Wireless
Shared spectrum, fading, and a MAC that is nothing like a wire.
Level: intermediate
What is it?
The wireless physical layer transmits information through radio waves over shared spectrum. Antennas, frequency bands, channel widths, modulation/coding, transmit power, fading, and interference influence achievable link quality.
Why does it matter?
Wireless links change with distance and environment and are shared among stations. Understanding the physical layer helps explain why signal strength alone is not enough to predict throughput or reliability.
Mental model
The transmitter creates a waveform, the radio channel changes it, and the receiver estimates the original symbols. Rate adaptation selects a robust or faster modulation/coding scheme based on conditions.
How it works
- Signal-to-noise ratio and interference affect which modulation and coding rates a receiver can decode reliably.
- Wider channels can raise peak data rate but use more spectrum and can be more vulnerable to interference or co-channel contention.
- Multipath fading and obstacles can alter signal quality even when distance changes little.
- Multiple-input multiple-output (MIMO) uses multiple antennas/streams or spatial techniques to improve throughput or robustness when conditions allow.
- A good RSSI value does not alone describe noise, airtime contention, retransmission rate, or backhaul constraints.
Steps
- The AP and station negotiate capabilities and choose a channel and supported rate modes.
- The transmitter encodes bits into modulation symbols and sends radio energy.
- The propagation environment adds attenuation, interference, and multipath effects.
- The receiver decodes symbols; rate adaptation and retransmissions respond to measured link conditions.
Real-world usage
- Diagnosing Wi-Fi performance across rooms or in dense deployments.
- Choosing channel width, AP placement, and client capabilities.
Common mistakes
- Assuming more transmit power always improves every client and reduces interference.
- Treating RSSI, bandwidth, and throughput as the same measurement.
- Ignoring airtime competition from other clients and neighboring networks.
Interview questions
Why can a strong RSSI still give poor Wi-Fi throughput?
Noise, contention, retransmissions, channel width, AP load, and the other endpoint's capabilities all affect goodput.
What is the trade-off of wider Wi-Fi channels?
They can increase peak bitrate, but consume more spectrum and may increase interference/co-channel contention risk.