Implementation and Performance Evaluation of two Reliable MAC ...

Implementation and Performance Evaluation of two Reliable MAC Layer Multicast Schemes for Wireless Local Area Network Imran Siddique and Waseem Ahmad

September 1, 2008 Master’s Thesis in Computing Science, 2*30 ECTS credits Supervisor at CS-UmU: Thomas Nilsson Examiner: Per Lindstr¨om

Ume˚ a University Department of Computing Science SE-901 87 UME˚ A SWEDEN

Abstract In the Wireless Local Area Network(WLAN), IEEE 802.11 is the most popular standard due to its low cost and easy deployment. However, IEEE 802.11 performs poorly for the multicast applications. Several problems exist for the multicast services in the wireless networks. Different approaches have proposed to improve the reliability and performance of multicast transmission in IEEE 8022.11 networks. In this thesis, two approaches are used to enhance reliability and performance of multicast traffic. EMCD is an algorithm which detects collision by pausing during transmission. PREMA is an algorithm which resolves the collision by jamming the channel. The simultion results present key parameters which affect the performance. The results show enhancements in the multicast services. EMCD performance is directly proportional to its collision detection ability Collision detection in EMCD is highly dependent on its collision detection interval (TCDI ). The comparative study has been done on simulation results, which shows PREMA achieves very high channel utilization and success probability by resolving the collisions. PREMA simply outperforms EMCD and IEEE 802.11 in all scenarios and independent of large network.

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Acknowledgments First of all thanks to Almighty Allah (The most Generous and most Beneficent). We are thankful to our supervisor Dr. Thomas Nilsson, for all his guidance and valuable suggestions. He inspired us as a teacher and researcher. He taught us to face and handle the challenges. We wish to become ambitious and committed like him. We are grateful to the Director of Studies, Dr. Per Lindstrom, for his remarkable cooperation and exceptional management for our studies. We are thankful to our friends who helped and support us during our study period. Especially we would like to admire Jahanzeb Tipu for his valuable suggestions. Thanks to Asrar and Shahid for their unforgettable company during studies in Ume˚ a. The most important we would like to express our heartiest gratitude to our parents and family members for their blessings, support and patience.

Imran Siddique, Waseem Ahmad.

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Contents 1 Problem Specification

1

1.1

Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1.2

Goal Of Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1.3

Tasks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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2 Introduction to IEEE 802.11 2.1

3

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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2.1.1

System Architecture and Service . . . . . . . . . . . . . . . . . .

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2.1.2

Distributed Coordination Function (DCF) . . . . . . . . . . . . .

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2.1.3

Collision Avoidance (CA) and Binary Exponential Backoff (BEB)

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2.1.4

An Example of DCF Operation . . . . . . . . . . . . . . . . . . .

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3 Limitations of IEEE 802.11 3.1

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Limitations of DCF Mechanism . . . . . . . . . . . . . . . . . . . . . . .

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3.1.1

Unfairness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3.1.2

Multicast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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4 Different Approaches for Reliable Wireless Multicast 4.1

Related Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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4.1.1

Collision Resolution . . . . . . . . . . . . . . . . . . . . . . . . .

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4.1.2

Collision Detection . . . . . . . . . . . . . . . . . . . . . . . . . .

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4.1.3

Channel Reservation . . . . . . . . . . . . . . . . . . . . . . . . .

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5 Prioritized Repeated Elimination Multiple Access 5.1

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Introduction to PREMA . . . . . . . . . . . . . . . . . . . . . . . . . . .

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5.1.1

Bursting and Elimination . . . . . . . . . . . . . . . . . . . . . .

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5.1.2

PREMA for IEEE 802.11 . . . . . . . . . . . . . . . . . . . . . .

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5.1.3

Optimal Parameters . . . . . . . . . . . . . . . . . . . . . . . . .

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5.1.4

Simple Scenarios of PREMA . . . . . . . . . . . . . . . . . . . .

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CONTENTS

6 Early Multicast Collision Detection 6.1 Introduction to EMCD . . . . . . . . . . . . . . . . . . . 6.2 How Protocol Works . . . . . . . . . . . . . . . . . . . . 6.2.1 Vanguard Transmission . . . . . . . . . . . . . . 6.2.2 Carrier Sensing . . . . . . . . . . . . . . . . . . . 6.2.3 Phase III - Jamming/Normal Transmission . . . 6.3 Collision Detection Interval . . . . . . . . . . . . . . . . 6.4 PHY and MAC Header . . . . . . . . . . . . . . . . . . 6.5 EMCD Behavior in Different Scenarios . . . . . . . . . . 6.5.1 Collision between two Multicast Senders . . . . . 6.5.2 Collision between Unicast and Multicast Senders 6.5.3 Undetectable Collisions . . . . . . . . . . . . . .

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8 Evaluation 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 Performance Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3 Simulation Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.1 Network Architecture . . . . . . . . . . . . . . . . . . . . . . . . 8.3.2 Physical Parameters . . . . . . . . . . . . . . . . . . . . . . . . . 8.4 Evaluation of EMCD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4.1 Throughput . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4.2 Collisions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5 Evaluation of PREMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.1 Effecting Parameter . . . . . . . . . . . . . . . . . . . . . . . . . 8.6 Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.6.1 Throughput Comparison of Multicast Traffic . . . . . . . . . . . 8.6.2 Throughput Comparison among Multicast and Unicast Stations

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9 Conclusion and Further Research 9.1 EMCD . . . . . . . . . . . . . . . 9.1.1 Advantages . . . . . . . . 9.1.2 Disadvantages . . . . . . . 9.2 PREMA . . . . . . . . . . . . . . 9.2.1 Advantages . . . . . . . .

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7 Design and Implementation 7.1 Introduction to GloMoSim . . . . . 7.2 Design and Implementation . . . . 7.2.1 Implementation of EMCD . 7.2.2 Working of EMCD . . . . . 7.2.3 Implementation of PREMA 7.2.4 Working of PREMA . . . .

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CONTENTS

9.3

9.2.2 Disadvantages . . . . . . Further Reading and Research 9.3.1 EMCD . . . . . . . . . . 9.3.2 PREMA . . . . . . . . .

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10 Summary

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A List of Abbreviations

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CONTENTS

List of Figures 2.1 2.2 2.3 2.4 2.5

System architecture of WLAN . . . . . . . . . . . . . . . The unicast and multicast services . . . . . . . . . . . . The basic mechanism for unicast and multicast in DCF. A relationship between Inter Frame Spaces . . . . . . . An example of DCF . . . . . . . . . . . . . . . . . . . .

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3.1 3.2

Unfairness between uplink and downlink . . . . . . . . . . . . . . . . . . Multicast ACK implosion. . . . . . . . . . . . . . . . . . . . . . . . . . .

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4.1

Hierarchal view of different approaches for reliable multicast [24] . . . .

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5.1 5.2 5.3 5.4

The basic access mechanism of PREMA . . . . . . . . . . A flowchart of PREMA . . . . . . . . . . . . . . . . . . . A collision scenario between two multicast stations . . . . A collision scenario between multicast and unicast station

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6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8

EMCD flow diagram . . . . . . . . . . . . . . . . . . . . . . . Early Multicast Collision Detection. . . . . . . . . . . . . . . Different vanguard transmissions by three multicast senders . Physical data packet . . . . . . . . . . . . . . . . . . . . . . . Collision detection between two multicast senders . . . . . . . Collision detection between unicast and multicast senders . . Undetectable collision between unicast and multicast senders Undetectable collision between two multicast senders . . . . .

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7.1 7.2 7.3

Sequence diagram of EMCD with successful transmission . . . . . . . . Sequence diagram of EMCD with unsuccessful transmission . . . . . . . Sequence diagram of PREMA . . . . . . . . . . . . . . . . . . . . . . . .

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8.1 8.2 8.3

EMCD throughput on different TCDI values. . . . . . . . . . . . . . . . EMCD throughput on different CW values. . . . . . . . . . . . . . . . . Undetection probability at different number of selectables . . . . . . . .

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LIST OF FIGURES

8.4 8.5 8.6 8.7 8.8

Detection probability at different number of selectables . Collision at MAC layer for vanguard transmissions . . . Prema results on different values of q and h parameters Throughput comparison of PREMA, EMCD and 802.11 Throughput comparison of PREMA, EMCD and 802.11.

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List of Tables 6.1

CIFS relation to Inter Frame Spaces . . . . . . . . . . . . . . . . . . . .

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7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8

Protocols and Models supported by GloMoSim at each layer . . . Important data members used in GlomoMacEmcd data structure Frame types used in Emcd MacFrameType data structure . . . . Important states used in Emcd MacStates data structure . . . . Important parameters of GlomoMacPrema . . . . . . . . . . . . . Some constant parameters used in implementation of PREMA . Frame types used in implementation of PREMA . . . . . . . . . MAC layer states used in implementation of PREMA . . . . . .

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8.1 8.2

EMCD parameters used in simulations . . . . . . . . . . . . . . . . . . . EMCD TCDI values used in simulation . . . . . . . . . . . . . . . . . . .

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LIST OF TABLES

Chapter 1

Problem Specification 1.1

Background

The Wireless Local Area Network (WLAN) standard, IEEE 802.11 is a popular technology due to its best effort services, low cost and ease of deployment. IEEE 802.11 became more and more popular due to its support for multimedia applications. Since multimedia applications require more bandwidth, therefore multicast services are used as a communication method in IEEE 802.11 networks. When a sender is transmitting the same data packet simultaneously to its neighbor or group of neighbors is known as multicast [22]. In IEEE 802.11, unicast traffic is protected by the Automatic Repeat Request (ARQ) mechanism which is based on acknowledgment (ACK). But in multicast traffic, the ARQ mechanism is not possible because multiple receivers try to send their ACKs at the same time to one sender which causes a feedback implosion. Packets losses due to overlapping cells and link adaptation are also considered as a problem creating issues in multicast traffic. Therefore different protocols are proposed to solve the prediscussed issues [24].

