1.7 Wireless Principles
Radio Frequency (RF) mechanics dictate how wireless data moves. Master frequency characteristics, channel selection constraints, and the structural design of corporate WLAN architectures.
Frequency Band Dynamics
2.4 GHz
Channels 1, 6, and 11- • 11 Channels total (US), only 3 are non-overlapping.
- • Longer Range (Lower frequencies penetrate walls better).
- • Slower speeds, highly susceptible to interference (microwaves, bluetooth).
5 GHz
Uncrowded Spectrum- • Up to 25 non-overlapping 20MHz channels.
- • Shorter Range (Higher frequencies are absorbed more easily).
- • Much faster throughput, much less crowded spectrum.
In the 2.4 GHz spectrum, each channel is 22 MHz wide but they are only spaced 5 MHz apart. This causes massive **Adjacent-Channel Interference (ACI)**. The only channels that have enough separation to operate side-by-side without overlapping are **1, 6, and 11**. When deploying adjacent Access Points in an office, you must alternate them strictly using this 1-6-11 pattern.
RF Signal Propagation Obstacles
Absorption
The RF signal travels into an object and gets trapped, converting into heat. Concrete walls, brick, and drywall absorb signals heavily, reducing strength on the other side.
Reflection
The RF signal bounces off a smooth, metallic surface, changing direction. This can cause multi-path distortion where the same signal arrives at the receiver at slightly different times.
Refraction
The RF signal bends as it passes through a medium with a different density (like glass or water). This changes the wave's velocity and direction, causing phase shifts.
Scattering
The RF signal hits an uneven surface (like foliage, rugged rocks, or rain) and shatters into multiple unpredictable, weaker directions, severely degrading quality.
WLAN Architecture Identifiers
Cisco tests heavily on the exact terminology used to describe wireless structural architecture. Ensure you can confidently distinguish between physical radio identifiers and logical roaming networks.