CWNA Chapter 6 – Wireless Networks and Spread Spectrum Technologies

My Notes from chapter 6 of the CWNA study guide 

Industrial, Scientific, and Medical bands (ISM)

  • ISM bands are defined by the ITU Telecommunication Standardization Sector (ITU-T)
  • All three of these bands are license-free bands, and there are no restrictions on what types of equipment can be used in any of them.
  • The frequency ranges of the ISM bands are as follows:
    • 900 MHz ISM band
      • Known as the industrial band
      • Is 26 MHz wide and spans from 902 MHz to 928 MHz
      • This band was used for wireless networking, but most now use higher frequencies  which are capable to greater throughput
      • Many parts of the 900 MHZ band are issued to Global System for Mobile Communications (GSM) for use by mobile phones
      • 802.11 radios do not operate in the 900 MHz ISM band
    • 2.4 GHz ISM band
      • Known as the scientific band
      • 2.4 GHz ISM band is 100 MHz wide and spans from 2.4 GHz to 2.5 GHz
      • The following wireless radios use this band:
        • 802.11 (FHSS radios or DSSS radios)
        • 802.11b (HR-DSSS radios)
        • 802.11g (ERP radios)
        • 802.11n (HT radios)
      • 2.4 GHz ISM band is also used by microwave ovens, cordless home telephones, baby monitors, and wireless video cameras
      • 2.4 GHz is heavily used and one of the biggest disadvantages of 802.11b/g/n 2.4 GHz radios is the potential for interference
      • Not every regulatory body will allow transition in the entire 2.4GHz ISM band
    • 5.8 GHz ISM band
      • Known as the medical band
      • 5.8 GHz ISM band is 150 MHz wide and spans from 5.725 GHz to 5.875 GHz.
      • Used by many of the same types of consumer products: baby monitors, cordless telephones, and cameras
      • Not uncommon f to confuse the 5.8 GHz ISM band with the U-NII-3 band  which spans from 5.725 GHz to 5.85 GHz
      • The United States has also always allowed OFDM transmissions on channel 165, which until April of 2014, resided in the 5.8 GHz ISM band
      • From the perspective of Wi-Fi channels, the 5.8 GHz ISM band is no longer relevant, however, many of the consumer devices that operate in the 5.8 GHz ISM band can cause RF interference with 802.11 radios that transmit in the U-NII-3 band

Unlicensed National Information Infrastructure bands (U-NII)

  • Wi-Fi radios that currently transmit in the 5 GHz U-NII bands include radios that use the following technologies
    • 802.11a (OFDM radios)
    • 802.11n (HT radios)
    • 802.11ac (VHT radios)
  • U-NII-1 (lower band)
    • 100 MHz wide and spans from 5.150 GHz to 5.250 GHz
    • Four 20 MHz 802.11 channels reside in the U-NII-1 band
    • FCC used to restrict U-NII-1 Band to indoor use – after 2004 this has been lifted
    • Prior to 2004 FCC required that Aps had permeant antennas to use this band, after 2004 allow detachable antennas, providing that the antenna connector is unique
  • U-NII-2 (middle band)
    • 100 MHz wide and spans from 5.250 GHz to 5.350 GHz.
    • Four 20 MHz 802.11 channels reside in the U-NII-2 band.
    • Radios that transmit in the U-NII-2 band must support dynamic frequency selection (DFS)
  • U-NII-2 Extended
    • 255 MHz wide and spans from 5.470 GHz to 5.725 GHz
    • Most 5 GHz 802.11 radios can transmit on a total of eleven 20 MHz 802.11 channels that reside in the U-NII-2 band
    • With 802.11ac , a new channel 144 has been added to the U-NII-2 Extended band – taking total to 12 channels
    • Radios that transmit in the U-NII-2 band must support dynamic frequency selection (DFS)
  • U-NII-3 (upper band)
    • 125 MHz wide and spans from 5.725 GHz to 5.850 GHz.
    • Typically used for outdoor point-to-point communications
    • Many of the countries in Europe do not use the U-NII-3 band for WLAN unlicensed communications
    • Five 20 MHz 802.11 channels reside in the U-NII-3 band
    • In April of 2014, the FCC expanded the size of the U-NII-3 band from 100 MHz to 125 MHz. Channel 165, formerly in the 5.8 GHz ISM band, is now available as part of the  U-NII-3 band
Band Frequency Channels
U-NII-1 (lower) 5.150 GHz–5.250 GHz 4
U-NII-2 (middle)  5.250 GHz–5.350 GHz 4
U-NII-2 Extended 5.47 GHz–5.725 GHz 12 (as channel 144 was added with 802.11ac)
U-NII-3 (upper) 5.725 GHz–5.825 GHz 4
  • Future U-NII bands
    • In Jan 2013 the FCC announced that 195 MHz of additional spectrum space would be made available for unlicenced use.
    • Two proposed U-NII bands are:
      • U-NII-2B:
        • 120 MHz wide band spans from 5.35 GHZ to 5.47 GHz
        • Six potential 20 MHz wide channels
      • U-NII-4
        • 75 MHz wide band spans from 5.85 GHZ – 5.925 GHz
        • 4 potential 20 MHz wide channels

