WLAN Fundamentals

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WLAN FundamentalsNirav dsouza 6002Siddharth suttraway 6042


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What is a Wireless LAN

A wireless local area network(LAN) is a flexible data communicationssystem implemented as an extension to, or as an alternative for, awired LAN.

Using radio frequency (RF) technology, wireless LANs transmit and receivedata over the air, minimizing the need for wired connections.

Thus, combining data connectivity with user mobility.


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Disadvantages of Using a

Wireless LAN and its

Practical Uses


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Benefits of Wireless LAN

Productivity, convenience, and cost advantages

Installation speed and simplicity.

Installation flexibility.

Reduced cost-of-ownership.

Mobility.

Scalability.


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Installation speed and simplicity

No cable to pull.

Eliminates current architecture obstacles.

Few transmitters/receivers for multiple for users.


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Installation flexibility

The network goes where wires cannot.

Not constrained by expensive walls.

Easy to add more computers and devices.


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Reduced cost-of-ownership

Mobile devices are less expensive than computer workstations.

Can Run Errands and stay in touch.

No need to build wiring closets.


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Mobility

Access to real-time information.

Supports productivity.

Provides service opportunities.

Promotes flexibility.


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Scalability

Spans a variety of topologies.

Configurations are easily changed.

Works over great distances.

Effective for wide range of user communities.

Small number of users with local needs.

Full infrastructure networks roaming over a broad area.

http://www.WirelessLAN.com
http://www.wirelesslan.com/http://www.wirelesslan.com/


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Disadvantage of Wireless LAN

Cost

Wireless network cards cost 4 times more than wired network cards.

The access points are more expensive than hubs and wires.

Signal Bleed Over

Access points pick up the signals of adjacent access points or overpower their

signal.


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Disadvantage of Wireless LAN

Environmental Conditions

Susceptible to weather and solar activity.

Constrained by buildings, trees, terrain.

Less Capacity

Slower bandwidth.

Limit to how much data a carrier wave can transmit without lost packets impacting

performance.

http://www.Compaq.com
http://www.compaq.com/http://www.compaq.com/


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Wireless LAN Applications

Medical Professionals

Education

Temporary Situations

Airlines

Security Staff

Emergency Centers

R t HUB B id & S it h


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Repeater, HUB, Bridge & Switch

REPEATER, HUB, BRIDGE ANDSWITCH

Repeater Hub Bridge & Switch


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Repeater

A repeater receives a signal, regenerates it, and passes

on.It can regenerate and retime network signals at the level to allow them to travel a longer distance on tmedia.

It operates at Physical Layer of OSI

The Four Repeater Rule for 10-Mbps Ethernet should

used as a standard when extending LAN segments.This rule states that no more than four repeaters can used between hosts on a LAN.

This rule is used to limit latency added to frame travel each repeater.

Repeater, Hub, Bridge & Switch



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HubHubs are used to connectmultiple nodes to a singlephysical device, whichconnects to the network.Hubs are actually multiportrepeaters.

Using a hub changes thenetwork topology from a linearbus, to a star.

With hubs, data arriving over

the cables to a hub port iselectrically repeated on all theother ports connected to thesame network segment, exceptfor the port on which the datawas sent.




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Bridge

Bridges are used to logically separatenetwork segments within the same

network.They operate at the OSI data link layer(Layer 2) and are independent of higher-layer protocols.

The function of the bridge is to makeintelligent decisions about whether ornot to pass signals on to the next

segment of a network.When a bridge receives a frame on the

network, the destination MAC address is

looked up in the bridge table to

determine whether to filter, flood, or

copy the frame onto another segment

Broadcast Packets are forwarded




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Switch

Switches are Multiport Bridges.

Switches provide a unique network segment onport, thereby separating collision domains.

Today, network designers are replacing hubs in wiring closets with switches to increase their neperformance and bandwidth while protecting existing wiring investments.

Like bridges, switches learn certain information the data packets that are received from vacomputers on the network.

Switches use this information to build forwarding to determine the destination of data being sent bcomputer to another computer on the network.




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Switches: Dedicated Access

Hosts have direct connection

to switchFull Duplex: No collisions

Switching: A-to-A and B-to-Bsimultaneously, no collisions

Switches can be cascaded toexpand the network


switch

A

AB

C


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Introduction to IEEE 802.11 Protocol


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Why Standard?

