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IEEE 802.11 Management Frames

IEEE 802.11 Management Frames. Gast ’ s Book (Chapter 4) Prof. Yu-Chee Tseng CS, NCTU. Outline. Fixed Fields Information Elements Management Frames Association and Probe. Introduction. Establishing the identity of a network station in a wired network is easy.

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IEEE 802.11 Management Frames

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  1. IEEE 802.11Management Frames Gast’s Book (Chapter 4) Prof. Yu-Chee Tseng CS, NCTU

  2. Outline • Fixed Fields • Information Elements • Management Frames • Association and Probe

  3. Introduction • Establishing the identity of a network station in a wired network is easy. • just drag wires from a central office to stations • Wireless network must create management feature to provide similar functionality.

  4. 3 Steps of Management • MS searches of connectivity • like finding a wired data jack on the wall • Network authenticates the MS • in wired network, this is provided by the cable itself • MS associates with the AP to gain access

  5. Structure of Management Frames • header: similar • frame body: (two types) • fixed fields: 10 types, fixed length • information elements: variable length, can be defined by newer version of 802.11, appear in specific order • These fields are building blocks of management frames (and will be assembled later).

  6. Fixed Fields These fields are building blocks of management frames (and will be assembled later).

  7. Fixed Field:Authentication Algorithm Number • 2 bytes to identify the type of authentication in the authentication process • only 2 values are defined currently • 0: Open System authentication • 1: Shared Key authentication • 2 - 65,535: reserved for future use

  8. Fixed Field: Authentication Transaction Sequence Number • authentication = multi-step process consisting of challenges and responses • 2 bytes to track the progress of the auth. exchange • 1 to 65,535 (never uses 0)

  9. Fixed Field: Beacon Interval • to indicate how frequent beacons sent • time unit (TU) = 1,024 us (about 1 ms) • beacon interval is commonly set to 100 TU (about 100 ms = 0.1 sec)

  10. Fixed Field:Capability Information • to advertise the network’s capability • in Beacon • in Probe Request/Response • stations that do not implement all features in the capability are not allowed to join

  11. ESS/IBSS: (mutually exclusive) • AP sets ESS = 1 and IBSS = 0 (infrastructure) • stations in IBSS sets ESS = 0 and IBSS = 1 (ad hoc mode) • Privacy: • 1 = required to use WEP for confidentiality • Short Preamble: • 1 = short preamble to support high-rate DSSS PHY in 802.11b • PBCC: • 1 = binary convolution coding modulation for high-rate DSS PHY in 802.11b • Channel Agility: • 1 = use channel agility to support high-rate DSS PHY in 802.11b

  12. CF Polling Bits: • STA can set CF capability in Association/Reassociation mgt. frame • AP can set CF capability in Beacon, Probe Response, Ass./Reass. Response,

  13. Fixed Field: Current AP Address • to indicate the MAC address of AP currently associated • STA can transmit this address when associating with a different AP to transfer the association and retrieve buffered frames from old AP.

  14. Fixed Field:Listen Interval • To indicate under PS mode, how often a STA will wake up to check buffered frames. • unit = one beacon interval • From this, AP can determine: • to estimate the resource required for buffering • may refuse to serve resource-intensive asso.

  15. Fixed Field:Association ID • An associated STA is given an Asso. ID to assist with control and mgt. functions. • 14 bits available (1-2,007 hosts) to be compatible with Duration/ID field

  16. Fixed Field:Timestamp • the number of microseconds the BSS has been active • for synchronization purpose • 64 bits (need > 580,000 years to wrap back)

  17. Fixed Field:Reason Code • STA may send Disass. or Deauth. frames in response to traffic when the sender has not properly joined the network with Reason Code. ex

  18. Fixed Field: Status Code • to indicate whether an operation succeed or fail, with proper Status Code ex

  19. Information Elements These fields are building blocks of management frames (and will be assembled later).

  20. Information Elements • generic format • Element ID and meanings

  21. Information Element:Service Set Identity (SSID) • allow network manager to assign a logical ID to the service set • STA may scan and join the network with a specified SSID • SSID is the same for all BSS composing an ESS • null SSID (known as broadcast SSID) is used in Probe Request frames to discover all 802.11 APs

