Wireless local area network IEEE802.11WLAN.ppt
- 1. Wireless LAN
- 2. Wireless LAN Advantages very flexible within the reception area 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... Disadvantages typically very low bandwidth compared to wired networks (1-10 Mbit/s) due to shared medium many proprietary solutions, especially for higher bit-rates, standards take their time (e.g. IEEE 802.11) products have to follow many national restrictions if working wireless, it takes a vary long time to establish global solutions like, e.g., IMT-2000
- 3. 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 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
- 4. Transmission Technologies to Set up WLANs Infrared 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 low bandwidth 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
- 5. Comparison: infrastructure vs. ad-hoc networks Infrastructure network Ad-hoc network AP AP AP wired network AP: Access Point
- 6. 802.11 - Architecture of an 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 Distribution System Portal 802.x LAN Access Point 802.11 LAN BSS2 802.11 LAN BSS1 Access Point STA1 STA2 STA3 ESS
- 7. 802.11 - Architecture of an 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 802.11 LAN IBSS2 802.11 LAN IBSS1 STA1 STA4 STA5 STA2 STA3
- 8. IEEE standard 802.11 mobile terminal access point fixed terminal application TCP 802.11 PHY 802.11 MAC IP 802.3 MAC 802.3 PHY application TCP 802.3 PHY 802.3 MAC IP 802.11 MAC 802.11 PHY LLC infrastructure network LLC LLC
- 9. 802.11 - Layers and functions PLCP Physical Layer Convergence Protocol clear channel assessment signal (carrier sense) PMD Physical Medium Dependent modulation, coding PHY Management channel selection, MIB Station Management coordination of all management functions PMD PLCP MAC LLC MAC Management PHY Management MAC access mechanisms, fragmentation, encryption MAC Management synchronization, roaming, MIB, power management PH Y DL C Station Management
- 10. 802.11 - Physical layer (classical) 3 versions: 2 radio (typ. 2.4 GHz), 1 IR data rates 1 or 2 Mbit/s FHSS (Frequency Hopping Spread Spectrum) spreading, despreading, signal strength, typ. 1 Mbit/s min. 2.5 frequency hops/s (USA), two-level GFSK modulation DSSS (Direct Sequence Spread Spectrum) DBPSK modulation for 1 Mbit/s (Differential Binary Phase Shift Keying), DQPSK for 2 Mbit/s (Differential Quadrature PSK) preamble and header of a frame is always transmitted with 1 Mbit/s, rest of transmission 1 or 2 Mbit/s chip sequence (11 symbols) : +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker code) max. radiated power 1 W (USA), 100 mW (EU), min. 1mW Infrared 850-950 nm, diffuse light, typ. 10 m range carrier detection, energy detection, synchronization
- 11. FHSS PHY packet format synchronization SFD PLW PSF HEC payload PLCP preamble PLCP header 80 16 12 4 16 variable bits Synchronization synch with 010101... pattern SFD (Start Frame Delimiter) 0000110010111101 start pattern PLW (PLCP_PDU Length Word) length of payload incl. 32 bit CRC of payload, PLW < 4096 PSF (PLCP Signaling Field) data of payload (1 or 2 Mbit/s) HEC (Header Error Check) CRC with x16 +x12 +x5 +1
- 12. DSSS PHY packet format synchronization SFD signal service HEC payload PLCP preamble PLCP header 128 16 8 8 16 variable bits length 16 Synchronization synch., gain setting, energy detection, frequency offset compensation SFD (Start Frame Delimiter) 1111001110100000 Signal data rate of the payload (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK) Service Length future use, 00: 802.11 compliant length of the payload HEC (Header Error Check) protection of signal, service and length, x16 +x12 +x5 +1 128 +16 + 8 + 8 + 16 + 16 = 192 bit a 1 Mbps -> 192 us (em 802.11b a partir de “signal“ pode ser a 2 Mbps)
- 13. 802.11 - MAC layer I - DFWMAC Traffic services Asynchronous Data Service (mandatory) exchange of data packets based on “best-effort” support of broadcast and multicast Time-Bounded Service (optional) implemented using PCF (Point Coordination Function) Access methods 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) Distributed Foundation Wireless MAC avoids hidden terminal problem DFWMAC- PCF (optional) access point polls terminals according to a list
- 14. 802.11 - MAC layer II 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 Tslot = 9; SIFS = 16; PIFS = 25; DIFS = 34 us t medium busy SIFS PIFS DIFS DIFS next frame contention direct access if medium is free DIFS
- 15. t medium busy DIFS DIFS next frame contention window (CW) (randomized back-off mechanism) 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 an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type) 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) CW = 7, 15, 31, 63, 127 if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness) slot time direct access if medium is free DIFS
- 16. 802.11 - Binary Exponential Backoff Stations choose their backoff time randomly from contention Window Ideal contention window size is trade-off between acceptable load and experienced delay Initial contention window size (CWmin) is 7 slots (backoff time between 0 and 7) After collision (no ack), contention window is “doubled” until CWmax = 255 is reached: 7 -> 15 -> 31 -> 63 -> 127 -> 255
- 17. 802.11 - competing stations - simple version (no RTS/CTS) t busy boe station1 station2 station3 station4 station5 packet arrival at MAC DIFS boe boe boe busy elapsed backoff time bor residual backoff time busy medium not idle (frame, ack etc.) bor bor DIFS boe boe boe bor DIFS busy busy DIFS boe busy boe boe bor bor
- 18. 802.11 - CSMA/CA access method II 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 t SIFS DIFS data ACK waiting time other stations receiver sender data DIFS contention
- 19. 802.11 – DFWMAC (Distributed Foundation Wireless MAC) Sending unicast packets 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 t SIFS DIFS data ACK defer access other stations receiver sender data DIFS contention RTS CTS SIFS SIFS NAV (RTS) NAV (CTS) NAV – Network Allocation Vector
- 20. Timing diagram of collision and successful transmission. (a) RTS/CTS collision, (b) RTS/CTS successful transmission, (c) Basic frame collision (d) Basic frame successful transmission ( note: in (a) and (c), crossed block represents collision).
