CS 268: Computer NetworkingL-9 WirelessWireless Intro• TCP on wireless links• Wireless MAC• Assigned reading• [BPSK97] A Comparison of Mechanism forImproving TCP Performance over WirelessLinks• [BDS+94] MACAW: A Media Access Protocolfor 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 supportstransmission in a certainfrequency range.• Outside this range, effects such asattenuation, .. degrade the signaltoo much• Transmit and receive hardwarewill try to maximize the usefulbandwidth in this frequencyband.• Tradeoffs between cost, distance,bit rate• As technology improves, theseparameters change, even for thesame wire.• Thanks to our EE friends5FrequencyGood BadSignalThe Nyquist Limit• A noiseless channel of width H can at mosttransmit a binary signal at a rate 2 x H.• E.g. a 3000 Hz channel can transmit data at arate of at most 6000 bits/second• Assumes binary amplitude encoding6Past the Nyquist Limit• More aggressive encoding can increase thechannel 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 themapart. 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 alimit on B and the S/N.89Free Space Loss Loss = Pt / Pr = (4π d)2 / (Gr Gt λ2)• Loss increases quickly with distance (d2).• Need to consider the gain of the antennas attransmitter and receiver.• Loss depends on frequency: higher loss withhigher 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 somesituationsMultipath Effects• Receiver receives multiplecopies of the signal, eachfollowing a different path• Copies can either strengthen orweaken each other.• Depends on whether they are inour out of phase• Small changes in location canresult in big changes in signalstrength.• Short wavelengths, e.g. 2.4 GHz 12 cm• Difference in path length cancause inter-symbol interference(ISI).1112Fading - Example• Frequency of 910 MHz or wavelength ofabout 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 oneperson 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 senseproblems• Relevant contentionat the receiver, notsender• Hidden terminal• Exposed terminal• Collision detectionproblems• Hard to build a radiothat can transmit andreceive at same timeABCABCDHidden Exposed• 4 design detailsContention is at the receiverCongestion is location dependentFairness through learning of congestion levelsPropagate synchronization information aboutcontention 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. (!)• Rates: 1, 2, 5.5, 11 Mbps• 802.11a: Faster, 5Ghz OFDM. Up to54Mbps• 802.11g: Faster, 2.4Ghz, up to 54Mbps26802.11 details• Fragmentation• 802.11 can fragment large packets (this is separatefrom IP fragmentation).• Preamble• 72 bits @ 1Mbps, 48 bits @ 2Mbps• Note the relatively high per-packet overhead.• Control frames• RTS/CTS/ACK/etc.• Management frames• Association request, beacons, authentication, etc.27Overview, 802.11 ArchitectureSTASTASTA STASTASTASTA STAAPAPESSBSSBSSBSSBSSExistingWired LANInfrastructureNetworkAd HocNetworkAd HocNetworkBSS: Basic Service SetESS: Extended Service Set28802.11 modes• Infrastructure mode• All packets go through a base station• Cards associate with a BSS (basic service set)• Multiple BSSs can be linked into an Extended ServiceSet (ESS)• Handoff to new BSS in ESS is pretty quick• Wandering around Soda and Cory Halls• Moving to new ESS is slower, may require re-addressing• Wandering from EECS networks to Airbears network• Ad Hoc mode•
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