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(1)AGH University of Science and Technology Faculty of Electrical Engineering, Automatics, Computer Science and Electronics. Ph.D. Thesis Katarzyna Kosek-Szott. An Analysis of IEEE 802.11 EDCA Ad-hoc Networks in the Presence of Hidden Nodes. Supervisor: Prof. dr hab. inż. Andrzej R. Pach.

(2) AGH University of Science and Technology Faculty of Electrical Engineering, Automatics, Computer Science and Electronics Department of Telecommunications Al. Mickiewicza 30, 30-059 Kraków, Poland tel. +48 12 6173937 fax +48 12 6342372 www.agh.edu.pl www.kt.agh.edu.pl. c Katarzyna Kosek-Szott, 2010 Copyright All rights reserved Printed in Poland.

(3) Acknowledgements It is a pleasure to thank those who made this thesis possible. First of all I would like to thank my supervisor Prof. Andrzej R. Pach for his valuable comments, patience, and continuous encouragement. I would also like to show my gratitude to my colleagues. Most of all I would like to thank Marek Natkaniec who supported me in a number of ways. I thank him for his invaluable advise and cooperation. I also express my gratitude to Lucjan Janowski who helped me understand Markov chain-based modelling and to Luca Vollero who was my co-supervisor during the FP6 NoE CONTENT project. Last but not least, I would like to show my gratitude to my Family, especially my husband Szymon, for their strong support, patience, and love. They deserve my deepest appreciation..

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(5) Abstract This dissertation deals with quality of service (QoS) provisioning in IEEE 802.11 ad-hoc networks with hidden nodes. In particular, it focuses on the Enhanced Distributed Channel Access (EDCA) function of the Medium Access Control (MAC) layer of the IEEE 802.11 standard. Currently the only solution recommended by the IEEE 802.11 standard to minimise the negative effects caused by hidden nodes is the four-way handshake mechanism. However, an analysis of this mechanism presented in this dissertation shows that its performance is unsatisfactory to ensure QoS in current ad-hoc IEEE 802.11 networks. Therefore, the following thesis is formulated and proved: It is possible to improve the performance of the EDCA (Enhanced Distributed Channel Access) function by modifying the IEEE 802.11 standard in order to improve the efficiency of adhoc networks with hidden nodes. Furthermore, in such networks, it is possible to improve the fairness of traffic prioritising for the access categories defined in the IEEE 802.11 standard. The thesis is proved by the proposed BusySiMOn mechanism, which implements an intelligent two-step reservation procedure combined with the advantages of EDCA service differentiation. It is shown that when BusySiMOn is applied, the overall throughput, fairness and maximum frame delay are improved in comparison to the four-way handshake mechanism. Additionally, a novel mathematical model of the IEEE 802.11 EDCA function is proposed. This model is compared with simulations as well as numerical results obtained for two other models presented in the literature. The comparison gives satisfactory results regardless of the offered load, number of nodes, or network configuration. The proposed model can be used for rapid estimation of throughput of ad-hoc networks for the basic and the four-way handshake based channel access. It is kept reasonably simple to make it attractive to network designers..

(6) vi. Keywords: ad-hoc, Enhanced Distributed Channel Access, EDCA, hidden nodes, mathematical analysis, quality of service, QoS, simulations.

(7) Contents 1 Introduction 1.1 Published Papers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Dissertation Structure . . . . . . . . . . . . . . . . . . . . . . . . . 2 Medium Access in IEEE 802.11 2.1 DCF . . . . . . . . . . . . . . . 2.2 EDCA . . . . . . . . . . . . . . 2.3 Probability of Collisions . . . . 2.4 Hidden Node Problem . . . . . 2.5 Exposed Node Problem . . . . 2.6 Mathematical Model of EDCA. . . . . . .. . . . . . .. . . . . . .. . . . . . .. 3 The MAC Layer and the Hidden Node works 3.1 Pure Contention-based Protocols . . . 3.1.1 MACA . . . . . . . . . . . . . 3.1.2 MACAW . . . . . . . . . . . . 3.1.3 MACA-BI . . . . . . . . . . . . 3.1.4 FAMA . . . . . . . . . . . . . . 3.1.5 Four-way Handshake . . . . . . 3.1.6 Hybrid Channel Access Scheme 3.1.7 AA . . . . . . . . . . . . . . . . 3.1.8 MACP . . . . . . . . . . . . . . 3.1.9 EDCA/RR . . . . . . . . . . . 3.2 Busy Tone Signal-based Protocols . . 3.2.1 RI-BTMA . . . . . . . . . . . . 3.2.2 Black Burst . . . . . . . . . . . 3.2.3 DBTMA . . . . . . . . . . . . . 3.2.4 PUMA . . . . . . . . . . . . . . 3.2.5 JMAC . . . . . . . . . . . . . .. . . . . . .. . . . . . .. . . . . . .. . . . . . .. . . . . . .. . . . . . .. . . . . . .. . . . . . .. . . . . . .. . . . . . .. . . . . . .. . . . . . .. . . . . . .. . . . . . .. . . . . . .. . . . . . .. 5 7 9 11 11 12 14 15 15 16. Problem in Ad-hoc Net. . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. 17 17 18 18 19 20 21 21 22 22 23 24 24 24 25 26 27.

(8) viii. CONTENTS. 3.3 3.4 3.5. 3.6 4 The 4.1 4.2 4.3 4.4. 4.5. 3.2.6 DUCHA . . . . . . . . . . . . . . . . . . . . . . . . . Power-aware Protocols . . . . . . . . . . . . . . . . . . . . . Directional Antenna-based Protocols . . . . . . . . . . . . . Multiple Channel-based Protocols . . . . . . . . . . . . . . . 3.5.1 MAC Protocols with a Common Control Channel . . 3.5.2 MAC Protocols without a Common Control Channel 3.5.3 Hybrid Protocols . . . . . . . . . . . . . . . . . . . . Chapter Summary . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. 28 30 30 30 30 30 31 31. Impact of Hidden Nodes on EDCA Performance Simulation Software (ns-2) . . . . . . . . . . . . . . . . . Simulation Setup . . . . . . . . . . . . . . . . . . . . . . Networks with Hidden Nodes . . . . . . . . . . . . . . . Networks with Hidden and Exposed Nodes . . . . . . . 4.4.1 Line Topology Networks . . . . . . . . . . . . . . 4.4.2 Ring Topology Networks . . . . . . . . . . . . . Chapter Summary . . . . . . . . . . . . . . . . . . . . .. 5 BusySiMOn - A Novel MAC Protocol 5.1 Discussion on EDCA Service Differentiation . . . 5.2 BusySiMOn . . . . . . . . . . . . . . . . . . . . . 5.3 Effectiveness of Channel Reservation . . . . . . . 5.4 Compatibility with EDCA . . . . . . . . . . . . . 5.5 Implementation in ns-2 . . . . . . . . . . . . . . . 5.6 Simulation Study . . . . . . . . . . . . . . . . . . 5.6.1 One AC per Node — Evaluation Criteria 5.6.2 One AC per Node — Overhead Study . . 5.6.3 One AC per Node — Saturation . . . . . 5.6.4 One AC per Node — Non-saturation . . . 5.6.5 Four ACs per Node — Saturation . . . . 5.7 Chapter Summary . . . . . . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . .. . . . . . . .. . . . . . . .. . . . . . . .. . . . . . . .. . . . . . . .. 35 35 36 36 41 41 45 51. . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . . .. 53 53 54 56 58 59 60 60 61 62 73 75 76. 6 Conclusions and Future Work. 79. Appendices. 81. Appendix A Results A.1 Saturation . . . . . . . . . . . . A.1.1 Overhead Study . . . . A.1.2 Star-topology Networks A.1.3 Ring-topology Networks A.1.4 Line-topology Networks. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. . . . . .. 83 84 84 85 89 91.

(9) CONTENTS. ix. A.2 Non-saturation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 Appendix B A Novel Mathematical Model of B.1 Background . . . . . . . . . . . . . . . . . . B.2 EDCA Access Parameters . . . . . . . . . . B.3 Analytical Model . . . . . . . . . . . . . . . B.3.1 Throughput . . . . . . . . . . . . . . B.3.2 Probability of a Busy Medium . . . B.3.3 Frame Blocking Probability . . . . . B.3.4 Frame Collision Probability . . . . . B.3.5 Frame Generation Probability . . . . B.3.6 Probability of Saturation . . . . . . B.3.7 Markov Chain Analysis . . . . . . . B.3.8 Delay and Service Time . . . . . . . B.4 Model Verification . . . . . . . . . . . . . . B.4.1 Simulated Scenarios . . . . . . . . . B.4.2 Model Cost . . . . . . . . . . . . . . B.5 Appendix Summary . . . . . . . . . . . . . Appendix C Changes in the ns-2 C.1 Four-way Handshake Support C.2 IEEE 802.11 Compliance . . . C.3 Duplicate Frame Handling . .. EDCA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .. 95 97 101 101 101 104 104 104 105 105 106 108 109 111 118 119. Code 121 . . . . . . . . . . . . . . . . . . . . . 121 . . . . . . . . . . . . . . . . . . . . . 126 . . . . . . . . . . . . . . . . . . . . . 128. Bibliography. 129. Streszczenie. 137.

