Wireless LAN Technology

Lecture 32

Wireless LAN Introduction The proliferation of laptop computers and

other mobile devices (PDAs and cell phones) created an obvious application level demand for wireless Local Area Networking.

Companies jumped in, quickly developing incompatible wireless products in the 1990’s.

Industry decided to entrust standardization to IEEE committee that dealt with wired LANs

IEEE 802 committee!!2

Defining a WLAN (Wireless Local Area Network) A communications network

that provides connectivity to wireless devices within a limited geographic area.

"Wi-Fi" is the universal standard for wireless networks and is the wireless equivalent of wired Ethernet networks. In the office, Wi-Fi networks are adjuncts to the wired networks. At home, a Wi-Fi network can serve as the only network since all laptops and many printers come with Wi-Fi built in, and Wi-Fi can be added to desktop computers via USB.

Wi-Fi is achieved with a wireless base station, called an "access point." Its antennas transmit and receive a radio frequency within a range of 30 to 150 feet through walls and other non-metal barriers.

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WLAN Design Goals Global, seamless operation Low power for battery use No special permissions or licenses needed to use the LAN Robust transmission technology Simplified spontaneous cooperation at meetings Easy to use for everyone, simple management Protection of investment in wired networks Security (no one should be able to read my data), privacy

(no one should be able to collect user profiles), safety (low radiation)

Transparency concerning applications and higher layer protocols, but also location awareness if necessary 4

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Early Wireless LANs Many standards = No standards

Limited or no encryption .5 to 2 Mbps throughput High NIC cost High AP cost Limited roaming

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Modern Wireless LANs IEEE standards based 128 bit encryption ≥ 11 Mbps throughput Low NIC cost Low AP cost Integrated roaming

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Pro and Cons of WLAN Advantages

Very flexible within the reception area Ad-hoc networks without previous planning possible (almost) no wiring difficulties (e.g. historic buildings, firewalls) More robust against disasters like, e.g., earthquakes, fire - or

users pulling a plug... Disadvantages

Typically very low bandwidth compared to wired networks due to shared medium

Many proprietary solutions, especially for higher bit-rates, standards take their time (e.g. IEEE 802.11n)

Products have to follow many national restrictions if working wireless, it takes a vary long time to establish global solutions like, e.g., IMT-2000 8

WLAN Deployments Medical Professionals Education Temporary Situations Airlines Security Staff Emergency Centers

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Wireless LAN Applications LAN Extension Cross-building interconnect Nomadic Access Ad hoc networking

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LAN Extension Wireless LAN linked into a wired LAN on

same premises Wired LAN

Backbone Support servers and stationary workstations

Wireless LAN Stations in large open areas Manufacturing plants, stock exchange trading

floors, and warehouses

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Single Cell LAN Extension

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Multiple-cell Wireless LAN

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Cross-Building Interconnect Connect LANs in

nearby buildings Wired or wireless LANs

Point-to-point wireless link is used

Devices connected are typically bridges or routers

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Cross-Building Interconnect Connect LANs in nearby buildings

Wired or wireless LANs Point-to-point wireless link is used Devices connected are typically bridges or

routers

Cisco Aironet 1300 and 1400 SeriesWireless Bridges

http://www.cisco.com/en/US/products/ps5861/prod_brochure09186a0080230777.html

Nomadic Access Wireless link between LAN hub and mobile

data terminal equipped with antenna Laptop computer or notepad computer

Uses: Transfer data from portable computer to office

server Extended environment such as campus users move around with portable computers access to servers on wired LAN

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Nomadic Access – Example

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Ad Hoc Networking Temporary peer-to-peer

network set up to meet immediate need

Example: Group of employees with

laptops convene for a meeting; employees link computers in a temporary network for duration of meeting

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Wireless LAN Requirements Throughput Number of nodes Connection to backbone LAN Service area Battery power consumption Transmission robustness and security Collocated network operation License-free operation Handoff/roaming Dynamic configuration 20

Wireless LAN Requirements Throughput. The medium access-control (MAC) protocol

should make as efficient use as possible of the wireless medium to maximize capacity.

Number of nodes. Wireless LANs may need to support hundreds of nodes across multiple cells.

