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UNIVERSITE LIBANAISE                     UNIVERSITE SAINT-JOSEPH
       (Faculté de Génie)                             (Fa...
A mes parents qui me sont les plus chers au monde, à ceux qui
m’ont donné, de leur effort, de leur sacrifice et de leur pa...
Remerciements
Je profite de cette occasion pour adresser mes premiers remerciements
à M. Wajdi Najem, doyen de l’école d’i...
Bassel Baker                                                                                                     DEA Résea...
Bassel Baker                                                                                                   DEA Réseaux...
Bassel Baker                                                                                          DEA Réseaux


      ...
Bassel Baker                                                                                              DEA Réseaux


Ta...
Bassel Baker                                                           DEA Réseaux


Wireless computing is a technology pr...
Bassel Baker                                           DEA Réseaux


 We will summarize our work in the conclusion.




  ...
Bassel Baker                                                                 DEA Réseaux


I.1- Introduction
    The IEEE ...
Bassel Baker                                                                DEA Réseaux



     A Wireless Medium
     Wh...
Bassel Baker                                                             DEA Réseaux




            Figure I.3: Three sta...
Bassel Baker                                                                DEA Réseaux



 An Extended Service Set ESS
T...
Bassel Baker                                                                 DEA Réseaux



I.3- Quality Of Service In WLA...
Bassel Baker                                                               DEA Réseaux


Another important reason to enhan...
Bassel Baker                                                                DEA Réseaux


               Chapter II: IEEE ...
Bassel Baker                                                              DEA Réseaux


back-off time of other stations ar...
Bassel Baker                                                                  DEA Réseaux



II.2.1.1.2- Hidden Node Probl...
Bassel Baker                                                             DEA Réseaux


follows. All four frames (RTS, CTS,...
Bassel Baker                                                                DEA Réseaux




            Figure II.4: Trans...
Bassel Baker                                                                  DEA Réseaux


Then, it is clear that if we w...
Bassel Baker                                                                  DEA Réseaux


II.2.1.2.2- DCF And PCF Coexis...
Bassel Baker                                                                  DEA Réseaux


2) Second, the cooperation bet...
Bassel Baker                                                                  DEA Réseaux


II.2.2.1- Hybrid Coordination ...
Bassel Baker                                                              DEA Réseaux



                Priority         ...
Bassel Baker                                                                  DEA Réseaux



The default EDCA parameters u...
Bassel Baker                                                                         DEA Réseaux

                        ...
Bassel Baker                                                               DEA Réseaux


AIFS (it’s the shortest IFS), so ...
Bassel Baker                                                                     DEA Réseaux


2) Second, it chooses the h...
Bassel Baker                                                                 DEA Réseaux


Using the above scheduling algo...
Bassel Baker                                                                     DEA Réseaux



II.2.2.3- FHCF (Fair HCF)
...
Bassel Baker                                                                                              DEA Réseaux


as...
Bassel Baker                                                                                                 DEA Réseaux

...
Bassel Baker                                                                   DEA Réseaux


                             ...
Bassel Baker                                                         DEA Réseaux



7) Mohammad Malli, Qiang Ni, Thierry T...
Bassel Baker                                                                   DEA Réseaux


                           Ch...
Bassel Baker                                                               DEA Réseaux


   instructions in the command wi...
Bassel Baker                                                                DEA Réseaux




                   Figure III....
Bassel Baker                                                                  DEA Réseaux



As regards to the delay, the ...
Bassel Baker                                                                     DEA Réseaux


The different traffic param...
Bassel Baker                                                                                     DEA Réseaux


           ...
Bassel Baker                                                              DEA Réseaux



• Channel Utilization And Number ...
Bassel Baker                                                                 DEA Réseaux


Note that the goodput is the da...
Bassel Baker                                                             DEA Réseaux




                           Figure...
A mes parents qui me sont les plus chers au monde, à ceux qui ...
A mes parents qui me sont les plus chers au monde, à ceux qui ...
A mes parents qui me sont les plus chers au monde, à ceux qui ...
A mes parents qui me sont les plus chers au monde, à ceux qui ...
A mes parents qui me sont les plus chers au monde, à ceux qui ...
A mes parents qui me sont les plus chers au monde, à ceux qui ...
A mes parents qui me sont les plus chers au monde, à ceux qui ...
A mes parents qui me sont les plus chers au monde, à ceux qui ...
A mes parents qui me sont les plus chers au monde, à ceux qui ...
A mes parents qui me sont les plus chers au monde, à ceux qui ...
A mes parents qui me sont les plus chers au monde, à ceux qui ...
A mes parents qui me sont les plus chers au monde, à ceux qui ...
A mes parents qui me sont les plus chers au monde, à ceux qui ...
A mes parents qui me sont les plus chers au monde, à ceux qui ...
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Transcript of "A mes parents qui me sont les plus chers au monde, à ceux qui ..."

