3.3 Wireless
📘CCNP Encore (350-401-ENCORE-v1.1)
Wireless networking is a shared radio environment. Unlike wired networks, wireless performance depends heavily on radio frequency (RF) behavior.
For the CCNP ENCOR exam, you must understand how RF signals work, how signal quality is measured, what affects wireless performance, and how client devices impact Wi-Fi design and operation.
1. RF (Radio Frequency) Power
What is RF Power?
RF power is the strength of the wireless signal transmitted by an access point (AP) or client device.
It is measured in dBm (decibels relative to 1 milliwatt).
Key Points about dBm
- dBm values are negative numbers in Wi-Fi
- Closer to 0 dBm = stronger signal
- More negative = weaker signal
Typical RF Power Levels
| dBm Value | Signal Strength |
|---|---|
| -30 dBm | Very strong (close to AP) |
| -50 dBm | Excellent |
| -60 dBm | Good |
| -67 dBm | Minimum for voice/video |
| -70 dBm | Minimum for data |
| -80 dBm | Poor |
| -90 dBm | Unusable |
Exam Notes
- Higher RF power increases coverage, but also increases interference
- Lower RF power reduces interference, but reduces coverage
- Enterprise wireless design uses controlled RF power, not maximum power
2. RSSI (Received Signal Strength Indicator)
What is RSSI?
RSSI measures how strong the received signal is at the receiver (client or AP).
- Vendor-specific value
- Often mapped to dBm
- Used by wireless clients to decide which AP to connect to
Why RSSI Matters
- Clients roam based on RSSI
- Low RSSI causes:
- Low data rates
- Retransmissions
- Packet loss
- Poor application performance
Exam Notes
- RSSI is not standardized
- dBm is preferred for accurate design and troubleshooting
- RSSI alone does not guarantee good performance
3. Noise
What is Noise?
Noise is unwanted RF energy that exists in the wireless environment.
Sources of noise in IT environments:
- Other wireless networks
- Bluetooth devices
- Wireless peripherals
- Non-Wi-Fi RF devices
Noise Floor
The noise floor is the background RF level in the environment.
- Measured in dBm
- Lower (more negative) is better
- Example:
- Noise floor: -95 dBm → good
- Noise floor: -85 dBm → poor
Exam Notes
- Noise affects all wireless devices
- High noise floor reduces signal quality
- Noise cannot be eliminated, only managed
4. SNR (Signal-to-Noise Ratio)
What is SNR?
SNR compares signal strength to noise level.
Formula
SNR = Signal Strength (dBm) – Noise Floor (dBm)
Example
- Signal: -60 dBm
- Noise: -90 dBm
- SNR = 30 dB → Excellent
SNR Quality Levels
| SNR (dB) | Quality |
|---|---|
| 40+ | Excellent |
| 25–40 | Good |
| 15–25 | Acceptable |
| <15 | Poor |
Why SNR is Critical
- Determines:
- Data rates
- Modulation type
- Reliability
- High RSSI with high noise = poor SNR
Exam Notes
- SNR is more important than RSSI
- Wireless performance depends primarily on SNR
5. Interference
What is Interference?
Interference occurs when multiple devices transmit on the same or overlapping frequencies.
Types of Interference
1. Co-Channel Interference (CCI)
- Multiple APs using the same channel
- Common in dense deployments
- Causes contention and reduced throughput
2. Adjacent-Channel Interference (ACI)
- APs using overlapping channels
- Worse than co-channel interference
- Causes corruption of frames
Exam Notes
- Co-channel interference is expected and managed
- Adjacent-channel interference should be avoided
- Proper channel planning is essential
6. Wireless Frequency Bands
2.4 GHz Band
- Longer range
- Lower data rates
- More interference
- Only 3 non-overlapping channels
Channels: 1, 6, 11
5 GHz Band
- Shorter range
- Higher data rates
- Less interference
- Many non-overlapping channels
- Supports wider channels (40, 80, 160 MHz)
6 GHz Band (Wi-Fi 6E)
- Very clean spectrum
- High performance
- Short range
- Requires Wi-Fi 6E-capable clients
Exam Notes
| Band | Key Exam Points |
|---|---|
| 2.4 GHz | Limited channels, high interference |
| 5 GHz | Preferred in enterprise networks |
| 6 GHz | New, clean, requires compatible clients |
7. Wireless Channels and Channel Width
Channel Widths
- 20 MHz
- 40 MHz
- 80 MHz
- 160 MHz
Trade-offs
- Wider channels:
- Higher throughput
- Fewer available channels
- More interference
- Narrow channels:
- More channels
- Better scalability
- Lower peak speeds
Exam Notes
- Enterprise designs usually prefer:
- 20 MHz or 40 MHz in high-density areas
- Wider channels are not always better
8. Client Device Capabilities
Why Clients Matter More Than APs
Wireless is client-driven:
- Client chooses:
- Which AP to join
- When to roam
- Data rates
- Band selection
Key Client Capabilities
1. Supported Bands
- 2.4 GHz only
- 2.4 + 5 GHz
- 2.4 + 5 + 6 GHz
2. Supported Standards
- 802.11n
- 802.11ac (Wi-Fi 5)
- 802.11ax (Wi-Fi 6 / 6E)
3. Spatial Streams
- 1×1, 2×2, 3×3, etc.
- More streams = higher potential throughput
4. Channel Width Support
- Some clients support only 20 or 40 MHz
- AP capabilities do not override client limitations
5. Roaming Behavior
- Clients decide when to roam
- Poor roaming causes:
- Sticky clients
- Performance drops
Exam Notes
- APs cannot force clients to roam
- Wireless design must consider lowest-capability clients
- Client limitations often cause performance issues
9. Relationship Between All RF Concepts
| Concept | What It Affects |
|---|---|
| RF Power | Coverage area |
| RSSI | Connection quality |
| Noise | Signal degradation |
| SNR | Data rates and reliability |
| Interference | Throughput and stability |
| Bands | Capacity and coverage |
| Channels | Interference management |
| Client capabilities | Overall user experience |
10. Exam-Focused Summary (Must Remember)
- dBm values are negative
- SNR is more important than RSSI
- High power ≠ good performance
- Interference reduces throughput
- 5 GHz is preferred in enterprise networks
- Channel planning prevents interference
- Wireless is client-controlled
- Design for client capabilities, not AP maximums
Final Exam Tip
For the CCNP ENCOR exam:
- Focus on how RF variables interact
- Understand cause-and-effect, not memorization
- Think in terms of design, performance, and troubleshooting
