1. CS 414 Indian Institute of Technology, Bombay CS 414 802.11

2. CS 414 Indian Institute of Technology, Bombay What is 802.11? ● Belongs to a group of 802.x IEEE LAN standards – 802.3 – 802.5 – 802.11 : Wireless LAN – Enhancements to original standard ● 802.11 a/b/g ● MAC is same, different PHY layers ● 802.11 e/n/h/i...

3. CS 414 Indian Institute of Technology, Bombay Operating Frequencies ● ISM band – RFs for industrial, scientific and medical – resevered internationally for free access – different regulations still exist in different countries – 802.11 b/g: 2.4 to 2.4835 Ghz – 802.11 a: 5.2 Ghz to 5.7 Ghz – Bluetooth: 2.4-2.4835 Ghz – 802.15.4: 2.4-2.4835 GHz

4. CS 414 Indian Institute of Technology, Bombay What does 802.11 specifiy? ● Protocol Layering (on board) ● PHY sub-layer – 802.11 a/b/g specfications ● MAC sub-layer – Independent of the PHY – Distributed Co-ordination Function (DCF) – Point Co-ordination Function (PCF) ● MAC management – syncrhonization, beacons, join, leave, handoff, power management...

5. CS 414 Indian Institute of Technology, Bombay 802.11 PHY ● 802.11b data-rates (modulation schemes) – 1Mbps (BPSK), 2Mbps (QPSK), 5.5Mbps (CCK),11Mbps (CCK) ● 802.11a and 802.11g data-rates: – 6, 9, 12, 18, 24, 36, 48, 54 Mbps – OFDM + BPSK/QPSK/16QAM/64QAM – 802.11g is backward compatible with 802.11b ● supports 802.11b data-rates and modulations

6. CS 414 Indian Institute of Technology, Bombay 802.11b/g Channels ● 2400 to 2483.5 Mhz ● Each channel: 22 MHz wide – centre frequency + width ● 11 total channels – 3 non-overlapping channels

7. CS 414 Indian Institute of Technology, Bombay 802.11a Channels ● 20 Mhz spacing ● 12 non-overlapping channels – 8 in most cases

8. CS 414 Indian Institute of Technology, Bombay 802.11 Architectures Basic Service Set/Area Independent BSS (Ad-hoc mode) Infrastructure BSS Access Point SSID: Service Set Identifier

9. CS 414 Indian Institute of Technology, Bombay Extended Service Set (ESS) AP1 AP2 AP3 backbone wired or wireless SSID of all APs in an ESS are same APs and Backbone act as bridges (layer-2 devices)

10. CS 414 Indian Institute of Technology, Bombay MAC Classification ● Multiplexing dimension – time, space, frequency, code ● Medium Control – Central ● Point Coordination Function (PCF) ● Contention-free ● Requires infrastructure networks – Distributed ● Distributed Coordination Function (DCF) ● CSMA/CA (Ethernet-like)

11. CS 414 Indian Institute of Technology, Bombay Ethernet CSMA/CD ● CSMA ? ● CD ?

12. CS 414 Indian Institute of Technology, Bombay Ethernet CSMA/CD ● Carrier Sense Multiple Access – Listen/poll medium before transmit – As soon as medium free, transmit (1-persistent) ● Sense and seize mode ● Collision Detection – Sender checks for collisions – If collision ● abort and resend ● exponential backoff

13. CS 414 Indian Institute of Technology, Bombay 802.11 CSMA/CA ● CSMA before transmissions ● Interested in detecting collisions where – at sender ? – at receiver ?

14. CS 414 Indian Institute of Technology, Bombay 802.11 CSMA/CA ● CSMA before transmissions ● Interested in detecting collisions where – at sender ? – at receiver ? – Cannot detect collisions at far-away receiver – Tx power at very high at transmitter ● Collision detection not possible in wireless ● Solution: Collision Avoidance

15. CS 414 Indian Institute of Technology, Bombay Hidden Node Problem ● A transmits – Is C able to listen? – Does C transmit? – What happens at B? A B C

16. CS 414 Indian Institute of Technology, Bombay Exposed Node Problem ● B transmits – A and C both receive – C wants to tranmit to D – Cannot (even if no collision with A) ● C is exposed to B A B C D

17. CS 414 Indian Institute of Technology, Bombay 802.11 Collision Avoidance – RTS: Reserves radio link, silences users in senders vicinity – CTS: Silences stations in receivers vicinity – Primarily addresses hidden-node problem RTS: Request to Send CTS: Clear to Send

18. CS 414 Indian Institute of Technology, Bombay RTS-CTS Overhead ● Extra handshake for every frame to be sent ● When should RTS/CTS not be used? ● RTS threshold SenderReceiver RTS CTS Frame ACK

19. CS 414 Indian Institute of Technology, Bombay CA and Exposed Node Problem ● Can RTS/CTS address the problem? ● What are assumptions?