  1. 1. UNIVERSITE LIBANAISE UNIVERSITE SAINT-JOSEPH (Faculté de Génie) (Faculté d'Ingénierie) Sous l'égide de l'Agence Universitaire de la Francophonie AUF Diplôme d'Etudes Approfondies Réseaux de télécommunications <Simulating 802.11 QoS features> Par <Bassel Baker> Encadré par : M. Mahmoud Doughan Soutenance le 20/12/2004 devant le jury composé de MM. Samir Tohmé Président Mohamad Zoaeter Membre Wajdi Najem Membre Imad Mougharbel Membre Nicolas Rouhana Membre Mahmoud Doughan Membre Maroun Chamoun Membre
  2. 2. A mes parents qui me sont les plus chers au monde, à ceux qui m’ont donné, de leur effort, de leur sacrifice et de leur patience et qui m’ont soutenu dans ma vie, je présente mes remerciements de tout mon coeur. Que Dieu nous les garde ! Avec tous ceux qui me sont chers, je voudrai partager ces joyeux moments. Bassel
  3. 3. Remerciements Je profite de cette occasion pour adresser mes premiers remerciements à M. Wajdi Najem, doyen de l’école d’ingénierie de Beyrouth et M. Mohamad Zoaeter, doyen de la faculté de génie de l’université libanaise, à M. Samir Tohmé et M. Mohamad Mougharbel, responsables scientifiques et administratifs du DEA. Je n’oublie pas de remercier tous les professeurs qui m’ont enseigné durant toute la durée du DEA. Mes remerciements vont également à tous les membres du jury pour leur attention, et spécialement, mes sincères remerciements vont à M. Mahmoud Doughan, mon encadrant de stage. Ses conseils utiles étaient nécessaires pour surmonter beaucoup de difficultés et bien enrichir le rapport, son encouragement bien orienté était à la base de ma réussite dans ce mémoire.
  4. 4. Bassel Baker DEA Réseaux  Table Of Contents Table Of Contents................................................................................................................1 List Of Figures.....................................................................................................................3 List Of Tables......................................................................................................................3 Introduction..........................................................................................................................4 Chapter I: WLAN: Architectures, Standards And QoS Overview.....................................6 I.1- Introduction...............................................................................................................7 I.2- WLAN System..........................................................................................................7 I.2.1- WLAN Components ..........................................................................................7 I.2.2- WLAN Standards.............................................................................................10 I.2.2.1- IEEE 802.11b............................................................................................10 I.2.2.2- IEEE 802.11a............................................................................................10 I.2.2.3- IEEE 802.11g............................................................................................10 I.2.2.4- Enhancements to IEEE 802.11a/b.............................................................10 I.3- Quality Of Service In WLAN Network..................................................................11 I.3.1- QoS Networking Overview..............................................................................11 I.3.2- Why QoS Is Very Important In WLAN Network?..........................................11 I.3.3- Is It Necessary To Enhance The QoS Features In WLAN Network?..............11 I.3.4- QoS Parameters In WLAN Network................................................................12 I.4- Conclusion...............................................................................................................12 References I...................................................................................................................12 Chapter II: IEEE 802.11 MAC Access Methods And QoS Involved................................13 II.1- Introduction............................................................................................................13 II.2- IEEE 802.11 MAC.................................................................................................13 II.2.1- IEEE 802.11 MAC Standards.........................................................................13 II.2.1.1- Distributed Coordination Function (DCF)...............................................13 II.2.1.1.1- Basic Mechanism..............................................................................13 II.2.1.1.2- Hidden Node Problem.......................................................................15 II.2.1.1.3- Optional RTS/CTS Mechanism (Hidden Node Solution)................15 II.2.1.1.4- DCF QoS Offers...............................................................................17 II.2.1.1.5- DCF Limitations...............................................................................17 II.2.1.2- Point Coordination Function (PCF).........................................................18 II.2.1.2.1- Basic Mechanism..............................................................................18 II.2.1.2.2- DCF And PCF Coexistence..............................................................19 II.2.1.2.3- PCF Offers For WLAN QoS Features..............................................19 II.2.1.2.4- PCF Limitations................................................................................19 II.2.2- IEEE 802.11e MAC Draft Standards..............................................................20 II.2.2.1- Hybrid Coordination Function (HCF).........................................................21 II.2.2.1.1- HCF Contention-Based Channel Access (EDCA)............................21 II.2.2.1.1.1- Basic Concept............................................................................21 II.2.2.1.1.2- How The Differentiation Is Exactly Realized?..........................21 II.2.2.1.1.3- EDCA Contention Parameters...................................................22 II.2.2.1.1.4- Internal Channel Access Contention And Its Solution..............23 II.2.2.1.2- HCF Controlled Channel Access (HCCA).......................................24 Page 1/54
  5. 5. Bassel Baker DEA Réseaux II.2.2.1.2.1- HCCA Versus PCF ...................................................................24 II.2.2.1.2.2- Basic Concept............................................................................24 II.2.2.1.2.3- HCF Scheduling Algorithm.......................................................25 II.2.2.1.2.4- HCF Admission Control Algorithm...........................................26 II.2.2.1.2.5- HCF Limitations........................................................................27 II.2.2.2- Adaptive EDCA (AEDCA)......................................................................27 II.2.2.3- FHCF (Fair HCF).....................................................................................28 II.2.2.3.1- Basic Concept...................................................................................28 II.2.2.3.2- QAP Scheduler..................................................................................28 II.2.2.3.3- Node Scheduler.................................................................................30 II.3- Conclusion.............................................................................................................31 References II..................................................................................................................31 Chapter III: Simulation And Results.................................................................................33 III.1- Introduction..........................................................................................................33 III.2- Simulations...........................................................................................................33 III.2.1- Simulation Software......................................................................................33 III.2.2- Simulation Experiments................................................................................34 III.2.2.1- DCF QoS Limitations.............................................................................34 III.2.2.1.1- Scenario Characteristics..................................................................34 III.2.2.2- EDCF Enhancement...............................................................................36 III.2.2.2.1- Scenario 1........................................................................................36 III.2.2.3- EDCA Versus AEDCA..........................................................................39 III.2.2.4- HCF Versus FHCF.................................................................................41 III.2.2.4.1- Scenario 2........................................................................................41 III.3- Conclusion............................................................................................................45 References III.................................................................................................................45 Chapter IV: Scenarios With Particular Cases, Analysis And Proposals...........................46 IV.1- Introduction..........................................................................................................46 IV.2- Particular Scenarios..............................................................................................46 IV.2.1- Varying The Number Of Audio Stations......................................................46 IV.2.2- Tuning Of The Contention Window Of A Background Flow.......................47 IV.3- Proposals For The Future Work...........................................................................48 IV.3.1- Enhanced PCF...............................................................................................49 IV.3.2- Extension For The Enhanced PCF (Mixed-Mode WLAN)...........................50 IV.4- Conclusion ...........................................................................................................52 References III.................................................................................................................52 Conclusion.........................................................................................................................53 Page 2/54
