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Technical Report No ENGINEERING FACULTY,UNIVERSITY OF PORTO Technical Report no: 1 Robson Costa Supervisor: Paulo Portugal (Ph.D.) Co-supervisor: Francisco Vasques (Ph.D.) Co-supervisor: Ricardo Moraes (Ph.D.) 2010, September c Robson Costa, 2010 Contents List of Figures ii List of Tables iii List of Abbreviations iv 1 Introduction1 1.1 Benefits . .2 1.2 Challenges . .2 2 IEEE 802.11 Standard4 2.1 IEEE 802.11 Medium Access Mechanisms . .5 2.1.1 DCF - Distributed Coordination Function . .6 2.1.2 PCF - Point Coordination Function . .7 2.1.3 EDCA - Enhanced Distributed Channel Access . .9 2.1.4 HCCA - HCF Controlled Channel Access . 11 3 IEEE 802.11n Amendment 14 3.1 PHY Enhancements . 15 3.1.1 MIMO - Multiple-Input Multiple-Output ................. 15 3.1.2 Channel-bonding . 17 3.2 MAC Enhancements . 18 3.2.1 Frame aggregation . 19 3.2.2 Block ACK . 21 3.2.3 Reverse Direction Protocol . 22 4 Review of Relevant Work 23 4.1 Real-Time communication in IEEE 802.11 . 23 4.1.1 CA - Collision Avoidance . 23 4.1.2 CS - Collision Solver . 26 4.1.3 CR - Collision Reducer . 27 4.2 Comparison of the solutions presented . 30 5 Conclusion 31 References 37 i List of Figures 2.1 Original IEEE 802.11 MAC architecture [1]....................5 2.2 IEEE 802.11e MAC architecture [2].........................5 2.3 Interframe spaces in the DCF and PCF mechanisms [1]. .6 2.4 DCF service [2]....................................6 2.5 PCF service [2]....................................8 2.6 CFP foreshortening [2]................................9 2.7 Interframe spaces in the EDCA mechanism [2]. 10 2.8 HCCA service [2]. ................................. 12 3.1 Receive diversity. 16 3.2 Transmit diversity. 16 3.3 Spatial Multiplexing. 17 3.4 DCF basic operation. 18 3.5 IEEE 802.11e TXOP and block ACK. 19 3.6 Two-level aggregation in IEEE 802.11n. 20 3.7 Block ACK in IEEE 802.11n. 21 3.8 Reverse Direction in IEEE 802.11n. 22 4.1 Comparative between approaches presented. 30 ii List of Tables 2.1 IEEE 802.1D to IEEE 802.11e map. 10 2.2 Access Category Medium Access Parameters. 11 3.1 Comparison between IEEE 802.11 network standard. 15 iii List of Abbreviations AC Access Category ACK Acknowledgement AIFS Arbitration Interframe Space BE Best-Effort Traffic BK Background Traffic CSMA Carrier Sense Multiple Access CSMA/CA Carrier Sense Multiple Access with Collision Avoidance CTS Clear-to-Send CW Contention Window DCF Distributed Coordination Function DIFS Distributed Interframe Space EDCA Controlled Channel Access EIFS Extended Interframe Space HCCA Enhanced Distributed Channel Access HCF Hybrid Coordination Function IEEE Institute of Electrical and Electronics Engineers IFS Interframe Space MAC Medium Access Mechanism PCF Point Coordination Function PHY Phyical Layer PIFS PCF Interframe Space QAP Quality of Service Access Point QoS Quality of Service RTS Request-to-Send iv LIST OF TABLES v SIFS Short Interframe Space TXOP Transmission Opportunity VI Video Traffic VO Voice Traffic Chapter 1 Introduction The industrial communication scenario is progressively evolving thanks to the availability of even more performing networks that may be employed at all the levels of factory automation systems. In particular, focusing on the lowest level (usually referred as factory floor), besides field-buses, which have been for long time the networks traditionally deployed, two other types of communication system are nowadays available: • Real-Time Ethernet (RTE) Communication Systems: can be classified as Networked Con- trol System (NCS), derived from well known IEEE 802.3 Ethernet [3], are based on full duplex switched configurations and exploit the impressive increasing of the communication speed in order to provide the performance required at the device level. • Wireless Communication Systems: can be classified as Wireless Networked Control Sys- tem (WNCS), available for industrial applications giving the chance to build on-the-fly con- trol systems allowing a greater flexibility, mobility and reduced costs when compared with the RTE-based solutions. The major requirement for industrial wireless networks is the support of timely communica- tion services. Therefore, the demand for high performance wireless networking with real-time (RT) capabilities is one of the most relevant research challenges in this domain. How nowadays the IEEE 802.11 Wireless Local Area Networks (WLANs) [2] is a de facto standard for wireless networks, the focus of this technical report is turned to it. In this chapter the main benefits and challenges when WLANs are utilized as real-time com- munications networks in the factory floor are presented. In the chapter2, is presented a brief introduction of IEEE 802.11 standard and yours main drawbacks when utilized as real-time com- munication system. The chapter3 presents the main enhancements provided by the IEEE 802.11n amendment [4]. The chapter4 presents a survey of state-of-the-art about solutions for real-time communications systems operating on top of IEEE 802.11 standard. Finally, in chapter5, the conclusions are presented. 