1 wireless fundamentals

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Radio Frequency Fundamentals


Loss (Attenuation)

• Decrease of amplitude or signal strength

– On a wire due to impedance

– Over the air-Free Space Path Loss

• Loss and gain are measured in Deibels

Pg 46


Multipath

•When multiple signals arrive at receiver due to different obstructions/effects on RF signal

•Difference in arrival is delay spread.

Pg 49

Downfade

Upfade

Nulling


2.4 Ghz channels

Pg 204


Overlapping vs. Non-Overlapping

Pg 206


5 Ghz channels

Pg 208


Throughput vs. Bandwidth

• Don’t confuse frequency Bandwidht (size of channels) and data bandwidht (speeds for transmission)– Also different from throughput, which is actual data

performance• Since wireless is half duplex, most of the time you get

50% or the “bandwidth”• Since it is shared, if 5 stations are sharing 20 Mbps,

each will get about 4Mbps of performance

Pg 210


What is SNR ?


Relation between SNR and Data rate

Rate (Mb/s) 1 2 5.5 11 6 9

SNR (dB) 4 6 8 10 4 5

Signal level (dBm) -81 -79 -77 -75 -81 -80

Rate (Mb/s) 12 18 24 36 48 54

SNR (dB) 7 9 12 16 20 21

Signal level (dBm) -78 -76 -73 -69 -65 -64


SNR VS Data rate


MIMO

Each MIMO system is defined with NxM antenna matrix

N – No of Tx antenna

M – No. of Rx antenna

E.g. 3x3 AP means physical presence of 3 transmit and 3 receive antenna.

Each Tx antenna transmits uniquely encoded data stream simultaneously. This

is called Spatial Stream (SS).


Spatial Multiplexing

• Each stream can use the same, or different modulation techniqus

A2B2

B1

A1

A

B1

2


Transmit Beamforming (TxBF)


Channel Bonding


Wide Channels


Bandwidth Total Subcarriers Subcarriers for Data

Multiplier

20 MHz 56 52 1

40 MHz 114 108 (108/52) = 2.07

80 MHz 242 234 (234/52) = 4.5

160 MHz 484 468 (468/52) = 9.0

FCC BandChannel Number

Available Bandwidth

Configurable 80 MHzchannels

Configurable 160 MHzchannels

UNII-1 36-48 5150-52502 1

UNII-2 52-64 5250-5350

UNII-2e 100-144 5470-5720 3 1

UNII-3 149-161 5745-58051 0

ISM 165 5825

Spectral efficiency and channel availability


Short guard interval

a

b

c

a

b

c

Receiv

ed

po

wer l

evel

time

A

B1

2

N N+1 N+2

N N+1 N+2

N N+1 N+2

Guard

intervalInter-symbol

interference

Previous 802.11 standards used a guard interval of 800nsec. 802.11n adds an option for 400nsec


Modulation Coding scheme (Set) ( MCS )

• New standards like, 11n and 11ac will use best combination of

Modulation and coding ( MCS ) to select the datarate ( 70 + in 11n and

10 in 11ac )

• MCS leads to a wide range of speeds depend on

• Channel condition,

• best combination of datarates,

• bonded channels,

• multiple spatial streams,

• different guard intervals

• modulation types and coding

• Modulation describes how many bits are contained within one

transmission time increment. Higher modulations pack more data into

the transmission, but they require much higher signal-to-noise ratios

• Error-correcting code adds redundant information in a proportion

described by the code rate.

• A code at rate R=1/2 transmits one user data bit (the numerator) for

every two bits (the denominator) on the channel.

• Higher code rates have more data and less redundancy at the cost of

not being able to recover from as many errors.


QAM

quadrature amplitude modulation (QAM) works by using the combination of amplitude level and phase shift to select one of many symbols

To identify each of the values, there will be a unique combination of phase

shift and quadrature (roughly speaking, the amplitude of a wave).

Ex : 16 QAM = 4 Phase shifts and 4 Quadrature

64 QAM = 8 Phase shifts and 8 Quadrature

256 QAM = 16 Phase shifts and 16 Quadrature

Each time a symbol is transmitted, it may take on one of the phase shifts

and one of amplitude levels combination.

With 256 QAM we can transmit 256 symbols at a time.


