802.11 Protocol Stack And Physical Layer 1ht20

Topic:

"The 802.11 Protocol Stack and Physical Layer”

Table of Contents 1. Abstraction 2. Introduction 2.1 Overview 2.2 Wireless Local Area Network 2.3 The Basic Structure of a Wireless LAN 2.4 Comparing a Wireless LAN to a Wired Network 2.5 The IEEE Standard

3. 802.11 Protocol Stack 3.1 Protocol Structure 3.2 Protocol Stack Architecture 3.2.1 Station (STA) Architecture: 3.2.2 Access-Point (AP) Architecture: 3.2.3 Basic Service Set (BSS): 3.2.4 Independent Basic Service Set (IBSS): 3.2.5 Infrastructure 3.2.6 Extended Service Set (ESS): 3.2.7 Service Set Identifier (SSID): 3.2.8 Basic Service Set Identifier (BSSID) 3.3 Protocol Stack for UNIX 3.4 Compare Overall Structure of 802.11b / 802.15.1 Coexistence Mechanism 3.5.1 MIH SAP Reference Model for 802.11 3.5.2 MIH SAP Reference Model for 802.16 3.6 Data Link Layer 3.6.1 for Time-Bounded Data 3.7 MAC Functional Description 3.7.1 MAC Architecture 3.8 Security

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3.8.1 Preventing Access to Network Resources 3.8.2 Eavesdropping

4. Physical Layer 4.1 The physical layer basics 4.2 PL Frame Fields 4.3 Infrared (IR) 4.4 Spread Spectrum 4.5 Frequency Hopping Spread Spectrum (FHSS) 4.6 Direct Sequence Spread Spectrum (DSSS) 4.6.1 DSSS Modulation 4.6.2 Transmit Frequencies 4.7 The IEEE 802.11a 4.7.1 Practice 802.11a 4.8 The IEEE 802.11b 4.8.1 Practice 802.11b 4.9Comparison of 802.11a and 802.11b

5. Conclusion 6. Abbreviation 7. Glossary

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Abstraction The writing of this Research Report was prompted to maintain two main developments of the IEEE 802.11 Standard physical protocol stack and Physical Layer enhanced from the developments in wireless communication in the past decade. First we had to do huge research activities in this topic. This has been a subject study since the sixties, so that during our exploring work we have selected a lot of materials and picked up the most visible things for the student to understand it more easily and clearly. So that we were concentrated to present the issue in modern wireless concepts in a coherent and unified manner and to illustrate the concepts in that way they are applied. The concepts can be structured into these levels: - Listing characteristics and modeling - Application of these concepts But of course there is interplay between these structures. So this Research report is written based on the material for the students in the sixth semester. Also in the end to understand better the terminology and the huge number of abbreviations explained and some definitions.

Introduction The past decade has seen many advances in physical-layer communication theory and their implementation in wireless systems. So that in this Research Report we are going to define and view fundamentals of wireless communication and that especially the IEEE 802.11 Standard and explain the advantages at a level that is accessible to our audience with a basic background. Wireless communication is one of the most vibrant areas in the communication field today. This is due to a confluence of several factors. First, there has been an explosive increase in demand for the tether less connectivity, driven so far mainly by cellular telephony but expected to be soon eclipsed by wireless data application. First there has been an explosive increase in demand for tether less connectivity, driven so far mainly by cellular telephony but expected to be soon eclipsed by wireless data applications. Second, the dramatic process in VLSI technology has enabled small area and low power implementation of sophisticated signal processing algorithms and coding techniques. Third, the success of second generation digital wireless standards and provide a concrete demonstration that good ideas from communication theory can have impact in practice. There are two fundamental aspects of wireless communication that make the problem challenging and interesting. These aspects are by and large not as significant in wire line communication. First the phenomenon of fading: the time variation of the channel strengths due to the small-scale effect of multipath fading, as well as large scale effects. Second, unlike the in the wired world where each transmitter-receiver pair can often be thought of as an isolated point-to point link, wireless s communicate over the air and there is significant interface between them. The original 802.11 standard specified three separate physical layers. Two are radiobased and one is infrared light-based. The original radio-based layers are spread spectrum: frequency hopping and direct sequence. These are all in the 2.4 GHz band. An additional 4

physical layer in the 5 GHz band was added with the 802.1a release, which is also radio-based: orthogonal frequency division multiplexing (OFDM). The latest release, 802.11g, added yet another PHY: complimentary code keying orthogonal frequency division multiplexing (CCK-OFDM). This is specified in both the 2.4 and the 5 GHz bands. Note that for 2 devices to be able to interact, they must conform to the same PHY layer. There are two sub layers in the 802.11 physical layers, the physical medium dependent layer (PMD) and the physical layer convergence procedure (PL). The PMD is the sub layer lowest on the stack. It transmits and receives bits over the air. The PHY layer has three basic functions. These are the carrier sense function, the transmit function, and the receive function. Overview Wireless technologies, in the simplest sense, enable one or more devices to communicate without physical connections – without requiring network cabling. Wireless technologies use radio transmissions as the means for transmitting data, whereas wired technologies use cables. Wireless technologies range from complex systems, such as WLANs and cell phones, to simple devices such as wireless headphones, microphones, and other devices that do not process or store information. In Computer network a substantial part is the Wireless Local Area Network (WLAN), is a closely grouped system of devices that communicate via radio waves instead of wires. Wireless LAN’s typically augment or replace wired computer networks, providing s with more flexibility and freedom of movement within the workplace. In a typical WLAN configuration, a transceiver—or access point—connects to the wired network from a fixed ilocation using a standard Ethernet cable. The access point receives, buffers, and transmits data between the components of the WLAN. For over a century, the IEEE-SA has offered an established standards development program that features balance, openness, due process, and consensus. The Institute of Electrical and Electronics Engineers Standards Association (IEEESA) is the leading developer of global industry standards in a broad-range of industries, including: Power and Energy; Biomedical and Healthcare; Information Technology; Telecommunications; Transportation; Nanotechnology; Information Assurance. We have discussed about them in this rapport. A protocol stack is a particular software implementation of a computer networking protocol suite. The are often used interchangeably. Strictly speaking, the suite is the definition of the protocols, and the stack is the software implementation of them. Security is one of the first concerns of people deploying a Wireless LAN; the 802.11 committee has addressed the issue by providing what is called WEP (Wired Equivalent Privacy) Authentication: A function that determines whether a Station is allowed to participate in network communication. The standard IEEE 802.11i is designed to provide secured communication of wireless LAN as defined by all the IEEE 802.11 specifications. IEEE 802.11i enhances the WEP (Wireline Equivalent Privacy); a technology used for many years for the WLAN security, in the areas of encryption, authentication and key management. The IEEE (define) 802.11 standard includes a common Medium Access Control (MAC) Layer, which defines protocols that govern the operation of the wireless LAN. In addition, 802.11 comprise several alternative physical layers that specify the transmission and reception of 802.11 frames.

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And as conclusion we will say that wireless networking has a promising future with 802.11 leading the way as the standard for adoption in local networking environments. 802.11 addresses mobility, security, reliability, and the dynamic nature of wireless LANS while keeping compatibility with 802-type legacy networks. Expect to see availability of 802.11 products increase dramatically in the near future as businesses discover the increased productivity provided by ‘untethered’ networks.

Wireless Local Area Network In Computer network a substantial part is the Wireless Local Area Network (WLAN), is a closely grouped system of devices that communicate via radio waves instead of wires. Wireless LAN’s typically augment or replace wired computer networks, providing s with more flexibility and freedom of movement within the workplace. s can access the company intranet or even the World Wide Web from anywhere on the company campus without relying on the availability of wired cables and connection. If information is the lifeblood of today's business environment, then wireless networks are its heart. Wireless LANs can pump information and data to executives in the boardroom and to employees in the warehouse. A wireless LAN (WLAN) is a flexible data communication system implemented as an extension or as an alternative for, a wired LAN within a building or campus. Using electromagnetic waves, WLAN’s transmits and receive data over the air, minimizing the need for wired connections. Thus, WLANs combine data connectivity with mobility, and, through simplified configuration, enable movable LANs. Over the last seven years, WLANs have gained strong popularity in a number of vertical markets, including the health-care, retail, manufacturing, warehousing, and academic arenas. These industries have profited from the productivity gains of using hand-held terminals and notebook computers to transmit real-time information to centralized hosts for processing. Today WLANs are becoming more widely recognized as a general-purpose connectivity alternative for a broad range of business customers. The U.S. wireless LAN market is rapidly approaching $1 billion in revenues. A wide variety of industries have discovered the benefits a WLAN can bring—not only to daily tasks but also to the balance sheet.

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The Basic Structure of a Wireless LAN In a typical WLAN configuration, a transceiver—or access point—connects to the wired network from a fixed location using a standard Ethernet cable. The access point receives, buffers, and transmits data between the components of the WLAN—whether laptops, printers, handheld devices, or any other wireless equipment—and the wired network infrastructure. A single access point can a small group of s and can function within a range of anywhere from 30 to several hundred feet. The access point can be installed anywhere in the facility as long as good radio coverage is maintained. s equipped with handheld devices or notebook computers can transmit data to the access point when within range. The wireless devices communicate with the network operating system via WLAN adapters, usually in the form of radio network interface cards (NICs), as in the case of notebook computers, ISA or PCI adapters for desktop computers, or similar devices integrated into handheld units.

Comparing a Wireless LAN to a Wired Network The speed at which a WLAN performs depends on the type and configuration of the devices within the network. The number of s, the distance between network components, the type of WLAN system in use, and the efficiency of the wired network elements all influence the overall speed and performance of a wireless network. Such factors also affect wired network speeds, but most commercial LANs operate at speeds from 10 megabits per second (10BaseT) to 100 Mbps (100BaseT). Wireless LAN components that use the 802.11a high data rate standard perform at speeds up to 54 Mbps, almost a five-fold increase from the performance of the 802.11b standard. Almost all mobile applications today lend themselves to deployment of an 802.11 WLAN infrastructure. Of the three main variations of 802.11, a plethora of applications and devices the 802.11b standard, which operates in the 2.4 GHz frequency range. Although this standard is much more widely implemented than its newer sister technologies, industry experts anticipate that it won’t be long before 802.11g and 802.11a exceed 802.11b in popularity. Wireless s recognize the benefits of the technology and need to know how to protect their business-critical data. These s—as well as those who hesitate to deploy wireless technology because of security concerns— stand to benefit from understanding the security options currently available, even as the industry moves aggressively to provide even more secure protocols. By working with a wireless vendor well-versed in security issues, companies can dramatically enhance the security of its wireless communications system.

