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Wireless Local Area Networks

TMTC MCSE S. T. LIANG. Wireless Local Area Networks. Department of Math. Computer Science Education Engineering Taipei Municipal University of Education , Taiwan 100 , R.O.C. Shih Tsung Liang stliang@ tmue .edu.tw. Introduction to IEEE 802.11 wireless LAN IEEE 802.11 MAC Operation

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Wireless Local Area Networks

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  1. TMTC MCSE S. T. LIANG Wireless Local Area Networks Department of Math. Computer Science Education EngineeringTaipei Municipal University of Education, Taiwan 100, R.O.C. Shih Tsung Liangstliang@tmue.edu.tw

  2. Introduction to IEEE 802.11 wireless LAN IEEE 802.11 MAC Operation IEEE 802.11 MAC Management IEEE 802.11 MAC Enhancement for QoS Support (IEEE 802.11e) Inter-Access Point Protocol (IAPP) (IEEE 802.11F) Table of Content

  3. TMUE MCSE S. T. LIANG Introduction to IEEE 802.11 wireless LAN Department of Math. Computer Science Education EngineeringTaipei Municipal Teachers College, Taiwan 100, R.O.C. Shih Tsung Liang stliang@tmtc.edu.tw

  4. What a Wireless LAN is ? Standardization of Wireless LAN IEEE 802.11 Physical Layer Evolutions Other IEEE Wireless Projects WLAN Driving Factors Wireless LAN Applications IEEE 802.11 WLAN Architecture IEEE 802.11 specified Services Services Invoked for a Mobile Station Outline

  5. What a Wireless LAN is ? • A WLAN can be considered as a wireless version of an Ethernet LAN • Main WLAN components : • Wireless Terminals (or Stations); • Access Points (linking the WLAN to other networks)

  6. Standardization of Wireless LAN • Wireless networks are standardized by IEEE • Under 802 LAN MAN standards committee server mobile terminal fixed terminal infrastructure network Application Application access point TCP TCP IP IP LLC LLC LLC 802.11 MAC 802.11 MAC 802.3 MAC 802.3 MAC 802.11 PHY 802.11 PHY 802.3 PHY 802.3 PHY

  7. Standardization of Wireless LAN • IEEE 802.11 Adopted in 1997. Defines: • MAC sublayer • MAC management protocols and services • Three Physical (PHY) layers • IR: Infra-Red • FHSS: Frequency Hopping Spread Spectrum radio, 2.4GHz band • DSSS: Direct Sequence Spread Spectrum radio, 2.4Ghz band

  8. 802.2 Logical Link Control Data Link Layer 802.1 Bridging ‧ ‧ ‧ 802.3 Medium Access 802.11 Medium Access ‧ ‧ ‧ Physical Layer 802.3 Physical 802.11 Physical 802.11b Physical 802.11a Physical 802.11g Physical IEEE 802.11 Physical Layer Evolutions

  9. IEEE 802.11 Physical Layer Evolutions Estimated Throughput Source: www.80211-planet.com

  10. IEEE 802.11 Physical Layer Evolutions Pros and cons ─ 802.11b Pros • Modest price. • Mature technology with many products available. • Throughput is adequate for most home and office applications. • In the best devices, throughput fluctuates little, out to the maximum range. Cons • Slowest throughput. • Less spectrum. • Only 3 channels available in 2.4GHz band. • Possible interference with other 2.4GHz devices (cordless phones, microwaves, garage-door openers) Source: www.80211-planet.com

  11. IEEE 802.11 Physical Layer Evolutions Pros and cons ─ 802.11a Pros • Higher throughput at short ranges. • Probably better for throughput-intensive multimedia applications than 802.11b. • 8 channels and OFDM technology, resulting in less interference among AP’s and more users. Cons • More expensive. • Less mature technology. • Shorter range and greater throughput fluctuation beyond 20 feet. • Require more AP’s • Primarily only in North America (b is worldwide) Source: www.80211-planet.com

  12. IEEE 802.11 Physical Layer Evolutions Pros and cons ─ 802.11g Pros • Backwards compatibility. • Throughput will be at least double that of 802.11b. • Range will be at least equal that of 802.11b. • Will use both DSSS and OFDM technologies Cons • Unavailable until early 2003. • Only 3 channels available in 2.4GHz band. • Possible interference with other 2.4GHz devices (cordless phones, microwaves, garage-door openers) Source: www.80211-planet.com

