wifi basic knowlege

WIRELESS TECHNICAL FUNDAMENTALS: TOPICS

• License Free Bands
  
  • 900 MHz
  • 2.4 GHz
  • 5 GHz

• RF Fundamentals
  
  • Full Duplex
  • Half Duplex

• Definitions
  
• Gain and Loss
• dBm vs. Watts
• Spread Spectrum Technologies
• Channels For FHSS
• Channels For DSSS Sytem
• DSSS Spectrum - IEEE 802.11b
• Overlapping Channels
• 802.1 Protocol Summary

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Conversions

1 MHz = 1,000,000 Hz
1 GHz = 1,000 MHz
1 GHz = 1,000,000,000 Hz

I.E. 
- 2400 MHz is the same as 2.4 GHz
- 5800 MHz is the same as 5800 GHz

Hz = Radio Spectrum
Bits = data
M = 1,000,000
G = 1,000,000,000

There is a total of 83.3 MHz of Spectrum available in the 2.4 GHz Band

LICENSE FREE BANDS

ISM
ISM: Industrial, Scientific, and Medical
UNI-I (or UNII)
Unlicensed National Information Infrastructure

• 902 to 928 MHz
  
• 2,400 to 2,483.5 MHz
  
• 5,725 to 5,850 MHz
  

• 5,150 to 5,250 MHz
  
• 5,250 to 5,350 MHz
  
• 5,725 to 5,825 MHz
  

 

900 MHz Band

• (902 to 928 MHz)
  
• Data Rates are up to 3 Mbps ( Typical 2 Mbps throughput)
  
• Non Line of Sight
  
• Unaffected by weather
  
• Only few Manufacturers for the products – proprietary technology
  
• Expensive – Compared to IEEE 802.X technologies

2.4 GHz Band

• Large bandwidth available (83.5 MHz)
  
• High data rates possible (up to 11 Mbps)
  
• With the 11g – 54 Mpps is possible 
  
• Equipment reasonably priced
  
• Unaffected by weather
  
• Line of Sight required for long range (greater the several hundred yards) 
  
• Used world-wide
  

5 GHz Band

• 5.725 – 5.850 GHz Spread Spectrum Band
  
• Virtually unlimited ERP on Point-to-Point links makes long range practical
  
• Very high data rates possible
  
• Equipment more expensive then 2.4 GHz equipment
  
• Still greatly underutilized
  
• High gain directional antennas focus signals
  
• Ideal for point-to-point wireless backbone

Unlicensed National Information Infrastructure

Band 1

Band 2

Band 3

FREQUENCY

5.15 - 5.25 GHz

5.25 - 5.35 GHz

5.725 - 5.825 GHz

TX POWER

23 dBm EIRP 
(200 mW)

30 dBm EIRP
(1 Watt)

36 dBm EIRP
(4 Watts)

APPLICATION

INDOOR USE ONLY

CAMPUS LINKS

LONG RANGE
(Several Miles)

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RF FUNDAMENTALS

RF Communication

Full Duplex

• Radio TX and RX data simultaneously
  
• Requires a pair of Frequencies (one for TX and one for RX)
  
• Fastest means of data transfer
  

Both radios are transmitting and receiving at the same time on different radio frequencies.

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Half Duplex

• Used by IEEE 802.11a,b,g and others
  
• Time Division Duplex (TDD)
  
• Bi-directional system
  
• Only one radio transmits at a time
  
• Radio buffers the data until it is its turns to transmit
  
• Operates only on one radio channel
  
• Typically less then half thru-put of the over-the-air data rate
  

Radios alternate transmit and receive on the same radio channel.

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DEFINITIONS

dB

Difference (or ratio) between two signal levels. Generally used used to describe the effect of system devices on signal strength.

dBm

A signal strength level. 0 dBm is defined as 1 mW of power. Small signals are negative numbers. (e.g. - 83 dBm)

dBi

The gain of an antenna relative to an isotropic radiator. Used in calculating ERP and range.

NOTE: dB, dBm and dBi are used because systems’ powers, gains and losses can be calculated by simply adding and subtracting.

