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  1. CS434/534: Topics in Networked (Networking) SystemsWireless Foundation: Wireless MAC; 802.11 MAC; New DesignsYang (Richard) YangComputer Science DepartmentYale University208A WatsonEmail: yry@cs.yale.eduhttp://zoo.cs.yale.edu/classes/cs434/

  2. Admin. • PS2 questions • Please make appointments with me to discuss potential projects

  3. Recap: Wireless PHY

  4. Recap: Link Layer Services • Framing • separate bits into frames; each frame has header, trailer and error detection/correction • Multiplexing/demultiplexing • use frame headers to identify src, dest • Media access control • implement sharing of wireless resources • Reliable delivery between adjacent nodes • seldom used on low bit error link (fiber, some twisted pair) • common for wireless links: high error rates

  5. Recap: MAC Protocols Taxonomy • Channel partitioning • SDMA, TDMA, FDMA and CDMA are basic mediapartitioning techniques • divide media into smaller “pieces” (space, time slots, frequencies, codes) for multiple transmissions to share • Non-channel partitioning • random access • “taking-turns”

  6. call setup from an MS RACH (request signaling channel)Slotted Aloha SDCCH (request call setup) AGCH (assign signaling channel) SDCCH (assign TCH) Recap: Aloha Protocol as a Random Access Protocol BTS MS A SDCCH message exchange Communication B

  7. Recap: Slotted Aloha • Advantages • Simple, decentralized random access protocol • Issues • Low efficiency • Only ~37% at optimal transmission rate • Even lower efficiency at non-optimal (fixed p) • No rate allocation/fairness

  8. Recap: Ethernet Fix for Efficiency • Introduce carrier sense (CS): do not interrupt others • Introduce collision detection (CD): instead of wasting the whole frame transmission time (a slot), we waste only the time needed to detect collision. • Introduce adaptive probability using exponential backoff(EB): reduce probability #collisions increases • If more collisions => p is high => should reduce p P: packet size, C: contention window C C C P

  9. Ethernet Fix: Carrier-Sense Multiple Access /Collision Detection/Exponential Backoff Carrier sense The Ethernet algorithm • get a frame from upper layer; • K := 0; n := 0; // K: control wait time; n: no. of collisions • repeat: • wait for K * 512 bit-time; • while (network busy) wait; • wait for 96 bit-time after detecting no signal; • transmit and detect collision; • if detect collision • stop and transmit a 48-bit jam signal; • n ++; • m:= min(n, 10), where n is the number of collisions • choose K randomly from {0, 1, 2, …, 2m-1}. • if n < 16 goto repeat • else give up • else • declare success Detect Collision Exponential backup Q: Does Ethernet alg work well in wireless?

  10. Outline • Recap • Wireless background • Frequency domain • Modulation and demodulation • Wireless channels • Wireless PHY design • Wireless MAC design • wireless access problem and taxonomy • wireless resource partitioning dimensions • media access protocols • ALOHA protocol • The Ethernet protocol • Hidden terminals in wireless

  11. The Hidden Terminal Problem • A is sending to B, but C cannot detect the transmission • Therefore C sends to B • In summary, A is “hidden” from C E D A B C

  12. CSMA/CD + Hidden Terminals • get a frame from upper layer; • K := 0; n := 0; // K: control wait time; n: no. of collisions • repeat: • wait for K * 512 bit-time; • while (network busy) wait; • wait for 96 bit-time after detecting no signal; • transmit and detect collision; • if detect collision • stop and transmit a 48-bit jam signal; • n ++; • m:= min(n, 10), where n is the number of collisions • choose K randomly from {0, 1, 2, …, 2m-1}. • if n < 16 goto repeat • else give up • else • declare success Hidden terminals => 0 goodput! Q: what is the outcome of CSMA/CD + hidden terminals,assume two senders with infinite backlog?

