wirelesstechnology

Wireless LAN Technology: Current State and FutureTrends

Zahed IqbalHelsinki University of Technology

Telecommunications Software and Multimedia [email protected]

Abstract

In this paper, a comprehensive overview of the current state and future trends ofWireless Local Area Network (WLAN) has been presented. This document stud-ies and compares two most competing commercialy potential WLAN technologies,namely IEEE 802.11 and ETSI HiperLAN. This study also addresses the challengesof their coexistance and convengences towards a global standards.

KEYWORDS: Wireless Local Area Network (WLAN), IEEE 802.11, ETSI,HiperLAN

1 Introduction

Wireless Local Area Network (WLAN) is a flexible data communication system that caneither replace or extend a wired LAN to provide added functionality. Using Radio Fre-quency (RF) technology, or Infrared (IR) WLANs transmit and receive data over the air,through wall, ceilings, and even cement structures, without wired cabling. A WLAN pro-vides all the features and benefits of traditional LAN technologies like Ethernet and TokenRing, but without the limitations of being connected by a cable. This provides greatlyincreased freedom and flexibility. [8]

Wireless Local Area Networks have been used increasingly in many critical applicationsover the past few years, particularly since 1997 when the first IEEE802.11 WLAN standardwas issued followed by its European competitor standard High Performance LAN (Hiper-LAN). In certain locations, the use of WLANs could save millions of dollars in cost anddeployment time when compared to permanent wired networks. In other locations, WLANservices are complimentary to existing wired LANs adding the advantage of user mobility.

Currently there exists an enormous number of Wireless LAN standards from different stan-dardization organs and they are competing to each other to a certain degree capable enoughto create a puzzling situation when to chose a wireless data communication solution. Themain problem is that there is not one unique standard like Ethernet with a guaranteed com-patibility between all standards and devices, but many proprietary standards pushed by

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Ad Hoc Mobile Wireless Networks – Research Seminar on Telecommunications Software, Autumn 2002HUT TML – Course T-110.557 – Publication ISBN 951-22-6309-2 ISSN 1456-7628 TML-C8http://www.tml.hut.fi/Studies/T-110.557/2002/papers/

each independent organs and incompatible between themselves. So many standards bringssome good aspect like lower product price but at the same time it introduces some chal-lenges concerning lack of compatibility and interoperability. The motivation of this paperis to identify the characteristics of those different standards and compare them.

Over all, this paper gives an overview of the current state of two wireless LAN standard,namely in the IEEE 802.11 and the HiperLAN, compare them and discusses the futuretrends of WLAN, as well as presents the challenges of their coexistence or convergencetowards a global standard.

1.1 Background

Over the past ten years or so an alternative to wired LAN structures has evolved in the formof the Wireless LAN. The first generation Wireless LAN products, operating in unlicensed900-928 MHz Industrial Scientific and Medical (ISM) band, with low range and through-put offering (500 Kbps), subjected to interference came to market with few success in someapplications. But they enjoyed reputation of being inexpensive due to break through devel-opment in semiconductor technologies, on the other hand the band become crowded withother products with in short period of time leaving no room for further development.

The second generation in 2.40-2.483 GHz ISM band WLAN products boosted by the de-velopment of semiconductor technology was developed by a huge number of manufactures.Using Spread spectrum technology and modern modulation schemes this generation prod-ucts were able to provide data rate up to 2 Mbps, but again the band become crowded sincemost widely used product in 2.4 GHz is microwave oven which caused interference.

Third generation product assembled with more complex modulation in 2.4 GHz band al-lows 11 Mbps data rate. In June 1997, the IEEE finalized the initial standard for wirelessLANs: IEEE 802.11. First fourth generation standard, HiperLAN, came as specificationfrom European Telecommunication Standard Institute (ETSI) Broadband Radio AccessNetwork (BRAN) in 1996 operating at 5 GHz band. Unlike the lower frequency bandsused in prior generations of WLAN products, the 5 GHz bands do not have a large "in-degenous population" of potential interferors like microwave ovens or industrial heatingsystem as was true in 900 MHz and 2.4 GHz [8]. In late 1999, IEEE published two supple-ments to the 802.11: 802.11b and 802.11a following the predecessor success and interestfrom the industry [2]. ETSIs next generation HiperLAN family, HiperLAN/2, proposed in1999 operating at same band with its predecessor, is still under development, the goal is toprovide high-speed (raw bit rate 54Mbps) communications access to different broadbandcore networks and moving terminals [8]. It is expected that 802.11b will compete withHiperLAN/1 and 802.11a will compete with HiperLAN/2 in near fut! ure.

1.2 Wireless LAN Topologies

The infrastructure mode and the ad-hoc mode are two most common topologies that aresupported by Wireless LAN. In HiperLAN terminology they are referred as "centralizedmode" and "direct mode", but the basic idea and how they work are same. The infrastruc-

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ture mode is some times called Basic Service Set (BSS), which rely on an Access Point(AP) that acts as a controller in each radio cell or channel. If station A want to commu-nicate with station B, it goes through the AP. This mode of operation is mainly used issuitable for business applications, both indoors and outdoors, where an area much largerthan a radio cell has to be covered. The access point performs several tasks, like connectingto wired network, bridging function to connect multiple WLAN cells or channels.

Ad-hoc modes are known as "peer-to-peer" mode in some literature. In this mode mobilenodes can form network among themselves without the help of any fixed or wireless in-frastructure like AP. It is principally used to quickly and easily build a network where noinfrastructure is available. A good example of the use of this mode could be in military, orconvention center to share file or some information sharing between users.

Another mode sometime referred in IEEE 802.11 standard is Extended Service Support(ESS), where multiple BSS are joined together to use the same channel to boost the aggre-gate throughput. Basically this mode is a set of BSS working together.

1.3 Outline of the paper

Chapter one starts with the general introduction of wireless local area network; it’s back-ground and problem statement. The network topologies, its use and benefits are also dis-cussed in chapter one.

Chapter two tells about the different WLAN technologies and standard in brief, mainlyIEEE 802.11 family standards and HiperLAN, considering the fact that they are commer-cially potential competitor to each other.

Chapter three deals with the WLAN layer architecture - this is related to the OSI lowerlayers (PHY, MAC). In physical layer level, different modulation techniques used by bothof the technologies are discussed; their strength and shortcomings are also taken in con-sideration when comparing them. The medium access control mechanism of competingstandards have been presented and compared.

