1 15-744: Computer Networking L-9 Wireless Wireless Intro • TCP on wireless links • Wireless MAC • Assigned reading • [BPSK97] A Comparison of Mechanism for Improving TCP Performance over Wireless Links • [BM09] In Defense of Wireless Carrier Sense • Optional • [BDS+94] MACAW: A Media Access Protocol for Wireless LAN’s 2 3 Wireless 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 interference 4 Overview • Wireless Background • Wireless MAC • MACAW • 802.11 • Wireless TCP2 Transmission Channel Considerations • Every medium supports transmission in a certain frequency range. • 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 rate • As technology improves, these parameters change, even for the same wire. • Thanks to our EE friends 5 Frequency Good Bad Signal The 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 encoding 6 Past the Nyquist Limit • More aggressive encoding can increase the channel bandwidth. • Example: modems • Same frequency - number of symbols per second • Symbols have more possible values 7 psk Psk+ AM Capacity 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. 83 9 Free 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 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 signals 10 Cellular 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 situations Multipath 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). 11 12 Fading - Example • Frequency of 910 MHz or wavelength of about 33 cm4 13 Overview • Wireless Background • Wireless MAC • MACAW • 802.11 • Wireless TCP Medium 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 fairness 14 15 Example 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 detected 16 CSMA/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 time A B C A B C D Hidden Exposed5 1-23-06 Lecture 3: Physical Layer 17 C F A B E D RTS RTS = Request-to-Send MACA (RTS/CTS) assuming a circular range 1-23-06 Lecture 3: Physical Layer 18 C F A B E D RTS RTS = Request-to-Send MACA (RTS/CTS) NAV = 10 NAV = remaining duration to keep quiet 1-23-06 Lecture 3: Physical Layer 19 C F A B E D CTS CTS = Clear-to-Send MACA (RTS/CTS) 1-23-06 Lecture 3: Physical Layer 20 C F A B E D CTS CTS = Clear-to-Send MACA (RTS/CTS) NAV = 86 1-23-06 Lecture 3: Physical Layer 21 C F A B E D DATA • DATA packet follows CTS. Successful data reception acknowledged using ACK. MACA (RTS/CTS) 1-23-06 Lecture 3: Physical Layer 22 C F A B E D MACA (RTS/CTS) Reserved area MACAW: Additional Design • ACK (needed for faster TCP transfers) • DS (needed since carrier sense disabled) RTS CTS Doesn’t hear CTS Hears RTS DS Hears DS DATA 23 RTS DS DATA RTS RRTS • Problem: CTS RTS Cannot send CTS Backoff Increases ACK RRTS RRTS prevents P2 from responding RTS CTS DS DATA RTS RTS lost X 247 MACAW: Conclusions • 8% extra overhead for DS and ACK • 37% improvement in congestion 25 26 Overview • Wireless Background • Wireless MAC • MACAW • 802.11 • Wireless TCP 27 IEEE 802.11 Overview • Adopted in 1997 Defines: • MAC sublayer • MAC management protocols and services • Physical (PHY) layers • IR • FHSS • DSSS 28 802.11 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 to 54Mbps • 802.11g: Faster, 2.4Ghz, up to 54Mbps • 802.11n: 2.4 or 5Ghz,
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