Wireless Nws

Wireless Nws

1. Types

   • Mobile Ad Hoc Nw (MANET)
     • Self-configurable nw
     • Wireless link
       • Connects mobile routers and hosts
   • Sensor Nw
     • nw of many, spatially distributed devices
     • sensors
       • Monitor conditions
         • Temp.
         • Humidity
         • Pollution level
       • Transmitter
       • Small microcontroller
       • Power source
         • eg : Batteries
   • Wireless Sensor Nw (WSN)
     • mesh nw
       • small sensor nodes
         • uses RF
     • Large scale
   • Mesh Nw
     • To Route btw nodes
       • Hops from node to node until connection is established
         • Continuous Connection
         • Reconfiguration around blocked paths
   • Wireless Mesh Nw

2. Elements

   • Base station
     • as relay
       • wired nw
       • w/less hosts
   • Wireless link
     • connection
       • B.S.
       • mobiles
   • Modes
     • Infrastructure mode
       • b.s. connects *mobile* devices to *wired* nw
       • handoff:
         • mobile dev. changes B.S.
     • Ad hoc mode
       • X B.S.
         • Nodes Tx within *link coverage*
           • Nodes organise themselves -> nw
             • Forms route among nodes

3. Wireless Link Characteristics

   • Differences
     • 1) Decreased signal strength
     • 2) Interference
     • 3) Multipath propagation
   • SNR and BER
     • Given phy layer
       • increase power
         • increase SNR
           • *DECREASES* BER
     • Given SNR
       • choose phy layer
         • Meets BER req.
         • Gives highest throughput
       • SNR can change with mobility
         • dynamically adapt phy layer
           • Modulation technique, etc
   • Eg: CDMA
     • code set parititioning
     • all users
       • same freq
       • diff 'chipping' seq
         • encodes data
           • encoded signal = ori data x chipping seq
     • Benefits
       • multiple users coexist
       • transmit at the same time
       • min. interference
   • 802.11 WLAN
     • Architecture
       • BSS (Basic Service Set)
         • aka cell
         • Infrastructure mode
           • w/less hosts
           • Access Points (B.S.)
         • Ad Hoc mode
           • Hosts only
     • Scanning
       • Active
         • 1. APs broadcast : beacon frames
           • 2. Host -> Selected AP : Association Req frame
             • 3. AP -> Host : Association Resp. frame
       • Passive
         • 1. H1 broadcast : Probe Req. frame
           • 2. AP-> Host : Probe Resp. frame
             • 3. Host -> Selected AP : Associatiation Req frame
               • 4. AP -> Host : Association Resp. frame
     • CSMA
       • sense b4 tx
         • difficult to sense all collisions
           • hidden terminal
           • fading effect
           • CSMA/CA (Collision Avoidance)
             • Protocol
               • Sender
                 • 1. Sense channel
                   • a) Idle for DIFS
                     • Tx entire frame
                   • b) Busy
                     • start rand backoff time
                       • Timer counts down while channel idle
                         • Timer expires -> Transmit
                           • If no ACK, increase rand backoff time, repeat b
             • Improv : usage of RTS and CTS packets
               • 1. sender-> BS : Req-to-send (RTS) via CSMA
                 • 2. BS broadcast : Clear-to-send ( CTS )
                   • 3. CTS heard by all nodes
                     • sender : transmit data frame
                     • others : defer transmissions

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