1.2

Goal Of Thesis

The main task of this thesis is to implement two protocols, Early Multicast Collision Detection (EMCD) [22] and Prioritized Repeated Eliminations Multiple Access (PREMA) [12] in IEEE 802.11. These protocols should be implemented in GloMoSim (Global Mobile Information System Simulator), written in the C language. The master thesis is a comparative study of these protocols aiming to make them more flexible for multicast. EMCD is an algorithm, designed for IEEE 802.11 networks. The objective is to implement EMCD for multicast in GloMoSim. A multicast sender performs the CCA (clear channel assessment) during its early pause in the transmission before sending the packet. If the channel is busy, the station aborts the transmission after a fixed time interval and schedules a retransmission. PREMA is a protocol that borrows the idea of bursting from EY-NPMA [5]. The bursting mechanism enhances the performance in unicast as well as in broadcast networks. PREMA was intended for a wireless broadcast network with few or many nodes [12]. PREMA consists of sensing the channel and prioritizing the nodes for sending

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Chapter 1. Problem Specification

data by the elimination process, which is based on different priority schemes. It makes PREMA more flexible and adaptable due to its quality of service (QoS), reliability and high performance achievement for the multicast transmission in multimedia applications. The goal of the thesis is to increase the reliability for multicast transmission, for different scenarios in PREMA. The objective is to implement PREMA by using backoff schemes and concept of bursting in order to achieve maximum throughput. The final task is to evaluate the performance of EMCD and PREMA by taking real time scenarios.

1.3

Tasks

The thesis work comprises of 40 points (20 x 2) and is divided as follow: – An in depth study about the multicast problems in IEEE 802.11 networks. – A comprehensive study of EMCD and PREMA. – A study of the design of GloMoSim. – Design and implementation of EMCD and PREMA. – Testing the implementation. – Design and simulate different scenarios to analyze the results.

Chapter 2

Introduction to IEEE 802.11 2.1

Introduction

Wireless networks are nowadays widely used and experienced a great success after the development of Internet. There are two types of networks used: centralized and distributed. A centralized network is centrally controlled by the access point (AP). A distributed network has no central point, a wireless station accesses the network using a access mechanism. Several standards exists for WLAN like IEEE 802.11 [4] and HiperLAN (High Performance Radio LAN)1 /and 2 [5]. IEEE 802.11 is also know as WiFi. It is the most popular and widely used standard due to its simplicity and low cost. HiperLAN/1 and 2 are not well known standards due to their complexities and are not considered to be a part of our study. In 1997, Institute of Electrical and Electronics Engineers (IEEE) released the 802.11 standard that also defines the Media Access Control (MAC) and physical (PHY) layer specification. There are three different types of PHY layer specifications described, Frequency Hopping Spread Spectrum (FHSS), Direct Sequence Spread Spectrum (DSSS) and Infra Red (IR). The FHSS and DSSS operate on the 2.4 GHz ISM (Industrial, Scientific and Medical) band, which is license free. FHSS and DSSS have a maximum data transfer rate of 2 Mbps. After 2 years, IEEE enhanced the physical layer specifications and introduced two new versions 802.11a [6] and 802.11b [7]. The 802.11b improves the DSSS physical layer, which operates on the 2.4 GHz, and archives the maximum data transmission rate of 11 Mbps. The 802.11a OFDM physical layer specification operates on the 5 GHz band with maximum data transmission rate up to 54 Mbps. However the MAC layer specification remains the same except for a few parameters which are dependent on the PHY layer. The MAC basically controls the access of a transmission on the medium. The MAC protocols are based on two different access mechanisms, the Distributed Coordination Function (DCF) and the Point Coordination Function (PCF). PCF is a polling based technique which centrally controls the channel and grants access based on polling. Whereas DCF is a multiple access technique based on the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) mechanism.

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2.1.1

Chapter 2. Introduction to IEEE 802.11

System Architecture and Service

IEEE 802.11 defines two different type of architectures called Basic Service Set (BSS) and Independent Basic Service Set (IBSS). The BSS infrastructure is formed when one or more wireless stations are associated with an AP. Several BSS are connected through a Distribution System (DS) that forms an Extended Service Set (ESS) as shown in fig 2.1. All the communication takes place through the AP regardless of the destination address. That means, stations can not communicate with each other directly. In contrast, stations can communicate with each other directly in an IBSS infrastructure, if they are within the range of each other. This allows the formation of a wireless ad-hoc network in the absence of any network infrastructure.

Figure 2.1: System architecture of WLAN IEEE 802.11 supports both unicast and multicast services. When a station sends a packet to a single destination is known as unicast and a single sender who transmits a packet to a group of receivers called ”group members”, as shown in fig. 2.2, is known as multicast.

Figure 2.2: The unicast and multicast services

2.1. Introduction

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Multicast is an important feature in WLAN, since it is used in several applications including audio and video streaming, multimedia conferencing, shared white boards, distance learning applications, multi player games etc [15]. IEEE 802.11 treats both services differently. There is no Automatic Repeat Request (ARQ) mechanism for multicast as it is used for retransmission in unicast. Therefore the IEEE 802.11 is not a reliable protocol for multicast.

2.1.2

Distributed Coordination Function (DCF)

DCF is the basic access mechanism for the IEEE 802.11 networks with Carrier Sense Multiple Access (CSMA). CSMA works as listen before talk scheme, that means the station senses the medium for a specific time interval called DIFS (DCF Inter Frame Space) before the transmission [11]. If the medium is idle then the transmission proceeds. Otherwise the station defers the access and waits until the medium becomes idle again, for a DIFS time period.

Figure 2.3: The basic mechanism for unicast and multicast in DCF. The unicast transmission is followed by an acknowledgment (ACK) and a retransmission technique to make sure that the packets have been received successfully. If the AP receives a unicast packet successfully, then it transmits an ACK after waiting for a short time period called Short Inter Frame Space (SIFS), as shown in fig 2.3. If there is no ACK received until the ACK timeout, the sender realizes that the packet has been lost and it schedules a retransmission. In the multicast transmission, the ACK and retransmission technique can not be applied due to undefined number of receivers as shown in fig 3.2. The sender can not receive the multiple ACKs at the same time. If there is no ACK then the collision can not be detected and there will be no retransmission. In the IEEE 802.11, all frame types do not have the same priority, therefore, Inter Frame Space (IFS) time intervals are defined [9]. It gives a priority access to the channel between the transmissions. The relation between inter frame spaces are shown in fig 2.4. There are three types of IFS defined in IEEE 802.11, as described below. – Short inter frame space (SIFS): has the highest priority and shortest time interval which comes between the packet and the ACK frame. It prevents the other stations to transmit while a sender is waiting for the ACK. – PCF inter frame space (PIFS): is shorter than the DIFS time interval used by the PCF, an optional mechanism, used in IEEE 802.11. The AP is centrally controlling the channel through polling of individual stations. In PCF, the AP waits for a

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PIFS time period which gives a priority access over the ordinary station, since they have to wait a longer time, at least for a DIFS time period. – DCF inter frame space (DIFS) : has the lowest priority and the largest time interval that comes prior to the packet transmission.

Figure 2.4: A relationship between Inter Frame Spaces The lengths of the IFSs are dependent on the physical layer specification and are measured in time slots. In the DSSS, the SIFS time period is equal to a half slot time and PIFS includes SIFS plus one slot time. The DIFS contains a SIFS plus two time slots time.

2.1.3

Collision Avoidance (CA) and Binary Exponential Backoff (BEB)

In the CSMA, if two or more stations are trying to access the medium at the same time that leads to a collision. To prevent this situation, a Collision Avoidance (CA) mechanism works along the CSMA and this is known as the CSMA/CA. In the WLAN, collision avoidance is used instead of Collision Detection (CD) which is used in Local Area Network (LAN) , e.g. IEEE 802.3 Ethernet [3]. Due to this, wireless networks are not capable of detecting the collisions. In a wired network, stations are capable of sending and receiving the packet at the same time [19], therefore, it is possible to detect the collisions. While in the wireless networks the stations can not receive a signal while transmitting, that is the basic characteristics of the wireless communication. The strength of the signal also decreases while it propagates. The other factors like interferences, noises and fading also affect the signal strength which makes it difficult for the sender to detect other signals in the presence of their own signal [19]. CSMA/CA relies on the Binary Exponential Backoff (BEB) algorithm to prevent the collisions. Stations have to wait for an extra contention time period after waiting the DIFS time period prior to the transmission. The station waits until the medium becomes idle for at least DIFS time period and selects a random value called backoff (BO) from a uniform interval [0, CW], where CW is the Contention Window. The station starts decreasing its backoff value by one for each time slot. A station with the smallest backoff value counts down to zero first, wins the access of the medium and starts the transmission. While other stations pause their remaining backoff value and wait until the medium becomes idle again for the DIFS time period. As the medium becomes idle for a DIFS, stations start deceasing their backoffs again where they paused, while a new station chooses a new backoff. When the transmission starts, the CW is set to be minimum value i.e.CWmin. After each collision the size of the CW becomes double , until it reaches its maximum value, i.e. CWmax. Since the CW size increases exponentially, therefore this algorithm is known as the Binary Exponential Backoff (BEB). By doubling the CW, increases the

2.1. Introduction

7

probability to select a larger backoff value at the same time and decreases the probability of further collisions. The values of CWmin and CWmax are dependent on the physical layer specifications. In IEEE 802.11b, the possible values of the CW are 15, 31, 63, 127, 255, 511 and 1023. A retransmit limit is defined, that means a frame can only be retransmitted to a limited number of times. If the retry limit reaches the maximum number of retries, the frame will dropped and the retry limit will be reset. After each successful transmission the CW is reset to the CWmin. If a sender has another frame to transmit then it selects a new backoff value from the reseted CW which is known as the post backoff. This ensures that there is at least a backoff interval between two consecutive transmissions. The CSMA/CA, reduces the risk of collisions but the collisions may still occur, if the backoff of two or more stations reaches zero at the same time or if two or more stations select the same backoff value. There is an ACK and retransmission mechanism for unicast packets and stations have the chance to retransmits. On the other hand, the condition is worse in the case of multicast. There is no ACK for a multicast packet, therefore we can not detect the collision and the packet is lost. Due to this the recovery mechanism can not be imposed for multicast packets. The CW for a multicast station is always set to the CWmin and doubling the CW is not possible, that also increases the risk of collisions.

