrastructure-based network. The lower sub layer of the MAC layer is the Distributed Coordination Function (DFC) to provide access to all traffic. PCF is built on top of DCF and exploits features of DCF to assure access for delay sensitive data traffic. PCF is an optional

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access method provided by the 802.11 standard. With PCF, a point coordinator within the access point controls all stations synchronously. Figue.2.9 shows the relationship between PCF and DCF within the MAC protocol.

Figure 2.9, PCF and DCF relationship within MAC protocol.

Within a time period called the contention free period all stations are polled one by one. For example, the point coordinator may first poll station A, and during a specific period of time station A can transmit data frames (and no other station can send anything). The point coordinator will then poll the next station and continue down the polling list, while listing each station to have a chance to send data. So there is no chance to collide data each other even no need extra RTS/CTS handshaking mechanism.

2.3.1 Access Procedure

The optional PCF is logically sits on the top of the DCF to provide an access of delay sensitive data and DCF provide media access for the normal data. The Contention Free Period (CFP) repetition interval is used to determine the frequency at which the PCF occurs. A Station (STA) has an option weather it is respond to a Contention-Free Poll (CF-Poll) or not .The CF-Poll is provided by an Access Point (AP). A CF-Pollable STA is selected by an active Point Coordinator (PC) according to its nature of frame i.e. whether it is delay sensitive data frame or normal data frame. RTS/CTS are not used by the CF-Pollable STAs and the PC in the CFP. When polled by the PC, a CF-Pollable STA may transmit only one MAC Protocols Data Unit (MPDU), which can be not only to the PC but also to any destination and gets the acknowledgement of the frame using “piggyback”. If the data frame is not acknowledged, the CF-Pollable STA will wait for the next poll again to retransmit the frame or it decides to retransmit during the contention period. Figure 2.10 shows the transmission from PC to STA and vice versa.

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Figure 2.10 illustrates the transmission of frames between the PC and a station, and vice

versa. If the medium is free or idle more than at least one PIFS then the AP transmits the beacon frame, which initiates the CFP repetition interval. There is an interval for at least one SIFS period between beacon frame and the transmit one of the following: a data frame, a CF-Poll frame, a Data + CF-Poll frame, or a CF-End frame. The above figure 2.10 shows the following steps of transmitting at CFP. If there is no traffic buffered and no polls to send at the PC, a CF-End frame shall be transmitted immediately after the initial beacon. All stations within the BSS update their NAV to the maximum length of the CFP at the beginning of each CFP repetition interval. A station can get only one chance to transmit data frame to PC during the CFP. If the PC receives a Data + CF-Ack frame from a station, the PC can send a Data + CF-Poll + CF-ACK frame to a different station, where the CF-ACK portion of the frame is used to acknowledge receipt of the previous data frame. The ability to combine polling and acknowledgement frames with data frames, transmitted between stations and the PC, has been designed to improve the efficiency.[18]. After waits a PIFS interval if PC fails to get ACK frame for the transmitting frame then PC adding the next station as polling lists. 2.4 Physical Layer

The 802.11 physical layer (PHY) is the interface between the MAC and the wireless media where frames are transmitted and received. The PHY layer is involved with three functions. First, the PHY builds an interface to exchange frames with the upper MAC layer for transmission and reception of data. Secondly, the PHY uses carrier frequency and spread spectrum modulation technique to transmit data frames over the media. Thirdly, the PHY provides a carrier sense indication back to the MAC to verify activity on the media. IEEE 802.11 provides three different types of modulation technique in PHY layer definitions.

Below we discuss each of them. The three different spread spectrum techniques are,

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1. Direct-Sequence Spread Spectrum

2. Frequency-Hopping Spread Spectrum

3. Infrared

Direct Sequence Spread Spectrum (DSSS)

The principle of Direct Sequence Spread Spectrum is to spread a signal on a larger frequency band by multiplexing it with a signature or code to minimize localized interference and background noise. To spread the signal, each bit is modulated by a code. In the receiver, the original signal is recovered by receiving the whole spread channel and demodulating with the same code used by the transmitter. There is not necessary to synchronization between transmitter and receiver. Table 2.2, illustrates the parameter sets of DSSS. The 802.11 Direct Sequence Spread Spectrum (DSSS) PHY also uses the 2.4 GHz radio frequency band.

Data rate Chipping code length

Modulation Symbol rate Bits / Symbol

1 Mbps 11 (Barker sequence)

DBPSK 1 Msps 1

2 Mbps 11 (Barker sequence)

DQPSK 1 Msps 2

5.5 Mbps 8 CCK DBPSK 11.375 Msps 4

11 Mbps 8 CCK DQPSK 1.375 Msps 8

Table 2.2 shows the parameter sets of DSSS

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Frequency Hopping Spread Spectrum (FHSS)

FHSS system makes use of a multiple channels, with the signal hopping from one channel to another based on a pseudo noise sequence. In the case of the IEEE 802.11 scheme, 1MHz channels are used. The number of channels available ranges from23 in Japan to 70 in the United States.

Data rate Modulation Symbol rate Bits / Symbol

1 Mbps Two-level GFSK 1Msps 1

2 Mbps Four-level GFSK 1Msps 2

Table 2.3 shows the parameter sets of FHSS

The details of the hopping scheme are adjustable. For example the minimum hop rate for the United States is 2.5 hops per second. The minimum hop distance in frequency is 6 MHz in North America and most of Europe and 5 MHz in Japan. For modulation the FHSS scheme uses two-level Gaussian FSK for the 1-Mbps system. The bits zero and one are encoded as deviations from the current carrier frequency. For 2 Mbps, a four-level GFSK scheme is used, in which four different deviations from the center frequency define the four 2-bit combinations

Infrared (IR)

The IEEE 802.11 infrared scheme is omni directional rather than point to point. A range of up to 20 m is possible. The modulation scheme for the 1Mbps data rate is known as16-PPM (pulse position modulation).

Data rate Modulation Symbol rate Bits / Symbol

1 Mbps 16-PPM 4 Msps 0.25

2 Mbps 4-PPM 4 Msps 0.5

Table 2.4 shows the parameter sets of IR

In this scheme each group of 4 data bits is mapped into one of the 16-PPM symbols, each symbol is string of 16 bits. Each 16bit string consists of fifteen 0s and one binary 1. For the 2-Mbps data rate each group of 2 data bits is mapped into one of four 4-bit sequences. Each sequence consists of three 0s and one binary 1. The actual transmission uses an intensity modulation scheme, in which the presence of a signal corresponds to a binary 1 and the absence of a signal corresponds to binary 0.

PHY Frame format

In this section, we will briefly discuss about physical layer frame format which is operate by Physical Layer Convergence Protocol (PLCP). Figure 2.11 illustrates the physical layer frame

29

format. There are two fields in PLCP Protocol Data Unit (PPDU).One is PLCP preamble which is 144 bits another is PLCP header which is 48 bits.

Figure 2.11 Shows the PHY layer frame format.

In PLCP preamble, Synchronization (Sync) field consists of 128 bits. It is filled in with predefined numbers and allows receiver to synchronize to the transmission. Another 16 bits, Start Frame Delimiter (SFD) field is used to mark the start of every frame. Combine of those above two fields are PLCP preamble (144 bits). In PLCP header, 8 bits Signal or Data rate (DR) field indicates how fast the data will be transmitted. The only two possible values for the June 1997 version of 802.11 are 00001010 for 1 Mbps DSSS and 00010100 for 2 Mbps DSSS. The next 8 bits Service field is reserved for future use. The next 16 bits Length field indicates the length of the ensuing MAC PDU. Last CRC field is used for error detecting which is 16 bits.The preamble and the PLCP header are transmitted at 1Mbps regardless of the current data transmission speed. But the data rate is not same for MPDU .The MPDU data rate is sent to the receiver at the rate specified in the services field. In the next section, some standards of IEEE 802.11 are discussed.