In Chapter four, the future trends in wireless area networking is presented, why the currenttechnologies are not adequate to meet the future requirements, how they can be addressedusing the new upcoming standards etc. Chapter five concludes this paper with findings andrecommendations; a coexistence of standards is also outlined.

2 Wireless LAN Standards Variants

An introduction to the wireless LAN standards, currently available from IEEE and ETSIhas been presented in this chapter.

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2.1 IEEE 802.11 Standard family

802.11

The 802.11 standard for WLAN operates at data rates up to 2 Mbps in the 2.4-GHz ISMband. The goal of this standard was to serve the same purpose as IEEE 802.3 for wiredEthernet to define an open standard for wireless networks so that the consumers no longerwas tied to a single vendor with proprietary technologies [10]. This standard describes thespecification of one Medium Access Control (MAC) layer and three physical layers: Fre-quency Hopping, Direct Sequence and diffuse infrared. The MAC has two main standardsof operation, a distributed mode (CSMA/CA), and a coordinated mode (polling mode.802.11 of course uses MAC level retransmissions, and also RTS/CTS and fragmentation.The physical layers and MAC layer will be discussed in next chapter in details.

This standard includes an optional but quite complex power management features, whichsupports two separate modes: Active mode and Power save mode. Power managementfeatures define functionality relating to how stations can enter into a power mode and thefunctionality relating to when another station desires to communicate with it during powersaving state, but the standard does not define when to enter or leave low power operatingstate, that is the reason why power management features are considered as complex.

The standard also includes optional authentication (open system and shared key) and en-cryption using Wired Equivalence Privacy (WEP) [11]. With the WEP enabled, the bodyof the data frame, not the header, is encrypted (RC4 symmetrical stream cipher 40 bit key)with a common key used for both encryption and decryption.

802.11b

The 802.11b is standards for WLAN operations at data rates up to 11 Mbps, real 4-6Mbps, in the 2.4 (2.4 to 2.4835) GHz ISM band, which provides 83 MHz spectrum. Thesame RF band of wireless spectrum used by cordless phone and microwave ovens. It isan expansion and much like of the IEEE 802.11 standards, supports transmission usingDSSS modulation. It allows transmission at such a speed at a distance of several hundredfeet. The distance depends on impediments, materials, environment and the line of sightfor IR based networks [3]. It was the first widely available WLAN technology to providespeeds similar to wired LAN. Organizations were quick to realize that the technology,operating at speeds of 11 Mbps, could very easily address most mainstreams, enterprise-wide applications such as e-mail messaging, database and internet access and traditionaloffice applications. Although 11 channels are available throughout this band, only threeof them are non-overlapping or clear ! channel, the occupied bandwidth of the spread-spectrum channel is 22 MHz spaced by 25 MHz apart. The available bandwidth decreasesif users roams more than 400 ft from and access point.

802.11b’s physical layer is slightly different than it’s predecessor while having same MAClayer. A High Rate extension of 802.11 in implemented in 802.11b called HR PHY toachieve higher bit rate which implements Complementary Code Keying (CCK) [10]. CCKis a set of 64 eight-bit code words used to encode data, it has unique mathematical proper-ties that allows them to be correctly distinguished from one another by a receiver even inthe presence of substantial noise and multipath interference. 802.11b standard is consid-

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ered as competitor of HiperLAN/1 technology.

802.11a

The 802.11a is standards for WLAN operations at data rates up to 54 Mbps in the 5 (5.15to 5.825) GHz Unlicensed National Information Infrastructure (UNII) band, which is de-signed to provide short-range, high-speed wireless networking communication. The MAClayer is same as 802.11 and 802.11b but it does not use spread spectrum technique in phys-ical layers, instead it uses another new modulation scheme called Orthogonal FrequencyDivision Multiplexing (OFDM), which is the basis for higher data rate. OFDM technol-ogy will be discussed in next chapter in details manner. This standard uses 300 MHz ofbandwidth, the spectrum is divided into 3 sections, and each section is having restrictionof maximum power uses. The first 100 MHz in lower frequency portion is restricted to amaximum power output of 50 mW, the second 100 MHz has a higher 250 mW maximumand the 3rd 100 MHz has maximum of 1.0 W output power intended for outdoor applica-tions [10, 17]. In each section there are four! channels available which gives all together12 channels, three times more than that of offered by 802.11b. The 802.11a standard hasa wide variety of high-speed data rates available: 6,9,12,18,24,36,48 and 54 Mbps; it ismandatory for all products to have 6Mbps, 12Mbps and 24Mbps rates [2]. This standardis considered as the most commercial competitor of HiperLAN/2 technology.

Others

802.11g takes the best features of 802.11a and 802.b. It will operate in 2.4GHz band butwill use OFDM as modulation scheme in physical layer. The original idea to have thisstandard is to maintain backward compatibility with 802.11b products with higher datarate (54 Mbps). 802.11e, 802.11f, 802.11h and 802.11i are the future 802.11 standardvariants and still waiting for standard to be ratified in future.

2.2 ETSI HiperLAN

The demand for broadband wireless communication for multimedia application support-ing higher data rate has lead development of new standards. ETSI has come up with stan-dardization of different kind of Broadband Radio Access Network (BRAN). One of thesestandards is HiperLAN, which will provide high-speed access to core broadband core net-works. [5] In this document HiperLAN type 1 and type 2 has been considered.