Multi Vender inter operability

Protects customer investment Economies of scale


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What is 802.11?

802.11 is an IEEE standard for MAC and Physical Layer for Wireless Loca

Area Network (WLAN).

IEEE 802 11 Wireless LAN Standa


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IEEE 802.11 Wireless LAN Standa

In response to lacking standards, IEEE developed the first internatio

recognized wireless LAN standard IEEE 802.11

IEEE published 802.11 in 1997, after seven years of work

Most prominent specification for WLANs

Scope of IEEE 802.11 is limited to Physical and Data Link Layers.


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802.11 Versions

802.11 - 1997

Maximum data rate: 2Mbps

2.4Ghz band

Indoor Range: 20 meters

Outdoor Range: 100 meters

Wide range of Physical layers

802.11a - 1999


5.1 - 5.8Ghz band

Indoor range: 35 meters

Outdoor range: 120 meters

54 Mbps


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802.11 Versions

802.11b - 1999


2.4Ghz band

Indoor Range: 38 meters

Outdoor Range: 140 meters

11 Mbps

802.11g - 2003

The current industry adopted specification


2.4Ghz band (backwards compatible with 802.11b)

Indoor range: 38 meters

Outdoor range: 140 meters

54 Mbps


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What is WiFi?

A trademark of the Wi-Fi Alliance

Founded in 1999 as WECA (Wireless Ethernet Compatibility Alliance).

More than 300 companies

WiFi certification warrants interoperability between different wirelessdevices

Ensures correct implementation IEEE 802.11

Tests the wireless components to their own terms of reference

Physical Media Defined by Origin
http://upload.wikimedia.org/wikipedia/en/3/32/Wi-Fi_Logo.svg


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Physical Media Defined by Origin802.11 Standard

Frequency-hopping spread spectrum

Operating in 2.4 GHz ISM band

Lower cost, power consumption

Most tolerant to signal interference

Direct-sequence spread spectrum

Operating in 2.4 GHz ISM band

Supports higher data rates

More range than FH or IR physical layers

Infrared

Lowest cost

Lowest range compared to spread spectrum

Doesnt penetrate walls, so no eavesdropping

Frequency Hopping Spread


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Frequency Hopping SpreadSpectrum

Signal is broadcast over seemingly random sof radio frequencies

Signal hops from frequency to frequency atfixed intervals

Receiver, hopping between frequencies insynchronization with transmitter, picks up

messageAdvantages

Efficient utilization of available bandwidth

Eavesdropper hear only unintelligible blips

Attempts to jam signal on one frequency succee

only at knocking out a few bits

Direct Sequence Spread Spectrum


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Direct Sequence Spread Spectrum

Each bit in original signal is represented by multiple bits in the transignal

Spreading code spreads signal across a wider frequency band DSSS is the only physical layer specified for the 802.11b specificatio

802.11a and 802.11b differ in use of chipping method

802.11a uses 11-bit barker chip

802.11b uses 8-bit complimentary code keying (CCK) algorithm


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Basic Service Set (BSS)


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Extended Service Set (ESS)

Portal

Distribution System

Authentication


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Authentication

Authentication

Prove identity to anotherstation.

Open system authentication

Shared key authentication

A sends

B responds with a text

A encrypt and send back

B decrypts and returns anauthenticationmanagement frame.

May authenticate anynumber of station.

Security Problem

A rogue AP SSID of ESS

Announce its presencewith beaconing

A active rogue reachhigher layer data if

unencrypted.

CSMA/CD vs. CSMA/CA


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CSMA/CD CSMA/Collision detection

For wire communication

No control BEFORE transmission Generates collisions

Collision Detection-How?

CSMA/CA CSMA/Collision Avoidance

For wireless communication

Collision avoidance BEFORE transmission Why avoidance on wireless?

Difference in energy/power for transmit & receive

Difficult to distinguish between incoming weak signanoise, and effects of own transmission


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Authentication Frame Sequence

Station APAuthentication Request 1

(authentication Type)

Authentication Response 1

Challenge Text

Authentication Request 2

Encrypted packet

Authentication Response 2

Pass/Fail


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RTS CTS Frame Sequence

Station AP

RTS

CTS

Data

ACK


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Comparison

Documents

WLAN Fundamentals