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Wireless Local Area Networks (WLAN) Part-1: IEEE802.11

Wireless Local Area Networks (WLAN) Part-1: IEEE802.11

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Wireless Local Area Networks (WLAN) Part-1: IEEE802.11

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  1. IT351: Mobile & Wireless Computing Wireless Local Area Networks (WLAN) Part-1: IEEE802.11 Objectives: • To provide a detailed study of the WLAN architecture and system operation

  2. Outline • Wireless LAN main uses, advantages, disadvantages • Classification of transmission technologies for WLAN • Classification of WLAN IEEE802.11 • Infrastructure networks • Ad Hoc networks • WLAN IEEE802.11 • Architecture • Protocols • Physical layer • MAC layer • MAC management • IEEE802.11-a/ b/... n

  3. Overview of the main chapters Chapter 10: Support for Mobility Chapter 9: Mobile Transport Layer Chapter 8: Mobile Network Layer Chapter 4: Telecommunication Systems Chapter 5: Satellite Systems Chapter 6: Broadcast Systems Chapter 7: Wireless LAN Chapter 3: Medium Access Control Chapter 2: Wireless Transmission

  4. Mobile Communication Technology according to IEEE WiFi 802.11a 802.11h Local wireless networks WLAN 802.11 802.11i/e/…/n/…/z/aa 802.11b 802.11g ZigBee 802.15.4 802.15.4a/b/c/d/e/f/g Personal wireless nw WPAN 802.15 802.15.5, .6 (WBAN) 802.15.3 802.15.3b/c 802.15.2 802.15.1 Bluetooth Wireless distribution networks WMAN 802.16 (Broadband Wireless Access) WiMAX + Mobility [802.20 (Mobile Broadband Wireless Access)] 802.16e (addition to .16 for mobile devices)

  5. Wireless LAN (WLAN) • Main uses: • Extension to existing LAN • Cross building interconnect • Nomadic access / ‘wireless hotspots’ • Ad Hoc networks • Main Standard is IEEE 802.11 • Wireless extension for Ethernet • Wi-Fi, Wireless-Fidelity, Alliance to certify products to the IEEE standard

  6. Characteristics of wireless LANs • Advantages • very flexible within the reception area, • allow for design of small independent devices (e.g. to be put in pockets) • Ad-hoc networks without previous planning possible • (almost) no wiring difficulties (e.g. historic buildings, firewalls) • more robust against disasters like, e.g., earthquakes, fire - or users pulling a plug... • Cost is independent of the number of users

  7. Characteristics of wireless LANs • Disadvantages • typically very low bandwidth compared to wired networks (1-10 Mbit/s) due to shared medium [now higher rates available] • high error rates, low quality • many proprietary solutions, especially for higher bit-rates, standards take their time (e.g. IEEE 802.11n) • products have to follow many national restrictions if working wireless, it takes a very long time to establish global solutions • Safety & security

  8. Design goals for wireless LANs • global, seamless operation • low power for battery use • no special permissions or licenses needed to use the LAN • robust transmission technology • simplified spontaneous cooperation at meetings • easy to use for everyone, simple management (plug & play) • protection of investment in wired networks • security (no one should be able to read my data), privacy (no one should be able to collect user profiles), safety (low radiation) • transparency concerning applications and higher layer protocols, but also location awareness if necessary • …

  9. Infrared (IR) At 900 nm wavelength, uses IR diodes, diffuse light, multiple reflections (walls, furniture etc.) Advantages simple, cheap, available in many mobile devices no licenses needed simple shielding possible Disadvantages interference by sunlight, heat sources etc. many things shield or absorb IR light , can not penetrate objects low bandwidth (115kbps – 4 Mbps) Example IrDA (Infrared Data Association) interface available everywhere Radio typically using the license free ISM band at 2.4 GHz Advantages experience from wireless WAN and mobile phones can be used coverage of larger areas possible (radio can penetrate walls, furniture etc.) Disadvantages very limited license free frequency bands shielding more difficult, interference with other electrical devices Example Many different products Classifications of transmission technologies: infrared vs. radio transmission

  10. IEEE802.x standards • 802 standards specify OSI layers 1 & 2 • Physical layer • Encoding/decoding signals • Preamble (for synchronization) • Bit transmission/reception • Link layer (Medium Access Control (MAC)) • Manage access to media • Assemble/disassemble frames • Addressing and error detection • Interface with higher layers

  11. ISM Unlicensed Frequency Bands

  12. IEEE 802.11 WLAN • 802.11 (Legacy, 1997) operates at 1-2 Mbps, with 3 methods • 1 infrared • 2 radio access (FHSS, DSSS) • Two Modes: • Infrastructure Mode (LAN extension) • Ad Hoc (wireless only) Wireless Communications and Networks, W. Stallings, Prentice Hall, N.J., 2001.