ERP

Effective Radiated Power, equal to the antenna gain added to the power into that antenna. (Technically, called EIRP.)

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Gain and Loss

Amplification

Signal Loss

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Remote Amplifier System

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Effective Radiated Power (ERP)

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dBm vs. Watts

dBm

0

1

2

3

6

10

15

20

30

33

36

40

50

60

Power

1.0 mW

1.3 mW

1.6 mW

2.0 mW

4.0 mW

10.0 mW

32.0 mW

100 mW

1 Watt

2 Watts

4 Watts

10 Watts

100 Watts

1.000 Watts

dBm = 10 log

 dBm = 10 log dBm = 10 log

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Example: Convert 250 mW to dBm

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10 log

 

250

 = 10 x 2.39 = 24dBm 

1 mW

 

Rule Of Thumb

Double/Half Power:

Add/Subtract 3dB

Ten Times/One-tenth Power:

Add/Subtract 10dB

Spread Spectrum Technologies

Frequency Hopping (FHSS)

• The radio carrier hops around the band.

Direct Sequence (DSSS)

• The radio carrier signal is “spread out” on a specific channel

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Channels For FHSS

Each data packet sent on one of 82 channels. 

• Next packet randomly hops to another channel.
• Signal itself not “spread” - occupies narrow bandwidth.
• Signals “hop” around interfering signals.

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Channels For DSSS System

802.11 Radio Channels
All Radios in One Network Operate on a Single Channel

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DSSS Spectrum - IEEE 802.11b

Direct Sequence Spread Spectrum (DSSS)
Frequency Channel in 2.4 GHz (IEEE 802.11)

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Overlapping Channels

802.11 Radio Channels Overlap

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802.1 Protocol Summary

Standard

Modulation

Air Data Rate

Frequency Band

802.11

FHSS
DSSS

2 Mbps

2.4 GHz

802.11a

OFDM

54 Mbps

UNII

802.11b

DSSS

11 Mbps

2.4 GHz

802.11g

OFDM
DSSS

54 Mbps
11 Mbps

2.4 GHz

802.1x

N/A

Authentication and
Access Control

All

802.11b WiFi Channels

In the United States and Canada there are 11 channels available for use in the 802.11b 2.4GHz WiFi Frequency range. This standard is defined by the IEEE.

ChannelLower FrequencyCenter FrequencyUpper Frequency    12.4012.4122.42322.4062.4172.42832.4112.4222.43342.4162.4272.43852.4212.4322.44362.4262.4372.44872.4312.4422.45382.4362.4472.45892.4412.4522.463102.4512.4572.468112.4512.4622.473

There are only 3 non-overlapping channels available in the 802.11b standard.These are Channels 1,6, and 11. For WiFi access points that are located near each other it is recommended that they each use one of the above non-overlapping channels to minimize the effects of interference.

This overlapping of 2.4GHz channels are better visualized from the chart below.

No. 20MHz is in 802.11b/g and 802.11a.

20MHz is a channel width. Bandwidth. Wider = more speed.

802.11b/g are standard at 20MHz bandwidth per channel.
802.11b/g channels are in the 2.4GHz band and always 20MHz.

802.11a channels are more numerous and are in the 5.8GHz band. The are standard at 20MHz.

40MHz bandwidth in 802.11n, if achievable as I dscussed above, about doubles the speed/capacity. 802.11n is mostly focused on the 2.4GHz band.

Either 2.4GHz or 5.8GHz can accomodate 20 and 40MHz bandwidth if the product implments all four combinations.

Since there are more channels and less usage, the 5.8GHz band is the best place to try 40MHz. This band is known as 802.11a.

802.11n is most commonly in the 2.4GHz band.

A very few products are now capable of 40MHz on 5.8GHz under the 802.11 standards.
Some may be dual-band but may operate 20MHz on 5.8GHz and 20/40MHz in 2.4GHz.

Use of 40MHz is imprudent at 2.4GHz and is an immature product technology at 5.8GHz at this time.