2.1.4

An Example of DCF Operation

Fig. 2.5 shows the operation of DCF for both the unicast and the multicast services. There are three stations, two multicast and one unicast, contending to access the medium. The packet of a multicast station (MC1) arrives at the MAC layer first and the station transmits after waiting the DIFS time period. Since there is no other packet in the queue, the backoff procedure is not used. Later on the packets of MC2 and unicast station (UC) arrive and the stations sense the medium but the medium is busy. They defer access and wait until the medium becomes idle. After the end of the transmission, the MC1 will not wait for an ACK. Because there is no ACK for multicast service, see section 2.1.2. All three stations wait for the DIFS and then select the backoff values. The MC1, MC2 and UC1 select the backoff values 11, 4 and 7 respectively and

Figure 2.5: An example of DCF

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start decreasing by one for each time slot. The MC2 has the shortest backoff value and therefore it reaches the backoff value zero first and it accesses the channel. The MC1 and UC1 pause their backoffs and wait for the medium to become idle again. Again all stations wait for the DIFS time period and MC2 selects a new backoff value that is 3. The MC1 and UC1 resume the count down of their backoff counters. The remaining backoff of UC1 and MC3 is the same, therefore they start transmission at the same time and it leads to a collision. The UC1 will wait until the ACK timeout, the multicast stations will immediately wait for the DIFS after the transmission is finished. They get access of the medium a bit early than the UC1 because they do not have to wait for the ACK timeout. The collision is not detectable for the multicast sender and the packet of MC2 is dropped and also the CW remains the same. After the ACK timeout the UC1 doubles its CW i.e. 63 and selects a new backoff value after waiting the DIFS. When UC1 get the access to the medium, it will retransmit the same packet, as before.

Chapter 3

Limitations of IEEE 802.11 IEEE 802.11 is a set of standards developed for WLAN. The different standards of IEEE 802.11 focus on the different aspects to improve the overall WLAN performance. For example 802.11e deals with QoS, 802.11i deals with security issues and etc [1]. In short, all these standards are dealing with several issues to overcome the problems. There are lot of limitations of these standards which are needed to be solved but are not included in our study. This chapter presents an overview of some limitations related to the multicast problems, which prevents the IEEE 802.11 standard to deal fairly with multicast traffic as it does with unicast traffic.

3.1

Limitations of DCF Mechanism

IEEE 802.11 has two basic access mechanisms as discussed earlier. DCF has at least the following limitations. – If multiple stations start their communication at the same time, many collisions occur, which lower the achieved bandwidth. – It treats all kind of data traffic in the same way and has no priority mechanism. This leads to the lacking of QoS in the IEEE 802.11 standard. – DCF serves all stations in the same fashion and makes no distinguish between Access Point (AP) and a normal station that leads to the unfairness in the downlink. – DCF is not suitable for multicast traffic due to the lackness of feedback or ACK implosion.

3.1.1

Unfairness

Since the AP and normal stations both are treated in the same way causes an unbalanced contention between uplink and downlink traffic. This is one of the major weakness found in the DCF mechanism in IEEE 802.11 standard. Unfairness in the contention between uplink and downlink stations increases with the increase of load of data traffic and becomes more evident when the load is exceeded from the maximum capacity of the 9

10

Chapter 3. Limitations of IEEE 802.11

system. There are 10 stations of packet size 1024 with the increasing load of 0 till 4 Mbps, used in the simulation as shown in fig 3.1. Where the maximum system capacity is 2 Mbps. The fig 3.1 shows uplink and downlink are attaining 100% fairness in the channel utilization for a certain period (until the load is approximately 1.4 Mbps), but after some time the downlink decreases dramatically and uplink traffic achieving more network resources approximately upto the desired level.

Figure 3.1: Unfairness between uplink and downlink

3.1.2

Multicast

Transmission of a single packet simultaneously to a station or group of stations is known as multicast. The IEEE 802.11 networks are based on the collision avoidance methods and are protected by the following mechanisms for unicast transmissions: 1)ARQ mechanism 2)backoff mechanism. The IEEE 802.11 network uses the ARQ (stop and wait) protocol for attaining the feedback to ensure reliable transmissions. But it is not possible for multicast traffic. Fig 3.2 illustrates the ACK problem in the multicast transmission. The factors that create problems in multicast are stated below. 1. Automatic Repeat Request (ARQ) is an error control method based on a feedback mechanism to indicate reliable receiving. In this method, additional check bits are used to ensure the reliable reception of a packet and request for retransmission of lost packet. ARQ is not feasible for the multicast traffic due to the ACK implosion. 2. In real time scenarios, both unicast and multicast traffic categories may exist in the same environment [22]. On collision, both have totally different ways to deal with the problem due to the ARQ protocol. In unicast, each cw is doubled on the collision occurrence but it is not feasible for the multicast case. Therefore, unfairness between unicast and multicast traffic arises.

3.1. Limitations of DCF Mechanism

11

3. When multicast streams are distributed in one ESS, the problem arises when multiple multicast streams share the same channel. In the ESS, multiple packet losses are due to the collisions in overlapping cells on the same channel.

Figure 3.2: Multicast ACK implosion. STA1 sends data packet to multiple receivers AP1, AP2, and AP3 as shown in the fig 3.2 and wait for ACK. The AP1, AP2 and AP3 receive packet and reply with ACKs at the same time which leads to the collision is called ACK implosion. The main problems for multicast traffic originate from a common source, poor reliable capability due to the failure in detection of lost multicast packets. Generally, lost packets or collisions are detected by the ACK or on the feedback, but it is not possible in the multicast as illustrated in fig 3.2. There are different solutions proposed to solve the issue of feedback [2]. When feedback problem is solved, new challenges arises that are listed below briefly. 1. Which approach is used for ACK (leader or token based approach)? 2. If it is leader based approach, then who is responsible for sending the ACK? 3. If one station does not receive the packet then what will happen, retransmission or ignore? Since retransmissions are not possible therefore contention window can not be adjusted dynamically.

12

Chapter 3. Limitations of IEEE 802.11

Chapter 4

Different Approaches for Reliable Wireless Multicast 4.1

Related Work

In this chapter, different approaches are discussed to solve the problems with multicast transmission. The algorithms are categorized according to their functionality and shown in the fig 4.1.

Figure 4.1: Hierarchal view of different approaches for reliable multicast [24]

14

Chapter 4. Different Approaches for Reliable Wireless Multicast

The main categories are briefly described below.

4.1.1

Collision Resolution

The collision resolution approach is based on certain attributes, taken from the HiperLAN (High Performance Radio LAN) European standard to support the IEEE 802.11 for multicast traffic. The main objective of this technique is to resolve the collisions before the transmission. This is a hybrid approach because it uses certain beneficial properties of parallel independent geometric distribution as explained in [12] and jamming for channel reservation. EY-NMPA[5], PREMA[12] and SEMA[23] are examples of this approach. Neither of these protocols are used for the multicast in the IEEE 802.11 networks. But, these protocols can be used to improve the multicast for IEEE 802.11 networks.

4.1.2

Collision Detection

Collision detection approach can be implemented by adapting different techniques. Basically, this approach is based on the feedback. Where the feedback can be achieved during the transmission or after the transmission. So, collision detection approach can be divided into two categories: 1) Carrier sensing during transmission and 2) Carrier sensing after transmission (ARQ based protocols). Lo and Mouftah [25], ROM[18] and EMCD are the examples for the collision detection during transmission. In this approach, a sender detects collisions by pausing during the transmission and senses the medium for a fixed time interval to detect other potential transmitting stations. If the sender detects the collision, it continues the transmission for a predetermined interval which is known as the collision detection interval or threshold time. The sender does not abort the transmission immediately because to inform the other senders about the collision. If the channel is sensed idle during the threshold time then sender continues with its normal transmission. Collisions can also be detected after transmission. ARQ based protocols are based on this technique to detect collisions. The protocols based on this technique modified the IEEE 802.11 MAC scheme. Kuri and Kusera[13] proposed a protocol, which is an example for this technique.