HiperLAN type 1 was designed to build ad-hoc networks; standard is quite simple, usessome advanced features and has already been ratified in 1996. The goal of this new stan-dard was to achieve higher data rates than 802.11. The main advantage of HiperLAN/1 isthat it works in a dedicated bandwidth 5.1-5.3 GHz, allocated in Europe, and so does nothave to include spread spectrum technology. It offers data rates up to 23.5 Mbps with 5fixed channels defined. The protocol uses a variant of CSMA/CA based on packet TTLand priority, and MAC level retransmission. [11]. The most important distinguishing char-acteristics of HiperLAN/1 is its centralized MAC, which supports QoS functions [1]. Theprotocol includes optional encryption and power management features in addition to Ad-hoc routing capability by means of which intermediate stations will automatically forwarddata packets through optimal routing with in the network if the destination seems to out

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of reach. One of the shortco! mings that HiperLAN/1 has is the lack of ability to providereal isochronous services. Although HiperLAN/1 provides a means of transporting timebounded services, it does not control or guarantee QoS on the wireless link. It is thus con-sidered a system for best-effort delivery of data. This is what motivated ETSI to develop anew generation of standards that support asynchronous data and time- critical services. [5]

On the other hand HiperLAN type 2 or HiperLAN/2 is total opposite of HiperLAN/1 ina sense that it was designed for managed infrastructure and wireless distribution system[11], operates on 5 GHz (5.470 to 5.725 GHz), dedicated in Europe and provides data ratesup to 54 Mbps in PHY layer. It is the first standard to be based on OFDM modulation, thebasis for higher data rates. It has been designed to provide high-speed seamless access toa variety of legacy backbone networks including 3G mobile core networks, AsynchronousTransfer Mode (ATM) networks and Internet Protocol (IP) -based networks in addition toprivate wireless LAN systems. Basic applications include data, voice and video, with spe-cific Quality of Service parameters taken into account. HiperLAN/2 provides connection-oriented data communication between MT and AP with strong security support for au-thentication and encryption. Authentication is based on the existence supporting functionssuch as directory service o! r something else. The user traffic on established connectionscan be encrypted to protect against for instance eavesdropping and man-in-middle attacks.HipetLAN/2 has a built in support for automatic frequency allocation, removing the needfor manual frequency planning. It includes power management mechanisms based on MTinitiated negotiation of sleeps periods; MT may request to AP at point of time to enter toa power save mode, AP differs all the pending packets mean to MT until the agreed sleepperiod expires.

HiperLAN/2 network consists typically of a number of Access Points (AP) each of whichcovers a certain geographic area. Together they form a radio access network with fullor partial coverage of an area of almost any size. The coverage areas may or may notoverlap each other, thus simplifying roaming of terminals inside the radio access network.Each AP serves a number of Mobile Terminals (MT) which have to be associated to it.HiperLAN/2 support two basic operations; Centralized mode where an AP is connected toa core network and serving several MT’s associated with it, Direct Mode where MAC isstill controlled by a central controller but this controller needs not necessarily be connectedto a core network. In direct mode the terminals can exchange directly via air whereas incentralized mode all traffic must pass the AP [14].

The basic protocol stack of HiperLAN/2 is on the AP side; it consists of the PHY layer onthe bottom, the Data Link Control (DLC) layer in the middle and the Convergence Layer(CL) on top. The Physical layer delivers a basic data transport function by providing meansof a baseband and modem and RF part. The baseband part will also contain and frowarderror correction function. The DLC layer consists of the Error Control function (EC), andRadio Link Control (RLC) sub-layer. [14] MAC and PHY layers are described in nextchapter.

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3 WLAN Protocol Layer Architecture

Wireless LAN Protocols are seen as logical layered architecture to comply with ISO model.As mentioned earlier, 802.11 standard family mostly deals with two lower layers of OSIarchitecture: the Data Link Layer (DLL) and PHY. In layered concept one lower layer isconsidered as service provider and the upper layer is so called service user. DLL is furtherdivided to sublayers due to simplifying conformant equipments; Logical Layer Control(LLC) and MAC. The initial idea was to use the same LLC developed for 802 compliantsystem and use upper layer protocols without the much concern that they differs signifi-cantly: one uses unreliable air medium and the other uses reliable wire media. Physicallayer and MAC layer of both wireless LAN standards are covered in the sections below.

3.1 Medium Access Control (MAC)

Basically MAC layer is a program that runs on a processor; it manages and maintainscommunications between radio Network Interface Card (NIC) and AP by coordinatingaccess to a shared radio channel. The goal of MAC layer is to provide access controlfunctions such as address coordination, frame check sequence generation and checking etcfor shared-medium PHYs [2]. The main purpose of the MAC protocol is to regulate theusage’s of the medium, and this is done through a channel access mechanism, the core ofMAC; a way to divide the main resource between nodes, the radio channel, by regulatingthe use of it.

An ideal MAC layer should provide the following features; Good throughput since thespectrum is scarce resource. Less delay due to the fact that there will be more and moretime-bounded multimedia applications. Transparency to different PHY layers. Fairnessto access because of unequal received power in fading channels. Low battery power con-sumption since the portable and mobile devices will be batteries powered. Maximum num-ber of nodes in a coverage area and less channel interference and off course security in anacceptable level [2].

In this section below IEEE 802.11 and HiperLAN MAC layer have been presented.

IEEE802.11 MAC Layer

IEEE802.11 uses distributed MAC protocol based on CSMA/CA as channel access mech-anism. CSMA/CA is used by most wireless LANs in the ISM bands, it specifies how thenode uses the medium: when to listen, when to transmit [19]. It is extremely unusualfor a wireless device to be able to receive and transmit simultaneously, that is the reasonwhy IEEE 802.11 uses Collision Avoidance (CA) rather than Collision Detection (CD) likeused for wired LAN. Since it is impractical for wireless devices to communicate with allother devices directly, IEEE802.11 implements a network allocation vector (NAV), a valuethat indicates to a station the amount of time that remains before the medium will becomeavailable. In that sense, NAV can be considered as virtual carrier sense mechanism. Bycombining physical carrier sense and virtual carrier sense mechanism, the MAC protocolimplements the CA portion of CSMA/CA access mechanism [14].

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As mentioned in chapter two, IEEE 802.11 supports one mandatory and two optional co-ordination function schemes; Distributed Coordinated Function (DCF) which is based onCSMA with collision avoidance (CSMA/CA) protocol, DCF with handshaking (CTS/RTS)and Point Coordinating Function (PCF) for time-bound multimedia services [2].

