WirelessLAN

WirelessLAN - Elements of a wireless network wireless hosts...

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1 Csci 232: Wireless LAN 1 Elements of a wireless network network infrastructure wireless hosts ± laptop, PDA, IP phone ± run applications ± may be stationary (non-mobile) or mobile ² wireless does not always mean mobility Csci 232: Wireless LAN 2 Elements of a wireless network network infrastructure base station ± typically connected to wired network ± relay - responsible for sending packets between wired network and wireless host(s) in its “area” ² e.g., cell towers for cellular networks, 802.11 access points Csci 232: Wireless LAN 3 Elements of a wireless network network infrastructure wireless link ± typically used to connect mobile(s) to base station ± also used as backbone link (WiMAX) ± multiple access protocol coordinates link access ± various data rates, transmission distance Csci 232: Wireless LAN 4 Elements of a wireless network network infrastructure infrastructure mode ± base station connects mobiles into wired network ± handoff: mobile changes base station providing connection into wired network Csci 232: Wireless LAN 5 Elements of a wireless network Ad hoc mode ± no base stations ± nodes can only transmit to other nodes within link coverage ± nodes organize themselves into a network: route among themselves Csci 232: Wireless LAN 6 Wireless Link Characteristics Differences from wired link …. ² decreased signal strength: radio signal attenuates as it propagates through media (path loss) ² interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well ² multipath propagation: radio signal reflects off objects, ground, arriving at destination at slightly different times …. make communication across (even a point to point) wireless link much more “difficult”
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2 Csci 232: Wireless LAN 7 Wireless network characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): A B C Hidden terminal problem ± B, A hear each other ± B, C hear each other ± A, C can not hear each other means A, C unaware of their interference at B A B C A’s signal strength space C’s signal strength Signal fading: ± B, A hear each other ± B, C hear each other ± A, C can not hear each other interferring at B Csci 232: Wireless LAN 8 Special challenge for Wireless MAC ± Collision detection is difficult • Transmission power is much higher than receiving power ± Hidden station problem • A station is not able to detect a potential competitor for the medium because the competitor is too far away • Example: A is transmitting to B. C cannot hear the transmission, thus falsely concludes that it can transmit. ± Exposed station problem • A station hears the on-going transmission, and falsely assume it cannot transmit • Example: B is transmitting to A. C hears the transmission, thus falsely concludes that it cannot transmit to D.
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WirelessLAN - Elements of a wireless network wireless hosts...

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