With wireless lans that idea does not work well 1

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With wireless LANs that idea does not work well. 1. Problems with multipath fading of a radio signal 2. Great deal of mobility , by mobility of wireless communication. 3. base station-to-base station movement . 4. Exposed and hidden stations problems From the outside, the entire system should look like a single Ethernet. The connection between the 802.11 system and the outside world is called Portal.
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43 IEEE standard 802.11 mobile terminal access point (BS) server fixed terminal application TCP 802.11 PHY 802.11 MAC IP 802.3 MAC 802.3 PHY application TCP 802.3 PHY 802.3 MAC IP 802.11 MAC 802.11 PHY LLC infrastructure network LLC LLC Unlike cellular systems (WAN) , each LAN cell has only one channel , covering the entire available bandwidth and covering all the stations in this cell. Typically, is 11 to 54 Mbps.
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44 FDMA (a) The original bandwidths. (b) The bandwidths raised in frequency. (b) The multiplexed channel.
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45 TDMA Compared to FDMA, TDMA offers a much more flexible scheme , No certain frequency is necessary, i.e., the receiver can stay at the same frequency the whole time. Uses one frequency . Simple receivers and transmitters . Many different MAC algorithms exist. Listening to different frequency channels at the same time is quite difficult, listening to many channels separated in time means TDMA+FDMA . Almost all MAC schemes for wired networks work according to this principle ( Ethernet, Token Ring ). Classical Aloha; Slotted Aloha
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46 Random Access protocols ( Review) Ethernet A node transmits at random (no a priory among nodes). If collide ”, they retransmit at random times The random access MAC protocol specifies how to detect collisions and how to recover from them ( via delayed retransmissions ) Random access MAC protocols: (a) ALOHA (b) SLOTTED ALOHA
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47 Collision Sender A Sender B Sender C Sender A Sender B Sender C Aloha (18%) Slotted Aloha (37%) Aloha Multiple Access
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48 Motivation for a specialized MAC A B C D A B C D Radio range a b Hidden station problem Exposed station problem Two Major Problems Possible interference at B
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49 The MACA protocol C E A B D RTS C E A B D CTS A sending an RTS to B ; B responding with a CTS to A . After CTS is received, A begins transmission 1. Anyone hearing RTS must remain silent until CTS to be transmitted back to A. 2. Anyone hearing CTR must remain silent during the upcoming data transmission, whose silent time determined from CTS. 3. C is within of A , but not within of B. D is within of B but not A . E hears both control messages and must be silent until the data frame is complete.
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50 The MACAW protocol Collision can still occur : B and C could both send RTS frames to A at the same time. In the collision, an unsuccessful transmitter waits a random amount of time and tries again later .
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