IEEE 802.11a

The IEEE 8020.11a specification, the 5-GHz band is used rather than the 2.4-GHz band. IEEE 802.11a specifications use a high effective multi-carrier modulation called Orthogonal Frequency Division Multiplexing (OFDM). Channels are orthogonal with each others so that interference and noise should be eliminated. OFDM uses multiple carrier signals at different frequencies sending some of the bits on each channel. Table 2.5 shows some parameter sets of IEEE 802.11a using OFDM multi-carrier modulation.

Release Date Op. Frequency Data Rate (Typ) Data Rate (Max) Range (Indoor)

October 1999 5 GHz 25 Mbit/s 54 Mbit/s ~30 meters (~98 ft)

Table 2.5 shows the parameter sets of IEEE 802.11a

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In the case of OFDM all of the sub-channels are treated as a single data source. The possible data rates for IEEE 802.11a are 6,9,12,18,24,36,48 and 54 Mbps. The system uses up to 52 sub-carriers that are modulated using BPSK, QPSK, 16-QAM, or 64-QAM, depending on the rate is required. Sub-carrier frequency spacing is 0.3125 MHz.

IEEE 802.11b

The 802.11b amendment to the original standard was ratified in 1999 and its maximum raw data rate is 11 Mbit/s. IEEE 802.11b operates with 2.4GHz frequency band and uses the same CSMA/CA media access method defined in the original standard. In practice the maximum throughput of IEEE 802.11b is about 5.9 Mbps using Transport Control Protocol (TCP) and 7.1 Mbps using User Datagram Protocol (UDP) due to the CSMA/CA protocol overhead. The IEEE 802.11b products came on the market very rapidly, since 802.11b is a direct extension of the DSSS (Direct-sequence spread spectrum) modulation technique, we have already discussed this Spread Spectrum technique on above section. Table 2.6 illustrates the parameter sets of IEEE 802.11b.

Release Date Op. Frequency Data Rate (Typ) Data Rate (Max) Range (Indoor)

October 1999 2.4 GHz 6.5 Mbit/s 11 Mbit/s ~30 meters (~98 ft)

Table 2.6 shows the parameter sets of IEEE 802.11b

Technically, the 802.11b standard uses Complementary Code Keying (CCK) as its modulation technique rather than OFDM or Spread Spectrum, which is a variation on Code Division Multiple Access (CDMA). It is grate advantage of IEEE 802.11b that the chipsets and products were easily upgraded to support its enhancements. The dramatic increase in throughput of 802.11b compared to the original standard along with substantial price reductions led to the rapid acceptance of 802.11b as the definitive wireless LAN technology.

IEEE 802.11g The IEEE 802.11g , 3rd standard was ratified in June 2003. IEEE 802.11g operates with 2.4 GHz band like 802.11b but operates at a maximum raw data rate of 54 Mbit/s like 802.11a. Due to protocols overhead, in practices maximum throughput of 802.11g is 27.9 Mbps. Another grate advantage of 802.11g is that the hardware is compatible with 802.11b hardware. Table shows some parameter sets of IEEE 802.11g.

Release Date Op. Frequency Data Rate (Typ) Data Rate (Max) Range (Indoor)

June 2003 2.4 GHz 24 Mbit/s 54 Mbit/s ~30 meters (~98 ft)

Table 2.7 shows the parameter sets of IEEE 802.11g

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The modulation scheme used in 802.11g is orthogonal frequency-division multiplexing (OFDM) for the data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbit/s, and reverts to (like the 802.11b standard) CCK for 5.5 and 11 Mbit/s and DBPSK/DQPSK+DSSS for 1 and 2 Mbit/s.

IEEE 802.11n The standard IEEE 802.11n is currently in under development. For multimedia communication the throughputs of this current standard try to enhance up to 540 Mbps. Table 2.8 shows the some parameter sets of IEEE 802.11n.

Release Date Op. Frequency Data Rate (Typ) Data Rate (Max) Range (Indoor)

Unfinished 2.4 GHz or 5 GHz 200 Mbit/s 540 Mbit/s ~50 meters (~165 ft)

Table 2.8 shows the parameter sets of IEEE 802.11n

2.5 Summary IEEE released 802.11 Wireless LAN standard in 1997 though began its journey early 1987. The first standard, IEEE 802.11b supported maximum data rate of 11 Mbps and next another improved standard IEEE 802.11a supported maximum data rate 54 Mbps. MAC is the lower part of OSI reference model is referred to as DLL. So the MAC layer makes an interface with the LLC and Physical (PHY) layer. PHY layer transmit data from transmitter to receiver which operate at ISM frequency band based on some modulation technique like spread spectrum (DSSS and FHSS), OFDM etc. MAC operates based on two types of access mechanism. Optional Point Coordination Function (PCF) is based on contention free access mechanism for delay sensitive data traffic. Centralized control polling based mechanism poll each stations at one time within contention period interval and no other pollable stations can get access the medium. So it is not possible collision among pollable stations and that is why RTS/CTS are not used in CFP. A CF-Pollable STA is selected by an active Point Coordinator (PC) according to its nature of frame i.e. whether it is delay sensitive data frame or normal data frame.

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DCF is basic access mechanism based on CSMA/CA.Before transmitting a frame, the station senses the medium, and if the medium is found idle at least for DIFS (DCF Inter-Frame Space) time period, the station starts transmission. Otherwise, if the medium is found busy during the DIFS period, the station defers access and chooses a random backoff value that specifies the additional time it has to wait after the medium becomes idle again. As the medium becomes idle for the DIFS time period again, the station starts decrementing its backoff time. If medium becomes busy during this backoff procedure, the station pauses the backoff timer, and resumes it as the medium becomes idle for the DIFS period again. The station is allowed to transmit as the backoff timer reaches to zero. The additional random backoff time helps avoiding collision, which is defined as the situation when two or more stations transmit at the same time and result in collision. As the destination station receives the frame, it acknowledges by sending back an ACK frame after SIFS (Short Inter-Frame Space) time period. SIFS, like DIFS, is one of the three inter-frame spaces (IFS) defined to control the medium access.An additional mechanism RTS/CTS is defined that allows the sender and receiver to handshake by exchanging RTS (Request To Send) and CTS (Clear To Send) frames prior to transmitting the data frame for overcome so called hidden node problem. There are many working groups for WLAN standard working on and already many standards in market available now. Some of the working groups work on the throughput issue of IEEE 802.11. The IEEE 02.11n standard is in under research and tries to enhance the throughput up to 554 Mbps.

33

Chapter 3 Overhead Analysis and Throughput Enhancement Schemes 3.1 Introduction There are lot of parameters involve in MAC design. Some of these parameters are given here according to priority order. Throughput, Delay, Bandwidth efficiency, Packet order preservation, Handoff/roaming between different services area are some of them. This thesis deals with mainly throughput and as well as delay parameter. The efficiency of medium access control (MAC) mechanism is based upon on designing goal. The IEEE 802.11 medium access control (MAC) employs mandatory contention-based channel access function called distributed coordination function (DCF), and an optional centrally controlled channel access function called point coordination function (PCF) [2]. The DCF method employ binary exponential back off using carrier sense multiple access with collision avoidance. Another PCF introduce centrally controlled polling based access mechanism. These two methods are already described in the previous chapter. In this chapter we will discuss about overhead and we will try to make a relationship between the overhead and throughput. In the next section we will go through the performance analysis of some proposed throughput enhancement schemes. 3.2 Overhead Analysis There are lot of control frames; management frames as well as data frames are involved during the transmission. All types of frames contain a specific amount of octets as header. A Frame Check Sequence (FCS) is extra checksum, which is added to all frames of communication protocols for error detection and correction. Cyclic Redundancy Check (CRC) is used mainly to do so. In Physical (PHY) layer also contain some overhead such as Physical Layer Convergence Protocol (PLCP) preamble and PLCP header. Inter Frame Space (IFS) is space between two consecutive frames that can help to separate these frames. In DCF function all data frames are separated by DIFS and acknowledgement frames are separated by SIFS. PIFS is used in PCF function, which we mention earlier. Random back off mechanism is used in DCF mode for avoid collision between frames. All parameters (i.e. MAC header, FCS, PHY preamble, PHY header, Inter-frame space (IFS), Back off time, ACK frame),which are mentioned above called overhead of IEEE 802.11 standard. All of these overheads are added to each frame during the transmission as a fixed manner. These overhead are the major fundamental issue for throughput enhancement. So there is need to analysis the overhead of MAC layer to provide good throughput. Theoretically, it is possible to increase the expected throughput efficiency is about 15% if only we consider header. Figure 3.1 shows the MAC header including FCS field is 28 octets and figure 3.2 shows the ACK frame is 14 octets as overhead.

