Wireless LAN Technology: Current State and Future Trends

Zahed Iqbal Helsinki University of Technology Telecommunications Software and Multimedia Laboratory [email protected]

Abstract

In this paper, a comprehensive overview of the current state and future trends of Wireless Local Area Network (WLAN) has been presented. This document studies and compares two most competing commercialy potential WLAN technologies, namely IEEE 802.11 and ETSI HiperLAN. This study also addresses the challenges of 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 can either 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 provides all the features and benefits of traditional LAN technologies like Ethernet and Token Ring, but without the limitations of being connected by a cable. This provides greatly increased freedom and flexibility. [8]

Wireless Local Area Networks have been used increasingly in many critical applications over the past few years, particularly since 1997 when the ﬁrst IEEE802.11 WLAN standard was 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 and deployment time when compared to permanent wired networks. In other locations, WLAN services are complimentary to existing wired LANs adding the advantage of user mobility.

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

1 Ad Hoc Mobile Wireless Networks – Research Seminar on Telecommunications Software, Autumn 2002 HUT TML – Course T-110.557 – Publication ISBN 951-22-6309-2 ISSN 1456-7628 TML-C8 http://www.tml.hut.ﬁ/Studies/T-110.557/2002/papers/ each independent organs and incompatible between themselves. So many standards brings some good aspect like lower product price but at the same time it introduces some challenges concerning lack of compatibility and interoperability. The motivation of this paper is 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 future trends of WLAN, as well as presents the challenges of their coexistence or convergence towards a global standard.

1.1 Background

Over the past ten years or so an alternative to wired LAN structures has evolved in the form of the Wireless LAN. The ﬁrst generation Wireless LAN products, operating in unlicensed 900-928 MHz Industrial Scientiﬁc and Medical (ISM) band, with low range and throughput offering (500 Kbps), subjected to interference came to market with few success in some applications. But they enjoyed reputation of being inexpensive due to break through development in semiconductor technologies, on the other hand the band become crowded with other 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 development of semiconductor technology was developed by a huge number of manufactures. Using Spread spectrum technology and modern modulation schemes this generation products were able to provide data rate up to 2 Mbps, but again the band become crowded since most 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 allows 11 Mbps data rate. In June 1997, the IEEE ﬁnalized the initial standard for wireless LANs: IEEE 802.11. First fourth generation standard, HiperLAN, came as speciﬁcation from European Telecommunication Standard Institute (ETSI) Broadband Radio Access Network (BRAN) in 1996 operating at 5 GHz band. Unlike the lower frequency bands used 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 heating system 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 interest from the industry [2]. ETSIs next generation HiperLAN family, HiperLAN/2, proposed in 1999 operating at same band with its predecessor, is still under development, the goal is to provide high-speed (raw bit rate 54Mbps) communications access to different broadband core networks and moving terminals [8]. It is expected that 802.11b will compete with HiperLAN/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 are supported by Wireless LAN. In HiperLAN terminology they are referred as "centralized mode" and "direct mode", but the basic idea and how they work are same. The infrastruc-

2 Ad Hoc Mobile Wireless Networks – Research Seminar on Telecommunications Software, Autumn 2002 HUT TML – Course T-110.557 – Publication ISBN 951-22-6309-2 ISSN 1456-7628 TML-C8 http://www.tml.hut.ﬁ/Studies/T-110.557/2002/papers/ 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 communicate with station B, it goes through the AP. This mode of operation is mainly used is suitable for business applications, both indoors and outdoors, where an area much larger than a radio cell has to be covered. The access point performs several tasks, like connecting to 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 mobile nodes can form network among themselves without the help of any ﬁxed or wireless infrastructure like AP. It is principally used to quickly and easily build a network where no infrastructure is available. A good example of the use of this mode could be in military, or convention 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 background and problem statement. The network topologies, its use and beneﬁts are also discussed in chapter one.

Chapter two tells about the different WLAN technologies and standard in brief, mainly IEEE 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 lower layers (PHY, MAC). In physical layer level, different modulation techniques used by both of the technologies are discussed; their strength and shortcomings are also taken in consideration when comparing them. The medium access control mechanism of competing standards have been presented and compared.

