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OECG – Chapter 22

745 Words. Plan about 4 minute(s) to read this.

The budding CCIE needs to understand the magic of radio waves to properly deploy wireless LANs.  Any people studying for their ham radio license “back in the day” will be familiar with much of this information.

Modulation

  • RF signals are analog, not digital.
  • A radio must turn digital data into analog radio waves, and back again.  This process is modulation/demodulation.
  • You can modulate an RF carrier signal by tweaking with amplitude, frequency or phase.
  • 802.11b modulates using phase shift keying (PSK).
  • 802.11a and g modulate using quadrature amplitude modulation (QAM).

RF Signal Characteristics

  • RF signals continually vary in time.  Frequency is the numbers of these cycles happening per second.
  • Bandwidth is modulating an RF signal, causing it to occupy a portion of frequency spectrum.
  • Amplitude is generally the RF signal’s power, often represented in watts.  The greater the signal amplitude the further you can go.
  • Because regulated wireless LANs speak so softly (milliwatts, usually), watts are an ineffective way of measuring power. Watts are then converted to dBm. dBm is a logarithmic value that references the signal power to 1mW.
  • dBm = 10 log (mW), ergo, 100mW = 20dBm.

Gain

  • The signal boost – the signal level at the device output, minus the signal level at the device input.
  • Amplifiers and antennas offer varying amounts of gain.
  • Attenuation is the opposite of gain.

Signal-To-Noise Ratio (SNR)

  • The difference between the signal and surrounding ambient noise level on a given RF channel.
  • > 40dBm – wonderful, heaven on earth, choirs of angels sing as you browse with reckless abandon.
  • 25-40 dBm – good, always connected to the AP, some of the angels go on strike, but life is good.
  • 15-25 dBm – not great, but always connected to the AP.  Browsing sluggish.  All angels have gone home, but you’re making your way.
  • 10-15 dBm – sometimes dropping from the AP.  Browsing sucks.  Demons poke at you with pitchforks.
  • 5-10 dBm – DOH!  No connection to the AP.  Too noisy.  You stare into the gates of Hades.  Satan beckons.

Spread Spectrum

  • Equipment using license-free spectrum must spread the signal out over a wide chunk to limit the likelihood of interfering with another user.
  • Frequency hopping spread spectrum (FHSS) – assuming a 2.4GHz band, in a coordinated fashion, senders and receivers tune across an 84MHz band in 2MHz channel increments, thus hopping from frequency to frequency.  This allows for data rates of 1 to 2 Mbps.
  • Direct sequence spread spectrum (DSSS) – uses a coding technique.  A chipping code represents a bit requiring transmission.  The signal rate is then increased by the number of bits in the chipping code, 11 total.  This process will spread the signal across the spectrum, in a 30MHz band.  This is the process used by 802.11b to increase data rates to 11Mbps.

Orthogonal Frequency Division Multiplexing (OFDM)

  • Not spread-spectrum.  Used by 802.11a and g.
  • OFDM divides a signal into 48 subcarriers in a 20-MHz channel.  This results in transmissions of 6, 9, 12, 18, 24, 36, 48, or 54 Mbps.
  • Data rates of 6, 12 and 24 Mbps are all required for 802.11 compliance.
  • OFDM is immune to multipath propagation, which can cause performance problems in spread-spectrum.
  • 802.11a uses various modulation types, dependent on the data rate.  6Mbps will use binary phase shift keying, whereas 54Mbps uses quadrature amplitude modulation.

FCC Rules

  • Effective Isotropic Radiated Power (EIRP) is used by the FCC to determine that a wireless LAN is compliant.
  • EIRP for 802.11b/g radio cards and APs can be up to 36dBm (transmitting at 30dBm, with a 6dBi antenna gain).

RF Interference

  • Interference corrupts wireless data frames, causing retransmissions.  This reduces throughput.
  • In the 2.4GHz band, there’s interference from all sorts of things, such as microwave ovens, cordless phones, Bluetooth, and other wireless networks.
  • Consider replacing a 2.4GHz phone with a 900MHz or 5.8GHz phone to reduce wireless LAN interference.

Multipath

  • This is when parts of the RF signal arrive at the destination at different times.  Some of the signal might have travels straight through, while other parts bounced off of objects in the way.
  • Multipath situations cause “intersymbol interference” (ISI), where the 802.11 receivers sees overlapping signals.  In a worst case scenario, the receiver won’t be able to sort out the overlap, forcing a retransmission.
  • Metal surfaces will act as a reflective surface for RF signals, causing multipath issues.
  • Spatial diversity antennas (ergo, 2 of them) can help with multipath problems.  An AP will receive RF on both antennas, and can do a calculation to optimize the signal, since the signal won’t arrive at both antennas at exactly the same time.