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WLANs typically use the unlicensed Industrial, Scientific, and Medical (ISM) radio frequency bands. In the United States, the ISM bands include the 900-MHz band (902–928 MHz), 2.4-GHz band (2400–2483.5MHz), and the 5.7-GHz band (5725–5850MHz). The most widely adopted WLAN standard around the world is 802.11 [28] today. IEEE 802.11 consists of a family of standards that defines the physical layers (PHY) and the Medium Access Control (MAC) layer of a WLAN, WLAN network architectures, how a WLAN interacts with an IP core network, and the frameworks and means for ing security and quality of service over a WLAN. The IEEE 802.11 standards family includes the following key standards:

The IEEE Standard For over a century, the IEEE-SA has offered an established standards development program that features balance, openness, due process, and consensus. The Institute of Electrical and Electronics Engineers Standards Association (IEEE-SA) is the leading developer of global industry standards in a broad-range of industries, including: • Power and Energy • Biomedical and Healthcare • Information Technology • Telecommunications • Transportation • Nanotechnology • Information Assurance 8

The following table lists highlights of the most popular sections of IEEE 802 and has links for additional information: 802

Overview

Basics of physical and logical networking concepts.

Bridging

LAN/MAN bridging and management. Covers management and the lower sub-layers of OSI Layer 2, including MAC-based bridging (Media Access Control), virtual LANs and port-based access control.

Logical Link

Commonly referred to as the LLC or Logical Link Control specification. The LLC is the top sub-layer in the data-link layer, OSI Layer 2. Interfaces with the network Layer 3.

802.3

Ethernet

"Granddaddy" of the 802 specifications. Provides asynchronous networking using "carrier sense, multiple access with collision detect" (CSMA/CD) over coax, twisted-pair copper, and fiber media. Current speeds range from 10 Mbps to 10 Gbps. Click for a list of the "hot" 802.3 technologies.

802.4

Token Bus

Disbanded

802.5

Token Ring

The original token-ing standard for twisted-pair, shielded copper cables. s copper and fiber cabling from 4 Mbps to 100 Mbps. Often called "IBM Token-Ring."

802.6

Distributed queue dual bus (DQDB)

"Superseded **Revision of 802.1D-1990 edition (ISO/IEC 10038). 802.1D incorporates P802.1p and P802.12e. It also incorporates and supersedes published standards 802.1j and 802.6k. Superseded by 802.1D-2004." (See IEEE status page.)

802.7

Broadband LAN Withdrawn Standard. Withdrawn Date: Feb 07, 2003. No Practices longer endorsed by the IEEE. (See IEEE status page.)

802.8

Fiber Optic Practices

Withdrawn PAR. Standards project no longer endorsed by the IEEE. (See IEEE status page.)

802.9

Integrated Services LAN

Withdrawn PAR. Standards project no longer endorsed by the IEEE. (See IEEE status page.)

802.10

Interoperable LAN security

Superseded **Contains: IEEE Std 802.10b-1992. (See IEEE status page.)

802.1

802.2

802.11

Wi-Fi

Wireless LAN Media Access Control and Physical Layer specification. 802.11a, b, g, etc. are amendments to the original 802.11 standard. Products that implement 802.11 standards must tests and are referred to as "Wi-Fi certified."

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802.11a

Specifies a PHY that operates in the 5 GHz U-NII band in the US - initially 5.15-5.35 AND 5.725-5.85 - since expanded to additional frequencies Uses Orthogonal Frequency-Division Multiplexing Enhanced data speed to 54 Mbps Ratified after 802.11b

802.11b

Enhancement to 802.11 that added higher data rate modes to the DSSS (Direct Sequence Spread Spectrum) already defined in the original 802.11 standard Boosted data speed to 11 Mbps 22 MHz Bandwidth yields 3 non-overlapping channels in the frequency range of 2.400 GHz to 2.4835 GHz Beacons at 1 Mbps, falls back to 5.5, 2, or 1 Mbps from 11 Mbps max.

802.11d

Enhancement to 802.11a and 802.11b that allows for global roaming Particulars can be set at Media Access Control (MAC) layer

802.11e

Enhancement to 802.11 that includes quality of service (QoS) features Facilitates prioritization of data, voice, and video transmissions

802.11g

Extends the maximum data rate of WLAN devices that operate in the 2.4 GHz band, in a fashion that permits interoperation with 802.11b devices Uses OFDM Modulation (Orthogonal FDM) Operates at up to 54 megabits per second (Mbps), with fallback speeds that include the "b" speeds

802.11h

Enhancement to 802.11a that resolves interference issues Dynamic frequency selection (DFS) Transmit power control (TPC)

802.11i

Enhancement to 802.11 that offers additional security for WLAN applications Defines more robust encryption, authentication, and key exchange, as well as options for key caching and preauthentication

802.11j

Japanese regulatory extensions to 802.11a specification Frequency range 4.9 GHz to 5.0 GHz

802.11k

Radio resource measurements for networks using 802.11 family specifications

802.11m

Maintenance of 802.11 family specifications Corrections and amendments to existing documentation 10

802.11n

Higher-speed standards -- under development Several competing and non-compatible technologies; often called "pre-n" Top speeds claimed of 108, 240, and 350+ MHz Competing proposals come from the groups, EWC, TGn Sync, and WWiSE and are all variations based on MIMO (multiple input, multiple output)

802.11x

Miss-used "generic" term for 802.11 family specifications

802.12

Demand Priority

Increases Ethernet data rate to 100 Mbps by controlling media utilization.

802.13

Not used

Not used

802.14

Cable modems

Withdrawn PAR. Standards project no longer endorsed by the IEEE.

802.15

Wireless Personal Area Networks

Communications specification that was approved in early 2002 by the IEEE for wireless personal area networks (WPANs).

802.15.1

Bluetooth

Short range (10m) wireless technology for cordless mouse, keyboard, and hands-free headset at 2.4 GHz.

802.15.3a

UWB

Short range, high-bandwidth "ultra wideband" link

802.15.4

ZigBee

Short range wireless sensor networks

Mesh network

Extension of network coverage without increasing the transmit power or the receiver sensitivity Enhanced reliability via route redundancy Easier network configuration - Better device battery life

802.16

Wireless Metropolitan Area Networks

This family of standards covers Fixed and Mobile Broadband Wireless Access methods used to create Wireless Metropolitan Area Networks (WMANs.) Connects Base Stations to the Internet using OFDM in unlicensed (900 MHz, 2.4, 5.8 GHz) or licensed (700 MHz, 2.5 – 3.6 GHz) frequency bands. Products that implement 802.16 standards can undergo WiMAX certification testing.

802.17

Resilient Packet IEEE working group description Ring

802.18

Radio IEEE 802.18 standards committee Regulatory TAG

802.19

Coexistence

802.15.5

IEEE 802.19 Coexistence Technical Advisory Group

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802.20

Mobile Broadband IEEE 802.20 mission and project scope Wireless Access

802.21

Media Independent Handoff

IEEE 802.21 mission and project scope

802.22

Wireless Regional Area

IEEE 802.22 mission and project scope

802.11 Protocol Stack A protocol stack is a particular software implementation of a computer networking protocol suite. The are often used interchangeably. Strictly speaking, the suite is the definition of the protocols, and the stack is the software implementation of them. Individual protocols within a suite are often designed with a single purpose in mind. This modularization makes design and evaluation easier. Because each protocol module usually communicates with two others, they are commonly imagined as layers in a stack of protocols. The lowest protocol always deals with "low-level", physical interaction of the hardware. Every higher layer adds more features. applications habitually deal only with the topmost layers (See also OSI model).

The protocols used by all the 802 variants, including Ethernet, have a certain commonality of structure. In the figure below we see a partial view of the 802.11 protocol stack. The physical layer corresponds to the OSI physical layer fairly well, but the data link layer in all 12

the 802 protocols is split into two or more sublayers. In 802.11, the MAC (Medium Access Control) sublayer determines how the channel is allocated, that is, and who gets to transmit next. Above it is the LLC (Logical Link Control) sublayer, whose job it is to hide the differences between the different 802 variants and make them indistinguishable as far as the network layer is concerned. We studied the LLC when examining Ethernet earlier in this chapter and will not repeat that material here. The 1997 802.11 standard specifies three transmission techniques allowed in the physical layer. The infrared method uses much the same technology as television remote controls do. The other two use short-range radio, using techniques called FHSS and DSSS. Both of these use a part of the spectrum that does not require licensing (the 2.4-GHz ISM band). Radio-controlled garage door openers also use this piece of the spectrum, so your notebook computer may find itself in competition with your garage door. Cordless telephones and microwave ovens also use this band. All of these techniques operate at 1 or 2 Mbps and at low enough power that they do not conflict too much. In 1999, two new techniques were introduced to achieve higher bandwidth. These are called OFDM and HRDSSS. They operate at up to 54 Mbps and 11 Mbps, respectively. In 2001, a second OFDM modulation was introduced, but in a different frequency band from the first one. Now we will examine each of them briefly.

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Protocol Structure In the figure below we can see the Wireless LAN by IEEE 802.11, 802.11a, 802.11b,802.11g, 802.11n801.11 protocol family MAC frame structure: 02 2 6 6 6 2 6 2312 Frame Address Duration Control 1 Frame Control Structure:

Address 2

Address 3

Se q

Address 4

Data

4 Check sum

2

2

4

1

1

1

1

1

1

1

1

Version

Type

Subtype

To DS

From DS

MF

Retry

Pwr

More

W

O

• • •

• • • • • • • • • • • •

Protocol Version - indicates the version of IEEE 802.11 standard. Type - Frame type: Management, Control and Data. Subtype - Frame subtype: Authentication frame, Deauthentication frame; Association request frame; Association response frame; Reassociation request frame; Reassociation response frame; Disassociation frame; Beacon frame; Probe frame; Probe request frame and Probe response frame. To DS - is set to 1 when the frame is sent to Distribution System (DS) From DS - is set to 1 when the frame is received from the Distribution System (DS) MF- More Fragment is set to 1 when there are more fragments belonging to the same frame following the current fragment Retry indicates that this fragment is a retransmission of a previously transmitted fragment. (For receiver to recognize duplicate transmissions of frames) Pwr - Power Management indicates the power management mode that the station will be in after the transmission of the frame. More - More Data indicates that there are more frames buffered to this station. W - WEP indicates that the frame body is encrypted according to the WEP (wired equivalent privacy) algorithm. - Order indicates that the frame is being sent using the Strictly-Ordered service class. Duration/ID (ID) Station ID is used for Power-Save poll message frame type. The duration value is used for the Network Allocation Vector (NAV) calculation. Address fields (1-4) - contain up to 4 addresses (source, destination, transmittion and receiver addresses) depending on the frame control field (the ToDS and FromDS bits).

