IEEE 802 Series 802.1 LAN/MAN Bridging & Management 802.2 Logical Link Control802.3 CSMA/CD (Ethernet)802.4 Token Bus802.5 Token Ring802.11 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

Wireless LAN

Single MAC 802.11 over multiple PHY

802.11 MAC Layer

2.4GHz FHSS1Mbps

2Mbps

2.4GHz DSSS1Mbps

2Mbps

Infrared

IR1Mbps

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

The 802.11 Protocol Stack

WLAN PHY

Part of the 802.11 protocol stack.

802.11g OFDM/54 Mbps

WLAN PHY

Standard 802.11a 802.11b 802.11g

Number of channels X

Interference X

Bandwidth X X

Power consumption X X

Range/ penetration X X

Upgrade/ compatibility

X

Price X X

X indicates superior technology or feature.

802.11 MAC Sublayer Protocol

WLAN PHY

The hidden station

problem

The exposed station

problem

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.

TerminologyFHSS Frequency Hopping Spread SpectrumDSSS Direct Sequence Spread SpectrumCSMA/CA : Collision AvoidanceDCF Distributed Coordination FunctionPCF Centralized Coordination FunctionHCF Hybrid Coordination FunctionEDCA Enhanced Distributive Channel AccessMMPDU MAC Management Protocol Data UnitIFS Inter-Frame SpaceSTA StationBSS Basic Service SetAP Access Point DS Distribution System

WLAN PHY

AP APSTASTA

STA STA

STA

STA

STA

BSS 1 BSS 2DS

Fading in Physical Layer

Multipathsmall scale

Shadowing ObstacleLarge scale

Doppler Effectmovement

WLAN PHY

Dispersion Pr=Pt·K/d2

Reflection ( 반사 )

Diffraction ( 회절 )

Scattering ( 산란 )

Refraction ( 굴절 )

Absorption Other channel Movement

antenna

transmitted

-10ς·log10[d/d0]+X

Grocery store ς = 1.8

Offices ς = 2.3-3

Path Loss (d)

Random noise

Doppler Effect

Doppler shift Doppler spreadΔfd ≤ v/λ

5.2GHz 3.6 km/hourv = 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

Delay spread : ISI and BC

Delay spread Tm>TsISI (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.

WLAN PHY

sender

receiver

Inter-symbol time, Ts

Delay spread, Tm

Combined Path Loss and Fading

WLAN PHY

Rece

ived

sig

nal st

ren

gth

(lo

g s

cale

)

Distance between transmitter and receiver

Large scale fading

Path loss

Small scale fading ≈λ

Diversity

Space diversity to reduce small-scale fadindSIMO (Single input multiple output) MIMO (Multiple input multiple output)

Time diversity : temporal duplicationFreq. diversity : spectrum duplicationRake receivers :

coherent by using multiple receivers

WLAN PHY

Spread spectrum

DSSS (Direct Sequence SS)

WLAN PHY

-1 1 -11 -1 1 -11 -11 -1 1 -11 -1 1 -1 1 -11 -1 1 -11 -1 1 -11 -1 1 -11

0 1 0 0

Decoded by inner production with chip sequence : Note that 1ⅹ1=(-1)ⅹ(-1)=1

Spread spectrum : 1/Td1/Tc (d: data, c: chip)

FHDS (Frequency hopping DS)

1

2

5

4

3

frequ

ency

time

OFDM (Orthogonal Freq. DM)

Nulls in the spectrum coincide with the center frequency of adjacent channel

Pro : without ISI, efficient bandwidth useCon : complexity (FFT), need accuracy in carrier frequency

WLAN PHY

4MbpsData

Stream

demultiplexed

1 Mbps

1 Mbps

1 Mbps

1 Mbps

modulationmodulationmodulationmodulation

frequency