20. CS 414 Indian Institute of Technology, Bombay 802.11 Carrier Sensing ● Physical Carrier Sensing ● Virtual Carrier Sensing ● MAC considers medium busy – If either sensing indicates medium busy

21. CS 414 Indian Institute of Technology, Bombay Physical Carrier Sensing ● CCA: Clear Channel Assesment ● Mode 1 – Energy detected greater than threshold ● Mode 2 – Detection of valid modulated signal ● Mode 3 – Combination of 1 and 2

22. CS 414 Indian Institute of Technology, Bombay Virtual Carrier Sensing ● Duration field in most 802.11 frames – NAV: Network Allocation Vector ● Amount of time medium will be reserved – Nodes cound down from NAV to 0 ● if NAV > 0, medium busy access to medium deferred contention window

23. CS 414 Indian Institute of Technology, Bombay Interframe Spacing ● SIFS: Short Interframe Space ● PIFS: PCF Interframe Space ● DIFS: DCF Interframce Space ● EIFS: Extended Interframce Space

24. CS 414 Indian Institute of Technology, Bombay Interframe Spacing ● SIFS: Short Interframe Space – Highest priority (rts/cts, acks) ● PIFS: PCF Interframe Space – Medium priority – To preempt contention-based traffic ● DIFS: DCF Interframce Space – Low priority – For asynchronous data transfer ● EIFS: Extended Interframce Space – After error in transmissions

25. CS 414 Indian Institute of Technology, Bombay Typical Timings ● Slot time = 20 us ● SIFS = 10 us ● PIFS = SIFS + Slot time ● DIFS = SIFS + 2 * Slot time ● EIFS = not fixed ● RTS ~ 24 bytes (PHY) + 20 bytes (MAC) – 352 us (1Mbps) ● CTS ~ 24 bytes (PHY) + 16 bytes (MAC) – 304 us (1Mbps)

26. CS 414 Indian Institute of Technology, Bombay Contention Based Access Using DCF – If medium idle for more than DIFS ● Transmit – If previous frame received without errors or NAV expired or If channel busy ● Wait till medium free for DIFS ● Choose backoff slot within contention window – Collision (No ACK) ● Increase Contention Window ● Choose backoff slot within contention window

27. CS 414 Indian Institute of Technology, Bombay Backoff Procedure A DIFS Frame B C D DIFS defer CWindow DIFS Frame CWindow DIFS Frame DIFS Frame ● Immediate access when medium is free >= DIFS ● When medium is not free, defer until the end of current frame trasnsmission + DIFS period ● To beging backoff procedure – Choose a random number in ( 0, Cwindow) – Use carrier sense to determine if there is activity during each slot – Decrement backoff time by one slot if no activity is detected during that slot ● Suspend backoff procedure if medium is determined to be busy at anytime during a backoff slot ● Resume backoff precedure after the end of current frame transmission source: P. Bhagwat IITK CS698T

28. CS 414 Indian Institute of Technology, Bombay Contention Window Size Previous Frame DIF S Previous Frame DIF S Previous Frame DIF S Previous Frame DIF S Previous Frame DIF S Previous Frame DIF S Previous Frame DIF S 31 slots 63 slots 127 slots 255 slots 511 slots 1023 slots Initial attempt 1 st ReTx 3 rd ReTx 2 nd ReTx 4 th ReTx 5 th ReTx 6 th ReTx

29. CS 414 Indian Institute of Technology, Bombay Why SIFS & DIFS? access to medium deferred contention window Due to SIFS CTS and ACKS get more priority than data frames

30. CS 414 Indian Institute of Technology, Bombay Error Recovery ● Positive ACKs – No ACKs mean retransmit ● All unicast data must be ACKed ● No ACKs for broadcast ● Retry – When on ACK – Collisions (cannot capture medium) – Increased contention window

31. CS 414 Indian Institute of Technology, Bombay Fragments Transmission ● Fragmentation: – Higher-level packets may need to be broken – Improves transmission reliability under noisy environments ● Using SIFS transmitter holds channel until end of fragment transmission burst ● If no ACK, source transmits from failed fragment after backoff and contention ● Duplicate fragments: responsibility of receiver to detect and discard ● Each fragment: – Same sequence number & Ascending fragment numbers Source Fragment 0 Destination SIFS ACK 0 SIFS Fragment 1 ACK 1 SIFS Fragment 2 ACK 2 SIFS DIFS SIFS Backoff window Fragment burst SIFS

32. CS 414 Indian Institute of Technology, Bombay Fragmentation with RTS/CTS ● Same as before ● NAV set to include duration for transmission of next fragment and ACK

33. CS 414 Indian Institute of Technology, Bombay 802.11 Frame Format PH Y CR C

34. CS 414 Indian Institute of Technology, Bombay Frame Control Field Type = 00 Management 0000 Association Request 0001 Association Response 0100 Probe request 1000 Beacon 1001 Traffic Indication Map 1010 Disassociation Type = 10 Data 0000 Data 0001 Data+CF-ACK 0010 Data+CF-Poll Type=01 Control 1011 RTS 1100 CTS 1101 ACK 1010 PS-Poll

35. CS 414 Indian Institute of Technology, Bombay Frame Control Fields Direction of traffic Fragments to follow indicator Frame retransmitted indicator Power save mode Buffered data ready for sleeping node Ordering of fragments Security?