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• • •

Sequence Control - consists of fragment number and sequence number. It is used to represent the order of different fragments belonging to the same frame and to recognize packet duplications. Data - is information that is transmitted or received. CRC - contains a 32-bit Cyclic Redundancy Check (CRC). Protocol Stack Architecture

Each computer, mobile, portable or fixed, is referred to as a station in 802.11 (Wireless Local Area Networks). The difference between a portable and mobile station is that a portable station moves from point to point but is only used at a fixed point. Mobile stations access the LAN during movement . Station (STA) Architecture: The Station Architecture is a device that contains the IEEE conformant MAC and PHY interface in the wireless medium, but on the other side it does not provide access to a distribution system. The Station Architecture is most often available in terminals like laptops, work-stations and it is implemented in the Avaya Wireless IEEE 802.11 PC-Card. • • • • • • • • •

The most important features and conditions of the Station Architecture are It has a driver interface like the Ethernet All protocol Stacks are virtually ed by the STA The Frame translation is done according to the IEEE STD 802.1H The IEEE 802.3 frames with this architecture are translated to 802.11 Via the Bridge Tunnel encapsulation scheme are encapsulated the Ethernet Types 8137 (Novell IPX) and 80F3 (AARP) All other Ethernet Types: encapsulated via the RFC 1042 (Standard for the Transmission of IP Datagram’s over IEEE 802 Networks) encapsulation scheme Maximum Data limited to 1500 octets Bridging to Ethernet is done transparently

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Access-Point (AP) Architecture: An Access Point is a device found within an IEEE 802.11 network which provides the point of interconnection between the wireless Station (laptop computer, PDA (Personnel Digital Assistant) etc.) and the wired network. The Access Point Architecture is a device that contains IEEE 802.11 conformant MAC and PHY interface to the wireless medium, and provides access to a distribution system for associated stations. Most often it contains infra-structure products that connect to wired backbones It is implemented in Avaya Wireless IEEE 802.11 PCCard when it is inserted in an AP-500 or AP-1000 The most important features and conditions of Access-Point (AP) Architecture: •

The Stations select an Access-Point and they associate with that. The Access point is a part that s roaming and also they provide time synchronization functions like beaconing. The Access-Point Architecture offers a Power Management . In a protocol stack architecture the traffic typically flows through the Access-Point. So that in the IBSS architecture takes place the direct Station-to-Station communication system.

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Basic Service Set (BSS): The Basic Service Set is a term used to describe the collection of Stations which may communicate together within an 802.11 WLAN (Wireless Local Area Network). The BSS may or may not include AP (Access Point) which provides a connection onto a fixed distribution system such as an Ethernet network. Two types of BSS exist; IBSS (Independent Basic Service Set) and Infrastructure Basic Service Set. When two or more stations come together to communicate with each other, they form a Basic Service Set (BSS). The minimum BSS consists of two stations. 802.11 LANs use the BSS as the standard building block.

In the BSS architecture a set of stations is controlled by a single “Coordination Function”, that is the logical function that determines when a station can transmit or receive. In this case we have similarity to a “cell” in the pre IEEE terminology also a BSS can have an Access-Point and that both in standalone networks and in building-wide configurations, or it just can run without and Access-Point but only in standalone networks. The diameter of the cells is twice the coverage-distance between two wireless stations. Independent Basic Service Set (IBSS): An Independent Basic Service Set also called ad hoc network is the simplest of all IEEE 802.11 networks in that no network infrastructure is required. As such, an IBSS is simply comprised of one or more Stations which communicate directly with each other. The contraction should not be confused with an Infrastructure BSS (Basic Service Set).A BSS that stands alone and is not connected to a base is called an Independent Basic Service Set (IBSS) or is referred to as an AdHoc Network. An ad-hoc network is a network where stations communicate only peer to peer. 17

There is no base and no one gives permission to talk. Mostly these networks are spontaneous and can be set up rapidly. Ad-Hoc or IBSS networks are characteristically limited both temporally and spatially. So that the Basic Service Set (BSS) forms a self-contained network in which no access to a Distribution System is available, or it is also similar to a BSS without an AccessPoint. One of the stations in the IBSS can be configured to “initiate” the network and assume the Coordination Function. The diameter of the cell is determined by coverage distance between two wireless stations.

Infrastructure When BSS's are interconnected the network becomes one with infrastructure. Infrastructure is established in the network when BSS are interconnected, so that the 802.11 infrastructures have several elements. Two or more BSS's are interconnected using a Distribution System or DS. This concept of DS increases network coverage. Each BSS becomes a component of an extended, larger network. Entry to the DS is accomplished with the use of Access Points (AP). An access point is a station, thus addressable. With help of the Access-Points data moves then between the BSS and the DS.

Extended Service Set (ESS): An Extended Service Set is comprised of a number of IEEE 802.11 BSS (Basic Service Set) and enables limited mobility within the WLAN (Wireless Local Area Network). Stations are able to move between BSS within a single ESS yet still remain “connected” to the fixed network and so continue to receive emails etc. As a Station moves into a new BSS, it will carry out a reassociation procedure with the new AP (Access Point). Creating large and complex networks using BSS's and DS's leads us to the next level of hierarchy, the Extended Service Set or ESS. The beauty of the ESS is the entire network looks like an independent basic service set to the Logical Link Control layer (LLC). This means that stations within the ESS can communicate or even move between BSS’s transparently to the LLC.

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It is the same here that the traffic always flows via Access-Point, and the diameter of the cell is double the coverage distance between two wireless stations Distribution System (DS).There is available a system to interconnect a set of Basic Service Sets and there is integrated a single Access-Point in a standalone network. In the wired network there are used cables to interconnect the Access-Points. In the wireless network are used wirelesses to interconnect the Access-Points. Example: of Extended Service Set (ESS) with single BSS and integrated DS

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Example: Extended Service Set (ESS) BSS’s with wired Distribution System (DS)

Example: Extended Service Set (ESS) BSS’s and wireless Distribution System (DS)

Service Set Identifier (SSID): The Service Set Identifier or Network Name is specified within IEEE 802.11 networks to identify a particular network. It is usually set by the setting up the WLAN and will be unique within a BSS (Basic Service Set) or ESS (Extended Service Set). The SSID may be broadcast from an AP within the wireless network to enable Stations to determine which network to “Associate” with. However, this feature should be disabled as it may assist hackers or wardrivers in gaining access to a private network. The most important things about the SSID are that it is 32 octets long and it is similar to “Domain-ID” in the pre-IEEE Wave LAN systems. So we can conclude that one network independent from that if it is ESS or IBSS it has always one SSID.

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Basic Service Set Identifier (BSSID) The BSSID is a 48bit identity used to identify a particular BSS (Basic Service Set) within one area. In the infrastructure BSS networks, the BSSID is the MAC (Medium Access Control) address of the AP and in Independent BSS or ad hoc networks, the BSSID is generated randomly. The BSSID identifies the cells and it is 6 octets long, that means that it is in the MAC address format. There is also visible a similarity to the NWID in the pre- IEEE Wave LAN systems. The value of the BSSID is the same as the MAC address of the radio in the AccessPoint. Protocol Stack for UNIX Also there are known developments for architectural enhancements for Unix-based servers to

provide a protocol stack for UNIX. To give a better idea how it looks like the figure below shows the basic components of the enhanced protocol stack architecture, with the new capabilities utilized either by -space agents or applications themselves. This architecture permits control over an application's inbound network traffic via policy-based traffic management; an adaptation/policy agent installs policies into the kernel via a special API. The policy agent interacts with the kernel via an enhanced socket interface by sending (receiving) messages to (from) special control sockets. The policies specify filters to select the traffic to be controlled, and actions to perform on the selected traffic. The figure shows the flow of an incoming request through the various control mechanisms.

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Compare Overall Structure of 802.11b / 802.15.1 Coexistence Mechanism An AWMA transmission control entity is integrated with the WLAN MAC layer and provides a Medium Free signal to the Bluetooth Baseband layer. This is a binary signal that gates when the WLAN and WPAN can each transmit packets. The 802.11b MAC and 802.15.1 LM + LC entities provide status information to the MEHTA control entities. The MEHTA control entity receives a per-transmission transmit request (TX Request) and issues a per-transmission transmit confirm (TX Confirm) to each stack to indicate whether the transmission can proceed. The TX Confirm carries a status value that is one of: allowed or denied. The TX Request and TX Confirm are discreet signals exchanged for every packet transmission attempt. Collaborative Coexistence Mechanism

802.11 Stack

Tx Enable

802.11 MAC

TDMA Control

Status

Tx Request Tx Confirm (status)

802.15.1 Stack

Tx Enable

Status

MEHTA Control

802.15.1 LM + LC

Tx Request Tx Confirm (status)

802.11 PL + PHY

802.15.1 Baseband

MIH SAP Reference Model for 802.11 The logical placement of the MIH Function in the 802.11 protocol stack for stations and access points is shown in the figure. It is similar to the 802.3, where the LLC SAP (LSAP) defines the interface of the MIH Function with the 802.11 data plane and can encapsulate MIH messages in data frames. However, since 802.11 does not currently Class 1 data frames, MIH messages can be transported over the 802.11 data plane only after the Mobile Node has associated with the 802.11 access point. Before the association between Mobile Node and access 22

point takes place, the L2 transport of MIH messages can rely on 802.11 management frames from the 802.11 management plane (MLME). The MIH MLME SAP defines the interface between the MIH Function and the MLME.

