7. Wireless Local Area Networks Characteristics IEEE 802.11 HIPERLAN, WATM, BRAN, HIPERLAN2 Bluetooth RF Comparison © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M7 – 1 Characteristics of Wireless LANs 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 – Quite cheap networking infrastructures possible Drawbacks: – Typically very low bandwidth compared to wired networks: 1-10 Mbit/s and error rates of about 10-4 instead of 10-12 – 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 – Lack of security, “open” air interface, War Driving © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M7 – 2 1 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 co-operation 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 © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M7 – 3 Comparison: Infrared vs. Radio Transmission Infrared: Radio: – Uses IR diodes, multiple reflections, – Typically using the license free ISM diffuse light, e.g., walls or furniture band at 2.4 GHz Advantages: Advantages: – Simple, cheap, available in many mobile – Experience from wireless WAN and devices mobile phones can be used – No licenses needed – coverage of larger areas possible, – Simple shielding possible e.g., radio can penetrate walls, furniture Drawbacks: – Interference by sunlight, heat sources Drawbacks: – Many things shield/absorb IR light, LoS – Very limited license free frequency – Low bandwidth (115 kbit/s … 4 Mbit/s) bands – Shielding more difficult, interference Example: with other electrical devices – IrDA (Infrared Data Association) interface available everywhere at Examples: 900 nm wave length – WaveLAN, HIPERLAN, Bluetooth © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M7 – 4 2 Comparison: Infrastructure vs. Ad-hoc Nets Infrastructure-based Network AP: Access Point AP AP Wired Network AP Ad-hoc Network © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M7 – 5 IEEE 802.11 — Architecture of an Infrastructure Network Station (STA): 802.11 LAN 802.x LAN – Terminal with access mechanisms to the wireless medium and radio contact to the access point STA1 BSS1 Basic Service Set (BSS): Access Portal – Group of stations using the same Point radio frequency Distribution System Access Point: Access – Station integrated into the wireless ESS Point LAN and the distribution system Portal: BSS2 – Bridge to other (wired) networks Distribution System: – Interconnection network to form one STA STA 2 802.11 LAN 3 logical network (ESS: Extended Service Set) based on several BSS © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M7 – 6 3 IEEE 802.11 — Architecture of an Ad-hoc Network Direct communication within a 802.11 LAN limited range: – Station (STA): STA1 • Terminal with access mechanisms STA IBSS1 3 to the wireless medium – Independent Basic Service Set (IBSS): • Group of stations using the same STA2 radio frequency IBSS2 STA5 STA4 802.11 LAN © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M7 – 7 IEEE Standard 802.11 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 © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M7 – 8 4 IEEE 802.11 — Layers and Functions MAC: MAC Management: – Access mechanisms, fragmentation, – Authentication, synchronization, encryption roaming, MIB, power management PLCP Physical Layer Convergence Protocol: PHY Management: – Clear Channel Assessment (CCA) – Channel selection, MIB signal (carrier sense) Station Management: PMD Physical Medium Dependent: – Coordination of all management – Modulation, coding functions LLC MAC MAC Management PLCP PHY Management PHY DLC PMD Station Management © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M7 – 9 IEEE 802.11 — Physical layer 3 versions: 2 radio (typical 2.4 GHz), 1 IR: – Data rates of 1 Mbit/s mandatory and 2 Mbit/s optional FHSS (Frequency Hopping Spread Spectrum): – Spreading, de-spreading, signal strength, typical 1 Mbit/s, 79 channels US/EU – Min. 2.5 frequency hops/s (USA), two-level GFSK (Gauß FSK) 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 – Chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker code), 11 Mhz chipping rate – Max. radiated power 1 W (USA), 100 mW (EU), min. 1mW Infrared: – 850-950 nm, diffuse light, typical 10 m range – Carrier detection, energy detection, synchronization © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M7 – 10 5 IEEE 802.11 — FHSS PHY Packet Format Synchronization: – Synchronization with 010101... pattern SFD (Start Frame Delimiter): – 0000110010111101 start pattern PLW (PLCP_PDU Length Word): – Length of payload including 32 bit CRC of payload, PLW < 4096 byte PSF (PLCP Signaling Field): – Data of payload (1 or 2 Mbit/s): 0 = 1 Mbit/s, 10 = 2 Mbit/s etc. HEC (Header Error Check) – CRC with x16+x12+x5+1 80 16 12 4 16 variable bit Synchronization SFD PLW PSF HEC Payload PLCP Preamble PLCP Header © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M7 – 11 IEEE 802.11 — DSSS PHY Packet Format Synchronization: – Synchronization, 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: – Future use, 00: 802.11 compliant Length: – Length of the payload (measured in ms) HEC (Header Error Check) – Protection of signal, service, and length, x16+x12+x5+1, ITU-T-CRC-16 standard 128 16 8 816 16 variable bit Synchronization SFD Signal Service Length HEC Payload PLCP Preamble PLCP Header © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M7 – 12 6 IEEE 802.11 — MAC Layer DFWMAC (1) Traffic services (Roaming, Authentication, Energy Savings): – Asynchronous Data Service (mandatory): • Exchange of data packets based on “best-effort” • Support of broadcast and multicast, however, no QoS support – Time-Bounded Service (optional): • Implemented using PCF 3 Access methods: – DFWMAC-DCF CSMA/CA (mandatory) (Distributed Foundation Wireless MAC): • Collision avoidance via randomized „back-off“ mechanism • Minimum distance between consecutive packets • ACK packet for acknowledgements (not for broadcasts) – DFWMAC-DCF with RTS/CTS (optional): • Avoids hidden terminal problem DCF: Distributed Coordination Function – DFWMAC- PCF (optional): PCF: Point Coordination Function • Access point polls terminals according to a list © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M7 – 13 IEEE 802.11 — MAC Layer DFWMAC (2) Priorities (determine waiting time before medium access): – Defined through different Inter Frame Spaces (IFS) – No guaranteed, hard priorities – SIFS (Short Inter Frame Spacing): • Highest priority, for ACK, CTS, polling response: DSSS 10 ms, FHSS 28 ms – PIFS (Point Coordination Function IFS): • Medium priority, for time-bounded service using PCF, polling of terminals, PIFS = SIFS plus time slot duration – DIFS (Distributed Coordination Function IFS): • Lowest priority, for asynchronous data service, DIFS = SIFS plus 2 time slots DIFS DIFS PIFS SIFS Medium Busy Contention Next Frame t Direct access if Clear Channel Assessment (CCA) medium is free ≥ DIFS © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M7 – 14 7 IEEE 802.11 — CSMA/CA Access Method (1) Contention Window DIFS DIFS (randomized back-off mechanism) Medium Busy Next Frame Direct Access if t medium is free ≥ DIFS Slot Time – 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 wait additionally 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) © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M7 – 15 IEEE 802.11 — Competing Stations/Simple 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 Station 5 t Medium not idle (frame, ack etc.) Busy boe Elapsed back-off time Packet arrival at MAC bor Residual back-off time © 2005 Burkhard Stiller and Jochen Schiller FU Berlin M7 – 16 8 IEEE 802.11 — CSMA/CA Access Method (2) Sending unicast packets: