Wireless Networking
A signal is any variation of a quantity that carries information. In wireless communication, this quantity is usually voltage or electromagnetic energy
A wave is a repeating disturbance that travels through space or a medium.
- Amplitude (A): Height of the wave → relates to power/strength.
- Wavelength (λ): Distance between repeating points (like crest-to-crest).
- Frequency (f): How many waves pass a point per second. Measured in Hz (Hertz).
- Speed (v): How fast the wave moves. For electromagnetic waves, this is the speed of light (~3×10⁸ m/s).
Relationship:
$ v = f \times \lambda $
This tells you if you know frequency, you can find wavelength, and vice versa.
Electromagnetic (EM) Waves
Wi-Fi uses electromagnetic waves. These are oscillating electric and magnetic fields that propagate through space.
Properties of EM waves:
- Travel at the speed of light in free space.
- Do not require a medium (can travel through vacuum, unlike sound waves).
- Characterized by frequency, wavelength, and amplitude.
Wi-Fi operates in the GHz range, so the waves are very high frequency and short wavelength (centimeters).
Frequency & Wavelength
Frequency is how many times something repeats per second.
a wave that wiggles 1 time per second = 1 Hz
So, frequency tells you how fast the wave oscillates. Faster waves can carry more information.
Units of frequency
Frequency is measured in Hertz (Hz) Named after Heinrich Hertz, the scientist who first proved that electromagnetic waves exist. Hertz just counts repetitions per second.
For very fast waves, we use these prefixes:
| Unit | Symbol | Value in Hz | Meaning / How many cycles per second | Typical Applications / Examples |
|---|---|---|---|---|
| Hertz | Hz | 1 Hz = 1 cycle per second | 1 oscillation per second | Very slow oscillations: heartbeats, pendulum clocks |
| Kilohertz | kHz | 1 kHz = 1,000 Hz | 1,000 cycles per second | AM radio (530-1700 kHz), old analog telephony |
| Megahertz | MHz | 1 MHz = 1,000,000 Hz | 1 million cycles per second | FM radio (88-108 MHz), TV broadcast, GPS, low Wi-Fi 2.4 GHz reference |
| Gigahertz | GHz | 1 GHz = 1,000,000,000 Hz | 1 billion cycles per second | Wi-Fi (2.4/5/6 GHz), mobile phones, radar, satellite comms |
| Terahertz | THz | 1 THz = 1,000,000,000,000 Hz | 1 trillion cycles per second | Infrared light, some spectroscopy, experimental comms |
| Petahertz | PHz | 1 PHz = 1,000,000,000,000,000 Hz | 1 quadrillion cycles per second | Visible light (~400-800 THz, equivalent to 0.4-0.8 PHz) |
| Exahertz | EHz | 1 EHz = 1,000,000,000,000,000,000 Hz | 1 quintillion cycles per second | Ultraviolet light, X-rays start (~1-10 EHz) |
| Zettahertz | ZHz | 1 ZHz = 1,000,000,000,000,000,000,000 Hz | 1 sextillion cycles per second | X-rays (~10²⁰ Hz) |
| Yottahertz | YHz | 1 YHz = 1,000,000,000,000,000,000,000,000 Hz | 1 septillion cycles per second | Gamma rays (~10²²-10²⁴ Hz) |
So when we say Wi-Fi operates at 2.4 GHz, it means the electromagnetic wave is oscillating 2.4 billion times every second. That wiggling is how data is encoded and sent over the air.
Observation: The higher the frequency:
- Shorter wavelength (distance between crests of the wave)
- More data can be carried
- But weaker penetration through walls…
Interaction with obstacles
When a wave hits an obstacle:
- Long wavelength (low frequency): can bend around objects and pass through walls better.
- Short wavelength (high frequency): tends to reflect, scatter, or get absorbed by obstacles.
Noise & Interference
Random unwanted signals that aren’t intentional comes from electronics, thermal motion of atoms, cosmic radiation, or power lines.
Usually low-level, continuous, unavoidable.
- Effect: decreases the Signal-to-Noise Ratio (SNR), which reduces the reliability of your communication.
$$ \text{SNR} = \frac{\text{Signal Power}}{\text{Noise Power}} $$
- Often expressed in decibels (dB): [ \text{SNR(dB)} = 10 \cdot \log_{10} \left( \frac{\text{Signal Power}}{\text{Noise Power}} \right) ]
- Higher SNR → cleaner signal → better communication
- Lower SNR → noisy signal → more errors and retransmissions
Analogy: Imagine you’re in a room:
-
Signal = your friend talking
-
Noise = people talking in the background
-
SNR = “how loud your friend is compared to everyone else”
- High SNR → easy to understand
- Low SNR → hard to hear, may misinterpret words
2. Examples in Wi-Fi
| Signal Strength | Noise Level | SNR (dB) | Effect on Wi-Fi |
|---|---|---|---|
| -30 dBm | -90 dBm | 60 dB | Excellent, very fast, reliable |
| -60 dBm | -90 dBm | 30 dB | Good, normal performance |
| -80 dBm | -90 dBm | 10 dB | Weak, slow, unstable |
| -90 dBm | -90 dBm | 0 dB | Signal = noise, almost unusable |
Note:
- dBm is a logarithmic measurement of power (decibels relative to 1 milliwatt).