4.1.3

Channel Reservation

In this approach, a station avoid other stations from accessing the channel until it finishes the transmission. The channel is reserved by different techniques depending upon the channel in which it is operated. For example, In black burst[20] the reservation message is a burst. The winner of the channel would be that station who will have the longest burst. The burst length is defined by the waiting time. So in a same way, the HiperLAN bursts are used for reserving the channel. On the other hand, in IEEE 802.11, RTS/CTS exchange packets are used for the reservation of a channel in the unicast transmission. But in multicast, multiple receivers are sending multiple CTS at the same time that causes feedback implosion. Therefore the channel reservation is applicable by following different techniques for the feedback procedure. Single CTS approach 1. Robust multicast [14],Simple Leader Base Protocol (SLBP) [2] and Beacon driven Leader Based Protocol (BLBP) [2] select one leader to send one CTS for the whole

4.1. Related Work

15

group. The selection of the leader is a big problem in these protocols. 2. The BMW [21] ensures the transmission reliability by unicast the RTS/CTS exchange packets with its neighbors on a round robin fashion. The BMW protocol becomes worse in the large networks. Multiple CTS approach Kuri and Kusera [13] proposed an intolerant approach to enable feedback feature in multicast traffic. A designated station transmits a positive ACK and if any station who has not received the multicast frame in that multicast group but detects the ACK will send a NACK (negative ACK). Due to this, it is a fault intolerant and is not feasible technique with the large networks.

16

Chapter 4. Different Approaches for Reliable Wireless Multicast

Chapter 5

Prioritized Repeated Elimination Multiple Access 5.1

Introduction to PREMA

PREMA is a collision resolution algorithm which resolves the channel conflicts prior to the actual packet transmission. PREMA is a simple bursting protocol that can be used for multicast transmissions. Elimination Yield Non-pre-emptive Prioritized Multiple Access (EY-NPMA) [5], was probably the first bursting protocol specified in HiperLAN/1. EY-NPMA is quite similar with the PREMA. EY-NPMA operates in three phases, 1- Prioritization phase, stations wait according to their priority before the transmission. 2- Bursting phase, stations transmit their bursts followed by a verification slot. Only that stations having longer bursts, survive and enter into the next phase. 3- Yield phase, the remaining stations perform yield (backoff). The station with the shortest yield time wins the channel and starts the transmission. PREMA operates with only bursting and elimination phase and repeat this phase to a certain times. PREMA starts by transmitting a burst. The burst length is sampled from the geometric distribution with a probability q. The station with the longest burst survives by sensing the medium idle and enters into the second elimination phase. The winner of h consecutive elimination phases finally accesses the medium.

5.1.1

Bursting and Elimination

PREMA is a simple bursting protocol that resolves the collisions between the multicast stations by using elimination phases. The stations are capable of performing operations, jamming the channel and the carrier sensing. In PREMA, a station transmits a burst rather than sending the actual packet. The burst contains no data and is used to transmit the noise on the channel, known as channel jamming. The elimination phase determines which station survives for the next phase by performing the carrier sensing. The station who have the longest burst and senses the medium idle survives while other stations defer their access. All stations sense the medium for at least DIFS time period before starting the transmission. If the medium is idle, the station starts transmitting the burst with a certain length sampled by the geometric distribution with a probability q. After the

18

Chapter 5. Prioritized Repeated Elimination Multiple Access

burst transmission, the station performs carrier sensing. If the medium is idle for at least one time slot that means the station have the longest burst and enters into the second phase. If the medium is not idle for a time slot the station realizes that the other stations have longer bursts. Then the station defer access and is eliminated from the bursting contention as shown in the fig 5.1.

Figure 5.1: The basic access mechanism of PREMA The remaining stations enter into the second phase. The elimination phase continues until the hth elimination. After the elimination phases there is large probability that there will be only a single winner.

5.1.2

PREMA for IEEE 802.11

PREMA is not implemented for the IEEE 802.11 standard. We have modified PREMA to work along with this standard. The backoff mechanism is introduced before the elimination phase. The multicast stations use the PREMA algorithm and unicast stations are treated in a same way as in 802.11. The fig. 5.2 describes the operation of PREMA.

5.1. Introduction to PREMA

19

Figure 5.2: A flowchart of PREMA

5.1.3

Optimal Parameters

The performance of PREMA is dependent on the h and q parameters. Where h is a PREMA threshold (number of elimination phases) and q is the probability used in the geometric distribution to sampled the burst length. The long burst and several number of elimination phases causes a waste of channel utilization and efficiency of the algorithm. The values of h and q should be optimized. The effect of these parameters are shown in the Section 8.5. The parameter are described detailedly in [12].

5.1.4

Simple Scenarios of PREMA

The behavior of PREMA is different in the different scenarios. Some of the scenarios are discuss below.

20


Collisions Resolution Between Multicast Senders The procedure starts in the same way as in the DCF. Fig. 5.3 illustrates an example scenario where six multicast stations are contending for the medium.

Figure 5.3: A collision scenario between two multicast stations Every station selects a random backoff value from the CW. The stations STA1, STA2, STA3, STA4, STA5 and STA6 select the backoff values 3, 3, 3, 6, 3 and 9 respectively and start to count down the values. The possibility of selecting the same BO value is high because of the CW size, which is small. The stations STA1, STA2, STA3 and STA5 select the same BO value, therefore they enter into elimination phase. In DCF, these stations, with the same backoff values, lead to the collision. These stations sample a burst length from the geometric distribution with parameter q and start transmitting the burst. STA4 and STA6 pause their backoffs and wait until the medium becomes idle. STA2 and STA3 sample a short burst and after sensing the medium busy, they defer their access. STA1 and STA4 sense the medium idle after transmitting the longest burst and then enter into the second elimination phase. In the second elimination phase only STA5 survives and enters into third elimination phase, while STA1 is eliminated because of the shorter burst. After the fourth elimination phase, the only survived station is STA5 who wins the access of the medium and starts the transmission. Collisions Resolution Between Unicast and Multicast Senders In this case, unicast stations are treated in the same way as in the DCF. There is no change in unicast technique. Fig. 5.4 shows the collision scenario between multicast and unicast stations.

5.1. Introduction to PREMA

21

Figure 5.4: A collision scenario between multicast and unicast station If both, unicast and multicast stations start transmission at the same time that leads to a collision. It is only possible if the backoff value of two or more stations reaches zero at the same time. The unicast stations start transmitting the packet while multicast starts transmitting the burst. At the end of first elimination phase, the multicast station realizes the collision and defers access. Unicast station continues with transmission and waits until ACK timeout. If the frame is collided then the station schedules for the retransmission of the frame. Since the multicast station transmits burst to contend for the medium, it saves the actual packet and attempts retransmission on failure. In normal DCF, the multicast packet is lost and there is no retransmission for multicast packets. There is a small delay before the actual packet transmission due to elimination phase but it is almost 99% guaranteed that there is only one winner after the elimination phases [12]. Therefore, it is a trade off between the delay and the reliability of the transmission.

22


Chapter 6

Early Multicast Collision Detection 6.1

Introduction to EMCD

Early multicast collision detection (EMCD) is an algorithm used by multicast senders to detect collisions among multicast and unicast senders. EMCD is based on the Rom [18] algorithm. EMCD detects collisions between multicast senders in the IEEE 802.11 network and also enables retransmission of the lost packets. Therefore, retransmission increases the reliability and reduces overhead associated to collision. Collision detection technique used in EMCD is effective in two ways: first, retransmissions is possible for the collided packet in multicast. Secondly adjusting the contention window (cw) for collided packet is also decreasing the number of collisions. The collision detection in EMCD makes it more adaptable to introduce different CW schemes to achieve higher throughput. Hence, EMCD is based on Rom algorithm differs from other collision detection protocols due to its collision detection mechanism. In EMCD, a multicast station detects potential transmitters during the transmission by introducing a pause and sensing the channel as opposed to the slotted CSMA which lost packets after the collision. The sender who detects a collision, continues the transmission for a predetermined interval referred to the collision detection interval (CDI) or threshold time rather than abort the transmission immediately. If the channel is sensed idle during the threshold time then the sender continues with the normal transmission. Lo and Mouftah [25] and Rom[18] proposed similar collision detection algorithms with a difference of choosing sub intervals. In the Rom algorithm, the pausing sub intervals is chosen from a uniform distribution. On the other hand, Lo and Mouftah assumes a non slotted CSMA with the same pausing sub interval for all senders.

6.2

How Protocol Works

The structure of the EMCD algorithm is shown in the fig 6.1.

Figure 6.1: EMCD flow diagram

Figure 6.2: Early Multicast Collision Detection. EMCD is divided into three phases as shown in the fig 6.2. The first phase is vanguard transmission Tv , the second phase is the carrier sensing phase and the third phase is jamming or main transmissionm . Fig 6.3 illustrates that, how EMCD algorithm works for different senders. There are three APs (AP1, AP2 and AP3), contending for transmission after the DIFS time period as shown in the fig 6.3. All senders select different Tv transmissions depend upon the division of the packet, which is explained in the next section. Fig 6.3 describes

6.2. How Protocol Works

25

that how multiple stations choose different Tv s and then during the pause, listen for the channel status which causes the Tm or jamming of the channel.

Figure 6.3: Different vanguard transmissions by three multicast senders

6.2.1

Vanguard Transmission

The basic idea is to divide the original packet into fragments: Vanguard transmission (Tv ) and jamming or main transmission (Tm ) packets. The size of Tv can be calculated from the given equation (6.1) [22]. Tv n

= Txmin + nT4 , ∈

(6.1)

U {0, (TCDI − Txmin )/T4 }

(6.2)

Txmin is minimum allowable transmission time for the Tv . If transmission is Txmin , then it means Tv consists of (PHY + MAC) headers without any data. The PHY and MAC headers are described in the section 6.4. Where ’n’ is the number of slots chosen randomly as shown in equation (6.2). T4 is the smallest time difference that can be detectable during the two Tv s, transmitted by two different senders at the same time. TCDI is the maximum allowable transmission time of the Tv plus Collision Inter Frame Space (CIFS). The parameter TCDI is explained in more detail in the section 6.3. The size of the Tv depends on the TCDI time.