  13. Classifications of IEEE802.11:infrastructure vs. ad-hoc networks infrastructure network AP: Access Point AP wired network AP AP ad-hoc network

  14. 802.11 - System architecture infrastructure network Portal Distribution System • 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 (ESS: Extended Service Set) based on several BSS • Each ESS has its own identifier ESSID 802.11 LAN 802.x LAN STA1 BSS1 Access Point Access Point ESS BSS2 STA2 STA3 802.11 LAN

  15. IEEE802.11: System architecture infrastructure network • The distribution system is not specified in IEEE802.11. • It could consist of IEEE LANs, wireless links or any other networks • It handles data transfer between different APs • To participate in a WLAN, you need to know the ESSID • Stations can select an AP and associate with it • The AP supports roaming (changing access points) • APs provide synchronization within a BSS, support power management, and can control medium access

  16. 802.11 – System architectureAd-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 • No specific node for data routing, or forwarding or exchange of topology information 802.11 LAN STA1 STA3 IBSS1 STA2 IBSS2 STA5 STA4 802.11 LAN

  17. IEEE802.11: Protocol architecture fixed terminal mobile terminal infrastructure network access point application application 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

  18. PLCP Physical Layer Convergence Protocol clear channel assessment signal (carrier sense) Service access point (SAP) PMD Physical Medium Dependent modulation, coding PHY Management channel selection, MIB maintenance Station Management coordination of all management functions, higher layer functions (interaction with distribution system) MAC access mechanisms, fragmentation, encryption MAC Management Association/de-association, synchronization, roaming, MIB (management Information Base), power management to save battery power, authentication mechanism 802.11 – Protocol Architecture Station Management LLC DLC MAC MAC Management PLCP PHY Management PHY PMD

  19. 802.11 - Physical layer (legacy) • 3 versions: 2 radio (typ. 2.4 GHz ISM), 1 IR • data rates 1 or 2 Mbit/s • All physical variants include the provision of the clear channel assessment (CCA). This is needed for MAC mechanisms. • The Physical layer a service access point (SAP) with 1 or 2 Mbits/s transfer rate to the MAC layer. • FHSS (Frequency Hopping Spread Spectrum) • spreading, despreading using different hopping sequences (79 hopping channels for North America and Europe) • Frequency Shift Keying (FSK) digital modulation

  20. 802.11 - Physical layer (legacy) • DSSS (Direct Sequence Spread Spectrum) • Spreading, despreading using 11-chip Barker code chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 • Phase Shift Keying (PSK) digital modulation • max. radiated power 1 W (USA), 100 mW (EU), min. 1mW • Robust against interference and multipath propagation • More complex compared to FHSS • Infrared • 850-950 nm, diffuse light, typ. 10 m range • Typically in buildings (classrooms, meeting rooms,..) • Frequency reuse is simple, a wall is enough for shielding

  21. 802.11 - MAC layer - DFWMAC • The MAC mechanisms are called Distributed Foundation Wireless Medium Access Control (DFWMAC) • Functions: medium access, support for roaming, authentication and power conservation • Traffic services • Asynchronous Data Service (mandatory) • exchange of data packets based on “best-effort” – no delay bounds • support of broadcast and multicast • Implemented using distributed coordination function (DCF) OR Point Coordination Function (PCF) • For both infrastructure and ad Hoc • Time-Bounded Service (optional) • implemented using PCF (Point Coordination Function) • Provides delay guarantees • For infrastructure 802.11 only

  22. MAC Layer • Asynchronous Data Service access method • DFWMAC-DCF CSMA/CA (mandatory) • collision avoidance via randomized „back-off“ mechanism • minimum distance between consecutive packets • ACK packet for acknowledgements (not for broadcasts) • DFWMAC-DCF w/ RTS/CTS (optional) • avoids hidden terminal problem • DFWMAC-PCF (optional) • Time-bounded Service access method • DFWMAC- PCF (optional) • access point polls terminals according to a list

