html5-img
1 / 90

Link Layer: Wireless Media Access Control

Link Layer: Wireless Media Access Control. Y. Richard Yang 11 /08/2012. Outline. Admin. and recap Wireless Media access control (MAC) Slotted Aloha Hidden terminal Example: 802.11 Hidden terminal with 802.11 revisited. Admin. Project check point I: next Tuesday

makaio
Download Presentation

Link Layer: Wireless Media Access Control

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Link Layer:Wireless Media Access Control Y. Richard Yang 11/08/2012

  2. Outline • Admin. and recap • Wireless Media access control (MAC) • Slotted Aloha • Hidden terminal • Example: 802.11 • Hidden terminal with 802.11 revisited

  3. Admin. • Project check point I: next Tuesday • Feedback on topics to cover

  4. Recap: Wireless Access Problem • Problem: single shared medium, hence if two transmissions overlap on all dimensions [time, space, frequency, and code], then it is a collision • Solution covered: Aloha (random access with collision recovery) A B

  5. Recap: Slotted Aloha [Norm Abramson] • Time is divided into equal size slots (= pkt trans. time) • Node with new arriving pkt: transmit at beginning of next slot with probability p • If collision: retransmit pkt in future slots with probability p, until successful. A B Success (S), Collision (C), Empty (E) slots

  6. Slotted Aloha Recap: Slotted Aloha: Goodput vs. Offered Load Curve S = throughput = “goodput” (success rate) 1.5 2.0 0.5 1.0 Define G = offered load = np • Goodput is optimal when offered load G (=p*n) = 1 • Goodput is less than 1/e (~37%) at optimal

  7. Recap: Slotted Aloha Dynamics n backlogged each transmits with prob. p m-n: unbacklogged each transmits with prob. pa

  8. desirable stable point successful transmission rate at offered load np + (m-n)pa new arrival rate:(m-n) pa undesirable stable point Recap: Slotted Aloha Dynamics • - assume a total of • m stations • pa << p • success rate is thedeparture rate, the rate the backlog is reducing dep. and arrival rate of backlogged stations n: number of backlogged stations m 0 offered load = 1 Slotted Aloha can have an undesirable stable point

  9. Summary of Problems of 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

  10. Ethernet Fix for Efficiency • Introduce collision detection: instead of wasting the whole frame transmission time (a slot), we waste only the time needed to detect collision. • Introduce adaptive probability: reduce probability of trans. as # of collisions increases • If more collisions => p is high => should reduce p P: packet size, C: contention window C C C P

  11. 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 Adapt Probability Q: Does Ethernet alg work well in wireless?

  12. Outline • Admin. and recap • Media access control • Slotted Aloha • Hidden terminal • Example: 802.11 • Hidden terminal with 802.11 revisited

  13. 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

  14. 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?

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

  21. Outline • Admin. and recap • Media access control • Slotted Aloha • Hidden terminal • Example: 802.11 • Hidden terminal with 802.11 revisited

  22. Portal Distribution System 802.11: Infrastructure Mode • Architecture similar to cellular • 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. 802.11 – MAC Traffic Services • Asynchronous Data Service (ADS) • Objective: exchange data packets 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 of bounded delay service • Approach: implemented TBS using point (access point) coordinated function (PCF)

  25. 802.11 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

  26. 802.11 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

  27. B1 = 25 wait data data wait B2 = 10 B2 = 20 Congestion Avoidance: Example busy B1 = 5 B2 = 15 busy B1 and B2 are backoff intervals at nodes 1 and 2 cw = 31

  28. 802.11 – 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 DIFS RTS data sender SIFS SIFS SIFS CTS ACK receiver DIFS NAV (RTS) data other stations NAV (CTS) t defer access new contention

  29. 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); 10 us in 802.11b • highest priority, for ACK, CTS, polling response • PIFS (PCF IFS); 30 us in 802.11b • medium priority, for time-bounded service using PCF • DIFS (DCF, Distributed Coordination Function IFS); 50 us in 802.11b • lowest priority, for asynchronous data service direct access if medium is free  DIFS

  30. 802.11 – Inter Frame Spacing

  31. 802.11: PCF for Polling (Infrastructure Mode) 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

  32. 802.11b Frame Format preamble 2 Sync SFD PLCP header MAC Data CRC Preamble (192 usec; or optional 96 short version) - Sync: alternating 0s and 1s (DSSS 128 bits) - SFD: Start Frame delimiter: 0000 1100 1011 1101 PLCH (Phsical Layer Convergence Procedure) Header - payload length - signaling field: the rate info. - CRC: 16 bit protection of header

  33. 802.11 – MAC Format • 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 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

  34. Example: 802.11b/ACK Timing • Suppose TCP with 1460 bytes data payload • TCP data frame size (not including preamble) • 1536 bytes (1460 + 40 TCP header + 36 802.11 header) • TCP ACK data frame size (not including preamble) • 76 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

  35. 802.11b/ACK Timing (1460B data)

  36. 802.11b/ACK Timing (1460B data)

  37. 802.11b/ACK Timing (1460B data)

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

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

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

  41. 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

  42. 802.11g Basic Timing (1460B data)

  43. Example: TCP/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

  44. 802.11g + CTS Timing (1460B data)

  45. Summary

  46. Outline • Admin. and recap • Media access control • Slotted Aloha • Hidden terminal • Example: 802.11 • Hidden terminal with 802.11 revisited

  47. Hidden Terminal with 802.11 No ACK Collision! Bob Alice

  48. Hidden Terminal with 802.11 Retransmission One more Collision Bob Alice

  49. 802.11 Setting: Two Collisions of Two Packets Pa Pa Pb Pb

More Related