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

2 Wireless LAN Standards Variants

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

3 Ad Hoc Mobile Wireless Networks – Research Seminar on Telecommunications Software, Autumn 2002 HUT TML – Course T-110.557 – Publication ISBN 951-22-6309-2 ISSN 1456-7628 TML-C8 http://www.tml.hut.ﬁ/Studies/T-110.557/2002/papers/

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 ISM band. The goal of this standard was to serve the same purpose as IEEE 802.3 for wired Ethernet to deﬁne an open standard for wireless networks so that the consumers no longer was tied to a single vendor with proprietary technologies [10]. This standard describes the speciﬁcation of one Medium Access Control (MAC) layer and three physical layers: Fre- quency Hopping, Direct Sequence and diffuse infrared. The MAC has two main standards of 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, which supports two separate modes: Active mode and Power save mode. Power management features deﬁne functionality relating to how stations can enter into a power mode and the functionality relating to when another station desires to communicate with it during power saving state, but the standard does not deﬁne when to enter or leave low power operating state, that is the reason why power management features are considered as complex.

The standard also includes optional authentication (open system and shared key) and encryption using Wired Equivalence Privacy (WEP) [11]. With the WEP enabled, the body of 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 4-6 Mbps, in the 2.4 (2.4 to 2.4835) GHz ISM band, which provides 83 MHz spectrum. The same RF band of wireless spectrum used by cordless phone and microwave ovens. It is an expansion and much like of the IEEE 802.11 standards, supports transmission using DSSS modulation. It allows transmission at such a speed at a distance of several hundred feet. The distance depends on impediments, materials, environment and the line of sight for IR based networks [3]. It was the ﬁrst widely available WLAN technology to provide speeds 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 traditional ofﬁce applications. Although 11 channels are available throughout this band, only three of 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 decreases if 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 MAC layer. A High Rate extension of 802.11 in implemented in 802.11b called HR PHY to achieve higher bit rate which implements Complementary Code Keying (CCK) [10]. CCK is 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 in the presence of substantial noise and multipath interference. 802.11b standard is consid-

4 Ad Hoc Mobile Wireless Networks – Research Seminar on Telecommunications Software, Autumn 2002 HUT TML – Course T-110.557 – Publication ISBN 951-22-6309-2 ISSN 1456-7628 TML-C8 http://www.tml.hut.ﬁ/Studies/T-110.557/2002/papers/ 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 (5.15 to 5.825) GHz Unlicensed National Information Infrastructure (UNII) band, which is designed to provide short-range, high-speed wireless networking communication. The MAC layer is same as 802.11 and 802.11b but it does not use spread spectrum technique in physical layers, instead it uses another new modulation scheme called Orthogonal Frequency Division Multiplexing (OFDM), which is the basis for higher data rate. OFDM technology will be discussed in next chapter in details manner. This standard uses 300 MHz of bandwidth, the spectrum is divided into 3 sections, and each section is having restriction of maximum power uses. The ﬁrst 100 MHz in lower frequency portion is restricted to a maximum power output of 50 mW, the second 100 MHz has a higher 250 mW maximum and the 3rd 100 MHz has maximum of 1.0 W output power intended for outdoor applications [10, 17]. In each section there are four! channels available which gives all together 12 channels, three times more than that of offered by 802.11b. The 802.11a standard has a wide variety of high-speed data rates available: 6,9,12,18,24,36,48 and 54 Mbps; it is mandatory for all products to have 6Mbps, 12Mbps and 24Mbps rates [2]. This standard is 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 but will use OFDM as modulation scheme in physical layer. The original idea to have this standard is to maintain backward compatibility with 802.11b products with higher data rate (54 Mbps). 802.11e, 802.11f, 802.11h and 802.11i are the future 802.11 standard variants and still waiting for standard to be ratiﬁed in future.