(10) Abbreviations AC. Access Category. ACK. Acknowledgement. ADDTS. Add Traffic Stream. AIFS. Arbitration Interframe Space. AIFSN. Arbitration Interframe Space Number. BB. Black Burst. BE. Best Effort. BK. Background. BTr. Receive-Busy Tone. BTt. Transmit-Busy Tone. BusySiMOn. Busy Signal-based Mechanism turned On. BusySiMOn v1 BusySiMOn version 1 BusySiMOn v2 BusySiMOn version 2 BusySiMOn v3 BusySiMOn version 3 CBR. Constant Bit Rate. CCC. Common Control Channel. CN. Competition Number. CSMA/CA. Carrier Sense Multiple Access with Collision Avoidance. CSR. Carrier Sensing Range.

(11) 2. CONTENTS. CTS. Clear to Send. CW. Contention Window. DCF. Distributed Coordination Function. DIFS. Distributed Coordination Function Interframe Space. DMBC. Distributed Multihop Binary Countdown. DS. Data Sending. EDCA. Enhanced Distributed Channel Access. EIFS. Extended Interframe Space. HCCA. Hybrid Coordination Function Controlled Channel Access. HR/DSSS. High Rate Direct Sequence Spread Spectrum. IFS. Interframe Space. MAC. Medium Access Control. NACK. Negative ACK. NAV. Network Allocation Vector. NCTS. Negative CTS. OSI. Open Systems Interconnection. PCF. Point Coordination Function. PHY. Physical. PIFS. Point Coordination Function Interframe Space. PLCP. Physical Layer Convergence Procedure. QoS. Quality of Service. RRTS. Request for RTS. RTR. Ready To Receive. RTS. Request to Send. SIFS. Short Interframe Space.

(12) SLRC. Station Long Retry Count. SSRC. Station Shot Retry Count. STP. Slot Time Period. TC. Traffic Category. TKN. Telecommunication Networks Group. TSPEC. Traffic Specification. TXOP. Transmission Opportunity. VI. Video. VO. Voice. WCC. Without Common Control Channel.

(13) 4. CONTENTS.

(14) Chapter 1. Introduction The IEEE 802.11 standard is currently one of the most popular wireless access technologies. It allows for quick and simple configuration of local, broadband networks at homes, in offices, or public places. It defines three types of networks: infrastructure, ad-hoc and mesh. This dissertation focuses on ad-hoc networks, which do not rely on pre-existing infrastructure, do not need complicated administration and may greatly facilitate Internet access. A basic example of an ad-hoc network is presented in Figure 1.1. It is a decentralised multi-hop network in which each node can communicate directly with other nodes within its communication range. Furthermore, each node participates in routing by forwarding traffic originating from other nodes.. Figure 1.1: Exemplary ad-hoc network Because of the half-duplex nature of radio transmissions the performance.

(15) 6. Introduction. of wireless ad-hoc networks heavily relies on Medium Access Control (MAC) protocols, i.e., protocols operating at the MAC sub-layer of the OSI model. Their goal is to define an efficient way of controlling access to the wireless radio channel and provide fair bandwidth sharing. The most popular wireless MAC protocol is CSMA/CA. In this scheme a transmitting node must check if the wireless channel is idle or busy. If the medium is busy, the node defers its transmission. When the medium is idle, the node transmits its data and waits for an acknowledgement. If no acknowledgement arrives then a collision at the receiver has occurred. Before resending its data, the transmitting node waits for the medium to be idle for a random number of predetermined time slots. Collisions may occur, especially if hidden nodes are present in the network. A basic example of the hidden node problem is illustrated in Figure 1.2. In the figure, node B is within the range of nodes A and C, while nodes A and C cannot sense each others’ transmissions. Therefore, every time when nodes A and C start their transmissions to B simultaneously a collision at B is unavoidable. In this case, nodes A and C are called hidden nodes.. A. B. C. Carrier Sensing Range. Collision. Transmission Direction. Figure 1.2: Exemplary scenario with two hidden nodes The most known solution currently used to minimise the negative effects of hidden nodes is the four-way handshake. This mechanism uses four different types of frames, i.e., Request to Send (RTS), Clear to Send (CTS), Data (DATA) and Acknowledgement (ACK) which are exchanged during the process of granting medium access. In the literature there are several concurrent solutions to the fourway handshake mechanism, however, none of them is broadly used. Additionally, it is the only mechanism recommended by the IEEE 802.11 standard to alleviate the problem of hidden nodes. With the growth of the popularity of the IEEE 802.11 standard, the number of available services also increased and the need for Quality of Service (QoS) provisioning became apparent. As a remedy to this problem, the HCF (Hybrid Coordination Function) Controlled Channel Access (HCCA) and the Enhanced Distributed Channel Access (EDCA) functions were proposed [24]. Both functions operate at the MAC layer. They define Traffic Categories (TCs) which allow assigning distinct priorities to different traffic types. The idea of TCs assures.

(16) 1.1 Published Papers. 7. better service for high priority traffic (e.g., Voice over IP) than for low priority traffic (e.g., e-mails). It has been shown that EDCA may not work properly if hidden nodes are present in the network, even if the four-way handshake is used to minimise their negative impact (Chapter 4). In particular it was shown that: • unhidden nodes are greatly favoured over hidden nodes in medium access, • the four-way handshake mechanism does not eliminate the unfairness in granting medium access, • for high priority traffic the performance of EDCA is the worst, • the presence of hidden nodes may cause significant degradation of network throughput. Such performance is unacceptable especially currently, when the role of QoS provisioning is growing. As a result the following thesis is proposed: It is possible to improve the performance of the EDCA (Enhanced Distributed Channel Access) function by modifying the IEEE 802.11 standard in order to improve the efficiency of adhoc networks with hidden nodes. Furthermore, in such networks, it is possible to improve the fairness of traffic prioritising for the access categories defined in the IEEE 802.11 standard. This dissertation describes a mechanism which can be used to improve the QoS provisioning in ad-hoc networks with hidden nodes. The new mechanism is based on both EDCA and the four-way handshake mechanism which assures interoperability with the IEEE 802.11 standard.. 1.1. Published Papers. The results presented in this dissertation were partially published in the following papers:. Journal Papers • K. Kosek-Szott, M. Natkaniec, A. R. Pach, A Simple but Accurate Throughput Model for IEEE 802.11 EDCA in Saturation and Non-saturation Conditions. Computer Networks (Elsevier), doi:10.1016 /j.comnet.2010.10.002..

(17) 8. Introduction. • K. Kosek, M. Natkaniec, A. R. Pach, Analiza symulacyjna sieci IEEE 802.11e o topologii gwiazdy w przypadku występowania stacji ukrytych (A Simulation Analysis of Star Topology IEEE 802.11e Networks in the Presence of Hidden Nodes), Telekomunikacja Cyfrowa, Technologie i Usługi. 2008/2009, no. 9, pp. 35-45. Conference Papers • K. Kosek-Szott, M. Natkaniec, A. R. Pach, BusySiMOn — a New Protocol for IEEE 802.11 EDCA-Based Ad-Hoc Networks with Hidden Nodes, IEEE GLOBECOM 2010, Miami, USA, 6-10 December 2010. • K. Kosek, M. Natkaniec, L. Vollero, Problems with Correct Traffic Differentiation in Line Topology IEEE 802.11 EDCA Networks in the Presence of Hidden and Exposed Nodes, Mobile Communications, ICCSA 2009, Seoul, Korea, 29 June-2 July 2009, LNCS 5593. • K. Kosek, M. Natkaniec, L. Vollero, Thorough Analysis of IEEE 802.11 EDCA in Ring Topology Scenarios with Hidden and Exposed Nodes, Wireless and Ad Hoc Networking, ICCSA 2009, Seoul, Korea, 29 June-2 July 2009, LNCS 5592. • K. Kosek, M. Natkaniec, S. Szott, Simulation Analysis of IEEE 802.11 EDCA Ad-hoc Networks, Society-Culture-Technology at the Dawn of the 21st Century, Krakow, Poland, 28-29 May 2009. • K. Kosek, M. Natkaniec, A. R. Pach, Analysis of IEEE 802.11e Line Topology Scenarios in the Presence of Hidden Nodes, ADHOC-NOW 2008, Sophia Antipolis, France, 10-12 September 2008, LNCS 5198. • K. Kosek, M. Natkaniec, L. Vollero, Thorough Analysis of 802.11e Star Topology Scenarios in the Presence of Hidden Nodes, IFIP NETWORKING 2008, Singapore, 5-9 May 2008, LNCS 4982. • K. Kosek, M. Natkaniec, L. Vollero, A. R. Pach, An Analysis of Star Topology IEEE 802.11e Networks in the Presence of Hidden Nodes, ICOIN 2008, Busan, Korea, 23-25 January 2008. • K. Kosek, M. Natkaniec, L. Vollero, A. R. Pach, Performance Analysis of 802.11e Networks with Hidden Nodes in a Star Topology, CCNC 2008, Las Vegas, USA, 10-12 January 2008. • K. Kosek, M. Natkaniec, L. Vollero, A. R. Pach, Simulation Study of 802.11e in the Presence of Hidden Terminals — a Star Topology Case, MedHocNet, Corfu, Greece, 13-15 June 2007..

