Pass the CWNP CWDP Wi-Fi Design CWDP-305 Questions and answers with CertsForce

In environments where access points (APs) are mounted on high ceilings (e.g., over 20 feet), it's essential to direct the wireless signal downwards towards the client devices on the floor. Patch antennas, which are directional antennas, are ideal for this scenario as they can focus the RF energy downward, providing better coverage and signal strength to the intended area.

The CWDP-305 Official Study and Reference Guide emphasizes the importance of antenna selection based on mounting height and coverage requirements:

"When APs are mounted on high ceilings, using directional antennas like patch antennas can help focus the RF energy towards the client devices, ensuring optimal coverage and performance."

The ability to locate a client device using WLAN infrastructure relies on the client actively transmitting frames that can be detected by multiple access points. These frames are typically management or data frames used during association, authentication, or normal operation.

If the client device is turned off, it is not transmitting any 802.11 frames. Therefore, the infrastructure cannot detect or triangulate its location using standard methods such as RSSI (Received Signal Strength Indicator) or time difference of arrival.

According to the CWDP-305 Official Study and Reference Guide (Chapter: Designing for Specific Applications), location-based services (LBS) are dependent on the active participation of the client device. Without beacon frames, probe requests, or any other 802.11 activity, no positioning data can be gathered.

“Client devices must be actively transmitting for their location to be calculated through triangulation or trilateration. If a device is powered off, it cannot be tracked by the WLAN infrastructure.”

— Reference: CWDP-305 Official Study and Reference Guide, Chapter on Designing for Specific Applications

In WLAN operations, two primary scanning methods are employed by client devices to discover available wireless networks:

Active Scanning:The client device actively sends out probe request frames on various channels. Access Points (APs) that receive these requests respond with probe response frames, providing information about the network. This method allows clients to discover networks even if they are not broadcasting their SSIDs.

Passive Scanning:The client device listens for beacon frames that are periodically transmitted by APs. These beacons contain information about the network, such as SSID, supported data rates, and security protocols. Passive scanning is less intrusive and does not generate additional traffic, but it may take longer to discover networks, especially if the beacon interval is lengthy.

Understanding these scanning methods is crucial for network design and troubleshooting, as they impact how quickly and efficiently clients can connect to available networks.

Each SSID broadcast by an AP adds additional management frame overhead, such as beacon frames. These beacon frames are transmitted periodically by the AP at the lowest mandatory data rate (e.g., 1 or 6 Mbps), which consumes airtime that would otherwise be used for client data.

According to CWDP-305:

“Each SSID increases beacon overhead. More SSIDs lead to greater airtime consumption by management traffic, which in turn decreases available throughput for actual client data traffic.”

Reducing the number of SSIDs is a best practice to minimize management overhead and optimize throughput in the wireless network.

— Reference: CWDP-305 Official Study and Reference Guide, Chapter on Infrastructure Design

The IEEE 802.11-2016 standard specifies two methods for Fast BSS Transition (FT) to facilitate fast and secure roaming:mrn-cciew

Over-the-Air (OTA):The client communicates directly with the target AP to perform the authentication exchange during roaming.

Over-the-Distribution System (Over-the-DS):The client communicates with the current AP, which then communicates with the target AP over the distribution system to perform the authentication exchange.

These methods are designed to reduce the time required for re-authentication during roaming, ensuring minimal disruption to ongoing sessions.

The CWDP-305 Official Study and Reference Guide states:

"802.11r defines two methods for fast BSS transition: Over-the-Air and Over-the-DS, both aimed at reducing latency during client roaming."

USB 3.0 operates with signaling rates up to 5 Gbps, which can emit electromagnetic interference in the 2.4 GHz frequency band. This interference can elevate the noise floor in the 2.4 GHz spectrum, leading to inaccurate readings during site surveys. The CWDP-305 study guide highlights this issue, emphasizing the importance of being aware of potential interference sources during surveys.

To mitigate this, it's recommended to use USB 2.0 devices or ensure that USB 3.0 devices are adequately shielded and positioned away from Wi-Fi antennas during surveys.

Comprehensive and Detailed Explanation:

For reliable Voice over Wi-Fi (VoWiFi) service, a minimum RSSI of -67 dBm is generally recommended. This level ensures sufficient signal strength to support the quality of service required for voice applications, providing adequate signal-to-noise ratio (SNR) and minimizing the likelihood of dropped calls or poor audio quality.

In the 5 GHz band, all Wi-Fi traffic (802.11a/n/ac/ax) uses OFDM or OFDMA modulation. If multiple APs use overlapping or identical channels, this results in Co-Channel Interference (CCI). Legacy technologies like HR/DSSS and ERP operate in 2.4 GHz and do not contribute to CCI in 5 GHz.

From CWDP-305:

“CCI occurs when multiple devices share the same channel. In 5 GHz, all compliant Wi-Fi traffic uses OFDM-based PHYs, so other nearby OFDM networks are a primary source of CCI.”

— Reference: CWDP-305 Official Study and Reference Guide, Chapter on Post-Design ValidationandTroubleshooting

Comprehensive and Detailed Explanation:

In a 3x3 802.11n configuration supporting two spatial streams over a 20 MHz channel with a short guard interval (400ns), the maximum data rate achievable is 144 Mbps. This is derived from the MCS (Modulation and Coding Scheme) index tables defined in the 802.11n standard, where MCS 15 corresponds to two spatial streams at 64-QAM modulation with a 5/6 coding rate. Therefore, the correct maximum MCS rate for this configuration is 144 Mbps.

Software-defined radios (SDRs) offer flexibility in RF operation. In dual-band APs, an SDR can be programmed to operate on either 2.4 GHz or 5 GHz, allowing dynamic optimization of the spectrum based on demand and interference.

From CWDP-305:

“Software-defined radios can be configured to operate in either band, allowing administrators to shift coverage based on band congestion or capacity requirements.”

This flexibility helps address issues in high-density deployments or when minimizing interference is critical.

— Reference: CWDP-305 Official Study and Reference Guide, Chapter on Infrastructure Design