Wireless Intro : Computer Networking. Wireless Challenges. Overview

Transcription

1 Wireless Intro : Computer Networking L-17 Wireless Overview TCP on wireless links Wireless MAC Assigned reading [BM09] In Defense of Wireless Carrier Sense [BAB+05] Roofnet (2 sections) Optional [BPSK97] A Comparison of Mechanism for Improving TCP Performance over Wireless Links (2 sections) [BDS+94] MACAW: A Media Access Protocol for Wireless LAN s 2 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 Cannot ignore the physical layer Lack of control over signal propagation channel Noisy lots of losses Distortion of signal due to multi-path, Doppler shifts,.. Interaction of multiple transmitters at receiver Collisions, capture, interference Very dynamic channels -> links Overview Wireless Overview Review Wireless MAC MACAW Carrier sense Roofnet 3 4 1

2 The Frequency Domain Signal = Sum of Sine Waves A (periodic) signal can be viewed as a sum of sine waves of different strengths. Corresponds to energy at a certain frequency Every signal has an equivalent representation in the frequency domain. What frequencies are present and what is their strength (energy) Again: Similar to radio and TV signals. = Amplitude X X Time Frequency X Transmission Channel Considerations The Nyquist Limit 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 Attenuation Good Frequency Bad 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 Signal 7 8 2

3 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 psk Psk + AM Capacity of a Noisy Channel Cannot 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 Free Space Loss Loss = P t / P r = (4 d) 2 / (G r G t 2 ) Loss increases quickly with distance (d 2 ). Path loss exponents is often higher 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 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)

4 Fading - Example What is the Bottom Line? Frequency of 910 MHz or wavelength of about 33 cm 13 Packet success rate depends on SINR at the receiver There are other factors Increasing transmit power increases chances of reception Or reduce interference Signal SINR = Noise + Interference SINR (db) Effect of Interference Experiment with two transmitters that are hidden from each other No carrier sense Variable-power transmitter fights with fixed-power interferer Results show strong packet capture: stronger transmitter wins Validates SINR model Interferer Source 15 Cellular Reuse Transmissions decay over distance Spectrum can be reused in different areas Different LANs Decay is 1/R 2 in free space, 1/R 4 in some situations 16 4

5 Different Ways of Controlling Access to Bands Higher Frequency Broadcast TV Wi-Fi (ISM) In licensed spectrum, users need a license to use that part of the spectrum Cellular, radio/tv, various federal agencies License typically provides exclusive use, i.e. license holder has full control over spectrum use in the band Commercial entities often pay for the license, e.g. through an auction In unlicensed spectrum, no user license required Various constraints are placed on the radio to improve coexistence between users E.g. transmit power, modulation, MAC, Devices must be licensed 17 How Do They Differ? In licensed spectrum, licensee has a lot of control Selects and operates the equipment Can manage network to meet quality standards E.g. control interference, admission control, In unlicensed spectrum, nobody is in control Heterogeneous technologies, e.g. WiFi, Bluetooth, cordless phones,.. Unmanaged equipment, deployments, Users do not understand technology Much more difficult to optimize performance! Overview Wireless Overview Review Wireless MAC MACAW Carrier sense Roofnet

6 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 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 Ethernet also has Collision Detection (CD) 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 Hidden A B C Exposed A B C D MACA (RTS/CTS) RTS = Request-to-Send RTS A B C D E F assuming a circular range

7 MACA (RTS/CTS) MACA (RTS/CTS) RTS = Request-to-Send CTS = Clear-to-Send RTS A B C D E F CTS A B C D E F NAV = 10 NAV = remaining duration to keep quiet MACA (RTS/CTS) MACA (RTS/CTS) DATApacket follows CTS. Successful data reception acknowledged using ACK. CTS = Clear-to-Send CTS A B C D E F DATA A B C D E F NAV =

8 MACA (RTS/CTS) Overview, Architecture Reserved area ESS AP Existing Wired LAN AP A B C D E F Infrastructure Network BSS BSS Ad Hoc Network BSS BSS Ad Hoc Network BSS: Basic Service Set ESS: Extended Service Set 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 Service Set (ESS) Handoff to new BSS in ESS is pretty quick Wandering around CMU Moving to new ESS is slower, may require readdressing Wandering from CMU to Pitt Ad Hoc mode Cards communicate directly. Perform some, but not all, of the AP functions Flavors b (WiFi) Frequency: Ghz DSSS Modulation: DBPSK (1Mbps) / DQPSK (faster) Orthogonal channels: 3 There are others, but they interfere. (!) Rates: 1, 2, 5.5, 11 Mbps a: Faster, 5Ghz OFDM. Up to 54Mbps, 19+ channels g: Faster, 2.4Ghz, up to 54Mbps n: 2.4 or 5Ghz, multiple antennas (MIMO), up to 450Mbps (for 3x3 antenna configuration)

9 Management Operations Scanning: looking for APs Can be active or passive Association/Reassociation Register with an AP why needed? Time synchronization Need to keep clocks in sync - controlled by AP Power management Allow client radios to sleep to save battery power Power Management in A station is in one of the three states Transmitter on Receiver on Both transmitter and receiver off (dozing) AP buffers packets for dozing stations AP announces which stations have frames buffered in its Beacon frames Dozing stations wake up to listen to the beacons If there is data buffered for it, it sends a poll frame to get the buffered data DCF ([RTS/CTS/]Data/ACK) Discussion RTS/CTS/Data/ACK vs. Data/ACK Why/when is it useful? What is the right choice Why is RTS/CTS not used?

10 Rate Adaptation Transmit Rate Selection spec specifies rates but not the algorithm for choosing a rate b 4 rates, a 8 rates, g 12 rates Each rate has different modulation and coding Transmission Rate Transmission Rate then Loss Ratio then Capacity Utilization throughput decreases either way need to get it just right Many algorithms proposed (more later) 47 Static Channel Mobile Channel Pedestrian 11 Mbps 54 Mbps Lower signal rates enable coverage of large additional area 5.5 Mbps 2 Mbps 1 Mbps 48 Mbps 36 Mbps 24 Mbps 18 Mbps 11 Mbps 5.5 Mbps 2 Mbps 1 Mbps

11 Overview Carrier Sense Wireless Overview Review Wireless MAC MACAW Carrier sense Roofnet Maybe Carrier Sense is Fine? Single Receiver, Sender and Interferer Assumes bitrate adaptation Based on Shannon s law Can we pick a carrier sense threshold that will have the right outcome? Note that it is easiest if a fixed threshold can be used

12 Interferer Position ABR Helps in Disagreements Carrier Sense + ABR Works Well Key Assumptions ABR == Shannon ABR is rarely this good Interference and ABR are both stable Interference may be bursty/intermittent

13 Overview Wireless Overview Review Wireless MAC MACAW Carrier sense Roofnet: presentation David W