2.2 ETSI HiperLAN

The demand for broadband wireless communication for multimedia application supporting higher data rate has lead development of new standards. ETSI has come up with standardization of different kind of Broadband Radio Access Network (BRAN). One of these standards is HiperLAN, which will provide high-speed access to core broadband core networks. [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, uses some advanced features and has already been ratified in 1996. The goal of this new standard was to achieve higher data rates than 802.11. The main advantage of HiperLAN/1 is that it works in a dedicated bandwidth 5.1-5.3 GHz, allocated in Europe, and so does not have to include spread spectrum technology. It offers data rates up to 23.5 Mbps with 5 fixed channels defined. The protocol uses a variant of CSMA/CA based on packet TTL and priority, and MAC level retransmission. [11]. The most important distinguishing characteristics of HiperLAN/1 is its centralized MAC, which supports QoS functions [1]. The protocol includes optional encryption and power management features in addition to Ad- hoc routing capability by means of which intermediate stations will automatically forward data packets through optimal routing with in the network if the destination seems to out

5 Ad Hoc Mobile Wireless Networks – Research Seminar on Telecommunications Software, Autumn 2002 HUT TML – Course T-110.557 – Publication ISBN 951-22-6309-2 ISSN 1456-7628 TML-C8 http://www.tml.hut.ﬁ/Studies/T-110.557/2002/papers/ of reach. One of the shortco! mings that HiperLAN/1 has is the lack of ability to provide real isochronous services. Although HiperLAN/1 provides a means of transporting time bounded services, it does not control or guarantee QoS on the wireless link. It is thus considered a system for best-effort delivery of data. This is what motivated ETSI to develop a new 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 in a 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 rates up to 54 Mbps in PHY layer. It is the first standard to be based on OFDM modulation, the basis for higher data rates. It has been designed to provide high-speed seamless access to a variety of legacy backbone networks including 3G mobile core networks, Asynchronous Transfer Mode (ATM) networks and Internet Protocol (IP) -based networks in addition to private wireless LAN systems. Basic applications include data, voice and video, with specific Quality of Service parameters taken into account. HiperLAN/2 provides connection- oriented data communication between MT and AP with strong security support for authentication and encryption. Authentication is based on the existence supporting functions such as directory service o! r something else. The user traffic on established connections can 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 need for manual frequency planning. It includes power management mechanisms based on MT initiated negotiation of sleeps periods; MT may request to AP at point of time to enter to a power save mode, AP differs all the pending packets mean to MT until the agreed sleep period expires.

HiperLAN/2 network consists typically of a number of Access Points (AP) each of which covers a certain geographic area. Together they form a radio access network with full or partial coverage of an area of almost any size. The coverage areas may or may not overlap 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 to a core network and serving several MT’s associated with it, Direct Mode where MAC is still controlled by a central controller but this controller needs not necessarily be connected to a core network. In direct mode the terminals can exchange directly via air whereas in centralized 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 on the 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 means of a baseband and modem and RF part. The baseband part will also contain and froward error correction function. The DLC layer consists of the Error Control function (EC), and Radio Link Control (RLC) sub-layer. [14] MAC and PHY layers are described in next chapter.

6 Ad Hoc Mobile Wireless Networks – Research Seminar on Telecommunications Software, Autumn 2002 HUT TML – Course T-110.557 – Publication ISBN 951-22-6309-2 ISSN 1456-7628 TML-C8 http://www.tml.hut.ﬁ/Studies/T-110.557/2002/papers/

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 OSI architecture: the Data Link Layer (DLL) and PHY. In layered concept one lower layer is considered as service provider and the upper layer is so called service user. DLL is further divided 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 compliant system 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. Physical layer 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 maintains communications between radio Network Interface Card (NIC) and AP by coordinating access to a shared radio channel. The goal of MAC layer is to provide access control functions such as address coordination, frame check sequence generation and checking etc for shared-medium PHYs [2]. The main purpose of the MAC protocol is to regulate the usage’s of the medium, and this is done through a channel access mechanism, the core of MAC; a way to divide the main resource between nodes, the radio channel, by regulating the use of it.

An ideal MAC layer should provide the following features; Good throughput since the spectrum is scarce resource. Less delay due to the fact that there will be more and more time-bounded multimedia applications. Transparency to different PHY layers. Fairness to 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 number of nodes in a coverage area and less channel interference and off course security in an acceptable 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 mechanism. CSMA/CA is used by most wireless LANs in the ISM bands, it speciﬁes how the node uses the medium: when to listen, when to transmit [19]. It is extremely unusual for a wireless device to be able to receive and transmit simultaneously, that is the reason why IEEE 802.11 uses Collision Avoidance (CA) rather than Collision Detection (CD) like used for wired LAN. Since it is impractical for wireless devices to communicate with all other devices directly, IEEE802.11 implements a network allocation vector (NAV), a value that indicates to a station the amount of time that remains before the medium will become available. In that sense, NAV can be considered as virtual carrier sense mechanism. By combining physical carrier sense and virtual carrier sense mechanism, the MAC protocol implements the CA portion of CSMA/CA access mechanism [14].

