• Objectives• Explain how nodes exc

• Objectives
• Explain how nodes exchange wireless signals
• Identify potential obstacles to successful wireless transmission and their repercussions, such as interference and reflection
• Understand WLAN (wireless LAN) architecture

• Objectives (cont’d.)
• Specify the characteristics of popular WLAN transmission methods, including 802.11 a/b/g/n
• Install and configure wireless access points and their clients
• Describe wireless WAN technologies, including 802.16 (WiMAX), HSPA+, LTE, and satellite communications
• Wireless Spectrum
• Is a continuum of the electromagnetic waves used for data and voice communication
– Waves are arranged by frequencies from lowest to highest
– Spans 9 KHz and 300 GHz
• Each type of wireless service can be associated with one area of the wireless spectrum
• Federal Communications Commission (FCC) oversees United States frequencies
– The organization responsible for the allocation, licensing, registration, and management of the RF bans not used for government and military purposes.
• ITU oversees international frequencies
– Air signals propagate across borders
• Characteristics of Wireless Transmission
• Signal transmission
• No fixed path, signals travel without guidance
• Antenna
– Used for both the transmission and reception of wireless signals
– To exchange information, two antennas must be tuned to the same frequency—this means share the same channel
• Antennas
• Radiation pattern: describes the relative strength over a three-dimensional area of all the electromagnetic energy the antenna sends or receives
• Directional antenna: transmits and receives signals to and from one general direction
– Used in a point-to-point link—i.e., satellite downlink
• Omnidirectional antenna: issues and receives wireless signals with equal strength and clarity in all directions—similar to how ripples radiate from the point where a rock is tossed into water
– Used when many different receivers must be able to pick up the signal and when the receiver’s location is highly mobile
– TV and radio stations used omnidirectional antennas, as do most cellular towers
• Antenna Range: geographical area that an antenna or wireless system can reach
• Antennas, cont.
• Directional Antennas:
– Parabolic
– Patch
– Sector
– Yagi-Uda array (Yagi)
• Omnidirectional (OD) Antennas:
– Dipole
– Domes and other wall or ceiling mounted antennas
• Signal Propagation
• LOS (line-of-sight)
– Signal travels in straight line
• Directly from transmitter to receiver
• Obstacles affect signal travel; signals may:
– Pass through them
– Be absorbed into them
– Be subject to three phenomena:
• Reflection: bounce back to source
• Diffraction: splits into secondary waves
• Scattering: diffusion or reflection in multiple different directions
• Signal Propagation (cont’d.)
• Multipath signals
– Wireless signals follow different paths to destination
– Caused by reflection, diffraction, & scattering
– Advantage
• Better chance of reaching destination
– Disadvantage
• Signal delay
• Signal Degradation
• Fading
– Variation in signal strength
• Electromagnetic energy scattered, reflected, diffracted
• Attenuation
– Signal weakens
• Moving away from transmission antenna
– Correcting signal attenuation
• Amplify (analog), repeat (digital)
• Noise
– Significant problem
• Wireless signals cannot depend on conduit or shielding to protect them from extraneous EMI (electromagnetic interference)
• Frequency Ranges
• 2.4-GHz band (older)
– Frequency range: 2.4–2.4835 GHz
– 11 unlicensed communications channels
– Susceptible to interference
• Unlicensed: no FCC registration required
• 5-GHz band (newer)
– Frequency bands
• 5.1 GHz, 5.3 GHz, 5.4 GHz, 5.8 GHz
– 24 unlicensed bands
– Used by weather and military radar communications
• WLAN equipment using this range of frequencies must be able to monitor and detect radar signals and, if one is detected, switch to a different channel automatically
• Channel
• Is a portion of a frequency range