6.2.2

Carrier Sensing

In the second phase, CIFS starts after the Tv . During the CIFS interval, the clear channel assessment (CCA) operation is initiated to listen for other potential transmitters. Then in the next phase, the decision of jamming or actual transmission comes, which is based on the outcome of the CCA operation. If the medium is idle, the senders continue with Tm otherwise jamming will be performed for a specific time. Carrier sensing is a method by which the station can determine the status of the channel by CCA operation. Physical layer performs the CCA operation and informs the MAC layer about the status of the channel. There are requirements specified in 802.11a WLAN for the CCA to detect the presence of other signals on the same channel.

26

Chapter 6. Early Multicast Collision Detection

The CIFS lower bound is defined by the time required to perform a correct CCA [6]. Generally, lower bound value ¡ 4µs is used for correct CCA operation. The correct CCA requires signal propagation time and the time required to switch from transmitting to receiving and vice versa. These terms (signal propagation and receive/transmit turnaround time) are hardware and physical configuration dependent respectively. Therefore, the CCA requires time to perform correct operation. The CIFS upper bound is defined according to SIFS, PIFS and DIFS time intervals. The IEEE 802.11a standard [6] defines the relation between Inter Frame Spaces, as listed in table 6.1. CIFS upper bound value must be less than other values. Overall CIFS value can be [3,4,5,...,11] to perform correct CCA. The suggested value for CIFS is 10µs in this algorithm. Inter Frame Space CIFS SIFS PIFS DIFS

Duration 10µs 16µs 25µs 34µs

Table 6.1: CIFS relation to Inter Frame Spaces [8].

6.2.3

Phase III - Jamming/Normal Transmission

During CIFS, if the medium is idle then the Tm is started by creating the packet according the following equation. Tm = pkt - Tv ; (6.3) pkt is the actual packet comes from network layer to the MAC layer. But if the medium is busy during the CIFS interval then the collided packet is created by the following equation and is transmitted. Collided packet = TCDI - (Tv +CIFS);

(6.4)

Collided packet jams the channel for a certain time period as calculated in equation (6.4). This jamming is actually based on the concept of acknowledging to every other station about the collision occurrence. After collided packet transmission, the retransmission is done by following the normal DCF procedure.

6.3

Collision Detection Interval

Collision Detection Interval (TCDI ) is important for this algorithm as shown in the fig 6.2. TCDI basically affects the performance of EMCD in three dimensions. First, the length of the TCDI determines how many unique transmissions are selectable by the multicast senders. Smaller value of TCDI offers limited selectable unique transmissions, that increases the probability of collisions by selecting the same transmission time. Secondly, greater TCDI value affects the collision detection ability with unicast transmission. But Tv also causes additional delay and more overhead. Fig 6.7 shows the possibility of the collision is undetectable by the longer Tv . Certainly, it also affects on delay and overhead if retransmission is scheduled in longer Tv .

6.4. PHY and MAC Header

27

Therefore, these factors demand for optimal threshold value investigation, that would be a better choice for the unique Tv s and it also does not cause longer Tv s.

6.4

PHY and MAC Header

The IEEE 802.11a [6] is the amended specification to IEEE 802.11. This specification describes a high speed physical layer (PHY) which operates in the 5 GHz band. The technique used in the 802.11a is the orthogonal frequency division multiplexing (OFDM) with a rate capabilities of 6, 9, 10, 12, 18, 24, 34, 36, 48 and 54 Mbits/s. The PHY header used in the PHY layer is called Physical Packet Data Unit (PPDU) as shown in the fig 6.4. PHY layer receives frame from the MAC layer and add more information before transmission. PHY header consists of three parts; preamble, signal and Data as shown in the fig 6.4. The preamble is used for the synchronization. Signal field is used to determine the frame length. The data field consists of frame which is received from the MAC layer. MAC frame consists of a MAC header and cyclic redundancy check (CRC) and data.

Figure 6.4: Physical data packet

6.5

EMCD Behavior in Different Scenarios

In this section, EMCD behavior is analyzed in different scenarios. First, the study about EMCD, how it works and detects the collisions among multiple multicasts, multicast and unicast senders. Secondly, discussion about those cases in which collision is not detected.

6.5.1

Collision between two Multicast Senders

Fig 6.5 shows two multicast senders that start their transmission at the same time and explains how the collision is detected between them. AP1 detects the collision before AP2 due to its smaller Tv . AP1 starts its carrier sensing phase before AP2 and finds the medium busy and jams the medium. If AP1 does not jam the medium then AP2 starts its carrier sensing phase and finds the medium idle. So jamming the medium, ensures that the potential transmitters also know about collision occurrence. At the end of TCDI , normal DCF procedure is invoked and retransmission is scheduled. CW is not doubled here due to different factors. When a station finds the medium idle, the normal transmission will be started.

6.5.2

Collision between Unicast and Multicast Senders

Fig 6.6 shows that simultaneous transmission from a unicast station and a multicast station leads to a collision. The AP senses the station and comes to know that the

28


Figure 6.5: Collision detection between two multicast senders medium is busy and it transmits the collided packet in order to jam the transmission. So the multicast sender detects the collision due to its EMCD collision sensing behavior, but unicast station detects collision by following the normal DCF mechanism. Retransmission is also scheduled for both multicast and unicast senders by normal DCF mechanism.

Figure 6.6: Collision detection between unicast and multicast senders

6.5.3

Undetectable Collisions

There are two possibilities in which EMCD can not detect collisions. First, if the unicast packet size is smaller than the Tv size as shown in fig 6.7. Secondly, if two multicast senders have the same size of the Tv and try to transmit them simultaneously, as illustrated in the fig 6.8.

Figure 6.7: Undetectable collision between unicast and multicast senders

6.5. EMCD Behavior in Different Scenarios

Figure 6.8: Undetectable collision between two multicast senders

29

30


Chapter 7

Design and Implementation 7.1

Introduction to GloMoSim

Global Mobile Information System Simulator (GloMoSim) [26] is a scalable simulation environment designed for large mobile and wireless communication networks, developed at The University of California, Los Angeles (UCLA) Parallel Computing Laboratory. GloMoSim is a discrete-event simulator which is capable of performing parallel simulation provided by the PARSEC (Parallel Simulation Environment for Complex Systems) [10]. PARSEC is a C based simulation language, also developed by the UCLA Parallel Computing Laboratory, designed for sequential and parallel execution of discrete-event simulation models [17]. Layers

Protocols

Mobility

Random waypoint, Random drunken, Trace based

Radio Propagation

Two ray and Free space

Radio Model

Noise Accumulating

Packet Reception Models

SNR bounded, BER based with BPSK/QPSK modulation

Data Link (MAC)

CSMA, IEEE 802.11 and MACA

Network (Routing)

IP with AODV, Bellman-Ford, DSR, Fisheye,

Transport

TCP and UDP

Application

CBR, FTP, HTTP and Telnet

LAR scheme 1, ODMRP, WRP

Table 7.1: Protocols and Models supported by GloMoSim at each layer GloMoSim can be used to simulate networks with up to thousand mobile nodes connected by a heterogeneous wireless networks that includes multicast, multi-hop ad hoc networks, asymmetric communication using direct satellite broadcast and traditional Internet protocols. GloMoSim is built by using a layered approach which is similar to the OSI seven layer network architecture and standard APIs are used in the simulation layers. That makes it easy to implement new protocols and models at different layers. The protocols supported by GloMoSim used by the different simulation layers are shown in table 7.1. GloMoSim is a discrete event driven simulator that means the execution of the

32

Chapter 7. Design and Implementation

simulation model consists of a set of events. The events occur and an appropriate action has been taken in its response. The event is defined as an incident which makes the root to change the state of the system. A certain event or a group of events may cause other events to be invoked and so on and that is how the simulation proceeds. Events can only occur at different time units and it is not allowed to occur between these time units. An event can be an arrival of packet at some layer or expires some time period. GloMoSim is freely available for educational purposes, so-called academic version and can be accessed from an educational domain. The academic version only supports the sequential execution of simulations.

7.2

Design and Implementation

In this section, an overview of design and implementation of EMCD and PREMA is presented. GloMosim supports IEEE 802.11 together with many other MAC protocols, but none of them is achieving successful results for multicast. Thus, the task was to design and implement the two reliable multicast schemes. The following section illustrates the important components used for the implementation of EMCD and PREMA.

7.2.1

Implementation of EMCD

EMCD is implemented at the MAC layer of GloMoSim to provide multicast functionality. GloMoSim is built by using the layered approach. It is really easy to understand and is more flexible to use than other common simulators like NS2 etc. Data Display Debugger (DDD v3.11) helped to understand the GloMoSim structure by follow the sequence of instructions at the different layers. The common notations which are used for the implementation of EMCD i.e D1 and D2 represents the Tv and Tm respectively. The following sections illustrates the important components used for the implementation of EMCD. GlomoMacEmcd GlomoMacEmcd is a data structure that holds important data members and represents the MAC of the entire station. GlomoMacEmcd uses the GlomoMac802 11 with some modifications as listed in the table 7.2. GlomoMacEmcd data structure clocktype CW FOR MULTICAST NODE ADDR destAddr Temp BOOL isChannelIdle BOOL isD1Transmitted float D1size float Txmin int payLoadSize int packetSize int collisions Table 7.2: Important data members used in GlomoMacEmcd data structure

7.2. Design and Implementation

33

CWFOR MULTICAST holds information about the CW which is used for the multicast stations. destAddr Temp is an address parameter which describes about the transmission (unicast or multicast). The two flags isChannelIdle and isD1Transmitted are used to kept the information about channel (busy or idle) and Tv . The decision of the Tm is based on the flag isD1Transmitted, if the flag is TRUE then Tm will be transmitted otherwise not. The four parameters D1size, Txmin, payLoadSize, packetSize and collisions are used to kept the information of the size of Tv , transmission minimum, packet size of Tv and payload size respectively. The collisions variable is used to contain the information of collisions detected on the MAC layer by the EMCD.