7 Ad Hoc Mobile Wireless Networks – Research Seminar on Telecommunications Software, Autumn 2002 HUT TML – Course T-110.557 – Publication ISBN 951-22-6309-2 ISSN 1456-7628 TML-C8 http://www.tml.hut.ﬁ/Studies/T-110.557/2002/papers/

As mentioned in chapter two, IEEE 802.11 supports one mandatory and two optional coordination function schemes; Distributed Coordinated Function (DCF) which is based on CSMA 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 passing mechanism, 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, it adapts quite well with the variable condition of trafﬁc and is quite robust against interference [2,19]. With DCF, 802.11 stations contend for access and attempt to send frames when 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 that there is no other transmission in progress on the wireless medium. However, if two stations detect the channel as free at the same time, a collision occurs, a Collision Avoidance mechanism, deﬁned in 802.11, takes care of reducing the probability of such collisions. As a part of CA, before starting a transmission, a station performs a back off procedures which sta! tes that it has to keep sensing the channel for an additional random time after detecting the channel is free for a minimum duration called DCF Inter Frame Space (DIFS). Only if the channel is idle for this additional random period, the station is allowed to initiate the transmission, this ensures that multiple stations wanting to send data don’t transmit at the same time. [14,20]

As mentioned earlier, with radio based LANs, a transmitting station can’t listen for collisions while sending data, mainly because the station can’t have it’s receiver on while transmitting 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 received frame. If the sending station does not receive an ACK after a speciﬁed period of time, the sending 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 the presence of a transmission already in progress and causes collisions. If RTS/CTS is enabled, a station will refrain from sending data frame until the station completes a RTS/CTS handshakes with another station, such as access point. A station initiates the process by sending a RTS frame. The access point receives the RTS and responds with CTS frame. The station must receive a CTS frame before sending the data frame. The CST also contains a time value that alerts other stations to hold off from accessing the medium while the station initiating the RTS transmits its data. [12] DCF with handshaking is 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 hidden station 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 controller called 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 standard deﬁnes the optional PCF where the access point grants access to individual stations to the

8 Ad Hoc Mobile Wireless Networks – Research Seminar on Telecommunications Software, Autumn 2002 HUT TML – Course T-110.557 – Publication ISBN 951-22-6309-2 ISSN 1456-7628 TML-C8 http://www.tml.hut.fi/Studies/T-110.557/2002/papers/ medium by polling the station, according to a polling list, during the contention free period and then switches to DCF mode. Stations can’t transmit frames unless the access point polls them first [20]. PCF has higher priority than DCF, because it may start transmission after 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, in which a CFP and the following CP forms a superframes. During the CFP, the PCF is used to ! access the media, while the DCF is used during the CP.[1,2] It is somewhat important to describe the general MAC frame format and how it forms data unit; MAC accepts MAC Service Data Unit (MSDU) from higher layers and adds headers and trailers to create MAC Protocol Data Unit (MPDU). The MAC may fragment MDSU into several frames (fragmentation), increasing the probability of each individual frame being delivered successfully [7]. The header, MSDU and trailer contain information like: address information, IEEE802.11-specific protocol information, information for setting the NAV, frame check sequence for verifying the integrity of the frame.

