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  • Wireless Common Sense

    Shifting the Collection of Prejudices

    Robert J. Berger Internet Bandwidth Development Consulting

    rberger@ibd.com

    1

  • 2

  • Common senseis the collection of

    prejudices acquired by age eighteen- Albert Einstein

    2

  • Tech You Grew Up with Shapes your Prejudices of Wireless Tech Capabilities

    3

  • Tech You Grew Up with Shapes your Prejudices of Wireless Tech Capabilities

    3

  • Claude Shannons Communications

    Theory Shows us the Reality beyond our prejudices

    4

  • C=W log (1+S/N)

    5

  • Shannons Theory

    where; C=channel capacity (bits/second)

    C=W log (1+S/N)

    5

  • Shannons Theory

    where; C=channel capacity (bits/second)W=Bandwidth (How much Spectrum we are using)

    C=W log (1+S/N)

    5

  • Shannons Theory

    where; C=channel capacity (bits/second)W=Bandwidth (How much Spectrum we are using)

    S=signal power, and N=noise power

    C=W log (1+S/N)

    5

  • Propagation Characteristics & Path Loss

    6

  • Propagation Characteristics & Path Loss

    Determines how much of the signal power gets to the receiver

    6

  • Propagation Characteristics & Path Loss

    Determines how much of the signal power gets to the receiver

    The higher the frequency- the less easily the signal propagates

    6

  • Propagation Characteristics & Path Loss

    Determines how much of the signal power gets to the receiver

    The higher the frequency- the less easily the signal propagates

    6

  • Propagation Characteristics & Path Loss

    Determines how much of the signal power gets to the receiver

    The higher the frequency- the less easily the signal propagates- The more buildings, trees, hills the more attenuation

    6

  • Propagation Characteristics & Path Loss

    Determines how much of the signal power gets to the receiver

    The higher the frequency- the less easily the signal propagates- The more buildings, trees, hills the more attenuation

    Physical Reality is such a Drag!!6

  • Radio & TV: Broadcast HIGH POWER & low frequency

    7

  • Radio & TV: Broadcast HIGH POWER & low frequencyMEGA WATTS of Power!!!

    7

  • Radio & TV: Broadcast HIGH POWER & low frequencyMEGA WATTS of Power!!!

    Spectrum in the 80 - 800 MHz range

    7

  • Radio & TV: Broadcast HIGH POWER & low frequencyMEGA WATTS of Power!!!

    Spectrum in the 80 - 800 MHz range50Khz - 6Mhz Channel Bandwidth

    7

  • Radio & TV: Broadcast HIGH POWER & low frequencyMEGA WATTS of Power!!!

    Spectrum in the 80 - 800 MHz range50Khz - 6Mhz Channel Bandwidth

    Gets Large Channel Capacity via Power and Significant Spectrum

    7

  • Tech from the 1930sVery simple analog modulationUses spectrum inefficiently

    Receiver has difficulty rejecting noise

    8

  • Cell Phones: Tiny Channel Capacity

    9

  • Cell Phones: Tiny Channel Capacity Low Signal Power & Advanced Modulation

    9

  • Cell Phones: Tiny Channel Capacity Low Signal Power & Advanced Modulation

    800 - 1900Mhz a bit more difficult propagation, but still good

    9

  • Cell Phones: Tiny Channel Capacity Low Signal Power & Advanced Modulation

    800 - 1900Mhz a bit more difficult propagation, but still good

    But only requires 4kbps Channel Capacity

    9

  • Cell Phones: Tiny Channel Capacity Low Signal Power & Advanced Modulation

    800 - 1900Mhz a bit more difficult propagation, but still good

    But only requires 4kbps Channel CapacityHigh speed data phones will have less coverage or need more basestations

    9

  • Past Experience Does Not Apply to WiFi

    10

  • Past Experience Does Not Apply to WiFi

    Uses Unlicensed Spectrum2.4Ghz & 5 - 6Ghz, Very Low Power

    10

  • Past Experience Does Not Apply to WiFi

    Uses Unlicensed Spectrum2.4Ghz & 5 - 6Ghz, Very Low Power

    Rapid comidification & evolution -Very low cost & advanced modulation

    10

  • Past Experience Does Not Apply to WiFi

    Uses Unlicensed Spectrum2.4Ghz & 5 - 6Ghz, Very Low Power

    Rapid comidification & evolution -Very low cost & advanced modulation

    Developed Bottoms Up like the Internet, not from Carriers or

    Broadcasters

    10

  • Unlicensed Wireless Constraints

    11

  • Unlicensed Wireless Constraints

    Original Spectrum considered Junk Spectrum2.4Ghz is absorption frequency of water5 - 6 Ghz propagates about 1/2 as well

    11

  • Unlicensed Wireless Constraints

    Original Spectrum considered Junk Spectrum2.4Ghz is absorption frequency of water5 - 6 Ghz propagates about 1/2 as well

