1. Studying 802.11 1 st lecture ~Overview of IEEE 802.11~ Tadashi YASUMOTO

2. In beginning To understand acronyms –Overwhelming numbers To understand the difference with Ethernet To understand the 802.11 MAC layer

3. What IEEE 802.11 is for. Allow WLAN to be identical to wired LAN –With mobility –With built-in power saving operations

4. IEEE 802.11 defines I.Physical layers II.MAC management protocols and service III.MAC sublayer

5. IEEE 802.11 defines I.Physical layers II.MAC management protocols and service III.MAC sublayer

6. Physical layers PHYs : Physical layers –Some kinds of layers 802.11b –HR/DSSS: a high-rate DSSS 802.11a –Based on OFDM; using orthogonal frequency division multiplex 802.11g –Using OFDM with backwards compatibility with 11b –Common specification CSMA/CA –Difference with CSMA/CD

7. Physical layers CSMA/CA: Carrier Sense Multiple Access with Collision Avoidance –Dispersion Contorol ( Centralization Control) –Difference with CSMA/CD Not always detect the collision –Features If the other node are in use of the channel –Wait for random delay time –And have the right to send

8. IEEE 802.11 defines I.Physical layers II.MAC management protocols and service III.MAC sublayer

9. MAC layer (1/2) MAC sublayer –LLC: the logical link control A common link layer used by lower-layer LAN tecnology –What is LLC -> next slide MAC management –802.1D Bridging –802.1Q Virtual LAN(VLAN)

10. MAC layer (2/2) LLC: Logical Links Control –One of the sublayer in Data Link Layer (L2 in OSI) –In 802, all of lower-layers are under LLC

11. 802.11 Architecture Components Types of Networks Services Other

12. 802.11 Architecture Components Types of Networks Services Other

13. Components DS APs STAs Medium

14. Components STAs: Stations Computing devices with wireless network interface(s) NICs familiar with in wired networks APs: Access points Wireless-to-wired bridging Wireless medium PHYs DS: Distribution system (later) Tracking the movements of mobile STAs

15. Components Distribution Systems –Provides services to STAs –Tracking where a STA is –Delivering frames Ethernet is a part of DS AP has at least two interfaces one is wired, the other is wireless with bridging egngine

16. 802.11 Architecture Components Types of Networks Services Other

17. Types of Networks(1/3) BSS: the basic service set –IBSS: independent BSS A set of STAs creating a short-lived network Direct communication and No relay function Sometimes referred to AD-HOC networks –Infrastructure BSS Using an AP Two hops communication with relay function –More transmission capacity than IBSS –Assist with STAs saving power STAs need to associate with an AP later

18. Types of Networks(2/3) ESS: Extended service area –An arbitrarily large size A set of BSSs With same SSID: service set identifier –Link with APs in layer 2

19. Types of Networks(3/3) Virtual APs –Some ESSs on same physical infrastructure To protect some different networks To create parallel networks with different security levels

20. 802.11 Architecture Components Types of Networks Services Other

21. Services STA services –Authentication –Deauthentication –Privacy –Data delivery Distribution services –Association –Reassociation –Disassociation –Distribution –Integration

22. Services Distribution services

23. Services Privacy (or Confidentiality) –Provide a level of protection WEP protocol: Wired Equivalent Privacy

24. 802.11 Architecture Components Types of Networks Services Other

25. Interaction between services –After Mobility support –3 types of transitions between AP s No transition BSS transition ESS transition

26. Other Interaction between services

27. Mobility support BSS transition –No need to be aware of STA's location the cooperation of AP s Same ESS

28. Mobility Support ESS transition –802.11 does not support... ESS1 ESS2

29. Workshop Presentation 2nd Ryo Nakajima

30. Wireless and 802.11 introduction Time scheduling in 802.11 Frame format in 802.11 Particular situation Frame translation between wire and wireless

31. 802.11 Wireless link –Many interfere Microwave ovens, node itself –Need power saving mode Mobile, battery operated Particular system: acknowledgement, RTS/CTS, CSMA/CA

32. Acknowledgement Each frame must be acknowledged –Or the sender will re-transmit the same frame

33. RTS/CTS The Hidden Node Problem –Only node2 can know both node1 and node3

34. RTS/CTS RTS/CTS keep other node silent

35. CSMA/CA CA: collision avoidance –Like CSMA/CD Wait until the channel become idle Details in Scheduling

36. Scheduling DCF (AD-HOC mode) –Basic time scheduling in 802.11 –To avoid collisions, use empty time PCF (AP) –Rare system –To avoid collisions, AP(Access Point) gives priority to each node in order HCF –Keep service quality, but now undertaking

37. DCF Carrier sensing –To find whether the channel is busy or not Time scheduling –To avoid collisions by simultaneous transmission

38. Carrier sensing Physical carrier sensing –Expensive electronics –Hidden nodes lurking everywhere Virtual carrier sensing –Use Network Allocation Vector(NAV) –NAV means time to be quiet –NAV is included almost all frames –So NAV can be heard every nodes

39. Time scheduling NAV Interframe Spacing –Waiting time after one frame transmitted

40. Interframe Spacing SIFS –Short range –To higher priority:RTS+CTS, Data+Ack, Fragment DIFS –Long range, standard –After Data+Ack, with Backoff time EIFS –To detect transmit failure –SIFS+Ack+DIFS

41. Time Schedule Example

42. Backoff Time to wait after DIFS –Random length –Each node determine by itself To lower collisions –Lower simultaneous beginning of transmission

43. Error Recovery No ack→retry Short retry counter and long retry counter –Incremented when frames are retransmitted –Can hear ack→set to 0 –If reach to the limit, discard that frames Backoff time become long to reduce transmission –If retry counter incremented, backoff time is doubled

44. General Frame format

45. Frame Control

46. Next Duration/ID

47. Duration/ID NAV PS-Poll+AID(Associated ID) –Node’s wake-up signal

48. –Next Address field

49. Address field Destination Address Source Address BSSID –AP’s wireless NIC’s MAC address Receiver Address Transmitter Address

50. Next Sequence Control

51. Sequence Control 4 bits Fragment number and 12 bits Sequence number All fragments have the same sequence number To detect duplicate frame

52. Next FCS

53. FCS Frame Check Sequence Calculate the value with hash function

54. Particular situation Group frames( broadcast multicast ) Fragment RTS/CTS Power saving Multirate support

55. Group frames No acknowledgement No fragment No NAV set

56. Fragment SIFS between fragment

57. RTS/CTS NAV=RTS+CTS+first_data+first_ack

58. Power saving Node send PS-poll to AP after wake up AP send data to node which is buffered –AP buffer data while node is sleeping –AP’s response: Immediate response or Deferred response Immediate: use SIFS Deferred: use DIFS+Backoff

59. Multirate Support How to choose the rate depends on the vendors Basic rate set: every station in the system must support Control frame that start a frame exchange –Such as RTS/CTS –One of the rates in the basic rate set –CTS no faster than RTS

60. Wireless→Wire 1 integrity check 2 BSSID (AP’s MAC) check 3 decrypt if needed 4 Fragment check 5 copy address and type 6 FCS recalculate

61. Wire→Wireless 1 Integrity check and destination node check 2 prepend SNAP header 3 power saving check 4 set sequence number, fragment number 5 encryption if needed 6 copy address 7 FCS recalculate