Layer 3 Mobility Protocol (L3MP), Higher-Layer Mobility Protocol, Handover Policy, Transport, Applications

802.21 Scope

MIH_SAP

Media Independent Handover (MIH) Function MIH_SME_SAP

MIH Event Service MIH Command Service MIH Information Service

SME

LSAP

MLME_SAP

Logical Link Control (LLC)

MLME_SAP

MLME MAC_SAP

MAC PHY_SAP

MLME_PLME_SAP

PHY

PLME

PLME_SAP

MIH SAP Reference Model for 802.16 The logical placement of the MIH Function in the 802.16 protocol stack is shown in the figure, so that we can compare better what is the difference between the 802.11 and 802.16. The MIH Function and the Network Control and Management System (NCMS) share the C_SAP and M_SAP for access to the mobility-management services of the Mobility Control Entity and Management Entity in the 802.16 Management Plane. The mechanisms for the direct encapsulation of MIH frames into 802.16 data frames may take multiple forms. The Service-Specific Convergence Sublayer instances currently available in the 802.16 standards and WiMAX only enable the encapsulation of IP packets and Ethernet frames. The only option available for L2 transport would be to first encapsulate the MIH messages into Ethernet frames with an MIH Ethertype value, and then mandate the adoption of Ethernet CS for 802.16 connections that carry the MIH messages. This approach limits both the efficiency of the L2 transport of MIH messages, and that since it imposes the addition of full Ethernet overhead – at least 18 bytes – to the MIH frame and the availability of L2 transport capabilities for MIH, since Ethernet CS is not ubiquitous. Alternatively, a solution that enables better efficiency and easier accessibility of L2 transport capabilities could become available with the possible standardization of the Generic Packet Convergence Sublayer (GPCS) recently proposed within 802.16g. With GPCS a more efficient

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LLC/SNAP encapsulation (8 bytes overhead) could create the needed room for the MIH Ethertype in 802.16 frame. Layer 3 Mobility Protocol (L3MP), Higher-Layer Mobility Protocol, Handover Policy, Transport, Applications

802.21 Scope

MIH_SAP

NCMS

Media Independent Handover (MIH) Function MIH Event Service MIH Command Service MIH Information Service CS_SAP

Service-Specific Convergence Sublayer (CS)

C_SAP

MAC_SAP

MAC Common Part Sublayer (MAC S) Security Sublayer

Management Plane M_SAP

PHY_SAP

Physical Layer (PHY)

Data Link Layer As an important part of the protocol stack, the data link layer within 802.11 consists of two sublayers: Logical Link Control (LLC) and Media Access Control (MAC). The 802.11 uses the same 802.2 LLC and 48-bit addressing as other 802 LANs, allowing for very simple bridging from wireless to IEEE wired networks, but the MAC is unique to WLANs. The 802.11 MAC is very similar in concept compared to the 802.3, which is designed to multiple s on a shared medium by having the sender sense the medium before accessing it. For 802.3 Ethernet LANs, the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) it is regulated from the protocol how Ethernet stations are going to establish access to the wire and how they detect and handle collisions that occur when two or more devices try to simultaneously communicate over the LAN. In an 802.11 WLAN, collision detection is not possible due to what is known as the “near/far” problem: to detect a collision, a station must be able to transmit and listen at the same time, but in radio systems the transmission drowns out the ability of the station to “hear” a collision. To for this difference, 802.11 use a slightly modified protocol known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) or the Distributed Coordination Function (DCF). CSMA/CA attempts to avoid collisions by using explicit packet acknowledgment (ACK), which means an ACK packet is sent by the receiving station to confirm that the data packet arrived intact. CSMA/CA works as follows. A station wishing to transmit senses the air, and, if no activity is detected, the station waits an additional, randomly selected period of time and then transmits if

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the medium is still free. If the packet is received intact, the receiving station issues an ACK frame that, once successfully received by the sender, completes the process. If the ACK frame is not detected by the sending station, either because the original data packet was not received intact or the ACK was not received intact, a collision is assumed to have occurred and the data packet is transmitted again after waiting another random amount of time. CSMA/CA thus provides a way of sharing access over the air. This explicit ACK mechanism also handles interference and other radio related problems very effectively. However, it does add some overhead to 802.11 that 802.3 does not have, so that an 802.11 LAN will always have slower performance than an equivalent Ethernet LAN. Another MAC-layer problem specific to wireless is the “hidden node” issue, in which two stations on opposite sides of an access point can both “hear” activity from an access point, but not from each other, usually due to distance or an obstruction.

RTS/CTS Procedure eliminates the “Hidden Node” Problem To solve this problem, 802.11 specify an optional Request to Send/Clear to Send (RTS/CTS) protocol at the MAC layer. When this feature is in use, a sending station transmits an RTS and waits for the access point to reply with CTS. Since all stations in the network can hear the access point, the CTS causes them to delay any intended transmissions, allowing the sending station to transmit and receive a packet acknowledgment without any chance of collision. Since RTS/CTS adds additional overhead to the network by temporarily reserving the medium, it is typically used only on the largest-sized packets, for which retransmission would be expensive from a bandwidth standpoint. Finally, the 802.11 MAC layer provides for two other robustness features: CRC checksum and packet fragmentation. Each packet has a CRC checksum calculated and attached to ensure that the data was not corrupted in transit. This is different from Ethernet, where higher-level protocols such as T handle error checking. Packet fragmentation allows large packets to be broken into smaller units when sent over the air, which is useful in very congested environments or when interference is a factor, since larger packets have a better chance of being corrupted. This technique reduces the need for retransmission in many cases and thus improves overall wireless network performance. The MAC layer is responsible for reassembling fragments received, rendering the process transparent to higher level protocols.

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for Time-Bounded Data Time-bounded data such as voice and video is ed in the 802.11 MAC specifications through the Point Coordination Function (PCF). As opposed to the DCF, where control is distributed to all stations, in PCF mode a single access point controls access to the media. If a BSS is set up with PCF enabled, time is spliced between the system being in PCF mode and in DCF (CSMA/CA) mode. During the periods when the system is in PCF mode, the access point will poll each station for data, and after a given time move on to the next station. No station is allowed to transmit unless it is polled, and stations receive data from the access point only when they are polled. Since PCF gives every station a turn to transmit in a predetermined fashion, a maximum latency is guaranteed. A downside to PCF is that it is not particularly scalable, in that a single point needs to have control of media access and must poll all stations, which can be ineffective in large networks. MAC Functional Description The 802.11 standard specifies a common medium access control (MAC) Layer, which provides a variety of functions that the operation of 802.11-based wireless LANs. In general, the MAC Layer manages and maintains communications between 802.11 stations (radio network cards and access points) by coordinating access to a shared radio channel and utilizing protocols that enhance communications over a wireless medium. Often viewed as the "brains" of the network, the 802.11 MAC Layer uses an 802.11 Physical (PHY) Layer, such as 802.11b or 802.11a, to perform the tasks of carrier sensing, transmission, and receiving of 802.11 frames.1 Before transmitting frames, a station must first gain access to the medium, which is a radio channel that stations share. The 802.11 standard defines two forms of medium access, distributed coordination function (DCF) and point coordination function (PCF). DCF is mandatory and based on the CSMA/CA (carrier sense multiple access with collision avoidance) protocol. With DCF, 802.11 stations contend for access and attempt to send frames when there is no other station transmitting. If another station is sending a frame, stations are polite and wait until the channel is free. As a condition to accessing the medium, the MAC Layer checks the value of its network allocation vector (NAV), which is a counter resident at each station that represents the amount of time that the previous frame needs to send its frame. The NAV must be zero before a station can attempt to send a frame. Prior to transmitting a frame, a station calculates the amount of time necessary to send the frame based on the frame's length and data rate. The station places a value representing this time in the duration field in the header of the frame. When stations receive the frame, they examine this duration field value and use it as the basis for setting their corresponding NAVs. This process reserves the medium for the sending station. An important aspect of the DCF is a random back off timer that a station uses if it detects a busy medium. If the channel is in use, the station must wait a random period of time before attempting to access the medium again. This ensures that multiple stations wanting to send data don't 1

http://www.javvin.com/wireless/MACAddress.html

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transmit at the same time. The random delay causes stations to wait different periods of time and avoids all of them sensing the medium at exactly the same time, finding the channel idle, transmitting, and colliding with each other. The back off timer significantly reduces the number of collisions and corresponding retransmissions, especially when the number of active s increases. 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 acknowledgement (ACK) if it detects no errors in the received frame. If the sending station doesn't receive an ACK after a specified period of time, the sending station will assume that there was a collision (or RF interference) and retransmit the frame. For ing time-bounded delivery of data frames, the 802.11 standard defines the optional point coordination function (PCF) where the access point grants access to an individual station to the medium by polling the station during the contention free period. Stations can't transmit frames unless the access point polls them first. The period of time for PCF-based data traffic (if enabled) occurs alternately between contention (DCF) periods. The access point polls stations according to a polling list, then switches to a contention period when stations use DCF. This process enables for both synchronous (i.e., video applications) and asynchronous (i.e., e-mail and Web browsing applications) modes of operation. MAC Architecture The new MAC access scheme described hereafter enhances the current 802.11 MAC. The MAC SAP is kept identical while the PHY SAP may be modified according to the capabilities of the PHY layer. As shown in Error: Reference source not found, the enhanced MAC layer is constituted of two Convergence sub-layers, LLC Convergence Sub-Layer (LLCCS) and Segmentation and Re-assembly (SAR), and two transfer sub-layers, MAC Intermediate SubLayer (MIS) and MAC Lower Sub-layer (MLS). The MAC SAP consistency is maintained by the LLCCS sub-layer. The MIS embeds the core transfer function of the MAC layer and is based on short fixed-size transfer units. The MIS also integrates the Error and Flow Control functions. The SAR sub-layer performs the adaptation between the variable size packet provided by the LLCCS and the transfer units managed by the MIS. The MLS sub-layer is in charge of building 802.11 compatible MPDUs from MIS transfer unit and signaling information, and delivers them to the PHY layer. In addition, it can implement the

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LLC

LLC MAC Packet Sequence Number Assignment

LLCCS

Sequence Number Assignment Fragmentation Encryption MPDU Header + CRC

Segmentation Segment Sequence Number Assignment

MAC

SAR Error and Flow Control

MIS Encryption MPDU Header Signalling Insertion

MLS

PHY

PHY

Extended MAC

Legacy 802.11

MAC Protocol Stack Comparison Security Security is one of the first concerns of people deploying a Wireless LAN; the 802.11 committee has addressed the issue by providing what is called WEP (Wired Equivalent Privacy) Authentication: A function that determines whether a Station is allowed to participate in network communication. The standard IEEE 802.11i is designed to provide secured communication of wireless LAN as defined by all the IEEE 802.11 specifications. IEEE 802.11i enhances the WEP (Wireline Equivalent Privacy); a technology used for many years for the WLAN security, in the areas of encryption, authentication and key management. IEEE 802.11i is based on the Wi-Fi Protected Access (WPA), which is a quick fix of the WEB weaknesses. The IEEE 802.11i has the following key components: 1. Temporal Key Integrity Protocol (TKIP): it is data-confidentiality protocol and it was designed to improve the security of products that were implemented through WEP. TKIP uses a message integrity code to enable devices to authenticate that the packets are coming from the claimed source, this code is called Michael. Also TKIP uses a mixing function to defeat weakkey attacks, which enabled attackers to decrypt traffic. 2. Counter-Mode/CBC-MAC Protocol (CCMP): a data-confidentiality protocol that is responsible for packet authentication as well as encryption. For confidentiality, CCMP uses AES in counter mode. For authentication and integrity, CCMP uses Cipher Block Chaining Message Authentication Code (CBC-MAC). In IEEE 802.11i, CCMP uses a 128-bit key. CCMP protects some fields that aren't encrypted. The additional parts of the IEEE 802.11 frame that get protected are known as additional authentication data (AAD). AAD includes the packets source and destination and protects against attackers replaying packets to different destinations.

