csci5211-wireless-lan-and-switching

csci5211-wireless-lan-and-switching - 802.11 Wi r el ess L...

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CSci5211: 802.11 Wireless and Bridges/ switches 1 802.11 Wireless LANs Brides and Switches 802.11 Wireless LAN LAN Bridges and Switches Readings Section 2.8 Section 3.2
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CSci5211: 802.11 Wireless and Bridges/ switches 2 IEEE 802.11 Wireless LAN 802.11b 2.4-5 GHz unlicensed radio spectrum up to 11 Mbps direct sequence spread spectrum (DSSS) in physical layer all hosts use same chipping code widely deployed, using base stations 802.11a 5-6 GHz range up to 54 Mbps 802.11g 2.4-5 GHz range up to 54 Mbps All use CSMA/ CA for multiple access All have base-station and ad-hoc network versions
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CSci5211: 802.11 Wireless and Bridges/ switches 3 Spread Spectrum Idea spread signal over wider frequency band than required originally deigned to thwart jamming Frequency Hopping transmit over random sequence of frequencies sender and receiver share… pseudorandom number generator seed 802.11 uses 79 x 1MHz-wide frequency bands
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CSci5211: 802.11 Wireless and Bridges/ switches 4 Spread Spectrum (cont) Direct Sequence for each bit, send XOR of that bit and n random bits random sequence known to both sender and receiver called n-bit chipping code 802.11 defines an 11-bit chipping code Random sequence: 0100101101011001 Data stream: 1010 XOR of the two: 1011101110101001 0 0 0 1 1 1
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CSci5211: 802.11 Wireless and Bridges/ switches 5 Base Station Approach Wireless host communicates with a base station base station = access point (AP) Basic Service Set (BSS) (a.k.a. “cell”) contains: wireless hosts access point (AP): base station BSS’s combined to form distribution system (DS)
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CSci5211: 802.11 Wireless and Bridges/ switches 6 Ad Hoc Network Approach No access point (i.e., base station) “peer-to-peer” mode wireless hosts communicate with each other to get packet from wireless host A to B may need to route through wireless hosts X,Y,Z Applications: “laptop” meeting in conference room, car interconnection of “personal” devices battlefield IETF MANET (Mobile Ad hoc Networks) working group
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CSci5211: 802.11 Wireless and Bridges/ switches 7 Wireless (802.11) MAC Similar to Ethernet Problem: hidden and exposed nodes A B C D
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CSci5211: 802.11 Wireless and Bridges/ switches 8 IEEE 802.11: Multiple Access Collision if 2 or more nodes transmit at same time CSMA makes sense: get all the bandwidth if you’re the only one transmitting shouldn’t cause a collision if you sense another transmission Collision detection doesn’t work: hidden terminal problem
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CSci5211: 802.11 Wireless and Bridges/ switches 9 IEEE 802.11 MAC Protocol: CSMA 802.11 CSMA: sender - if sense channel idle for DIFS sec. then transmit entire frame (no collision detection) -if sense channel busy then binary backoff 802.11 CSMA receiver - if received OK return ACK after SI FS (ACK is needed due to hidden terminal problem) DIFS: distributed interframe spacing SIFS: short interframe spacing NAV: network allocation vector
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CSci5211: 802.11 Wireless and Bridges/ switches 10 Collision Avoidance (CA) Mechanisms Problem:
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