IEEE 802.11: A Fast Reassociation procedure in the Point Coordination Function

1. IEEE 802.11: A Fast Reassociation procedure in the Point Coordination Function

2. Standards • IEEE 802.11 • Finalized in June of 1997 and revised in 1999 • (In-building) Operation Range = 50~150 m • Quality of Service • (optional) Point Coordination Function

3. Standards (Cont.) • IEEE 802.11 • IEEE Std 802.11a • Orthogonal frequency domain multiplexing (OFDM) radio, delivering up to 54 Mbps data rates • IEEE Std 802.11b • Extension to the DSSS PHY in the 2.4 GHz band, delivering up to 11 Mbps data rates

4. Standards (Cont.) • IEEE 802.11 • IEEE Std 802.11g • Orthogonal frequency domain multiplexing (OFDM) radio in the 2.4 GHz band, delivering up to 54 Mbps data rates • IEEE Std 802.11i • Enhance the current 802.11 MAC to provide improvements in security.

5. Standards (Cont.) • IEEE 802.11 • IEEE Draft 802.11f • To be specified as the Inter-Access Point Protocol, handles the registration of APs within a network and the exchange of information when a user is roaming among coverage areas supported by different vendors' access points. It will help with fast hand-off from AP to AP.

6. Standards (Cont.) • IEEE 802.11 • IEEE Draft 802.11n • Standard for Enhancements for Higher Throughput. • IEEE Draft 802.11e • Medium Access Control (MAC) Quality of Service (QoS) Enhancements

7. 802.11 Architecture • Basic Service Set (BSS): A group of stations that can directly communicate with each other • Independent BSS (IBSS) • Infrastructure BSS • Extended Service Set (ESS)

8. 802.11 Architecture (Cont.)

9. Independent BSS • One and only one Basic Service Set • Ad hoc network • Direct Communication • Limited coverage area

10. Infrastructure BSS • Infrastructure • Access Point • Analogous the base station in a cellular communication network • Provide specific services and range extension

11. Extended Service Set (ESS) • Distribution System • Interconnect multiple BSSs to form an ESS, Extended Service Set • Not part of 802.11 standard • Could be IEEE 802.3 LANs, wireless, other networks • Only distributed System Services are defined

12. IEEE 802.11 Services • Station Services • Authentication/Deauthentication • Prove identity • Similar to the physically connecting to the network cable • Privacy • Provide an equivalent level of protection as that provided by a wired network • WEP: wired equipment privacy • Data delivery

13. IEEE 802.11 Services (Cont.) • Association/Reassociation/Disassocation • Association service is usually invoked once, when the mobile station enters the WLAN for the first time, • While reassociation service shall include the information about the AP with which a mobile station has been previously associated. • Distribution • An AP determines how to deliver the frames it receives uses the distribution services • Integration • Connect the IEEE 802.11 WLAN to other LANs

14. IEEE 802 System View

15. 802.11 MAC Entity

16. 802.11 Protocol Entities

17. MAC Functional Description • 802.11 MAC • Regulate the usage of the media. • Provide the same interface with other 802.x families to the upper LLC layer.

18. MAC Functional Services • Data Service: carrier sense multiple access with collision avoidance (CSMA/CA) • Distribution coordination function (DCF) • Asynchronous data delivery services • Point coordination function (PCF): optional • Time bounded data delivery services • Fragmentation/Defragmentation

19. DCF: CSMA/CA • CSMA: carrier sense multiple access • Physical carrier sense: physical layer • Virtual carrier sense: MAC layer • Network Allocation Vector (NAV) • CA: collision avoidance • Random backoff procedure

20. CSMA/CA: Without RTS/CTS • NAV (Network Allocation Vector) • Indicate the amount of time that must elapse until the current transmission is complete • Update the duration field of packets

21. CSMA/CA with RTS/CTS

22. CSMA/CA with RTS/CTS (Cont.) • RTS/CTS • Resolve hidden node • Fast collision inference • Cannot used for broadcast/multicast immediate address • NAV(Network Allocation Vector) • All STAs receiving a valid frame shall update their NAV in the Duration Field. • All of the RTS/CTS/DATA frames contains NAV field. • Use of RTS/CTS is optional but must be implemented • Use of RTS/CTS is controlled by a RTS_Threshold parameter per station • To limit overhead for short frames

23. RTS/CTS Overhead Impact • RTS/CTS can be automatically disabled by thresholding on frame length through dot11RTSThreshold. • IEEE 802.11 Std 1999 Edition Annex D • The default value is 2347.