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Figure 3.1 MAC frame format within the DS. i.e Not valid for AP to AP transmission.

Figure 3.2 MAC Ack frame format. During the transmission, a PLCP preamble and a PLCP header are added to an MPDU to create a PPDU. The PPDU format of the IEEE 802.11a PHY is shown in Figure 3.3, which includes PLCP preamble, PLCP header, MPDU, tail bits, and pad bits. The PLCP preamble field, with the time duration of tPLCPPreamble, is composed of 10 repetitions of a short training sequence (0.8µs).

Figure 3.3 PPDU frame format of IEEE 802.11a OFDM PHY. The PLCP header, except the 16 bits SERVICE field, with the duration of tPLCP_SIG, constitutes a single OFDM symbol. The 6 tail bits are used for convolution codec and the pad bits are used to make the resulting bit string into a multiple of OFDM symbols. Each OFDM symbol interval is 4µs and denoted by tSymbol. The 16-bits SERVICE field of the PLCP header and the MPDU (along with six tail bits and pad bits), represented by DATA, are transmitted at the data rate specified in the RATE field. In the table 3.1, we show the PHY OFDM characteristic.

35

Characteristics Value Comments

tSlotTime 9 µs Slot time tSIFS 16 µs SIFS time tDIFS 34 µs DIFS=SIFS+2 ×slot

CWmin 15 Minimum contention window size CWmax 1023 Maximum contention window size

tPLCPPreamble 16 µs PLCP preamble duration tPLCP_SIG 4 µs PLCP SIGNAL field duration

tSymbol 4 µs OFDM symbol interval Table 3.1 PHY OFDM IEEE 802.11a characteristics. For this analysis we will consider the following assumptions for simplicity a) only one active sender and only one active receiver operate at DCF mode within BSS. b) The channel is error-free or idea channel. c) In the transmitter end frames always must have wait for transmission. d) Each frame is in equal of size. Note that data frames can be transmitted at any data rate but control frames including ACK frame must be operated at predefined BSS rate set so that all stations within the same network understood the same control rate. For example, if the basic rate set is defined by the BSS is {12Mbps, 24Mbps, and 36Mbps} and data rate is 28 Mbps then the ACK frame transfer rate will be 24 Mbps. Now for IEEE 802.11a parameter set, the transmission time of data frame or MAC Protocol Data Unit (MPDU) with L byte payload

tSymbolmBsP

LSIGtPLCPletPLCPpreabTdata *)(

8/)616(28_

+++++=

tSymbolmBpS

LSIGtPLCPbletPLCPpreamTdata *)(

75.30_

+++= ………………………….(1)

and

tSymbolnBpS

SIGtPLCPbletPLCPpreamTACK *)(

8/)816(14_

++++=

tSymbolnBpS

SIGtPLCPbletPLCPpreamTACK *)(

75.16_

++= ……………………………..(2)

where BpS(m) and BpS(n) represent byte per symbol rate of data frame and ACK frame respectively. So the number of bytes in a symbol

BpS (m) = m/2 …………………………………………………… (3)

m is the PHY data rate in Mbps.

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If the data frames are transmitted at m Mbps in PHY and ACK frame are transmitted at n Mbps then the throughput performance of the system can be calculated as follows:

ACKdataBackoffthroughput TtSIFSTiTttDIFS

LnmT++++

=)(

*8),( ……………………(4)

Here (i-1)th retransmission attempt of random backoff time is defined as follows:

[ ] tSlotTimeCWCWiTti

Backoff *2

max,1)1min.(2min)(1 −+=

−

……………………………(5)

Random back off delay is the unit of slot time and this random integer is picked up from a uniform distribution of the interval [0,CW] where CW is the current contention window size. The initial contention window size is CWmin and maximum contention window size is CWmax. But in this case CW is always in CWmin. Because there is only one active sender and only one active receiver station is in BSS. The below figure 3.4 shows the analysis result of above analysis at 32Mbps data rate and ACK frame transmission rate is 24Mbps. We observe that the throughput performance is extremely poor when the payload size is small at constant data rate. For example, using 100 byte payload only achieves about 3Mpbs throughput over 54Mbps link. But the throughput performance is good almost 22Mbps at payload size 1500 byte.

0 500 1000 1500 2000 25000

5

10

15

20

25Throughput Vs Payload

Payload in Bytes

Thr

ough

put

32Mbps

Figure: 3.4 show Throughput performance vs. payload size at 32Mbps data rate.

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In figure 3.5 shows the throughput as a function of payload size at different data rate. In this figure, throughput increases when the data rate increase at fixed payload size. Suppose at fixed data payload size 500 Byte the throughput increases from 12Mbps to 17 Mbps when the data rate increases from 24Mbps to 54Mbps. Again we observe that at higher data rate the throughput performance is poor than the lower data rate. For example at 1000 byte packet size the data rate difference between first consecutive two curves is 15Mbps and the throughput difference between these two consecutive curves is 8Mbps. Again at 1000 byte packet size the data rate difference between 2nd and 3 rd consecutive two curves is 30 Mbps but the throughput difference between these two curves is 9Mbps. So it is easy to make a comment that the throughput wills independent of data rate after certain value which will more clear from below figure 3.6 and figure 3.7. In figure 3.6, shows the relationship between throughput and data rate at fixed payload size.

0 100 200 300 400 500 600 700 800 900 10000

5

10

15

20

25

30

35

40Throughput Vs Payload

Payload in Bytes

Thr

ough

put

9Mbps

24Mbps

54Mbps108Mbps

216Mbps

Figure: 3.5 shows throughput as a function of payload in bytes at different data rate. In figure 3.6 indicates that at higher data rate throughput are independent upon data rate even the data rate is infinitely high. We observe that the throughput almost the same for all data rate after 225Mbps at 200 byte fixed payload size. Another figure 3.7 shows throughput as a function of data rate at different payload size.

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0 50 100 150 200 250 300 350 4000

1

2

3

4

5

6

7

8

9

10Throughput Vs Data Rate

Data Rate in Mbps

Thr

ough

put

L=200 Byte

Figure 3.6 shows the throughput as a function of data rate at 200 Byte fixed payload size.

0 100 200 300 400 500 600 700 800 9000

5

10

15

20

25

30

35

40

45Throughput Vs Data Rate

Data Rate in Mbps

Thr

ough

put

100 Byte

300 Byte

500 Byte700 Byte

900 Byte

Figure 3.7 shows the throughput as a function of data rate at different payload size.