Emcd MacFrameType Emcd MacFrameType uses 802 11MacFrameType with enhancements as listed in table 7.3. There are three MAC frame types used in the Emcd MacFrameType data structure. EMCD D1, EMCD D2, EMCD COLLIDE are the frames types which represents Tv , Tm and collided transmissions. Frame types EMCD D1 EMCD D2 EMCD COLLIDE Table 7.3: Frame types used in Emcd MacFrameType data structure

EMCD MacStates In EMCD, different states are defined in which a station can be switched. These states are transmitting and waiting states. More states are added to the original standard of the GloMoSim as listed in table 7.4. Tv is transmitted by setting the state to EMCD X D1. After Tv , control comes to the second phase of listening by changing state to EMCD S WFIFS. After CIFS time period, state is changed with EMCD S WFNEXTIFS and TransmitDataFrame() method is called. Now, if the channel is sensed idle then Tm will be done otherwise collided transmission will be happened by changing their states to EMCD X D2 and EMCD D2PktCollided. Different States EMCD S WFIFS EMCD S WFNEXTIFS EMCD X D1 EMCD X D2 EMCD X D2PktCollided Table 7.4: Important states used in Emcd MacStates data structure

34

7.2.2


Working of EMCD

The design is based on the EMCD functionality. In this algorithm, a transmission is divided into two parts, first transmission takes place and a pause follows after a fixed time interval. Secondly, the channel is sensed for a while, if the channel is idle then the transmission is done. As discussed earlier that solution is based on three major phases. The work is to implement EMCD algorithm at the MAC layer of GloMoSim. The functionality of the MAC layer starts from the point when a packet arrives from the network queue to the MAC layer. The control moves forward by setting the states and enters into the backoff mechanism if network queue is not empty. If the queue is found empty then the control directly goes for the transmission of the packet after changing its state.

Figure 7.1: Sequence diagram of EMCD with successful transmission


35

Vanguard Transmission The role of the EMCD is involved from the point where traffic is decided to be a unicast or multicast. The function of the EMCD algorithm starts from the point when traffic is detected to multicast in the function MacEmcdTransmitDataFrame() as shown in the fig 7.1. The packet is taken from the queue and a new packet is created according the equation 6.1 and transmitted.

Figure 7.2: Sequence diagram of EMCD with unsuccessful transmission

36


Pause and Channel Sensing When the control arrives at the function TransmissionHasfinished(), the next phase of sensing starts. In the function MacEmcd D1PKT Transmitted() which is called by TransmissionHasfinished(), WFIFS state setting and timer initialization is done by calling MacEmcdSetState() and EmcdStartTimer() functions respectively. The CIFS value used in EmcdStartTimer() is fixed which is 10µS to carry out a proper CCA operation. The channel is sensed twice here before and after calling of EmcdStartTimer() function. Collided/Main Transmission The start of third phase is the point when the control arrives at the function MacEmcdTransmitDataFrame() after channel sensing twice. If the channel is sensed idle then control goes to the second section of the function of TransmitDataFrame(), generates the D2 packet which is known as Tm and transmits that as shown in the last section of fig 7.1. But if the channel is reported to be busy then D2 collided packet is generated and is transmitted. The last section of fig 7.2 is showing the sequence to deal with D2 collided packet.

7.2.3

Implementation of PREMA

The PREMA is implemented on the MAC layer in GloMoSim v2.03. IEEE 802.11 and some other MAC protocols are currently supported by GloMoSim. Our task was to design and implement PREMA on the existing DCF mechanism. The following section describes the important components that are used for the implementation of PREMA. GlomoMacPrema The GlomoMacPrema is modified version of GlomoMac802 11 data structure by adding the features of PREMA. The important parameters of GlomoMacPrema is illustrated in table 7.5. PREMA Data Structure int state int prevState clocktype CW clocktype BO int idleSlot BOOL isAFirstBurst long BurstLength Table 7.5: Important parameters of GlomoMacPrema idleSlot kejpg the information of number of eliminations that how many eliminations of particular station have been done. isAFirstBurst is used as tag to perform certain tasks, like reseting the values etc. As name indicates the BurstLength is used to kept the length of burst which is sampled from the geometric distribution. Some constant parameters are also defined, as shown in table 7.6 with default values. PREMA THRESHOLD is a total number of elimination phases and PI is used in geometric distribution.


37

PREMA Parameters PREMA THRESHOLD 4 PREMA PI 0.5 Table 7.6: Some constant parameters used in implementation of PREMA PREMA MacFrameType There are many frame types defined at the MAC layer. Some modified frame types for PREMA are describes in table 7.7. PREMA Frame Types PREMA BURST PREMA BROADCAST Table 7.7: Frame types used in implementation of PREMA

Prema MacStates There are several states are defined in GloMoSim. The MAC layer states are divided into three categories, wait for response states, transmission states and miscellaneous states. There are three states added for PREMA in GloMoSim, as illustrated in table 7.8. PREMA PREMA PREMA PREMA

States X BURST X MULTICAST X WFCS

Table 7.8: MAC layer states used in implementation of PREMA PREMA X BURST and PREMA X MULTICAST are the transmission states. The station enters into these states when it going to transmit a burst or multicast the frame. PREMA S WFCS indicates the time period for carrier sensing comes between two elimination phases.

7.2.4

Working of PREMA

The working of PREMA is divided into three phases, transmission of the burst, carrier sensing and transmission of the actual packet. The sequence of PREMA with functions and some important parameters, is shown in fig 7.3. When the frame arrives at the MAC layer, the station attempts to go to the DIFS by performing the carrier sensing. If the medium is idle, the state of the system changes and the DIFS time period starts. If there is only one frame in the queue then the backoff will be zero and directly goes for the transmission. Otherwise the backoff timer starts and transmits packet when backoff reaches to zero. At this point, PREMA operation starts.

38


Figure 7.3: Sequence diagram of PREMA Burst Transmission After the backoff time period, the station enters into the bursting phase as shown in fig 7.3. The PREMA gets the frame from the network layer. Only multicast stations enter into the bursting phase. The multicast stations sample the burst length by calling the Geometric Distribution function. After changing to the state ’PREMA X ’ BURST, the transmission of burst starts with the sampled length. Carrier Sensing The radio layer informs the change of status to the MAC layer. The MAC layer calls PREMA to take appropriate actions. The radio status informs that the transmission of the burst has finished. The next step of PREMA is to perform the carrier sensing after waiting a very short time period. It is because the minimum signal detection


39

time is approximately 4µs and it is useless to perform carrier sensing immediately after the transmission. The state is changing to PREMA S WFCS. As the time expires the station performs the carrier sensing by verifying the radio status. If the medium is idle the station performs another bursting and then carrier sensing until the specified repeated eliminations. Frame Transmission After the repeated eliminations, the winner starts the transmission of the actual frame. Again PREMA calls the network layer but this time the frame is dequeued. Since the retransmission is not possible for multicast frame so there is no meaning to be still in queue. The state changes to PREMA X BROADCAST and the transmission of the dequeued frame starts.

40


Chapter 8

Evaluation 8.1

Introduction

In this chapter, the performance evaluation of EMCD and PREMA is analyzed and conclusions are made on the results of the simulated scenarios. The evaluation presentation is divided into three parts, 1) EMCD performance evaluation, 2) PREMA performance evaluation and 3)comparison between them. In first section, the EMCD is simulated with some simple scenarios to observe the role of affecting parameters to ensure the reliability of multicast transmissions. In the second section, PREMA is simulated and analyzed with simple scenarios and highlights its performance affecting parameters for the multicast transmission. In the third section, the comparative evaluation of EMCD, PREMA and 802.11 is presented by adapting a more realistic approach in the simulations. Last section describes who is more suitable for multicast traffic in real time scenarios.

8.2

Performance Parameters

The performance evaluation is presented in the following QoS metrics. – Throughput Throughput is the rate of data transferred over the time and is expressed in bits per second. It is really difficult to show the multicast throughput precisely, because in multiple receivers, few of them may not receive packets. Therefore in these simulations, the packet received by one receiver in a multicast group is considered to be received by all receivers. Hence, aggregated throughput is taken for the result presentation. – Collision A collision is happened, when two or more senders try to transmit data at the same time. Collisions result in retransmissions which also lead to the wastage of bandwidth and decreases the throughput. – Channel Utilization It is an achieved throughput presented in percentage, is less ambiguous term for the presentation of achieved throughput.

42

8.3 8.3.1

Chapter 8. Evaluation

Simulation Model Network Architecture

In simulations, the Basic Service Set (BSS) infrastructure has been used which consists of different number of APs and stations. Hence, all the communication takes place through the AP, therefore, the stations are not able to communicate directly.

8.3.2

Physical Parameters

IEEE 802.11a has been used for simulations which is based on the Orthogonal FrequencyDivision Multiplexing (OFDM) specification. There are some parameters that are used for OFDM specification as listed in table 8.1. Parameter

Value

T4

4µs

TCDI

112µs

CIFS

10µs

DIFS

34µs

SIFS

16µs

CWmin

15

CWmax

1023

PLCP Preamble

16µs

Signal

4µs

ACK

24µs

Distance between Access Points

50 m

Transmit Power

40 mW

Simulation Time

50µs

Rate used for multicast

6Mbps

Rate used for unicast

6Mbps

CCA Time

4µs

Table 8.1: EMCD parameters used in simulations

8.4 8.4.1

Evaluation of EMCD Throughput

In this section the performance of EMCD is measured in terms of throughput. EMCD is an early collision detection method which pauses transmission and listen for channel. Therefore the performance of EMCD is highly dependent on the duration of the TCDI as discussed in section 6.3. Another important factor is backoff mechanism which is used for retransmission and also affects the system throughput. Collision Detection Interval Different TCDI values and increase in stations from (2-10) have been used in this simulation. The ratio between TCDI and number of selectable are listed in the table 8.2.

