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

Transcription

1 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 Lindström Umeå University Department of Computing Science SE UMEÅ SWEDEN

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3 Abstract In the Wireless Local Area Network(WLAN), IEEE is the most popular standard due to its low cost and easy deployment. However, IEEE 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 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 (T CDI ). 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 in all scenarios and independent of large network.

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5 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 Umeå. 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. iii

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7 Contents 1 Problem Specification Background Goal Of Thesis Tasks Introduction to IEEE Introduction System Architecture and Service Distributed Coordination Function (DCF) Collision Avoidance (CA) and Binary Exponential Backoff (BEB) An Example of DCF Operation Limitations of IEEE Limitations of DCF Mechanism Unfairness Multicast Different Approaches for Reliable Wireless Multicast Related Work Collision Resolution Collision Detection Channel Reservation Prioritized Repeated Elimination Multiple Access Introduction to PREMA Bursting and Elimination PREMA for IEEE Optimal Parameters Simple Scenarios of PREMA v

8 vi CONTENTS 6 Early Multicast Collision Detection Introduction to EMCD How Protocol Works Vanguard Transmission Carrier Sensing Phase III - Jamming/Normal Transmission Collision Detection Interval PHY and MAC Header EMCD Behavior in Different Scenarios Collision between two Multicast Senders Collision between Unicast and Multicast Senders Undetectable Collisions Design and Implementation Introduction to GloMoSim Design and Implementation Implementation of EMCD Working of EMCD Implementation of PREMA Working of PREMA Evaluation Introduction Performance Parameters Simulation Model Network Architecture Physical Parameters Evaluation of EMCD Throughput Collisions Evaluation of PREMA Effecting Parameter Comparison Throughput Comparison of Multicast Traffic Throughput Comparison among Multicast and Unicast Stations 49 9 Conclusion and Further Research EMCD Advantages Disadvantages PREMA Advantages

9 CONTENTS vii Disadvantages Further Reading and Research EMCD PREMA Summary 55 A List of Abbreviations 57

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11 List of Figures 2.1 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 Unfairness between uplink and downlink Multicast ACK implosion Hierarchal view of different approaches for reliable multicast [24] 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 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 Sequence diagram of EMCD with successful transmission Sequence diagram of EMCD with unsuccessful transmission Sequence diagram of PREMA EMCD throughput on different T CDI values EMCD throughput on different CW values Undetection probability at different number of selectables ix

12 x LIST OF FIGURES 8.4 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 Throughput comparison of PREMA, EMCD and

13 List of Tables 6.1 CIFS relation to Inter Frame Spaces 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 EMCD parameters used in simulations EMCD T CDI values used in simulation xi

14 xii LIST OF TABLES

15 Chapter 1 Problem Specification 1.1 Background The Wireless Local Area Network (WLAN) standard, IEEE is a popular technology due to its best effort services, low cost and ease of deployment. IEEE 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 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 , 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 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 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

16 2 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 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.

17 Chapter 2 Introduction to IEEE 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 [4] and HiperLAN (High Performance Radio LAN)1 /and 2 [5]. IEEE 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 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 a [6] and b [7]. The b improves the DSSS physical layer, which operates on the 2.4 GHz, and archives the maximum data transmission rate of 11 Mbps. The a 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.

18 4 Chapter 2. Introduction to IEEE System Architecture and Service IEEE 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 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

19 2.1. Introduction 5 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 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 is not a reliable protocol for multicast Distributed Coordination Function (DCF) DCF is the basic access mechanism for the IEEE 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 , 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 , 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 The AP is centrally controlling the channel through polling of individual stations. In PCF, the AP waits for a

20 6 Chapter 2. Introduction to IEEE 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 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 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

21 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 b, the possible values of the CW are 15, 31, 63, 127, 255, 511 and 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 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 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

22 8 Chapter 2. Introduction to IEEE 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.

