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Berkeley COMPSCI 268 - L-9 Wireless

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CS 268: Computer NetworkingWireless IntroWireless ChallengesOverviewTransmission Channel ConsiderationsThe Nyquist LimitPast the Nyquist LimitCapacity of a Noisy ChannelFree Space LossCellular ReuseMultipath EffectsFading - ExampleSlide 13Medium Access ControlExample MAC ProtocolsCSMA/CD Does Not WorkMACAWFairness in MACAWMACAW: Additional DesignSlide 20RRTSMACAW: ConclusionsSlide 23IEEE 802.11 Overview802.11 particulars802.11 detailsOverview, 802.11 Architecture802.11 modes802.11 Management OperationsScanning & JoiningAssociation in 802.11Time Synchronization in 802.11Power Management in 802.11IEEE 802.11 Wireless MAC802.11 DCF (CSMA)802.11 DCF (RTS/CTS)DiscussionSlide 38Slide 39TCP Problems Over Noisy LinksConstraints & RequirementsChallenge #1: Wireless Bit-ErrorsPerformance DegradationProposed SolutionsApproach Styles (End-to-End)Approach Styles (Split Connection)Split-Connection Congestion WindowApproach Styles (Link Layer)Hybrid Approach: Snoop ProtocolSnoop OverviewSnoop Protocol: CH to MHSlide 52Slide 53Slide 54Slide 55Slide 56Slide 57Slide 58Slide 59Slide 60Slide 61Performance: FH to MHSlide 63CS 268: Computer NetworkingL-9 WirelessWireless Intro•TCP on wireless links•Wireless MAC•Assigned reading•[BPSK97] A Comparison of Mechanism for Improving TCP Performance over Wireless Links•[BDS+94] MACAW: A Media Access Protocol for Wireless LAN’s23Wireless Challenges•Force us to rethink many assumptions•Need to share airwaves rather than wire•Don’t know what hosts are involved•Host may not be using same link technology•Mobility•Other characteristics of wireless•Noisy  lots of losses•Slow•Interaction of multiple transmitters at receiver• Collisions, capture, interference•Multipath interference4Overview•Wireless Background•Wireless MAC•MACAW•802.11•Wireless TCPTransmission Channel Considerations•Every medium supports transmission in a certain frequency range.•Outside this range, effects such as attenuation, .. degrade the signal too much•Transmit and receive hardware will try to maximize the useful bandwidth in this frequency band.•Tradeoffs between cost, distance, bit rate•As technology improves, these parameters change, even for the same wire.•Thanks to our EE friends5FrequencyGood BadSignalThe Nyquist Limit•A noiseless channel of width H can at most transmit a binary signal at a rate 2 x H.•E.g. a 3000 Hz channel can transmit data at a rate of at most 6000 bits/second•Assumes binary amplitude encoding6Past the Nyquist Limit•More aggressive encoding can increase the channel bandwidth.•Example: modems•Same frequency - number of symbols per second•Symbols have more possible values7pskPsk+ AMCapacity of a Noisy Channel•Can’t add infinite symbols - you have to be able to tell them apart. This is where noise comes in.•Shannon’s theorem:•C = B x log(1 + S/N)•C: maximum capacity (bps)•B: channel bandwidth (Hz)•S/N: signal to noise ratio of the channel•Often expressed in decibels (db). 10 log(S/N).•Example:•Local loop bandwidth: 3200 Hz•Typical S/N: 1000 (30db)•What is the upper limit on capacity?•Modems: Teleco internally converts to 56kbit/s digital signal, which sets a limit on B and the S/N.89Free Space Loss Loss = Pt / Pr = (4 d)2 / (Gr Gt 2) •Loss increases quickly with distance (d2).•Need to consider the gain of the antennas at transmitter and receiver.•Loss depends on frequency: higher loss with higher frequency.•But careful: antenna gain depends on frequency too•For fixed antenna area, loss decreases with frequency•Can cause distortion of signal for wide-band signals10Cellular Reuse•Transmissions decay over distance•Spectrum can be reused in different areas•Different “LANs”•Decay is 1/R2 in free space, 1/R4 in some situationsMultipath Effects•Receiver receives multiple copies of the signal, each following a different path•Copies can either strengthen or weaken each other.•Depends on whether they are in our out of phase•Small changes in location can result in big changes in signal strength.•Short wavelengths, e.g. 2.4 GHz  12 cm•Difference in path length can cause inter-symbol interference (ISI).1112Fading - Example•Frequency of 910 MHz or wavelength of about 33 cm13Overview•Wireless Background•Wireless MAC•MACAW•802.11•Wireless TCPMedium Access Control•Think back to Ethernet MAC:•Wireless is a shared medium•Transmitters interfere•Need a way to ensure that (usually) only one person talks at a time.•Goals: Efficiency, possibly fairness1415Example MAC Protocols•Pure ALOHA•Transmit whenever a message is ready•Retransmit when ACK is not received•Slotted ALOHA•Time is divided into equal time slots•Transmit only at the beginning of a time slot•Avoid partial collisions•Increase delay, and require synchronization•Carrier Sense Multiple Access (CSMA)•Listen before transmit•Transmit only when no carrier is detected16CSMA/CD Does Not Work•Carrier sense problems•Relevant contention at the receiver, not sender•Hidden terminal•Exposed terminal•Collision detection problems•Hard to build a radio that can transmit and receive at same timeABCABCDHidden Exposed•4 design details1. Contention is at the receiver2. Congestion is location dependent3. Fairness through learning of congestion levels4. Propagate synchronization information about contention periodsMACAW17Fairness in MACAW•Channel capture in MACA•Backoff doubled every collision•Reduce backoff on success•Solution: Copy backoffs•This does not always work as wanted18MACAW: Additional Design•Multiple Stream Model•ACK (TCP transfer!)19MACAW: Additional Design•DS•Because carrier sense disabledRTSCTSDoesn’t hear CTSHears RTSDSHears DSDATA20RTSDSDATARTSRRTS•Problem:CTSRTSCannot send CTSBackoff IncreasesACKRRTSRRTS prevents P2 from respondingRTSCTSDSDATARTSRTS lostX21MACAW: Conclusions•8% extra overhead for DS and ACK•37% improvement in congestion•Future work:•Multicast support•Copying backoff2223Overview•Wireless Background•Wireless MAC•MACAW•802.11•Wireless TCP24IEEE 802.11 Overview•Adopted in 1997Defines:•MAC sublayer •MAC management protocols and services•Physical (PHY) layers•IR •FHSS•DSSS25802.11 particulars•802.11b (WiFi)•Frequency: 2.4 - 2.4835 Ghz DSSS•Modulation: DBPSK (1Mbps) / DQPSK (faster)•Orthogonal channels: 3•There are others, but they interfere.


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Berkeley COMPSCI 268 - L-9 Wireless

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