8.4. Evaluation of EMCD

43

The CW is fixed to 15. The rest of values are used in the simulation according to the table 8.1. Collision Detection Interval TCDI

No. of Selectable

72

0

80

2

112

10

152

20

200

30

Table 8.2: EMCD TCDI values used in simulation

Figure 8.1: EMCD throughput on different TCDI values. Discussion on Result The fig 8.1 shows the results and further illustrates that how much the TCDI is important in the performance of EMCD. The simulation result is described on the following three points. – Smaller TCDI Value Smaller TCDI values lead to small number of choices for unique transmissions. At the TCDI 72, the choice for unique transmission is zero and the packet with physical and MAC header is transmitted, which leads to the undetectable collisions, that is illustrated in fig 6.8. As there is no successful transmission because

44


of collisions, so the throughput is zero. At the TCDI 80, the choice for unique transmission is 3. Since the detection ratio is increased due to more choices for unique transmissions as shown in fig 8.1. Due to the increase in detection ratio, the throughput is also increased. – Higher TCDI Value Higher TCDI value leads to more choices for the unique transmissions but it also causes increasing in the size of that unique transmission. When the collision occurs for that longer unique transmission, it causes longer delay and decrease in throughput. Fig 8.1 shows the decrease in throughput with increase of selectables e.g., 20 and 30. – Optimal TCDI Value As stations between 2 to 10 are utilizing the best channel resources on the selectable value 10 with TCDI value 112. Fig 8.1 result can be summarized as there is a trade off relation between the number of stations and the number of selectable. In other words, throughput is proportional to the collision detection ability. Contention Window (CW) value Optimal CW is one of the major factor that also increases the channel utilization. It always depends on the CW value. In this algorithm, the focus is about the effect of the retransmission on throughput but not about different contention schemes. Same values has been used in the simulation, which are listed in table 8.2. For this simulation, the number of stations increases from 10 to 100, and the CW values are 15, 31, 63, 100. The value for the TCDI is 112.

Figure 8.2: EMCD throughput on different CW values.

8.4. Evaluation of EMCD

45

Discussion on Results Fig 8.2 shows surprising results, the higher CW values (63,100) show the high throughput when the number of stations are few and constantly decreasing with the increase of number of stations. On the other hand, smaller CW values (15,31) achieve low throughput in the start and later maintains its throughput. If CW values are higher then the probability of collision is very low which improves performance of EMCD. But with the increase in the number of stations, the probability of collision is also increased. So at the higher CW value, the contention phase is relatively large that leads to the probability of longer delays and in smaller values of CW, the case is inverse. Inconsistent results in this figure show the aggregated affect of both TCDI and CW value on the throughput.

8.4.2

Collisions

In this section we study the comparison between detection and undetection collision probability at different number of selectables. The results are taken in Matlab by using following formulas. Undetectable U ndetectionP robability = (1/N )k−1

Figure 8.3: Undetection probability at different number of selectables

(8.1)

46


Detectable DetectionP robability = 1 − (1/N )k−1

(8.2)

Figure 8.4: Detection probability at different number of selectables Here N represents the number of stations and k represents the number of slots available or selectables. Discussion on Results The results are presented in fig 8.3 and fig 8.4. The ideal conditions for simulation is assumed. Queue is full and is always ready to send the packets. Only multicast stations are assumed. The collision detection capability increases with the increasing number of stations and the number of selectables. The detection capability is zero when there is no choice for the unique transmission, and the undetection is the inverse of detection capability. TCDI Collisions The simulation for collision detection on the MAC layer is the same simulation having the same parameters used for the throughput in the fig 8.1. The Figure is clearly showing the relationship between detection ratio and throughput. As we discussed in section 8.4.1, throughput increases with the increase in detection ability. The detection ability is high at the optimal TCDI value 112 as shown in the fig 8.5.

8.5. Evaluation of PREMA

47

Figure 8.5: Collision at MAC layer for vanguard transmissions

8.5 8.5.1

Evaluation of PREMA Effecting Parameter

This section presents the performance evaluation of PREMA on the bases of throughput. The evaluation of PREMA enables us to observe the effectiveness of parameters h and q, as described in section 5.1.3, and helps to find out the optimal value for these parameters. A simple scenario is considered in order to get optimal parameters for PREMA. In this simulation the number of stations is fixed to 50. The value of parameter q is increasing from .1 to .9 during the simulation. Therefore, eight different values of h are tested for each corresponding values of q. Similar results are presented in [12]. Discussion on Result As Gerger et al. suggests [12] and fig 8.6 shows that the optimal values of h and q is 4 and 0.5, respectively. The throughput is at maximum at these points. Lower values of q gives lower throughput. By using the lower value of q, the burst length is large and the throughput is wasted. As we show in the graph, the throughput is increased by the increase of q up to certain point 0.8 and then decreases. On the other hand, for the lower values of h means that less number of elimination phases, can not resolves the conflicts completely and throughput is low. By the increase of elimination phases,

48


Figure 8.6: Prema results on different values of q and h parameters throughput increases. From the 4th line of h as shown in fig 8.6 , there is no significant increase in throughput. By using four elimination phases, there is large probability of a single winner, so that the remaining elimination phases are wasted. In general, for the optimal lower value of h corresponds to the higher value of q and vice versa.

8.6

Comparison

In this section, the comparison of EMCD, PREMA and 802.11 is presented. The aim of this study is to distinguish the algorithms and to focus, which one is more reliable and efficient for the multicast traffic. The results are presented into two sections. In the first section, the throughput comparison for multicast traffic is presented. In the second section, more realistic scenarios are used with fixed number of broadcast stations and varying number of unicast stations.

8.6.1

Throughput Comparison of Multicast Traffic

The simulation is done with increasing number of stations from (10-100). The optimal values are used for this simulation to make the comparison. The EMCD TCDI optimal value 112 is used for this simulation. The optimal values used for PREMA are h=4 and q=0.5. The rest of values which are used are mentioned in table 8.2. Discussion on Results The PREMA outperforms the 802.11 and EMCD in all scenarios as shown in the fig 8.8 . The repeated elimination phases give the high throughput. The probability of a unique winner for the transmission after the eliminations is approximately 99%. The PREMA sustains the throughput during the simulation, and has effect of increase in the number of stations. That shows clearly the PREMA is the best option for the multicast traffic to any scale of network.

8.6. Comparison

49

Figure 8.7: Throughput comparison of PREMA, EMCD and 802.11

8.6.2

Throughput Comparison among Multicast and Unicast Stations

There are 10 multicast stations and increasing number of unicast station from 10 till 50, used in this simulation. The simulation starts from 10 multicast stations and 10 unicast stations are added simultaneously. The other values used in the simulation are listed in the table 8.1. last simulation result. The results are presented in fig 8.8. Both are part of the same simulation scenario.

Figure 8.8: Throughput comparison of PREMA, EMCD and 802.11. Discussion on Results As shown in the fig 8.8, fairness among the unicast and multicast traffic is relatively higher for PREMA. The results show that EMCD multicast stations are utilizing more channel resources as compared to other algorithms. The reason for this behavior that EMCD does not double CW value on collisions. Therefore, EMCD wins more channel access.

50


Chapter 9

Conclusion and Further Research In this thesis, We have studied several existing problems in order to use the multicast traffic in IEEE 802.11 networks. The lack of the ARQ protocol, unfairness among unicast and multicast traffic, and multiple multicast traffic in the same ESS causes high collision rate and poor performance. In this regard, IEEE 802.11 is unsuitable option for the multicast traffic in wireless networks. EMCD and PREMA are the focus of our study from different approaches for dealing with the multicast problems. The simulation results present the key parameters to improve the performance of both algorithms.The comparison between EMCD, PREMA and 802.11 algorithm is also presented.

9.1

EMCD

EMCD minimizes the overall packet loss rate and enhances throughput. EMCD performance is highly dependent on the TCDI value and at some extent on the CW value. The results are concluded and summarized as. – As shown in the results, the EMCD performance has a direct proportion to its detection capability. Detection ability is highly dependent on the throshlod value. – TCDI has a trade-off relation with the network. So, it always demands investigation of optimal TCDI value according to the scale of network. – EMCD algorithm has combined effect of two phases to solve the collision, Phase-I backoff phase and Phase-II EMCD collision detection. So its efficiency is dependent on the TCDI and CW value.

9.1.1

Advantages

– EMCD achieves high channel utilization in small scale networks. – EMCD based on collision detection on feedback which may minimize the hidden terminal problem in the multicast traffic[24].

52

Chapter 9. Conclusion and Further Research

9.1.2

Disadvantages

– EMCD does not work very well with the increase of the network load. – EMCD introduces additional overhead. – EMCD is expensive and complex to perform CCA operation during transmission. EMCD is harder to implement due to a pause during transmission, sense the channel and then again transmission.

9.2

PREMA

PREMA is a collision resolution algorithm that resolves collisions by jamming the channel with brusts. Two parameters h and q are used for the elimination phases and the probability q is used in the geometric distribution to sample the burst length, respectively. PREMA resolves the collision between multicast stations as well as between unicast and multicast. The conclusions of the results are summarized as.

9.2.1

Advantages

– PREMA achieves very high channel utilization and success probability. – The nature of PREMA is memoryless, i.e. each elimination phase is independent, that provides good fairness. – Burst length and the number of elimination phases are adjustable according to the scenario to achieve the maximum channel utilization. – PREMA performs well in large scale networks.

