© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module 2-1 © 2002, Cisco Systems, Inc. All rights reserved.

2 Radio Frequency Spread Spectrum Technology 2-1

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module 2-3

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module 2-4 Objectives Upon completion of this module, you will be able to perform the following tasks: Define facts and characteristics of each spread spectrum technology. Identify facts on multipath distortion. Identify the process a wireless client adapter card undergoes while associating to an access point. Define multipath distortion and identify why diversity antennas are used on access points. Define basic facts on Orthogonal Frequency Division Multiplexing (OFDM).

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module 2-5 Unlicensed Frequency Bands Extremely Low Very Low MediumHighVery High InfraredVisible Light Ultra- violet X-Rays Audio AM Broadcast Short Wave RadioFM Broadcast Television Infrared wireless LAN Cellular (840 MHz) NPCS (1.9 GHz) MHz 26 MHz 5 GHz (IEEE ) HiperLAN HiperLAN a 2.4 – GHz 83.5 MHz (IEEE ) b and g Ultra High Super High

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module 2-6 Three Wireless Technologies The Laws of Radio Dynamics: Higher Data Rates = Shorter Transmission Range Higher Power Output = Increased Range, but Lower Battery Life Higher Frequency Radios = Higher Data Rates Shorter Ranges b a g 2.4 GHz5 GHz2.4 GHz WorldwideUS/APWorldwide 11 Mbps54 Mbps Frequency Band Availability Maximum Data rate Other Services (Interference) Cordless Phones Microwave Ovens Wireless Video Bluetooth Devices HyperLAN Devices

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module 2-7

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module 2-8 Worldwide Availability

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module 2-9 IEEE Standard IEEE became a standard in July GHz at 2 Mbps Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS) IEEE a and b became standards in September a – 5 GHz at 54 Mbps OFDM b – 2.4 GHz at 11 Mbps DSSS IEEE g is scheduled to be ratified in g – 2.4 GHz at 54 Mbps OFDM promises true vendor interoperability

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module 2-10 What Is WLAN RF Technology? Data sent over the air waves Two-way radio communications (half duplex) Same radio frequency for sending & receiving (transceiver) No licensing required for Cisco Aironet Wireless products (in most countries)

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module 2-11 Transmitting a Signal The goal of sending data over RF is to: Send as much data as far, and as fast as possible Transmitting more data across the airwaves on a signal More frequency spectrum is used or Complex modulation techniques are used

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module 2-12 Frequency Bandwidth More information means more frequency spectrum is used 3K 175K 4500K Bandwidth in KHz TV Signal FM Radio Signal CB Radio Signal

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module 2-13 Signal Strength Strong Med Weak Low Med High Noise Level Modulation Complex modulation Better signal strength Less coverage area Complex modulation schemes compress data Better (quieter) phone line needed for higher speed More noise, less speed

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module MHz DSSS Scheme More data rate, more frequency Three 215 Kbps Two 344 Kbps One 860 Kbps

802.11b Modulation © 2002, Cisco Systems, Inc. All rights reserved. 15

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module b Radio Modulation Cisco Aironet Access Points Three different types of modulation Depending upon the data rate: –Binary Phase Shift Keyed (BPSK) –Quadrature Phase Shift Keying (QPSK) –Complementary Code Keying (CCK) BPSK Modulation Example

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module b Direct Sequence Modulation Each data bit becomes a string of chips (chipping sequence) transmitted in parallel across a wide frequency range Minimum chip rate per the FCC is 10 chips for 1 and 2 Mbps (BPSK/QPSK) and 8 chips for 11 Mbps (CCK) data rates b uses 11 chips If the data bit was: 1001 Chipping code is : 1= = Transmitted data would be:

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module GHz Channel Sets Americas Europe, Middle East and Asia Center Frequency 2412 MHz 2417 MHz 2422 MHz 2427 MHz 2432 MHz 2437 MHz 2442 MHz 2447 MHz 2452 MHz 2457 MHz 2462 MHz 2467 MHz 2472 MHz 2484 MHz 2412 MHz 2417 MHz 2422 MHz 2427 MHz 2432 MHz 2437 MHz 2442 MHz 2447 MHz 2452 MHz 2457 MHz 2462 MHz 2467 MHz 2472 MHz 2484 MHz Japan Israel XXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXX XXXXXXXXXXXXXX Regulatory Domain Channel Identifier

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module 2-19

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module 2-20 Channels- 2.4 GHz DSSS 11 chips per bit means each bit sent redundantly 11 Mbps data rate 3 access points can occupy same area 11 Channels – each channel 22 MHz wide 1 set of 3 non-overlapping channels 14 Channels – each channel 22 MHz wide 4 sets of 3 non-overlapping channels, only one set used at a time

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module b Access Point Coverage 1 Mbps DSSS 5.5 Mbps DSSS 11 Mbps DSSS 2 Mbps DSSS

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module b Scalability Blue = 11 Mbps Green = 11 Mbps Red = 11 Mbps Total Bandwidth = 33 Mbps!!!

