Wireless Networks and Wireless Networks and Spread Spectrum Spread Spectrum

TechnologiesTechnologies

Industrial, Scientific, and Medical Industrial, Scientific, and Medical (ISM) Bands(ISM) Bands

ISM bands are:◦902-928 MHz (26 MHz wide)

Industrial ◦2.4000-2.4835 GHz (83.5 MHz wide)

Scientific◦5.725-5.875 GHz (150 MHz wide)

MedicalThese are all license-free bands (no

restrictions on equipment)

Industrial, Scientific, and Medical Industrial, Scientific, and Medical (ISM) Bands(ISM) Bands900 MHz ISM Band

◦Used for wireless networking in past, but now wireless uses higher frequencies (higher throughput)

◦ In some parts of world this frequency is allocated to GSM cellular phones

2.4 GHz ISM Band◦Most common band used for wireless networking◦Also used for microwaves, cordless home phones,

baby monitors, and wireless video cameras

Industrial, Scientific, and Medical Industrial, Scientific, and Medical (ISM) Bands(ISM) Bands

5.8 GHz ISM Band◦Used for: baby monitors, cordless phones,

cameras◦Preferred spectrum for long distance wireless

bridging

Unlicensed National Information Unlicensed National Information Infrastructure Bands (UNII)Infrastructure Bands (UNII)

802.11a amendment uses three bands of UNII frequencies, known as lower, middle, and upper UNII bands◦Lower Band (UNII-1)

100 MHz wide and spans from 5.150-5.250 GHz Indoor communications only Maximum allowed output of 40 mW at the IR

◦Middle Band (UNII-2) 100 MHz wide and spans from 5.250-5.350 GHz Indoor and outdoor communications Maximum allowed output of 200 mW

Unlicensed National Information Unlicensed National Information Infrastructure Bands (UNII)Infrastructure Bands (UNII)

Upper Band (UNII-3)◦100 MHz wide and spans 5.725-5.825 GHz◦Outdoor point-to-point communications◦Maximum allowed output power is 800 mW at

IR

Narrowband and Spread Spectrum

Narrowband Transmission◦Uses very little bandwidth◦Higher power◦AM/FM radio stations

Spread Spectrum◦Uses more bandwidth◦Spreads data across frequencies◦Very low power levels

Makes it less likely to cause interference with other systems not required to be licensed by local regulatory body usually

Frequency Hopping Spread Spectrum (FHSS)

FHSS◦ Transmits data using small frequency carrier space, then

hops to another small frequency carrier◦ Uses frequency for set period of time = dwell time

When dwell time expires, switches frequencies◦ Hopping Sequence

Instead of transmitting on one set channel or finite frequency space, FHSS radio card transmits on a sequence of sub-channels called hops.

Hop time is a measurement of amount of time it takes for transmitter to change from one frequency to next

802.11/b/g Channels

14 Separate Channels in 2.4 GHz ISM band

Most channels overlap, but 1, 6, and 11 do not◦ To not overlap, must be separated by at least

five channels or 25 MHz.

Direct Sequence Spread Spectrum Direct Sequence Spread Spectrum (DSSS)(DSSS) “Unlike FHSS where the transmitter jumped between

frequencies, DSSS is set to one channel.” Data that is transmitted is therefore spread across the frequencies that are part of the specified channel.◦ This process of spreading data across frequencies of a

channel is known as data encoding.◦ Two types of data encoding are Barker Code and

Complementary Code Keying(CCK). As we add more information to our data being transmitted

to guard against data corruption, it is known as processing gain.

ModulationModulation After the data is encoded(chipping method), the

transmitter than needs to create a carrier signal containing the chips.

Differential Binary Phase Shift Keying (DBPSK) – uses two phase shifts, one to represent the “0” chip and another to represent the “1” chip.

Differential Quaternary Phase Shift Keying (DQPSK) – to offer faster throughput, DQPSK uses four phase shifts. In the process it allows each of the four phases to modulate two chips i.e. (00, 01, 10, 11) thus doubling the speed.

Packet Binary Convolutional Code (PBCC) – this is a modulation technique that supports data rates of 5.5, 11, 22, and 33 Mbps.

Orthogonal Frequency Division Orthogonal Frequency Division Multiplexing (OFDM)Multiplexing (OFDM) This is one of the most popular wired/wireless technologies

in use. ◦ It is capable of transmitting up to speeds of 54Mbps ◦ OFDM is NOT a spread spectrum technology◦ OFDM actually transmits across 52 separate frequencies, referred

commonly to as subcarriers Frequency range of each subcarrier is 312.5 KHz Since there are so many subcarriers the overall data rates are

HIGHER ISI is LESS LIKELY to occur because delay spread is a smaller % of the

symbol period◦ OFDM technology is MORE RESISTANT to negative effects of multipath

than DSSS and FHSS. Convolution Coding – a form of error correction that is

performed to make OFDM more resistant to narrowband interference.

Throughput vs. BandwidthThroughput vs. Bandwidth A specified frequency band is your bandwidth, or the space in

which your communication is limited. Many factors account for how much Bandwidth you really can

use. Example in book: “Because of the half-duplex nature of the

medium and the overhead generated by CSMA/CA, the actual aggregate throughput is typically 50 % or less of the data rate…For example, if a data rate is 54 Mbps, due to CSMA/CA the aggregate throughput might be about 20 Mbps. If five client stations were all downloading the same file from an FTP server at the same time, the perceived throughput for each client station would be about 3 Mbps under ideal circumstances.”

Then take into consideration other overhead costs such as security…