3.6 GHz band

  • Frequency range of 3.65 GHz to 3.7 GHz.
  • Included limitations when used near certain satellite earth stations
  • Designed to operate in any 5 MHz, 10 MHz, or 20 MHz channel
  • Although the project was designed for use in the United States, it was carefully designed to be able to operate in other countries without the need to ratify a new amendment

4.9 GHz band

  • Frequency range of 4.94 GHz to 4.99 GHz in the United States for public safety organizations to use for the protection of life, health, or property.
  • Licensed band and is reserved strictly for public safety
  • 802.11j amendment was ratified, providing support for the 4.9 GHz to 5.091 GHz frequency range for use in Japan

Future Wi-Fi frequencies

  • 60 GHz
    • Speeds up to 7 Gbps
    • Difficulty penetrating through walls
    • Will not be backward compatible with other 802.11 technology
    • Wi-Fi Alliance designated the WiGig certification to test interoperability of products that operate in the 60 GHz band
  • White-Fi
    • Wi-Fi technology in the unused television RF spectrum also known as TV white space

Narrowband and spread spectrum

Chapter6-1.png

  • Two primary radio frequency (RF) transmission methods:
    • Narrowband
      • Uses very little bandwidth to transmit the data that it is carrying
      • Intentional jamming or unintentional interference of this frequency range is likely to cause disruption in the signal
    • Spread spectrum
      • Uses more bandwidth than is necessary to carry its data
      • It is typically less susceptible to intentional jamming or unintentional interference from outside sources, unless the interfering signal was also spread across the range of frequencies used by the spread spectrum communications
  • Multipath interference
    • Think of Multipath interference as an Echo of your first word arriving over the top of your second word
    • If the delay spread is too great, data from the reflected signal may interfere with the same data stream from the main signal; this is referred to as intersymbol interference (ISI).
    • Spread spectrum systems are not as susceptible to ISI because they spread their signals across a range of frequencies 802.11 (DSSS) and 802.11b (HR-DSSS) can tolerate delay spread of up to 500 nanoseconds
    • Prior to 802.11n and 802.11ac MIMO technology, multipath had always been a concern

Frequency hopping spread spectrum (FHSS)

  • works is that it transmits data by using a small frequency carrier space, then hops to another small frequency carrier space and transmits data, then to another frequency, and so on

chapter6-2

  • Hopping sequence
    • Uses a predefined sequence
    • Each time the hop sequence is completed, it is repeated
    • Original IEEE 802.11 standard mandates that each hop is 1 MHz in size
  • Dwell time
    • Amount of time that the FHSS system transmits on a specific frequency before it switches to the next frequency in the hop set
    • Regulatory bodies typically limits the amount of dwell time
  • Hop time
    • Not a specified period of time but rather a measurement of the amount of time it takes for the transmitter to change from one frequency to another
  • Modulation
    • Uses Gaussian frequency shift keying (GFSK) to encode the data

Direct sequence spread spectrum (DSSS)