  6. 6. Bassel Baker DEA Réseaux  List Of Figures Figure I.1: 802.11 WLAN occupy the PHY layer and the MAC sub-layer..................7 Figure I.2: Four stations connected to an AP within a BSS................................................8 Figure I.3: Three stations connected together within an IBSS............................................9 Figure I.4: Two BSSs connected together by the DS within the ES...................................9 Figure II.1: A basic DCF time diagram transmission........................................................14 Figure II.2: The hidden node problem...............................................................................15 Figure II.3: The RTS/CTS frames protects the data transmission ....................................16 Figure II.4: Transmission of multiple fragments using SIFS............................................17 Figure II.5: PCF and DCF alternation process..................................................................18 Figure II.6: PCF and DCF alternation process..................................................................19 Figure II.7: IEEE 802.11e inter-frame space (IFS) relationship.......................................22 Figure II.8: Internal collision for different access categories............................................24 Figure II.9: 802.11e beacon interval used in HCF scheduling algorithm..........................25 Figure II.10: Queue length evolution for a TS:.................................................................28 Figure III.1: Each station send three types of traffic for each other..................................34 Figure III.2: Throughput with respect to network load........................................35 Figure III.3: Mean Delay with respect to network load.....................................35 Figure III.5: Latency in EDCF mode.................................................................................37 Figure III.6: Latency distribution comparison in EDCF mode..........................................38 Figure III.7: EDCA and AEDCA comparison...................................................................39 Figure III.8: Goodput Comparison....................................................................................40 Figure III.9: Latency distribution......................................................................................41 Figure III.10: 15 stations sending three types of traffic....................................................42 Figure III.11: Latency in HCF mode.................................................................................43 Figure III.12: Latency in FHCF mode...............................................................................43 Figure III.13: Latency distribution in HCF mode..............................................................44 Figure III.14: Latency distribution in FHCF mode........................................................44 Figure IV.1: Varying the number of Audio Flow in EDCF mode................................47 Figure IV.2: Varying the value of Contention Window....................................................48 Figure IV.3: A congested WLAN cell...............................................................................49 Figure IV.4: Minimization of high congestion in AP........................................................50 Figure IV.5: A Mixed-Mode WLAN Network..................................................................51 Figure IV.6: Per-flow Throughput variation.....................................................................52  List Of Tables Page 3/54
  7. 7. Bassel Baker DEA Réseaux Table II.1: Mapping between user priorities and Access Categories................................22 Table II.2: Default EDCA parameters...............................................................................23 Table III.1: Traffic parameters..........................................................................................37 Table III.2: Latency results................................................................................................38 Table III.3: IFQ drops in EDCF mode...............................................................................39 Table III.4: Description of different traffic streams..........................................................41 Table III.5: Traffic characteristics.....................................................................................42 Table IV.1: Mean Latency.................................................................................................47  Introduction Page 4/54
  8. 8. Bassel Baker DEA Réseaux Wireless computing is a technology providing users with network connectivity without any necessity of wires. In the recent past, IEEE 802.11 wireless LAN has emerged as one prevailing wireless technology throughout the world and will also play an important role in the future fourth-generation wireless and mobile communication systems. On the other hand, 802.11 technologies provide a cheap and flexible wireless access capability, so it is very easy to deploy an 802.11 WLAN in offices, hospitals, campuses, airports, stock markets, etc. During our days, the number of multimedia applications increases rapidly such as voice, audio and high-speed video services which need to a high QoS level guarantee such as guaranteed bandwidth, bounded delay and jitter, but the current 802.11 standard still use a Distributed Coordination Function mode to access the MAC layer. In this mode, all WLAN users have the same probability to get the air medium which is not convenient to the real time applications. Then, in order to give some priority access for the real time applications, the IEEE 802.11 WLAN Group had proposed some MAC access modes which implement the service differentiation mechanism. So, our work aims to study the QoS in WLAN network, and by simulation, it tries to show the enhancement of the proposed access methods. Our project includes four chapters and it is structured as following: o Chapter I: We will start by a general overview about WLAN network, its architecture, its components, the QoS level in these networks and the problems involved. o Chapter II: We will detail the IEEE 802.11 standard mechanisms which are DCF and PCF modes and we will present the proposed mechanisms especially EDCA and AEDCA, HCF and FHCF detailing their concepts, the advantages and the disadvantages of each one. Some comparisons will also be achieved. o Chapter III: We will prove, using the Network Simulator software and the extended distribution, how the proposed mechanisms enhance the WLAN network performance by studying many quality of service features especially the packet delay which is a critical feature of the real time applications. We will present many different comparative graphs which aim to show that the new access methods give an acceptable QoS for real time applications and, based on it, we can send a voice and a video application with an acceptable delay. o Chapter IV: The last chapter includes an analysis of particular WLAN scenarios. We will present the simulation results and we will propose some WLAN procedures which aim to more reduce the congestion problem, and so, to more improve the QoS in some WLAN cells. That constructs our proposals for the future work in the WLAN domain. Page 5/54
  9. 9. Bassel Baker DEA Réseaux We will summarize our work in the conclusion. Chapter I: WLAN: Architectures, Standards And QoS Overview Page 6/54
  10. 10. Bassel Baker DEA Réseaux I.1- Introduction The IEEE 802.11 WLAN Network is one of the most deployed technologies on the world and it’s able to be an important alternative among the wireless network in the future generations. This Chapter aims to present the aspects of the wireless system architecture and the standards that had been published or studied until our days. We show also the need to enhance the quality of service in the WLAN system due to its big importance in the future generation networks. I.2- WLAN System Wireless technology is the method of delivering data from one point to another without using physical wires. A wireless Local Area Network WLAN is a flexible data communication system implemented as an extension, or an alternative for a wired LAN within a warehouse, factory, building or campus. The figure I.1 shows the OSI model part occupied by 802.11 WLAN. Figure I.1: 802.11 WLAN occupy the PHY layer and the MAC sub-layer The wireless system includes many components that we will briefly introduce in the next paragraph. I.2.1- WLAN Components The WLAN network is based on the cellular architecture (series of interconnected cells) and it consists of the following:  Wireless Devices Or Stations The wireless client receiver is needed to connect a computing device (wireless station) like a laptop, a Personal Digital Assistant (PDA) or desktop to the wired network via an access point (AP).  An Access Point AP Or It’s Called Also The Base Station Bs It is occupied by an antenna and it is physically connected to a conventional wired Ethernet network. The AP provides a wireless access to the clients as its name suggests, it is comparable to the Ethernet switch and operates in half-duplex mode. Page 7/54
  11. 11. Bassel Baker DEA Réseaux  A Wireless Medium Which is scarce in the WLAN network and the stations contend always to get it in the DCF mode.  A Basic Service Set BSS The fundamental building block for the 802.11 architecture is the Basic Service Set BSS. The BSS is designed as a group of stations under the direct control of a single coordination function contained within a centralized access point. The geographical area covered by a BSS is known as a Basic Service Area (BSA). The figure I.2 shows a BSS service set connected to the network backbone. For each station, we have a station adapter SA. Figure I.2: Four stations connected to an AP within a BSS If there are no connections back to a wired network, we will obtain as we called an independent basic service set (IBSS) (Figure I.3). Page 8/54
  12. 12. Bassel Baker DEA Réseaux Figure I.3: Three stations connected together within an IBSS  A Distribution System DS Even some WLAN networks are composed by one cell and one AP (or without any AP), the most WLAN networks are composed by many cells and the all APs are interconnected by a backbone which is called a Distribution System (DS).The Distribution system forms the spine of the WLAN, making the decisions whether to forward traffic to one BSS to the wired network or back out to another AP or BSS. Figure I.4: Two BSSs connected together by the DS within the ES Page 9/54
  13. 13. Bassel Baker DEA Réseaux  An Extended Service Set ESS The Extended Service Set comprises multiple BSSs that are connected using a common Distribution System DS (figure I.4). It has the appearance of one large BSS to the Logical Link Layer LLC of each station. All of these components working together providing a seamless mesh which give wireless devices the ability to roam around the WLAN looking for all intents and purposes like wired device. I.2.2- WLAN Standards In 26th June 1997, the Institute of Electrical and Electronics Engineers (IEEE) creates the first WLAN standards: IEEE 802.11. Unfortunately, 802.11 only supports a maximum data rate of 2Mbps, too slow for most applications, for this reason, ordinary 802.11 wireless products are no longer manufactured. I.2.2.1- IEEE 802.11b IEEE expanded on the original 802.11 standard in 16th September 1999, creating the 802.11b specification (also referred to Wi.fi) that supports a data rates up to 11 Mbps comparable to traditional Ethernet. Note that 802.11b uses the same radio signaling frequency 2.4GHZ as the original 802.11 standard but it adopts a new modulation technique called Complementary Code Scheme (CCK) which allows the speed increase. I.2.2.2- IEEE 802.11a This standard supports a data rates up to 54Mbps and signals in a regulated 5GHZ range. 802.11a is a second extension to the original 802.11 standard and is created by the IEEE group at the same time to 802.11b, but due to its higher cost, 802.11a fits predominately in the business market whereas 802.11b better serves the home market. Compared to 802.11b, the higher frequency used by 802.11a (5 GHZ) limits its range and it is meaning that the 802.11a has more difficulty penetrating walls and other obstructions. Since 802.11a and 802.11b utilize two different frequencies then these two technologies are incompatible with each other. I.2.2.3- IEEE 802.11g The success of the IEEE 802.11b standard has lead to a huge interest to continuing to improve the standard. 802.11g is a new standard that attempts to combine the best of both 802.11a and 802.11b standards. It supports a data rates up to 54 Mbps, it uses the 2.4GHZ frequency and it is compatible with 802.11b. I.2.2.4- Enhancements to IEEE 802.11a/b A number of enhanced WLAN standards are developed by the IEEE WLAN group (WG). The most important and interested one for our subject is the IEEE 802.11e that has been designed to enhance the 802.11 MAC with the view to improve and manage Quality Of Service QoS and provide a class of service concept to WLAN network. Page 10/54