Connection to backbone LAN. In most cases, interconnection with stations on a wired backbone LAN is required. For infrastructure wireless LANs, this is easily accomplished through the use of control modules that connect to both types of LANs. There may also need to be accommodation for mobile users and ad hoc wireless networks.

Service area. A typical coverage area for a wireless LAN has a diameter of 100 to 300 m. 21

Wireless LAN Requirements Battery power consumption. Mobile

workers use battery-powered workstations that need to have a long battery life when used with wireless adapters. This suggests that a MAC protocol that requires mobile nodes to monitor access points constantly or engage in frequent handshakes with a base station is inappropriate. Typical wireless LAN implementations have features to reduce power consumption while not using the network, such as a sleep mode. 22

Wireless LAN Requirements Transmission robustness and

security. Unless properly designed, a wireless LAN may be interference-prone and easily eavesdropped. The design of a wireless LAN must permit reliable transmission even in a noisy environment and should provide some level of security from eavesdropping.

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Wireless LAN Requirements Collocated network operation. As

wireless LANs become more popular, it's quite likely that two or more wireless LANs will operate in the same area or in some area where interference between the LANs is possible. Such interference may thwart the normal operation of a MAC algorithm and may allow unauthorized access to a particular LAN.

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Wireless LAN Requirements License-free operation. Users would prefer to

buy and operate wireless LAN products without having to secure a license for the frequency band used by the LAN.

Handoff/roaming. The MAC protocol used in the wireless LAN should enable mobile stations to move from one cell to another.

Dynamic configuration. The MAC addressing and network management aspects of the LAN should permit dynamic and automated addition, deletion, and relocation of end systems without disruption to other users. 25

Wireless LAN Categories Infrared (IR) LANs Spread spectrum LANs Narrowband microwave

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Wireless LANs

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Strengths of Infrared Over Microwave Radio

Spectrum for infrared virtually unlimited Possibility of high data rates

Infrared spectrum unregulated Equipment inexpensive and simple Reflected by light-colored objects

Ceiling reflection for entire room coverage Doesn’t penetrate walls

More easily secured against eavesdropping Less interference between different rooms

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Drawbacks of Infrared Medium

Indoor environments experience infrared background radiation Sunlight and indoor lighting Ambient radiation appears as noise in an

infrared receiver Transmitters of higher power required

Limited by concerns of eye safety and excessive power consumption

Limits range

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IR Data Transmission Techniques

Directed Beam Infrared Ominidirectional Diffused

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Directed Beam Infrared Used to create point-to-point links Range depends on emitted power and

degree of focusing Focused IR data link can have range of

kilometers Cross-building interconnect between bridges

or routers

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Ominidirectional Single base station within line of sight of all

other stations on LAN Station typically mounted on ceiling Base station acts as a multiport repeater

Ceiling transmitter broadcasts signal received by IR transceivers

IR transceivers transmit with directional beam aimed at ceiling base unit

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Diffused All IR transmitters focused and aimed at a

point on diffusely reflecting ceiling IR radiation strikes ceiling

Reradiated omnidirectionally Picked up by all receivers

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Spread Spectrum WLANConfiguration

usually use multiple-cell arrangement adjacent cells use different center

frequencies

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Spread Spectrum WLANsTransmission Issues

licensing regulations differ between countries USA FCC allows in ISM band:

spread spectrum (1W), very low power (0.5W) 902 - 928 MHz (915-MHz band) 2.4 - 2.4835 GHz (2.4-GHz band) 5.725 - 5.825 GHz (5.8-GHz band)

2.4 GHz also in Europe and Japan

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Narrowband Microwave LANs Use of a microwave radio frequency band

for signal transmission Relatively narrow bandwidth Licensed Unlicensed

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Licensed Narrowband RF Licensed within specific geographic areas

to avoid potential interference Motorola - 600 licenses in 18-GHz range

Covers all metropolitan areas Can assure that independent LANs in nearby

locations don’t interfere Encrypted transmissions prevent

eavesdropping

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Unlicensed Narrowband RF RadioLAN introduced narrowband wireless