(18) 1.2 Dissertation Structure. 9. Workshop Papers • K. Kosek, Problems with Providing QoS in EDCA Ad-hoc Networks with Hidden and Exposed Nodes, IEEE INFOCOM 2009 Student Workshop, Rio de Janeiro, Brazil, 19-25 April 2009. • K. Kosek, Overview of the Hidden Terminal Problem in IEEE 802.11 Networks, CONTENT PhD Student Workshop, Leganes, Spain, 13 February 2007.. 1.2. Dissertation Structure. This dissertation is structured as follows. Chapter 2 describes parts of the IEEE 802.11 standard relevant to this dissertation, i.e., the EDCA function and its predecessor — DCF. Chapter 3 constitutes a survey of MAC layer protocols alleviating the hidden node problem. Chapter 4 shows the destructive impact of hidden nodes on the performance of EDCA in different network topologies. A novel MAC layer protocol is described in Chapter 5. The dissertation is concluded in Chapter 6. Additionally, Appendix A contains detailed simulation results, Appendix B describes a new mathematical model of EDCA, and Appendix C includes code samples illustrating changes to the used simulator..

(19) 10. Introduction.

(20) Chapter 2. Medium Access in IEEE 802.11 In the first version of the IEEE 802.11 standard (released in 1997) there were two medium access functions — Distributed Coordination Function (DCF) and Point Coordination Function (PCF). Next, QoS was introduced to the IEEE 802.11 MAC through EDCA and HCCA functions. DCF is the fundamental access method of the IEEE 802.11 MAC and EDCA is an enhanced variant of DCF. These functions were designed for both ad-hoc and infrastructure networks. PCF and HCCA are alternatives designed for infrastructure networks. Therefore, in this chapter only DCF and EDCA are described. For more details on PCF and HCCA the reader is referred to [24]. Parts of this chapter have been published in [35, 38, 41].. 2.1. DCF. DCF uses the CSMA/CA protocol. Before each transmission a node must sense the medium and determine if another node is currently transmitting. If the medium is idle the transmission may take place. If the medium is busy, the node must defer until the ongoing transmission is finished. After deferral, the node must select a random Backoff interval. The Backoff interval counter is decremented every time slot during which the medium is idle. In order to minimise the probability of collisions the node may send additional Request to Send (RTS) and Clear to Send (CTS) frames to reserve the medium. This procedure is called the four-way handshake and is described in more detail in Section 3.1.5. The time diagram of the DCF medium access procedure is illustrated in Figure.

(21) 12. Medium Access in IEEE 802.11. 2.1, in which each event is preceded by an appropriate Interframe Space (IFS) time interval: DCF IFS (DIFS) — used prior to transmission of DATA and RTS frames as well as before Backoff countdown, Short IFS (SIFS)— used prior to transmission of an ACK frame, the subsequent fragment of a DATA frame or a CTS frame. Immediate Access if Medium Idle ≥ DIFS. Contention Window. DIFS. DIFS Busy Medium SIFS. Backoff Slots. DATA. SIFS ACK. Decrement Backoff when Medium is Idle. Defer Access. Figure 2.1: The DCF channel access procedure [24]. The length of the Backoff interval is computed in the following way: h i Backof f = Random 0, CW. (2.1). where CW is the current Contention Window size. The CW parameter initially equals CWmin . After each unsuccessful transmission attempt it is doubled, until it reaches the CWmax value. After CWmax is reached, CW remains unchanged until it is reset. The number of allowed retransmissions is limited by the Station Long Retry Count (SLRC) for the basic DCF channel access and by the Station Shot Retry Count (SSRC) for the four-way handshake. Exemplary DCF parameters (defined for High Rate Direct Sequence Spread Spectrum — HR/DSSS, commonly known as IEEE 802.11b) are given in Table 2.1. Table 2.1: DCF parameter set for HR/DSSS. 2.2. DIFS 50 µs. SIFS 20 µs. Slot 10 µs. CWmin 31. CWmax 1023. SSRC 7. SLRC 4. DATA max 4095 B. ACK/CTS 112 b. RTS 144 b. EDCA. In networks with heterogeneous traffic the QoS requirements of each service should be carefully taken into account. In particular, in the case of simultaneous transmissions of voice and data traffic the delay constraints of the voice service should be primarily met. To achieve this goal voice traffic should have a certain.

(22) 2.2 EDCA. 13. priority over data traffic. Within wireless ad-hoc networks it is the EDCA function of the IEEE 802.11 standard which was designed to satisfy this requirement. The EDCA function defines several QoS enhancements to the legacy IEEE 802.11 DCF. It introduces four Access Categories (ACs): Voice (VO), Video (VI), Best Effort (BE), and Background (BK). VO has the highest and BK has the lowest priority. Inside a QoS node each frame of a particular traffic stream is mapped into an appropriate AC and then it is buffered into a hardware queue (Figure 2.2).. Classifier: Mapping to ACs Higher Priority. Lower Priority. VO. VI. BE. BK. Backoff [VO]. Backoff [VI]. Backoff [BE]. Backoff [BK]. Virtual Collision Handling. Transmission Attempt. Figure 2.2: EDCA implementation model. To provide traffic differentiation each AC has a different set of the following medium access parameters: the CW minimum (CWmin [AC]) and maximum (CWmax [AC]) size, the arbitration interframe space number (AIF SN [AC]), and the transmission opportunity limit (T XOPLimit [AC]). The functions of the access parameters are as follows. CWmin [AC] and CWmax [AC] determine the number of Backoff slots in the following way: ". . Backof f [AC] = Random 0, min 2 (CWmin [AC]+1)−1, CWmax [AC] k. # (2.2). where k is the number of times the currently transmitted frame has collided. AIF SN [AC] determines the minimum time interval before a frame transmission.

(23) 14. Medium Access in IEEE 802.11. and each decrement of the Backoff counter: AIF S[AC] = AIF SN [AC] × Te + SIFS. (2.3). where Te is the duration of a single slot time. T XOPLimit [AC] allows for the consecutive transmission of several frames after gaining medium access, known as contention free bursting. This parameter is optional. The EDCA parameter set for HR/DSSS is presented in Table 2.2. Table 2.2: EDCA parameter set for HR/DSSS AIF SN 2 2 3 7. Access Category Voice Video Best Effort Background. CWmin 7 15 31 31. CWmax 15 31 1023 1023. T XOPLimit 3.264 ms 6.016 ms 0 0. The impact of these parameters on the channel access procedure is shown in Figure 2.3. Every node is assigned the right to transmit after the medium was sensed idle for AIF S[AC] and when the Backoff time has elapsed. Therefore, the smaller the AIF SN [AC] and the CW sizes, the higher the probability of being granted access to the wireless medium before other ACs. Immediate Access if Medium Idle ≥ AIFS[AC]. AIFS[AC] Contention Window. AIFS[AC] Busy Medium SIFS. Backoff Slots. Defer Access. DATA. SIFS ACK. Decrement Backoff when Medium is Idle. Figure 2.3: The EDCA channel access procedure [24]. 2.3. Probability of Collisions. Two types of collisions may occur during the EDCA channel access procedure — virtual and physical. A virtual collision happens when more than one AC in a given node is granted the right to transmit at the same time. In such a case, a node is obliged to transmit the higher priority frame and delay the lower priority ones. This procedure is called virtual collision handling (Figure 2.2) and was not present in DCF. A physical collision occurs when two or more nodes start their transmissions over the wireless medium simultaneously (this type of.

(24) 2.4 Hidden Node Problem. 15. collisions occurs also for DCF). After a node receives a collided (erroneous) frame the medium must be idle for an Extended IFS (EIFS) under DCF or for EIFS − DIFS + AIF S[AC] under EDCA, before another transmission may take place. EIFS is given by the following equation EIFS = SIFS + DIFS + ACKTxTime. (2.4). where ACKTxTime is the time required to transmit an entire ACK frame (together with its PHY overhead). The number of physical collisions may be especially high if hidden nodes are present within the network. Therefore, a solution which minimises the probability of collisions caused by hidden nodes is desirable.. 2.4. Hidden Node Problem. The hidden node problem was first described in 1975 by Kleinrock and Tobagi [31]. Since then it has been studied in many papers [2, 4, 9, 15, 16, 17, 21, 22, 27, 54, 65, 70, 71, 74]. It is considered as one of the major problems in the performance of wireless networks because it may lead to severe performance degradation and unfairness in accessing the medium. Therefore, there is a number of papers describing methods of minimising the negative impact of the hidden node problem on the performance of ad-hoc networks. They are described in more detail in Chapter 3. The hidden node problem was illustrated in Figure 1.2. The source of the problem is the inability of hidden nodes (nodes A and C) of sensing the ongoing transmission of a transmitter (nodes C and A, respectively). They assume that the medium is idle and start their own transmissions causing collisions at node B. After they realise that the collisions occurred they initialise the Backoff procedure [24]. When the Backoff counter decreases to zero, they retransmit their frames. Obviously, there is no guarantee that the retransmitted frame will be successfully received by the receiver. Therefore, the number of retransmissions in networks with hidden nodes may be indeterminate [54].. 2.5. Exposed Node Problem. Another problem appearing in multi-hop ad-hoc networks is the exposed node problem. It is illustrated in Figure 2.4. Node B transmits data to node A. This transmission is also sensed by node C which defers its transmission to D, even though it would not cause a collision..