  14. 14. Bassel Baker DEA Réseaux I.3- Quality Of Service In WLAN Network I.3.1- QoS Networking Overview The Quality Of service (QoS) refers to the capability of a network to provide better service to selected network traffic over various technologies. The primary goal of QoS is to provide including dedicated bandwidth, controlled jitter and latency which that required by a real-time and interactive traffic like audio, video or other multimedia applications and improved loss characteristics. Also important is making sure that providing priority for one or more flows does not make other flows fail. QoS technologies provide the elemental building blocks that will be used for future business applications in campus, WLAN and service provider networks. To provide a better service to a certain flows, we can either raising the priority of the flow or limiting the priority of another flow. This method is called service differentiation and it will be used by one of the enhanced QoS drafts in this project. I.3.2- Why QoS Is Very Important In WLAN Network? Net access through hotspots at airports, hotels and coffee shops, via the high-speed wireless Internet access service known as WiFi, is rapidly becoming common. In wireless home and office networks where voice, video and audio will be delivered, quality of service and multimedia support are critical, and are essential ingredients to offer customers video on demand, audio on demand, voice over IP, and high speed Internet access. In the near future, all consumer electronic devices at home, such as a Video Cassette Recorder (VCR), TVs and microwave ovens, can be equipped for and connected to QoS-enabled wireless networks. One of the most promising home networking candidates is IEEE 802.11 WLAN. All that has been said guides us to consider that the QoS features are crucial for WLAN networks. I.3.3- Is It Necessary To Enhance The QoS Features In WLAN Network? The typical question on the characteristics on the WLAN concerns the quality of service, and how it is supported in these networks. But the IEEE 802.11 standards still use the Distributed Coordination Function during which all WLAN stations have the same probability to get the WLAN medium, so if there are a voice, a video and a background flows transmission in a WLAN cell, even if a voice application has much more priority than the background flow, especially in packet delay, it won’t get the medium when it should do, so unfortunately, the WLAN standards does not provide, actually, the required quality of service that guarantee a better service for specific packets or customers. On the other hand, wireless network is vulnerable to errors due to the nature of the wireless link layer environment. In addition, the collision problem in wireless communication leads us to try to reduce the collision rate in order to guarantee the QoS required by the multimedia applications. Page 11/54
  15. 15. Bassel Baker DEA Réseaux Another important reason to enhance the QoS WLAN is the future generation network that will combine all network technologies in one IP network, and the WLAN integration in this huge network necessitates an equivalent QoS level between used technologies. I.3.4- QoS Parameters In WLAN Network When we want to study the QoS in WLAN network, we must focus our work on many various numbers of QoS metrics such as: 1) Access delay It is the time from when a packet reaches a MAC layer until it is successfully transmitted. Since the IEEE 802.11 WLAN use the CSMA/CA access channel mechanism at the MAC layer, then the access delay is all the more as great than a number of frame retransmissions is great. 2) Throughput It is the amount of data transferred from one place to another or processed in a specified amount of time. The throughput is the most important QoS metrics not only in WLAN network, but also in all other networks. 3) Channel utilization It presents the amount of time in which the medium are effectively used for data transmission. 4) Collision rate It is the amount of frame (or packet) collisions that occur in a network per time unit, generally one minute or one second. I.4- Conclusion In this chapter, we have presented the WLAN network architecture, the IEEE 802.11 standards and their characteristics. We have showed that the QoS is a critical element for the WLAN networks. Also, the quality of service overview is introduced and we have enumerated the QoS metrics in WLAN, these metrics will be our reference criteria in the simulation part of our project. In the next chapter, we will study, in details, the MAC channel access mechanisms standardized by IEEE 802.11 Working Group, and the proposed enhanced schemes. References I 1) Dr. Mark Davis, “The 802.11 family of WLAN Standards”, School of Electronic and Communication Engineering. 2) Bastien Alexandre, Le Baron Julien, David Vincent, LeFevre Fabrice, “Wireless Norme IEEE 802.11”, Licence pro Réseaux 2002/2003. 3) www.cisco.com/univercd/cc/tt/doc/cisintwk/ito_doc/qos.htm 4) www.ecs.umass.edu/ece/wireless 5) www.qosforum.com Page 12/54
  16. 16. Bassel Baker DEA Réseaux Chapter II: IEEE 802.11 MAC Access Methods And QoS Involved II.1- Introduction In the chapter I, we saw that the WLAN QoS features should be enhanced in order to satisfy the QoS requirements for the real-time applications such as voice, audio, video and other multimedia applications and to join the all IP network in the future generation, but we didn’t show, in details, the limitations of the MAC access channel mechanisms that it is used actually in WLAN network. This work will be done in this chapter. The QoS offers and QoS limitations for each mechanism will be also studied, too. Next, we are going to study the new mechanisms that are proposed by IEEE WLAN group and we will present their offers in term of QoS features and a comparison between them will be presented too. II.2- IEEE 802.11 MAC II.2.1- IEEE 802.11 MAC Standards The MAC layer is primarily responsible for controlling access to the wireless medium. IEEE 802.11 MAC employs a mandatory Distribution Coordination Function DCF and an optional Point Coordination Function PCF channel access mechanisms. These mechanisms determine when a station is permitted to transmit over the air channel. In this section, we will study these two methods and show what they present in term of WLAN networks performance. II.2.1.1- Distributed Coordination Function (DCF) II.2.1.1.1- Basic Mechanism DCF is the fundamental MAC method used in IEEE 802.11 WLAN network and it is based on a CSMA/CA mechanism where the principle is “listen before talking”. The high dynamic attenuation of a radio signals makes practically very difficult for a radio transceiver to listen to other signals while transmitting, which is essential for the collision detection part for the CSMA/CD. This is the mean reason which leads the WLAN designers to use the CSMA/CA mechanism and not the CSMA/CD mechanism that is used in the Ethernet technology. In the DCF mechanism, a station, with a frame to transmit, monitors the channel activities until an idle (no transmission) period equal to a Distributed Inter-Frame Space (DIFS) is detected. After sensing an idle DIFS, the station waits for a random back-off interval before transmitting. The back-off counter is an amount of time chosen, at each transmission, in the range (0, CW-1) in terms of time slot: Backoff _ time = random(0, CW ) * slot _ time . Where CW (CW for Contention Window) is the current back-off window size; CW is the range [CWmin , CWmax ] . In the IEEE 802.11, CWmin = 7 and CWmax = 255 . The back-off counter is decremented in terms of time slot as long as the channel is sensed idle. The counter is stopped when a transmission is detected on the channel, and reactivated when the channel is sensed idle again for more than a DIFS. In this manner, the stations deferred from channel access because their back-off time was larger than the Page 13/54
  17. 17. Bassel Baker DEA Réseaux back-off time of other stations are given higher priority when they resume their transmission attempts. Finally, the station transmits its frame when the back-off time reaches zero. After each successfully transmission, the contention window is reset to a CWmin and after detecting a collision, the contention window is doubled (exponentially back-off), so the risk of further collisions is reduced. The figureII.1 shows a time diagram for a basic DCF function procedure. Note that the DIFS interval is supposed including also the back-off time. Figure II.1: A basic DCF time diagram transmission The other stations, that sense transmitting frame, set their Network Allocation Vector (NAV) properly in order to defer access to the channel during this transmission. NAV is a parameter that is updated with the other terminal’s transmission duration, and it is used for a virtual carrier sensing. The transmission duration is declared in data, RTS or CTS frames. (The RTS or CTS frames will be explained latter in this chapter). The period in which we have a DCF function mode is called the Contention Period (CF). At the very first transmission attempt, CW equals the initial back-off window size, CWmin. After each unsuccessful transmission, CW is doubled until a maximum back-off window size value, CWmax, is reached. After the destination station successfully receives the transmitted frame, it transmits an acknowledgment frame (ACK) following a Short Inter-Frame Space (SIFS) time as shown in the above figure. If the transmitting station does not receive the ACK within a specified ACK timeout, or if it detects the transmission of a different frame on the channel, it reschedules the frame transmission according to the previous back-off rules. Page 14/54
  18. 18. Bassel Baker DEA Réseaux II.2.1.1.2- Hidden Node Problem A hidden node is one that is within the range of the receiver, but out of range of the sender, or is within the range of the sender but out of range of the receiver. In figure II.2, the node A is transmitting to the node B. Meanwhile, the node A has a packet to transmit to the node B, the node C senses an idle channel as it is out of range of the node A, therefore it starts transmission which causes collision at the node B, which is in range of both A and C. The node C is hidden to the node A. As we see, the hidden node problem increases the collisions, therefore it reduces the efficiency. Figure II.2: The hidden node problem Note that the collision can be caused by the following two reasons: 1) Two ore more stations starts their transmission at the same time after waiting to the channel to become idle. 2) Two or more hidden terminals transmit simultaneously. II.2.1.1.3- Optional RTS/CTS Mechanism (Hidden Node Solution) To reduce the hidden node problem, an optional four-way data transmission mechanism, Request To Send/Clear To Send (RTS/CTS), is also defined in DCF. With RTS/CTS, before transmitting a data frame, a short RTS frame is transmitted (This frame includes the source, the destination and the transmission time including transmitted packet and its acknowledgment). The RTS frame also follows the back-off rules introduced in the DCF basic mechanism. If the RTS frame succeeds, the receiver station responds with a short CTS frame that includes the same information about transmission time in the RTS frame. Each station receives either the RTS frame or the CTS frame and so it sets its NAV properly after this reception. Using this information, the station can know when the current transmission will finish and the channel will be idle for the next time. When the source receives successfully the CTS frame, it starts transmitting its frame being sure that the channel is reserved for itself during all the frame duration. Briefly, the short RTS and CTS frames reserve the channel for the data frame transmission that Page 15/54