  23. 802.11 - MAC layer • Priorities • defined through different inter frame spaces • no guaranteed, hard priorities • SIFS (Short Inter Frame Spacing) • highest priority, for ACK, CTS, polling response • PIFS (PCF IFS) • medium priority, for time-bounded service using PCF • DIFS (DCF, Distributed Coordination Function IFS) • lowest priority, for asynchronous data service DIFS DIFS PIFS SIFS medium busy contention next frame t direct access if medium is free  DIFS

  24. 802.11 - CSMA/CA access method I • station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment) • if the medium is free for the duration of a DCF Inter-Frame Space (DIFS), the station can start sending • if the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off time (collision avoidance, multiple of slot-time) • if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness) contention window (randomized back-offmechanism) DIFS DIFS medium busy next frame t direct access if medium is free  DIFS slot time (20µs)

  25. 802.11 - competing stations - simple version DIFS DIFS DIFS DIFS boe bor boe bor boe busy station1 boe busy station2 busy station3 boe busy boe bor station4 boe bor boe busy boe bor station5 t medium not idle (frame, ack etc.) busy boe elapsed backoff time packet arrival at MAC bor residual backoff time

  26. 802.11 – CSMA/CA • The contention window (CW) size affect the performance of the MAC scheme • A small CW ensures shorter access delay but the probability of collision increases (more than one station can have the same backoff time) • The contention window starts with a minimum value then doubles each time a collision occurs up to a maximum value (e.g. 7, 15, 31,63, 127, 255). • This is called the exponential backoff algorithm (already used in CSMA/CD)

  27. 802.11 - CSMA/CA • Sending unicast packets • station has to wait for DIFS before sending data • receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC) • automatic retransmission of data packets in case of transmission errors (The sender has to compete again) DIFS data sender SIFS ACK receiver DIFS data other stations t waiting time contention

  28. 802.11 – DFWMAC with RTS/CTS (method II) • Sending unicast packets • To solve the problem of hidden terminal • station can send RTS with reservation parameter after waiting for DIFS (reservation determines amount of time the data packet needs the medium) • acknowledgement via CTS after SIFS by receiver (if ready to receive) • sender can now send data at once, acknowledgement via ACK • other stations store medium reservations distributed via RTS and CTS in the NAV (net allocation vector) DIFS RTS data sender SIFS SIFS SIFS CTS ACK receiver DIFS NAV (RTS) data other stations NAV (CTS) t defer access contention

  29. 802.11 – DFWMAC with RTS/CTS (cont.) • The scheme reserves the medium for one user (virtual reservation scheme) • RTS/CTS can result in a non-negligible overhead causing a waste of bandwidth and higher delay • A threshold based on frame size can be used to determine when to use the additional mechanism and when to disable it • To reduce the bit error-rates in transmission, fragmentation can be used. However, for RTS/CTS scheme all fragments are sent by one RTS. Each fragment reserve the medium for the next fragment.

  30. DFWMAC-PCF with polling – Method III (almost never used) • The two previous methods cannot guarantee a maximum delay or minimum bandwidth • PCF provides time-bounded service • It requires an access point that control medium access and polls the single nodes • Ad Hoc network can’t use this function so it provides only best-effort service • The point coordinator in the access point splits the access time into super frame periods. • A super frame comprises an contention-free period and a contention period • If only PCF is used and polling is distributed evenly, the bandwidth is also distributed evenly – static centrally controlled TDMA with TDD transmission • Much overhead if nodes have nothing to send.

  31. 802.11 - Frame format • Types: control frames, management frames, data frames • Sequence numbers • important against duplicated frames due to lost ACKs • Addresses • receiver, transmitter (physical), BSS identifier, sender/receiver (logical) • Miscellaneous • Duration (to set the NAV), checksum, frame control, data bytes 2 2 6 6 6 2 6 0-2312 4 Frame Control Duration/ ID Address 1 Address 2 Address 3 Sequence Control Address 4 Data CRC bits 1 1 1 1 1 1 2 2 4 1 1 Protocol version Type Subtype To DS From DS More Frag Retry Power Mgmt More Data WEP Order

