Yang Xiao, PhD, is a Professor in the Department of Computer Science at The University of Alabama. He is a Senior Member of the IEEE and currently serves as Editor in Chief of International Journal of Security and Networks (IJSN), International Journal of Sensor Networks (IJSNet), and International Journal of Telemedicine and Applications (IJTA). His areas of research include security, telemedicine, sensor networks, and wireless networks. He has published more than 300 papers in major journals, refereed conference proceedings, and books related to the aforementioned areas.

Yi Pan, PhD, is a Yamacraw Professor in the Department of Computer Science at Georgia State University. His research interests include parallel and distributed computing, optical networks, wireless networks, and bioinformatics. His work on computing using reconfigurable optical buses has inspired extensive subsequent work by many researchers, and his research results have been cited by more than 100 researchers worldwide. He is a co-holder of three U.S. patents (pending) and five provisional patents; has coedited eleven books or conference proceedings; and has published more than 130 research papers.

Emerging Wireless LANs, Wireless PANs, and Wireless MANs

IEEE 802.11, IEEE 802.15, 802.16 Wireless Standard Family

John Wiley & Sons

Copyright © 2009 John Wiley & Sons, Inc.
All right reserved.
ISBN: 978-0-471-72069-0

Chapter One

IEEE 802.11 MEDIUM ACCESS CONTROL AND PHYSICAL LAYERS

KAVEH GHABOOSI, MATTI LATVA-AHO, and YANG XIAO

1.1 INTRODUCTION

A wireless local area network (WLAN) is an information system intended to offer diverse location-independent network service access to portable wireless devices using radio waves instead of wired infrastructure. In corporate enterprises, WLANs are typically deployed as the ultimate connection between an existing cable infrastructure network and a cluster of mobile clients, giving them wireless access to the shared resources of the corporate network across a building or campus setting. Fundamentally, WLANs liberate customers from reliance on hard-wired access to the network backbone, giving them anywhere, anytime network services access. The pervasive approval of WLANs depends upon industry standardization to ensure product compatibility and reliability among various brands and manufacturers. Among existing system architectures, the IEEE 802.11 family is the most popular and accepted standard concerning medium access control (MAC) and physical (PHY) layers in WLANs; therefore, in this chapter, we briefly overview its basic features in both aforementioned layers. We start our investigation with the MAC layer and its fundamental components. Supported network types, different network services, and media access schemes are covered, accordingly. Subsequently, the physical layer and its basic characteristics are discussed. Different technologies, including frequency hopping (FH), direct-sequence spread spectrum (DSSS) and its high rate (HR) counterpart (i.e., HR/DSSS), and orthogonal frequency division multiplexing (OFDM), recommended for the IEEE 802.11 physical layer are then explored. As a result, this chapter can be assumed as a comprehensive overview of the IEEE 802.11 standard.

1.2 IEEE 802.11 MAC PROTOCOL

In 1997, the IEEE 802.11 working group (WG) proposed the IEEE 802.11 WLAN standard and, subsequently, a revised version was released in 1999. The primary medium access scheme in IEEE 802.11 MAC is the distributed coordination function (DCF), a contention-based protocol which is based on the carrier sense multiple-access/collision avoidance (CSMA/CA) protocol. In the DCF, mobile terminals should contend for the shared wireless channel, and as a result, the medium access delay for each station (STA) cannot be bounded in heavy-traffic-load circumstances. Thus, the DCF is capable of offering only asynchronous data transmission on a best effort (BE) basis. In order to support real-time traffic such as voice and video, the point coordination function (PCF) scheme has been advised as a noncompulsory option. Basically, the PCF is based on a centralized polling scheme for which a point coordinator (PC) residing in an access point (AP) provides contention-free services to the associated stations in a polling list. In addition to the IEEE 802.11 standard, there is a well-known book by Gast which is considered as a complete scientific review of 802.11 families. Due to the popularity of the aforementioned book, we will use it frequently throughout the section to refer the reader to more technical issues and discussions.

Recently, considerable interest in wireless networks supporting quality of service (QoS) has grown noticeably. The PCF is already available in IEEE 802.11 to offer QoS but has not yet been implemented in reality due to its numerous technical limitations and performance drawbacks. For that reason, the 802.11 WG initiated IEEE 802.11e activity to develop the existing 802.11 MAC to facilitate support of QoS. Regarding the 802.11e amendment, not only the IEEE 802.11e standard but also recognized introductory and survey papers will be used repeatedly as key references. We cite many technical issues from these works and the references therein to more appropriately explain 802.11/802.11e-based system features.

1.2.1 Categories of 802.11 Networks

The key constructing component of an 802.11 network is the basic service set (BSS), a group of wireless terminals that communicate with each other over a common radio channel. Data transmission is accomplished within a basic service area, defined by radio propagation characteristics of wireless channel. BSSs come in two categories, as illustrated in Fig. 1.1.