  13. Hidden Terminals • Why cannot senders C and A detect collisions or potential collisions? • Collision is spatially dependent • C/A is at a different location than B • Only receiver can detect a collision happened or potential collisions A B C

  14. Roadmap: Wireless MAC • Problem: single shared medium, hence if two transmissions overlap on all dimensions [time, space, frequency, and code], then it is a collision +CS+CD+EB Slotted ALOHA Ethernet ? Hidden-terminalCollision detection/prevention

  15. Solution I: Receiver Notifies Collision Happened • Solution: receiver sends ACK to sender to indicate a collision happened or not • If no ACK from receiver, sender assumes a collision

  16. Solution II: Receiver Signals Potential Collision • Receiver sends busy-tone • Used in CDPD (cellular digital packet data) • The base station sends a busy tone on the down link when receiving data

  17. DATA RTS CTS CTS Solution III: Receiver Signals Potential Collision Using Virtual Carrier Sense/ACK • Short signaling packets (virtual carrier sense) • Sender: RTS (request to send) • Receiver: CTS (clear to send) • contain sender address, receiver address, transmission duration, called network allocation vector (NAV) • A node keeps quiet for NAV in CTS A B C D

  18. Comparisons: Media Access Techniques Handling Hidden Terminals • Slotted Aloha • very simple to implement but low efficiency • CSMA/CD (Ethernet alg.) • hidden terminals can cause 0 goodput • CSMA/CD + ACK • simple to implement • low efficiency (CD is not effective)

  19. Comparisons: Media Access Techniques Handling Hidden Terminals • Busy tone • simple to implement but need a channel for busy signal • Virtual carrier sensing (RTS/CTS) • higher efficiency when a collision occurs (not waste the whole duration) • But energy consumption can be high because a node needs to monitor the environment all the time • Idle:receive:send: 1:1.05:1.4 [Stemm and Katz]; Digitan 2 Mbps WLAN 1:2:2.5 • many measurements show that overhead hurts performance

  20. Outline • Recap • Wireless background • Frequency domain • Modulation and demodulation • Wireless channels • Wireless PHY design • Wireless MAC design • wireless access problem and taxonomy • wireless resource partitioning dimensions • media access protocols • ALOHA protocol • The Ethernet protocol • Hidden terminals in wireless • IEEE 802.11

  21. IEEE 802.11 (Requirements) • Design for small coverage (e.g. office, home) • Use un-licensed spectrum • High data-rate applications • Ability to integrate real time applications and non-real-time applications (implications?)

  22. Portal Distribution System 802.11: Infrastructure Mode • Components • networks station (STA) • terminal with access mechanisms to the wireless medium and radio contact to the access point • access point (AP) • station integrated into the wireless LAN and the distribution system • basic service set (BSS) • group of stations using the same AP • portal • bridge to other (wired) networks • distribution system • interconnection network to form one logical network (EES: Extended Service Set) based on several BSS 802.11 LAN 802.x LAN STA1 BSS1 Access Point Access Point ESS BSS2 STA2 STA3 802.11 LAN

  23. The IEEE 802.11 Family

  24. 802.11b PHY Format Long preamble & header (192 usec; or optional 96 short version) Preamble & header always transmitted at 1Mbps DBPSK Preamble - Sync: alternating 0s and 1s (DSSS 128 bits) - SFD: Start Frame delimiter: 0000 1100 1011 1101 PLCH (Phsical Layer Convergence Procedure) Header - SIGNAL: the rate info - SERVICE: mostly future use - payload length - CRC: 16 bit protection of header

  25. 802.11 – MAC Format bytes 2 2 6 6 6 2 6 0-2312 4 Frame Control Duration/ ID Address 1 Address 2 Address 3 Sequence number Address 4 Data CRC bits 1 1 1 1 1 1 2 2 4 1 1 Protocol version Type Subtype To DS From DS More Frag Retry Power Mgmt More Data WEP Order • Types • control frames, management frames, data frames • Sequence numbers • important against duplicated frames due to lost ACKs • Addresses • receiver, transmitter (physical), BSS identifier, sender (logical) • Miscellaneous • sending time, checksum, frame control, data

  26. 802.11 – MAC • Asynchronous Data Service (ADS) • Objective: exchange data based on distributed random access • Approach: Implement ADS using distributed coordinate function (DCF): • DCF CSMA/CA (mandatory) • - collision avoidance via randomized “back-off“ • ACK packet for acknowledgements/detection • DCF w/ RTS/CTS (optional) • additional virtual “carrier sensing • Time-Bounded Service (TBS) • Objective: Exchange data with bounded delay service • Approach: implemented TBS using point (access point) coordinated function (PCF)

  27. 802.11 ADS/DCF: CSMA/CA • CSMA: Listen before transmit • Collision avoidance • when transmitting a packet, choose a backoff interval in the range [0, CW] • CW is contention window • Count down the backoff interval when medium is idle • count-down is suspended if medium becomes busy • Transmit when backoff interval reaches 0