802.11a Modulation © 2002, Cisco Systems, Inc. All rights reserved. 23

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module 2-24 Comparing the Technologies a Data Rates Modulation with Sub Channels BPSK Data Rate Per Subchannel (Kbps) BPSK QPSK QPSK 375 Total Data Rate (Mbps) QAM 64QAM QAM

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module a Uses Orthogonal Frequency Division Multiplexing (OFDM) Modulation Channel sampled at 20 MHz 64-sample (3.2us) symbols 16-sample (0.8us) cyclic prefix/guard interval 250 symbols per second Of 64 sub-carriers: 12 zero sub-carriers (In black) on sides and center –Side is frequency guard band leaving 16.5 MHz occupied BW –Center sub-carrier is zero for DC offset/carrier leak rejection 48 data sub-carriers (In green) per symbol 4 pilots sub-carriers (In red) per symbol for synchronization/tracking

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module a 5GHz Frequency Bands The figure above shows the center frequency of the channels. The frequency of the channel is 10 MHz either side of the dotted line. There is 5 MHz of separation between channels. Lower and Middle UNII Bands: 8 Carriers in 200 MHz / 20 MHz Spacing

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module a Channel Sets Americas include: Argentina Australia Austria Brazil Canada Chile Columbia Denmark France Mexico New Zealand Panama Peru Sweden United Kingdom United States Venezuela

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module a Access Point Coverage OFDM 54 Mbps 48 Mbps 36 Mbps 24 Mbps 18 Mbps 12 Mbps 09 Mbps 06 Mbps

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module a Scalability (Indoor UNII-1 and 2) 54 Mbps Total Bandwidth = 432 Mbps!!! 8 non-overlapping channels 54 Mbps

Authentication © 2002, Cisco Systems, Inc. All rights reserved. 30

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module Protocol Association Process Access Point B Access Point A Initial Connection to an Access Point 1. Client sends probe request via RF. 2. APs (A/B) send probe response via RF. Client evaluates probe response and selects best AP (A). 3. Client sends authentication request to selected AP (A) via RF. 4. AP (A) confirms authentication via RF packet. 5. Client sends association request to selected AP (A) via RF packet. 6. AP (A) confirms association request via RF packet and registers client.

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module Protocol Roaming/Re-association Process Access Point B Access Point A Initial Connection to an Access Point 1. Client is currently associated to AP (A), but continually listens for beacons from all APs via RF packets. Client evaluates beacons and selects best AP (B). 2. Client sends association request to selected AP (B) via RF packet. 3. AP (B) confirms association request via RF packet and registers client. 4. AP (B) informs AP (A) via Ethernet packet of the clients re-association with AP (B).

Multipath © 2002, Cisco Systems, Inc. All rights reserved. 33

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module 2-34 Multipath Distortion Occurs when an RF signal has more than one path between a receiver and a transmitter RF take more than one path Multiple signals cause distortion of the signal Can cause high signal strength yet low signal quality Ceiling Floor TXRX Obstruction Time Received Signals Combined Results Time

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module 2-35 Diversity and Multipath In a multipath environment, signal null points are located throughout the area Moving the antenna slightly will allow you to Move out of a null point Receive the signal correctly Dual diversity antennas typically mean if one antenna is in a null, the other one will not be, therefore providing better performance in multipath environments TX RX1 RX2 Ceiling Obstacle

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module 2-36

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module 2-37 OFDM Is the Antidote for Inter-Symbol Interference Ways to minimize inter-symbol interference: Reduce the symbol rate, but data rate usually goes down too Equalizers, but equalization is processor intensive Solution: Transmit over multiple carrier frequencies in parallel (Orthogonal Frequency Division Multiplexing) Frequency

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module 2-38 Summary Upon completion of this module, you will be able to perform the following tasks: Define facts and characteristics of each spread spectrum technology. Identify facts on multipath distortion. Identify the process a wireless client adapter card undergoes while associating to an access point. Define multipath distortion and identify why diversity antennas are used on access points. Define basic facts on Orthogonal Frequency Division Multiplexing (OFDM).

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module 2-39 Review Questions What does ISM stand for? What are the unlicensed frequency bands? What causes multipath distortion? What are the three modulations techniques that are utilized in all b radios? Why is it important to choose a single vendors access point for the wireless backbone? What is achieved by co-locating access points and how many access points can you co-locate in an b environment?

© 2002, Cisco Systems, Inc. All rights reserved. AWLF 3.0Module 2-40 Review Questions (cont.) Why are diversity antennas used on the Cisco Aironet Access Points? How does OFDM encoding scheme work? Why is OFDM signal not affected by inter- symbol interference? a has how many non-overlapping channels? What is the highest aggregate data rate for an a system?