Wifi Fundamentals

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)

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. 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.

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.)

Gain and Loss Amplification Signal Loss Remote Amplifier System

Effective Radiated Power (ERP) Return To Top Of Page

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 Example: Convert 250 mW to dBm 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

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.

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

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

Overlapping Channels 802.11 Radio Channels Overlap

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