  28. B1 = 25 wait data data wait B2 = 10 B2 = 20 802.11 ADS/DCF: CSMA/CA Example busy B1 = 5 B2 = 15 busy B1 and B2 are backoff intervals at nodes 1 and 2 cw = 31

  29. 802.11 ADS/DCF: CSMA/CA Backoff • IEEE 802.11 contention window CWis adapted dynamically depending on collision occurrence • after each collision, CW is doubled • thus CW varies from CWmin to CWmax

  30. 802.11 ADS/DCF: RTS/CTS + ACK • Sender sends RTS with NAV (Network allocation Vector, i.e. reservation parameter that determines amount of time the data packet needs the medium) • Receiver acknowledges via CTS (if ready to receive) • CTS reserves channel for sender, notifying possibly hidden stations • Sender can now send data at once, acknowledgement via ACK • Other stations store NAV distributed via RTS and CTS RTS data sender CTS ACK receiver NAV (RTS) data other stations NAV (CTS) t defer access new contention

  31. 802.11 TBS/PCF: Polling (Infrastructure Mode) D D point coordinator U polled wireless stations NAV NAV contention free period t medium busy contention period D: downstream poll, or data from point coordinator U: data from polled wireless station

  32. DIFS DIFS PIFS SIFS medium busy contention next frame t Integrating PCF and DCF • Using different inter frame spacing values to implement priority • SIFS (Short Inter Frame Spacing) • highest priority, for ACK, CTS, polling response • PIFS (PCF IFS) • medium priority, for time-bounded service using PCF • DIFS (DCF, Distributed Coordination Function IFS) • lowest priority, for asynchronous data service direct access if medium is free  DIFS

  33. 802.11 – Inter Frame Spacing

  34. Example: RTS-CTS-data-ACK DIFS RTS data sender SIFS SIFS SIFS CTS ACK receiver DIFS NAV (RTS) data other stations NAV (CTS) t defer access newcontention

  35. Example: PIFS SIFS PIFS D D point coordinator SIFS U polled wireless stations NAV NAV contention free period t medium busy contention period D: downstream poll, or data from point coordinator U: data from polled wireless station

  36. Outline • Recap • Wireless background • Frequency domain • Modulation and demodulation • Wireless channels • Wireless PHY design • Wireless MAC design • wireless access problem and taxonomy • wireless resource partitioning dimensions • media access protocols • ALOHA protocol • The Ethernet protocol • Hidden terminals in wireless • IEEE 802.11 • Design • Timing example

  37. Example: 802.11b/ACK Timing Analysis • Suppose TCP with 1460 bytes data payload • TCP data frame size (not including preamble) • 1536 bytes (1460 + 40 TCP/IP header + 36 802.11 header) • TCP ACK data frame size (not including preamble) • 76 TCP/IP header bytes • 802.11b ACK frame size 14 bytes • Suppose 802.11b at the highest rate • 8 bits per symbol • 1.375 Msps http://www.andrews.edu/~swensen/Wifi%20Throughput.pdf

  38. 802.11b/ACK Timing (1460B data)

  39. 802.11b/ACK Timing (1460B data)

  40. 802.11b/ACK Timing (1460B data)

  41. 802.11b/ACK Timing (1460B data)

  42. 802.11b/ACK Timing (1460B data)

  43. 802.11b/ACK Timing (1460B data)

  44. Example: 802.11g/ACK Timing • Suppose 802.11g at the highest rate (54Mbps) • symbol duration: 4 usec; 216 bits/symbol • 20 usec preamble; 6 usec“signal extension time” at the end of each frame http://www.andrews.edu/~swensen/Wifi%20Throughput.pdf

  45. Example: 802.11g/ACK Timing • Suppose 802.11g at the highest rate (54Mbps) • symbol duration: 4 usec; 216 bits/symbol • 20 usec preamble; 6 usec“signal extension time” at the end of each frame • Suppose TCP with 1460 bytes data payload • data: 57 (=1536*8/216) symbols; ACK: 3 (=76*8/216) symbols • 802.11b ACK frame size 14 bytes • 1 symbol http://www.andrews.edu/~swensen/Wifi%20Throughput.pdf

  46. 802.11g Basic Timing (1460B data)

  47. Example: 802.11g + CTS • RTS/CTS uses 802.11b DIFS (50 usec) and long preamble (192 usec) • RTS/CTS uses 802.11b frame coding • 20 bytes RTS • 14 bytes CTS http://www.andrews.edu/~swensen/Wifi%20Throughput.pdf

  48. 802.11g + CTS Timing (1460B data)

  49. Summary