36. CS 414 Indian Institute of Technology, Bombay Address Fields ● 48-bit addresses ● DA : Destination address (sends to upper layer) ● SA : Source address (gets data from upper layer) ● RA: Receiver address (who should process the frame?) ● TA: Transmitter address (backbone transmitter)

37. CS 414 Indian Institute of Technology, Bombay Beacons and Probes ● Stations need to know presence of AP other stations (BSS) ● Scanning – Passive: Switch channels and listen for beacon – Active: Send probe and wait for response on each channel AP Station beacons AP Station probe request probe response

38. CS 414 Indian Institute of Technology, Bombay Beacon Frame ● AP timing information (64-bit) ● Beacon period (100 ms) ● Capability information – ESS/IBSS, Privacy, Slot time... ● SSID ● PHY parameters ● Traffic Indication Map (TIM) ● Supported data rates

39. CS 414 Indian Institute of Technology, Bombay Authentication & Association ● Station in 3 modes (w.r.t an AP) – unauthenticated/unassociated – authenticated/unassociated – authenticated/associated ● Optimize: Pre-authenticate for speedup ● Station can be only associated to one AP

40. CS 414 Indian Institute of Technology, Bombay Reassociation ● For stations moving across BSSs in the same ESS or ● After temporary disconnects ● Include address of current associated AP – New AP can contact old to get association data (buffered frames)

41. CS 414 Indian Institute of Technology, Bombay When to switch APs?

42. CS 414 Indian Institute of Technology, Bombay When to switch APs? ● Signal Strength based – One-time – Periodic scan – Out-of-range ● Load on AP – # users associated already

43. CS 414 Indian Institute of Technology, Bombay PS-Poll ● Power-Save Poll ● After wakeup poll AP for buffer frames

44. CS 414 Indian Institute of Technology, Bombay RTS Frame 00 01 1101 0000000 NAV RARA TA FC S

45. CS 414 Indian Institute of Technology, Bombay CTS/ACK Frame 00 01 0011 0000000 NAV RARA FC S 1011 AC K

46. 802.11 PHY MAC Protcol Data Unit (MPDU) MAC Protcol Data Unit (MPDU) PLCP header MAC Protcol Data Unit (MPDU) PLCP header MAC Protcol Data Unit (MPDU) Sender Receiver Physical Media Dependent (PMD) layer PMD layer MAC PHY High rate (DSSS) PHY 11, 5.5 Mbps 802.11b Direct Sequence Spread Spectrum (DSSS) PHY 1,2 Mbps Frequency Hopping Spread Spectrum (FHSS) PHY 1, 2 Mbps Infrared (IR) PHY 1,2 Mbps Higher rate (DSSS) PHY 20+ Mbps 802.11g 2.4 GHz Orthogonal Frequency Division Multiplexing (OFDM) PHY 6,9,12,18,24,36,48,54 Mbps 802.11a 5.7 GHz Source: IITK 2003, P Bhagwat CS698T

47. CS 414 Indian Institute of Technology, Bombay Physical Layer 802.11 PHY (revisited) ● PLCP: Physcial Layer Convergance Procedure – Glue between MAC frames and PHY – Adds PHY headers ● PMD: Physical Medium Dependent sublayer – Transmit bits from PLCP over wireless ● CCA (Clear Channel Assesment) PLCP PMD Data Link Layer (MAC, LLC)

48. CS 414 Indian Institute of Technology, Bombay PHY Frame Format SYNC 128 bits SFD 16 bits Signal 8 bits Service 8 bits Length 16 bits CRC 16 bits PLCP Preamble PLCP Header MPDU PPDU ● SFD: Start Frame Delimiter ● Signal: Tx Rate (modulation type) ● Service (reserved) ● Length= Time to Tx frame ● CRC: Protection for header

49. CS 414 Indian Institute of Technology, Bombay 802.11 Performance ● 802.11b data rates – 1, 2, 5.5 and 11 Mbps – throughput at MAC-level?

50. CS 414 Indian Institute of Technology, Bombay 802.11 Throughput ● PLCP Preamble + Header: 24 bytes ● RTS: 20 bytes, CTS: 14 bytes ● MAC Header: 28-34 bytes ● IP Header: 20 bytes ● TCP Header: 20 bytes ● UDP Header: 8 bytes ● Retransmissions, ACKs ● Large packet overhead!

51. CS 414 Indian Institute of Technology, Bombay Measured Throughput

52. CS 414 Indian Institute of Technology, Bombay

53. MIT RoofNet

54. CS 414 Indian Institute of Technology, Bombay

61. 802.11 Measurement-based Characterization Channel 1 Channel 8 Channel 11 ● AP setting ● 5 APs ● 5 monitors