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. WLAN PHY PHY MAC sublayer LLC sublayer Network layer

2. Wireless LAN Single MAC 802.11 over multiple PHY 802.11 MAC Layer 2.4GHz FHSS 1Mbps 2Mbps 2.4GHz DSSS 1Mbps 2Mbps Infrared IR 1Mbps 2Mbps 802.11a 5 GHz OFDM 6, 9, 12, 18, 24, 36, 48, 54 Mbps 802.11a 2.4 GHz DSSS 5.5, 11 Mbps PHY Layer * λ=0.125m

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

4. 802.11g OFDM/54 Mbps WLAN PHY Standard802.11a802.11b802.11g Number of channels X Interference X Bandwidth XX Power consumption XX Range/ penetration XX Upgrade/ compatibility X Price XX X indicates superior technology or feature.

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

6. RTS and CTS WLAN PHY 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.

7. 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 WLAN PHY AP STA BSS 1 BSS 2 DS

8. Fading in Physical Layer Multipath small scale Shadowing Obstacle Large scale Doppler Effect movement WLAN PHY  Dispersion  P r =P t ·K/d 2  Reflection ( 반사 )  Diffraction ( 회절 )  Scattering ( 산란 )  Refraction ( 굴절 )  Absorption  Other channel  Movement antenna transmitted -10ς·log 10 [d/d 0 ]+X Grocery store ς = 1.8 Offices ς = 2.3-3  Path Loss (d) Random noise

9. Doppler Effect Doppler shift  Doppler spread Δf d ≤ v/λ 5.2GHz 3.6 km/hour v = 3.6km/h = 1m/s (walking speed) λ = 3 ⅹ 10 8 [m/s]/5.2 ⅹ 10 9 [1/s]=0.058m (Δf d ) max =1/0.058=17.3 Hz Coherent time T C = 1/Δf d Slow fading T C >T S, fast fading T C <T S T S : inter-symbol time WLAN PHY

10. Delay spread : ISI and B C Delay spread T m >T s  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 B C ≈ 1/T m > f 1 -f 2 Two frequency components experience similar attenuation if f 1 -f 2 << B C, coherent bandwidth. WLAN PHY sender receiver Inter-symbol time, Ts Delay spread, Tm

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

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 DSSS (Direct Sequence SS) WLAN PHY 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0100  Decoded by inner production with chip sequence : Note that 1 ⅹ 1=(-1) ⅹ (-1)=1  Spread spectrum : 1/T d  1/T c (d: data, c: chip)  FHDS (Frequency hopping DS) 1 2 5 4 3 frequency time

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 WLAN PHY 4Mbps Data Stream demultiplexed 1 Mbps modulation frequency