(25) 16. Medium Access in IEEE 802.11. A. B. C. D. Deferred Transmission. Carrier Sensing Range Transmission Direction. Figure 2.4: Exemplary scenario with two exposed nodes The exposed node problem influences the spatial reuse and causes bandwidth under-utilisation [81]. This problem is signalised because, apart from topologies with only hidden nodes, topologies with both hidden and exposed nodes are studied in this dissertation. This allows analysing ad-hoc networks from a wider perspective. Exemplary solutions to the exposed node problem are described in [12] and [56].. 2.6. Mathematical Model of EDCA. During the course of studies it occurred that there are no mathematical models which sufficiently address all the important aspects of EDCA. Therefore, a novel EDCA model is proposed in Appendix B, which supports correct Backoff countdown, proper handling of frames, and allows for traffic differentiation with the use of EDCA access parameters (AIF SN , CW M IN and CW M AX ). Additionally, the model distinguishes the medium blocking and frame blocking probabilities and includes a new method of modelling AIF S differentiation. Furthermore, it can be used with the basic and the four-way handshake (also referred to as RTS/CTS) channel access methods. The model performs well for a small and large number of nodes, small and large frame sizes and for different traffic types. Finally, the model is kept reasonably simple to make it attractive for network designers..

(26) Chapter 3. The MAC Layer and the Hidden Node Problem in Ad-hoc Networks A number of MAC protocols addressing the problem of hidden nodes have been proposed in the literature. Some of them were created strictly to cope with the problem of hidden nodes, others address this problem while concentrating mainly on different issues of wireless ad-hoc networks. In general, these protocols can be classified as: pure contention-based, busy tone signal-based, multiple channelbased, power-aware, and directional-antenna-based. In this chapter the first two families of protocols are described in detail because they constitute a background for the proposed MAC protocol described in Chapter 5. The other families are shortly presented to give a comprehensive overview of the protocols related to the hidden node problem.. 3.1. Pure Contention-based Protocols. Pure contention-based protocols can be divided into three groups: sender-initiated, receiver-initiated and hybrid. They have the following two major advantages. Firstly, nodes can use standard hardware with a single transceiver, which is inexpensive and easily available. Secondly, if standard frames are used then compatibility with IEEE 802.11 [24] is achieved..

(27) 18. 3.1.1. The MAC Layer and the Hidden Node Problem in Ad-hoc Networks. MACA. Multiple Access with Collision Avoidance (MACA) [30] is a sender-initiated protocol which first introduced two fixed-size signalling frames (RTS and CTS) in order to alleviate the hidden node problem. The RTS frame is sent by a sender to a receiver (Figure 3.1). Every time when the sender’s neighbouring nodes overhear the RTS frame they must defer their transmission. The RTS frame contains information on the length of the planned transmission. As a reply to the RTS frame, the receiver must send the CTS frame, which also contains information on the transmission length. As a result, after the neighbouring nodes overhear the CTS frame, they must defer for the length of the expected transmission. Moreover, if two RTS frames collide which each other, each sending node must wait for a randomly chosen Backoff interval before invoking its transmission again. This procedure is repeated as long as one of the senders overhears CTS from its receiver. For the situation presented in Figure 1.2, if node A sent an RTS frame to node B, node B would immediately send a CTS frame in reply (only if it was not involved in another transmission). The CTS frame would be overheard by nodes A and C. Node C would defer its transmission and no collision would occur.. RTS. DATA. Sender. Time. CTS Receiver. Figure 3.1: MACA operation One of the biggest disadvantages of MACA is the lack of acknowledgements of a successful transmission at the MAC layer. With MACA all retransmissions must be performed by the transport layer which increases the overall transmission delay. This makes the protocol unsuitable for delay sensitive traffic, e.g., voice and video.. 3.1.2. MACAW. MACA for Wireless (MACAW) [5] is a sender-initiated protocol built on the basis of MACA. This mechanism uses four signalling frames (RTS, CTS, ACK, and Data Sending — DS). The DS frame is used to inform deferring nodes about the length of the subsequent DATA frame. In comparison to MACA the acknowledgements at the MAC layer significantly decrease the delay of a transmission, which allows for faster error recovery..

(28) 3.1 Pure Contention-based Protocols. 19. MACAW can also take advantage of RRTS (Request for RTS). This frame is sent whenever a deferring node receives an RTS. After the successful contention during the next contention period it sends the RRTS frame to the sender of the RTS which immediately responds with an RTS frame. All other nodes overhearing RRTS must defer for long enough to hear the successful RTS/CTS exchange.. 3.1.3. MACA-BI. MACA By Invitation (MACA-BI) [62] is another mechanism based on MACA. Instead of the RTS and CTS frames it uses a Ready To Receive (RTR) frame. The RTR frame is simply a renamed CTS frame. It serves as the polling frame (Figure 3.2) and is sent by the receiver to the sender. Therefore, MACA-BI is a receiver-initiated protocol.. DATA Time. Sender RTR Receiver. Figure 3.2: MACA-BI operation In MACA-BI a sender cannot send data before being asked, therefore, a receiver needs to have a built-in traffic prediction algorithm so as to know when to ask the sender for its data. To achieve this, the authors of MACA-BI propose piggybacking the information regarding the frame queue length and data arrival rate in the sender’s DATA frame. Additionally, whenever an RTR frame has not been received by the sender for a given time it can send an explicit RTS packet (in such a case the mechanism changes into MACA). Therefore, MACA-BI is suitable for networks with predictable traffic patterns as its performance degrades to MACA in case of periods of inactivity, which are common for bursty traffic. Furthermore, the authors of MACA-BI stress that the control frames may collide with each other and/or DATA frames and lead to protocol failures. Recovery from such a situation is possible only by using ACK frames, however, explicit acknowledgements are not implemented in MACA-BI. In 2009 an enhancement of MACA-BI, called Slotted MACA-BI [57], was proposed. The new scheme divides the wireless channel into fixed slots of equal size. The length of each slot is equal to the sum of the end-to-end propagation delay and the RTR frame size..

(29) 20. 3.1.4. The MAC Layer and the Hidden Node Problem in Ad-hoc Networks. FAMA. There is a family of sender-initiated Floor Acquisition Multiple Access (FAMA) protocols which includes: FAMA with Non-persistent Carrier Sensing (FAMANCS) and FAMA for Non-persistent Packet Switching (FAMA-NPS) [14]. Both protocols require the sender to obtain control of the floor (i.e., the wireless channel) before it is allowed to send any data. The goal is to avoid collisions at the receiver side. The reservation of the wireless channel is done with the use of the RTS/CTS exchange. The authors of FAMA claim that it is possible for the signalling frames to collide, however, DATA frames do not suffer from collisions. In order to obtain the medium, a sending node transmits RTS using either carrier sensing (it uses the CSMA protocol to send RTS) or packet sensing (it uses the ALOHA [3] protocol for the RTS transmission). The receiver responds with a CTS frame long enough to avoid any hidden nodes’ transmissions (Figure 3.3). This behaviour corresponds to a single channel Busy Tone Multiple Access (BTMA) scheme [64] which uses a busy tone signal sent on a separate busy tone channel to signalise transmission on a data channel. A. RTS. CTS. B C. RTS. Jamming Time. Figure 3.3: FAMA operation [14] FAMA-NCS is similar to the four-way handshake protocol (described in Section 3.1.5). The main difference between the two mechanisms is that FAMA-NCS uses longer CTS frames. On the other hand, FAMA-NPS does not require nodes to sense the medium before transmission. Additionally, it uses RTS and CTS frames of the same length. The time of transmission of these frames is longer than maximum round-trip delay. The authors assume that FAMA-NPS may help avoid problems caused by hidden nodes only in fully connected networks in which CTS frames are transmitted just once (i.e., every hidden sender recognises at once the node which has acquired the wireless channel). Therefore, it is the FAMANCS which is recommended by the authors because, in their opinion, it addresses the hidden node problem more effectively. However, FAMA-NCS requires each node to hear the interference to keep silent for a period of a maximum data unit. As a consequence, when the RTS/CTS negotiation fails or the transmitted DATA frames are short this solution is ineffective..