23 Chapter 3 Limitations of IEEE IEEE is a set of standards developed for WLAN. The different standards of IEEE focus on the different aspects to improve the overall WLAN performance. For example e deals with QoS, i 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 standard to deal fairly with multicast traffic as it does with unicast traffic. 3.1 Limitations of DCF Mechanism IEEE 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 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 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 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

24 10 Chapter 3. Limitations of IEEE 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 Multicast Transmission of a single packet simultaneously to a station or group of stations is known as multicast. The IEEE 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 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.

25 3.1. Limitations of DCF Mechanism 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.

26 12 Chapter 3. Limitations of IEEE

27 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]

28 14 Chapter 4. Different Approaches for Reliable Wireless Multicast The main categories are briefly described below Collision Resolution The collision resolution approach is based on certain attributes, taken from the Hiper- LAN (High Performance Radio LAN) European standard to support the IEEE 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 networks. But, these protocols can be used to improve the multicast for IEEE networks 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 MAC scheme. Kuri and Kusera[13] proposed a protocol, which is an example for this technique 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 , 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

29 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.

30 16 Chapter 4. Different Approaches for Reliable Wireless Multicast

31 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 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

32 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 PREMA for IEEE PREMA is not implemented for the IEEE 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 The fig. 5.2 describes the operation of PREMA.

33 5.1. Introduction to PREMA 19 Figure 5.2: A flowchart of PREMA 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] Simple Scenarios of PREMA The behavior of PREMA is different in the different scenarios. Some of the scenarios are discuss below.

34 20 Chapter 5. Prioritized Repeated Elimination Multiple Access 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.

35 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.

36 22 Chapter 5. Prioritized Repeated Elimination Multiple Access

37 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 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.

38 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 T v, the second phase is the carrier sensing phase and the third phase is jamming or main transmission m. 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 T v transmissions depend upon the division of the packet, which is explained in the next section. Fig 6.3 describes

39 6.2. How Protocol Works 25 that how multiple stations choose different T v s and then during the pause, listen for the channel status which causes the T m or jamming of the channel. Figure 6.3: Different vanguard transmissions by three multicast senders Vanguard Transmission The basic idea is to divide the original packet into fragments: Vanguard transmission (T v ) and jamming or main transmission (T m ) packets. The size of T v can be calculated from the given equation (6.1) [22]. T v = T xmin + nt, (6.1) n U{0, (T CDI T xmin )/T } (6.2) T xmin is minimum allowable transmission time for the T v. If transmission is T xmin, then it means T v 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). T is the smallest time difference that can be detectable during the two T v s, transmitted by two different senders at the same time. T CDI is the maximum allowable transmission time of the T v plus Collision Inter Frame Space (CIFS). The parameter T CDI is explained in more detail in the section 6.3. The size of the T v depends on the T CDI time Carrier Sensing In the second phase, CIFS starts after the T v. 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 T m 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 a WLAN for the CCA to detect the presence of other signals on the same channel.

40 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 a 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 Duration CIFS 10µs SIFS 16µs PIFS 25µs DIFS 34µs Table 6.1: CIFS relation to Inter Frame Spaces [8] Phase III - Jamming/Normal Transmission During CIFS, if the medium is idle then the T m is started by creating the packet according the following equation. T m = pkt - T v ; (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 = T CDI - (T v +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 (T CDI ) is important for this algorithm as shown in the fig 6.2. T CDI basically affects the performance of EMCD in three dimensions. First, the length of the T CDI determines how many unique transmissions are selectable by the multicast senders. Smaller value of T CDI offers limited selectable unique transmissions, that increases the probability of collisions by selecting the same transmission time. Secondly, greater T CDI value affects the collision detection ability with unicast transmission. But T v also causes additional delay and more overhead. Fig 6.7 shows the possibility of the collision is undetectable by the longer T v. Certainly, it also affects on delay and overhead if retransmission is scheduled in longer T v.

41 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 T v s and it also does not cause longer T v s. 6.4 PHY and MAC Header The IEEE a [6] is the amended specification to IEEE This specification describes a high speed physical layer (PHY) which operates in the 5 GHz band. The technique used in the a 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 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 T v. 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 T CDI, 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 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