LAN in 1995 Uses unlicensed ISM spectrum Used at low power (0.5 watts or less) Operates at 10 Mbps in the 5.8-GHz band Range = 50 m to 100 m

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Original 802.11 Physical Layer-DSSS

Direct-sequence spread spectrum (DSSS) 2.4 GHz ISM band at 1 Mbps and 2 Mbps up to seven channels, each 1 Mbps or 2 Mbps,

can be used depends on bandwidth allocated by various

national regulations 13 in most European countries one in Japan

each channel bandwidth 5 MHz encoding scheme DBPSK for 1-Mbps and DQPSK

for 2-Mbps using an 11-chip Barker sequence42

Original 802.11 Physical Layer-FHSS

Frequency-hopping spread spectrum makes use of multiple channels signal hopping between multiple channels based on

a pseudonoise sequence 1-MHz channels are used

hopping scheme is adjustable 2.5 hops per second in United States 6 MHz in North America and Europe 5 MHz in Japan

two-level Gaussian FSK modulation for 1 Mbps four-level GFSK modulation used for 2 Mbps

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Original 802.11 Physical Layer-Infrared

omnidirectional range up to 20 m 1 Mbps uses 16-PPM (pulse position modulation)

4 data bit group mapped to one of 16-PPM symbols each symbol a string of 16 bits each 16-bit string has fifteen 0s and one binary 1

2-Mbps has each group of 2 data bits is mapped into one of four 4-bit sequences each sequence consists of three 0s and one binary 1

intensity modulation is used for transmission

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Comparison: infrared vs. radio transmission

Infrared uses IR diodes, diffuse light, multiple

reflections (walls, furniture etc.) Advantages

simple, cheap, available in many mobile devices

no licenses needed simple shielding possible

Disadvantages interference by sunlight, heat

sources etc. many things shield or absorb IR light low bandwidth

Example IrDA (Infrared Data Association)

interface available everywhere

Radio typically using the license free ISM

band at 2.4 GHz Advantages

experience from wireless WAN and mobile phones can be used

coverage of larger areas possible (radio can penetrate walls, furniture etc.)

Disadvantages very limited license free frequency

bands shielding more difficult,

interference with other electrical devices

Example Many different products

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Comparison: Infrastructure vs. ad-hoc Networks

infrastructure network

ad-hoc network

APAP

AP

wired network

AP: Access Point

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In response to lacking standards, IEEE developed the first internationally 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.

IEEE 802.11 Wireless LAN Standard

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Appliance Interoperability Fast Product Development Stable Future Migration Price Reductions The 802.11 standard takes into account

the following significant differences between wireless and wired LANs:

Power Management Security Bandwidth

Benefits of 802.11 Standard

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IEEE 802 LAN Standards Family

IEEE 802.3CarrierSense

IEEE 802.4TokenBus

IEEE 802.5TokenRing

IEEE 802.11Wireless

IEEE 802.2Logical Link Control (LLC)

PHYOSI Layer 1(Physical)

Mac

OSI Layer 2(Data Link)

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802.11 Protocol Stack

Part of the 802.11 protocol stack. 51

LiFi LiFi is transmission of data through illumination

by taking the fiber out of fiber optics by sending data through a LED light bulb that varies in intensity faster than the human eye can follow.

Li-Fi is the term some have used to label the fast and cheap wireless communication system, which is the optical version of Wi-Fi. The term was first used in this context by Harald Haas in his TED Global talk on Visible Light Communication. “At the heart of this technology is a new generation of high brightness light-emitting diodes

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LiFi LIFI Solid-State Plasma Lighting

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Comparison Between Current and Future Wireless Technology

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Summary WLAN Applications Wireless Requirements WLAN Classifications DSSS and Frequency Hopping in WLANs IEEE Standardization 802.11 Physical Layer FHSS 802.11 Physical Layer DSSS Comparison of Infrared with Radio Transmission Infrastructure vs. ad-hoc Network 802.11 Benefits 802.11 Protocol Stack 55

2nd Part of the LectureGNU Radio

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What is GNU Radio?Basic ConceptsGNU Radio Architecture & PythonDial Tone Example