  13. Other IEEE Wireless Projects • MAC Layer enhancements • IEEE 802.11e ─ QoS • Addresses Quality of Service issues • Will enable differentiated traffic servicing, based on the requirements of the specific traffic type • IEEE 802.11i─ Security • Higher (user) level authentication • Advanced security algorithms • Addresses existing 802.11 security issues • Multi-Vendor Access Point Interoperability • IEEE 802.11f─ IAPP • Addresses issues with roaming between unrelated (different networks) Access Points

  14. Wireless LAN Driving Factors • Increased demand for mobile computing • Productivity increases when the network can be accessed seamlessly from multiple locations within the premises or around outside hotspots • Cost savings comparing to wired networks (for cables, cable deployment, network installation / administration / maintenance) • Communication in areas with deployment constraints (e.g., historical buildings) • Easiness to set-up temporary, ad-hoc networks (e.g., for meeting rooms, emergencies) • No new-wires solution for multimedia Home Networks(audio/video/data streaming for Set-Top Boxes and/or multimedia data pad, straightforward network set-up) • 802.11b has been widespread accepted for usage in corporate networks, remote working and business travels (hotels, airports, convention centers). Upgrade to 802.11g will be expected.

  15. Wireless LAN Applications • Enterprise • Wired LAN replacement, ad-hoc networks (NICs for PCs, printers, switches, and other office appliances) • Multiple cell coverage, high user density, roaming • Home • Networking for fixed (Residential Gateways, Set-Top Boxes) portable (Laptops) and mobile (Notebook) terminals • Distribution of digital video, Internet broadband access, sharing of PC peripherals, … • Education • Cost effective network access to teachers and students anywhere within the school from mobile and fixed terminals • Retail / Manufacturing • Inventory, prices “management” (labeling, shelf audits, updates), customer aid for shopping lists, POS/cash register downloads • Hotels • Seamless connectivity for guest rooms and meeting rooms • Public Access Points

  16. 802.11 WLAN Architecture infrastructure network AP: Access Point AP AP wired network AP ad-hoc network

  17. Portal Distribution System 802.11 Infrastructure Network • Station (STA) • terminal with access mechanisms to the wireless medium and radio contact to the access point • Basic Service Set (BSS) • group of stations using the same radio frequency • Access Point • station integrated into the wireless LAN and the distribution system • 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

  18. 802.11 Ad-hoc Network • Direct communication within a limited range • Station (STA):terminal with access mechanisms to the wireless medium • Independent Basic Service Set (IBSS):group of stations using the same radio frequency • Single-hop only 802.11 LAN STA1 STA2 IBSS1 STA3 STA4 IBSS2 STA5 802.11 LAN

  19. Services specified by IEEE 802.11 • Station Services (SS) • Authentication • De-authentication • Privacy • MSDU delivery • Distribution System Services (DSS) • Association • Disassociation • Re-association • Distribution • Integration *note* 1. All conformant stations (including APs) provide SS 2. APs provide access to DSS

  20. Services specified by IEEE 802.11 • Station Services (SS) • Authentication • De-authentication • Privacy • MSDU delivery • Distribution System Services (DSS) • Association • Disassociation • Re-association • Distribution • Integration • Used by all stations to establish their identities to stations with which they will communicate • IEEE 802.11 provides link-level authentication (not end-to-end) • IEEE 802.11 requires mutually acceptable, successful, authentication (or no data can be delivered) • A station may be authenticated with many other stations at any given instant • Preauthentications are allowed

  21. Services specified by IEEE 802.11 • Station Services (SS) • Authentication • De-authentication • Privacy • MSDU delivery • Distribution System Services (DSS) • Association • Disassociation • Re-association • Distribution • Integration • Used to terminate an existing authentication • Deauthentication shall cause the station to be disassociated • May be invoked by either authenticated party (AP or non-AP) • Deauthentication is not a request but a notification

  22. Services specified by IEEE 802.11 • Station Services (SS) • Authentication • De-authentication • Privacy • MSDU delivery • Distribution System Services (DSS) • Association • Disassociation • Re-association • Distribution • Integration • IEEE 802.11 specifies an optional privacy algorithm, WEP, to perform the encryption of message • WEP stands for Wired Equivalent Privacy • The default privacy state for all 802.11 stations is “in the clear”

  23. Services specified by IEEE 802.11 • Station Services (SS) • Authentication • De-authentication • Privacy • MSDU delivery • Distribution System Services (DSS) • Association • Disassociation • Re-association • Distribution • Integration • To provide reliable delivery of data frames

  24. Services specified by IEEE 802.11 • Station Services (SS) • Authentication • De-authentication • Privacy • MSDU delivery • Distribution System Services (DSS) • Association • Disassociation • Re-association • Distribution • Integration • Initiated by an mobile station to make a logical connection with an AP, so the AP can accept data frames from/to the station. • At any given instant, a station may be associated with no more than one AP

