CWDP CWDP QoS and Voice/Video over WLAN 2 — Questions and Answers
Question 1: What is jitter in the context of VoIP over WLAN, and what is the typical acceptable maximum?
- Variation in packet arrival times; under 50 ms (Correct answer)
- Total round-trip delay; under 150 ms
- Packet loss rate; under 1%
- Signal level fluctuation; under 10 dBm
Correct 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.
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)?
- 8 kbps
- 64 kbps
- 87.2 kbps with headers (Correct answer)
- 256 kbps
Correct 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.
Question 3: Which 802.11 power save mechanism is incompatible with low-latency VoIP requirements?
- WMM Power Save (U-APSD)
- Active mode (no power save)
- Legacy 802.11 Power Save Mode (PSM) (Correct answer)
- Target Wake Time (TWT)
Correct 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.
Question 4: What WMM feature specifically helps VoIP handsets conserve battery while maintaining low latency?
- EDCA
- U-APSD (Unscheduled Automatic Power Save Delivery) (Correct answer)
- TXOP bursting
- Block ACK
Correct 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.
Question 5: In a high-density VoWLAN deployment, what call admission control (CAC) mechanism prevents AP oversubscription?
- Assigning all VoIP to 2.4 GHz only
- Limiting maximum associations per AP
- Using TSPEC-based admission control defined in 802.11e (Correct answer)
- Disabling WMM on voice SSIDs
Correct 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.
Question 6: Which deployment practice best reduces co-channel interference impacting VoIP quality in a multi-AP environment?
- Using all APs on the same channel
- Implementing proper channel reuse planning with non-overlapping channels (Correct answer)
- Increasing transmit power on every AP
- Deploying APs only on 2.4 GHz
Correct 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.
CWDP CWDP QoS and Voice/Video over WLAN 2
What is jitter in the context of VoIP over WLAN, and what is the typical acceptable maximum?