CWDP CWDP QoS and Voice/Video over WLAN 2

Question 1

What is jitter in the context of VoIP over WLAN, and what is the typical acceptable maximum?

Accepted Answer

Variation in packet arrival times; under 50 ms

Answer

Jitter is the variance in packet inter-arrival delay; values above 50 ms cause audible distortion in VoIP calls.

Question 2

A CWDP designer is planning for 20 simultaneous VoIP calls per AP. What minimum data rate should be supported per call (G.711 codec)?

Accepted Answer

87.2 kbps with headers

Answer

G.711 at 64 kbps payload plus RTP/UDP/IP/802.11 headers totals approximately 87.2 kbps per call.

Question 3

Which 802.11 power save mechanism is incompatible with low-latency VoIP requirements?

Accepted Answer

Legacy 802.11 Power Save Mode (PSM)

Answer

Legacy PSM buffers frames at the AP until the client polls, introducing unpredictable latency that disrupts real-time voice.

Question 4

What WMM feature specifically helps VoIP handsets conserve battery while maintaining low latency?

Accepted Answer

U-APSD (Unscheduled Automatic Power Save Delivery)

Answer

U-APSD allows clients to trigger buffered delivery from the AP at will, balancing power savings with low-latency delivery.

Question 5

In a high-density VoWLAN deployment, what call admission control (CAC) mechanism prevents AP oversubscription?

Accepted Answer

Using TSPEC-based admission control defined in 802.11e

Answer

TSPEC (Traffic Specification) admission control allows the AP to accept or reject new voice streams based on available bandwidth.

Certified Wireless Design Professional (CWDP) Practice Test