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What is GNU Radio?Software toolkit for signal

processing Software radio construction Rapid development Cognitive radio

USRP (Universal Software Radio Peripheral) Hardware frontend for sending

and receiving waveforms

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GNU Radio Components

Hardware Frontend Host Computer

RF Frontend(Daugtherboard)

ADC/DAC andDigital Frontend(USRP)

GNU RadioSoftware

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GNU Radio Software

Opensource software (GPL) Don't know how something works? Take a look! Existing examples: 802.11b, Zigbee, ATSC

(HDTV), OFDM, DBPSK, DQPSK

Features Extensive library of signal processing blocks

(C++/ and assembly) Python environment for composing blocks (flow

graph)60

GNU Radio Hardware

Sends/receives waveforms USRP Features

USB 2.0 interface (480Mbps) FPGA (customizable) 64Msps Digital to Analog converters (receiving) 128Msps Analog to Digital converteres (transmitting) Daughterboards for different frequency ranges

Available Daughterboard 400-500Mhz, 800-1000Mhz, 1150-1450Mhz, 1.5-

2.1Ghz, 2.3-2.9Ghz61

GNU Radio Hardware Schematic

RX/TXDaughterboard

ADC/DAC

Host Computer

FPGAUSBInterface

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Basics: Blocks Signal Processing Block

Accepts 0 or more input streams

Produces 0 or more output streams

Source: No input noise_source,signal_source,usrp_source

Sink: No outputs audio_alsa_sink,usrp_sink

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Basics: Data Streams

Blocks operate on streams of data

1 5 3

3 7 9

4 12 12

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Basics: Data Types

Blocks operate on certain data types char, short, int, float,

complex Vectors

Input Signature: Data types for input

streams Output Signature:

Data types for output streams

Two streamsof float

One streamof complex

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Basics: Flow Graph

Blocks composed as a flow graph Data stream flowing from sources to sinks

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GNU Radio Architecture

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GNU radio has provided some useful APIs What we are interested in at this time is how to use the

existing modules that has been provided in GNU radio project to communicate between two end systems

GNU Radio Architecture

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GNU Radio Architecture - software

How these modules co-work? Signal processing block and flow-graph

C++: Extensive library of signal processing blocks Performance-critical modules

Python: Environment for composing blocks Glue to connect modules Non performance-critical modules

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GNU Radio Architecture – software(2)

Python scripting language used for creating "signal flow graphs“

C++ used for creating signal processing blocks

An already existing library of signaling blocks

The scheduler is using Python’s built-in module threading, to control the ‘starting’, ‘stopping’ or ‘waiting’ operations of the signal flow graph.

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GNU Radio Architecture – software(3)

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GNU Radio Companion

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GNU Radio Companion (Cont'd)

GNU Radio CompanionDesign flow graphs

graphicallyGenerate runnable code

Demonstration

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Dial Tone Example

Sine Generator (350Hz)

Sine Generator (440Hz)

Audio Sink

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Dial Tone Example#!/usr/bin/env python

from gnuradio import grfrom gnuradio import audiofrom gnuradio.eng_option import eng_optionfrom optparse import OptionParser

class my_top_block(gr.top_block): def __init__(self): gr.top_block.__init__(self)

parser = OptionParser(option_class=eng_option) parser.add_option("-O", "--audio-output", type="string", default="", help="pcm output device name. E.g., hw:0,0") parser.add_option("-r", "--sample-rate", type="eng_float", default=48000, help="set sample rate to RATE (48000)") (options, args) = parser.parse_args () if len(args) != 0: parser.print_help() raise SystemExit, 1

sample_rate = int(options.sample_rate) ampl = 0.1

src0 = gr.sig_source_f (sample_rate, gr.GR_SIN_WAVE, 350, ampl) src1 = gr.sig_source_f (sample_rate, gr.GR_SIN_WAVE, 440, ampl) dst = audio.sink (sample_rate, options.audio_output) self.connect (src0, (dst, 0)) self.connect (src1, (dst, 1))

if __name__ == '__main__': try: my_top_block().run() except KeyboardInterrupt: pass

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Summary

What is GNU Radio? Basic Concepts GNU Radio Architecture & Python Dial Tone Example

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