  25. Services specified by IEEE 802.11 • Station Services (SS) • Authentication • De-authentication • Privacy • MSDU delivery • Distribution System Services (DSS) • Association • Disassociation • Re-association • Distribution • Integration • Being invoked whenever an existing association is to be terminated • AP may invoke disassociation to inform stations that AP no longer provide the link • Stations shall attempt to disassociate whenever they leave a network

  26. Services specified by IEEE 802.11 • Station Services (SS) • Authentication • De-authentication • Privacy • MSDU delivery • Distribution System Services (DSS) • Association • Disassociation • Re-association • Distribution • Integration • Being invoked to “move” a current association from one AP to another

  27. Services specified by IEEE 802.11 • Station Services (SS) • Authentication • De-authentication • Privacy • MSDU delivery • Distribution System Services (DSS) • Association • Disassociation • Re-association • Distribution • Integration • When an AP receives a frame, it invoke the distribution service to determine the “output” point that corresponds to the desired recipient

  28. Services specified by IEEE 802.11 • Station Services (SS) • Authentication • De-authentication • Privacy • MSDU delivery • Distribution System Services (DSS) • Association • Disassociation • Re-association • Distribution • Integration • When the “output” point is determined to be a portal, the integration function should be invoked • The integration function is responsible for accomplishing whatever is needed to deliver a message from (to) the DSM to (from) the integrated LAN media • (e.g., frame format translation)

  29. Relationships between Services • A STA keeps two state variables for each STA with which direct communication via the WM is needed: • Authentication state • Association state • The current state existing between the source and destination station determine the IEEE 802.11 frame types that may be exchanged between that pair of STAs STA1 STA2 STA0 (AP)

  30. Relationships between Services Class 1 State 1: Unauthenticated, Unassociated Class 1 Frames Successful Authentication De-Authentication Notification State 2: Authenticated, Unassociated Class 1&2 Frames Class 2 Successful Association or Re-association Disassociation Notification Class 3 State 3: Authenticated, Associated Class 1,2,&3 Frames De-Authentication Notification

  31. Services Invoked for a Mobile Station f Move c a e AP 3 b As the station find AP1, it will authenticate and associate with AP1. As the station moves, it may pre-authenticate with AP2 Station may re-associate with AP2 The re-association would cause AP2 to notify AP1 of new location of the station AP2 is disassociated with station The station would need to find AP3 and authenticate and associate with AP3 AP 2 AP 1 d back

  32. TMTC MCSE S. T. LIANG IEEE 802.11 MAC Operation Department of Math. Computer Science Education EngineeringTaipei Municipal Teachers College, Taiwan 100, R.O.C. Shih Tsung Liangstliang@tmtc.edu.tw

  33. MAC Sublayer and OSI Reference Model MAC Sublayer Functionality MAC Architecture DCF PCF Coexist of PCF and DCF MAC Frame Formats Outline

  34. MAC sublayer and OSI reference model Success for it is odd !! LLC MAC PHY IEEE 802 OSI

  35. to provide a reliable MSDU delivery to control access to wireless medium Distributed Coordination Function（DCF） Point Coordination Function（PCF） to provide authentication and privacy for data delivery MAC provides a privacy service called Wired Equivalent Privacy（WEP）encryption MAC Sublayer Functionality

  36. Physical MAC Architecture Reguired for Contention-Free services Used for Contention Services and basis of PCF Point Coordination Function (PCF) MAC Extent Distribution Coordination Function （DCF）

  37. CSMA/CA Error Recovery Mechanisms DCF Access Procedure DCF

  38. Why CSMA/CD doesn’t work? The hidden terminal problem! CSMA/CA STA1 STA2 STA3 STA1can communicate with only STA2. STA2 can communicate with STA1and STA3. STA3 can communicate with only STA2. The frame from STA1to STA2 can be corrupted by a transmission initiated by STA3. The STA3 did not know the ongoing transmission from STA1to STA2

  39. To cope with the hidden terminal problem Medium reservation through the exchange of RTS and CTS frames prior to the actual data CSMA/CA RTS CTS STA2 STA3 STA1 Area cleared by RTS (Request To Send) Area cleared by CTS (Clear To Send)

  40. MAC-Level Acknowledgement Wireless media are noisy and unreliable The source needs to make sure the frame has been correctly received by the destination If the source does not receive the ACK, the source will retransmit the frame CSMA/CA