(30) 3.1 Pure Contention-based Protocols. 3.1.5. 21. Four-way Handshake. SIFS. SIFS. RTS. SIFS. Sender 1. DIFS/AIFS. In order to minimise the negative effects of hidden nodes on network performance the IEEE 802.11 standard (described in Chapter 2) assumes a four-way handshake exchange. The four-way handshake is a sender-initiated mechanism which involves four different types of frames: RTS, CTS, DATA and ACK as illustrated in Figure 3.4.. DATA. CTS. ACK. Receiver 1 NAV (RTS). BACKOFF. RTS. SIFS. Sender 2. SIFS. NAV (CTS) DIFS/AIFS. Other. DATA. CTS. Receiver 2 DEFER Intended Transmission (Detected Busy Medium). Figure 3.4: Four-way Handshake Mechanism Each time an ad-hoc node wants to transmit data the wireless channel has to be sensed idle for a predefined time period (i.e., DIFS for DCF and AIFS for EDCA). If the medium is busy, the node must set its Backoff timer (similarly as described in Chapter 2). When the Backoff timer expires the node is allowed to start the four-way handshake. The RTS and CTS frames include information on the duration of the intended transmission. This allows other nodes to appropriately set their Network Allocation Vectors (NAVs)1 and minimise the number of collisions in the network.. 3.1.6. Hybrid Channel Access Scheme. The New Hybrid Channel Access Scheme for Ad Hoc Networks [68] is designed so as to adapt its channel access scheme and take advantage of either senderinitiated (SI) or receiver-initiated (RI) handshakes (Figure 3.5). The SI mode is the default mode and the RI mode is triggered only if the SI fails. The authors claim that their scheme fits within the IEEE 802.11 standard, is simple and does not introduce any new control frames. 1 NAV is a counter which determines how long a node must deffer from accessing the wireless medium..

(31) 22. The MAC Layer and the Hidden Node Problem in Ad-hoc Networks. SI Mode Threshold Exceeded (Failed RTSs) Empty Queue. RI Setup CTS Received. SI Mode Packets with RI flag cleared. Packets with RI flag. RI Associated. RI Mode. Sender. Receiver. Figure 3.5: Sender/Receiver statuses [68] The sender/receiver pair decides on the RI mode when the sender sends the same RTS frame for more than half of the time allowed by the IEEE 802.11 standard. In most cases, if the receiver is unable to send a response to the RTS frame the contention around it is very severe. Therefore, according to authors, it is better to let the sender change to the RI Setup mode. In this mode the sender sets the RI flag in every frame it transmits to the receiver. In this way it requests the receiver to change to the RI mode. If the sender does not receive a CTS frame from the receiver it can assume that the receiver is down. However, if the CTS frame is received, the sender enters the RI associated mode and sets the RI flag in each DATA frame it sends to the receiver. The RI flag can be cleared only when the sender does not have any more data to send. After the overload conditions are mitigated, the sender/receiver pair returns to the SI mode. The two most important advantages of the described behaviour are the reduced number of collisions (also caused by hidden nodes) and shorter queueing delays [68].. 3.1.7. AA. Advance Access (AA) [76] is a sender-initiated protocol which uses modified RTS and CTS frames in order to reserve the wireless channel. Each RTS and CTS frame contains an appropriate lag time for the intended transmission which separates these control frames from their associated DATA frames (Figure 3.6). This allows for scheduling simultaneous transmissions which are not possible if the standard DCF function is used. Additionally, ACK frames can be detached from their associated DATA frames or piggybacked on RTS/CTS frames (Figure 3.6).. 3.1.8. MACP. Multiple Access Collision Prevention (MACP) [78] is a family of sender-initiated protocols in which nodes use prohibiting signals in order to compete with other.

(32) 3.1 Pure Contention-based Protocols. A. B. 23. C. B. RTS. D. DATA Time. A. CTS. ACK. RTS. C. DATA. RTS Time. D. CTS. CTS. Figure 3.6: Simultaneous transmissions with AA [76]. nodes. The competition among nodes is based on the idea of Distributed Multihop Binary Countdown (DMBC). In each round of DMBC competing nodes select appropriate Competition Numbers (CNs). A node with the longest CN wins the competition. After the competition is won MACP uses a traditional four-way handshake mechanism to send DATA frames. The authors of MACP claim that the protocol can be extended to support QoS by assigning large CNs only to high priority frames. Finally, they admit that with two transceivers the MACP protocol can achieve better results.. 3.1.9. EDCA/RR. EDCA with Resource Reservation (EDCA/RR) [43] is an enhancement of the IEEE 802.11 EDCA function. In order to reserve the wireless channel EDCA/RR uses Add Traffic Stream (ADDTS) requests and ADDTS responses. The ADDTS requests contain Traffic Specification (TSPEC), which describes characteristics of traffic flows, e.g., data rate, delay bound [24]. This is done in order to transfer the admission control and scheduling mechanisms designed for infrastructure networks into ad-hoc networks. The operation of EDCA/RR is presented in Figure 3.7. Node C defers its transmission after it is informed by node B about A’s transmission time..

(33) 24. The MAC Layer and the Hidden Node Problem in Ad-hoc Networks ADDTS request. A. ADDTS response DATA. B. ADDTS response. C. DATA. Figure 3.7: EDCA/RR [43]. 3.2. Busy Tone Signal-based Protocols. All busy tone signal-based protocols take advantage of one or more busy tone signals. These signals are used to keep hidden nodes silent during transmission. They can be divided into single and multiple channel-based. Single channel protocols use standard hardware and are partially or fully interoperable with the IEEE 802.11 standard. Multiple channel-based protocols require more complex hardware. The main advantage of busy tone signal-based protocols is that busy tones can be recognised more easily than traditional MAC frames. Additionally, several of these protocols support QoS.. 3.2.1. RI-BTMA. The Receiver-Initiated Busy Tone Multiple Access (RI-BTMA) scheme [72] divides the total bandwidth into two channels — one for the transmission of data, and one for the busy tone signal. Prior to DATA transmission, the sender sends a preamble which identifies the receiver on the data channel. The preamble can be sent only if the channel is idle and, thus, if the channel is busy the preamble transmission must wait a random time. After the preamble is successfully received, the receiver immediately sends a busy tone signal on the busy tone channel. This signal informs the sender of a successful preamble transmission and reservation of the data channel. Such behaviour prevents hidden nodes from transmitting data on the data channel. Then, the sender transmits its data. During the reception of data, the receiver keeps sending the busy tone on the dedicated channel to keep other nodes silent.. 3.2.2. Black Burst. The Black Burst protocol [60] provides a bounded time delay for real-time traffic in ad-hoc networks. It extends the basic CSMA access method by introducing pulses of energy called Black Bursts (BBs). Nodes sending real-time traffic use BBs to contend for accessing the wireless channel. The lengths of BBs are proportional to the time the nodes had to wait for the channel to become idle, measured from the first attempt to access the channel until the beginning of a transmission..

(34) 3.2 Busy Tone Signal-based Protocols. 25. After transmitting its BB, the node waits for a specified time interval to see if any other node is transmitting a longer BB. If the channel is idle after this interval the node can immediately transmit its real-time frame. Otherwise, it waits for the next channel access cycle and repeats the algorithm. A round-robin discipline among nodes transmitting real-time frames is enforced, which results in bounded access delays. The Black Burst protocol can also support asynchronous data transmissions. In such a case nodes sending asynchronous traffic use a longer IFS than nodes sending real-time traffic. Real-time frames are always favoured over asynchronous data frames due to the BB contention scheme. In order to reduce overhead and provide robustness against hidden nodes, the Black Burst protocol can operate with the use of negative acknowledgements. Each time a receiver does not receive an expected real-time frame it may send a negative acknowledgement to the sender in the form of an invitation mini-frame. The main advantage of the Black Burst protocol is that it can be combined with IEEE 802.11 DCF and implemented, with minor modifications, in current WLAN cards.. 3.2.3. DBTMA. The Dual Busy Tone Multiple Access (DBTMA) protocol [12] was designed on the basis of the BTMA [64] and RI-BTMA [72] protocols. The goal of DBTMA is to meet the needs and requirements of ad-hoc networks. The authors show that the network utilisation of DBTMA is about twice as that of RTS/CTS-based schemes. DBTMA divides the single common channel into two sub-channels — a data channel and a control channel. No synchronisation between nodes is required. DATA frames are transmitted on the data channel, while control frames (i.e., RTS/CTS) are transmitted on the control channel. Additionally, two narrowband tones (Receive-Busy Tone — BTr and Transmit-Busy Tone — BTt) are added to the control channel with enough spectral separation in order to separate the directions of forward and reverse communication. A basic time diagram for the DBTMA protocol is presented in Figure 3.8. The functionality of the protocol is given next. The sender may send its RTS on the control channel to set up a transmission request only after it did not sense any BTr on the control channel. It must also keep on sensing the BTr signal during the transmission of its RTS frame (to check if other nodes did not send a BTr during this time). If it senses a BTr during this period it will defer its transmission even after the reception of a CTS frame from the receiver. When the receiver receives the RTS frame, it senses BTt on the control channel. Every time when there is no BTt (i.e., other nodes in the receiver’s area do not transmit.