  32. 802.11 - Frame format • Frame Control • Protocol version: 2 bits • Type (management 00, control 01, data 10) • Subtype (e.g. Management- association 0000, beacon 100 Control – RTS 1011, CTS 1100) • More fragments: 1 if another fragment to follow • Retry: 1 if retransmission of an earlier frame • Power Management: 1 if the station will go to power save mode • More Data: A sender has more data to send • Wired Equivalent Privacy (WEP): Standard security mechanism applied • Order: frame must be processed in strict order bytes 2 2 6 6 6 2 6 0-2312 4 Frame Control Duration/ ID Address 1 Address 2 Address 3 Sequence Control Address 4 Data CRC bits 1 1 1 1 1 1 2 2 4 1 1 Protocol version Type Subtype To DS From DS More Frag Retry Power Mgmt More Data WEP Order

  33. MAC address format DS: Distribution System AP: Access Point DA: Destination Address SA: Source Address BSSID: Basic Service Set Identifier RA: Receiver Address TA: Transmitter Address

  34. 802.11 - MAC management MAC management plays a central role in an IEEE802.11 as it controls all the functions related to system integration, i.e., integration of a wireless station into a BSS, formation of an ESS, synchronization of stations,..etc The major functions are: • Synchronization • try to find a WLAN and stay within it • synchronization of internal clock (timing synchronization function (TSF) • For power management • For coordination of PCF (super frame) • For synchronization of hopping sequence in FHSS systems • Generation of beacon signals

  35. 802.11 - MAC management (cont.) • Power management • To control transmitter activity for power conservation • sleep-mode without missing a frame • periodic sleep, frame buffering, traffic measurements • Association/Re-association • integration into a WLAN • roaming, i.e. change networks by changing access points • scanning, i.e. active search for a network • MIB - Management Information Base • managing, read, write, update

  36. Synchronization using a Beacon (infrastructure) • Within a BSS, timing is conveyed by the periodic transmission of a beacon frame • A beacon contains a timestamp and other management information (identification of BSS, power management, roaming) • In infrastructure-based networks, the beacon is sent by the access point periodically. However, it may be delayed if medium is busy, but beacon interval is not shifted if one beacon is delayed. • The time stamp is used by a node to adjust its local clock beacon interval (20ms – 1s) B B B B access point busy busy busy busy medium t B value of the timestamp beacon frame

  37. Synchronization using a Beacon (ad-hoc) • Each node maintains its own timer and starts transmission of a beacon frame after the beacon interval • Using random backoff algorithm, one beacon only wins • All other stations adjust their internal clock according to the received beacon beacon interval B1 B1 station1 B2 B2 station2 busy busy busy busy medium t B value of the timestamp beacon frame random delay

  38. Power management • Power-saving mechanisms are crucial for wireless devices • Standard WLAN protocols assume that stations are always ready to receive data. This permanent readiness consumes much power • Idea: switch the transceiver off if not needed • States of a station: sleep and awake • Timing Synchronization Function (TSF) • stations wake up periodically at the same time • Buffering of data at senders • Senders announce destination during wake periods • Longer off periods save battery life but reduce average throughput and increase delay

  39. Power Management • Infrastructure • Access point buffers all frames destined for stations operating in power-save mode • With every beacon sent, a Traffic Indication Map (TIM) is transmitted • TIM contains a list of unicast receivers transmitted by AP • Beacon interval = TIM interval • Additionally, the AP maintains a Delivery Traffic Indication Map (DTIM) • list of broadcast/multicast receivers transmitted by AP • DTIM interval = multiple of TIM interval • The TSF assures that sleeping stations will wake-up periodically and listen to the beacon and TIM • If TIM indicates a unicast frame buffered for a station, the station stay awake to receive it • Stations always stay awake for muti-cast/ broadcast transmission • Stations also wake-up when they have frames to be transmitted

  40. T D awake TIM DTIM data transmission to/from the station B p d broadcast/multicast PS poll Power saving with wake-up patterns (infrastructure) TIM interval DTIM interval D B T T d D B access point busy busy busy busy medium p d station t

  41. Power saving with wake-up pattern (Ad hoc) • Ad-hoc • Ad-hoc Traffic Indication Map (ATIM) • announcement of receivers by stations buffering frames • more complicated - no central AP • collision of ATIMs possible (scalability?) • APSD (Automatic Power Save Delivery) • new method in 802.11e replacing above schemes