- 21. Fragmentation t SIFS DIFS data ACK1 other stations receiver sender frag1 DIFS contention RTS CTS SIFS SIFS NAV (RTS) NAV (CTS) NAV (frag1) NAV (ACK1) SIFS ACK2 frag2 SIFS
- 23. DFWMAC-PCF II (cont.) t stations‘ NAV wireless stations point coordinator D3 NAV PIFS D4 U4 SIFS SIFS CFend contention period contention free period t2 t3 t4 CFend - contention free period end
- 24. 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 (logical) Miscellaneous sending time, checksum, frame control, data Frame Control Duration/ ID Address 1 Address 2 Address 3 Sequence Control Address 4 Data CRC 2 2 6 6 6 6 2 4 0-2312 bytes Protocol version Type Subtype To DS More Frag Retry Power Mgmt More Data WEP 2 2 4 1 From DS 1 Order bits 1 1 1 1 1 1 MAC header + trailer = 34 octets
- 25. MAC address format scenario to DS from DS address 1 address 2 address 3 address 4 ad-hoc network 0 0 DA SA BSSID - infrastructure network, from AP 0 1 DA BSSID SA - infrastructure network, to AP 1 0 BSSID SA DA - infrastructure network, within DS 1 1 RA TA DA SA DS: Distribution System AP: Access Point DA: Destination Address SA: Source Address BSSID: Basic Service Set Identifier RA: Receiver Address (AP) TA: Transmitter Address (AP)
- 26. Endereços MAC 802.11 00 – adhoc DA, SA, BSSID 01 – wired to wireless DA, BSSID, SA 10 – wireless to wired BSSID, SA, DA 11 – via wireless (bridge) RA, TA, DA, SA SA TA RA DA SA DA SA BSSID DA (BSSID) SA BSSID DA DA,SA DA,SA DA,SA DA,SA
- 27. Special Frames: ACK, RTS, CTS Acknowledgement Request To Send Clear To Send Frame Control Duration Receiver Address Transmitter Address CRC 2 2 6 6 4 bytes Frame Control Duration Receiver Address CRC 2 2 6 4 bytes Frame Control Duration Receiver Address CRC 2 2 6 4 bytes ACK RTS CTS
- 28. 802.11 - MAC management Synchronization try to find a WLAN, try to stay within a WLAN timer etc. Power management sleep-mode without missing a message periodic sleep, frame buffering, traffic measurements Association/Reassociation integration into a LAN roaming, i.e. change networks by changing access points scanning, i.e. active search for a network MIB - Management Information Base managing, read, write (SNMP)
- 29. Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS05 7.29 Synchronization using a Beacon (infrastructure) beacon interval t medium access point busy B busy busy busy B B B value of the timestamp B beacon frame (BSSID, Timestamp)
- 30. Synchronization using a Beacon (ad-hoc) t medium station1 busy B1 beacon interval busy busy busy B1 value of the timestamp B beacon frame station2 B2 B2 random delay
- 31. Power management Idea: switch the transceiver off if not needed States of a station: sleep and awake Timing Synchronization Function (TSF) stations wake up at the same time Infrastructure Traffic Indication Map (TIM) list of unicast receivers transmitted by AP Delivery Traffic Indication Map (DTIM) list of broadcast/multicast receivers transmitted by AP 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?)
- 32. Power saving with wake-up patterns (infrastructure) TIM interval t medium access point busy D busy busy busy T T D T TIM D DTIM DTIM interval B B B broadcast/multicast station awake p PS poll p d d d data transmission to/from the station PS – Power Saving
- 33. Power saving with wake-up patterns (ad-hoc) awake A transmit ATIM D transmit data t station1 B1 B1 B beacon frame station2 B2 B2 random delay A a D d ATIM window beacon interval a acknowledge ATIM d acknowledge data
- 34. Scanning Scanning involves the active search for a BSS. IEEE 802.11 differentiates between passive and active scanning. Passive scanning - listening into the medium to find other networks, i.e., receiving the beacon of another network issued by access point. Active scanning - sending a probe on each channel and waiting for a response. Beacon and probe responses contain the information necessary to join the new BSS.
- 35. Active Scanning
- 36. 802.11 - Roaming No or bad connection? Then perform: Scanning scan the environment, i.e., listen into the medium for beacon signals or send probes into the medium and wait for an answer Reassociation Request 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
- 37. Roaming: Active Scanning / Authentication/ Reassociation
- 38. Handoff with IAPP (Inter Access Point Protocol), IEEE 802.11f