HiperLAN MAC layer

The medium access control protocol is based on a dynamic Time-Division Multiple Access and Time-Division Duplex (TDMA/TDD) scheme with centralized control (CC). The time- slotted structure of the medium allows for simultaneous communication in both downlink and uplink within the same time frame. The basic MAC frame structure on the air interface has 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 as uplink(UL) and random access. All data from both AP and MTs is transmitted in dedicated time slots, except for the random access channel where the duration of the other fields is dynamically adapted to the current traffic situation. The BCH contains control information that is sent in every MAC frame to mainly enable some RRC functions. The FCH contains an exact description on allocation of resources within the current MAC frame. The ACH conveys information on previous random access attempts. Downlink, uplink and directlink phases consists of two types of PDUs: long PDUs and short PDUs. The long PDUs hace a size of 54 bytes and contains control or user data. The payload is 49.5 bytes and the remaining 4.5 bytes are used for PDU type, a sequence number and cyclic redundancy check. Long PDUs are reffered to as Long transport CHannel (LCH). Short PDUs, 9 byte size, contains resource request, ARQ messages etc and refered to as Short transport CHannel (SCH) Traffic from multiple connection to/from one MT could be multiplexed onto one so called PDU train. [4,9,10] Whenever a MT has data to transmit on a certain DLC connection; it initially requests capacity by sending Resource Request (RR) to the AP. The RR contains the number of pending LCH PDUs that the MT currently has for the particular DLC connection. The MT may use contention slots to send RR message based on slotted ALOHA scheme. By varying the number of contention slots, the AP may decrease the access delay. If a collision occurs, the MT will be informed about it in the ACH in the next frame. The MT will then back off a random number of access slots. After sending the RR to the AP, the MT goes into a contention free period mode where that AP schedules for the MT for transmission opportunities. The scheduling of resources is performed in the AP. From time to time the AP may poll the MT for more [12]. Information concerning the MTs current pending PDUs, the MT may also inform the AP about the new status by sending a RR via RCH.[4]

9 Ad Hoc Mobile Wireless Networks – Research Seminar on Telecommunications Software, Autumn 2002 HUT TML – Course T-110.557 – Publication ISBN 951-22-6309-2 ISSN 1456-7628 TML-C8 http://www.tml.hut.ﬁ/Studies/T-110.557/2002/papers/

3.2 Physical Layer

The PHY is the interface between the MAC and wireless media, which transmit and receives data frames over a shared wireless media. The PHY provides three levels of functionality: First, provides a frame exchange between the MAC and PHY under the control of the Physical Layer Convergence Procedure (PLCP), a sublayer between MAC and Phys- ical Medium Dependent Layer (PMD). Secondly, the PHY uses signal carrier and spread spectrum 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 on the medium. [7]

In this section a comprehensive overview of the basics of different PHY layers techniques used for different Wireless LAN standards are discussed and compared. IEEE802.11 standard actually specifies a choice of three different PHY layers, any of which can underline a single MAC layer. Specifically, the standard provides for an optical-based PHY that uses Infrared light to transmit data and two RF-based PHYs that leverage different types of spread-spectrum radio communications. The IR PHY will typically be limited in range and most practically implemented within a single room. The RF-based PHYs meanwhile, can be 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 and relies on Pulse Position Modulation (PPM). The IR technology is cheaper, simpler and widely used but the problem is that it can not penetrate obstacles and needs direct line of sight of communications. The RF PHYs includes Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS) choices. Both of these techniques use Spread spectrum technology, which trades bandwidth for reliability. The goal is to use more bandwidth than the system really needs for transmission to reduce the impact of localized interference on the system by artificially spreading the transmission band so that transmitted signal can be accurately received and decoded in face of noise. But they differ significantly the way they work. In the 2.4 GHz band, the regulation specifies that systems 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 transmitter abruptly changes in accordance with a pseudo random code sequence. The FHSS method works by dividing the 2.4GHz bandwidth into 75 subchannels, each having 1MHz bandwidth. 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-hopping pattern, minimizing the chance of two pairs using the same subchannel. A minimum hop rate of 2.5 hops per second is speciﬁed for the United States. The limitation of this method is introduced by the (1MHz) bandwidth of each of the subchannels, which allows a maximum throughput of 2Mbps. This situation is made worst by the hopping overhead limiting this 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