(35) 26. The MAC Layer and the Hidden Node Problem in Ad-hoc Networks BTt RTS. DATA. Sender. Time CTS Receiver. BTr. Figure 3.8: DBTMA time diagram data on the data channel) the receiver replies with a CTS frame and turns on the BTr. Otherwise it keeps silent. After receiving the CTS frame the sender node turns on BTt on the signalling channel and starts its transmission. When the transmission is finished, the sender turns off its BTt. The use of busy tones helps to avoid any unwanted transmission on the data channel as every node that senses BTr or BTt will not start its own transmission. Regarding hidden nodes, it is the BTr signal which helps to prevent their simultaneous transmissions. Additionally, since hidden nodes can reply to RTS requests they are allowed to use the channel together with unhidden nodes. The main advantage of DBTMA is that it gives a simple solution to the hidden node problem. The busy tone signals do not require much bandwidth and they are easy to decode. However, DBTMA also has several flaws. First of all, it does not use ACKs and it is not backward compatible with the IEEE 802.11 standard. Furthermore, it requires additional transceivers and channels. It is also difficult to optimally divide the spectrum between the control and data channels. It should be done adaptively depending on the current traffic load. Finally, to avoid interchannel interference, there must be enough spectral separation between the data and control channels.. 3.2.4. PUMA. The Priority Unavoidable Multiple Access (PUMA) protocol [52] enhances DCF to support strict priority isochronous traffic transmission in the ad-hoc mode. In PUMA every active node measures SIFS, PIFS(Point Coordination Function IFS), and DIFS intervals (SF IS < P IF S < DIF S [24]) after the end of each frame transmission in order to start its own transmission. If the medium is determined to be idle for an interval exceeding PIFS, the sender proceeds with an isochronous transmission by sending an appropriate JAM signal of the length of one slot to inform other nodes. The PIFS interval was selected by the authors.

(36) 3.2 Busy Tone Signal-based Protocols. 27. Busy Medium. PIFS. of PUMA in order to silence all legacy IEEE 802.11 nodes which need to sense the wireless channel to be idle for a DIFS before each transmission (cf., Chapter 2). Upon receiving JAM the nodes defer their transmissions until the reception of an RTS or CTS frame to update their NAVs. After successful channel reservation, the sender transmits its DATA frame in a collision-free manner. Figure 3.9 illustrates a typical isochronous frame transmission for PUMA. In order to increase the efficiency of PUMA in networks with high load and a large number of contending nodes, a Backoff scheme called Double Increment Double Decrement (DIDD) [51] is used as the default Backoff mechanism.. JAM. BACKOFF. RTS Time. Figure 3.9: PUMA operation for isochronous traffic [52]. 3.2.5. JMAC. The Jamming-based MAC (JMAC) [75] protocol divides the entire bandwidth into two sub-channels (S and R, Figure 3.10) with a ratio of α : (1 − α) (the method of choosing α is given in [75]). Channel S is used for the transmission of RTS and DATA frames. Channel R is used for the transmission of ACK and CTS frames. For each channel, JMAC employs one half-duplex transceiver. The basic access procedure of JMAC is presented in Figure 3.10. The R channel must be sensed idle for a DIFS period before an RTS frame can be transmitted. Otherwise, it performs the Backoff procedure. After a successful transmission of the RTS frame, the sender must wait for a CTS frame. After the reception of the RTS frame the receiver responds with a CTS frame and starts listening on the S channel for data. Then, after finishing the data transmission, the sender awaits for an ACK frame. While waiting for the CTS and ACK frames the sender node jams the S channel. This is done with the same purpose as the reservation mechanism in IEEE 802.11 (i.e., sending RTS). The main difference is that the medium is jammed as long as needed. On the other hand, the receiver during the reception of a DATA frame on the S channel jams the R channel in order to prevent other nodes from sending RTS frames on the S channel. The sender will stop jamming only after the RTS/CTS exchange fails and the receiver will stop jamming only when the data transmission appears unsuccessful. The presented mechanism helps to effectively block the hidden nodes which are two-hops away from the sender node. However, it requires complex hardware (with two half-duplex transceivers)..

(37) The MAC Layer and the Hidden Node Problem in Ad-hoc Networks. SIFS. 28. RTS. Channel R. Sender. Receiving JAM. SIFS. Channel R. Receiving ACK. Receiving JAM. Receiving DATA. CTS. ACK. Jamming SIFS. Receiver. Jamming. Receiving JAM. Receiving CTS. Receiving RTS. Channel S. DATA. Jamming. DIFS. Channel S. Figure 3.10: Basic access procedure of JMAC [75]. 3.2.6. DUCHA. Data Channel 2nd Control Channel. CTS. NACK Period. SIFS. RTS. SIFS. 1st Control Channel. DIFS. Dual-Channel MAC Protocol for Multihop Ad Hoc Networks (DUCHA) [82] is another protocol which utilises two separate channels. The protocol introduces the Negative CTS (NCTS) and the Negative ACK (NACK) busy tone signals. NCTS is transmitted over the control channel and is used to inform of the remaining time of the ongoing DATA transmission. The NACK tone is sent by the receiver to inform of a data transmission failure. The protocol does not use ACK frames. The basic time diagram of DUCHA is presented in Figure 3.11.. DATA. Busy Tone. (Correct Transmission). 2nd Control Channel. Busy Tone. (Erronous Transmission). Figure 3.11: DUCHA time diagram [82].

(38) 3.2 Busy Tone Signal-based Protocols. 29. DUCHA functions as follows. Before the transmission of an RTS frame the control channel must be idle for at least a DIFS period. If the channel is busy for a given time (i.e., equal to or longer than an RTS transmission) a node must defer its transmission to avoid collision with a CTS frame. Otherwise it transmits the RTS. Then, if the data channel is idle, the receiver of the RTS frame transmits the CTS frame without checking the state of the control channel. After a correct reception of the CTS frame, if the control channel is idle, the sender starts transmitting its data. However, if both channels are busy upon the reception of the RTS frame, the receiver ignores the received RTS to avoid a collision. Finally, if the control channel has been idle for at least the duration of a CTS transmission and the data channel is busy, the receiver transmits NCTS. If the sender receives NCTS, it defers the data transmission for the time given in NCTS. Otherwise, it assumes a collision, defers the transmission, and doubles its Backoff window. Every time a CTS frame is sent over the control channel the receiver must wait for a DATA frame. Whenever it starts receiving DATA it also transmits the out of band busy tone signal over the signalling channel in order to prevent possible hidden nodes from starting their transmissions on the data channel. The receiver has a timer which helps to decide on finishing the reception of the DATA frame. It can also inform the sender about a transmission failure caused by the channel error using NACK (i.e., the lengthened busy tone signal). The sender assumes a successful data transmission if there is no NACK sensed during the NACK period. Additionally, any node located in the carrier sensing range of the sender or the receiver is not allowed to receive data during the NACK and data transmission periods. An example of solving the hidden node problem by the DUCHA protocol is presented in Figure 3.12, in which the dashed lines represent the carrier sensing ranges while the solid lines represent the transmit ranges. Every time when B starts receiving data from A it broadcasts a busy tone signal over the signalling channel. Thanks to this behaviour, node D (which is hidden from A) will hear B’s tone. It will not transmit over the data channel in order to avoid interference with B’s reception and the problem of hidden nodes will be mitigated.. A. B. C. D. Figure 3.12: Hidden node problem in DUCHA [82].

(39) 30. 3.3. The MAC Layer and the Hidden Node Problem in Ad-hoc Networks. Power-aware Protocols. The main goal of power-aware protocols is to decrease the energy consumption of wireless nodes. They can be combined with busy tones or can take advantage from using multiple channels in order to additionally combat the problem of hidden nodes. Examples of such protocols include: PAMAS [58], PCMA [49], PCM [29], RICK [77], DRCE [80], SSPC [66], M-VRMA [79], and [47].. 3.4. Directional Antenna-based Protocols. Directional antenna-based protocols allow for simultaneous data transmission and reception in order to increase spatial reuse. They minimise the probability of frame collisions (also caused by hidden nodes) and usually they have higher network efficiency than the legacy IEEE 802.11. Examples of such protocols include: DMAC [10], CDR-MAC [32], CDMAC [67], RDMAC [8], MARS [42], and MCDA [7].. 3.5. Multiple Channel-based Protocols. Multiple channel-based protocols use multiple channels to increase the overall network throughput, reduce the number of collisions (also caused by hidden nodes), and decrease transmission delays. They can take advantage from using busy tone signals and load balancing to additionally improve their performance. They can be divided into three categories: protocols with a common control channel, protocols without a common control channel, and hybrid solutions which combine the advantages of the previous two categories. Each category is shortly described next.. 3.5.1. MAC Protocols with a Common Control Channel. MAC protocols with a common control channel (CCC) are proposed as means to reduce collisions between different kinds of frames by exploiting the advantage of separation of the signalling and the control channels. The transmission of different kinds of frames over different channels can help meaningfully reduce the number of collisions. Examples of such protocols include: M-VRMA [79], MAC-SCC [45], SAM-MAC [19], CCM-MAC [50], PAMAS [58], DRCE [80], and MCDA [7].. 3.5.2. MAC Protocols without a Common Control Channel. MAC protocols without a common control channel (WCC), in contrary to the CCC solutions, arrange different channels for RTS, CTS, DATA, and ACK frame.