DCF, the fundamental access method, is used to support asynchronous message passingmechanism, delivering the best effort services, but no bandwidth and latency guarantee.The main advantages of DCF are its suitability for network protocols such as TCP/IP, itadapts quite well with the variable condition of traffic and is quite robust against inter-ference [2,19]. With DCF, 802.11 stations contend for access and attempt to send frameswhen there is no other station transmitting. The protocol starts by listening on the channel,stations delivers MAC service data unit (MSDU) of arbitrary lengths after detecting thatthere is no other transmission in progress on the wireless medium. However, if two sta-tions detect the channel as free at the same time, a collision occurs, a Collision Avoidancemechanism, defined in 802.11, takes care of reducing the probability of such collisions. Asa part of CA, before starting a transmission, a station performs a back off procedures whichsta! tes that it has to keep sensing the channel for an additional random time after detectingthe channel is free for a minimum duration called DCF Inter Frame Space (DIFS). Only ifthe channel is idle for this additional random period, the station is allowed to initiate thetransmission, this ensures that multiple stations wanting to send data don’t transmit at thesame time. [14,20]

As mentioned earlier, with radio based LANs, a transmitting station can’t listen for col-lisions while sending data, mainly because the station can’t have it’s receiver on whiletransmitting the frame. As a result, the receiving station needs to send an acknowledge-ment (ACK), by checking CRC of the received packet, if it detects no error in the receivedframe. If the sending station does not receive an ACK after a specified period of time, thesending station will assume that there was a collision and retransmit the frame or fragment.

As an optional feature, the 802.11 standard includes Request-to-Send/Clear-to-Send(RTS/CTS) mechanisms to reduce so called "hidden station" problem - where a station,believing the channel to be idle, begins transmitting without successfully detecting thepresence of a transmission already in progress and causes collisions. If RTS/CTS isenabled, a station will refrain from sending data frame until the station completes aRTS/CTS handshakes with another station, such as access point. A station initiates theprocess by sending a RTS frame. The access point receives the RTS and responds withCTS frame. The station must receive a CTS frame before sending the data frame. TheCST also contains a time value that alerts other stations to hold off from accessing themedium while the station initiating the RTS transmits its data. [12] DCF with handshakingis an overhead to the protocol; [1] has presented, based on the study performed by K. C.Chen, that throughput reduces by 63% du! e to CTS/RTS overhead if there is no hiddenstation problem.

Priority based access is another way to gain access to the medium. This is a contention-free access protocol usable on infrastructure network configuration containing a controllercalled point coordinator with access point; this mode is referred as Point Coordinated Func-tion (PCF) [10]. For supporting time-bound delivery of data frames, the 802.11 standarddefines the optional PCF where the access point grants access to individual stations to the

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medium by polling the station, according to a polling list, during the contention free periodand then switches to DCF mode. Stations can’t transmit frames unless the access pointpolls them first [20]. PCF has higher priority than DCF, because it may start transmissionafter a shorter duration than DIFS; this time space is called PCF Inter Frame Space (PIFS).With PCF a Contention Free Period (CFP) and Contention Period alternate over time, inwhich a CFP and the following CP forms a superframes. During the CFP, the PCF is usedto ! access the media, while the DCF is used during the CP.[1,2] It is somewhat impor-tant to describe the general MAC frame format and how it forms data unit; MAC acceptsMAC Service Data Unit (MSDU) from higher layers and adds headers and trailers to createMAC Protocol Data Unit (MPDU). The MAC may fragment MDSU into several frames(fragmentation), increasing the probability of each individual frame being delivered suc-cessfully [7]. The header, MSDU and trailer contain information like: address information,IEEE802.11-specific protocol information, information for setting the NAV, frame checksequence for verifying the integrity of the frame.

HiperLAN MAC layer

The medium access control protocol is based on a dynamic Time-Division Multiple Accessand Time-Division Duplex (TDMA/TDD) scheme with centralized control (CC). The time-slotted structure of the medium allows for simultaneous communication in both downlinkand uplink within the same time frame. The basic MAC frame structure on the air interfacehas fixed duration of 2 ms and comprise fields for broadcast control (BCH), frame control(FCH), access feedback control (ACH), data transmission in downlink (DL) as well asuplink(UL) and random access. All data from both AP and MTs is transmitted in dedicatedtime slots, except for the random access channel where the duration of the other fields isdynamically adapted to the current traffic situation. The BCH contains control informationthat is sent in every MAC frame to mainly enable some RRC functions. The FCH containsan exact description on allocation of resources within the current MAC frame. The ACHconveys information on previous random access attempts. Downlink, uplink and directlinkphases consists of two types of PDUs: long PDUs and short PDUs. The long PDUs hacea size of 54 bytes and contains control or user data. The payload is 49.5 bytes and theremaining 4.5 bytes are used for PDU type, a sequence number and cyclic redundancycheck. Long PDUs are reffered to as Long transport CHannel (LCH). Short PDUs, 9byte size, contains resource request, ARQ messages etc and refered to as Short transportCHannel (SCH) Traffic from multiple connection to/from one MT could be multiplexedonto one so called PDU train. [4,9,10] Whenever a MT has data to transmit on a certainDLC connection; it initially requests capacity by sending Resource Request (RR) to theAP. The RR contains the number of pending LCH PDUs that the MT currently has forthe particular DLC connection. The MT may use contention slots to send RR messagebased on slotted ALOHA scheme. By varying the number of contention slots, the AP maydecrease the access delay. If a collision occurs, the MT will be informed about it in theACH in the next frame. The MT will then back off a random number of access slots. Aftersending the RR to the AP, the MT goes into a contention free period mode where thatAP schedules for the MT for transmission opportunities. The scheduling of resources isperformed in the AP. From time to time the AP may poll the MT for more [12]. Informationconcerning the MTs current pending PDUs, the MT may also inform the AP about the newstatus by sending a RR via RCH.[4]

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3.2 Physical Layer

The PHY is the interface between the MAC and wireless media, which transmit and re-ceives data frames over a shared wireless media. The PHY provides three levels of func-tionality: First, provides a frame exchange between the MAC and PHY under the controlof the Physical Layer Convergence Procedure (PLCP), a sublayer between MAC and Phys-ical Medium Dependent Layer (PMD). Secondly, the PHY uses signal carrier and spreadspectrum modulation to transmit data frames over the media under the control of PMD.Thirdly, the PHY provides a carrier sense indication back to the MAC to verify activity onthe medium. [7]

In this section a comprehensive overview of the basics of different PHY layers techniquesused for different Wireless LAN standards are discussed and compared. IEEE802.11 stan-dard actually specifies a choice of three different PHY layers, any of which can underlinea single MAC layer. Specifically, the standard provides for an optical-based PHY thatuses Infrared light to transmit data and two RF-based PHYs that leverage different types ofspread-spectrum radio communications. The IR PHY will typically be limited in range andmost practically implemented within a single room. The RF-based PHYs meanwhile, canbe used to cover significant areas and indeed entire campuses when deployed in cellular-like configurations.