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3.IEEE 802.1x: offers an effective framework for authenticating and controlling traffic to a protected network, as well as dynamically varying encryption keys. 802.1X ties a protocol called EAP (Extensible Authentication Protocol) to both the wired and wireless LAN media and multiple authentication methods. 4. EAP encapsulation over LANs (EAPOL)– it is the key protocol in IEEE 802.1x for key exchange. Two main EAPOL-key exchanges are defined in IEEE 802.11i. The first is referred to as the 4-way handshake and the second is the group key handshake. Because IEEE 802.11i has more than one data-confidentiality protocol, IEEE 802.11i provides an algorithm for the IEEE 802.11i client card and access point to negotiate which protocol to use during specific traffic circumstances and to discover any unknown security parameters.

Protocol Structure - IEEE 802.11i: WLAN Security Standards Preventing Access to Network Resources This is done by the use of an Authentication mechanism where a station needs to prove knowledge of the current key; this is very similar to the Wired LAN privacy, on the sense that an intruder needs to enter the premises (by using a physical key) in order to connect his workstation to the wired LAN. Eavesdropping Eavesdropping is prevented by the use of the WEP algorithm, which is a Pseudo Random Number Generator (PRNG), initialized by a shared secret key. This PRNG outputs a key sequence of pseudo-random bits equal in length to the largest possible packet, which is combined with the outgoing/incoming packet producing the packet transmitted in the air. The WEP algorithm is a simple algorithm based on RSA?s RC4 algorithm, which has the following properties: Reasonable strong: Brute-force attack to this algorithm is difficult because of the fact that every frame is sent with an Initialization Vector, which restarts the PRNG for each frame. Self Synchronizing: The algorithm synchronized again for each message, this is needed in order to work on a connectionless environment, where packets may get lost (as any LAN).

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Physical Layer The IEEE (define) 802.11 standard includes a common Medium Access Control (MAC) Layer, which defines protocols that govern the operation of the wireless LAN. In addition, 802.11 comprise several alternative physical layers that specify the transmission and reception of 802.11 frames. The physical layer basics To know the physical layer terminology we need to understand the essential intricacies of 802.11. GFSK is a modulation scheme in which the data are first filtered by a Gaussian filter in the Baseband, and then modulated with a simple frequency modulation. 2 and 4 bit represent the number of frequency offsets used to represent data symbols of one and two bits, respectively. DBPSK is phase modulation using two distinct carrier phases for data signaling providing one bit per symbol. DQPSK is a type of phase modulation using two pairs of distinct carrier phases, in quadrature, to signal two bits per symbol. The differential characteristic of the modulation schemes indicates the use of the difference in phase from the last change or symbol to determine the current symbol's value, rather than any absolute measurements of the phase change. Both the FHSS and DSSS modes are specified for operation in the 2.4 GHz industrial, scientific and medical (ISM) band, which has sometimes been jokingly referred to as the interference suppression is mandatory band because it is heavily used by various electronic products. The third physical layer alternative is an infrared system using near-visible light in the 850 nm to 950 nm range as the transmission medium. At the forefront of the new WLAN options that will enable much higher data rates are two supplements to the IEEE 802.11 standard: 802.11b and 802.11a, as well as a European Telecommunications Standards Institute (ETSI) standard, High Performance LAN (HIPERLAN/II). Both 802.11 and HIPERLAN/II have similar physical layer characteristics operating in the 5 GHz band and use the modulation scheme orthogonal frequency division multiplexing (OFDM), but the MAC layers are considerably different. The focus here, however, will be to compare the physical layer characteristics of 802.11a and 802.11b. With HIPERLAN/II sharing several of the same physical properties as 802.11a, many of the same issues will apply. Another standard that warrants mention in this context is IEEE 802.11g. With a ruling from the Federal Communications Commission that will now allow OFDM digital transmission technology to operate in the ISM band and the promise of interoperability with a large installed base of 802.11b products, the 802.11g extension to the standard begins to garner the attention of WLAN equipment providers. Although not detailed here, it will offer data rates equal to or exceeding 22 Mb/s with products available late in 2002. Each of the five permitted transmission techniques makes it possible to send a MAC frame from one station to another. They differ, however, in the technology used and speed achievable. The infrared option uses diffused (i.e., not line of sight) transmission at 0.85 or 0.95 microns. Two speeds are permitted: 1 Mbps and 2 Mbps. At 1 Mbps, an encoding scheme is used in which a

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group of 4 bits is encoded as a 16-bit codeword containing fifteen 0s and a single 1, using what is called Gray code. This code has the property that a small error in time synchronization leads to only a single bit error in the output. At 2 Mbps, the encoding takes 2 bits and produces a 4-bit codeword, also with only a single 1, that is one of 0001, 0010, 0100, or 1000. Infrared signals cannot penetrate walls, so cells in different rooms are well isolated from each other. Nevertheless, due to the low bandwidth (and the fact that sunlight swamps infrared signals), this is not a popular option. As with other 802.11 Physical layers, 802.11b includes Physical Layer Convergence Procedure (PL) and Physical Medium Dependent (PMD) sub-layers. These are somewhat sophisticated that the standard uses to divide the major functions that occur within the Physical Layer. The PL prepares 802.11 frames for transmission and directs the PMD to actually transmit signals, change radio channels, receive signals, and so on. PL Frame Fields The PL takes each 802.11 frame that a station wishes to transmit and forms what the 802.11 standard refers to as a PL protocol data unit (PPDU). The resulting PPDU includes the following fields in addition to the frame fields imposed by the MAC Layer: Sync. This field consists of alternating 0s and 1s, alerting the receiver that a receivable signal is present. The receiver begins synchronizing with the incoming signal after detecting the Sync. Start Frame Delimiter. This field is always 1111001110100000 and defines the beginning of a frame. Signal. This field identifies the data rate of the 802.11 frame, with its binary value equal to the data rate divided by 100Kbps. For example, the field contains the value of 00001010 for 1Mbps, 00010100 for 2Mbps, and so on. The PL fields, however, are always sent at the lowest rate, which is 1Mbps. This ensures that the receiver is initially uses the correct demodulation mechanism, which changes with different data rates. Service. This field is always set to 00000000 and the 802.11 standard reserves it for future use. Length. This field represents the number of microseconds that it takes to transmit the contents of the PPDU, and the receiver uses this information to determine the end of the frame. Frame Check Sequence. In order to detect possible errors in the Physical Layer header, the standard defines this field for containing 16-bit cyclic redundancy check (CRC) result. The MAC Layer also performs error detection functions on the PPDU contents as well. PSDU. The PSDU, which stands for Physical Layer Service Data Unit, is a fancy name that represents the contents of the PPDU (i.e., the actual 802.11 frame being sent). Don't expect to see the physical layer fields with 802.11 analyzers from AirMagnet and Wildpackets, however. The 802.11 radio card removes these fields before the resulting data is processed by the MAC Layer and offered to the analyzer for viewing. Next, we come to HR-DSSS (High Rate Direct Sequence Spread Spectrum), another spread spectrum technique, which uses 11 million chips/sec to achieve 11 Mbps in the 2.4-GHz band. It is called 802.11b but is not a follow-up to 802.11a. In fact, its standard was approved first and it got to market first. Data rates ed by 802.11b are 1, 2, 5.5, and 11 Mbps. The two slow rates run at 1 Mbaud, with 1 and 2 bits per baud, respectively, using phase shift modulation (for compatibility with DSSS). The two faster rates run at 1.375 Mbaud, with 4 and 8 bits per baud, respectively, using Walsh/Hadamard codes. The data rate may be dynamically adapted during

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operation to achieve the optimum speed SEC. 4.4 WIRELESS LANS 295 possible under current conditions of load and noise. In practice, the operating speed of 802.11b is nearly always 11 Mbps. Although 802.11b is slower than 802.11a, its range is about 7 times greater, which is more important in many situations. An enhanced version of 802.11b, 802.11g, was approved by IEEE in November 2001 after much politicking about whose patented technology it would use. It uses the OFDM modulation method of 802.11a but operates in the narrow 2.4- GHz ISM band along with 802.11b. In theory it can operate at up to 54 MBps. It is not yet clear whether this speed will be realized in practice. What it does mean is that the 802.11 committee has produced three different high-speed wireless LANs: 802.11a, 802.11b, and 802.11g (not to mention three lowspeed wireless LANs). One can legitimately ask if this is a good thing for a standards committee The 802.11 physical layer (PHY) is the interface between the MAC and the wireless media where frames are transmitted and received. The PHY provides three functions. First, the PHY provides an interface to exchange frames with the upper MAC layer for transmission and reception of data. Secondly, the PHY uses signal carrier and spread spectrum modulation to transmit data frames over the media. Thirdly, the PHY provides a carrier sense indication back to the MAC to activity on the media. 802.11 provides three different PHY definitions: Both Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS) 1 and 2 Mbps data rates. An extension to the 802.11 architecture (802.11a) defines different multiplexing techniques that can achieve data rates up to 54 Mbps. Another extension to the standard (802.11b) defines 11 Mbps and 5.5 Mbps data rates (in addition to the 1 and 2Mbps rates) utilizing an extension to DSSS called High Rate DSSS (HR/DSSS). 802.11b also defines a rate shifting technique where 11 Mbps networks may fall back to 5.5 Mbps, 2 Mbps, or 1 Mps under noisy conditions or to interoperate with legacy 802.11 PHY layers. Infrared (IR) The Infrared PHY utilizes infrared light to transmit binary data either at 1 Mbps (basic access rate) or 2 Mbps (enhanced access rate) using a specific modulation technique for each. For 1 Mbps, the infrared PHY uses a 16-pulse position modulation (PPM). The concept of PPM is to vary the position of a pulse to represent different binary symbols. Infrared transmission at 2 Mbps utilizes a 4 PPM modulation technique. Spread Spectrum Spread spectrum is a technique trading 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 media. Spread spectrum spreads the transmitted bandwidth of the resulting signal, reducing the peak power but keeping total power the same.