39

In figure 3.7 shows the five curves for five-payload size. Payload sizes are 100 bytes, 300 bytes, 500 bytes, 700 bytes and 900 bytes. If we compare between the curves of 100 bytes payload and 900 bytes payload then we see that the throughput of the curve of 100 bytes payload is independent with data rate above 300 Mbps where as the throughput of the curve of 900 bytes payload is independent with data rate 650 Mbps. In case of small payload, throughput is really suffering or almost independent from the data rate. 3.3 Throughput Enhancement Schemes There are lot of existing throughput enhancement schemes. Some of them are PHY layer based and some of them are MAC layer based. PHY layer enhancement only deals with data rate already we mention previously. Some higher data rate schemes of IEEE 802.11 has proposed, such as Bit Loading (BL) approach [11], the double clock rate approach [19], the double sub carrier number approach [19], the 4096-QAMOFDM approach [19] and the OFDM/SDM, Multiple Input and Multiple Output (MIMO) system approach [19] etc. But already we observed from above section that data rate is not the main parameter to achieve maximum throughput. Recently new Task Group (TG), called TGn, was established within the IEEE 802.11 standardization Working Group (WG) with the goal of developing a Higher Throughput extension of the current WLAN standard. The main goal of this group is to achieve the throughput performance of over 100 Mbps, which is approximately 4 times higher than current IEEE 802.11a and IEEE 802.11g WLANs.[4] So working group is perusing IEEE 802.11n for higher throughput instead of higher data rate at MAC and PHY enhancement. In this chapter, we shall discuss some potential proposed schemes for throughput enhancement of IEEE 802.11 especially we focused on overhead reduction method in the next section. Here we introduce and propose following efficient MAC strategies Direct Link Protocol (DLP): DLP was proposed in IEEE 802.11e [10]. In the legacy standard, if SATs are operating in infrastructure mode, all stations communicate each other’s via AP. When the communication is only between two stations, which are belongs to the same BSS then channel resources are wasted. In this case in order to improve efficiency, a direct link between peer stations is allowed in 802.11e. DLP creates an active direct link between two stations after exchanging all necessary information like transmission rate sets, security elements between them. Block Acknowledgement Protocol (BAP): BAP was proposed in IEEE 802.11e [10]. It is also called Burst Transmission and Acknowledgment (BTA) [3], Contention-Free Burst (CFB) [14], Group Transmission and Acknowledgements (GTA) [15], Delayed Group Acknowledgement (DGA) and etc. The key idea is that not positive acknowledgement for each transmitted frames from receiver rather than first transmitted all frames to receiver then acknowledge them all by single frame. We will discuss this BTA mechanism in below section in detail. Aggregation [4]: There is lot of small frames transmits on Internet. Some times the MAC and PHY overhead much bigger than the original payload size. In this case, multiple small frames are aggregated with each others and then transmit. Remember that the aggregated frame size is not bigger than the threshold of the channel quality. Frames can be concatenated if they are available, and have the same source and destination addresses. In below section we discuss it in detail.

40

Without Acknowledgement: Where frame loss is tolerable but delay is very sensitive. In thus case Without Acknowledgement schemes is applicable. For some applications such as frames with audio UDP packets as payload, delay is much more important, whereas some frame-loss is tolerable. Therefore, we can adopt ‘without acknowledgement’ mechanism to become more efficient. Piggyback: The idea of the piggyback mechanism is that a receiver station is allowed to piggyback a data frame to the sender station once if the receiver station has a frame to send to the sender. This type of mechanism is very useful for immediate acknowledgement such as TCP ACK packets. If a TCP receiver successfully received a packet from a TCP sender, then it is essential to send an acknowledgement packet back to the TCP sender immediately. But if the acknowledgement packet from TCP receiver can’t access the media channel immediately, then this delay of TCP acknowledgement causes retransmission from TCP sender again. This proposed piggyback mechanism can remove this above drawback of retransmission from TCP sender by piggybacked acknowledgement to the sender immediately. So the overall performance of the system will be improved greatly. 3.3.1 Frame Aggregation The frame aggregation method is very simple to enhance the system throughput of IEEE 802.11 wireless networks. The frame aggregation is to aggregate the multiple frames into a single MAC frame until the aggregated frame is not bigger than the threshold of channel quality. So the impact of the fixed overheads is relatively reduced and enhances the performance of throughput, which was analyzed earlier. The task of the frame aggregation can be made simpler by performing frame aggregation above the MAC SAP, which lies between MAC and LLC.[4] . So it is not necessary to modify the current MAC and PHY layer. We can aggregate multiple IP packets as a single supper frame, which is smaller than transmission threshold above the MAC layer. So MAC layer can deals with this aggregate super frame as ordinary MSDU. Figure 3.8 shows the original MAC frame and Figure 3.9 shows the MAC aggregate supper frame.

Figure: 3.8 shows the original MAC frame with LLC layer contains. In this figure 3.8, the existing MAC frame where the Data field consists only one IP packet, LLC header, and Sub Network Access Protocol (SNAP) header [17]. When the upper layer LLC frame forwarded to the MAC layer, a MAC header and 32 bits (4 octets) FCS are added with this data frame. So there are fixed amount of overhead add with each data frame and the throughput really suffering if the data packet is relatively small. In figure 3.9, shows that how the multiple Ether- frames aggregate in a single MAC frame. To do this some new fields add with original data frame. Those fields are Count fields and Size fields, which mention the number of aggregation packets and size of the corresponding packets respectively. The

41

aggregated supper frame is send to the MAC as a normal MSDU like above. Here we mention that all packets have the same source and destination address and the MAC aggregate frame must be smaller than the threshold∗ value of channel quality.

Figure 3.9 shows the MAC aggregate supper frame as a single MSDU. Now we will analysis the packet size distribution for all IP packets. Figure 3.10 shows the packet size distribution which chart is from the measurement taken in the IEEE 802.11 standard meeting room in the mooring of July 22nd, 2003. Form this pie chart, we see 49.40 % packets are below 64 bytes and 26.45 % are within 64 to 127 bytes. From this it can be seen that over 70% of packets are small, under 128 bytes. It is about 15 % packets are over 1024 bytes. So small size packets are dominating over big size packets and degrade the performance of total system throughput.

Figure: 3.10 show the packet size distribution for all IP packets.∗

Now we will try to evaluation of the performance of frame aggregation schemes. To evaluate the performance of the frame aggregation schemes we assume the following assumptions.

∗ The length of concatenation should be smaller than a threshold [9].

<64

64-127

128-255

256-511512-1023

1024-2024

49.40 % = <64 Byte

26.45 % = 64-127 Byte

6.27 % = 128-255 Byte

2.38 % = 256-511 Byte

1.17 % = 512-1023 Byte

14.31 % = 1024-2047 Byte

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There is only one active station in the BSS and assume that channel is ideal i.e no collision will be occurred during the transmission. Due to one active station is present in BSS so we do not use RTS/CTS mechanism in this simulation. We follow the above packet size distribution in figure 3.10, which is published at IEEE standard meting. In paper [4], they assume equal size of packets for the shake of simplicity to simulate this frame aggregation scheme, which is not so realistic. Here we try to simulate this frame aggregation schemes as above packets size distribution. Again we assume that packets are always available in sender buffer to be transmitted.

0 50 100 150 200 250 300 350 400 450 5000

5

10

15PHY data rate vs Throughput(Without Frame aggregation)

Data rate in Mbps

Thr

ough

put

Figure: 3.11 indicates data rate verses throughput graph without frame aggregation schemes. In figure 3.11 shows the throughput in various PHY data rate when the packets size is selected as random nature with uniform distribution. Here the slope of first 50 Mbps PHY data rate is very large but after that the slope decreases fast with data rate. This indicates that throughput does not depend on very much at higher data rate. In lower data rate it has very good effect on throughput. In figure 3.12 shows the PHY data rate verses throughput with frame aggregation schemes. In this figure we noticed that throughput improved much more as compare with figure 3.11 that is without frame aggregation schemes. Another thing is that throughput depends all over the range of PHY data rate equally. i.e the slope of this curve are almost equal over the range of PHY data rate. If we compare these two figures, In 100 Mbps PHY data rate throughput is about 17 Mbps without frame aggregation schemes where as in frame aggregation schemes the throughput is about more than 45 Mbps at the same PHY data rate.

43

0 50 100 150 200 250 300 350 400 450 5000

10

20

30

40

50

60

70

80PHY data rate vs Throughput(With Frame aggregation)

Data rate in Mbps

Thr

ough

put threshold 2304

Figure: 3.12 show the data rate vs throughput with frame aggregation when threshold 2304.