    Cant interfere, but must accept interference

    11

  • Unlicensed Wireless Constraints

    Original Spectrum considered Junk Spectrum2.4Ghz is absorption frequency of water5 - 6 Ghz propagates about 1/2 as well

    Cant interfere, but must accept interferenceVery low power, but lots of spectrumVery low power, but lots of spectrum

    11

  • Unlicensed Wireless Constraints

    Original Spectrum considered Junk Spectrum2.4Ghz is absorption frequency of water5 - 6 Ghz propagates about 1/2 as well

    Cant interfere, but must accept interferenceVery low power, but lots of spectrum

    Very short distance with high bandwidth

    11

  • Average WiFi Link Budget

    12

  • Average WiFi Link BudgetAP to Client limited by weakest linkAvg clients Signal Power: 15dBmAvg AP Sensitivity 1Mbps: -94dBm

    12

  • Average WiFi Link BudgetAP to Client limited by weakest linkAvg clients Signal Power: 15dBmAvg AP Sensitivity 1Mbps: -94dBmLink Budget avg client to AP:

    signal power + xmit antenna gain - free space path loss + receiv antenna gain - receive

    sensitivity = Margin

    12

  • Average WiFi Link BudgetAP to Client limited by weakest linkAvg clients Signal Power: 15dBmAvg AP Sensitivity 1Mbps: -94dBmLink Budget avg client to AP:

    signal power + xmit antenna gain - free space path loss + receiv antenna gain - receive

    sensitivity = Margin15dBm + 2dB - (-84) + 7dB - (-94dBM) =

    34 dBm Margin at 150 Meters 12

  • Link Margin

    150

    125

    100 75 50 25

    24 2628

    3134

    40

    35 3739

    4245

    51

    1 Mbps 11 Mbps

    Meters

    Link

    Mar

    gin

    13

  • Link MarginThe Free Space Loss calculations are optimistic

    150

    125

    100 75 50 25

    24 2628

    3134

    40

    35 3739

    4245

    51

    1 Mbps 11 Mbps

    Meters

    Link

    Mar

    gin

    13

  • Link MarginThe Free Space Loss calculations are optimistic

    Realistic distance of a laptop to an AP is ~30m in the open

    150

    125

    100 75 50 25

    24 2628

    3134

    40

    35 3739

    4245

    51

    1 Mbps 11 Mbps

    Meters

    Link

    Mar

    gin

    13

  • Link MarginThe Free Space Loss calculations are optimistic

    Realistic distance of a laptop to an AP is ~30m in the open

    At 150 Meters, One good tree will knock out your link 150 125 100 75 50 25

    24 2628

    3134

    40

    35 3739

    4245

    51

    1 Mbps 11 Mbps

    Meters

    Link

    Mar

    gin

    13

  • Real World Constraints

    14

  • Real World ConstraintsWiFi originally designed for indoor LAN

    14

  • Real World ConstraintsWiFi originally designed for indoor LAN

    WiFi CSMA does not handle large numbers of clients accessing an AP

    14

  • Real World ConstraintsWiFi originally designed for indoor LAN

    WiFi CSMA does not handle large numbers of clients accessing an AP

    Nearby clients and APs not associated to an SSID can cause contention

    14

  • Real World ConstraintsWiFi originally designed for indoor LAN

    WiFi CSMA does not handle large numbers of clients accessing an AP

    Nearby clients and APs not associated to an SSID can cause contention

    Asymmetrical Power causes client misbehavior

    14

  • Real World ConstraintsWiFi originally designed for indoor LAN

    WiFi CSMA does not handle large numbers of clients accessing an AP

    Nearby clients and APs not associated to an SSID can cause contention

    Asymmetrical Power causes client misbehaviorObstructions:

    One Tree: 20dB One Wall: 10dB

    14

  • WiFi has to Evolve Around Limitations

    15

  • WiFi has to Evolve Around Limitations

    802.11 Phy has already evolved from 1Mbps to 802.11n with 125Mpbs

    15

  • WiFi has to Evolve Around Limitations

    802.11 Phy has already evolved from 1Mbps to 802.11n with 125Mpbs

    Distance not really improved (but more bandwidth for same short distance)

    15

  • WiFi has to Evolve Around Limitations

    802.11 Phy has already evolved from 1Mbps to 802.11n with 125Mpbs

    Distance not really improved (but more bandwidth for same short distance)

    Contention by nearby nodes not addressed yet (802.11s may, Proprietary extensions today)

    15

  • WiFi Recapitulates Ethernet Evolution

    16

  • WiFi Recapitulates Ethernet Evolution

    Originally Considered a Toy

    16

  • WiFi Recapitulates Ethernet Evolution

    Originally Considered a ToyStarted out as LAN evolved to MAN

    16

  • WiFi Recapit

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