MCS Index

MCS index value Modulation Code rate (R)

0 BPSK 1/2

1 QPSK 1/2

2 QPSK 3/4

3 16-QAM 1/2

4 16-QAM 3/4

5 64-QAM 2/3

6 64-QAM 3/4

7 64-QAM 5/6

8 256-QAM 3/4

9 256-QAM 5/6


Speed with Combo of MCS, CH width and SS

MCS value20 MHz data

rate (1SS, short GI)

Spatial stream multiplication factor

Channel width multiplication

factor

Maximum 40 MHz rate (8 SS,

short GI)

Maximum 80 MHz rate (8 SS, short GI)

Maximum 160 MHz rate (8 SS,

short GI)

MCS 0 7.2 Mbps

x2 for 2 streamsx3 for 3 streamsx4 for 4 streamsx5 for 5 streamsx6 for 6 streamsx7 for 7 streamsx8 for 8 streams

x1.0 for 20 MHzx2.1 for 40 MHzx4.5 for 80 MHz

x9.0 for 160 MHz

120.0 Mbps 260.0 Mbps 520.0 Mbps

MCS 1 14.4 240.0 520.0 1040.0

MCS 2 21.7 360.0 780.0 1560.0

MCS 3 28.9 480.0 1040.0 2080.0

MCS 4 43.3 720.0 1560.0 3120.0

MCS 5 57.8 960.0 2080.0 4160.0

MCS 6 65.0 1080.0 2340.0 4680.0

MCS 7 72.2 1200.0 2600.0 5200.0

MCS 8 86.7 1440.0 3120.0 6240.0

MCS 9 96.3 1600.0 3466.7 6933.3


802.11 n

• 2.4 Ghz AND 5 Ghz

• High Throughput-HT

• Improved OFDM

– Instead of using 48 sub carriers, it uses 52 sub carriers

• MIMO technology with OFDM

– Increased throughput and greater range

– Radio Chains

• spatial multiplexing

• Transmit beam forming

– Can use max 4 SS

• Channel Bonding

– Uses (20 + 20) MHz in 2.4 GHz and 40MHz in 5 GHz channels

• Short god interval

– Uses 400 ns Guard interval

• Block acknowledgement


IEEE 802.11 n

300 Mbps with

2 Streams

600 Mbps with

4 Streams

450 Mbps with

3 Streams


802.11 ac

• Wide Channels 80 MHz channels (contiguous)

160 MHz channels (contiguous or two non-contiguous 80 MHz slices)

256-QAM (Quadrature amplitude modulation)

Provides a 33% increase in throughput over the 64-QAM used in 11n

• Short Guard Interval ( 400 ns )

• Up to 8 spatial streamsCompared to a maximum of 4 spatial streams in 11n

Up to 8 spatial streams in both single-user and multi-user modes

• Downlink Multi-user MIMOAllows 1 AP to transmit unique data to multiple stations simultaneously

• Independent virtual carrier sense in sub-channels


Speed comparison

Technology 20 MHz[a] 40 MHz 80 MHz 160 MHz

802.11b 11 Mbps

802.11a/g 54 Mbps

802.11n (1 SS) 72 Mbps 150 Mbps

802.11ac (1 SS) 87 Mbps 200 Mbps 433 Mbps 867 Mbps

802.11n (2 SS) 144 Mbps 300 Mbps

802.11ac (2 SS) 173 Mbps 400 Mbps 867 Mbps 1.7 Gbps

802.11n (3 SS) 216 Mbps 450 Mbps

802.11ac (3 SS) 289 Mbps 600 Mbps 1.3 Gbps 2.3 Gbps[b]

802.11n (4 SS)[c] 289 Mbps 600 Mbps

802.11ac (4 SS) 347 Mbps 800 Mbps 1.7 Gbps 3.5 Gbps

802.11ac (8 SS) 693 Mbps 1.6 Gbps 3.4 Gbps 6.9 Gbps

http://chimera.labs.oreilly.com/books/1234000001739/ch02.html#ftn.id449471




If the client’s user credentials are successfully authenticated, the AS will send a Master Session Key (MSK) to the AP and Client.

802.1X/EAP

The MSK cannot be sent as a cleartext, which eventually will cause a security threat.

So the MSK has to be sent to the AP and Client in a secure way as a cipher text. This goal is achieved by EAP, Extensible Authentication Protocol.

The 802.1X authentication in which EAP is used to protect the authentication data is called as 802.1X/EAP.

802.1X/EAP authentication occurs in three phases -Initiation -EAP Negotiation -Authentication


4 Way Handshake

• The RSNA process creates multiple keys

– Group Master Key (GMK)

– Pairwise Master Key (PMK)• PMK can also be created from a Pre-Shared Key (PSK)


Deriving PTK and GTK


Roaming


5 Steps of Roaming


Example of Roaming


Types of Roaming


L2 Roaming


L3 Roaming


PMK Caching

36


OKC

37


What is IEEE 802.11K

1 wireless fundamentals

Engineering

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