Frame Transmission
Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) is used in 802.11 WLANs to avoid collisions.
The Distributed Coordination Function (DCF) handles the transmission of frames.
If one station is currently transmitting, a station wishing to transmit must wait for the current station to finish plus the length of the DCF Inter-frame Space (DIFS) and a random back-off timer before it may transmit.
Service Sets
Service Set Identifiers (SSIDs) are used to logically group related wireless clients.
Service set types:
Independent Basic Service Set (IBSS) – An ad-hoc network where all clients communicate directly
Basic Service Set (BSS) – Access is centralized on an access point
Extended Service Set (ESS) – An access point bridged the wireless network to a wired network
An SSID can be mapped to a VLAN on an Ethernet network.
Radio Frequency
2.4 GHz band = 2.412 – 2.484 GHz
5 GHz band = 5.150 – 5.825 GHz
Types of interference:
Reflection – Signal is reflected off an object
Refraction – Bending of a signal as it passes through material of varying density
Absorption – Signal strength weakens as it passes through an object
Scattering – A signal is reflected in many different directions
Diffraction – The bending of a signal around an object which partially blocks its path
Fresnel zones – The elliptical sphere of space which must remain clear between two line-of-sight wireless transmitters to prevent diffraction
Measurements of signal strength:
dB – Logarithmic ratio to a reference signal
dBm – Reference to a 1.0 mW signal
dBw – Reference to a 1.0 W signal
Receivers are generally rated in negative dBm, noting their sensitivity.
Antenna gain is expressed in dBi, referenced to a theoretical isotropic antenna which propagates a signal evenly in all directions.
Effective Isotropic Radiated Power (EIRP) = Tx power (dBm) + antenna gain (dBi) – cable loss (dB).
WLAN Antenna Categories
Omnidirectional – Distributes signal in a donut-shaped area; low gain
Semi-Directional – Produces an elongated but broad coverage area in one direction, includes Yagi antennas; medium gain
Directional – Directs signal to a single point; high gain
WLAN Standards
Frame Types
Management Frames – Used for service advertisement and membership management
Beacons
Client association
Client authentication
Control Frames – Control traffic flow
Probe request/response
RTS/CTS messages
Data Frames – Contain data payload
WLAN frames have a 32-byte header and 4-byte trailing checksum.
802.11b
Operates on the 14 channels within the 2.4GHz Industrial, Scientific, Medical (ISM) band.
Only channels 1, 6, and 11 are non-overlapping.
Direct Sequence Spread Spectrum (DSSS) modulation allows for varying speeds: 1.0, 2.0, 5.5, and 11.0 Mbps. Higher data rates require stronger signal strength.
DSSS speeds can be mixed among clients within an AP cell, allowing each client to transmit at its fastest potential.
802.11g
Expands upon 802.11b with greater speeds and more complex modulation.
802.11g operates on the same frequencies and channels as 802.11b.
Orthogonal Frequency Division Multiplexing (OFDM) allows for additional speeds of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps.
802.11g is backward compatible with 802.11b, but if an 802.11b client joins an 802.11g cell, all clients must fall back to 802.11b.
802.11a
Shares the same data rates and modulation techniques as 802.11g, but is not compatible with it or 802.11b.
Operates on the 5 GHz Unlicensed National Information Infrastructure (U-NII) band.
The U-NII was divided by the FCC into three smaller bands:
Lower band – 5.5 to 5.25 GHz; indoor use
Middle band – 5.25 – 5.35 GHz; indoor and outdoor use
Upper band – 5.725 – 5.825 GHz; outdoor use
Four non-overlapping channels are offered within each band (12 total).
Other Standards
802.11e – QoS for WLANs
802.11i – Security enhancements
802.11n – Improvements for higher throughput