0 50 100 150 200 250 300 350 400 450 5000

10

20

30

40

50

60

70PHY data rate vs Throughput(With Frame aggregation)

Data rate in Mbps

Thr

ough

put

threshold 2000

Figure: 3.13 shows data rate verses throughput with frame aggregation when threshold 2000

44

To above simulation of frame aggregation schemes in figure 3.12 we consider the threshold value of channel quality is 2304 octets. But the throughput could vary for more realistic traffic situations. The practical maximum packet size is about 1500 octets, which is bounded by the maximum size of Ethernet frame, while the maximum MSDU of IEEE 802.11 is 2304 octets. If we consider the threshold values of channel quality are less than 2304 octets, which decrease the system throughput. In figure 3.13 we simulate data rate verses throughput when the threshold vale is 2000 octets. To do that we assume no fragmentation will occur i.e no Ethernet frame is as bigger than 2000 octets. 3.3.2 Burst Transmission and Acknowledgement (BTA) In IEEE 802.11 MAC protocols, each transmitted frame is acknowledged from receiver before transmitting again. This is robust and natural but introduce quite amount of overhead which decrease the system throughput. In order to reduce acknowledgement overhead a new mechanism is introduced. This is Burst Transmission and Acknowledgement (BTA). In the BTA mechanism, before any acknowledgement a series of data frames which is called burst frames to be transmitted. The sender sends a burst acknowledgement request (BurstAckReq) frame after sending a burst of frames, and the receiver must respond it by sending the burst acknowledgement (BurstAck) frame. The burst acknowledgement frame is the information frame of all correctly received data frame. All the frames are separated by an SIFS period. In figure 3.14 shows the basic mechanism of BTA. Here we mention that typical BTA sequence is working on contention-free period.

Figure 3.14 shows the basic mechanism of BTA. Before sending burst frames first the sender have to win a transmission opportunity (TXOP)

using a channel access mechanism. A TXOP is time duration between the starting time of transmission onto the Wireless Media (WM) and maximum time for the station. The TXOP mechanism is specified in the IEEE 802.11e MAC enhancements for QoS [10]. Figure 3.15, figure 3.16 and figure 3.17 shows the BurstAckReq frame, BTA MAC header and BurstAck

frame respectively.

Figure: 3.15 BTA burst acknowledgement request (BurstAckReq) frame.

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The receiver controls the length of each burst and the amount of burst. In figure 3.16 we see that the MAC format is a little different from the original MAC data frame. The length of Sequence Control (SC) field is 2 octets and there are two subfields of SC field. One is fragment number which is 4 bits another is Sequence Number. Therefore, the maximum number of fragmented MSDU is 16 from one original MSDU. If the size of the original MSDU is larger than the fragmentation threshold then this frame fragmentation will occur. We have already mentioned it in the previous section. Another important field is 32 octets Ack Bitmap field, where each bit can acknowledge one fragment frame (MPDU). It is very difficult to say in prior that the MSDU will be fragmented or not.

Figure: 3.16 show the MAC header of BTA frame.

Figure: 3.17 show the burst acknowledgement (BurstAck) frame. To acknowledge one MSDU, 2 octets in the Ack Bitmap field are needed. So the Ack Bitmap field can acknowledge up to 32/2=16 MSDU's. The sequence control field defines an MPDU sequence number equal to (Sequence Number * 16) + Fragment Number. If the BurstAck indicates that there are missing some of MPDU and not received correctly then the sender will retry that MPDU. Note that the MPDU’s is within the appropriate retry limit. The receiver will maintain a burst acknowledgement records and as well as corresponding transmitter address. In order to get the maximum throughput, we assume the following assumptions. Firstly, all MSDU’s are less than or equal to fragmentation threshold so that no fragmentation will occur. Because more fragmentations will introduce more overhead including frame headers and SIFS spaces, which decrease the throughput. Secondly, the burst length and the number of bursts, the higher the throughput is, since the acknowledgement overhead is saved much more when the number of frames in a burst is larger. Thirdly, the channel is an ideal channel without errors, and all stations are within radio range. Finally, we assume that the TXOP needs zero overheads to obtain. Such an assumption potentially increases the maximum throughput a little bit, makes equation derivations simpler, and avoids the introduction of the whole QoS scheme and MAC enhancement for IEEE 802.11 [12]. Let the CF-Poll (without data) frame is a zero length data frame. Now we try to evaluate the throughput of BTA mechanism with above following assumptions. Let Lr = 22 denote the size of the burst acknowledgement request (BurstAckReq) in bytes according to figure 3.15, La = 56 denote the size of the burst acknowledgement (BurstAck) in bytes according to figure 3.17, and Tr denote the time

46

required to transmit the burst acknowledgement request frame, Ta denote the time required to transmit the burst acknowledgement frame then we have following equation for Maximum Throughput (MT)

)2(**8

+++++=

NbTsifsTaTrTdataNbTpoNbLdataMT ……………………………..(6)

and Minimum Delay (MD)

TsifsTdataMD += ………………………………………………………………….(7) Where Tpo and Nb denote that the time required transmitting contention free poll (CF-Poll) and number of bursts respectively. According to paper [3] for the BTA mechanism, a theoretical throughput upper limit (TUL_BTA) and a theoretical delay lower limit (DLL_BTA) exist. The TUL_BTA and DLL_BTA are independent of data rate (even data rate goes infinitely high). TUL_BTA and DLL_BTA are define as follows

)2())(3(*8_

++++=

NbTsifsTphyTpNbNbLdataBTATUL ……………………………………(8)

and

TsifsBTADLL =_ …………………………………………………………………….(9) For the shake of simplicity we assume that all bursts are equal in length and the data rate and control rate are equal.

47

Figure 3.18 shows the BTA throughput verses payload when the data rate are different. If we compare between figure 3.18 and figure 3.5, we see the big improvement of throughput using BTA mechanism. For example at 500 fixed payload sizes, the throughput of normal access mechanism and BTA mechanism are 19Mbps and 47Mbps respectively. It is about 55% throughput gain is possible using BTA mechanism.

0 100 200 300 400 500 600 700 800 900 10000

10

20

30

40

50

60

70

80

90

100BTA Throughput Vs Payload

Payload in Bytes

Thr

ough

put

9Mbps

24Mbps

54Mbps108Mbps

216Mbps

Figure: 3.18 show the Maximum Throughput (MT) verses payload in bytes. 3.4 Summary The efficiency of IEEE 802.11 MAC protocols is based on its design. There are lot of parameters involved in this design. Throughput is one of them. Overhead is the fundamental issue in case of MAC throughput. Because the throughput are almost independent of PHY layer data rate when data rate goes infinitely high. For enhancement of throughput, MAC overhead must be reduced. There are lot of MAC overhead exist in IEEE 802.11 protocols. MAC headers, FCS, PHY preamble, PHY headers, Inter-frame space, back off time, Ack frames are denoted as overhead. In overhead analysis section, we provided an overhead study for efficient MAC strategy of IEEE 802.11a extension. The existing MAC protocols show that throughput is notoriously poor as compare with PHY data rate. There are lot of proposed schemes for throughput enhancement of IEEE 802.11. Most of them are regarding PHY layer data rate. But we saw earlier that only PHY data rate is not very important parameters for MAC enhancement. Therefore, in this chapter we discussed two schemes especially on overhead reduction of existing MAC protocol. One is Frame Aggregation and another is Burst Transmission and Acknowledgement (BTA). In frame aggregation scheme in order to reduce the MAC/PHY overheads of IEEE 802.11 WLAN by aggregating multiple packets into a single MAC fame. Because the throughput are really suffers due to small size of packets.

48

Important information is that small size IP packets are dominating the all other packets. It is about 50% Internet packets are below 100 bytes. That is why frame aggregation scheme may be an important component of IEEE 802.11n. The frame aggregation can be performed above the MAC SAP very easily with simple device driver modifications. In existing MAC protocols, all transmitted frame are acknowledged by the receiver end. This issue is about extra overhead. Due to reduce acknowledgement overhead Burst Transmission and Acknowledgement (BTA) scheme is proposed. We saw our performance study of BTA that the overhead has been greatly reduced and the performance has been greatly improved.

