L21 - Wireless LANs Increasing in popularity Some LANs use...

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Wireless LANs Increasing in popularity. Some LANs use base station, some don’t. 802.11is the standard number for wireless LANs. It allows for different physical layer methods . Show Fig. 4-25
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Physical layer alternatives for 802.11: 1. Infrared - Diffused, 1 or 2 Megabits/second, isolated to a room. 2. Frequency Hopping Spread Spectrum (FHSS) 79 channels, 1 Mhz each. Hop by a pseudorandom hopping pattern known to sender and receiver. Dwell time at one frequency - adjustable up to 400 milliseconds per hop.
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Some thoughts on 79 - channel frequency hopping : If only one is sending, the data rate is slow compared to 79 mhz total bandwidth. Data rate only about 1 mb/s But if there are two or more simultaneous senders, they may occasionally collide in one or more hops. If there is one hop per packet, each collision is a packet loss. If there are multiple hops per packet, there is a high probability part of a packet will be lost. Redundancy in the packet could recover some partial loss. Extra packets could recover some lost packets. But extra packets add to the traffic.
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Physical layer alternatives for 802.11, continued: 3. Direct Sequence Spread Spectrum - 11 chips/bit, for 1 mb/s for 11 mbaud pulse rate. Also a 2 mb/s version available. 4. 802.11a - Orthogonal Frequency Division Multiplexing (OFDM) - Up to 54 megabits/second, sending simultaneously on multiple frequencies. (Operates in 5 Ghz band) 52 different frequencies, 48 for data. With fading, some frequencies could be avoided, or redundancy used to recover lost bits.
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