  42. 802.11 - Roaming • No or bad connection? Then perform: • Scanning • scan the environment, • Passive scanning: listen into the medium for beacon signals • Active scanning: send probes into the medium and wait for an answer • Reassociation Request • Choose best AP (e.g. based on signal strength) • station sends a request to one or several AP(s) • Reassociation Response • success: AP has answered, station can now participate • failure: continue scanning • AP accepts Reassociation Request • signal the new station to the distribution system • the distribution system updates its data base (i.e., location information) • typically, the distribution system now informs the old AP so it can release resources • Fast roaming – 802.11r • e.g. for vehicle-to-roadside networks

  43. Data rate 1, 2, 5.5, 11 Mbit/s, depending on SNR User data rate max. approx. 6 Mbit/s Transmission range 300m outdoor, 30m indoor Max. data rate ~10m indoor Frequency DSSS, 2.4 GHz ISM-band Security Limited, WEP insecure, SSID (service set identifier) Availability Many products, many vendors Connection set-up time Connectionless/always on Quality of Service Typ. Best effort, no guarantees (unless polling is used, limited support in products) Manageability Limited (no automated key distribution, sym. Encryption) Special Advantages/Disadvantages Advantage: many installed systems, lot of experience, available worldwide, free ISM-band, many vendors, integrated in laptops, simple system Disadvantage: heavy interference on ISM-band, no service guarantees, slow relative speed only WLAN: IEEE 802.11b

  44. Data rate 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR User throughput (1500 byte packets): 5.3 (6), 18 (24), 24 (36), 32 (54) 6, 12, 24 Mbit/s mandatory Transmission range 100m outdoor, 10m indoor E.g., 54 Mbit/s up to 5 m, 48 up to 12 m, 36 up to 25 m, 24 up to 30m, 18 up to 40 m, 12 up to 60 m Frequency Free 5.15-5.25, 5.25-5.35, 5.725-5.825 GHz ISM-band Security Limited, WEP insecure, SSID Availability Some products, some vendors Connection set-up time Connectionless/always on Quality of Service Typ. best effort, no guarantees (same as all 802.11 products) Manageability Limited (no automated key distribution, sym. Encryption) Special Advantages/Disadvantages Advantage: fits into 802.x standards, free ISM-band, available, simple system, uses less crowded 5 GHz band Disadvantage: stronger shading due to higher frequency, no QoS WLAN: IEEE 802.11a

  45. WLAN: IEEE 802.11– current developments • 802.11j: Extensions for operations in Japan • Changes of 802.11a for operation at 5GHz in Japan using only half the channel width at larger range • 802.11-2007: Current “complete” standard • Comprises amendments a, b, d, e, g, h, i, j • 802.11k: Methods for channel measurements • Devices and access points should be able to estimate channel quality in order to be able to choose a better access point of channel • 802.11m: Updates of the 802.11-2007 standard • 802.11n: Higher data rates above 100Mbit/s • Changes of PHY and MAC with the goal of 100Mbit/s at MAC SAP • MIMO antennas (Multiple Input Multiple Output), up to 600Mbit/s are currently feasible • However, still a large overhead due to protocol headers and inefficient mechanisms • 802.11p: Inter car communications • Communication between cars/road side and cars/cars • Planned for relative speeds of min. 200km/h and ranges over 1000m • Usage of 5.850-5.925GHz band in North America • 802.11r: Faster Handover between BSS • Secure, fast handover of a station from one AP to another within an ESS • Current mechanisms (even newer standards like 802.11i) plus incompatible devices from different vendors are massive problems for the use of, e.g., VoIP in WLANs • Handover should be feasible within 50ms in order to support multimedia applications efficiently

  46. WLAN: IEEE 802.11– current developments • 802.11s: Mesh Networking • Design of a self-configuring Wireless Distribution System (WDS) based on 802.11 • Support of point-to-point and broadcast communication across several hops • 802.11T: Performance evaluation of 802.11 networks • Standardization of performance measurement schemes • 802.11u: Interworking with additional external networks • 802.11v: Network management • Extensions of current management functions, channel measurements • Definition of a unified interface • 802.11w: Securing of network control • Classical standards like 802.11, but also 802.11i protect only data frames, not the control frames. Thus, this standard should extend 802.11i in a way that, e.g., no control frames can be forged. • 802.11y: Extensions for the 3650-3700 MHz band in the USA • 802.11z: Extension to direct link setup • 802.11aa: Robust audio/video stream transport • 802.11ac: Very High Throughput <6Ghz • 802.11ad: Very High Throughput in 60 GHz • Note: Not all “standards” will end in products, many ideas get stuck at working group level • Info:,,