49

Chapter 4 Proposed Throughput Enhancement Schemes 4.1 Introduction Delivering for multimedia applications like High-Definition Television (HDTV), DVD are the main goal of IEEE 802.11WLAN. By virtue of MIMO technology PHY layer data rate is sufficiently high. Some IEEE 802.11n proposal expects to support 600Mbps [21]. It has been shown that MAC utilizes less than 20% of the transportation ability provided by the PHY layer.[22] There are two main dimension of enhancement of IEEE 802.11 MAC protocol. One is the MAC throughput and another is Quality of Services (QoS). Throughput is notoriously poor due to some overhead of MAC and PHY layer though the PHY data rate is sufficiently high. QoS is the important issue for delay sensitive multimedia traffic. In this chapter we will introduce a new MAC mechanism that reducing overhead for enhancement of throughput. To do so we consider IEEE 802.11e MAC legacy, which provide QoS support in WLAN networks. The IEEE 802.11e supplement introduces the hybrid coordination function (HCF) as the medium access control scheme. It is the combination compatible with the legacy 802.11 DCF and PCF and provides stations with prioritized and parameterized QoS access to the wireless medium. HCF is the combination of both the contention-based and the contention free access methods, where the contention-based channel access mechanism is called as the enhanced distributed channel access (EDCA) and its contention-free counterpart is known as the HCF Control Channel Access (HCCA). EDCA has been designed for support of prioritized traffic similar to DiffServ, whereas HCCA supports parameterized traffic similar to IntServ Here in this chapter we will concentrate on EDCA access. Two important MAC overhead reduction components that we have discussed earlier (frame aggregation and BTA) are combined together to improve MAC efficiency in our throughput enhancement proposal. 4.2 Overview of Extended DCF (EDCF) In IEEE 802.11 MAC has two mode of access the wireless medium as we mention previous chapter. An optional second mode, Point Coordination Function (PCF), supports time-sensitive traffic flows in wireless medium. In infrastructure mode operation, wireless access points periodically send beacon frames to communicate network identification and management parameters specific to the wireless network. Between two consecutive beacon frames, PCF divide the time into a contention-free period and a contention period. With PCF enabled, a station can transmit data during contention-free polling periods. Due to unpredictable time of transmission, PCF is not accepted widely. It is very difficult to distinguish traffic types or sources before entering the DCF and PCF access mode. So the IEEE developed enhancements in 802.11e to both coordination modes to facilitate QoS. The enhancement to DCF - Enhanced Distribution Coordination Function (EDCF) - introduces the

50

concept of traffic categories. Each station has eight traffic categories, or priority levels. Using EDCF, stations try to send data after detecting the medium is idle and after waiting a period

Figure: 4.1 Access mechanism of EDCA. of time defined by the corresponding traffic category called the Arbitration Inter frame Space (AIFS). Figure 4.1 show that the access mechanism of EDCF. Graphical representation of Access Categories (AC) with the parallel backoff entities is shown in Figure 4.1. A low-priority traffic category will have a bigger AIFS than a high-priority traffic category. Thus AIFS time duration will differentiate various types of traffic category. Stations with higher-priority traffic must wait shorter than those with lower-priority traffic to access the medium. To avoid same category internal collisions, the station counts down an additional random number of time slots, known as a contention window, before attempting to transmit data. If another station transmits before the countdown has ended, the station waits for the next idle period. Then it continues the countdown until it zero. EDCF establishes a probabilistic priority mechanism to allocate bandwidth based on traffic categories. One of the crucial concepts introduced by 802.11e draft is Transmission Opportunity (TXOP) defined as an interval of time when a particular 802.11e station (referred to as QSTA) has the right to initiate transmissions onto the wireless medium. A TXOP is defined by a starting time and a

51

maximum duration. Transmission opportunity obtained via contention based medium access is referred to as EDCA TXOP. The duration of EDCA TXOP is limited by a QoS Basis Services Set (QBSS) -wide TXOP limit. This EDCA TXOP limit is indicated in beacon frames, but in contention free period the duration of polled TXOP is specified in the poll frame. Within the limits of each EDCA TXOP, a QSTA is allowed to transmit multiple frames from the same AC with a SIFS gap between an ACK and the subsequent frame transmission as depicted in the Figure 4.3. Figure 4.2 shows the basic access mechanism of EDCA technique.

Figure 4.2 EDCA access mechanisms.

Figure 4.3 EDCA TXOP frames transmissions The 802.11e protocol also defines a maximum MSDU lifetime per AC, which defines the maximum time a frame can remain in the MAC state. Once the maximum lifetime has expired then the frame is dropped from MAC without transmission. This feature can be useful in real time applications.

52

4.3 Block Acknowledgement schemes in EDCA

In IEEE 802.11e, there are two types of channel access procedure. One is contention based EDCA and another is hybrid coordination HCCA with contention based access mechanism. In EDCA procedures, it is not necessary to transmit single MSDU after accessing channel. The station, which wins the contention gains, can transmit one or more MPDUs (eventually belonging to different MSDUs) within the TXOP duration. We have already discussed BTA mechanism in the previous chapter and also evaluated its performance. In this section we will adopt Block Acknowledgement (BA) schemes in EDCA of IEEE 802.11e. The crucial feature of IEEE 802.11e is TXOP. TXOP is defined by the starting time and the maximum duration. The starting time is represented by the time when the medium is determined to be available under the EDCA access rules; the maximum duration is limited by a threshold, called TXOP limit. In EDCA rules fragmentation is allowed if whenever the MSDU transmission duration length is more than the TXOP limit. But in our scheme we do not allow fragmentation due to introduce more overheads. In order to reduce ACK overhead a new block acknowledges a scheme is introduced in the 802.11e. Block Acknowledgement is important feature of the new IEEE 802.11e standard that can improve channel efficiency. Basically, the block ACK mechanism allows a block of MPDUs to be transmitted, each data burst separated by a SIFS period, and to be acknowledged by a final aggregated ACK frame, called block ACK. The block ACK contains detail information about the reception of the whole block via a corresponding acknowledgement bitmap, and this aggregated ACK frame is transmitted after an explicit transmitter request. This request is performed through a new control frame, called block ACK request frame. Note that both the block ACK request and the block ACK control frames are transmitted with same data transmissions rate. The transmitter can terminate the block ACK agreement by sending specific notification frame to the receiver. Figure 4.4 show the block acknowledgement features with EDCA in IEEE 802.11e.

Figure 4.4 Block acknowledgement schemes in EDCA mode. The maximum number of frames in the block is specified in the setup phase, according to the recipient reordering buffer size up to the number of MPDUs that can generated by 64 MSDUs (i.e., 16 *64 = 1024). Note that the block size does not depend on the TXOP limit. In fact, the originator can split the MPDUs belonging to the same block across different TXOPs.

53

Therefore, a data block can be composed of several data bursts. The block ACK request is finally transmitted in the same TXOP, which conclude the block, or in a subsequent TXOP. As a normal ACK, the block ACK reception is expected before the expiration of the block ACK timeout. If there is no response, the block ACK request is retransmitted until the maximum retry limit is reached. In order to quickly identify collisions and to reduce the probability of other stations transmitting during each TXOP, the head-of-burst (HOB) frame of each new data burst requires to be protected with an immediate ACK. The data burst protection mechanism can also be based on the RTS/CTS frame exchange. In fact, after the successful reception of the CTS frame, no other station can try to access the channel during the time interval specified in the RTS/CTS duration fields. 4.3.1 Assumptions and Analytical frame works There are two types of Block Ack mechanisms: immediate and delayed. Immediate Block Ack is suitable for high-bandwidth, low latency traffic while the delayed Block Ack is suitable for applications that tolerate moderate latency. In this section we will try to calculate maximum throughput of both immediate block ack and delayed block ack schemes. Regarding this we will take the following assumptions a) channel are error free or ideal b) all frame are small enough that does not support fragmentation, because fragmentation introduce more extra overhead which decrease the system throughput c) Only one active transmitter station and only one active receiver station in the radio range so that no collision will occur. d) Frames are always available in the transmitter buffer queue. For the shake of simplicity we again assume that all frames are in equal in length. For immediate acknowledgement schemes, If an AC is allocated a TXOP with time duration Ttxop and the size of MPDU is Ldata in bytes, then the number of MPDU that fit within TXOP limit is calculated by

…………………………………………..(1)