  19. 19. Bassel Baker DEA Réseaux follows. All four frames (RTS, CTS, data and ACK) are separated by a Short Inter-frame space SIFS time (DIFS=SIFS+2*slot time), so no station can interrupt the current transmission. Another advantage to RTS/CTS mechanism is that it allows us to prevent a loss of a lot of time in case of a long frame, keeping the transmission going on while collision is taking place. o Using Of RTS/CTS Mechanism The overhead of sending RTS/CTS frames becomes considerable and would compromise the overall performance when data frames sizes are small, then using of RTS/CTS mechanism is much helpful when the actual data size is large compared to the size of RTS/CTS frames. The Figure II.3 shows the DCF mechanism with the RTS/CTS optional method: Figure II.3: The RTS/CTS frames protects the data transmission o Packet fragmentation case In the case of a large packet, the packet fragments are transmitted on the channel separated by a SIFS time, so no packet can interrupt the current transmission. Each fragment is acknowledged separately (Figure II.4) else the fragment is retransmitted before any other fragments, keeping the sequence in order, and enhancing the throughput efficiency. Page 16/54
  20. 20. Bassel Baker DEA Réseaux Figure II.4: Transmission of multiple fragments using SIFS PIFS is an interval of time concerning an optional channel access mechanism that we are going to explain later in the next paragraph. II.2.1.1.4- DCF QoS Offers DCF mechanism offers some QoS characteristics that may be useful for some applications and there are the following: 1) When system load is low, CSMA/CA mechanism provides a simple and fair solution, since all stations have an equal probability of accessing the medium. 2) DCF is ideal for the best effort traffic. 3) DCF doesn’t need to an Access Point for controlling medium access. II.2.1.1.5- DCF Limitations DCF mode doesn’t support any QoS guarantees, only a best-effort service is provided. Typically, time-bounded applications such Voice over IP (VoIP), or videoconferencing require specified bandwidth, low delay and jitter, but can tolerate some losses. The point is that in DCF, all the stations compete for the channel with the same priorities. There is no differentiation mechanism to guarantee bandwidth, packet delay and jitter for high- priority multimedia flows. In the following, some points that enlighten the QoS limitations in the DCF mode: 1) The equal access probabilities are not desirable among stations with different priority frames such as audio, video and FTP transmission. 2) The DCF method does not guarantee for queuing delays since a station can reserve the channel during the all transmitting time without any limitations, so it is not optimal for time-bounded applications. 3) The DCF mode does not offer a guarantee in bandwidth because we have a large throughput fluctuation. In addition, DCF throughput is degraded when the system load is high since all stations contend to obtain the channel and the collision overhead will make it low. Page 17/54
  21. 21. Bassel Baker DEA Réseaux Then, it is clear that if we want to make the IEEE 802.11 WLAN networks able to support the real-time applications with a good performance, we have to integrate some kinds of service differentiation in the channel access mechanism. II.2.1.2- Point Coordination Function (PCF) The PCF is an optional centrally controlled channel access function that was defined in order to support time-bounded services and let stations have priority access to the wireless medium. II.2.1.2.1- Basic Mechanism In this mode, a Point Coordinator (PC), normally located in the AP (so this mode in only usable on the infrastructure mode), senses the medium idle for a PCF Inter-Frame Space (PIFS) period of time and then transmits a beacon frame to initiate a CPF (Contention Free Period) period. After a SIFS time, the PC sends a poll frame to a station to ask to transmit a frame. Note that the beacon frame is a small frame used to initiate as we called a super-frame. A beacon frame and super-frame concepts will be more detailed in the next paragraph (DCF and PCF coexistence). After receiving the poll frame from the PC, the station with a frame to transmit, acknowledges the successful reception after a SIFS period and may transmit only one frame whish can be for any destination. If the PC does not receive any response from the polled station after waiting for PIFS, it may poll another station or ends the CFP period using a control frame called CF-End frame. When the destination receives a data frame, an ACK is returned to the source station, after a SIFS time (figure II.5). Figure II.5: PCF and DCF alternation process As we see in this mechanism, there is no contention between stations since a station can transmit only if it gets polled, for this reason, we call that a Contention Free Period (CFP) period. Note here that PIFS=SIFS+ 1*slot time, then the PCF mode has higher priority than DCF mode since it is adopting a shorter Inter-Frame Space: SIFS<PIFS<DIFS. This gives the AP absolute priority to transmit before any of the stations try to contend using DCF. Page 18/54
  22. 22. Bassel Baker DEA Réseaux II.2.1.2.2- DCF And PCF Coexistence In a long run, the transmission time is always divided into repetition intervals called super-frames. Each super-frame starts with a beacon frame and the remaining time is further divided into the CFP in which the DCF works and the CP in which the PCF works. The aim of the coexistence of the DCF and PCF is that, in DCF method, a priority flow can wait a long time before getting successfully the channel, whereas when PCF starts, it will give directly the channel to the highest priority flow based on a prioritized list stations as we explained above. But since the PCF is an optional mechanism, then if it is not active, the super-frame will not include it. However, the beacon frame is always sent no matter whether the PCF is active or not. o Beacon frame: It is a management frame for synchronization, power management and delivering parameters. It contains the duration of CFP and the CP. In a BSS mode, an AP sends a beacon frame while in the IBSS, any mobile station that is configured to start an IBSS will begin sending beacon frame. Beacon frames are generated at regulated intervals called Target Beacon Transmission Times (TBTT). The figure II.6 shows PCF and DCF alternation. Figure II.6: PCF and DCF alternation process Since the AP has to wait for previous transmission to resume before transmitting the beacon frame, the CFP repetition interval may be not constant. II.2.1.2.3- PCF Offers For WLAN QoS Features The PCF mode was introduced to be suitable for time-bounded services, and since it has higher priority to DCF mode we can say that it can offer, if it is active, some priority service for WLAN applications. II.2.1.2.4- PCF Limitations Although PCF has been designed to support time-bounded multimedia applications, this mode has three main problems that lead to poor QoS performance. 1) First, the central polling scheme is questionable. All the communications between two stations in the same BSS have to go through the AP. Thus, when this kind of traffic increases, a lot of channel resources are wasted, this is inefficiency in large networks. Page 19/54
  23. 23. Bassel Baker DEA Réseaux 2) Second, the cooperation between CP and CFP modes may lead to unpredictable beacon delays. The AP schedules the next beacon transmission at next TBTT but it has to contend to access the channel, the beacon can then be transmitted when the medium has been found idle for an interval of time longer than a PIFS. Hence, depending on whether the wireless medium is idle or busy around the TBTT, the beacon frame may be delayed. In the current 802.11 legacy standard, stations are allowed to transmit even if the frame transmission cannot terminate before the upcoming TBTT. The duration of the beacon to be sent after the TBTT defers the transmission of data frame during CFP, which may severely impact the QoS performance of multimedia applications. In the worst case, the maximum delay of the beacon frame can be 4.9 ms in 802.11a, and the average delay of a beacon frame can reach up to 250 μs. 3) Third, it’s difficult to predict the transmission time of a polled station. A polled station is allowed a frame of any length between 0 and 2304 bytes (the maximum MAC service data unit size MSDU) which may introduce variable transmission time. Furthermore, the physical rate of the polled station can change according to the varying characteristics of the channel, so the AP is not able to predict in a precise manner the transmission time, this prevents the AP to provide a guaranteed delay and jitter performance for other stations presented in the polling list during the rest of the CFP. 4) Finally, we shouldn’t forget the hidden station problem which is not solved yet in this mode. II.2.2- IEEE 802.11e MAC Draft Standards o Why? The explosive growth of multimedia applications in the recent years required the QoS requirements support such as guaranteed delay, jitter and bandwidth for these applications. However, the original IEEE 802.11 WLAN has been mainly designed for data applications and does not provide any QoS support for multimedia applications. Actually, and based on the limitations of DCF and PCF which have been described on the previous sections, no traffic prioritization mechanism is done in MAC layer in the WLAN networks and without this, high data rate alone is not sufficient to meet QoS requirements imposed by some applications such as real-time voice, audio and video. Thus, the IEEE 802.11 WLAN Group decides to introduce a new mechanism that allows a simple traffic prioritization as a part of the medium access mechanism (Differentiated Service) and provides end-to-end per-connection QoS with support of streaming and centralized scheduling. o The enhanced WLAN components and the involved compatibility The stations that support QoS enhancements are called QoS stations (QSTA), and there are associated with a QoS enhanced access point (QAP) in a QoS BSS (QBSS) or in a (QIBSS) without a QAP. Since the QSTA implements a superset of STA functionality, the QSTA may associate with a non-QoS AP in a non-QoS BSS to be backward compatible by providing the original MAC data service. Page 20/54
