Introduction to Wireless Networking

REFERENCES

CCRI Engineering and Technology Jbernardini 1

Introduction to Wireless Networking

Module-5Physical Layer Access Methods and

Spread Spectrum

CCRI ENGINEERING AND TECHNOLOGY

Jerry Bernardini


REFERENCES


REFERENCES


• CWTS Certified Wireless Technology Specialist Official Study Guide , Chapter-5

• CWNA Certified Wireless Network Administration Official Study Guide (PWO-104), David Coleman, David Westcott, 2009, Chapter-6

• The California Regional Consortium for Engineering Advances in Technological Education (CREATE) project

• Spread Spectrum Scene http://www.sss-mag.com/primer.html

http://www.sss-mag.com/primer.html

REFERENCES


Chapter Objectives


• Define concepts which make up the functionality of RF and spread spectrum technology

• Define and differentiate between the following physical layer (PHY) wireless technologies

REFERENCES


IEEE 802.3 CSMA/CD vs. IEEE 802.11 CSMA/CA


• CSMA/CD is for wired collision handling• CSMA/CA is for wireless collision handling• CSMA = Carrier Sense Multiple Access• CD = Collision Detection• CA = Collision Avoidance• Why do collisions occur?

– Answer = Two or more stations transmit at the same time• Why is it important to detect or avoid collisions?

– Answer = Because there is data loss and retransmission is necessary

• Wired networks are designed for the transmitting station to detect most collisions

• Many collisions will not be detected by Wireless networks – therefore avoid collisions

REFERENCES


IEEE 802.11 Collision Handling CSMA/CA


• In CSMA/CA a Wireless node that wants to transmit performs the following sequence:

1. Listen on the desired channel.

2. If channel is idle (no active transmitters) it sends a packet.

3. If channel is busy the node waits random time until transmission stops and then waits an additional time period.

4. If the channel is still idle at the end of the time period the node transmits its packet otherwise it repeats the process defined in 3 above until it gets a free channel.

5. Additional support mechanisms such as ACK, RTS/CTS can be used but increase overhead noticeably.

REFERENCES


CSMA/CA and ACK


• CSMA/CA also reduces collisions via explicit frame acknowledgment

• Acknowledgment frame (ACK): Sent by receiving device to sending device to confirm data frame arrived intact

• If ACK not returned, transmission error assumed• CSMA/CA does not eliminate collisions and does not solve hidden

node problem

REFERENCES


Two Kinds of Carrier Sensing Mechanisms


• Physical Carrier Sense– Uses Clear Channel Assessment (CCA) – Is the RF energy on the channel above a threshold? – If CCA>threshold --->wait for CCA< threshold before transmitting– Checks received signal strength using RSSI– RF energy from a hidden node could be missed

• Virtual Carrier Sense– Uses the Network Allocation Vector (NAV) in each station– NAV is a timer that determines if station can contend for RF medium– NAV >0 --->wait for count down to NAV=0– NAV=0 --->use CCA to check for RF energy on medium– IF NAV=0 and CCA > threshold --->station resets NAV>0 and waits

REFERENCES


Network Access Methods


• Reserving Time for Data Transmission Using Distributed Coordination Function (DCF)– Employs a contention period for devices competing to

send data on the wireless network

REFERENCES


CSMA/CA Request to Send/Clear to Send


• Request to Send/Clear to Send (RTS/CTS) protocol: Option used to solve hidden node problem– Significant overhead upon the WLAN with transmission of

RTS and CTS frames• Especially with short data packets

– RTS threshold: Only packets that longer than RTS threshold transmitted using RTS/CTS

REFERENCES


IEEE 802.11 -Half Duplex Communication


• Effects of Half Duplex on Wireless Throughput– Half Duplex: two way communication that occurs in only

one direction at a time• Effective halves the max bit rate

REFERENCES


Telecommunication Channel


• Channel - a path along which information in the form of an electrical signal passes.