- Signal strength must be higher than noise to maintain communication.
dBm
- dBm = decibels relative to 1 milliwatt. It’s a way of expressing power on a logarithmic scale instead of a linear scale.
Formula:
$$ \text{Power(dBm)} = 10 \cdot \log_{10}\left(\frac{\text{Power (mW)}}{1,\text{mW}}\right) $$
- Reference point: 0 dBm = 1 milliwatt of power.
Key points:
- Positive dBm → power greater than 1 mW
- Negative dBm → power less than 1 mW
- Every 10 dB increase → 10× more power
- Every 10 dB decrease → 10× less power
2. Why logarithmic?
- Wireless power can vary from millionths of a milliwatt to hundreds of milliwatts.
- Logarithms let us express huge ranges compactly and compare ratios easily.
- Example: 100 mW = 20 dBm, 0.1 mW = -10 dBm
Common Wi-Fi signal levels in dBm
| Signal Strength (dBm) | Power (mW) | Quality / Range | Comment |
|---|---|---|---|
| 0 dBm | 1 mW | Very strong | Only very close to AP |
| -10 dBm | 0.1 mW | Extremely strong | Almost at transmitter |
| -30 dBm | 0.001 mW | Excellent | Close range Wi-Fi, strong |
| -50 dBm | 0.00001 mW | Very good | Typical indoor close to AP |
| -60 dBm | 0.000001 mW | Good | Usable, maybe moderate distance |
| -70 dBm | 0.0000001 mW | Fair | Edge of coverage indoors |
| -80 dBm | 0.00000001 mW | Weak | Might drop packets, slower speed |
| -90 dBm | 0.000000001 mW | Very weak | Almost unusable |
| -100 dBm | 0.0000000001 mW | Unusable | Too weak to connect |
Remember: lower (more negative) dBm = weaker signal.
Relationship with SNR
SNR uses signal power (dBm) minus noise power (dBm):
$$ \text{SNR(dB)} = \text{Signal(dBm)} - \text{Noise(dBm)} $$
-
Example:
- Signal = -60 dBm
- Noise = -90 dBm
- SNR = -60 - (-90) = 30 dB → good connection
5. Practical meaning in Wi-Fi
- Wi-Fi routers usually transmit 15-20 dBm (≈30-100 mW).
- Distance, walls, and interference reduce received power → lower dBm at the client device.
- Devices measure RSSI (Received Signal Strength Indicator) → usually mapped to dBm.
- Signal below -80 dBm → slower, unreliable Wi-Fi.
In short:
dBm tells you the absolute strength of your wireless signal on a logarithmic scale relative to 1 milliwatt. Negative numbers mean weak signals, positive numbers mean very strong signals.
Interference
Deliberate or other-device signals using the same frequency or overlapping frequencies, can be co-channel (same channel) or adjacent-channel (nearby frequency).
can causes collisions, retransmissions, reduced throughput.
Example:
- Another Wi-Fi network on the same channel, your packets may collide.
- Microwave oven emits strong EM radiation at 2.4 GHz, temporarily corrupts Wi-Fi packets.
Bluetooth devices in one room usually don’t interfere seriously because of clever techniques:
Frequency hopping
Classic Bluetooth hops rapidly between 79 channels in the 2.4 GHz band (or 40 in BLE).
Each device only transmits on a tiny fraction of the spectrum at any given time. Even if two devices collide occasionally, it’s brief, the hopping moves them to a clean frequency.
2. Low power & short range
Bluetooth devices use very low transmission power, the signals are weak outside a few meters.
Multiple devices can coexist because their signals are too weak to drown each other out most of the time.
Error correction & retransmission
Bluetooth has built-in error detection and automatic retransmission, so occasional collisions don’t break the connection.
So you can have several headphones, keyboards, or mice in the same room and they mostly work fine.
RF Spectrum Overview
The Radio Frequency (RF) spectrum is like the electromagnetic highway. Every frequency is a lane, and different services use different lanes:
- AM/FM radio: low frequency, long range, slow data.
- TV signals: mid-range frequency moderate range, decent data.
- Wi-Fi: high frequency (GHz), short range, very high speed.
higher frequency waves carry more data but travel shorter distances. This is why 5 GHz Wi-Fi is faster but doesn’t go as far as 2.4 GHz.