  24. 24. Bassel Baker DEA Réseaux II.2.2.1- Hybrid Coordination Function (HCF) o HCF Introduction Based on which has been detailed above, QoS for WLAN MAC has received much attention, and in May 2000, the 802.11 WLAN Group initiated the 802.11e Task Group. This group was charged with enhancing the 802.11 MAC to improve and manage QoS. The 802.11e proposed a new MAC layer coordination function called: the Hybrid Coordination Function (HCF). HCF uses a contention-based channel access method, also called the Enhanced Distributed Channel Access (EDCA) (or the Enhanced Distributed Coordination Function (EDCF)) that operates concurrently with an HCF Controlled Channel Access (HCCA) method. o HCF Characteristics One main new feature of HCF is the concept of transmission opportunity (TXOP), which refers to an instance during which a given QSTA has the right to send data frames. The aim of introducing TXOP is to limit the time interval during which a QSTA is allowed to transmit frames. Thus, the problem of unpredictable transmission time of a polled station in PCF is solved. A TXOP is called either EDCA TXOP when it’s obtained by winning a successful EDCA contention or HCCA TXOP when it is obtained by receiving a QoS CF-poll frame from the QAP. In order to limit delay, the maximum value of TXOP is bounded by a value, namely TXOPLimit, which is determined by the QAP. A QSTA can transmit packets as long as its TXOPLimit has not expired. QAP allocates an uplink TXOP to a QSTA by sending a QoS CF-poll frame, while no specific control frame is required for downlink. In the following, we will describe the queue-based 802.11e HCF methods: II.2.2.1.1- HCF Contention-Based Channel Access (EDCA) II.2.2.1.1.1- Basic Concept EDCA is designed to provide prioritized QoS by enhancing DCF. It can be seen from the basic DCF mechanism that we can vary many parameters to provide channel access differentiation: The defer time DIFS, CWmin and CWmax, based on which the random back- off timer is generated. II.2.2.1.1.2- How The Differentiation Is Exactly Realized? Before entering the MAC layer, each data packet received from higher layers is assigned a specific user priority value. But how can we tag such a priority value for each packet? The response is by an implementation issue and it’s detailed as following: At the MAC layer, EDCA defines four FIFO queues, namely, access categories (ACs). These ACs must to be implemented in each 802.11e station (the station that works with 802.11e mode). Then each data packet from the higher layer along with a specific user priority value between 0 and 7 (8 different priorities are supported in EDCA) will be mapped into a corresponding AC using a mapping table (tableII.1). As shown in this table, different kinds of applications such as background traffic, best-effort traffic, video traffic and voice traffic are mapped to different AC queues (AC_BK, AC_BE, AC_VI, AC_VO respectively). Page 21/54
  25. 25. Bassel Baker DEA Réseaux Priority AC Designation 1 AC_BK Background 2 AC_BK Background 0 AC_BE Best effort 3 AC_VI Video 4 AC_VI Video 5 AC_VI Video 6 AC_VO Voice 7 AC_VO Voice Table II.1: Mapping between user priorities and Access Categories AC_VO is the highest priority and AC_BK is the lowest one. II.2.2.1.1.3- EDCA Contention Parameters Each AC behaves as a single DCF contenting entity and each entity has its own contention parameters (CWmax[AC], CWmin[AC], AIFS[AC] and TXOPLimit[AC]) which are announced by the QAP periodically via beacon frames. The AIFS (Arbitrary Distributed Inter-Frame Space) is a defer time which characterizes every AC. Basically, the smaller values of CWmax[AC], CWmin[AC], AIFS[AC] and TXOPLimit[AC] give a shorter access delay for the corresponding AC, and thus the higher priority to access the medium. The AIFS’s length is arbitrary and it’s determined by: AIFS[ AC ] = SIFS + AIFSN [ AC ]. Slot time Where AIFSN [ AC ] , called the arbitration inter-frame space number, represents the number of slot time for an AC. For an example, DCF uses AIFSN = 2 to compute DIFS, so DIFS=SIFS+2.slot time. Figure II.7 shows us different IFSs and their values with respect to slot times. Figure II.7: IEEE 802.11e inter-frame space (IFS) relationship Page 22/54
  26. 26. Bassel Baker DEA Réseaux The default EDCA parameters used by QSTAs are suggested in tableII.2. ACs CWmin CWmax AIFSN TXOPLimit TXOPLimit (802.11b) (802.11a/g) BK CWmin CWmax 7 0 0 BE CWmin CWmax 3 0 0 VI  CWmin + 1  CWmin 2 6.016ms 3.008ms  − 1  2  VO  CWmin + 1   CWmin + 1  2 3.008ms 1.504ms  − 1  − 1  4   2  Table II.2: Default EDCA parameters After waiting for an idle time interval of AIFSN [ AC ] , each AC has to wait for a random back-off time ( CWmin [ AC ] ≤ back - off time ≤ CWmax [ AC ] ). The purpose of using different contention parameters for different queues is to give low-priority traffic a longer back-off time than high-priority traffic. In the following, two different CW ranges that show two different access channel priorities: 1) High-priority: CW range=(8,31). 2) Low priority (Best- effort): CW range=(32,128). II.2.2.1.1.4- Internal Channel Access Contention And Its Solution With introducing of EDCA mode, a new channel access contention mechanism appears other the external one. Since, in EDCA, back-off timers of different ACs in one QSTA are random values, then they may reach zero at the same time which causes the internal collision. In order to avoid those internal collisions, EDCA introduces a scheduler inside every QSTA to allow only the highest priority AC to transmit a packet. As a result, EDCA aims to support prioritized QoS for multimedia applications. Figure II.7 shows an internal channel access contention among traffic of different priorities inside the same station. Page 23/54
  27. 27. Bassel Baker DEA Réseaux AC3 AC2 AC1 AC0 Multiple Backoff Instances ----- Multiple Transmission Queues ----- Backoff Backoff Backoff Backoff ----- Virtual Collision Figure II.8: Internal collision for different access categories II.2.2.1.2- HCF Controlled Channel Access (HCCA) The original PCF mode suffers from many serious problems especially the unlimited transmission time of polled station. So, in order to provide strict and parameterized QoS support regardless the traffic conditions, IEEE 802.11e proposed the HCF Controlled Channel Access (HCCA) mechanism. II.2.2.1.2.1- HCCA Versus PCF HCCA uses a poll-and-response mechanism similar to PCF, but there are many differences between these two mechanisms. For example, HCCA is more flexible than the PCF, it means that the QAP can start HCCA during both CFP and CP where PCF is only allowed in CFP. In addition, HCCA solves the three main problems of PCF: 1) A direct link between peer stations is allowed in 802.11e, where stations can communicate each other without going through AP in HCCA. 2) An 802.11e QSTA is not allowed to transmit a packet if the frame transmission cannot be finished before the next beacon, whish solves the beacon delay problem with PCF. 3) A TXOPLimit is used to bind the transmission time of a polled station. II.2.2.1.2.2- Basic Concept The figure II.8 shows an example of an 802.11e beacon interval which is composed of alternated modes of optional CFP and mandatory CP. After an optional period of CFP, the mechanisms of EDCA and HCCA which are used in CAP duration, alternate in a beacon interval as shown in figureII.8. During CP, QAP is allowed to start several contention-free bursts, called Controlled Access Period (CAP) at any time after detecting channel as being idle for a time interval of PIFS, and since PIFS is shorter than DIFS and Page 24/54
  28. 28. Bassel Baker DEA Réseaux AIFS (it’s the shortest IFS), so QAP has a higher probability to start HCCA at any instant during a CP than other contending QSTAs. Figure II.9: 802.11e beacon interval used in HCF scheduling algorithm Although HCCA can provide more strict QoS support than EDCA, the latter is still mandatory in 802.11e for supporting QoS specification exchange between QSTAs and QAP. For this purpose, the maximum duration of HCCA in an 802.11e beacon interval is bounded by a variable, TCAPLimit. II.2.2.1.2.3- HCF Scheduling Algorithm A simple HCF scheduling algorithm is suggested as a reference design in the 802.11e specification, providing a parameterized QoS support based on the contract between QAP and corresponding QSTAs. Before any data transmission, a traffic stream has first to be established and each QSTA is allowed to have no more than eight traffic stream with different priorities. To avoid confusion with the EDCA parameters, traffic streams and ACs are separated and use different MAC queues. In order to setup the traffic stream connection, a QSTA must send a QoS request frame containing the corresponding traffic specification (TSPEC) to the QAP. A TSPEC describes the QoS parameters requirement of a traffic stream such as: 1) Mean data rate, 2) The MSDU size, 3) The delay bound and 4) The maximum Required Service Interval (RSI). The last parameter refers to the maximum time duration between the start of successive TXOPs that can be tolerated by the application. The simple 802.11e HCF scheduling algorithm is summarized as follows: 1) First, on receiving all these QoS requests, the QAP scheduler determines the minimum value of all the maximum RSIs required by the different traffic streams. Page 25/54
  29. 29. Bassel Baker DEA Réseaux 2) Second, it chooses the highest sub-multiple value of the 802.11e beacon interval duration as the selected service interval (SI), which is less than the minimum of all the maximum RSIs. (For example, if the beacon interval is 500ms and the three maximum RSI values are 150ms, 275ms and 200ms, the QAP scheduler will choose 125ms as selected SI which is the highest sub-multiple of beacon interval (500ms) and is smaller than the minimum value of the three maximum RSIs (150ms in this example)). Note that the selected SI refers to the time between the start of successive TXOPs allocated to a QSTA, which is the same for all the stations. 3) Third, the 802.11e beacon interval is cuted into several SIs and QSTAs are polled accordingly during each selected SI. As soon as the SI is determined, the QAP scheduler computes the different TXOP values allocated to the different traffic streams for different QSTAs, which are TXOP1, TXOP2, TXOP3…as shown in the figureII.3. Suppose the mean data rate request of the applications from traffic stream j in the QSTA i is α ij and the nominal MSDU size for this queue is M i, j , then the number of packet arriving in the traffic stream during the selected SI can be approximately computed as follows:  α ij SI  N i, j =    M i, j    Thus the QAP scheduler computes the allocated TXOP, Ti, j for the traffic stream j in the QSTA i as follows:  N i, j M i, j M  Ti, j = max  R + O, max + O    R  Where R is the PHY (physical) layer transmission rate and M max is the maximum MSDU size (2304 bytes). O refers to the transmission overheads due to PHY/MAC layer frame headers, IFSs, ACKs and poll frames. O is computed generally as: 2SIFS + TACK ( TACK is the duration to transmit an acknowledgment packet) and it’s in time units. 4) Fourth, the QAP scheduler sums all the TXOP values of different traffic streams in a ji QSTA i as: TXOPi = ∑T j =1 i, j , where ji is the number of active traffic streams in QSTA i. Then, the QAP scheduler allocates the time interval of TXOPi to QSTA i, and allows the QSTA to transmit multiple frames during this time interval. In this way, the QAP scheduler is supposed to allocate the corresponding TXOP for transmitting all the arriving frames during the selected SI. Thus, the QAP scheduler is expected to control the delays. II.2.2.1.2.4- HCF Admission Control Algorithm An admission control algorithm is also suggested in the simple HCF scheduler. We will describe, in the following, how the QAP can control the traffic flows: Page 26/54
