Designing Cisco Wireless Networks (WLSD) Free Sample Questions

Solid concrete introduces significant RF attenuation, typically ranging from 12 to 15 dB at 5 GHz, making it one of the most obstructive common building materials. Drywall typically introduces 3-4 dB, and cubicle partitions introduce 1-2 dB.

First, calculate total devices: 400 students * 2.5 devices = 1000 devices. Next, calculate active devices: 1000 * 0.40 (40%) = 400 active devices. Finally, divide by the radio threshold: 400 / 35 = 11.42. Since you cannot have a fraction of a radio, round up to the nearest whole number, which is 12 radios.

Cisco's large-public-venue design guidance says a proper RF design with directional antennas is a must for very high density, while omnidirectional antennas (internal or external) must generally be avoided because of their large co-channel interference footprint. Overhead coverage is preferred wherever possible: narrow-beam directional (stadium) antennas on catwalks or the roof structure illuminate individual seating sections as small, well-defined cells. Where no suitable mounting locations exist, APs under the seats aimed up at the seating area are a valid but uncommon alternative: the crowd's bodies attenuate the signal and isolate each cell, allowing more APs and more capacity, although coverage differs greatly between a full and an empty venue. Low-gain dipoles in a mesh across the bleachers, or omnidirectional antennas with RxSOP disabled to maximize overlap, would increase co-channel interference.

Cisco's voice WLAN design targets at the cell edge are an RSSI of -67 dBm or better (with about 20% cell overlap so that at least two APs are heard at -67 dBm), a noise floor no higher than -92 dBm for an SNR of 25 dB, and packet loss of 1% or less (plus channel utilization under 40% and 802.11 retries under 20%). End to end, Cisco QoS design guidance targets one-way latency of 150 ms or less and average jitter under 30 ms for voice.

Predictive surveys are only as good as the data entered into them. Attenuation depends on metal content, moisture, thickness and conductivity: Cisco lists drywall at about 3 dB, brick about 10 dB and concrete about 12 dB, and rates metal as very high attenuation, so lead-lined walls and dense metal medical equipment block far more signal than the 3 dB drywall assumed in the model. The remediation is to correct the model with measured wall losses, add APs to close the gaps (for example inside the shielded rooms rather than relying on penetration from the corridor), and validate with a post-deployment survey against the voice targets (-67 dBm with enough overlap that two APs are heard). Raising AP power is not a remedy, because the battery-powered badges cannot transmit back through the same walls.

A 100% duty cycle means the transmitter is sending RF energy constantly without pausing. Analog devices such as wireless video cameras have a constant, always-on signal (Cisco shows video-camera duty cycles of 90-100%), so Wi-Fi devices that hear it above the Clear Channel Assessment (CCA) threshold defer indefinitely; in a study cited by Cisco, an analog video camera 25 feet from the AP or client degraded Wi-Fi throughput by 100 percent (no ability to connect). A microwave oven, by contrast, cycles on and off with the AC power (about a 50% duty cycle) and is used in short bursts, and a BLE beacon sends brief, narrow, low-duty-cycle bursts. Design response: locate and remove or relocate the camera, or keep the affected APs and clients on channels or a band it does not occupy; CleanAir ED-RRM can move an AP off a channel hit by such a high-duty-cycle interferer.