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Frequency Hopping Spread Spectrum (FHSS) In FHSS the total frequency band is split into a number of channels. The broadcast data is spread across the entire frequency band by hopping between the channels in a pseudo random fashion. Frequency-hopping spread spectrum (FHSS) is a spread-spectrum method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver. A spread-spectrum transmission offers three main advantages over a fixed-frequency transmission: Spread-spectrum signals are highly resistant to noise and interference. The process of recollecting a spread signal spreads out noise and interference, causing them to recede into the background. Spread-spectrum signals are difficult to intercept. A Frequency-Hop spread-spectrum signal sounds like a momentary noise burst or simply an increase in the background noise for short Frequency-Hop codes on any narrowband receiver except a Frequency-Hop spread-spectrum receiver using the exact same channel sequence as was used by the transmitter. Spread-spectrum transmissions can share a frequency band with many types of conventional transmissions with minimal interference. The spread-spectrum signals add minimal noise to the narrow-frequency communications, and vice versa. As a result, bandwidth can be utilized more efficiently. Frequency Hopping utilizes a set of narrow channels and "hops" through all of them in a predetermined sequence. For example, the 2.4 GHz frequency band is divided into 70 channels of 1 MHz each. Every 20 to 400 msec the system "hops" to a new channel following a predetermined cyclic pattern. The 802.11 Frequency Hopping Spread Spectrum (FHSS) PHY uses the 2.4 GHz radio frequency band, operating with at 1 or 2 Mbps data rate. FHSS (Frequency Hopping Spread Spectrum) uses 79 channels, each 1- MHz wide, starting at the low end of the 2.4-GHz ISM band. A pseudorandom number generator is used to produce the sequence of frequencies hopped to. As long as all stations use the same seed to the pseudorandom number generator and stay synchronized in time, they will hop to the same frequencies simultaneously. The amount of time spent at each frequency, the dwell time, is an adjustable parameter, but must be less than 400 msec. FHSS’ randomization provides a fair way to allocate spectrum in the unregulated ISM band. It also provides a modicum of security since an intruder who does not know the hopping sequence or dwell time cannot eavesdrop on transmissions. Over longer distances, multipath fading can be an issue, and FHSS offers good resistance to it. It is also relatively insensitive to radio interference, which makes it popular for building-to-building links. Its main disadvantage is its low bandwidth. The third modulation method. Frequency hopping relies on frequency diversity to combat interference. This is accomplished by multiple frequency, code selected, FSK. Basically, the incoming digital stream is shifted in frequency by an amount determined by a code that spreads the signal power over a wide bandwidth. In comparison to binary FSK, which has only two possible frequencies, FHSS may have 2*10^20 or more.

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The FHSS transmitter is a pseudo-noise PN code controlled frequency synthesizer. The instantaneous frequency output of the transmitter jumps from one value to another based on the pseudo-random input from the code generator. Varying the instantaneous frequency results in an output spectrum that is effectively spread over the range of frequencies generated.

Fig.1 FHSS Spectrum In this system, the number of discrete frequencies determines the bandwidth of the system. Hence, the process gain is directly dependent on the number of available frequency choices for a given information rate. Another important factor in FHSS systems is the rate at which the hops occur. The minimum time required to change frequencies is dependent on the information bit rate, the amount of redundancy used, and the distance to the nearest interference source. Direct Sequence Spread Spectrum (DSSS)

Direct Sequence Spread Spectrum is based on the multiplying of the baseband signal data with a broadband spreading code. The result is termed the chip rate. The characteristics of the broadband spreading code are that of pseudorandom noise. Consequently the receiver synchronized to the code will obtain the narrowband signal. All other receivers will see the spread signal as white or colored noise. In contrast, frequency-hopping spread spectrum pseudo-randomly retunes the carrier, instead of adding pseudo-random noise to the data, which results in a uniform frequency distribution whose width is determined by the output range of the pseudo-random number generator. In telecommunications, direct-sequence spread spectrum is a modulation technique where the transmitted signal takes up more bandwidth than the information signal that is being modulated, which is the reason that it is called spread spectrum. DSSS has the following features: for generating spread-spectrum transmissions by phase-modulating a sine wave pseudo randomly with a continuous string of pseudo noise code symbols, each of duration much smaller than a bit. A signal structuring technique utilizing a digital code sequence (PN Sequences) having a chip rate much higher than the information signal bit rate. Each information bit of a digital signal is transmitted as a pseudorandom sequence of chips.

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The principle of Direct Sequence is to spread a signal on a larger frequency band by multiplexing it with a signature or code to minimize localized interference and background noise. To spread the signal, each bit is modulated by a code. In the receiver, the original signal is recovered by receiving the whole spread channel and demodulating with the same code used by the transmitter. The 802.11 Direct Sequence Spread Spectrum (DSSS) PHY also uses the 2.4 GHz radio frequency band. It is also a part of the 802.11 b and g standards. Note that in the original 802.11 standard, either FHSS or DSSS may be used. DSSS (Direct Sequence Spread Spectrum) is also restricted to 1 or 2 Mbps. The scheme used has some similarities to the CDMA system, but differs in other ways. Each bit is transmitted as 11 chips, using what is called a Barker sequence. It uses phase shift modulation at 1 Mbaud, transmitting 1 bit per baud when operating at 1 Mbps and 2 bits per baud when operating at 2 Mbps. 802.11b uses DSSS to disperse the data frame signal over a relatively wide (approximately 30MHz) portion of the 2.4GHz frequency band. This results in greater immunity to radio frequency (RF) interference as compared to narrowband signaling, which is why the Federal Communications Commission (FCC) (define) deems the operation of spread spectrum systems as license free. For years, the FCC required all wireless communications equipment operating in the ISM bands in the U.S. to use spread spectrum, but in May 2002, that rule was dropped as new technologies emerged. The first of the high-speed wireless LANs, 802.11a, uses OFDM (Orthogonal Frequency Division Multiplexing) to deliver up to 54 Mbps in the wider 5GHz ISM band. As the term FDM suggests, different frequencies are used—52 of them, 48 for data and 4 for synchronization—not unlike ADSL. Since transmissions are present on multiple frequencies at the same time, this technique is considered a form of spread spectrum, but different from both CDMA and FHSS. Splitting the signal into many narrow bands has some key advantages over using a single wide band, including better immunity to narrowband interference and the possibility of using noncontiguous bands. A complex encoding system is used, based on phase-shift modulation for speeds up to 18 Mbps and on QAM above that. At 54 Mbps, 216 data bits are encoded into 288-bit symbols. Part of the motivation for OFDM is compatibility with the European HiperLAN/2 system (Doufexi et al., 2002). The technique has a good spectrum efficiency in of bits/Hz and good immunity to multipath fading. This is probably the most widely recognized form of spread spectrum. The DSSS process is performed by effectively multiplying an RF carrier and a pseudo-noise (PN) digital signal. First the PN code is modulated onto the information signal using one of several modulation techniques (eg. BPSK, QPSK, etc). Then, a doubly balanced mixer is used to multiply the RF carrier and PN modulated information signal. This process causes the RF signal to be replaced with a very wide bandwidth signal with the spectral equivalent of a noise signal. The demodulation process (for the BPSK case) is then simply the mixing/multiplying of the same PN modulated carrier with the incoming RF signal. The output is a signal that is a maximum when the two signals exactly equal one another or are "correlated". The correlated signal is then filtered and sent to a BPSK demodulator. The signals generated with this technique appear as noise in the frequency domain. The wide bandwidth provided by the PN code allows the signal power to drop below the noise threshold without loss of information. The spectral content of an SS signal is shown in Fig. 1. Note that this is just the spectrum of a BPSK signal with a (sin x / x) 2 form.

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Fig. 1 BPSK DSSS Spectrum The bandwidth in DSSS systems is often taken as the null-to-null bandwidth of the main lobe of the power spectral density plot (indicated as 2Rc in Fig. 1). The half power bandwidth of this lobe is 1.2 Rc, where Rc is the chip rate. Therefore, the bandwidth of a DSSS system is a direct function of the chip rate; specifically 2Rc/RINFO. This is just an extension of the previous equation for process gain. It should be noted that the power contained in the main lobe comprises 90 percent of the total power. This allows a narrower RF bandwidth to accommodate the received signal with the effect of rounding the received pulses in the time domain. One feature of DSSS is that QPSK may be used to increase the data rate. This increase of a factor of two bits per symbol of transmitted information over BPSK causes an equivalent reduction in the available process gain. The process gain is reduced because for a given chip rate, the bandwidth (which sets the process gain) is halved due to the two-fold increase in information transfer. The result is that systems in a spectrally quiet environment benefit from the possible increase in data transfer rate.

DSSS Modulation The modulator converts the spread binary signal into an analog waveform through the use of different modulation types, depending on which data rate is chosen. For example with 1Mbps operation, the PMD uses differential binary phase shift keying (DBPSK). This isn't really as complex as it sounds. The modulator merely shifts the phase of the center transmit frequency to distinguish a binary 1 from a binary 0 within the data stream. For 2Mbps transmission, the PMD uses differential quadrature phase shift keying (DQPSK), which is similar to DBPSK except that there are four possible phase shifts that represents every two data bits. This is a clever process that enables the data stream to be sent at 2Mbps while using the same amount of bandwidth as the one sent at 1Mbps. The modulator uses similar methods for the higher, 5.5Mbps and 11Mbps data rates.