9.2.2

Disadvantages

– PREMA is not a good option to deal with the hidden terminal problem – PREMA is expensive too in terms of power loss during its jamming session.

9.3

Further Reading and Research

There are several issues with the proposed algorithms, that needs to be further investigated. Some of them are listed below

9.3.1

EMCD

– EMCD in terms of bursting efficiency [16]. – EMCD starts with contention phase and then moves to the collision detection phase to accomplish its task. Therefore it is dependent on both values. To improve the efficiency in EMCD, further study is needed to do research on alternative solutions about combining both phases (contention + collision detection).

9.3. Further Reading and Research

9.3.2

53

PREMA

– PREMA outperforms EMCD and IEEE 802.11 in throughput but is expensive in power utilization. SEMA is the motivated study for this issue [16].

54

Chapter 9. Conclusion and Further Research

Chapter 10

Summary In this thesis, our aim was to study the multicast services in IEEE 802.11 networks. IEEE 802.11 is a popular technology in the wireless communication world due to its low cost and ease of deployment. Mostly DCF basic access mechanism is used for the IEEE 802.11 networks, which is based on CSMA/CA ”listen before transmit” scheme. In this study, problems associated with multicast in IEEE 802.11 networks have been investigated. The lack of ARQ protocol, unfairness among unicast and multicast traffic and multiple multicast traffic in the same ESS leads to high collisions and poor performance. Different approaches have been studied to overcome the problems in multicast transmissions. There are a diversity of algorithms to solve the problems which are organized under the collision resultion, collision detection and channel reservation approaches. The focus of this study is on the two algorithms EMCD and PREMA using collision detection and collision resultion approaches respectively. EMCD is the collision detection method which performs listening operation during transmission to send a packet. EMCD is composed of three phases: 1.) Tv 2.) Carrier sensing 3.) main/collided transmission. The TCDI value is the key parameter to attain high performance. Correct values of CW also enhances performance. PREMA is a simple collision resolution algorithm that uses bursting and repeated elimination to resolve the channel conflicts for multicast transmission. Burst is used for channel jamming, and elimination phase determines the surviving station with the longest burst. PREMA is modified and integrated with IEEE 802.11, so that multicast transmission is handled by PREMA and unicast by IEEE 802.11. There are two optimal parameters of PREMA, h and q, where h is the number of elimination phases and q is the probability used in the geometric distribution to sample the burst length. PREMA resolves the collisions between multicast stations as well as between unicast and multicast. The results presented in this study, show that EMCD and PREMA improves the performance of multicast. The comparative study shows that PREMA outperforms EMCD and IEEE 802.11 in all scenarios due to its independence relevant to the scale of the network.

56

Chapter 10. Summary

Appendix A

List of Abbreviations AP AIFS AODV AP API ARQ BEB BER BO BPSK BSS BLBP CA CBR CCA CIFS CSMA/CA CW CWmax CWmin DCF DDD

Access Point Arbitration Inter Frame Space Ad-hoc On-demand Distance Vector Access Point Application Programming Interface Automatic Repeat Request Binary Exponential Backoff Bit Error Rate Back off Binary Phase Shit-Keying Basic Service Set Beacon Driven Leader Based Protocol Collision Avoidance Constant Bit Rate Clear Channel Assesment Collision Inter Frame Space Carrier Sense Multiple Access / Collision Avoidance Contention Window Contention Window Maximum Contention Window Minimum Distributed Coordination Function Data Display Debugger

58

DIFS DS DSSS EMCD ESS EY-NPMA FTP FHSS GloMoSim HiperLan HCF HTTP IBSS IP ISM IR IFS MAC Mbps MC ODMRP OFDM PARSEC PCF PIFS PREMA QoS SEMA SIFS SLBP SNR STA TCDI TCP TIFS Txmin UC UCLA UDP WFIFS Wifi WLAN WRP

Chapter A. List of Abbreviations

DCF Inter Frame Space Distribution System Direct Sequence Spread Spectrum Early Multicast Collision Detection Extended Service Set Elimination Yield Non-Preemptive Prioritized Multiple Access File Transfer Protocol Frequency Hopping Spread Spectrum Global Mobile Information System Simulator High Performance Radio Lan Hybrid Coordination Function Hypertext Transfer Protocol Independent Basic Service Set Internet Protocol Industrial Scientific and Medical Infra Red Inter Frame Space Medium Access Control Mega bit per second Multicast Station On-Demand Multicast Routing Protocol Orthogonal frequency-division multiplexing Parallel Simulation Envoirnment for Complex Systems Point Coordination Function Point Inter Frame Space Prioritized Repeated Elimination Multiple Access Quality of Service Silent Elimination Multiple Access Short Inter Frame Space Simple Leader Base Protocol Signal to Noise Ration Station Collision Detection Interval Transmission Control Protocol Time Medium- Inter Frame Space Transmission Minimum Unicast Station University of California, Los Angeles User Datagram Protocol Wait for Inter Frame Space Wireless Fidelity Wireless Local Area Network Wireless Routing Protocol

References [1] IEEE 802.11. http://en.wikipedia.org/wiki/IEEE 802.11, accessed 2008-05-15. [2] Multicast MAC extensions for high rate real-time traffic in Wireless LANs. http://www.nt.uni-saarland.de/projects/mac/, accessed 2008-02-15. [3] IEEE Std. 802.3, Part 3: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications. 1985. [4] IEEE Std. 802.11, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. 1997. [5] Broadband Radio Access Networks (BRAN); High Performance Radio Local Area Network (HiperLAN) type 1; functional specification. 1.2.1(ESTI), 1998. [6] IEEE Std. 802.11a, Supplement to Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: Higher-Speed Physical Layer Extension in the 5 GHz Band. 1999. [7] IEEE Std. 802.11b, Supplement to Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: Higher-Speed Physical Layer Extension in the 2.4 GHz Band. 1999. [8] Wireless LAN Medium Access Control (MAC) and Physical Layer Specifications, 1999. www.csse.uwa.edu.au/adhocnets/802.11-1999.pdf , accessed 2007-05-12. [9] I. Aad, C. Castelluccia, and R.A. INRIA. Differentiation mechanisms for IEEE 802.11. INFOCOM 2001. Twentieth Annual Joint Conference of the IEEE Computer and Communications Societies. Proceedings. IEEE, 2001. [10] Rajive Bagrodia, Richard Meyer, Mineo Takai, Yu an Chen, Xiang Zeng, Jay Martin, and Ha Yoon Song. PARSEC: A Parallel Simulation Environment for Complex Systems. IEEE Computer, 31(10):77–85, October 1998. [11] Jahanzeb Farooq and Bilal Rauf. Implementation and Evaluation of IEEE 802.11e Wireless LAN in GloMoSim. pages 1–8, Department of Computing Science, Umea University, Umea, Sweden, 2006. [12] Thomas Nilsson Greger Wikstrand. Prioritized Repeated Elimination Multiple Access: A Novel Protocol for Wireless Networks. Conference of the IEEE Computer Communications., 27(INFOCOM.), 2008. 59

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REFERENCES

[13] S.K Kasera J. Kuri. Reliable multicast in multi-access wireless LANs. Eighteenth Annual Joint Conference of the IEEE Computer and Communications Societies. Proceedings. IEEE, 7(INFOCOM ’99.):760–767, 1999. [14] Jean Tourrilhes. Robust Broadcast: Improving the reliability of broadcast transmissions on CSMA/CA. Hewlett Packard Laboratories, page 5, 2002. [15] J. Kuri and S.K. Kasera. Reliable Multicast in Multi-Access Wireless LANs. Wireless Networks, 7(4):359–369, 2001. [16] Thomas Nilsson. Distributed Multiple Access and Service Differentiation Algorithms for Wireless Networks. Umea University, SE-901 87 Ume˚ a Sweden, PHD Thesis, Dept. of Computing Science, Ume˚ a University, January 2008. [17] J. Nuevo. A Comprehensible GloMoSim Tutorial, 2003. [18] Raphael Rom. Local area and multiple access networks. Computer Science Press, Inc., New York, NY, USA, 1986. [19] Jochen Schiller. Mobile Communications. Addison-Wesley, second edition, 2003. [20] J.L. Sobrinho and AS Krishnakumar. Medium Access Control Layer. Bell Labs Technical Journal, 10:173, 1996. [21] Gerla .M Tang .K. MAC reliable broadcast in ad hoc networks. Military Communications Conference, 2001. MILCOM 2001. Communications for Network-Centric Operations: Creating the Information Force. IEEE, 2(2):1008– 1013, 2001. [22] Jerry Eriksson Thomas Nilsson, Greger Wikstrand. Collision detection method for multicast transmissions in CSMA/CA networks. Wireless Communications and Mobile Computing, 7(6):795 – 808, 7 Jul 2006. [23] Lennart Bondesson Thomas Nilsson, Greger Wikstrand. Silent Elimination Multiple Access: An Efficient Channel Brusting Protocol. [24] Greger Wikstrand. Human Factors and Wireless Network Applications More Bits and Better Bits. Technical Report UMINF 06.34, Dept. of Computing Science, Ume˚ a University, Ume˚ a, Sweden, 2006. [25] H. Wing Lo, Mouftah. Collision Detection and Multitone Tree Search for MultipleAccess Protocols on Radio Channels. Selectd Areas in Communications, 5(6):1035 – 1040, Jul 1987. [26] X.Zeng, R. Bagrodia, and M. Gerla. GloMoSim: A Library for the Parallel Simulation of Large scale Wireless Networks. In Proceedings of the 12th Workshop on Parallel and Distribution Simulation PADS, 1998.

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