  • Was defined in the original 802.11 standard to provide 1 & 2 Mbps on the 2.4GHz ISM Band
  • 802.11b 5.5 and 11 Mbps speeds are known as High-Rate DSSS (HR-DSSS)
  • 802.11b is backwards compatible with 802.11 DSSS
  • 802.11b devices are not capable of transmitting using FHSS; therefore, they are not backward compatible with 802.11 FHSS devices
  • DSSS is set to one channel. The data that is being transmitted is spread across the range of frequencies that make up the channel.
  • DSSS data encoding
    • To provide resilience against data corruption, each bit of data is encoded and transmitted as multiple bits of data
    • Task of adding additional, redundant information to the data is known as processing gain
    • The system converts the 1 bit of data into a series of bits that are referred to as chips
    • This sequence of chips is then spread across a wider frequency space. Although 1 bit of data might need only 2 MHz of frequency space, the 11 chips will require 22 MHz of frequency carrier space
    • When the Barker code is used, as many as 9 of the 11 chips can be corrupted, yet the receiving radio will still be able to interpret the sequence and convert them back into a single data bit
  • Modulation
    • Differential binary phase shift keying (DBPSK) utilizes two phase shifts, one that represents a 0 chip and another that represents a 1 chip

Packet Binary Convolutional Code (PBCC)

  • Is a modulation technique that supports data rates of 5.5, 11, 22, and 33 Mbps;
  • PBCC modulation was originally defi ned as optional under the 802.1b amendment
  • 802.11g amendment allowed for two additional optional ERP-PBCC modulation modes with payload data rates of 22 and 33 Mbps

Orthogonal Frequency Division Multiplexing (OFDM)

chapter6-3

  • One of the most popular communications technologies, used in both wired and wireless communications
  • OFDM is not a spread spectrum technology, even though it has similar properties
  • OFDM actually transmits across 52 separate, closely and precisely spaced frequencies, often referred to as subcarriers
  • Forty-eight of the subcarriers are used to transmit data
  • The other four are used as references for phase and amplitude by the demodulator, allowing the receiver to synchronize itself as it demodulates the data in the other subcarriers
  • Convolutional coding
    • To make OFDM more resistant to narrowband interference, a form of error correction known as convolutional coding is performed.
    • is a forward error correction (FEC) that allows the receiving system to detect and repair corrupted bits
  • Modulation
    • OFDM uses binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK) phase modulation for the lower ODFM data rates
    • Higher data rates use Quadrature amplitude modulation (QAM) modulation
    • Constellation diagram, also known as a constellation map, is a two dimensional diagram often used to represent QAM modulation.

chapter6-4

2.4 GHz channels

  • The IEEE 802.11-2012 standard divides the 2.4 GHz ISM band into 14 separate channels
  • Regulatory bodies determine which channels are available to be used in each country
  • Channels are designated by their centre frequency
  • DSSS and HR-DSSS 802.11 radios are transmitting, each channel is 22 MHz wide and is often referenced by the centre frequency +/- 11 MHz
  • Within the 2.4 GHz ISM band, the distance between channel centre frequencies is only 5 MHz, because of this the channels will have overlapping frequency space

chapter6-5

  • Non overlapping channels

chapter6-6

5 GHz channels

  • 5 GHz U-NII bands: U-NII-1, U-NII-2, U-NII-2 Extended, and U-NII-3.
  • The original three U-NII bands each had four non-overlapping channels with 20 MHz separation between the centre frequencies. A fifth channel was recently added to U-NII-3.
  • U-NII-2 Extended band has twelve non-overlapping channels with 20 MHz of separation between the centre frequencies (U-NII-2 Extended was 11 channels for many years until channel 144 was added with 802.11ac was released)
  • The channels that you can use are regulated by local regulatory bodies

chapter6-7

  • DFS is required in U-NII-2 and U-NII-2E channels to avoid interference with radar

chapter6-8

Adjacent, non-adjacent, and overlapping channels

  • When deploying a WLAN, it is important to have overlapping cell coverage for roaming to occur. However, it is just as important for these coverage cells not to have overlapping frequency space
  • A channel reuse pattern is needed because overlapping frequency space causes degradation in performance

Throughput vs. bandwidth

  • Frequency band is the bandwidth
  • Data encoding, modulation, medium contention, encryption, and many other factors also play a large part in data throughput
  • Care should be taken not to confuse frequency bandwidth with data bandwidth
  • OFDM 802.11a radios can transmit at 6, 9, 12, 18, 24, 36, 48, or 54 Mbps, yet the frequency bandwidth for all the U-NII band channels is the same for all of these speeds
  • Because of the half-duplex nature of the medium and the overhead generated by CSMA/CA, the actual aggregate throughput is typically 50 percent or less of the data rates for 802.11a/b/g legacy transmissions, and 60-70 percent of the data rates for 802.11n/ac transmissions.
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