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Transmit Frequencies The transmitter's modulator translates the spread signal into an analog form with a center frequency corresponding to the radio channel chosen by the . The following identifies the center frequency of each channel: Channel 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Frequency (GHz) 2.412 2.417 2.422 2.427 2.432 2.437 2.442 2.447 2.452 2.457 2.462 2.467 2.472 2.484

Various countries limit the use of these channels. For example, the U.S. only allows the use of channels 1 through 11, and the U.K. can use channels 1 through 13. Japan, however, authorizes the use all 14 channels. This complicates matters when deg international public wireless LANs. In that case, you need to choose channels with the least common denominator. After RF amplification takes place based on the transmit power you've chosen (100mW maximum for the U.S.), the transmitter outputs the modulated DSSS signal to the antenna in order to propagate the signal to the destination. The trip in route to the destination will significantly attenuate (define) the signal, but the receiver at the destination will detect the incoming Physical Layer header and reverse (demodulate and dispread) the process implemented by the transmitter The IEEE 802.11a While 802.11a was approved in September 1999, new product development has proceeded much more slowly than 802.11b. This is due to the cost and complexity of implementation. This standard uses 300 MHz of bandwidth in the 5 GHz unlicensed national information infrastructure (UNII) band. The spectrum is divided into three domains, each having restrictions imposed on the maximum allowed output power (see Figure 1). The first 100 MHz in the lower frequency portion is restricted to a maximum power output of 50 mW. The second 100 MHz has a higher 250 mW maximum, while the third 100 MHz, which is mainly intended for outdoor applications, has a maximum of 1.0 W power output. OFDM operates by dividing the transmitted data into multiple parallel bit streams, each with lower relative bit rates and modulating separate narrowband carriers, referred to as sub-carriers.

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The sub-carriers are orthogonal, so each can be received without interference from another. 802.11a specifies eight non-overlapping 20 MHz channels in the lower two bands; each of these are divided into 52 sub-carriers (four of which carry pilot data) of 300-kHz bandwidth each. Four non-overlapping 20 MHz channels are specified in the upper band. The receiver processes the 52 individual bit streams, reconstructing the original high-rate data stream. Four complex modulation methods are employed, depending on the data rate that can be ed by channel conditions between the transmitter and receiver. These include BPSK, QPSK, 16-QAM, and 64QAM. Quadrature amplitude modulation is a complex modulation method where data are carried in symbols represented by the phase and amplitude of the modulated carrier. 16-QAM has 16 symbols. Each represents four data bits. 64-QAM has 16 symbols with each representing four data bits. BPSK modulation is always used on the four pilot sub-carriers. Although it adds a degree of complication to the Baseband processing, 802.11a includes forward error correction (FEC) as part of the specification. FEC, which does not exist within 802.11b, enables the receiver to identify and correct errors made during transmission by sending additional data along with the primary transmission. This nearly eliminates the need for retransmissions when packet errors are detected. The data rates available in 802.11a are noted in Table 2, together with the type of modulation and the coding rate. 802.11a products are expected to begin arriving in the first half of 2002. Some of the companies developing chipset solutions for 802.11a are touting the availability of operational modes that exceed the 54 Mb/s stated in the specification. Of course, because faster data rates are out of the specification's scope, they require the use of equipment from a single source throughout the entire network. Considering the composite waveform resulting from the combination of 52 sub-carriers, the format requires more linearity in the amplifiers because of the higher peak-to-average power ratio of the transmitted OFDM signal. In addition, better phase noise performance is required because of the closely spaced, overlapping carriers. These issues add to the implementation cost of 802.11a products. Application-specific measurement tools aid in the design and troubleshooting of OFDM signals and systems.

Practice 802.11a Design of devices using 802.11a with OFDM signals and operating at 5 GHz will bring new challenges in testing, particularly because the data rate will be increasing by a factor of five and using the same bandwidth (20 MHz) to do it. The high peak-to-average power ratio representative of multicarrier OFDM signals dictates the need for highly linear and efficient amplifiers, as well as a method to characterize them. Transmitted signals such as OFDM, which do not have a constant power envelope, are not wellcharacterized by peak-to-average power ratio. This metric is not useful, as the true peak power may not occur often. It is usually more meaningful for OFDM signals to associate a percentage probability with a power level.

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A more meaningful method for viewing OFDM signal power characteristics uses the complementary cumulative distribution function (CCDF). This metric links a percentage probability to a power level. In this measurement, an instrument with time-gating capability is used to select only the active portion of the burst (see Figure 2 lower trace). If time gating were not used, the periods when the burst is off would reduce the average power calculation. The CCDF, which is simply the more common cumulative distribution function (CDF) subtracted from 1.0, shows the number of decibels above the average power on the horizontal axis, and percent probability on vertical axis (see Figure 2 upper trace). A CCDF measurement would be made over several bursts to improve the accuracy of the measurement. The IEEE 8102.11b 802.11b, which was approved by the IEEE in 1999, is an extension of the 802.11 DSSS system previously mentioned and s higher 5.5 and 11 Mb/s payload data rates in addition to the original 1 and 2 Mb/s rates. Products are now widely available, and the installed base of systems is growing rapidly. 802.11b also operates in the highly populated 2.4 GHz ISM band (2.40 to 2.4835 GHz), which provides only 83 MHz of spectrum to accommodate a variety of other radiating products, including cordless phones, microwave ovens, other WLANs, and personal area networks (PANS). This makes susceptibility to interference a primary concern. The occupied bandwidth of the spread-spectrum channel is 22 MHz, so the ISM band accommodates only three non-overlapping channels spaced 25 MHz apart. To help mitigate interference effects, 802.11b designates an optional frequency agile or hopping mode using the three non-overlapping channels or six overlapping channels spaced at 10 MHz. 802.11b uses eight-chip complementary code keying (CCK) as the modulation scheme to achieve the higher data rates. Instead of the Barker codes used to encode and spread the data for the lower rates, CCK uses a nearly orthogonal complex code set called complementary sequences. The chip rate remains consistent with the original DSSS system at 11 Mchip/s, while the data rate varies to match channel conditions by changing the spreading factor and/or the modulation scheme. To achieve data rates of 5.5 and 11 Mb/s, the spreading length is first reduced from 11 to eight chips. This increases the symbol rate from 1 Msym/s to 1.375 Msym/s. For the 5.5-Mb/s bit rate with a 1.375 MHz symbol rate, it is necessary to transmit 4 bits/symbol (5.5 Mb/s/1.375 Msym/s) and for 11 Mb/s, an 8 bits/symbol. The CCK approach taken in 802.11b, which keeps the QPSK spread-spectrum signal and still provides the required number of bits/symbol, uses all but two of the bits to select from a set of spreading sequences and the remaining two bits to rotate the sequence. The selection of the sequence, coupled with the rotation, represents the symbol conveying the four or eight bits of data. For all 802.11b payload data rates, the preamble and header are sent at the 1 Mb/s rate.

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Practice 802.11b

The 20 MHz-wide bandwidth of WLAN signals makes power envelope measurements difficult because most spectrum analyzers have resolution bandwidth filters that are limited to 10 MHz or less. Therefore, the signal is considerably attenuated by the time the power is measured within the instrument. Vector signal analyzers are available with information bandwidths that are considerably greater than 20 MHz, making WLAN signal analysis more accurate. The 802.11b standard uses error vector magnitude (EVM) as a measure of modulation quality. This measurement has become common for most wireless applications. The underlying philosophy of EVM is that any signal deteriorated by a noisy channel can be represented as the sum of an ideal signal and an error signal. The test instrument determines the error signal by reconstructing the ideal signal based on detected signal information and subtracting it from the actual signal at each sample point. Comparison of 802.11a and 802.11b A drawback of the 5 GHz band, which has received considerable attention, is its shorter wavelength. Higher-frequency signals will have more trouble propagating through physical obstructions encountered in an office (walls, floors, and furniture) than those at 2.4 GHz. An advantage of 802.11a is its intrinsic ability to handle delay spread or multipath reflection effects. The slower symbol rate and placement of significant guard time around each symbol, using a technique called cyclical extension, reduces the inter-symbol interference (ISI) caused by multipath interference. (The last one-quarter of the symbol pulse is copied and attached to the beginning of the burst. Due to the periodic nature of the signal, the junction at the start of the original burst will always be continuous.) To contrast, 802.11b networks are generally rangelimited by multipath interference rather than the loss of signal strength over distance. When it comes to deployment of a wireless LAN, operational characteristics have been compared to those of cellular systems, where frequency planning of overlapping cells minimizes mutual interference mobility and seamless channel handoff. The three non-overlapping frequency channels available for IEEE 802.11b are at a disadvantage compared to the greater number of channels available to 802.11a. The additional channels allow more overlapping access points within a given area while avoiding additional mutual interference. Both 802.11b and 802.11a use dynamic rate shifting where the system will automatically adjust the data rate based on the condition of the radio channel. If the channel is clear, then the modes with the highest data rates are used. But as interference is introduced into the channel, the radio will fall back to a slower, albeit more robust, transmission scheme. Network planning is critical to the development of an optimized system. Each network must be customized to satisfy the planned applications and the physical environment. Requirements must be researched and well-documented, including anticipated roaming and data rates needed for applications to be used at specific locations. A site survey must be thorough and realistic to