The infrastructure BSS networks are primary for mobile stations to access the Internet via an AP so that, in most of cases, communications between two stations within the same service set do not happen. In communications between mobile stations in the same service set, the AP deployment acts as an intermediate node for all information exchanges comprising communications. In other words, any data communication between two wireless clients should take two successive hops, i.e., source STA to AP and AP to destination STA. Obviously, the exploitation of APs in infrastructure networks brings two major advantages. On the one hand, no restriction is placed on the physical distance between mobile stations. On the other hand, allowing straight communication between wireless terminals would apparently preserve system capacity but at the cost of increased physical and MAC layer complexity. The most important functions of an AP are to assist stations in accessing the Internet and help save battery power in associated wireless stations. If a mobile terminal is in the power-saving (PS) mode, the AP buffers those frames destined to reach the station during the period it will be in PS status. When the terminal exits the PS mode, the AP forwards the cached data frames to the station one by one. Hence, APs evidently play a key role in infrastructure networks to make implementation of PS mechanisms possible.

In the independent BSS (IBSS), mobile stations are allowed to communicate directly. Characteristically, IBSSs are composed of a few stations configured for a particular goal and for a short period of time. IBSSs are sometimes referred to as ad hoc BSSs or simply ad hoc networks.

IEEE 802.11 allows wireless networks of arbitrary size to be installed and utilized by introducing the extended service set (ESS) concept. Basically, the ESS is constructed by chaining neighboring BSSs and requires a backbone system that provides a particular set of services. Figure 1.2 illustrates an ESS as a combination of three neighboring BSSs. Switching between adjacent BSSs while being connected to the system is called a handoff. Stations with the same ESS are able to communicate with each other even if they are not in the same BSS or are moving from one point to another. For associated stations within an ESS, a wireless network should behave as if it were a single layer 2 local area network (LAN). In such an architecture, APs are similar to layer 2 bridges; consequently, the backbone network should be a layer 2 network as well (e.g., Ethernet). Several APs in a single area may be connected to a single switch or can even use virtual LANs (VLANs) if the link layer connection spans a larger area. 802.11 supplies link layer mobility within an ESS, but only if the backbone network is a single link layer domain, such as a shared Ethernet or a VLAN.

Theoretically, extended service areas are the highestlevel abstraction supported by 802.11 wireless networks. In order to let non-802.11 network devices use the same MAC address to exchange data traffic with an associated station somewhere within the ESS, APs should mimic an absolute cooperative system. In Fig. 1.2, the illustrated gateway uses a single MAC address to deliver data frames to the targeted mobile stations in different BSSs. This is the MAC address of an AP with which the intended wireless station has been already associated. As a result, the gateway is unaware of the actual location of a tagged wireless terminal and relies only on the corresponding AP to forward data traffic. The backbone network to which APs are connected is called the distribution system (DS) since it makes delivery of information to and from the outside world possible.

It should be noted that technically different types of 802.11 networks may coexist at the same time. For instance, IBSSs might be constructed within the basic service area of an AP. Coexisting infrastructure BSSs and IBSSs should share the same radio channel capacity and, as a result, there may be adverse performance implications from colocated BSSs as well.

1.2.2 IEEE 802.11 Networks Services

Generally the IEEE 802.11 standard provides nine dependent services: Three of these services are dedicated distinctively to data transfer purposes while the remaining ones are explicitly devoted to management operations enabling network systems to keep track of mobile stations and react in different circumstances accordingly.

The distribution service is exploited in infrastructure networks to exchange data frames. Principally, the AP, upon receiving a MAC protocol data unit (MPDU), uses this service to forward it to the intended destination station. Therefore, any communication with an AP should use a distribution service to be possible. Integration is a specific service provided by the distribution system that makes connection with a non-802.11 network possible. The integration function is not expressed technically by the standard, except in terms of the services it should offer.

MAC frame delivery to the associated terminals will not be possible unless the association service ensures that the AP and connected stations can work together and use the network services. Consequently, the distribution system is able to use the registration information to determine the AP with which a specific mobile station has been associated. In other words, unassociated wireless terminals are not permitted to obtain any service from the whole system. In an ESS, when a mobile station moves between different BSSs, there should be a set of handoffs to be accomplished in order to keep the station connected to the system. Reassociation is generally initiated by a wireless terminal once the signal strength indicates that a different association is necessary. This means that handoff and reassociation requests are never commenced by APs. Upon completion of reassociation, the distribution system renews its location records to reflect the latest information about reachability of the mobile station. To terminate an existing association, wireless stations may possibly use the so-called disassociation service. Upon invocation of disassociation, any mobility information stored in the distribution system corresponding to the requesting station is removed at once.