The infrared PHY provides for 1-Mbps-peak data rates with a 2-Mbps rate optional andrelies on Pulse Position Modulation (PPM). The IR technology is cheaper, simpler andwidely used but the problem is that it can not penetrate obstacles and needs direct lineof sight of communications. The RF PHYs includes Direct Sequence Spread Spectrum(DSSS) and Frequency Hopping Spread Spectrum (FHSS) choices. Both of these tech-niques use Spread spectrum technology, which trades bandwidth for reliability. The goal isto use more bandwidth than the system really needs for transmission to reduce the impactof localized interference on the system by artificially spreading the transmission band sothat transmitted signal can be accurately received and decoded in face of noise. But theydiffer significantly the way they work. In the 2.4 GHz band, the regulation specifies thatsystems have to use one of the two main spread spectrum techniques: FHSS or DSSS.[16]

Frequency Hopping Spread Spectrum (FHSS)

In frequency hopping spread spectrum systems, the carrier frequency of the transmitterabruptly changes in accordance with a pseudo random code sequence. The FHSS methodworks by dividing the 2.4GHz bandwidth into 75 subchannels, each having 1MHz band-width. The sender and receiver agree on a subchannel hopping pattern and the data is sent.Each sender/receiver pair in the network medium selects a different frequency-hoppingpattern, minimizing the chance of two pairs using the same subchannel. A minimum hoprate of 2.5 hops per second is specified for the United States. The limitation of this methodis introduced by the (1MHz) bandwidth of each of the subchannels, which allows a maxi-mum throughput of 2Mbps. This situation is made worst by the hopping overhead limitingthis method to a small throughput. [1,13]

Direct Sequence Spread Spectrum (DSSS)

The DSSS seems to be the most promising physical layer in the IEEE 802.11 standard and

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it is relatively simple to implement. In this scheme a narrow band carrier is modulatedby a code sequence. The carrier phase of the transmitted signal is abruptly changed inaccordance with this code sequence [13]. This method divides the 2.4GHz band into 14twenty-two MHz subchannels and no hopping between subchannels occurs. Data is sentthrough one 22MHz channel and special technique "chipping" is used to compensate forchannel noise. Chipping simply converts raw bit data into redundant bit patterns called"chips", which provide a form of error checking and correction at the receiver side, mini-mizing the need for retransmission. The resulting data is then modulated onto the carrierusing either Differential Binary Phase Shift Keying or Differential Quadrature Phase ShiftKeying. By spreading the data bandwidth over a much wider frequency band, the powerspectral density of the signal i! s reduced by the ratio of the data bandwidth to the totalspread bandwidth. In a DSSS receiver the incoming spread spectrum data is fed to a cor-relator where it is correlated with a copy of the pseudo-random spreading code used at thetransmitter. Since noise and interference are by definition de-correlated from the desiredsignal, the desired signal is then extracted from a noisy channel. The usual implementationof DSSS in the 2.4GHz band employee a 13 MHz wide channels to carry a 1 MHz signal.Channels are centered at 5 MHz spacing, giving significant overlap. [8] The advantage ofthis technique is that it reduces the effect of narrow band sources of interference.

A comparisons of the above is necessary to have better understanding of the 802.11 PHYtechnologies. In terms of complexity, the DSSS is more complicated than FSSS whichallows lower implementation cost, in terms of bandwidth sharing, the two technologiesperforms really differently. The same is true in terms of resistance to interference. DSSSseems to have a lower overhead on the air. Transmission time in DSSS is shorter since itdoes not require spending time to change frequency of the channel unlike FSSS. An IEEE802.11 standard does not strictly express which PHY to use and hence it leaves the issueopen for the manufacture to come up with different incompatible PHYs in products.[1]

Orthogonal Frequency Division Multiplexing and 5 GHz WLAN Physical Layer

Orthogonal Frequency Division Multiplexing (OFDM) physical layer delivers up to 54Mbps data rates in the 5MHz band. The OFDM physical layer commonly referred to as802.11a and HiperLAN/2, will likely become the basis for high-speed wireless LANs. Itis worth mentioning that OFDM is not really a modulation scheme rather it is a codingor transport scheme. OFDM divides a single digital signal across 1000 or more signalcarriers simultaneously. The signals are sent at right angles to each other so they do notinterfere with each other. The benefits of OFDM are high spectral efficiency, resiliency toRF interference, and lower multipath distortion. The orthogonal nature of OFDM allowssub-channels to overlap, having a positive effect on spectral efficiency. The sub-carrierstransporting information are just far enough apart to avoid interference with each other,theoretically. [9,10]

OFDM has been selected as modulation scheme for HiperLAN/2 and 802.11a due to goodperformance on highly dispersive channels. The key feature of the physical layer is to pro-vide modes with different code rates and modulation schemes, which are selected by linkadaptation. The interleaved data is subsequently mapped to data symbols according to ei-ther a BPSK, QPSK 16QAM of 64-QAM scheme. The OFDM modulation is implementedby means of inverse FFT. 48 data symbols and 4 pilots are transmitted in parallel in the

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form of one OFDM symbol.[9,10]

3.3 A bit Comparisions

In this section a comparison among 802.11 variants and as well as between HiperLAN and802.11 has been presented.

The main difference between IEEE 802.11a and HiperLAN/2 occurs at MAC layer. Dis-tributed CSMA/CA is commonly used as access mechanism in 802.11a but a centralizedTDMA approach where an AP informs MT at which point in time the MAC frame theyare allowed to transmit their data is used in case of HiperLAN/2. Time slots are allocateddynamically depending on the need for transmission resources. In HiperLAN/2 MAC lay-ers; if channel access in not possible it enters in three phases: prioritization, elimination,and yield which differ from 802.11 standards in a way that 802.11 uses back off method tohandle the contention situation.

HiperLAN/2 supports QoS for packet delivery by assigning a priority by the applicationand the packet lifetime. The residual lifetime of a packet with its priority determines itschannel access priority. In IEEE 802.11, a limited QoS (best effort) is supported withPCF which is an optional and nonstandard feature. In true sense presently 802.11a doesnot support QoS but work is underway to incorporate it into the standard. It is worthmentioning here that 802.11a is connectionless in nature, on the other hand HiperLAN/2is connection oriented.