  22. Information Element:Supported Rate • which rates are supported • some mandatory, some optional • indicated by the most significant bit • 2 = 1 Mbps, 4 = 2 Mbps, 11 = 5.5 Mbps, 22 = 11 Mbps

  23. Information Element:FH Parameter Set • dwell time: amount of time in a channel (in TU) • hop set: set of hopping patterns • hop pattern: which hopping pattern in the hop set • hop index: current position in the hop sequence

  24. Information Element:DS Parameter Set • which channel when DSSS is used

  25. Information Element:CF Parameter Set • transmitted in Beacon by AP to advertise the CFP parameters • optional

  26. Information Element:Traffic Indication Map (TIM) • to indicate which low-power STAs have buffered traffics waiting to be picked up • partial virtual bitmap = 8 ~ 2,008 bits • each bit for one association ID • 1 = traffic buffered

  27. DTIM count: • when will the next DTIM frame arrives • DTIM is for buffered broadcast/multicast • unit = beacon interval • DTIM period: • period of DTIMs (unit = beacon interval) • Bitmap Control: • bit 0 is for traffic indication of Asso.ID 0 (i.e., multicast traffic) • the remaining 7 bits indicate the offset of the start of Virtual Bitmap • so we can only transmit a portion of the virtual bitmap • saving wireless bandwidth

  28. Information Element:IBSS Parameter Set • to indicate the period of IBSS Beacons in an ad hoc network • unit = TU • the period is contained in ATIM (ATIM = Announcement TIM)

  29. Information Element:Challenge Text • Shared-key authentication requires STA successfully decrypt an encrypted challenge. • to be filled in the Challenge Text

  30. Management Frames Fixed fields and information elements will be used in management frames

  31. Management Frames: Introduction • Fixed fields and information elements will be used in the body of management frames to convey information. • Frame types: • Beacon, Probe Request, Probe Response, ATIM, Disassociation, Deauthentication, Asso. Request, Reasso. Request, Asso. Response, Reasso. Response, Authentication

  32. Mgt. Frame: Beacon • FH and DS Parameter Sets are mutually exclusive.

  33. Beacon Generation byAPs and ad hoc networks by AP: Beacon by contention (D1 = random backoff) Busy medium by IBSS:

  34. Mgt. Frame: Probe Request • SSID: to request a specific network • if “broadcast SSID” is used, any network is fine • Supported Rates

  35. Mgt. Frame: Probe Response • If a Probe Request encounters a network with compatible parameters, the network sends Probe Response. • In IBSS, beacon transmission is distributed. The STA who sent the previous Beacon replies the Probe Response.

  36. Mgt. Frame: ATIM (for IBSS) • When a STA has buffered frames for a low-power receiver, it sends ATIM frame during the delivery period to notify the sleeping STA (in DA field).

  37. Mgt. Frames: Disassociation and Deauthentication • Disassociation: to end an asso. • Deauthentication: to end an authentication relationship

  38. Mgt. Frame: Association Request • Once a STA identifies a compatible network, it may send an Asso. Req. • The AP will verify the STA’s parameters

  39. Mgt. Frame: Reassociation Request • When roaming between BSSs under the same ESS, Reasso. Req. can be sent. • Reasso. differs from Asso. in that it contains the old AP’s address, so the new AP will contact the old AP to pick up possible buffered frames at the old AP.

  40. Mgt. Frames: Asso./Reasso. Response • To respond to the earlier request

  41. Mgt. Frame: Authentication • exchanged between AP and STA for authentication purpose. • auth. algo. no: • auth. trans. seq. no: to track the progress • depending on what auth. algo. is used

  42. Association/Authentication andProbe/Response

  43. A STA keeps two state variables for each STA: Authentication state Association state Possible States

  44. Probe Request/Response • Active scanning when missing APs • infrastructure mode: probe response generated by AP; • IBSS mode: probe response generated by the STA which generated the last beacon.

  45. Ad Hoc Network: Beacon vs. Probe STA STA STA STA STA STA STA STA (first beacon) (second beacon) Beacon STA (probe request) (response) STA STA Probe request Probe response STA

  46. For each channel, PROBE is sent. • Probe Responses from all possible APs are collected, until Max_Probe_Response_Time. • Then the best quality AP may be chosen.

  47. Summary • Building blocks of management frames: • fixed field • information elemenet • Management frames • A probe request/response example

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