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adequately characterize the RF environment of the proposed wireless network in of range, channel interference and delay spread. It would be unrealistic to expect to realize the full data rate capability (54 Mb/s) of 802.11a if the access points of an existing 802.11b network optimized to operate at full speed (11 Mb/s) — were simply replaced. But as has been shown, 802.11a is faster than 802.11b at any range. Cost vs. performance requirements need thorough analysis during the network planning stage to arrive at the appropriate implementation decision. Testing is critical to any product development process. WLAN products require that special attention be given to design verification and characterization because standardized operation across multivendor products may be required. To provide an efficient development environment, test tools are available to quickly diagnose problems and isolate them throughout all design segments. These tools can be used within the manufacturing process to generate and analyze production metrics for process and product improvement. Even during these lean economic times, when there is a reduced demand for technology products, the new, but already robust WLAN market is projected to grow by an order of magnitude over the next five years. These wireless networks will require increasing data rates to provide the simultaneous distribution of Internet data, high-quality video and audio in the office or at home. In addition to higher data rates, it is almost a foregone conclusion that end-s will be demanding continuous improvements in functionality, ease-of-use and reliability. Conclusion Wireless networking has a promising future with 802.11 leading the way as the standard for adoption in local networking environments. 802.11 addresses mobility, security, reliability, and the dynamic nature of wireless LANS while keeping compatibility with 802-type legacy networks. Expect to see availability of 802.11 products increase dramatically in the near future as businesses discover the increased productivity provided by ‘untethered’ networks. 802.11-based networks have seen widespread deployment across many fields, mainly due to the physical conveniences of radio-based communication. This deployment, however, was predicated in part on the expectation of confidentiality and availability. This paper addressed the availability aspect of that equation. We examined the 802.11 MAC layer and described the architecture and the main functions of the MAC as part of the protocol stack also we made a few comparisons. WE think that with the comparisons we have offered an interesting issue and that the description has more efficiency. Security as a part of the protocol stack has also been mention because the protocol stack as a part of the software is the first part in what people are interested in. The widespread acceptance of WLANs depends on industry standardization to ensure product compatibility and reliability among the various manufacturers. The Institute of Electrical and Electronics Engineers (IEEE) ratified the original 802.11 specification in 1997 as the standard for wireless LANs. That version of 802.11 provides for 1 Mbps and 2 Mbps data rates and a set of fundamental signaling methods and other services. The most critical issue affecting WLAN demand has been limited throughput. The data rates ed by the original 802.11 standard are too slow to most general business requirements and have slowed adoption of WLANs. Recognizing the critical need to

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higher data-transmission rates, the IEEE recently ratified the 802.11b standard (also known as 802.11 High Rate) for transmissions of up to 11 Mbps. Global regulatory bodies and vendor alliances have endorsed this new high-rate standard, which promises to open new markets for WLANs in large enterprise, small office, and home environments. With 802.11b, WLANs will be able to achieve wireless performance and throughput comparable to wired Ethernet. Today’s business environment is characterized by an increasingly mobile workforce and flatter organizations. Employees are equipped with notebook computers and spend more of their time working in teams that cross functional, organizational, and geographic boundaries. Much of these workers’ productivity occurs in meetings and away from their desks. s need access to the network far beyond their personal desktops. WLANs fit well in this work environment, giving mobile workers much-needed freedom in their network access. With a wireless network, workers can access information from anywhere in the corporation—a conference room, the cafeteria, or a remote branch office. Wireless LANs provide a benefit for IT managers as well, allowing them to design, deploy, and enhance networks without regard to the availability of wiring, saving both effort and dollars. Businesses of all sizes can benefit from deploying a WLAN system, which provides a powerful combination of wired network throughput, mobile access, and configuration flexibility. The economic benefits can add up to as much as $16,000 per —measured in worker productivity, organizational efficiency, revenue gain, and cost savings—over wired alternatives. So at the end we hope that we have offered a general overview of the IEEE 802.11, especially protocol stack and physical layers, parts that are defined in the IEEE Standard. The IEEE 802.11 is a huge topic and the Standards we can free from the internet.

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Abbreviation WLAN – Wireless Local Area Network LAN - Local Area Network MAC - Medium Access Control PHY – Physical Layer ISM - Industrial, Scientific, and Medical CSMA/CD - Carrier Sense, Multiple Access with Collision Detect ISO - International Standards Organization DSSS - Direct Sequence Spread Spectrum QoS - Quality Of Service T - Transmit power control DFS - Dynamic frequency selection MIMO - Multiple Input, Multiple Output LLC - Logical Link Control DS - Distribution System WEP - Wired Equivalent Privacy NAV - Network Allocation Vector CRC - Cyclic Redundancy Check IEEE-SA - Institute of Electrical and Electronics Engineers Standards Association EVM - Error Vector Magnitude CCK - complementary code keying CDF - Cumulative Distribution Function CCDF - Complementary Cumulative Distribution Function FEC - Forward Error Correction QAM DQPSK - Differential Quadrature Phase Shift Keying DBPSK - Differential Binary Phase Shift Keying PN – Pseudo Noise RF – Radio Frequency OFDM - Orthogonal Frequency Division Multiplexing FFC - Federal Communications Commission DSSS - Direct Sequence Spread Spectrum FHSS - Frequency Hopping Spread Spectrum PPM - Pulse Position Modulation HR-DSSS - High Rate Direct Sequence Spread Spectrum PPDU - PL Protocol Data Unit PMD - Physical Medium Dependent PL - Physical Layer Convergence Procedure

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Glossary ISO - International Standards Organization The International Standards Organization is responsible for a wide range of standards, including those relevant to networking. The ISO developed the OSI (Open System Interconnection) reference model which is a popular networking reference tool. WLAN - Wireless Local Area Network This is a generic term covering a multitude of technologies providing local area networking via a radio link. Examples of WLAN technologies include Wi-Fi (Wireless Fidelity), 802.11b and 802.11a, HiperLAN, Bluetooth, IrDA (Infrared Data Association) and DECT (Digital Enhanced Cordless Telecommunications) etc. WiMAX - Worldwide Interoperability for Microwave Access The term WiMAX has become synonymous with the IEEE 802.16 suite of standards. These define the radio or air interface within two broad radio bands 2GHz to 11GHz (IEEE 802.16a) and 10GHz - 66GHz (IEEE 802.16c) although initial interest is confined to the line of sight bands - 2.5GHz, 3.5GHz and 5.8GHz. It is anticipated that WiMAX will be used initially as a backhaul connection with other technologies such as Wi-Fi being used to cover the “final mile”. Wi-Fi - Wireless Fidelity Wi-Fi is an interoperability standard developed by WECA (Wireless Ethernet Compatibility Alliance) and issued to those manufacturers whose IEEE 802.11a and 802,11b equipment has ed a suite of basic interoperability tests. Equipment ing these tests carries the Wi-Fi logo. Note: Wi-Fi not WiFi. MAC - Medium Access Control Media Access Control is the lower of the two sublayers of the Data Link Layer. In general , MAC handles access to a shared medium, and can be found within many different technologies. For example, MAC methodologies are employed within Ethernet, GPRS, and UMTS etc.

Physical Link A Physical Link is the connection between devices. DSSS - Direct Sequence Spread Spectrum Direct Sequence Spread Spectrum is based on the multiplying of the Baseband signal data with a broadband spreading code. The result is termed the chip rate. The characteristics of the broadband spreading code are that of pseudorandom noise. Consequently the receiver synchronized to the code will obtain the narrowband signal. All other receivers will see the spread signal as white or colored noise.

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QoS - Quality of Service The performance of a communications channel or system is usually expressed in of QoS (Quality of Service). Depending upon the communication system, QoS may relate to service performance, SNR (Signal to Noise Ratio), BER (Bit Error Ratio), maximum and mean throughput rate, reliably, priority and other factors specific to each service.

T - Transmission Control Protocol Transmission Control Protocol is a reliable octet streaming protocol used by the majority of applications on the Internet. It provides a connection-oriented, full-duplex, point to point service between hosts.

LLC - Logical Link Control In the GPRS system the LLC protocol provides a highly reliable ciphered logical link between the MS (Mobile Station) and SGSN (Serving GPRS Node). It is independent of the underlying radio interface protocols enabling the introduction of alternative GPRS radio solutions with minimal changes to the Network Switching System.

CRC - Cyclic Redundancy Code A linear error code that is generated using a polynomial function on the data to be sent, the remainder from the process being the CRC. This is sent along with data so that a parity check of the received data can be conducted. IEEE - Institute of Electrical and Electronics Engineers The Institute of Electrical and Electronics Engineers is a professional organization whose activities include the development of communications and network standards.

IEEE 802.11a Part of the IEEE 802.11 family of specifications, this wireless local area network technology is comprised of a high speed physical layer operating in the 5GHz unlicensed band and s data rates up to 54Mbps. Equipment operating in accordance with the IEEE specifications and ing the Alliances interoperability tests is able to display the Wi-Fi logo. Several manufacturers have developed equipment which is capable of operating in accordance with both IEEE 802.11a and IEEE 802.11bspecifications.

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IEEE 802.11b Part of the IEEE 802.11 family of specifications, IEEE 802.11b is currently the most popular wireless networking technology. The equipment operates in the 2.4GHz unlicensed band and utilizes HR/DSSS (High Rate - Direct Sequence Spread Spectrum) enabling data rates of up to 11Mbps to be achieved. Equipment operating in accordance with the IEEE specifications and ing the Wi-Fi Alliances interoperability tests is able to display the Wi-Fi logo. Several manufacturers have developed equipment which is capable of operating in accordance with both IEEE 802.11a and IEEE 802.11b specifications.

IEEE 802.11 - Wireless This is an IEEE (Institute of Electrical and Electronic Engineers) technical standard covering WLAN (Wireless Local Area Network) technology. The standards have been divided into sub groups with 802.11b currently the most common. This provides a wireless communication at up to 11Mbps and operates within the 2.4GHz ISM (Industrial Scientific and Medical) band. The 802.11a based equipment is now commercially available and provides data rates up to 54Mbps and operates in the 5GHz ISM band. IEEE 802.11 networks are comprised of Stations, Wireless Medium, AP (Access Points) and a DS (Distribution System). AP - Access Point An Access Point is a device found within an IEEE 802.11 network which provides the point of interconnection between the wireless Station (laptop computer, PDA (Personnel Digital Assistant) etc.) and the wired network. ESS - Extended Service Set An Extended Service Set is comprised of a number of IEEE 802.11 BSS (Basic Service Set) and enables limited mobility within the WLAN (Wireless Local Area Network). Stations are able to move between BSS within a single ESS yet still remain “connected” to the fixed network and so continue to receive emails etc. As a Station moves into a new BSS, it will carry out a resuscitation procedure with the new AP (Access Point). SSID - Service Set Identifier The Service Set Identifier or Network Name is used within IEEE 802.11 networks to identify a particular network. It is usually set by the setting up the WLAN (Wireless Local Area Network) and will be unique within a BSS (Basic Service Set) or ESS (Extended Service Set). The SSID may be broadcast from an AP (Access Point) within the wireless network to enable Stations to determine which network to “Associate” with. However, this feature should be disabled as it may assist “hackers, or wardrivers” in gaining access to a private network.

SAP - Service Access Point A conceptual point where a protocol layer offers access to its services to the layer above or below.

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