• Usually a range of contiguous frequencies involved in supporting information transmission

Bandwidth

Amplitude

FrequencyChannel

CenterChannel Frequency

REFERENCES


RF Bands for Wireless Networks


• ISM- Industrial Scientific and Medical – Three Bands– 900 MHz band– 2.4 GHz band– 5 GHz Band

• UNII- Unlicensed National Information Infrastructure– 5 GHz band– UNII-1 (Lower)– UNII-2 (middle)– UNII2 Extended– UNII-3 (Upper)

REFERENCES


DSSS USA Channel Allocation


• 14 Channels available• 11 Channels in the United States

2.401 GHz 2.473 GHz

1 2 3 4 5 6 7 8 9 10 11

Amplitude

Freq.

Channels

REFERENCES


DSSS 3 Non-overlap Channels


2.401 GHz 2.473 GHz

Ch 1 Ch 6 Ch 11(2.412 GHz) (2.437GHz) (2.462 GHz)

Amplitude

Freq.

2401 MHz

22 MHz

3MHz

2426 MHz2423 MHz

REFERENCES


5 GHz Band and Channels


REFERENCES


Introduction to Spread Spectrum


• Spread Spectrum – a telecommunications technique in which a signal is transmitted in a bandwidth considerably greater than the frequency content of the original information.

Frequency

Amplitude Narrowband

Wideband

REFERENCES


Narrow Band and Spread Spectrum Communications


• Narrowband Vs. Spread Spectrum Communication– Narrowband and Spread Spectrum are two examples of

how devices can communicate using radio frequency

REFERENCES


4-Types Spread Spectrum


• Time Hopping, (THSS)• Frequency Hopping, (FHSS)• Direct Sequence Spread Spectrum, (DSSS)• Hybrid, DSSS/FHSS• Original IEEE 802.11 wireless LAN standard:

– Frequency-hopping spread spectrum (FHSS)– Direct-sequence spread spectrum (DSSS)

• High Rate/ Direct-Sequence Spread Spectrum (HR/DSSS)

• DSSS and HR/DSSS Channels

REFERENCES


Uses of Spread Spectrum


• Military - For low probability of interception of telecommunications.

• Civil/Military - Range and positioning measurements. GPS – satellites.

• Civil Cellular Telephony.• Civil Wireless Networks – 802.11 and Bluetooth.

REFERENCES


Frequency Hopping Patent


• Hedy Lamarr and composer George Antheil, patent number 2,292,387 , circa 1942

• A Hollywood cocktail party with Navy officers present

REFERENCES


Frequency Hopping Spread Spectrum (FHSS)


• FHSS - Acronym for frequency-hopping spread spectrum. 802.11, Bluetooth, & HomeRF.

Freq.

Amp.

1 2 3 4

Frequency Hop Sequence: 1, 3, 2, 4

Wide BandChannel

REFERENCES


Frequency Hopping Spread Spectrum – Simplistic View


REFERENCES


FHSS System Block Diagram


Frequency Synthesizer

CarrierFrequency

DataBuffer

SequenceGenerator

MixerMod

AntennaFHSS

1 23 4

1 23 4

REFERENCES


Direct Sequence Spread Spectrum (DSSS)


Freq.

Amp.

1 2 3 4

DSSS BandChannel

1 Signal

REFERENCES


DSSS System Block Diagram


CarrierFrequency

DataBuffer

Pseudo –Noise

Generator

Antenna

CarrierGenerator

11-bit Barker Code

Mixer

Encoder

DSSS

Chipping Code

Mod

10110111000

REFERENCES


Comparing FHSS & DSSS


Frequency HoppingSpread Spectrum,

FHSS802.11

Direct SequenceSpread Spectrum,

DSSS802.11b

Dwell Time400 mS Higher Cost No

Dwell Time Lower Cost

LowerThroughput

(2 or 3 Mbps)

LowerInteroperabili

ty

HigherThroughput (11 Mbps)

HigherInteroperabili

ty

Better Immunity to Interference

More User Density (79)

Poorer Immunity to Interference

Less User Density (3)

REFERENCES


Orthogonal Frequency Division Multiplexing (OFDM)


• Frequency division multiplexing (FDM) is a technology that transmits multiple signals simultaneously over a single transmission path, such as a cable or wireless system.