10 Ad Hoc Mobile Wireless Networks – Research Seminar on Telecommunications Software, Autumn 2002 HUT TML – Course T-110.557 – Publication ISBN 951-22-6309-2 ISSN 1456-7628 TML-C8 http://www.tml.hut.fi/Studies/T-110.557/2002/papers/ it is relatively simple to implement. In this scheme a narrow band carrier is modulated by a code sequence. The carrier phase of the transmitted signal is abruptly changed in accordance with this code sequence [13]. This method divides the 2.4GHz band into 14 twenty-two MHz subchannels and no hopping between subchannels occurs. Data is sent through one 22MHz channel and special technique "chipping" is used to compensate for channel 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, minimizing the need for retransmission. The resulting data is then modulated onto the carrier using either Differential Binary Phase Shift Keying or Differential Quadrature Phase Shift Keying. By spreading the data bandwidth over a much wider frequency band, the power spectral density of the signal i! s reduced by the ratio of the data bandwidth to the total spread 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 the transmitter. Since noise and interference are by definition de-correlated from the desired signal, the desired signal is then extracted from a noisy channel. The usual implementation of 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 of this 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 PHY technologies. In terms of complexity, the DSSS is more complicated than FSSS which allows lower implementation cost, in terms of bandwidth sharing, the two technologies performs really differently. The same is true in terms of resistance to interference. DSSS seems to have a lower overhead on the air. Transmission time in DSSS is shorter since it does not require spending time to change frequency of the channel unlike FSSS. An IEEE 802.11 standard does not strictly express which PHY to use and hence it leaves the issue open 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 54 Mbps data rates in the 5MHz band. The OFDM physical layer commonly referred to as 802.11a and HiperLAN/2, will likely become the basis for high-speed wireless LANs. It is worth mentioning that OFDM is not really a modulation scheme rather it is a coding or transport scheme. OFDM divides a single digital signal across 1000 or more signal carriers simultaneously. The signals are sent at right angles to each other so they do not interfere with each other. The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multipath distortion. The orthogonal nature of OFDM allows sub-channels to overlap, having a positive effect on spectral efficiency. The sub-carriers transporting 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 good performance on highly dispersive channels. The key feature of the physical layer is to provide modes with different code rates and modulation schemes, which are selected by link adaptation. The interleaved data is subsequently mapped to data symbols according to either a BPSK, QPSK 16QAM of 64-QAM scheme. The OFDM modulation is implemented by means of inverse FFT. 48 data symbols and 4 pilots are transmitted in parallel in the

11 Ad Hoc Mobile Wireless Networks – Research Seminar on Telecommunications Software, Autumn 2002 HUT TML – Course T-110.557 – Publication ISBN 951-22-6309-2 ISSN 1456-7628 TML-C8 http://www.tml.hut.ﬁ/Studies/T-110.557/2002/papers/ 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 and 802.11 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 centralized TDMA approach where an AP informs MT at which point in time the MAC frame they are allowed to transmit their data is used in case of HiperLAN/2. Time slots are allocated dynamically depending on the need for transmission resources. In HiperLAN/2 MAC layers; 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 to handle the contention situation.

HiperLAN/2 supports QoS for packet delivery by assigning a priority by the application and the packet lifetime. The residual lifetime of a packet with its priority determines its channel access priority. In IEEE 802.11, a limited QoS (best effort) is supported with PCF which is an optional and nonstandard feature. In true sense presently 802.11a does not support QoS but work is underway to incorporate it into the standard. It is worth mentioning here that 802.11a is connectionless in nature, on the other hand HiperLAN/2 is connection oriented.

Packet forwarding is optional in IEEE 802.11 where as HiperLAN/2 supports packet forwarding destined for other nodes. Having a set of convergence layer, HiperLAN/2 offers connectivity to several legacy core networks like ATM, 3G mobile system in addition to Ethernet, 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 allows wireless 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 missing from 802.11a that’s prohibit its legal use in Europe.

As mentioned in chapter two, 802.11 only encrypts the frame body leaving complete header unencrypted and available to eavesdropping. Besides, this standard does not describe the key distribution and key negotiation method which may lead manufactures to end up with incompatible solutions. HiperLAN/2 supports strong security features with support for individual authentication and per session keys, including support to use either pre-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 kind of security features is available to 802.11.

802.11a deﬁnes two separate modes for power management: Active mode and a Power Save mode. But it does not specify when a station may enter or leave a low power operating state, only deﬁne how the transition is to take place. In HiperLAN/2 power saving is based on 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, which means 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 differs slightly. They use different training sequences in the preamble. The training symbols used for channel estimation are the same but those sequences provided for synchronization are different. The MAC PDUs can be of variable lengths in 802.11a, HiperLAN/2 PDUs are of ﬁxed size.