(40) 3.6 Chapter Summary. 31. transmissions in a flexible way. The goal is to reduce the overall number of collisions. Examples of such protocols include: ICSMA [26], MMAC [59], and PCMA [49].. 3.5.3. Hybrid Protocols. Hybrid multiple channel-based protocols combine the advantages of the CCC and WCC methods. An example of such a protocol is described in [47].. 3.6. Chapter Summary. This chapter gave a detailed overview of the following two types of MAC protocols alleviating the hidden node problem: pure contention-based and busy tone signal-based. The following three protocol types were additionally mentioned to create a comprehensive view of the solutions related to the hidden node problem: power-aware, directional antenna-based, and multiple channel-based. The main advantages and disadvantages of all five MAC protocol types are gathered in Table 3.1. Additionally, Table 3.2 contains a detailed comparison of a number of protocols (ordered by their year of publication) alleviating the problem of hidden nodes. Even though many protocols have been proposed in the literature, only the legacy four-way handshake mechanism has become broadly used and implemented in wireless devices. Currently it is the only mechanism recommended by the IEEE 802.11 standard to minimise the negative effects caused by hidden nodes. Therefore, in the following chapter the effectiveness of the four-way handshake mechanism is analysed in multiple EDCA-based networks of different topologies with hidden nodes..

(41) 32. The MAC Layer and the Hidden Node Problem in Ad-hoc Networks. Table 3.1: Advantages and disadvantages of different MAC protocol types for networks with hidden nodes Protocol Type. Advantages. Disadvantages. Pure contention-based (e.g., MACA, four-way handshake, EDCA/RR). Standard hardware. Interoperability with IEEE 802.11 if the standard RTS and CTS frames are used.. Medium/large signalling overhead. Slow/very slow channel reservation. Often not suitable for delay sensitive traffic.. Single channel busy tone-based (e.g., PUMA). Standard hardware. Easy recognition of busy tones. Partial or full interoperability with IEEE 802.11. QoS support. Quick channel reservation is possible.. Increased signalling overhead. Legacy nodes may be assigned a lower priority (PUMA).. Multiple channel-based (e.g., DBTMA, SAM-MAC, CCM-MAC). Separation of data and control traffic to reduce collisions. Possibility of load balancing and use of busy tones. Simultaneous transmissions in the same region without interference. Higher network efficiency than legacy IEEE 802.11.. Power-aware (e.g., DRCE, PCM, SSPC). Decreased energy consumption. Can be combined with busy tones or can take advantage of multiple channels.. Directional antenna-based (e.g., RDMAC, MARS, MCDA, DMAC). Simultaneous data transmission and reception increases spatial reuse. Minimised probability of collisions. Higher network efficiency than IEEE 802.11.. Assignment of separate channels must be done in real-time. Nodes must sometimes be synchronised. Hardware complexity because of additional channels and transceivers. Channel gain of data and control channels may be different. Nodes with a large number of transceivers (e.g., one per channel) are expensive while nodes equipped with a single transceiver are inefficient. Difficult interoperability with existing IEEE 802.11. Large signalling overhead. Slow channel reservation. No QoS support. Signal fading may degrade performance. Reducing the power of ACK transmission may lead to increased number of collisions due to decreased carrier sensing range. Additional hardware complexity. Large signalling overhead. Slow channel reservation. New kinds of hidden nodes, higher directional interference and deafness. Performance decreases with node mobility. Additional hardware complexity. In most cases large signalling overhead and slow channel reservation. Performance strongly dependent on network topology. No QoS support. Performance can be deteriorated by the side-lobe problem..

(42) Large. Large. Medium/ Large. Medium. Large. RICK [77]. DRCE [80]. EDCA/RR [43]. CDR-MAC [32]. Medium. PUMA [52]. MACP [78]. Medium. PCM [29]. Medium. Large. DMAC [10]. Small. Large. DBTMA [12]. AA [76]. Large. PCMA [49]. M-VRMA [79]. None. Standard. Modified RTS/CTS. Modified RTS/CTS. RTS/CTS. Extended RTS/CTS frames.. Complex (directional transmission). Standard. Very large. Large. Large. Complex (power control, additional transceivers). RTS/CTS. Medium. Complex (power control). Binary countdown. Large. Standard. Large. Large. Large. Large. Large. Large. Large. Large. Medium. Large. Large. Overhead. DMBC. Standard. Standard. JAM, RTS and CTS. RTS/CTS. Complex (power control). Complex (power control, two transceivers) Complex (two transceivers) Complex (directional and omnidirectional transmission). Standard. Standard. Standard. Standard. Hardware. RTS/CTS. RTS/CTS. RTS/CTS. RTS/CTS. RTS/CTS. Circular directional transmission of RTS, directional antennas with predefined number of beams, multiple RTS transmissions, each node maintains location table.. Additional signalling frames, transmission power control, two separate channels.. Extended RTS/CTS frames, additional signalling frame. Additional level of channel access competition based on DMBC. Changed Backoff control and countdown competition, added power control.. Extended RTS/CTS frames.. Noise and interference level measurement, busy tone signals determine the maximum possible transmit power level. Out-of-band signalling, busy tones, no ACK frame. Frames are transmitted using directional antennas, medium is sensed using omni-directional antennas, each node maintains a Directional NAV Table. RTS/CTS frames sent using maximum transmit power, ACK sent with minimum transmit power. Additional JAM signal for isochronous traffic, modified control frames, modified Backoff mechanism.. —. Black bursts. Medium. Variable number of black bursts, dependent on time before channel is idle.. Four-way Handshake [24]. RTS/CTS. Long CTS frames.. RTS/CTS. Additional signalling overhead.. Channel Reservation Method. Small. Modification. Table 3.2: Comparison of different MAC protocols for networks with hidden nodes. Small. Black Burst [60]. MACAW [5] FAMA-NCS [14]. Protocol Name. Required IEEE 802.11 Change. Slow. Slow. Slow. Variable. Variable. Slow. Slow. Slow. Slow. Slow. Slow. Slow. Slow. Variable. Slow. Slow. Channel Reservation Time. No. Yes. Yes. Yes. No. Yes. Yes. Yes. No. No. No. No. Yes. Yes. No. No. QoS. 2007. 2006. 2005. 2004. 2004. 2003. 2003. 2002. 2002. 2002. 2002. 2001. 1999. 1996. 1995. 1994. Year. 3.6 Chapter Summary 33.

(43) Required IEEE 802.11 Change. Large. Large. Medium. Large. Large. Large. Medium. Protocol Name. DMACPCDR [61]. SAM-MAC [19]. CCM-MAC [50]. MARS [42]. RDMAC [8]. Slotted MACA-BI [57]. SSPC [66]. RTS and CTS replaced by the Ready-To-Receive frame, utilises slotted channel. Power control of Data, RTS and CTS frames, changed format of RTS and CTS frames.. Smart usage of directional and omni-directional antennas, additional signalling.. Large. Complex (directional and omnidirectional transmission). RTS/CTS. RTR. Large. Complex (power control). Complex (directional and omnidirectional transmission). Medium. Large. Complex (smart antennas). RTRT/RTS/ CTS. Standard. Very large. Standard. RTS/CTS/ DCTS/IFI/ CFM. RTS/CTS. Very large. Complex (two transceivers) Large. Overhead. Hardware. RTS/CTS. RTS/CTS. Each node equipped with GPS, smart usage of omni-directional and directional antennas, rotation of receiving antenna beams. Balances traffic over multiple channels, two half-duplex transceivers for each node, additional signalling. Additional control frames: decide-channel-to-send (DCTS), information-to-inform (ITI), confirm (CFM). Additional signalling (Ready-to-Receive-and-Transmit frame), changed RTS frame format, smart antennas.. Channel Reservation Method. Modification. Table 3.2 – Continued. Slow. Medium. Slow. Slow. Slow. Slow. Slow. Channel Reservation Time. No. No. No. No. No. No. No. QoS. 2009. 2009. 2009. 2009. 2009. 2008. 2008. Year. 34 The MAC Layer and the Hidden Node Problem in Ad-hoc Networks.