Where Tmpdu and Tack are the time required to transmit a single MPDU and Acknowledgement respectively. So the throughput of immediate acknowledgement

txopslottime

datathroughput TTCWAIFS

NLT

+×+××

=8

……………………………………….(2)

Where CW is the mean of Contention Window and Tslottime is the time duration of each slot. For delayed acknowledgement, we assume that the BlockAckReq(BAR) frame and BlockAck (BA) frame are in the same TXOP. Figure 4.4 shows the frame exchange sequence. So the number of MPDU in TXOP is given by

+−×+−

=SIFST

TSIFSTTN

mpdu

babartxop 2……………………………………………….(3)

and the throughput is given by

+×+

=TackSIFSTmpdu

TtxopN2

54

txopslottime

datathroughput TTCWAIFS

NLT

+×+××

=8

…………………………………………..(4)

Where Tbar and Tba are the time duration to transmit BlockAckReq (BAR) and BlockAck (BA) respectively. In next chapter, we will see the performance graph of above both immediate and delayed acknowledge schemes. In next chapter we will evaluate the performance of block acknowledgement schemes in EDCA. 4.4 Frame Aggregation and Block Acknowledgement (FABA) schemes 4.4.1 Introduction Earlier this year, IEEE Working Group meeting approved the first draft of IEEE 802.11n [33], in which the data rate is expected to be as high as 600Mbps. Also the throughput performance is the main objective of the upcoming IEEE 802.11n standardization. For multimedia applications, it is recommended that throughput must be greater than 100 Mbps as measured at the MAC SAP (Service Access Point), i.e., the interface between the MAC and the higher layer. Industry also seeking better QoS at the MAC layer. A QoS amendment of IEEE 802.11 MAC, IEEE 802.11e [10], aims to provide service differentiations to different traffic types. Due to MIMO-OFDM technique, now the physical layer data rate (PHY data rate) is sufficiently high as compare with MAC throughput. So researchers are trying to enhance the efficiency of MAC throughput as well as QoS. Already there are lot of MAC enhancement proposals discussing in IEEE standardization committee and some of them take as standardization. We already mention some possible components of upcoming IEEE 802.11n standardization. The 802.11n specification adopts the MAC schemes to ensure backward compatibility with existing IEEE 802.11 specifications. In the section, we propose a new medium access control (MAC) scheme for the next-generation high-speed WLANs such as IEEE 802.11n, which is the combination of two important components and also try to evaluate the throughput performance of this new MAC mechanism in the next chapter. 4.4.2 Mechanism of Frame Aggregation and Block Acknowledgement (FABA) Schemes

The poor performance upcoming of wireless LAN compare to wire network is low throughput. The mainly reason is that the large amount of overheads add with the MAC and PHY layers, we already know from above section. In this scheme, we pay attention to improve the throughput performance of wireless LAN by reducing MAC overheads. To ensure the QoS, we use EDCA part of legacy IEEE 802.11e.

Both the frame aggregation and block acknowledgement (BA) are popular schemes for reducing MAC and PHY layer overheads. In section 3.3.1 and 3.3.2 of chapter 3, we have already discussed these both frame aggregation and block acknowledgement schemes respectively and also evaluated the throughput performance. Individually the throughput performance of these schemes is highly satisfactory. To achieve higher throughput for IEEE 802.11n standard, we combine the above two popular components in our proposed FABA schemes.

55

To describe the mechanism, we will consider three phases of FABA scheme. Firstly frame aggregation phase, secondly channel access phase then finally block acknowledgement phase. In frame aggregation, multiple small frames aggregate into a single MAC frame up to threshold value. There are four possible aggregation mechanisms: A-MSDU, A-MPDU, A-PPDU, and PPDU bursting. They belong to different levels: MSDU aggregation is between LLC (Logical Link Control) and MAC, MPDU aggregation is between MAC and PHY, and PPDU aggregation and PPDU bursting are at PHY [34]. The task of frame aggregation can be made simpler if it performs above the MAC SAP. Even without any change of hardware, it can be implemented. It is possible to implement by changing only devices driver. Above MAC SAP, we can aggregate multiple IP packets into a single chunk, and send it to the MAC as a single frame or more exactly MAC Service Data Unit (MSDU). Therefore, the MAC layer can deal with the aggregated frame as a normal MSDU [4]. Figure of the frame aggregation scheme is shown in figure 3.9 of the previous chapter. From figure 3.10 shows that IP packet size distribution. The measurement was taken in the IEEE 802.11 standard meeting room in the mooring of July 22nd, 2003. Form this pie chart, we see 49.40 % packets are below 64 bytes and 26.45 % are within 64 to 127 bytes. From this it can be seen that over 70% of packets are small, under 128 bytes. It is about 15 % packets are over 1024 bytes. So small size packets are dominating over big size packets and degrade the performance of total system throughput. In frame aggregation phase, it is obviously increase the throughput performance because of greatly reduce PHY and MAC overhead. In access to the wireless media phase, we consider the EDCA backoff procedure of IEEE 802.11e standard to ensure the QoS. By adopting this mechanism, the aggregated frames will have the same destination address and same level of QoS priority. First small frames are aggregated above MAC-SAP, which has same distination address and same level of QoS priority then this aggregated frame contend for the backoff priode. The duration of the backoff periode are vary in accroding to AC of EDCA mechanism because of the arbitration inter frame spacing (AIFS), is different for each AC. In below figure 4.5 show the total FABA mechanism.

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Figure: 4.5 Frame aggregation and Block Acknowledgement (FABA) Schemes. Each AC content its backoff duration as like individuals STA, which is competing with each other’s and access channel. In IEEE 802.11e, there is a mechanism, burst transmission, provided for STA to transmit a number of frames in a TXOP. While it provides an approach to save the contention time of each frame, there are still overheads in each transmitting frame [31]. Transmitting longer frames may have a better throughput than transmitting shorter frames, adopting the concatenation mechanism; the system can achieve the throughput of transmitting longer frames. Another overhead the contention time of each frame will remove using burst transmission mechanism. But an important overhead, like acknowledgement overhead should be removed. So do that Block Acknowledgement mechanism is introduced in FABA scheme. We have seen the BTA mechanism in the previous chapter. In TXOP duration, multiple frame can transmit with single backoff periode. When all frames transmitted, a special frame called BrustAckReq frame (BAR) frame transmitted to the receiver so that let the receiver knows burst transmission is ended and acknowledgement should be sent to the transmitter via BrustAck frame(BA). Figure 4.6 and figure 4.7 shows the frame format of BrustAckReq frame (BAR) and BrustAck frame(BA) respectively. There are two ways the block acknowledgement schemes can be implemented. One is immediate acknowledgement and another is delayed acknowledgement. We already discussed those two types block acknowledgement in section 4.3.1. For some multimedia applications such as VoIP, immediate block acknowledgement is preferred because of short delay and jitter and those multimedia applications where throughput is more important than delay and jitter, delayed block acknowledgement is preferred. However in both case, overhead is greatly reduced and consequently improvement of throughput is high. Both BurstAckReq (BAR) and BurstAck (BA) frame are transmitted at the same rate used for data transmissions.

