Wifi basics

How Wireless Works#

As Radio Frequency (RF)
Travels in a portion of electromagnetic spectrum (RF is part of electromagnetic spectrum)
What is electromagnetic spectrum?
- The range of all possible electromagnetic radiation
- Waves that can move through matter or space

Electromagnetic Spectrum (in decreasing wavelength top to down)

                    - - Broadcast band (10^3 meters or 1 kilometer) 
                    | -
                    | - Radio (10^0 m or 1 meter)
Radio Spectrum    --| -
                    | - Radar (10^-3 m or 1 millimeter) 
                    | -
                    - - Microwaves 
                    | -
                    | - Infrared (IR) (10^-6 m or 1 micrometer )
                    | - <--- Visible lights to human (700 nanometers to 400 nanometers)
                    | -
Optical Spectrum  --| - Ultraviolet (UV)
 (can be seen       | -
 using some tool)   | - X-rays (10^-9 m or 1 nanometer)
                    | -
                    | - Gamma rays
                    | -
                    - - Cosmic rays (10^-12 m or 1 picometer)

- WiFi access point transmits RF signal at a particular wavelength - The characteristic of wavelength and environment determines how far the signal can travel and deliver data

Wireless SWOTs#

(think w.r.t. a Wi-Fi client)

Strength#

Convenience (access without a wire)
Mobility (clients can move)
Productivity (access from various locations)
Deployment (no cabling)
Extendability (easily scale up & down)
Expense (lower ownership & maintenance cost)

Weakness#

Security (data travels through RF and can be captured by unknows in between; need encyption)
Range (is limited)
Speed (usually no faster than 54 Mbps - will improve in near future)
Complexity (wavelenght, channel, band, spectrum etc)

Opportunity#

Increased Speed (802.11ac promise drastic improvement)
Better Security (continues to improve, VLAN, etc.)

Threat#

Usual security threats
- Denial of Service (DoS)
- Rogue WAPs
- Poorly configured WAPs
- Sniffing
- Wireless Driver Attacks

Wireless Signal Characteristics#

(Defined by law of Physics)

Wavelength#

What: Distance between one complete oscillation (from positive crest to negative valley & back)
Unit:

Frequency#

What: No. of times a signal occurs in a given time period
Unit: Hz (by Heirich Rudolf Hertz)
A cycle that occurs 1 time in 1 second = 1 Hz
A cycle that occurs 500 times in 1 second = 500 Hz
Frequency is inversly proportional to Wavelength

Amplitude#

What: The strength or power of the signal
Unit:

Phase#

What: The relationship between 2 or more signals
Unit: Degree
If peaks are exactly aligned, they are in same phase else out of phase
2 signal in same phase results in much greater signal strength (potentially as much as twice the amplitude)
2 signal exactly 180 degree out of phase cancel outs each other

Wireless Components#

Simply
- Wireless access point (WAP)
  - various types
  - based on 802.11
  - utilizes diff. freq.
- Wireless client (Receiver)
  - diff. NIC (Network Interface Card) have diff. performance
In-depth
- Frequency
  - e.g. 2.4GHz vs 5GHz
- Environment
  - Lighting, Fog, Humidity etc
- RF interference
  - 2.4GHz range is shared by phone, microwaves, and other devices as well
- Electrical interference
  - Computer, appliances, fan etc.
- Obstacles
- Signal delivery
  - Diff. signal technologies have their pros & cons
  - FHSS, DSSS, OFDM etc.

Measuring Wireless Power#

Major performance concerns

Coverage
- Distance
- Area
Performance
- Speed of data transfer
- Simultaneous connections

This boils down to power. Understanding power and signal measurement helps. Especially in design phase.

Things about measurement

A lot of things are measured as relative & absolute/actual value
power is measure as absolute value
unit is watt (W)
where 1 milliwatt (mW) = 1000th of a watt
most 802.11 equipments uses 300 mW of transmit power or less
max power is limited by FCC in the US
However, changes in power is measured in decibels (dB)
- decibel is unit of comparision, not unit of power
- to represent diff. between two power value
- e.g. diff. between power of two transmitters
- e.g. output of a transmitter/antenna Vs signal received by the receiver/client
A unit bel is the ratio of a 10 to 1 difference (came from Bell Labs)
A decibel is 1/10 of a bel (i.e. 100 to 1 ratio diff.)
$dB = 10log_{10}(\frac{A}{B})$ where $A$ & $B$ are power values

Example:

                                  ┌───────┐
                          10mW    │       │
                       ┌─────────►│Laptop1│
                       │          │       │
┌────────────┐         │          └───────┘
│            │  100mW  │
│Access Point├─────────┤
│            │         │
└────────────┘         │          ┌───────┐
                       │  1mW     │       │
                       └─────────►│Laptop2│
                                  │       │
                                  └───────┘

Q. Here, what is the diff. in signal received by two laptops?
A. Laptop1's signal is 1 bel or 10 decibel more than Laptop2.
Because, Laptop1=10mW; Laptop2=1mW
Thus, $dB = 10log_{10}(\frac{10}{1}) = 10$

Q. Here, what is the diff. in signal transmitted by AP vs received by Laptop2?
A. AP's signal is 20 dB more than Laptop2.
Because, AP=100mW; Laptop2=1mW
Thus, $dB = 10log_{10}(\frac{100}{1}) = 20$

Ref: https://support.huawei.com/enterprise/en/doc/EDOC1000113315/c3242b10/power-and-signal-strength