1 wireless fundamentals

CONFIDENTIAL
© Copyright 2014. Aruba Networks, Inc. All rights reserved
Radio Frequency Fundamentals

CONFIDENTIAL © Copyright 2014. Aruba Networks, Inc. All rights reserved
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
A2
B2
B1
A1
A
B
1
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 Band
Channel
Number
Available
Bandwidth
Configurable
80 MHz
channels
Configurable
160 MHz
channels
UNII-1 36-48 5150-5250
2 1
UNII-2 52-64 5250-5350
UNII-2e 100-144 5470-5720 3 1
UNII-3 149-161 5745-5805
1 0
ISM 165 5825
Spectral efficiency and channel availability

Short guard interval
a
b
c
a
b
c
Receivedpowerlevel
time
A
B
1
2
N N+1 N+2
N N+1 N+2
N N+1 N+2
Guard
interval
Inter-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 value
20 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 streams
x3 for 3 streams
x4 for 4 streams
x5 for 5 streams
x6 for 6 streams
x7 for 7 streams
x8 for 8 streams
x1.0 for 20 MHz
x2.1 for 40 MHz
x4.5 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 streams
Compared 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 MIMO
Allows 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

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

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