1 Introduction 2 1.1 Benefits The main benefits on the use of wireless networks in real-time communication systems when compared with the RTE-based solutions are the greater flexibility, mobility and reduced costs that this kind of network can provide. • Flexibility: supporting the trends towards reconfigurable production systems that need to be adaptable to changing need of the market [5]. Parallel to flexible automation concepts on application level, wireless networks provide utmost flexibility on the communication side. • Mobility: which is advantageous in areas where nodes are widely scattered or where wires can be installed only with great difficulties due to hostile environments. Mobility in this respect does not necessarily mean that nodes may go wherever they want; they can as well stay within a strictly confined area (e.g., robotic arm). • Reduced Costs: when flexibility and mobility of a line of production is increased, hence their costs will drop due to reduced downtime for changes that may be necessary and also the reduction of wiring used. 1.2 Challenges The utilization of WLANs to industrial applications presents a number of additional chal- lenges compared to home or enterprise applications, the most severe being to strict requirements about transmission delays and frame loss ratio. In enterprise Voice-over-IP (VoIP) applications can tolerate transmissions latencies up to 150 ms and up to 1 percent [6] data corruption on the exchanged frames, thanks to adaptive play-out control and error concealment algorithms. By contrast, factory automation systems usually require shorter cycle times, that usually range (about) from 1 to 10 ms but could be well bellow 1 ms for specific applications (e.g., motion con- trol). In addition, minimizing communication jitters is also important for many control systems based on cyclic operation. Another important requirement in automated control systems are deterministic performance guarantees. In industrial networks, runtime performance degradation in not an option for mission- critical applications. This requirement is also enforced during device roaming, which leads to the requirements for real-time handover. Therefore, at present, wireless communications are used in industrial environments mainly to enable simpler and more cost-effective maintenance and diag- nostics functions. According to Cena et al. in [7], the wireless solutions and products today available on the market are generally considered unsuitable for implementing distributed control applications and systems, in particular when real-time is one of the key issues. This is mainly due to three reasons: • communication over radio channels is very sensitive to electromagnetic interferences (often present in abundance in industrial environments), which may cause excessively high error rates; 1.2 Challenges 3 • interference may be generated as a consequence of overlapping wireless networks, that are out of the sphere-of-control1 of the real-time system; • the nature of the random access scheme used by the WLANs, namely, the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), prevents that this kind of network presents a deterministic behavior. As a consequence, in wireless networks there is not any guarantee that a frame will be delivered timely to the intended target(s), and not even that it will be delivered at all. Thus, no upper bound can be ensured on transmission latency, that is usually unacceptable in most control systems. 1The concept “inside/outside“ sphere-of-control was defined by Kopetz [8]. Whenever a real-time entity is in the sphere-of-control of a subsystem, it belongs to a subsystem that has the authority to change all the value of this real-time entity. Outside its sphere-of-control, the value of the entity can be observed, but cannot be modified. Chapter 2 IEEE 802.11 Standard With the authorization of the use of three frequency bands in 1955 by the Federal Communi- cations Commission (FCC), U.S. agency responsible for regulating the use of frequency spectrum, in 1990 the Institute of Electrical and Electronics Engineers (IEEE) established a committee to define a standard for wireless connectivity which had its first standard approved in 1997 under the name of IEEE 802.11 [1] with nominal data rates of 1 and 2 Mbps; In 1999 it was approved the IEEE 802.11b [9] and 802.11a [10], which use the frequencies of 2.4 and 5 GHz and are capable of nominal rates of transmission of 11 and 54 Mbps, respectively. The 802.11b standard, while achieving lower transmission rates, gained a greater share of the market than 802.11a, the reasons for this were basically two: first, the 802.11b interfaces were less expensive than the 802.11a and, second, 802.11b implementations was introduced on the market before than the 802.11a implementations. In 2003, the standardization committee of the IEEE approved the IEEE 802.11g [11] standard that, like 802.11b, works in the 2.4GHz frequency, but reaches up to 54 Mbps of nominal rate of transmission.
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