  30. 30. Bassel Baker DEA Réseaux Using the above scheduling algorithm, the total fraction of transmission time reserved for K TXOPi HCCA of all K QSTAs in an 802.11e beacon interval can be computed as: ∑i =1 SI . In order to decide whether or not a new request from a new traffic flow can be accepted in HCCA, the QAP scheduler only needs to check if the new request of TXOPk+1 plus all the current TXOP allocations are lower than or equal to the maximum fraction of time that can be used by HCCA. Then, the QAP must to check the following inequation: TXOPk +1 K TXOPi TCAPLimit +∑ ≤ SI i =1 SI TBeacon Where TCAPLimit is the maximum duration bound of HCCA and TBeacon represents the length of a beacon interval. II.2.2.1.2.5- HCF Limitations In figure II.8, each QSTA is polled once per SI according to the HCF scheduling. This scheduling algorithm assumes that all types of traffic are CBR, so the queue length increases linearly according to the constant application data rate. However, a lot of real- time applications, such as videoconferencing, have variable bit rate (VBR) characteristics and for this type of traffic, the sending rate and packet size are varying, then some packet may not be transmitted during the TXOP due to a higher sending rate than that specified in the TSPEC even if the mean transmission rate of these applications is lower than the rate specified in the QoS requirements, in this case, latency of these packets can not be controlled by the QAP and some packets may be dropped. Then, in order to enhance HCF mechanism and lead it suitable with VBR applications, some adaptive schemes that take into account fluctuation of traffic transmission rates are thus necessary. In the following, we will present two adaptive schemes which aim to enchance EDCA as well as HCF. II.2.2.2- Adaptive EDCA (AEDCA) While EDCA service differentiation is an important QoS enhancement for DCF, it is not enough to provide strict QoS support for delay-bounded multimedia applications. In addition to that, EDCF performance decreases rapidly with high load due to its high contention rate. So, an adaptive scheme which is called Adaptive EDCA (AEDCA) was required to solve these problems. In AEDCA, after each successful transmission, the CWs of different Access Categories (ACS) do not reset to CWmin as in EDCA. Instead, the CWs are updated according to the estimated channel collision rate which takes into account the varying traffic conditions. Our subject isn’t localized in the study of AEDCA, but we preferred, in our project, to present the AEDCA enhancement, just to present the efficiency of this proposition. In the next chapter, we will present the convenient simulation results showing the comparison between EDCA and AEDCA. Page 27/54
  31. 31. Bassel Baker DEA Réseaux II.2.2.3- FHCF (Fair HCF) o Why? Based on the above discussion, a Fair HCF is proposed by IEEE 802.11e working group. FHCF aims to enhance the HCF basic scheme in order to support the QoS requirements for a VBR applications type such as videoconferencing. II.2.2.3.1- Basic Concept FHCF is composed of two schedulers: The QAP scheduler and the node scheduler. The QAP scheduler estimates the varying queue length for each QSTA before the next SI and compares this value with the ideal queue length. The figure II.9 shows a comparison b between these two queue lengths. q i Figure II.10: Queue length evolution for a TS: (0). Ideal queue length case. (1). Estimated queue length evolution II.2.2.3.2- QAP Scheduler First, the QAP scheduler computes the ideal queue length of the TS queue i for each QSTA at the beginning of the next SI:  i Mj  α i . SI - ∑ N j .   R + 2 SIFS + ACK    ideal  j =1   qi = Mi The ideal queue length refers to the queue size at the beginning of the next SI which was zero at the end of the current TXOP. We should note that the ideal queue length evolution Page 28/54
  32. 32. Bassel Baker DEA Réseaux assumption is used by the IEEE 802.11e HCF referenced scheduling scheme, which is valid only when the sending rate of the application is strictly CBR. Second, when a QSTA sends a QoS data packet, the QAP uses the QoS control field of e e the IEEE 802.11e header to record its queue length qi at the end of the TXOP. Let t i be the corresponding time at the end of current TXOP which is also recorded by the QAP est scheduler. Using this information, the QAP scheduler will be able to estimate qi , the queue length of the TS i at the beginning of the next SI as follow: qi est = ( α i . SI - t i e + qi )e Mi If the sending rate and packet size of the specification change, the above queue length estimation may not be accurate. To solve this problem, the FHCF scheme uses a window of w already known real queue length measurements to adjust the estimation. With a bigger value of w we can use more previous records for estimation and improve the accuracy, but the complexity will increase. Then, during the n-th SI, the QAP computes n ∆ i the absolute value of the difference between qi b ,real (the real queue length at the b ,est beginning of TXOP of the i-th TS) and qi , (the estimation of this queue length): n b , real b ,est ∆ i = qi − qi Figure II.10 shows that for a typical VBR video traffic, the sending rate almost follows a n Gaussian law and packets have a fluctuating size. Thus ∆ i also follows the same n Gaussian law with an expected value of 0. By recording a window of w ∆ i , and then by n (n) a corrective term that accounts for the variability of ∆ i , the QAP b ,est adding to qi scheduler is able to improve its estimation of the queue length at the beginning of the next polling of the TS: n− j ∑∆ j i qib.new ( n ) = qib.new ( n ) + .est j =n − w w Third, the QAP compares the real queue length estimation to the ideal queue length (which was zero at the end of the current TXOP) at the beginning of the next SI. It est computes the numbers of additional packets, DN i which is the difference between the estimated queue length and the ideal case: Page 29/54
  33. 33. Bassel Baker DEA Réseaux n −1 ∑∆ j i = qib,,new ( n ) − qib ,ideal ( n ) = qiest ( n ) − qiideal ( n ) + est est j =n−w DN i w Where qi ( n ) and qi ( n ) are given by the above equations. Then, the QAP computes est ideal est the additional required time t i (which may be positive or negative) for each TS of each QSTA and reallocates the corresponding TXOP duration according to the estimation of est DN i : est Mi  = DN iest . ti + 2 SIFS + ACK   R  Then it computes TP , the sum of all the positives values, TN , the absolute vale of the sum of all negative values, and T r , the remaining time of HCCA duration after allocating all the TXOPs computed in one SI using the ideal case. If TP − TN > T , it means that the r scheduler is not able to allocate all the time it expected according to the estimations, and est all the additional time t i have to be reduced. In order to be fair for all the flows, the scheduler reduces each positive additional time with a percentage β (chosen negative to est correspond to a reduction) of t i . On the other hand, each negative additional time is increased by the same percentage β of t i , where β is expressed by: est ( TP − T N ) − T r β =− TP + T N add Finally, the effective additional time t i allocated to the TS i is equal to: tiadd = (1+ β ) t iest est if t i ≥ 0 (1− β ) t iest if t i est ≤ 0 When it is time for the QAP to poll a QSTA, the QAP scheduler computes the sum of all the normal TXOPs and the effective additional time allocated to the different TSs in a QSTA. II.2.2.3.3- Node Scheduler The node scheduler plays also a very important role since it has to redistribute the additional allocated time to the different TSs within a node. It performs almost the same computations as the QAP. Suppose that the number of active TS of a given polled QSTA is p (1 ≤ p ≤ 8) . First the node scheduler in this QSTA computes N i , the number of packets to transmit in the i-th TS, and the time required to transmit a packet according to its allocated TXOP T and the number of packets the QSTA should transmit from each TS, it evaluates the remaining time T r that can be reallocated: Page 30/54
  34. 34. Bassel Baker DEA Réseaux p M  T = T − ∑ N i . i + 2 SIFS + ACK  r i =1  R  Since each QSTA knows exactly its own TS queue sizes at the beginning of the polling, it is able to estimate more precisely its queue size at the end of the TXOP and consequently the required additional time per TS. Using this information, the node scheduler performs the same computations as the QAP scheduler (calculation of the corresponding TXOP duration according to the estimation of DN i , calculation of β and the effective est ). The difference is that the coefficient β may be positive if the QSTA add additional time ti has more time than required to send all its packets or negative if, on the contrary, the remaining time is less than required. Thus, just after the CF-Poll reception, the QSTA can redistribute additional time to its different TSs with the option to add more time to each TS if β is positive. II.3- Conclusion In this chapter, we have presented the old MAC access mechanisms, DCF and PCF, the advantages and the limitations of each one as well as the new MAC access mechanisms which are proposed by IEEE 802.11 WLAN Group. In addition to that, we have demonstrated, analytically, that these new mechanisms enhance the QoS features as regards to the contention problem, the channel utilization and the priority level supporting. In the next chapter, we aim to show, by simulation, the advantages to HCF mechanisms. References II 1) Yiang Xiao, “IEEE 802.11 e : QoS Provisioning At The MAC Layer”, University Of Menphis 2) Pierre Ansel, Qiang Ni, Thierry Turletti, “FHCF : A Fair Scheduling Scheme for 802.11 WLAN”, Institut National de Recherche en Informatique et en Automatique, July 2003 3) Anders Lindgren, Andreas Almquist, Olov Schelén, “Quality of Service Scheme For IEEE 802.11 Wireless LANs- An Evaluation”, Division Of Computer Science and Networking, Department Of Computer Science and Electrical Engineering, Luleå University Of Technology, 2003 4) Qiang Ni, Thierry Turletti, “QoS Support For IEEE 802.11 Wireless LAN”, 2004 5) Pierre Ansel, Qiang Ni, Thierry Turletti, “An Efficient Scheduling Scheme For IEEE 802.11e”, March 2004 6) Fernando Santos González, “Analysis Of QoS Using IEEE 802.11e For WLANs”, Diploma thesis in LINKÔPING University Page 31/54
  35. 35. Bassel Baker DEA Réseaux 7) Mohammad Malli, Qiang Ni, Thierry Turletti, Chadi Barakat, “Adaptive Fair Channel Allocation for QoS Enhancement in IEEE 802.11 Wireless LANs”, Projet Planète, INRIA-Sophia Antipolis, France 8) Anders Lindgren, Andreas Almquist, Olov Schelén, “Evaluation Of Quality Of Service Schemes for IEEE 802.11 Wireless LANs”, Division Of Computer Science and Networking, Department Of Computer Science and Electrical Engineering, Luleå University Of Technology Page 32/54
  36. 36. Bassel Baker DEA Réseaux Chapter III: Simulation And Results III.1- Introduction In the chapter II, we have detailed the different MAC access mechanisms and we have proved how the EDCF mechanism provide a service differentiation by supporting 8 different priorities and, on the other hand, how the HCF approach will support better the QoS sensitive applications requirements. In addition to that, we have seen how the FHCF scheme can be considered a more fair approach than HCF one and so it provides an acceptable QoS elements to the different priorities. In this chapter, we aim to show, by simulation, these MAC access mechanism behaviors using different convenient scenarios. We will present the simulation results comparing the performance of the existing and proposed MAC implementations. III.2- Simulations III.2.1- Simulation Software o Why Did We Use NS? For our simulation, we will use the Network Simulator ns software. Ns is an open source code simulator, it is easy to extend in order to add new functionalities. In addition to that, ns gives the ability to visualize the circulation of packets in the network using the network animator tool (NAM) which provides a dynamic representation protocols and gives a rich infrastructure for developing new ones. o Procedures, Problems And Advices About The NS Distribution We have used the ns distribution indicated on the first reference at the end of this chapter, but we have met a lot of problems during our work and we didn’t succeed to get the results before passing all simulation errors. 1) Firstly, we have to install the Linux system paying attention to the compatibility issue, since some software isn’t compatible with a specific Operating System. One of the compatibility problems i have to pay your attention to is that this distribution needs a gcc 2.96 version. GCC is a GNU Compiler Collection, else you will get a compiling c error. 2) After untar the file (extraction), i followed the instructions in the Readme file and the first command i should type is: make depend, it worked well. 3) After that, i have to install this distribution by executing the command: make install. But i obtained an error which it wasn’t solved before making the following change: In the ~ns/ns2/random.cc file, you have to change the instruction: “random int_32” into “long int”. After this change, you will install successfully the distribution. 4) The setting of the PATH is one of the problems i encountered. The student have to know, after setting, in the command window the PATH variable, that he should add these PATHs in a file namely: .bashrc in order to fix these PATHs, else the Page 33/54