Packet forwarding is optional in IEEE 802.11 where as HiperLAN/2 supports packet for-warding destined for other nodes. Having a set of convergence layer, HiperLAN/2 offersconnectivity to several legacy core networks like ATM, 3G mobile system in addition toEthernet, where as 802.11a supports Ethernet only.

802.11a needs to support Dynamic Frequency Selection (DFS), Total Power Control (TPC)and priority to compete with HiperLAN/2 in Europe. DFS is a protocol, which allowswireless application to dynamically respond to radio interference by changing channels,and TPC is a mechanism of using low power modulation. Both of these features are missingfrom 802.11a that’s prohibit its legal use in Europe.

As mentioned in chapter two, 802.11 only encrypts the frame body leaving completeheader unencrypted and available to eavesdropping. Besides, this standard does not de-scribe the key distribution and key negotiation method which may lead manufactures toend up with incompatible solutions. HiperLAN/2 supports strong security features withsupport for individual authentication and per session keys, including support to use eitherpre-shared keys or PKI along with DES/3DES. HiperLAN/2 also defines two Ids of com-municating nodes uniquely identifying any stations to accomplish security. No such kindof security features is available to 802.11.

802.11a defines two separate modes for power management: Active mode and a PowerSave mode. But it does not specify when a station may enter or leave a low power operatingstate, only define how the transition is to take place. In HiperLAN/2 power saving is basedon MT initiated negotiation of sleep periods.

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The entire band of 802.11a is does not operate in a consistent output power level, whichmeans that the client performance will vary drastically from client to client [6].

Even though physical layers of both HiperLAN/2 and 802.11a are very similar, but they dif-fers slightly. They use different training sequences in the preamble. The training symbolsused for channel estimation are the same but those sequences provided for synchronizationare different. The MAC PDUs can be of variable lengths in 802.11a, HiperLAN/2 PDUsare of fixed size.

The major advantages of 802.11a compared to 802.11b are higher throughput rates andincreased channel support, both of which result in greater added bandwidth. But 802.11band 802.11a are incompatible because of different modulation scheme in physical layers(DSSS vs. OFDM).

The three non-overlapping frequency channels available for IEEE.11b are at disadvantagescompared to the greater number of channels available to 802.11a.

Higher frequency signals (5 GHz in 802.11a) will have, due to shorter wavelength, moretrouble propagating through physical obstruction compared than those at 2.4 GHz used in802.11b. An advantage of 802.11a is its intrinsic ability to handle delay spread or multi-path reflection effects. To contrast, 802.11b networks are generally range-limited by multi-path interference rather than the loss of signal strength over distance. [17]

802.11b devices operating in 2.4 GHz are vulnerable to RF interference with BlueTooth,Microwave or similar devices operating in the same frequency band compare to 802.11aoperating in 5 MHz band. A greater number of AP will be required, in order to providegreater coverage with desired higher speed, in outdoor environment with 802.11a compareto 802.11b.

802.11b allows a maximum output of 1000mW of transmit power, 802.11a provides fora different maximum transmit power (40mW, 200mW and 800mW) output in each U-NIIband.

802.11b products and chipsets are available in volume in marketplace; for enterprise, homeand office use. Price has been reduced significantly so that the laptop, PDA and cell phonevendors are integrating 802.11 clients to their products. The Wireless Ethernet Compatibil-ity Alliance (WECA), a vendor consortium, tests products compatibility; which has greatlyincreased the consumers confidence. The vendors that compete in the enterprise marketspace are Cisco (Aeronet 1200), Enterasys, Proxima/Lucent (AP-2000), Intel, Nokia etc.Intel has announced that its Banais Pentium 4 will incorporate 802.11b in 2003 and PCchipset makers are beginning to integrate 802.11b into motherboards. On the other handthere are few shipping 802.11a products and most use Atheros AR5000 chipset. Moreover,chipset verdors (like Intersil,Texas instruments) have begun sourcing multi-mode chipsetsthat combine 802.11a, .11b and .11g in the same NIC.

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4 Future Trends and Upcoming Additions

Wireless systems are evolving towards the development of broadband applications, includ-ing multimedia services in a way to compete with wired LAN systems. It is expected thatusers will eventually demand the development of new applications with broadband accessand bit rates higher that 2 Mbps, including broadband WLANs, multimedia, and inter-active broadcasts in a global environment based on terrestrial and satellite systems. Thisnecessity will give rise to a fourth generation systems however the scarcity of availablespectrum will pose serious obstacle to the development. This section presents the futuredevelopment trends of wireless system.

The aim of today’s research effort is to provide high bandwidth wireless data commu-nication system with similar performance, reliability and security compared to its wiredcounterpart.

As wireless technology matures, newer features and functionality will continue to be madeavailable. Standardization organizations, like IEEE, ETSI, are providing continuous effortto meet new demands from user by introducing new standards as well as minimizing short-comings of the previous standards. This includes performance fine-tuning, like smotherand seamless roaming capabilities as well as QoS and most importantly security features.These standards are currently in development, and will sit atop of existing ones (802.11a,802.11b 802.11g, and HiperLAN), delivering more robust performance Wireless LAN. Fu-ture IEEE standards like; 802.11g is high-rate extension to 802.11b allowing for data ratesup to 54 Mbps in the 2.4 MHz ISM band and full ratification expected by early 2003.802.11e will eventually add QoS to WLANs, 802.11f will improve the handover from APto AP as users roam, 802.11h will address European approvals, 802.11i will address secu-rity aspects. These feat! ures will be available by the end of year 2002.

IEEE 802.15.1 is wireless area personal networking stnadards, conditionally approved, andbased on the Bluetooth specification, operating in the 2.4 GHZ ISM band.

IEEE WirelessHUMAN (Wireless High-speed Unlicensed MAN) project is developingstandards for fixed wireless access in license exempt band. WirelessHUMAN will bebased on modification of IEEE 802.16 MAC layer, while the physical layer will be basedon OFDM mechanism of IEEE 802.11a or similar standards. Research effort is going onto identify the key parameters of two WLAN, IEEE802.11a and HiperLAN/2 and applica-bility of these two standards for WirelessHUMAN systems.

The HomeRF Shared Wireless Access Protocol (HomeRF SWAP) was intended to be aless costly version of the original IEEE 802.11 standard. It is a simplified implementationof IEEE 802.11’s FSSS option, and includes DECT features in order to support both dataand voice. This standard is also an evolving one.