The major advantages of 802.11a compared to 802.11b are higher throughput rates and increased channel support, both of which result in greater added bandwidth. But 802.11b and 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 disadvantages compared to the greater number of channels available to 802.11a.

Higher frequency signals (5 GHz in 802.11a) will have, due to shorter wavelength, more trouble propagating through physical obstruction compared than those at 2.4 GHz used in 802.11b. An advantage of 802.11a is its intrinsic ability to handle delay spread or multipath reﬂection effects. To contrast, 802.11b networks are generally range-limited by multipath 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 802.11a operating in 5 MHz band. A greater number of AP will be required, in order to provide greater coverage with desired higher speed, in outdoor environment with 802.11a compare to 802.11b.

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

802.11b products and chipsets are available in volume in marketplace; for enterprise, home and office use. Price has been reduced significantly so that the laptop, PDA and cell phone vendors are integrating 802.11 clients to their products. The Wireless Ethernet Compatibil- ity Alliance (WECA), a vendor consortium, tests products compatibility; which has greatly increased the consumers confidence. The vendors that compete in the enterprise market space 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 PC chipset makers are beginning to integrate 802.11b into motherboards. On the other hand there are few shipping 802.11a products and most use Atheros AR5000 chipset. Moreover, chipset verdors (like Intersil,Texas instruments) have begun sourcing multi-mode chipsets that 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, including multimedia services in a way to compete with wired LAN systems. It is expected that users will eventually demand the development of new applications with broadband access and bit rates higher that 2 Mbps, including broadband WLANs, multimedia, and inter- active broadcasts in a global environment based on terrestrial and satellite systems. This necessity will give rise to a fourth generation systems however the scarcity of available spectrum will pose serious obstacle to the development. This section presents the future development trends of wireless system.

The aim of today’s research effort is to provide high bandwidth wireless data communication system with similar performance, reliability and security compared to its wired counterpart.

As wireless technology matures, newer features and functionality will continue to be made available. Standardization organizations, like IEEE, ETSI, are providing continuous effort to meet new demands from user by introducing new standards as well as minimizing shortcomings of the previous standards. This includes performance ﬁne-tuning, like smother and 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 rates up 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 AP to AP as users roam, 802.11h will address European approvals, 802.11i will address security 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, and based on the Bluetooth speciﬁcation, operating in the 2.4 GHZ ISM band.

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

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

Wireless ATM (WATM) is a technology that is currently under development in cooperation work 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 able to 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 the users of wireless systems access to the broadband services [1].

Another important aspect of development of wireless communications systems is ﬁxed wireless services, or Wireless Local Loop (WLL) services, which will allow ﬁrst and easy way 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 performance keeping in mind the fact that terminal have to be able to adapt the forth coming new services and applications. There is currently a wide variety of terminals available ranging from cellular phone to handheld PC to PDA. Each of them is specializing in certain application areas, some of them are purely terminals for communications and some of them are terminal for data processing. This brings problem in interoperability between standards and technologies and users are burdened to carry more than one device. However, work is 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 processing from the air interface through the application is performed in software, can be considered as part of this kind of solutions enabling both communications and data processing features in a single ! device. The goal of software radio is to create a communications systems in which any aspects of the signal processing can be dynamically modified to adapt to changing environmental conditions, traffic constraints, user requirements and infrastructure limitations. The coupling of wide band digitization with application level software running on a general-purpose processor allows for the modification of a greater range of functionality than any existing solutions [18].

New wireless standards and technologies are continuously evolving, a range of IR and RF standards has been proposed already to satisfy users demands. It is highly unlikely that all of them will survive in the long term, but most likely the widely accepted wireless standard for 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 different source that will impact wireless LAN market and the choice of user products. This chapter provides some conclusion presenting drawbacks and shortcomings of existing standards and comparing them.

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

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

15 Ad Hoc Mobile Wireless Networks – Research Seminar on Telecommunications Software, Autumn 2002 HUT TML – Course T-110.557 – Publication ISBN 951-22-6309-2 ISSN 1456-7628 TML-C8 http://www.tml.hut.ﬁ/Studies/T-110.557/2002/papers/ wireless LAN user will demand for wired equivalent security in wireless environment as well. WEP and use of stronger encryption has made security a bit less issue but still not enough and adequate. Manufactures will bring new products with additional RF and security features, assuring that all terminals are equipped with the right software release have automated some of this, but most solutions are still vendor proprietary.

