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IEEE 802 Series

IEEE 802 Series. 802.1 LAN/MAN Bridging & Management 802.2 Logical Link Control 802.3 CSMA/CD (Ethernet) 802.4 Token Bus 802.5 Token Ring 802.11 Wireless LAN/MAN 802.11a (FHSS), b (DSSS), e (QoS), p (Vehicle) 802.15 Wireless Personal Area Networks Bluetooth, UWB, etc. Network layer.

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IEEE 802 Series

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  1. IEEE 802 Series • 802.1 LAN/MAN Bridging & Management • 802.2 Logical Link Control • 802.3 CSMA/CD (Ethernet) • 802.4 Token Bus • 802.5 Token Ring • 802.11 Wireless LAN/MAN • 802.11a (FHSS), b (DSSS), e (QoS), p (Vehicle) • 802.15 Wireless Personal Area Networks • Bluetooth, UWB, etc. Network layer LLC sublayer MAC sublayer PHY WLAN PHY

  2. Wireless LAN • Single MAC 802.11 over multiple PHY PHY Layer * λ=0.125m

  3. The 802.11 Protocol Stack • Part of the 802.11 protocol stack. WLAN PHY

  4. 802.11g OFDM/54 Mbps X indicates superior technology or feature. WLAN PHY

  5. 802.11 MAC Sublayer Protocol The hidden station problem The exposed station problem WLAN PHY

  6. RTS and CTS A sending an RTS to B. RTS (Request To Send) : “A” doesn’t know that “B” is ready for listen or that “B” is communicating with “D.” B responding with a CTS to A. CTS (Clear To Send) : “I am talking with others.” Imagine there are two 802.11 end users (Station A and Station B) and one access point. Station A and Station B can't hear each other because of high attenuation (e.g., substantial range), but they can both communicate with the same access point. Because of this situation, Station A may begin sending a frame without noticing that Station B is currently transmitting (or vice versa). This will very likely cause a collision between Station A and Station B to occur at the access point. As a result, both Station A and Station B would need to retransmit their respective packets, which results in higher overhead and lower throughput. WLAN PHY

  7. AP AP STA STA STA STA STA STA STA Terminology • FHSS Frequency Hopping Spread Spectrum • DSSS Direct Sequence Spread Spectrum • CSMA/CA : Collision Avoidance • DCF Distributed Coordination Function • PCF Centralized Coordination Function • HCF Hybrid Coordination Function • EDCA Enhanced Distributive Channel Access • MMPDU MAC Management Protocol Data Unit • IFS Inter-Frame Space • STA Station • BSS Basic Service Set • AP Access Point • DS Distribution System BSS 2 BSS 1 DS WLAN PHY

  8. Fading in Physical Layer • Multipath • small scale • Shadowing • Obstacle • Large scale • Doppler Effect • movement transmitted antenna • Dispersion • Pr=Pt·K/d2 • Reflection (반사) • Diffraction (회절) • Scattering (산란) • Refraction (굴절) • Absorption • Other channel • Movement -10ς·log10[d/d0]+X Grocery store ς = 1.8 Offices ς = 2.3-3 • Path Loss (d) Random noise WLAN PHY

  9. Doppler Effect • Doppler shift  Doppler spread Δfd ≤ v/λ • 5.2GHz 3.6 km/hour • v = 3.6km/h = 1m/s (walking speed) • λ = 3ⅹ108[m/s]/5.2ⅹ109[1/s]=0.058m • (Δfd)max =1/0.058=17.3 Hz • Coherent time TC = 1/Δfd • Slow fading TC>TS, fast fading TC<TS • TS : inter-symbol time WLAN PHY

  10. Delay spread : ISI and BC • Delay spread Tm>TsISI (Inter-Symbol Interference) • Small office (20-50 ns), Shopping mall (50-100 ns) • Factory (100-200 ns), • Urban (~1μs), Rural <25 μs • Coherent bandwidth BC ≈ 1/Tm > f1-f2 • Two frequency components experience similar attenuation if f1-f2 << BC, coherent bandwidth. Inter-symbol time, Ts sender Delay spread, Tm receiver WLAN PHY

  11. Combined Path Loss and Fading Received signal strength (log scale) Path loss Large scale fading Small scale fading ≈λ Distance between transmitter and receiver WLAN PHY

  12. Diversity • Space diversity • to reduce small-scale fadind • SIMO (Single input multiple output) • MIMO (Multiple input multiple output) • Time diversity : temporal duplication • Freq. diversity : spectrum duplication • Rake receivers : • coherent by using multiple receivers WLAN PHY

  13. Spread spectrum -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 • DSSS (Direct Sequence SS) 0 1 0 0 • Decoded by inner production with chip sequence : Note that 1ⅹ1=(-1)ⅹ(-1)=1 • Spread spectrum : 1/Td1/Tc (d: data, c: chip) • FHDS (Frequency hopping DS) frequency 4 2 1 5 3 time WLAN PHY

  14. OFDM (Orthogonal Freq. DM) • Nulls in the spectrum coincide with the center frequency of adjacent channel • Pro : without ISI, efficient bandwidth use • Con : complexity (FFT), need accuracy in carrier frequency modulation 4Mbps Data Stream 1 Mbps modulation demultiplexed 1 Mbps modulation 1 Mbps modulation 1 Mbps frequency WLAN PHY

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