Wireless ATM (WATM) is a technology that is currently under development in cooperationwork between ATM forum and the ESTI BRAN. The aim of this work is to develop three-subnetwork standard - HiperLAN/2, HIPERACCESS and HIPERLINK - that will be ableto support ATM and most of the needs of broadband mobile systems [1].

Meanwhile, the Information Society of Technology Research Program of the European

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Union is developing the mobile broadband system (MBS). The MBS would provide theusers of wireless systems access to the broadband services [1].

Another important aspect of development of wireless communications systems is fixedwireless services, or Wireless Local Loop (WLL) services, which will allow first and easyway to add subscriber to local telephone networks at a low cost of installation and mainte-nance.

A change in functionality of wireless terminal is expected to achieve best performancekeeping in mind the fact that terminal have to be able to adapt the forth coming new ser-vices and applications. There is currently a wide variety of terminals available rangingfrom cellular phone to handheld PC to PDA. Each of them is specializing in certain appli-cation areas, some of them are purely terminals for communications and some of them areterminal for data processing. This brings problem in interoperability between standardsand technologies and users are burdened to carry more than one device. However, workis going on to develop new generation of terminals, which will provide both functionality[1]. Software radio, a wireless communication system in which all of the signal processingfrom the air interface through the application is performed in software, can be consideredas part of this kind of solutions enabling both communications and data processing featuresin a single ! device. The goal of software radio is to create a communications systems inwhich any aspects of the signal processing can be dynamically modified to adapt to chang-ing environmental conditions, traffic constraints, user requirements and infrastructure lim-itations. The coupling of wide band digitization with application level software running ona general-purpose processor allows for the modification of a greater range of functionalitythan any existing solutions [18].

New wireless standards and technologies are continuously evolving, a range of IR and RFstandards has been proposed already to satisfy users demands. It is highly unlikely that allof them will survive in the long term, but most likely the widely accepted wireless standardfor data communication, WLAN will enjoy tremendous growth in future.

5 Conclusion and Findings

In previous chapters we have seen that there are a number of standards from differentsource that will impact wireless LAN market and the choice of user products. This chapterprovides some conclusion presenting drawbacks and shortcomings of existing standardsand comparing them.

One of the important purposes of standardization process is to make sure that the productfrom different vendor and manufactures work in a manner that they are interoperable andinterchangeable. IEEE 802.11 standards unfortunately offers three different PHY layerimplementation: FHSS, DSSS and IR, they are fundamentally incompatible, three classesof 802.11 compliant devices can not interoperate.

Security will become more of a concern as a large portion of company’s critical informa-tion is made available to WLAN users. The current wireless LAN standards offer veryunsatisfactory level of security and one could not truly trust them. It is expected that

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wireless LAN user will demand for wired equivalent security in wireless environment aswell. WEP and use of stronger encryption has made security a bit less issue but still notenough and adequate. Manufactures will bring new products with additional RF and secu-rity features, assuring that all terminals are equipped with the right software release haveautomated some of this, but most solutions are still vendor proprietary.

Microwave ovens and 2.4 GHz portable phones may interfere with 802.11b devices. Inan enterprise office environment, this is not usually a big issue but the problem is in thescenario where APs being bought in ad-hoc fashion and are not under control, the cost ofsolving interference problem is high. 2.4 GHz bands are unlicensed by the FCC so channelcoordination and lower AP power output are the only workarounds. And since Bluetooth-enabled devices appears in numbers, there could be a clash as Bluetooth shares the samespectrum.

As mention in chapter three, DCF with handshaking is an overhead to the IEEE 802.11protocol; it has been found that throughput reduces by 63% due to CTS/RTS overhead ifthere is no hidden station problem.

The time required for SIFS and DIFS in 802.11 standard is independent on the PHY modesand so it affects the higher data rates more, this is not the case in HiperLAN/2.

IEEE 802.11 a needs a better priority mechanisms in order to support real time services[10]. It needs to standardize support for dynamic frequency selection, Power control andpriority mechanisms in order to compete with HiperLAN/2 in performance and to be al-lowed for use in Europe. Although HiperLAN/2 seems to be a promising technology, thereare still no products on the market.

Since, a 5 GHz signal is attenuated more than a 2.4 GHz signal, this fact makes 802.11a’srange smaller resulting a smaller cell sizes and needs for more APs.

The non-overlapping channels are used to determine overall capacity for a section of abuilding. Although 802.11b has 11 available channels, only 3 of them are non-overlapping,one can maximize the total capacity in an area using repeating triad of these channels withcarefully laying out coverage area. On the other hand 802.11a has 8 non-overlappingchannels, which makes clearly superior [16].

The presence of FHSS in 802.11 network in a vicinity of a DSSS network will degradethe performance of the DSSS network [18]. The emerging HomeRF operating in 2.4 GHzband will interfere with a 802.11 FHSS network. Bluetooth products are now thought to beespecially disruptive to 802.11 DSSS products, in particular to the new generation 802.11High Data Rate 2.4 GHz DSSS products. Recently published simulations indicate that abusy Bluetooth network co-located with an 802.11 High Data Rate network will reducethe throughput of the 802.11 network by about 45% under the most optimistic assumptions[18]. However, it seems that increasing utilization of the 2.4GHz band will only increasethe severity of the problem.

HiperLAN/2 and IEEE 802.11 are the two promising standards that most probably will becompeting on the market in a near future. It has been seen that throughput of 802.11a isdependent on the packet size. Increasing packet size means better throughput. With differ-ent packet sizes HiperLAN/2 throughput remains stable comparing to 802.11a [10]. Study

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conducted in [9] has seen that HiperLAN/2 and IEE802.11a have similar throughput per-formances only in case where the PSDU size in 802.11a is 4096 bytes. For both standardsoverheads due to preambles, header fields and ACK frames were considered.

It has been shown in [10] that HiperLAN/2 has the possibility to obtain better maximumthroughput than the IEEE802.11 family, but it is worth to keep in mind that the simulationsand calculation has been seen from MAC point of view which might be different fromend-to-end throughput that user will experience.

