08-11-2012, 05:35 PM
Wireless Networking Technologies WLAN, WiFi Mesh and WiMAX
Wireless Networking.ppt (Size: 7.09 MB / Downloads: 222)
Wireless networks
Access computing/communication services, on the move
Wireless WANs
Cellular Networks: GSM, GPRS, CDMA
Satellite Networks: Iridium
Wireless LANs
WiFi Networks: 802.11
Personal Area Networks: Bluetooth
Wireless MANs
WiMaX Networks: 802.16
Mesh Networks: Multi-hop WiFi
Adhoc Networks: useful when infrastructure not available
Limitations of the mobile environment
Limitations of the Wireless Network
limited communication bandwidth
frequent disconnections
heterogeneity of fragmented networks
Limitations Imposed by Mobility
route breakages
lack of mobility awareness by system/applications
Limitations of the Mobile Device
short battery lifetime
limited capacities
Wireless v/s Wired networks
Regulations of frequencies
Limited availability, coordination is required
useful frequencies are almost all occupied
Bandwidth and delays
Low transmission rates
few Kbits/s to some Mbit/s.
Higher delays
several hundred milliseconds
Higher loss rates
susceptible to interference, e.g., engines, lightning
Always shared medium
Lower security, simpler active attacking
radio interface accessible for everyone
secure access mechanisms important
Factors affecting wireless system design
Frequency allocations
What range to operate? May need licenses.
Multiple access mechanism
How do users share the medium without interfering?
Antennas and propagation
What distances? Possible channel errors introduced.
Signals encoding
How to improve the data rate?
Error correction
How to ensure that bandwidth is not wasted?
Frequencies for mobile communication
VHF-/UHF-ranges for mobile radio
simple, small antenna for cars
deterministic propagation characteristics, reliable connections
SHF and higher for directed radio links, satellite communication
small antenna, focusing
large bandwidth available
Wireless LANs use frequencies in UHF to SHF spectrum
some systems planned up to EHF
limitations due to absorption by water and oxygen molecules (resonance frequencies)
weather dependent fading, signal loss caused by heavy rainfall etc.
Antennas: isotropic radiator
Radiation and reception of electromagnetic waves, coupling of wires to space for radio transmission
Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference antenna
Real antennas always have directive effects (vertically and/or horizontally)
Radiation pattern: measurement of radiation around an antenna