• Orthogonal means to establish right angle relationships between frequencies

• OFDM spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies and null out of channel sidebands

f1f3

f2

f4

REFERENCES


OFDM Features


• Used in IEEE 802.11a (OFDM) and IEEE 802.11g (ERP-OFDM) and IEEE 802.11n (HT-OFDM) amendments

• Allows for much higher data rate transfers than DSSS and HR/DSSS

• Up to 54 Mbps for OFDM, ERP-OFDM and 300-600 Mbps for HT-OFDM

• OFDM functions in either the 2.4 GHz ISM or the 5 GHz UNII bands

• The channel width is smaller than DSSS or HR/DSSS• The width of an OFDM channel is only 20 MHz compared to

22 MHz for DSSS

REFERENCES


Orthogonal Frequency Division Multiplexing (OFDM)


• OFDM operates in either the 2.4 GHz ISM or the 5 GHz UNII bands

• The width of an OFDM channel is only 20 MHz compared to 22 MHz for DSSS or HR/DSSS

REFERENCES


Multiple Input/Multiple Output (MIMO) Channels


• MIMO networks can operate in both the 2.4 GHz ISM and 5 GHz UNII bands

• Capable of either 20 or 40 MHz–wide channels• Wider channels mean more data can be transmitted

over the RF medium simultaneously• In the 2.4 GHz ISM band, there is only one 40 MHz–

wide channel without any adjacent-channel overlap

REFERENCES


Multiple Input/Multiple Output (MIMO)


• Used by IEEE 802.11n devices• Wider channels mean more data

can be transmitted over the RF medium simultaneously

• In the 2.4 GHz ISM band, there is only one 40 MHz–wide channel without any adjacent-channel overlap

REFERENCES


MIMO Throughput and Features


• Allows for data rates up to 600 Mbps• Current data rates up to 450 Mbps• More throughput, reliable, predictable• Lower latency for mobile communications• More consistent coverage and throughput for mobile

applications• MIMO networks can operate in both the 2.4 GHz

ISM and 5 GHz UNII bands• Capable of either 20 MHz or 40 MHz–wide channels

REFERENCES


IEEE 802.11n Features


• Uses three modes of OFDM – 20MHz and 40 MHz bands– Data rates up to 600 Mbps

• Non-HT mode– OFDM– Backward compatibility to a, b, g

• HT mixed mode– Supports OFDM and ERP-OFDM

• Greenfield mode– Only ERP-OFDM– Highest data rates

• Channel Bonding

REFERENCES


Co-Location WLAN systems


• Co-location of IEEE 802.11b HR/DSSS and IEEE 802.11a/g/n OFDM Systems

REFERENCES


Co-Location WLAN systems


• Adjacent-channel and Co-channel Interference– Adjacent-channel and co-channel interference

• WLAN/WPAN Coexistence – IEEE 802.11 wireless LANs can be affected when co-

located with WPAN devices

REFERENCES


Encoding and Modulation


• Encoding - To change or translate one bit stream into another.

• Modulation – Appling information on a carrier signal by varying one or more of the signal's basic characteristics - frequency, amplitude and phase.DBPSK (Differential Binary Phase Shift Keying) DQPSK (Differential Quaternary PSK)

REFERENCES


Modulation


• Carrier signal is a continuous electrical signal– Carries no information

• Three types of modulations enable carrier signals to carry information– Height of signal– Frequency of signal– Relative starting point

• Modulation can be done on analog or digital transmissions

REFERENCES


Analog vs. Digital Transmissions


Analog Signal = A signal that has continuously varying voltages, frequencies, or phases. All amplitude values are present from minimum to maximum signal levels.

Digital Signal = A signal in which information is carried in a limited number of different discrete states or levels; High/Low, One/Zero, 1/0

REFERENCES


Analog and Digital Modulation


• Analog Transmission use analog carrier signals and analog modulation.

• Digital Transmission use analog carrier signals and digital modulation.