  37. 37. Bassel Baker DEA Réseaux instructions in the command window will not work well and he will have to add these PATHs after each starting of its system. Due to these errors and many others errors, I spent a lot of my time which i could use to enhance my personnel work. So, I hope that these errors will be fixed in order to simplify the students work. III.2.2- Simulation Experiments III.2.2.1- DCF QoS Limitations As we said in chapter I, the standard DCF scheme does not support any QoS guarantees. The point is that all the stations compete for the channel with the same priorities. So, there is no differentiation mechanism to guarantee packet delay or jitter of the high- priority flows. To illustrate the DCF problem, the INRIA Group had made the following simulation using the ns2 simulator. III.2.2.1.1- Scenario Characteristics A WLAN cell includes a variable number of stations which use an ad-hoc mode and all hear each other (1-hop distance). To illustrate the non-differentiation of service in DCF mode, the INRIA Group considered that each station transmits three types of traffic (Audio, Video and Background flows) to each other. The figure III.1 shows the case of the network including 8 stations. So, each station aims to transmit these three types of flows to each other station. Figure III.1: Each station send three types of traffic for each other The audio data rate was 8Kbytes/s PCM audio flow, the video data rate was 80 Kbytes, whereas the background data rate was 128 Kbytes/s. Using DCF, these stations will contend to get the channel as we have explained in the chapter II. To vary the network load, we increase the number of QSTA stations from 2 to 18. The figure III.2 and figure III.3 show the simulation results for the throughput and delay. Page 34/54
  38. 38. Bassel Baker DEA Réseaux Figure III.2: Throughput with respect to network load A simple studying of the above figure shows that the average throughput of the three flows is quasi-stable when the number of stations is equal to 10. 10 stations correspond to a network load equal to 70%, after this value, it is normally that the throughput of any flow will decrease if there isn’t any priority concept in this access method. Figure III.3: Mean Delay with respect to network load The mean throughput of the audio, video and background flow corresponds, approximately, to the data rate of each one respectively. But when the number of station is larger than 10, we remark that the throughput of all three flows decrease very fast and especially in the background case since the background flow has the greatest weight of data. The reduction arrives to the 40% when the number of station arrives to 18 which mean that the network isn’t able to deliver more than 60% of the all throughput. Page 35/54
  39. 39. Bassel Baker DEA Réseaux As regards to the delay, the above figure shows a constant delay for each of the considered flow as long as the number of stations is less than 10 and this delay will increase up to 420 ms when the network includes 18 stations. The above analysis proves that there is neither throughput nor delay differentiation between these three flows, since they share the same queues. That was the base idea behind EDCF scheme (8 priority traffics distributed on 8 different queues supported inside the same station). III.2.2.2- EDCF Enhancement To illustrate the EDCF service differentiation, we have to simulate a network including different kinds of traffic (different priorities) and get different values of QoS elements for each priority. The DCF mechanism will behave equally with these different traffics. In our simulation, we have used the ns distribution code which implements the EDCF, HCF and FHCF schemes. This distribution is published on the net and it is actually available on the site: http://www-sop-inria.fr/planete/qni/fhcf/ III.2.2.2.1- Scenario 1 The first scenario includes three kinds of traffics: An audio traffic, which has the highest priority (6), a Poisson traffic which has a medium priority (4) and the background traffic which has the lowest priority (1). The figure III.4 shows a virtual WlAN cell including an access point AP, and 12 wireless nodes. The first 6 wireless nodes send an audio traffic labeled by a red arrow, the second 6 wireless nodes send a Poisson flow labeled by a blue arrow and all wireless nodes send a background traffic labeled by a pink arrow. Figure III.4: 12 stations send many types of traffic Page 36/54
  40. 40. Bassel Baker DEA Réseaux The different traffic parameters are summarized in the table III.1 Node Application Data rate Packet CWmin CWmax Max SI CWOffset (Kbps) Size (ms) (bytes) 1-6 Audio 64 160 7 15 50 2 7-12 Poisson 1520 380 15 31 50 2 1-12 Background 341.3 256 31 1023 50 2 Table III.1: Traffic parameters The simulation time is 10 s, and all the flows start at 2.9s in order to give a complete time for the ARQ requests. We run the tcl script on EDCF mode, and we obtain a trace file. The analysis of the trace file gives us the following results: • Latency: As regards to the latency parameter which is a very important QoS element, the following figure (figure III.5) shows the latency curves for three different flow types. Latency Audio Flow Poisson Flow Background Flow 1200 Latency (ms) 900 600 300 0 0 5 10 15 Simulation Time (s) Figure III.5: Latency in EDCF mode The following table (table III.2) gives us some useful latency parameter values for these tree traffic types. Page 37/54
  41. 41. Bassel Baker DEA Réseaux Application Max Min Mean Latency Latency Latency (ms) (ms) (ms) Audio 5.768 0.132 0.936 Poisson 109.445 0.174 32.326 Background 1182.71 0.262 322.612 Table III.2: Latency results As we remark, the background flow has the greatest mean latency since it has the lowest priority (greatest CWmin) and the Audio flow has the lowest mean latency since it has the greatest priority (lowest CWmin). So, the simple analysis for this parameter shows how EDCF differentiates clearly between different traffic types. • Latency Distribution: The figure III.6 shows a comparison between the latency distributions of the three types of traffic: Audio, Poisson and Background flows. Latency Distribution Audio Flow Poisson Flow Background Flow 120 Cumulative % of packets 100 80 60 40 20 0 0 300 600 900 1200 Latency (ms) Figure III.6: Latency distribution comparison in EDCF mode The above figure shows that the Background delay packets are completely uncontrolled; whereas the Audio and the Poisson delay packets are bounded by 110 ms. The figure III.5 and figure III.6 show how EDCF starves the low priority traffic (Background Flow) especially at high load (after some seconds of simulation), and we can say that if we increase the QSTA number, for example if we have 5 stations instead of 3 stations send an Audio flow, we will obtain a much higher latency of the background flow. This is the major disadvantage of the EDCF scheme that must to be solved. Page 38/54
  42. 42. Bassel Baker DEA Réseaux • Channel Utilization And Number Of Collision: The channel is a very scarce resource in WLAN network. In spite that the EDCF mode provides a hard service differentiation with respect to the traffic priorities, but the collision number in the above scenario is very high. During 10 s of simulation, we had 8176 times of collision. In addition to that, the following table gives us the number of packet dropped in each application type: Application type Number of IFQ drops Audio (6 Flows) 0 Poisson (6 Flows) 1080 Background (12 Flows) 3669 Table III.3: IFQ drops in EDCF mode In the following, we will show a comparative study realized by the INRIA Group between the standard EDCA scheme and the Adaptive scheme called AEDCA; The AEDCA enhancement in different QoS features will be proved. III.2.2.3- EDCA Versus AEDCA In the simulation [Reference III.2], we vary the channel load by increasing the QSTA number where each QSTA sends three types of traffic (Audio, Video and Background flows) to the same QSTA which is the access point. The simulation procedure lasts 15 s and the most simulation results are averaged over 5 simulations. The figure III.7 shows the total Goodput variation and the collision number variation with respect to the number of QSTA (network load). Figure III.7: EDCA and AEDCA comparison a) Total goodput variation b) Collision number variation Page 39/54
  43. 43. Bassel Baker DEA Réseaux Note that the goodput is the data rate (as throughput) but at MAC level, it means that the throughput designates only the delivery of data packets (efficiency data) but the goodput designates the delivery of any kind of packets including data packets, routing packets and control packets. The figure III.7 a) shows that AEDCA improves significantly the total goodput of EDCA, mainly in high load situations (when the network includes 16 stations). This is because AEDCA reduces the number of collisions a lot in both moderate and high load rate cases. If we decide to study the behavior of the access mode with each type flow separately, the figure III.8 a) shows that the AEDCA gives a constant goodput for multimedia flows (video and voice flows), whatever the channel load while the goodput of video flow degrades in moderate load case in EDCA. In addition to that, we can remark that with AEDCA scheme, the degradation goodput of the background flow is less than in EDCA case (figure III.8; b)). Figure III.8: Goodput Comparison a) AEDCA scheme b) EDCA scheme As regards to the packet delay, the figure III.9 shows the latency distribution with AEDCA (left) and EDCA (right) schemes when the channel load corresponds to the 14 QSTA. Note that on the cumulative latency distribution curve, the perfect result correspond to the y-axis representing 100% of packets with zero latency, in spite of this result is practically impossible. So as much as the scheme provides a vertical line close to the y-axis, it will be the best one. Page 40/54
  44. 44. Bassel Baker DEA Réseaux Figure III.9: Latency distribution a) AEDCA scheme b) EDCA scheme The above figures allow us to give the table III.4. Schemes Voice packets Video Background (During 1.5ms) packets packets (During (During 4ms) 1.7s) AEDCA 90% 90% 90% EDCA 41% 5.7% 3% Table III.4: Description of different traffic streams Note that the improvement of the delay performance comes from the fact that AEDCA obtains lower collision rate and less idle back-off time than ECDA. III.2.2.4- HCF Versus FHCF As we detailed in chapter II, the FHCF scheme consists of 2 different schedulers: The QAP scheduler that distributes TXOPs to the different QSTAs and the node scheduler that redistributes its allocated TXOP to the different TSs accepted in HCCA. So, to evaluate the QAP scheduler performance, we have to simulate a network in which each station send one type of traffic (scenario 2). We have to see how the QAP efficiency is sufficient to distribute the convenient amount of time (TXOP) for each station (each flow). III.2.2.4.1- Scenario 2 This scenario is designed to evaluate the performance of the QAP scheduler. The network topology (figure III.10) includes 15 QSTAs and 1 QAP with only one TS per station. We have 5 QSTAs which send a high priority on-off audio traffic (64kbps), another 5 QSTAs Page 41/54

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