(44) Chapter 4. The Impact of Hidden Nodes on EDCA Performance This chapter presents the impact of hidden nodes on EDCA performance. Three different topologies are discussed in detail — star, line, and ring. The first topology contains hidden nodes and does not have any exposed nodes. Line and ring topologies contain both hidden and exposed nodes. Parts of this chapter were published in [33, 36, 37, 38].. 4.1. Simulation Software (ns-2). All simulations were performed using the popular ns-2 simulator [1] patched with an improved version of the EDCA module implemented by TKN — the Telecommunication Networks Group of the Technical University Berlin [63]. A brief description of the major changes introduced (during the course of this dissertation) to the TKN module to make it more standard compliant can be found in [34]. The improvements crucial to this dissertation are the following: • Four-way handshake (cf., Section 3.1.5) support — this mechanism was not fully implemented. • Handling of duplicate frames — these frames were not handled correctly. • Backoff — the range from which the random Backoff values were generated did not comply with the IEEE 802.11 standard..

(45) 36. The Impact of Hidden Nodes on EDCA Performance. The source code relevant to these changes can be found in Appendix C.. 4.2. Simulation Setup. The simulation study was done under the assumptions that all nodes send traffic with a varying sending rate to measure both non-saturation and saturation conditions. The HR/DSSS was used as the physical (PHY) layer type and EDCA was used at the MAC layer. Additionally, in order to combat the hidden node problem, the four-way handshake mechanism was employed. The general PHY and MAC layer parameters used during simulations are given in Table 4.1. Finally, in all figures presented in this chapter the 95% confidence intervals do not exceed ± 2%.. Table 4.1: General simulation parameters IEEE 802.11 PHY Parameters Basic rate Propagation delay PHY overhead SIFS interval ACK/CTS frame Data frame Distance between neighbouring nodes Access Category Voice Video Best Effort Background. 4.3. 1 Mb/s 2 µs 192 b 10 µs 112 b 1000 B. Data rate Slot time MAC header DIFS RTS frame Traffic model. 11 Mb/s 20 µs 32 B 50 µs 160 b CBR. 200 m. Carrier sensing range. 263 m. 802.11 EDCA Parameters AIF SN CWmin 2 2 3 7. 7 15 31 31. CWmax 15 31 1023 1023. Networks with Hidden Nodes. The simulation analysis of networks with hidden (but without exposed nodes) was performed for three star topology networks illustrated in Figure 4.1. Node N0 was the only unhidden node. All other nodes were hidden from each other. Networks with nodes transmitting traffic of equal and mixed ACs were analysed..

(46) 4.3 Networks with Hidden Nodes. 37 (c). (a). N1 N2. N1. N0. (b) N1. N2. N4. N0. Transmission Direction. N2. Carrier Sensing Range. N3. N3. N0. Figure 4.1: Star topology networks.. Nodes Transmit Traffic with Equal ACs Figures 4.2-4.4 illustrate throughput as a function of the total offered load for different ACs with each AC simulated separately. Hidden nodes obtain almost the same throughput. Therefore, for the clarity of presentation, only their mean throughput is shown for each simulation and it is denoted as HA (Hidden Average). The throughput of N0 is different. Therefore, it is presented separately for each simulation and is denoted as N0. 450 400 Throughput [KB/s]. 350 300 250 200 150 100 50 0 0. 500. 1000. 1500. 2000. 2500. 3000. 3500. 4000. Offered Load [KB/s] N0 (VO). HA (VO). N0 (VI). HA (VI). N0 (BE). HA (BE). N0 (BK). HA (BK). Figure 4.2: Three-node star. ACs tested separately. Four-way handshake enabled..

(47) 38. The Impact of Hidden Nodes on EDCA Performance. 450 400 Throughput [KB/s]. 350 300 250 200 150 100 50 0 0. 500. 1000. 1500. 2000. 2500. 3000. 3500. 4000. Offered Load [KB/s] N0 (VO). HA (VO). N0 (VI). HA (VI). N0 (BE). HA (BE). N0 (BK). HA (BK). Figure 4.3: Four-node star. ACs tested separately. Four-way handshake enabled.. 400. Throughput [KB/s]. 350 300 250 200 150 100 50 0 0. 500. 1000. 1500. 2000. 2500. 3000. 3500. Offered Load [KB/s] N0 (VO) N0 (BE). HA (VO) HA (BE). N0 (VI) N0 (BK). HA (VI) HA (BK). Figure 4.4: Five-node star. ACs tested separately. Four-way handshake enabled.. The main conclusions for all star topology networks are the following: • The unhidden node always has considerably higher throughput than any other node. This is because it can sense all other nodes’ transmissions and avoid collisions. • The throughput values achieved by the hidden nodes are unsatisfactory..

(48) 4.3 Networks with Hidden Nodes. 39. • For high priority traffic the hidden nodes obtain much lower throughput than for low priority traffic. This can be explained by the assignment of EDCA parameters. VO and VI have much lower CW values than BE and BK. This causes the number of collisions to be much higher for high priority traffic than for low priority traffic. • The unfairness between nodes is most severe under saturation. Nodes Transmit Traffic with Mixed ACs Figure 4.5 and 4.6 illustrate throughput as a function of the total offered load for different ACs for networks with nodes transmitting traffic of mixed priorities.1 Mixed traffic priorities are analysed in order to verify how they influence the analysed network performance. In the first configuration N0 transmits low priority traffic, in the second — N0 transmits high priority traffic. 300. Throughput [KB/s]. 250 200 150 100 50 0 0. 500. 1000. 1500. 2000. 2500. 3000. 3500. Offered Load [KB/s] N1 (BK), RTS off. N2 (BE), RTS off. N3 (BE), RTS off. N4 (VI), RTS off. N5 (BK), RTS off. N1 (BK), RTS on. N2 (BE), RTS on. N3 (BE), RTS on. N4 (VI), RTS on. N5 (BK), RTS on. Figure 4.5: Five-node star. Unhidden node transmits low priority traffic.. Detailed conclusions for the first configuration are the following: • The hidden node transmitting VI (N4) obtains the highest throughput. • There is severe unfairness between the unhidden node and the hidden nodes sending low priority traffic. In particular, the unhidden node sending BK achieves higher throughput than N2 and N3 sending higher priority traffic 1 Only the most interesting results are presented here. For more results for star topology networks with mixed priorities see [36]..

(49) 40. The Impact of Hidden Nodes on EDCA Performance. (BE). This is because the unhidden node can sense all other nodes’ transmissions while N2 and N3, being hidden from each other, cannot sense their transmissions. This causes collisions. • The unfairness between nodes is most severe under saturation. 400. Throughput [KB/s]. 350 300 250 200 150 100 50 0 0. 500. 1000. 1500. 2000. 2500. 3000. 3500. Offered Load [KB/s] N1 (VO), RTS off. N2 (BK), RTS off. N3 (BE), RTS off. N4 (VI), RTS off. N5 (VO), RTS off. N1 (VO), RTS on. N2 (BK), RTS on. N3 (BE), RTS on. N4 (VI), RTS on. N5 (VO), RTS on. Figure 4.6: Five-node star. Unhidden node transmits high priority traffic.. Detailed conclusions for the second configuration are the following: • Hidden nodes transmitting high priority traffic (N4, N5) successfully transmit hardly any data. This is because their frames collide which each other. • There is strong unfairness between nodes sending VO traffic — the unhidden node considerably outperforms the hidden one. This is because the unhidden node, unlike the hidden one, can sense all other nodes’ transmissions and avoid collisions. • The unfairness between nodes is most severe under saturation. Conclusions Regarding Star Topology Networks To summarise, EDCA does not perform satisfactory for star topology networks, even if the four-way handshake mechanism is enabled. In each analysed configuration there is strong unfairness between hidden and unhidden nodes, regardless of the transmitted AC. The unfairness is the strongest for high priority ACs and it is most visible under saturation..

(50) 4.4 Networks with Hidden and Exposed Nodes. 4.4. 41. Networks with Hidden and Exposed Nodes. The simulation analysis of networks with hidden and exposed nodes was performed for three different line topology networks and for five different ring topology networks. In this section the results for only two distinct ACs are shown (VO and BK). Their comparison gives the overall scope of EDCA behaviour because it can be assumed that nodes with VI behave similarly to nodes with VO, and nodes with BE behave similarly to nodes with BK. The differences between their behaviour is only quantitative but not qualitative (cf., Section 4.3).. 4.4.1. Line Topology Networks. The line topology is illustrated in Figure 4.7. Each node can only detect transmissions of its two nearest neighbours. Nodes at the ends of the line have only one neighbour. All other nodes have two neighbours.. ... N0. N1. Transmission Direction. ... Nn. Transmission Range. Figure 4.7: Line topology.. In the performed analysis the number of nodes changes from 4 (numbered from left to right N0-N3) to 6 (N0-N5). Each AC is simulated separately2 . In order to combat the hidden node problem the RTS/CTS mechanism is used3 . Additionally, for the sake of clarity of the presented figures, if two nodes obtain similar throughput it is presented as a single mean value.. 2 Results for mixed ACs can be found in [35]. They are not presented here because they differ from the results gathered with the use of a newer version of the improved ns-2 simulator, cf., Section 5.6 3 Results for the basic channel access can be found in [33].