• Modem (MOdulator/DEModulator): Used when digital signals must be transmitted over analog medium– On originating end, converts distinct digital signals into

continuous analog signal for transmission– On receiving end, reverse process performed

• WLANs use digital modulation of analog signals (carrier signal)

REFERENCES


Frequency and Period


REFERENCES


Analog Modulation


• Amplitude: Height of carrier wave• Amplitude modulation (AM): Changes amplitude so

that highest peaks of carrier wave represent 1 bit while lower waves represent 0 bit

• Frequency modulation (FM): Changes number of waves representing one cycle– Number of waves to represent 1 bit more than number of

waves to represent 0 bit• Phase modulation (PM): Changes starting point of

cycle– When bits change from 1 to 0 bit or vice versa

REFERENCES


Analog Modulation


Amplitude modulation (AM) – Carrier frequency varies in amplitude

Frequency modulation (FM) – Carrier frequency varies in frequency

Phase modulation (PM) – Carrier varies in phase

REFERENCES


Digital Modulation


• Advantages over analog modulation:– Better use of bandwidth– Requires less power– Better handling of interference from other signals– Error-correcting techniques more compatible with other

digital systems• Unlike analog modulation, changes occur in discrete

steps using binary signals– Uses same three basic types of modulation as

Amplitude shift keying (ASK)

REFERENCES


Frequency vs. Phase Shift Key Modulation


Frequency shift keying (FSK)

Phase shift keying (PSK)

REFERENCES


Throughput vs. Data Rate


• Data Rate = Total Data Rate through system• Throughput = Data Payload Rate• Data Rate = Data Payload Rate + Overhead• Overhead = Coding + Modulation+ Bandwidth +

Hardware + Software + Retransmission(errors)

5 Mbps Throughput 11 Mbps Data Rate 5 Mbps Throughput

REFERENCES


Analog vs. Digital Bandwidth


• Analog Bandwidth – Frequency in Khz,Mhz (1 Mhz)• Digital Bandwidth – bits per second (11 Mbps)• Wireless Bandwidth – Frequency Space made

available to network devices (22 Mhz)

Frequency

Amplitude

Bandwidth

Digital Bandwidth (Average Bit Rate)

REFERENCES


Quadrature phase shift keying (QPSK)


REFERENCES


16-QAM Modulation


REFERENCES


64-QAM - 64-level Quadrature Amplitude Modulation


REFERENCES


Spread Spectrum Comparisons


PHY Data Rates Frequency Band

Standards Max Colocated WLANs

Max Total Service Area

Data Rate

FHSS 1 or 2 Mbps 2.4 GHz ISM IEEE 802.11 1997

79 max,12 practical

24 Mbpspractical

DSSS 1 or 2 Mbps 2.4 GHz ISM IEEE 802.11 1997

2 or 3 6 Mbps

HR/DSSS

1, 2, 5.5, or11 Mbps

2.4 GHz ISM IEEE802.11b1999

3 33 Mbps

ERP 1-54 Mbps 2.4 GHz ISM IEEE 802.11g 2003

3 162 Mbps

OFDM 6-54 Mbps 5 GHz U-NII IEEE 802.11a 1999

23 648 Mbps

REFERENCES


Data Rates and Throughput Estimates


PHY Standards Data Rate Throughput

FHSS IEEE 802.11-1997 1–2 Mbps 0.7–1 Mbps

DSSS IEEE 802.11-1997 1–2 Mbps 0.7–1 Mbps

HR/DSSS IEEE 802.11b-1999 1, 2, 5.5, and 11 Mbps 3–6 Mbps

ERP IEEE 802.11g-2003 1, 2, 5.5, 11, 6, 9, 12, 18, 24, 36, 48, 54 Mbps

3–29 Mbps

OFDM IEEE 802.11a-1999 6, 9, 12, 18, 24, 36, 48, 54 Mbps 3–29 Mbps

HT IEEE 802.11n-2009 1–600 Mbps (with 4 spatial streams)

~ 100 Mbps

REFERENCES


Review


• Access methods • WLANs have no way of detecting collisions, so they

use CSMA/CA• Wireless LANs use half-duplex communication• Physical Layer Specifications• DSSS and HR/DSSS channels• OFDM• MIMO• WLAN Co-location

Date post:	23-Feb-2016
Category:	Documents
Upload:	kitty
View:	46 times
Download:	1 times

Introduction to Wireless Networking

Documents