  41. 4-way MAC frame exchange protocol CSMA/CA Source Destination RTS Collision Protect!! CTS who protect me? (size is the key!!) Data ACK

  42. More about 4-way handshake RTS and CTS may be disabled by the dot11RTSThreshold attribute in the MIB (Management Information Base) If frame length > dot11RTSThreshold → 4-way frame exchange with RTS and CTS If frame length≤dot11RTSThreshold → frame exchange without RTS and CTS The defaultdot11RTSThresholdis 128 In environments STAs can hear from each other, a higher dot11RTSThreshold can reduce the bandwidth consumption on RTS and CTS CSMA/CA

  43. Carrier Sense Mechanism Physical carrier sense Physical layer carrier sense Similar to 802.3 Check for Medium status (Idle/Busy) Virtual carrier sense Mac layer carrier sense Network Allocation Vector (NAV) A countdown counter to record the amount of time remains before wireless channel clear (i.e. NAV=0→clear) CSMA/CA

  44. MAC control logic CSMA/CA Wait for frame to transmit NAV=0 ? Flag=0 Flag=1 Note: The period of time immediately following a busy medium is the highest probability of collision ccurring. Many stations may be waiting for the medium to become idle and attempt to transmit at the same time. Thus whenever the station sensing a busy medium, a random backoff time is used. Check PHY N Medium Idle? Y Collision ? N Y Wait IFS Transmit Frame Flag==0 ? Still Idle ? N Y Y N Random Backoff Time

  45. CSMA/CA • Random backoff time • Backoff time=Random()*aSlotTime • Random():a uniform distributed integer randomly selected from [0,CW], where CW is contention window • For each unsuccessful frame transmission, CW doubles (from CWmin to CWmax) • CW  2 CW+1 • Reduces the collision probability

  46. Error Recovery Mechanisms • Errors (interference, collision) • STA sends an RTS but not receive the CTS • STA sends a data frame but not receive the ACK • Retransmission with retry limit • shortRetryLimit : frame length≤dot11RTSThreshold • longRetryLimit : frame length > dot11RTSThreshold

  47. DCF Access procedure • Interframe space (IFS) • SIFS: Short InterFrame Space • Used for immediate response actions (e.g., ACK, CTS) • PIFS: PCF InterFrame Space • Used by centralized controller in PCF scheme when using polls • DIFS: DCF InterFrame Space • Used by distribution coordination function (DCF) for asynchronous frames contention • EIFS: Extended InterFrame Space • Used by the DCF after indication of the erroneous frame (e.g., FCS error) • Reception of an error-free frame during the EIFS causes the access using EIFS is terminated and normal medium access (using DIFS) continues shortest longest

  48. DIFS Immediate access when medium is free >= DIFS Contention Window PIFS DIFS SIFS Busy Backoff Next Frame Medium Window Slot Time Defer Access Select Slot and decrement backoff as long as medium is idle DCF Access procedure • Basic Access Method

  49. DCF Access procedure • Example of backoff procedure DIFS DIFS DIFS backoff=12 backoff=7 backoff=3 busy STA 1 backoff=5 busy STA 2 DIFS busy STA 3 backoff=9 backoff=4 busy STA 4 • After MSDU arriving at MAC, STA 3 senses medium free for DIFS, so it initiates transmission • immediately without backoff interval • For STA 1,2, and 4, their DIFS intervals are interrupted by STA 3. Thus, the backoff • Intervals for STA 1, 2, and 4, are generated randomly (e.g., 12, 5, and 9, respectively) • After transmission of STA 2, the remaining backoff interval of STA 1 is (12-5) = 7. • After transmission of STA 2, the remaining backoff interval of STA 4 is (9-5) = 4. • After transmission of STA 4, the remaining backoff interval of STA1 is (7-4) = 3.

  50. DCF Access procedure • Example of backoff procedure (continue) DIFS DIFS DIFS backoff=9 backoff=4 busy STA 1 backoff=5 backoff=20 backoff=16 busy STA 2 DIFS busy STA 3 backoff=5 backoff=18 backoff=14 busy busy STA 4 • STA 3 senses medium free for DIFS and initiates transmission immediately • For STA 1,2, and 4, their DIFS intervals are interrupted by STA 3. Thus, the backoff • Intervals for station 1, 2, and 4, are generated randomly (e.g., 9, 5, and 5, respectively) • Collision occurs between STA 2 and 4. • After the collision of STA 2 and 4, the remaining backoff interval of station 1 is (9-5) = 4. • The backoff Intervals for retransmission of STA 2, and 4, are generated randomly (e.g., 20 and 18, respectively). (tend to be larger the initial attempt)

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