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BrustAckReq frame (BAR)

Burst Ack Request Bits:0-3 4-15

FC DU RA TA

TID R

CRC

Figure: 4.6 BurstAckReq frame from sender to receiver. BrustAck frame(BA)

Burst Ack Control Bits:0-3 4-14 15

FC DU RA TA

TID R W

SC BM CRC

Figure: 4.7 BurstAck frame from receiver to sender. 4.4.3 Algorithm for FABA schemes. Pseudo code of FABA Schemes if (all frames have same source and destination address) Aggregation Phase: Initialisation, i=0; aggre_frame=0; while(aggre_frame< fragmen_thresh) if (frame[i]+aggre_frame < fragmen_thresh and have same level priority of QoS ) aggre_frame= aggre_frame+frame[i] i=i+1; do Contention Phase: According to QoS priority level Select AC (Access Category) switch ( QoS priority level) case AC0: AIFS [AC0]; cwindow( ); break;

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case AC1: AIFS [AC1]; cwindow( ); break; case AC2: AIFS [AC2]; cwindow( ); break; case AC3: AIFS [AC3]; cwindow( ); Block Acknowledgement Phase: if (Immediate acknowledgement) after SIFS duration block acknowledge for current aggregate frame; if (delayed acknowledgement) after transmitting all bursts within TXOP block acknowledgement for whole bursts. 4.5 Summary In this chapter, we discussed about extended DCF (EDCF) and block acknowledgement schemes based on IEEE 802.11e EDCA mode. Also a new throughput enhancement MAC mechanism is discussed, called Frame Aggregation and Block Acknowledgement (FABA). In this MAC enhancement schemes two important MAC enhancement components are combined with each other based on EDCA of IEEE 802.11e. Overhead can greatly negative impact on MAC efficiency. Overhead must be reduced to improve MAC efficiency. FABA mechanism reduce MAC overhead, IFS overhead, contention overhead, PHY preamble and ACK overhead. Frame aggregation provides an excellence mechanism to reduce PHY and MAC overhead due to small size packet. Again block acknowledgement reduce acknowledgement overhead. So this FABA mechanism greatly improves MAC performance by reducing overhead.

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Chapter 5

Performance Evaluation using simulation 5.1 Introduction The previous chapter we described the access mechanism of IEEE 802.11e and block acknowledgement schemes. Also we introduced a new mechanism for throughput enhancement, which is called FABA schemes. This is the combination of possibly two important components of IEEE 802.11n. Here we will try to evaluate the performance of legacy IEEE 802.11, frame aggregation, BTA schemes and also the new FABA mechanism using simulation. All above models are implemented through computer code written in the Matlab 7.1. To model these schemes we consider some necessary assumption in the next section 5.2 and simulate these results in the section 5.3. However, it is important that how the performance of FABA schemes is better than others above schemes. 5.2 Necessary Assumptions For the calculation of maximum theoretical throughput and evaluate the performance of legacy IEEE 802.11 and enhanced FABA schemes, we will consider the following assumptions ○ there is only one active station in the radio range. No other station within the BSS. ○ all packets have the same source and destination address. ○ Packets are always available in the buffer queue. ○ channel is always error free (i.e ideal). So there are no collisions during transmission. ○ all packets are small enough that no fragmentation will be allowed.(i.e packets are smaller than the fragmentation threshold)

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5.3 Simulating Result

5.3.1 Frame aggregation schemes

0 5 0 0 1 0 0 0 1 5 0 0 2 0 0 00

1 0

2 0

3 0

4 0

5 0

6 0

7 0Th ro u g h p u t V s P a y lo a d

P a y lo a d in B y te s

Thr

ough

put

9 M b p s

2 4 M b p s

5 4 M b p s1 0 8 M b p s

2 1 6 M b p s

Figure: 5.1 Frame aggregation schemes, Payload vs Throughput at different PHY data rate

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0 100 200 300 400 500 600 700 800 9000

10

20

30

40

50

60

70

80

90Throughput Vs Data rate

Data rate in Mbps

Thr

ough

put

Payload 100 Bytes

Payload 500 Bytes

Payload 1000 BytesPayload 1500 BytesPayload 2000 Bytes

Figure: 5.2 Frame aggregation scheme, PHY data rate vs Throughput at different payload in byte.

0 100 200 300 400 5000

5

10

15PHY data rate vs Throughput(Without Frame aggregation)

Data rate in Mbps

Thr

ough

put

Random packet size with uniform distribution

Figure: 5.3 show the PHY data rare vs Throughput where no frame aggregation scheme is implemented. Random packet size is chosen.

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5.3.2 BTA or Block Acknowledgement Scheme This below simulation is block acknowledgement scheme in EDCA mode of IEEE 802.11e. Here we assume that only one AC is active. i.e all traffic are in same access priority .

0 500 1000 15000

10

20

30

40

50

60

70

80

90

100

9 Mbps

24 Mbps

54 Mbps108 Mbps

216 Mbps

Figure: 5.4 show Payload in byte vs Throughput at different data rate. 5.3.3 FABA Schemes Here in simulation, we aggregate all packets up to the threshold value. Due to randomize packets length the following graph does not show smooth curves.

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0 100 200 300 400 500 600 700 800 900 10000

50

100

150

200

250PHY data rate vs Throughput(FABA Schemes)

Data rate in Mbps

Thr

ough

put

Figure 5.5: show the PHY data rate vs Throughput.

0 100 200 300 400 500 600 700 800 900 10000

20

40

60

80

100

120

140

160

180

200PHY data rate vs Throughput(FABA Schemes)

Data rate in Mbps

Thr

ough

put

Data Burst 1

Data Burst 2

Data Burst 3Data Burst 4

Data Burst 5

Figure: 5.6 show the PHY data rate vs Throughput at different number of Burst within TXOP duration.

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5.4 Summary In this chapter, we simulate some proposed schemes and also simulate a new MAC enhancement scheme. To simulate those we first consider some assumptions in the section 5.2. In figure 5.1, figure 5.2 and figure 5.3, we noticed that the throughput performance of block acknowledgement scheme and frame aggregation scheme are much better than the legacy existing MAC protocols. From figure 5.2, another observation is that the throughput performance is independent with PHY data rate. Especially small packet sizes are more suffer than the bigger packets size. For example, 100, 500 and 1000 byte payloads are independent at 100, 600 and 700 Mbps data rate respectively. However those above two schemes are not sufficient enough for multimedia applications. Those both two schemes provide below 100 Mbps throughput. But industries are seeking above 100 Mbps. Here we proposed a new MAC enhancement scheme is called FABA, which is able to provide as sufficient enough for multimedia applications. Figure 5.5 and figure 5.6 shows the simulation of FABA scheme. Figure 5.5 shows the PHY data rate versus Throughput of FABA scheme and figure 5.6 shows PHY data rate versus throughput at different number of burst within TXOP duration.

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Chapter 6 Conclusions and Future Work 6.1 Conclusions This research mainly focuses on the throughput enhancement of legacy IEEE 802.11 MAC protocol. The two parameters, throughput and QoS are now main challenge for multimedia applications in wireless networks. Throughput parameter are very poor as compare with PHY data rate, even data rate goes infinite high. After certain PHY data rate, throughput are almost independent of data rate when data packets are fixed size. This is only due to large amount overhead adding with every data packets. So overhead reduction is the best solution to enhance the throughput for next generation communication. Here we discussed two important overhead reduction mechanisms, which sufficiently improve the throughput performance. One is frame aggregation and another is block acknowledgement scheme. In performance analysis graph of both schemes, shows that the throughput performance are below 100 Mbps. As we mention previous chapter that industries are seeking higher data rate for multimedia applications. i.e more than 100 Mbps. So we make an easy comment that the above two important components are not sufficient enough for IEEE 802.11n individually. To fulfil the throughput performance of IEEE 802.11n, we proposed a new MAC mechanism. The new FABA MAC mechanism is the combination of above two important components, which improve the throughput performance as our desire. In this research work we also simulate the FABA scheme, which shows the throughput performance more than 160 Mbps when the number of burst within TXOP duration is 5 and PHY data rate 600 Mbps. 6.2 Future work There are several important areas for future work. This research work is only based on the best-case scenario. i.e only we try to figure out the theoretical maximum throughput. So it is needed to more study on saturation case scenario, when the large number of stations are involved. We consider EDCA schemes to evaluate block acknowledgement schemes in our FABA scheme. But in our simulation we consider only one AC. So QoS study in combination with throughput may be another great issue for IEEE 802.11n. Here we consider the channel is error free i.e always ideal and we take 2304 byte as fragmentation threshold for performance evaluation. But in reality channel is always error porn. So there is a big issue that how the fragmentation threshold depend on channel quality or channel condition. Frame aggregation should be in proper layer due to QoS ensures. Here frame aggregation is considering only when the source and destination address are same for all packets. If the source and destination address are different then the aggregation schemes need further studied to find out the proper aggregation layer.

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