115-744: 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•[BDS+94] MACAW: A Media Access Protocol for Wireless LAN’s2Wireless Challenges• Force us to rethink many assumptions• Need to share airwaves rather than wireD’tk htht i ld•Don’t know what hosts are involved• Host may not be using same link technology• Mobility• Other characteristics of wireless• Noisy Æ lots of losses•Slow3Slow• Interaction of multiple transmitters at receiver• Collisions, capture, interference• Multipath interferenceOverview• Wireless Background• Wireless MAC• MACAW• 802.114• Wireless TCP2Transmission Channel Considerations• Every medium supports transmission in a certain frequency range.•Outside this range effects such asGood Bad•Outside this range, effects such as attenuation, .. degrade the signal too much• Transmission and receive hardware will try to maximize the useful bandwidth in this frequency band.• Tradeoffs between cost, distance, bit rateFrequencybit rate• As technology improves, these parameters change, even for the same wire.• Thanks to our EE friends5SignalThe 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 valuespsk7pskPsk+ 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:•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 Hzp• 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.83Free Space LossLoss = Pt/ Pr= (4π d)2 / (GrGtλ2) 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 too9•But careful: antenna gain depends on frequency too• For fixed antenna area, loss decreases with frequency• Can cause distortion of signal for wide-band signalsCellular Reuse• Transmissions decay over distance• Spectrum can be reused in different areas• Different “LANs”• Decay is 1/R2in free space, 1/R4in some situations10Multipath 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 strengthstrength.• Short wavelengths, e.g. 2.4 GHz Æ 12 cm• Difference in path length can cause inter-symbol interference (ISI).11Fading - Example12• Frequency of 910 MHz or wavelength of about 33 cm4Overview• Wireless Background• Wireless MAC• MACAW• 802.1113• 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 fairness14Example 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 synchronization15• Carrier Sense Multiple Access (CSMA)• Listen before transmit• Transmit only when no carrier is detectedCSMA/CD Does Not Work• Carrier sense problems•Relevant contentionHidden Exposed•Relevant contention at the receiver, not sender• Hidden terminal• Exposed terminal• Collision detection problemsABCABC16problems• Hard to build a radio that can transmit and receive at same timeCD5• 4 design details1. Contention is at the receiver2CiiliddMACAW2.Congestion is location dependent3. Fairness through learning of congestion levels4. Propagate synchronization information about contention periods17Fairness in MACAW• Channel capture in MACA• Backoff doubled every collisionRdbkff•Reduce backoffon success• Solution: Copy backoffs• This does not always work as wanted18MACAW: Additional Design• Multiple Stream Model•ACK19MACAW: Additional Design•DS• Because carrier sense disabledRTSCTSDoesn’t hear CTSHears RTSDSHears DSDATA206RTSRRTS• Problem:ACKRTSDSDATARTSCTSRTSCannot send CTSBackoff IncreasesACKRRTSRRTS prevents P2 from respondRTSDSDATARTSXRTSCTSDSDATARTSRTS lostX21MACAW: Conclusions• 8% extra overhead for DS and ACK• 37% improvement in congestionpg• Future work:Mlti t t•Multicast support• Copying backoff22Overview• Wireless Background• Wireless MAC• MACAW• 802.1123• Wireless TCPIEEE 802.11 Overview• Adopted in 1997Defines:• MAC sublayer• MAC management protocols and services• Physical (PHY) layers•IR 24•FHSS• DSSS7802.11 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 to25•802.11a: Faster, 5Ghz OFDM. Up to 54Mbps• 802.11g: Faster, 2.4Ghz, up to 54Mbps802.11 details• Fragmentation• 802.11 can fragment large packets (this is separate from IP fragmentation)from IP fragmentation).• Preamble• 72 bits @ 1Mbps, 48 bits @ 2Mbps• Note the relatively high per-packet overhead.• Control framesRTS/CTS/ACK/ t26•RTS/CTS/ACK/etc.• Management frames• Association request, beacons, authentication, etc. Overview, 802.11 ArchitectureESSSTASTASTASTASTA STAAPAPBSSBSSExisting Wired LANInfrastructure Network27STASTASTASTABSS BSSAd Hoc NetworkAd Hoc NetworkBSS: Basic Service SetESS: Extended Service Set802.11 modes• Infrastructure mode• All packets go through a
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