Learning Objectives
- Understand the basic properties of radio frequency (RF) waves
- Calculate signal-to-noise ratio (SNR) and understand its impact on throughput
- Explain how frequency, wavelength, and attenuation affect wireless performance
RF Basics
Wireless networking is built on radio frequency (RF) waves — electromagnetic energy that travels through the air. Understanding RF behavior is essential for designing and troubleshooting Wi-Fi networks.
Three fundamental properties define any RF wave:
- Frequency — The number of wave cycles per second, measured in Hz. Wi-Fi operates at 2.4 GHz, 5 GHz, and 6 GHz.
- Wavelength — The physical distance between wave peaks. Wavelength = c / frequency (where c is the speed of light). Higher frequencies have shorter wavelengths.
- Amplitude — The wave's strength, which determines signal power.
The relationship between frequency and wavelength is inverse: 2.4 GHz waves are about 12.5 cm long, while 5 GHz waves are about 6 cm. This is why 5 GHz signals don't travel as far — shorter wavelengths are more easily absorbed by obstacles.
Signal Strength and Attenuation
RF signals weaken as they travel through air and obstacles. This is called attenuation. Different materials cause different amounts of attenuation:
| Material | Attenuation at 2.4 GHz | Attenuation at 5 GHz | |----------|----------------------|----------------------| | Drywall | 3 dB | 5 dB | | Glass | 2 dB | 4 dB | | Brick | 8 dB | 15 dB | | Concrete | 15 dB | 25 dB | | Metal | 25+ dB | 35+ dB |
Notice that 5 GHz signals suffer more attenuation through every material. This is a key reason why 2.4 GHz has better range but 5 GHz offers higher throughput — the trade-off between range and speed.
SNR: Signal-to-Noise Ratio
The most important metric in wireless networking is SNR — the difference in dBm between the received signal and the background noise floor.
- SNR = Signal (dBm) - Noise (dBm)
- Higher SNR means a clearer signal and supports higher modulation rates
- Below ~20 dB SNR, throughput drops significantly as the radio switches to more robust (but slower) modulation
Wi-Fi clients dynamically adjust their data rate based on SNR. At high SNR (40 dB+), a client can use 1024-QAM or 4096-QAM modulation, packing more bits per symbol. At low SNR (10–15 dB), the radio falls back to QPSK or BPSK, which are much slower but more resilient to errors.
What is the SNR in dB?
Free Space Path Loss and the Inverse Square Law
As RF waves travel outward from the transmitter, their power spreads over an expanding sphere. This is the inverse square law — doubling the distance reduces signal power by one-quarter (6 dB loss).
Free Space Path Loss (FSPL) at a given distance and frequency:
FSPL (dB) = 20 log₁₀(d) + 20 log₁₀(f) + 32.44
Where d is distance in km and f is frequency in MHz. At 2.4 GHz, FSPL at 100 meters is about 80 dB. At 5 GHz, the same distance loses about 86 dB. This 6 dB difference means 5 GHz signals have roughly one-quarter the power at the same distance — another reason higher frequencies have shorter effective range.
A Wi-Fi signal measures -65 dBm at the receiver with a noise floor of -88 dBm. What is the SNR?
Why does 5 GHz Wi-Fi have shorter range than 2.4 GHz?
Key Takeaways
- Higher frequency = shorter wavelength = more attenuation through obstacles
- SNR = Signal - Noise; higher SNR enables faster modulation rates
- Free space path loss increases with both distance and frequency
- The inverse square law means every doubling of distance loses 6 dB of signal
- Wi-Fi clients dynamically adjust data rate based on SNR