Microwave ovens and 2.4 GHz portable phones may interfere with 802.11b devices. In an enterprise ofﬁce environment, this is not usually a big issue but the problem is in the scenario where APs being bought in ad-hoc fashion and are not under control, the cost of solving interference problem is high. 2.4 GHz bands are unlicensed by the FCC so channel coordination 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 same spectrum.

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

The time required for SIFS and DIFS in 802.11 standard is independent on the PHY modes and 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 and priority mechanisms in order to compete with HiperLAN/2 in performance and to be allowed for use in Europe. Although HiperLAN/2 seems to be a promising technology, there are 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’s range 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 a building. 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 with carefully laying out coverage area. On the other hand 802.11a has 8 non-overlapping channels, which makes clearly superior [16].

The presence of FHSS in 802.11 network in a vicinity of a DSSS network will degrade the performance of the DSSS network [18]. The emerging HomeRF operating in 2.4 GHz band will interfere with a 802.11 FHSS network. Bluetooth products are now thought to be especially disruptive to 802.11 DSSS products, in particular to the new generation 802.11 High Data Rate 2.4 GHz DSSS products. Recently published simulations indicate that a busy Bluetooth network co-located with an 802.11 High Data Rate network will reduce the 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 increase the severity of the problem.

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

16 Ad Hoc Mobile Wireless Networks – Research Seminar on Telecommunications Software, Autumn 2002 HUT TML – Course T-110.557 – Publication ISBN 951-22-6309-2 ISSN 1456-7628 TML-C8 http://www.tml.hut.ﬁ/Studies/T-110.557/2002/papers/ 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 standards overheads 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 maximum throughput than the IEEE802.11 family, but it is worth to keep in mind that the simulations and calculation has been seen from MAC point of view which might be different from end-to-end throughput that user will experience.

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

It is most likely that 802.11g may offer too little, too late. The 802.11g products are not coming at least before 2Q of 2003 and by this time 802.11a products will mature enough and price will fall at 802.11b level. But 802.11g promises to be backward compatible with 802.11b, 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 products based on 802.11b when deployed at the same cell. 802.11a, 802.11b are not accepted on a worldwide basis. Japan permits use of only small band containing half of band while Europe is holding HiperLAN/2. The solution for having a global standard may come via several events. First, integration of DFS and TPC in 802.11h standard to satisfy European union regulators, which is deﬁnitely an advantage of HiperLAN/2. Second, manufactures are considering using both 5.2 and 5.8 GHz, which could operate, in more worldwide locations.

Existing 802.11b products operate in the 2.4 GHz frequency spectrum, resulting in potential RF interference with Bluetooth products. 802.11a products, however, will operate mostly in the relatively empty 5 GHz bands, encouraging a happy coexistence between wireless LANs and Bluetooth devices.

Frequency Sharing Rules (FSR) or frequency etiquettes provide the fair coexistence of the two 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 the U-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 and 802.11g, HiperLAN/2, but the 802.11b will remain the standards of choice for another two years [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 but not with 802.11a. This is some kind of interoperability dilemma that will some how impact on the development of wireless LAN standard to become a global one.

17 Ad Hoc Mobile Wireless Networks – Research Seminar on Telecommunications Software, Autumn 2002 HUT TML – Course T-110.557 – Publication ISBN 951-22-6309-2 ISSN 1456-7628 TML-C8 http://www.tml.hut.ﬁ/Studies/T-110.557/2002/papers/

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

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

References

[1] Asunción Sntamaría & Francisco J. & López-Hernández; Wireless LAN Standard and Applications, 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; SAMS Publishing; 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 wide WLAN 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/ziqbal/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 comparison 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 performance 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.

18 Ad Hoc Mobile Wireless Networks – Research Seminar on Telecommunications Software, Autumn 2002 HUT TML – Course T-110.557 – Publication ISBN 951-22-6309-2 ISSN 1456-7628 TML-C8 http://www.tml.hut.ﬁ/Studies/T-110.557/2002/papers/

[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 and HIPERLAN/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 Wireless Networking;

[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.