In terms of maturity of standards and products, 802.11b chipsets are plentiful and so areproducts both in small office and home environment and enterprise. Laptops, PDAs andlatest cell phones are integrating 802.11b clients and the interoperability with client andAP are fairly good with the exception of security. On the other hand only few vendorsare shipping 802.11a products so far, but very recently some giant vendors are consideringto manufacture heavily 802.11a products, which is definitely a good move towards thesupport of this superior technology.

It is most likely that 802.11g may offer too little, too late. The 802.11g products are notcoming at least before 2Q of 2003 and by this time 802.11a products will mature enoughand price will fall at 802.11b level. But 802.11g promises to be backward compatible with802.11b, it is seems highly likely that multiple standard will proliferate.

Challenges and Co-existence

802.11a based products have to begin shipping in volume and at process close to productsbased on 802.11b when deployed at the same cell. 802.11a, 802.11b are not accepted ona worldwide basis. Japan permits use of only small band containing half of band whileEurope is holding HiperLAN/2. The solution for having a global standard may come viaseveral events. First, integration of DFS and TPC in 802.11h standard to satisfy Europeanunion regulators, which is definitely an advantage of HiperLAN/2. Second, manufacturesare considering using both 5.2 and 5.8 GHz, which could operate, in more worldwidelocations.

Existing 802.11b products operate in the 2.4 GHz frequency spectrum, resulting in po-tential RF interference with Bluetooth products. 802.11a products, however, will operatemostly in the relatively empty 5 GHz bands, encouraging a happy coexistence betweenwireless LANs and Bluetooth devices.

Frequency Sharing Rules (FSR) or frequency etiquettes provide the fair coexistence of thetwo broadband communication standards (HiperLAN/2 and 802.11a) are discussed in [15].The etiquette will allow the spectrum efficient and fair co-existence of the standards in theU-NII and license exempt bands at 5 GHz, under consideration of QoS.

It is expected that even given the rollout of newer WLAN standards like 802.11a and802.11g, HiperLAN/2, but the 802.11b will remain the standards of choice for another twoyears [6]. On the other hand 802.11a provides no backward compatibility with 802.11b,where as 802.11g which is coming soon should be backward compatible with 802.11b butnot with 802.11a. This is some kind of interoperability dilemma that will some how impacton the development of wireless LAN standard to become a global one.

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Some chipset vendors have begun to invest in developing dual-mode chipsets that combineboth 802.11a and 802.11b; in this way one can manage to use both with the combo a/bclients rather than carrying around two separate NICs. At home and traveller infrastructurewith 802.11b and in enterprise office with 802.11a and this way both standard can coexistsfor sometime.

If the current 802.11 WLAN standard can not offer the degree of interoperability thatwill allow WLAN users to "mix and match" equipment from different vendors or ensurecoexistence with similar other WLAN products [18].

References

[1] Asunción Sntamaría & Francisco J. & López-Hernández; Wireless LAN Standard andApplications, 2001; Artech House Publishers; pages 3-7,45-77,93-94, 109-112,186-187.

[2] Jim Geier; Wireless LANs, second Edition; 2002; ISBN: 0-672-32058-4; SAMSPublishing; pages 19-20,35-40,94-100.

[3] Jaidev Bhola; Wireless LANs Demystified ; ISBN: 0071387846; McGraw-Hill Profes-sional; pages 30-32.

[4] Göran Malmgren & Jamshid Khun et. al.; HiperLAN Type 2 - An emerging world wideWLAN standard; http://www.issls-council.org/proc00/papers/6_3.pdf.

[5] Zahed Iqbal; Introduction to HiperLAN/2; Telecom business Seminar, Fall 2001; De-partment of Computer Science and Engineering; Helsinki University of Technology;http://www.hut.fi/z̃iqbal/hyperlan2.doc.

[6] Tiberio Massaro; Understanding WLAN Standards; June 2002;http://www.signaservices.com/PDF’s/WBT_2-5_(Massaro).pdf.

[7] Mustafa Ergen; IEEE 802.11 Standard; University of California; June 2002;http://www.eecs.berkeley.edu/ ergen/docs/ieee.pdf.

[8] Wireless Local Area Networks: Issues in Technology and Standards;http://www.radiolan.com/ds/WP-WLAN

[9] Angela Doufexi & Simon Armour & Peter Karlson et. al; A compar-ison of HiperLAN/2 and IEEE 802.11a; Center for communication Re-search, University of Bristol, UK, Telia Research AB, Malmoe, Sweden;http://www.magisnetworks.com/pdf/industry/standards_comparison.pdf.

[10] Tor Arne Birkeland & Frode Fekjaer Nilsosson; Limitations in per-formance for WLAN technologies; Agder University of College, 2002;http://siving.hia.no/ikt02/ikt6400/g07/files/Poster

[11] Jean Tourrilhes, Hewlett Packard; Wireless Overview - Some Wireless LAN standards;http://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Linux.Wireless.std.html.

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[12] Jim Geier. Understanding 802.11 Frame Types; August 15, 2002; http://www.80211-planet.com/tutorials; referred at 25th October 2002.

[13] Harold E. Price, NK6K; Digital Communications; http://www.sss-mag.com/pdf/ssprice.pdf.

[14] Mika Kasslin & Nico van Waes; Applicability of IEEE802.11a andHIPERLAN/2 for WirelessHUMAN Systems; NOKIA Research Center;http://wirelessman.org/human/contrib/80216hc-00_09.pdf.

[15] Stefan Mangold & Mohammed Hmaimou & Harianto Wijaya; Communication Net-works, Aachen University of Technology; http://www.comnets.rwth-aachen.de/ smd.

[16] Angela Champness; Understanding the benefit of IEEE 802.11; January 2002;http://www.steinkuehler.de/wavelan_802-11_Benefits.htm; referred at 14.10.2002.

[17] John Hansen; 802.b/a - A physical medium comparison; February 2002;http://images.rfdesign.com/files/4/0202Hansen32.pdf.

[18] Vanu G. Bose & Alok B. Shah & Michale Ismert; Software Radios for WirelessNetworking;

[19] Jean Tourrilhes, Hewlett Packard; Wireless Overview - The MAC Level;http://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Linux.Wireless.mac.html.

[20] Jim Geier; 802.11 MAC Layer Defined; June 15, 2002; http://www.80211-planet.com/tutorials; referred at 25th October 2002.

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wirelesstechnology

Documents

mt initiated negotiation

higher data rate

physical layer delivers

higher data rates

power management features

power save mode

contention free period

radio access network