CWDP QoS and Voice/Video over WLAN Flashcards

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

Answer: Variation in packet arrival times; under 50 ms

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

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

Answer: 87.2 kbps with headers

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

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

Answer: Legacy 802.11 Power Save Mode (PSM)

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

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

Answer: U-APSD (Unscheduled Automatic Power Save Delivery)

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

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

Answer: Using TSPEC-based admission control defined in 802.11e

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

Which deployment practice best reduces co-channel interference impacting VoIP quality in a multi-AP environment?

Answer: Implementing proper channel reuse planning with non-overlapping channels

Proper channel reuse ensures co-channel APs are separated enough that their interference falls below the noise floor for VoIP clients.