Authentication is an obligatory prerequisite to association due to the fact that only authenticated users are authorized to use the network resources. If the APs of a distribution system have been configured in such a way as to authenticate any station, then the system is called an “open system or an “open network.” These kinds of wireless networks can be found, for instance, at university campuses. Deauthentication terminates an authenticated relationship between an AP and a wireless station. Since authentication is required before system resources utilization, a side effect of deauthentication is termination of any existing association.

IEEE 802.11 offers a noncompulsory privacy service called wired equivalent privacy (WEP). WEP is not iron-clad security; in fact, it can be easily disabled. In response, the IEEE 802.11i task group (TG) is seeking an enhanced and stronger security scheme to be included in the next generation of 802.11 equipments. IEEE 802.11i, known as WiFi-protected access version 2 (WPA2), is an amendment to the 802.11 standard specifying security mechanisms for wireless networks. It makes use of the advanced encryption standard (AES) block cipher, while WEP and WPA (an earlier version) use the RC4 stream cipher. The 802.11i architecture contains the following components: 802.1X for authentication [entailing the use of extensible authentication protocol (EAP) and an authentication server], the robust security network (RSN) for keeping track of associations, and the AES-based counter mode with cipher block-chaining message authentication code protocol (CCMP) to provide confidentiality, integrity, and origin authentication. Another important element of the authentication process is an innovative four-way handshake.

The MPDU is a fancy name for 802.11 MAC frames. The MPDU does not, however, include PHY layer convergence procedure (PLCP) headers. On the other hand, the MAC service data units (MSDUs) are only composed of higher level data units [e.g., Internet protocol (IP) layer]. For instance, an 802.11 management frame does not contain an MSDU. Wireless stations provide the MSDU delivery service, which is responsible for getting the data to the actual recipient.

1.2.3 IEEE 802.11 Media Access Schemes

In what follows, we discuss the medium access rules defined in the 802.11 standard and its corresponding amendments. We begin the discussion with the contention-based 802.11 DCF access scheme. Subsequently, a few paragraphs are dedicated to the 802.11 PCF, which is a contention-free channel acquisition technique. Finally, the supplementary QoS-aware amendment of the IEEE 802.11 standard, i.e., the 802.11e hybrid coordination function (HCF), is explored.

1.2.3.1 IEEE 802.11 DCF. The fundamental IEEE 802.11 access scheme is referred to as the DCF and operates based upon a listen-before-talk (LBT) approach and CSMA/CA.

As indicated, MSDUs are transmitted using MPDUs. If the wireless station chooses to fragment a long MSDU into a number of MPDUs, then it should send the long MSDU through more than one MPDU over the radio system. 802.11 stations deliver MSDUs following a media detection procedure dealing with an idle wireless channel that can be acquired for data transmission. If more than one station senses the communication channel as being idle at the same time, they might commence their frame transmissions simultaneously, and inevitably a collision occurs subsequently. To minimize the collision risk, the DCF uses carrier sense functions and a binary exponential backoff (BEB) mechanism. In particular, two carrier sense schemes, namely physical and virtual carrier sense functions, are employed to simultaneously resolve the state of the radio channel. The former is offered by the physical layer and the latter by the MAC layer, called network allocation vector (NAV). The NAV records the duration that the medium will be busy based upon information announced before the control/data frames are captured over the air interface. If either function indicates a busy medium, the medium is considered busy (i.e., reserved or occupied); if not, it is considered idle. Subsequent to detection of wireless medium as idle, for a so-called DCF interframe space (DIFS) time duration, stations continue sensing the channel for an extra random time period called a backoff period. The wireless station begins traffic delivery whenever the shared medium remains idle over this further random time interval. The backoff time is determined by each station as a multiple of a pre defined slot time chosen in a stochastic fashion. This means that a fresh independent random value is selected for every new transmission. In the BEB algorithm, each station chooses a random backoff timer uniformly distributed in an interval [0, CW – 1], where CW is the current contention window size. It decreases the backoff timer by 1 for every idle time slot. Transmission is started whenever the backoff timer reaches zero. When frame transmission fails due to any reason, the station doubles the CW until it reaches the maximum value [CW.sub.max]. Afterward, the tagged station restarts the backoff procedure and retransmits the MAC frame when the backoff counter reaches zero. If the maximum transmission retry limit is reached, the retransmission should be stopped, the CW should be reset to the initial value [CW.sub.min], and the MAC frame is simply discarded. At the same time as a wireless station is counting down its backoff counter, if the radio channel becomes busy, it suspends its backoff counter decrement and defers from the media acquisition until the medium again becomes idle for a DIFS.

(Continues…)

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