📘 CCNA 200-301 v1.1
1.11 Describe wireless principles
1.11.c RF
1. What is RF?
RF stands for Radio Frequency.
It refers to the range of electromagnetic waves used to transmit data wirelessly.
In networking, RF signals are used by Wi-Fi, Bluetooth, and other wireless technologies to send and receive data through the air — instead of cables.
These signals are measured in hertz (Hz), which means cycles per second.
- 1 Hz = 1 cycle per second
- 1 MHz (megahertz) = 1 million cycles per second
- 1 GHz (gigahertz) = 1 billion cycles per second
So, when you hear Wi-Fi works at 2.4 GHz or 5 GHz, it means it’s using radio waves that oscillate 2.4 or 5 billion times per second.
2. RF in Wireless Networks
Wi-Fi networks (IEEE 802.11 standards) use RF signals to send and receive data between:
- Wireless clients (like laptops, phones, printers) and
- Access points (APs) or wireless routers
Each Wi-Fi device has a radio and an antenna that:
- Converts digital data into RF signals for transmission
- Receives RF signals and converts them back into digital data
3. RF Spectrum and Frequency Bands
The RF spectrum is a wide range of frequencies available for wireless communication.
Wi-Fi typically uses two main frequency bands (and a third one in newer standards):
| Band | Frequency Range | Common Use | IEEE Standards |
|---|---|---|---|
| 2.4 GHz | 2.4 – 2.4835 GHz | Used by older and some newer Wi-Fi devices | 802.11b/g/n |
| 5 GHz | 5.150 – 5.825 GHz (divided into sub-bands) | Used by newer Wi-Fi devices | 802.11a/n/ac |
| 6 GHz | 5.925 – 7.125 GHz | Latest, less interference | 802.11ax (Wi-Fi 6E) |
Note: GHz = billions of cycles per second.
4. RF Characteristics
RF waves have several important physical characteristics that affect wireless communication.
For the CCNA exam, you must understand the following key terms:
a. Frequency
- The number of times a signal wave completes a cycle in one second.
- Higher frequency = faster data rate but shorter range.
- Lower frequency = longer range but slower data rate.
Example:
- 2.4 GHz has a longer range, but slower speed and more interference.
- 5 GHz has a shorter range, but faster speed and less interference.
b. Wavelength
- The distance a signal travels in one cycle.
- It is inversely related to frequency: Wavelength=Speed of LightFrequency\text{Wavelength} = \frac{Speed\ of\ Light}{Frequency}Wavelength=FrequencySpeed of Light
- Higher frequencies have shorter wavelengths (can carry more data but less distance).
- Lower frequencies have longer wavelengths (can travel farther).
c. Amplitude
- The height or strength of the signal wave.
- Measured in dBm (decibels milliwatts).
- A stronger amplitude (signal) helps maintain communication over longer distances.
Example in IT:
When a Wi-Fi signal shows –50 dBm, it’s strong.
When it shows –90 dBm, it’s weak and may disconnect.
d. Phase
- Phase means the position of a signal wave at a specific point in time.
- Two signals can be in phase (aligned) or out of phase (misaligned).
- Wireless devices use phase for techniques like MIMO (Multiple Input Multiple Output) and beamforming to improve speed and reliability.
5. RF Propagation (How RF Signals Travel)
RF signals travel through the air but are affected by the environment.
You must understand the following RF behaviors:
| Behavior | Description | Effect |
|---|---|---|
| Reflection | Signal bounces off a surface (like a wall or metal object). | Causes multiple copies of the same signal (multipath interference). |
| Refraction | Signal bends when passing through materials like glass. | Changes signal direction or speed. |
| Diffraction | Signal bends around an obstacle. | Helps signals reach areas that are not in line of sight. |
| Scattering | Signal breaks into smaller waves when it hits rough surfaces or small objects. | Causes signal loss or distortion. |
| Absorption | Materials like concrete or water absorb the signal energy. | Reduces signal strength (attenuation). |
6. RF Interference
Interference happens when two or more RF signals overlap on the same or nearby frequencies, causing disruption.
Types of Interference:
- Co-channel interference:
Occurs when multiple access points use the same channel. - Adjacent-channel interference:
Happens when channels that overlap (like 2.4 GHz channels 1, 6, and 11) are used too closely. - Non-Wi-Fi interference:
Other devices (like microwaves, Bluetooth devices, or cordless phones) also use 2.4 GHz and can cause interference.
How to minimize interference:
- Use non-overlapping channels (1, 6, 11 in 2.4 GHz band).
- Use 5 GHz or 6 GHz bands where possible.
- Maintain good channel planning and AP placement.
7. RF Power and Signal Measurement
Wi-Fi engineers measure RF power using decibels (dB) and dBm.
| Term | Description | Example |
|---|---|---|
| dB (decibel) | A ratio used to compare two power levels. | +3 dB = double power, –3 dB = half power. |
| dBm (decibel-milliwatt) | Absolute measurement of power (based on 1 mW). | –30 dBm (very strong), –90 dBm (very weak). |
| SNR (Signal-to-Noise Ratio) | Difference between signal power and noise power. | Higher SNR = better performance. |
A good Wi-Fi signal usually has:
- RSSI (signal strength): –30 to –67 dBm
- SNR: 25 dB or more
8. Antenna and Gain
The antenna converts electrical signals into RF waves and vice versa.
- Omnidirectional antennas send signals in all directions (used in most access points).
- Directional antennas focus the signal in one direction (used in long-distance links).
Gain measures how well an antenna focuses energy in a specific direction.
It’s measured in dBi (decibels isotropic).
Higher dBi = more focused signal = longer distance in one direction.
9. RF Channels
Each frequency band is divided into channels to organize communication and avoid interference.
For example:
- 2.4 GHz band has 14 channels, but only 1, 6, and 11 are non-overlapping.
- 5 GHz band has many channels (20 MHz wide each), divided into:
- UNII-1, UNII-2, UNII-2 Extended, UNII-3 sub-bands.
Each channel can carry one Wi-Fi network signal. Using the same or overlapping channels causes performance drops.
✅ Summary for CCNA Exam
| Concept | Key Point |
|---|---|
| RF | Radio waves that carry wireless data |
| Frequency | Determines speed and range |
| Wavelength | Inversely related to frequency |
| Amplitude | Signal strength (measured in dBm) |
| Phase | Used in advanced transmission methods like MIMO |
| Propagation | Signal behaviors: reflection, refraction, diffraction, scattering, absorption |
| Interference | Overlapping channels or devices causing disruption |
| Power & Measurement | dB, dBm, SNR, RSSI |
| Antenna & Gain | Determines signal direction and distance |
| Channels | Organized frequencies to separate networks |