24. Backoff Procedure • Random backoff procedure • Decrement its backoff time by aSlotTime when free . • Stop the backoff procedure when busy . Backoff Time = INT ( CW * Random( )) * aSlotTime

25. Backoff Procedure (Cont.)

26. Backoff Procedure (Cont.)

27. Optional Point Coordination Function (PCF)

28. Optional Point Coordination Function (PCF) (Cont.) • Access method • Virtual carrier-sense mechanism with access priority mechanism. • Virtual carrier-sense • PCF should distribute information within Beacon frames to gain control of the medium by setting the network allocation vector in STAs. • Access priority • Contention-free access

29. Contention Free operation

30. Polling • In a CFP, the PC sends poll frames to stations asking whether they have frames to send. • If a polled station has a frame to send to the AP, it can send it to the AP now.

31. 802.11 Protocol Entities

32. MAC Management Layer • Synchronization • Timing Synchronization Function (TSF) • Power Management • sleeping without missing any messages • Power Management functions • periodic sleep, frame buffering, Traffic Indication Map • Association and Reassociation • Joining a network • Moving from one AP to another

33. Association in Infrastructure WLAN AP A AP B • Each Station is Associated with a particular AP • Stations 1, 2, and 3 are associated with Access Point A • Stations 4 and 5 are associated with Access Point B

34. Association description • Association is requested by MHs. • Initializing association service when sending data via AP. • The act of becoming associated invokes the association service, which provides the STA to AP mapping to the DS. • How the mapping information provides by the association service is stored and managed within the DS is not specified by the 802.11 standard.

35. Reassociation • Association + Reassociation  support BSS-transition • Reassociation is request by MHs. • Invoked when a STA “move” a current association from one AP to another. • Plus IAPP (802.11f) to support wired transmission protocol.

36. Disassociation • Invoked whenever an existing association is to be terminated in a BSS. • The disassociation is a notification, not a request. • Invoked by MHs or APs.

37. Motivation • Mobility issue • The trend of light-weighted mobile stations. • Vertical handoff in heterogeneous wireless access network. • When a mobile station is in communication and under high mobility, insuring fast continuous services is required. • Real-Time (e.g. voice over IP) which is time bound. • Reassociation service is processed when a mobile station moves towards different BSS. • It is a necessary service for BSS-transition. It is critical to design a mechanism to insure that the reassociation delay will be reduced. protocol improvement ~~~~~

38. Reassociation within DCF/PCF mode • Reassociation is active at DCF when PCF is applied in original IEEE 802.11. • It may reach a unlimited time-bound when channel load is high. • We propose a fast reassociation scheme operating in the PCF.

39. IEEE 802.11 wireless LAN association procedure for multimedia applications IEEE 1999 MICOM [Acro99] Another reassociation procedure proposed by Acro

40. Proposed reassociation operation

41. Detailed request/respond slots

42. Performance Analysis 1)System model 2)Analysis

43. System model • System has upper limit capacity • Reference: “Throughput and Delay limits of IEEE 802.11,”IEEE Communications Letters , vol. 6, no. 8, 2002 Aug. • Assumption 1: System has a maximal number of MSs n. • Assumption 2: Each MSs always has traffic (signaling and data) to transmit.

44. DCF backoff stage modeling

45. Probability obtained from previous queueing model • b ij – i represents backoff stage, j represents backoff window size, j = 2^iW0 • The probabilistic backoff status in each mobile station. • b ij – The Probability of stable state • τ: The stationary prob. of a mobile station transmitting in a randomly chosen slot time. • p : conditional collision probability, that is, the probability of a collision seen by a packet transmitting on the channel. Given n users.

46. Traffic model

47. Prob. obtained from previous queueing model • State (i, j) – i represents external MS arrival events, j represents active MS within this BSS. • Pij – The probability of each state in the stable situation. • Psh(i ,j) – The probability of any external arrival MS that finish reassociation procedure successfully • Psh(i, j) = (i, 1)x τx(1- τ)^i+j-1 • Pshall (i, j)– In the state(i, j), the probability of all external arrival that finish reassociation procedure successfully • Pshall = ∑ (h=0  i) Psh(h, i+j-h)

48. Example • i = 5, j = 6. Psh(5,6) = (5, 1)x τx(1- τ)^5+6-1 • i =4, j = 7. So, Pshall =Psh(5,6)xPsh(4,7)x … xPsh(0,11)

49. Cont. • Psysh – The probability of all external arrival MS that finish reassociation procedure • Psysh = ∑(i=0n) ∑(j=0n) ∑ (h=0  i) • Pij x Psh(h, i+j-h)

50. Prob. in the PCF • Ppcf –In the PCF, the probability of all external arrival MSs finish reassociaton procedure successfully • Ppcf = ∑(i=0n) ∑(j=0n) Pij x m(m-1)……(m-i+1)/m^i m : request(respond)-mini slot size within a packet-transmission slot.