Chap 2 Radio Frequency Fundamentals.pptx

CMPN 371 Chapter Two Radio Frequency Fundamentals 2 Objectives CSMA/CA vs. CSMA/CD Radio frequency characteristics Radio frequency dispersion and loss CSMA/CA vs. CSMA/CD Ethernet networks (IEEE 802.3) use a form of collision management known as collision detection (CD). Wireless networks use a different form of collision management known as collision avoidance (CA). 3 CSMA/CA vs. CSMA/CD The carrier sense (CS) in CSMA means that the devices will attempt to sense whether the physical medium is available before communicating. The multiple access (MA) indicates that multiple devices will be accessing the physical medium. In a CD implementation of CSMA, when a collision is detected, both devices go silent for a pseudo-random period of time. In WLANS CD won’t work so we have to go to CA (Collision Avoidance) 4 “As a WLAN engineer, it is important to have an understanding of RF propagation behaviors so access points are deployed in the proper location, to make sure the proper type of antenna is chosen, and for monitoring the health of the wireless network.” 5 Power of the signal received depends on 3 factors Path loss Dissipation of radiated power; depends on the distance Shadowing Caused by the obstacles between the transmitter and the receiver Attenuates the signal absorption, reflection, scattering, diffraction Multipath Constructive and destructive addition of multiple signal components 6 Radio Frequency Characteristics Wavelength Frequency Amplitude Phase 7 Frequency Dispersion 9 Absorption 10 Reflection 11 Scattering 12 Refraction As signal travels through an object 13 Diffraction Object in the path of the signal 14 In combination Attenuation Strength of signal falls off with distance over transmission medium Attenuation factors for unguided media: Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal Signal must maintain a level sufficiently higher than noise to be received without error Attenuation is greate