350-801 · Question #262
An engineer implements a new Cisco UCM based telephony system per these requirements: - The local Ethernet bandwidth is sized based on the total bandwidth per call - A G.736 codec is used. - The bit r
The correct answer is D. 87.2 kbps. To calculate total Ethernet bandwidth for a VoIP call, all protocol header overhead must be added to the voice payload. Given: voice payload = 160 bytes per 20ms packet, packetization rate = 1000ms ÷ 20ms = 50 packets/second. Header sizes: RTP = 12 bytes, UDP = 8 bytes, IP = 20 b
Question
An engineer implements a new Cisco UCM based telephony system per these requirements:
- The local Ethernet bandwidth is sized based on the total bandwidth
per call
- A G.736 codec is used.
- The bit rate is 64 kbps
- The codec sample interval is 10 ms.
- The voice payload size is 160 bytes per 20 ms.
What should the size of the Ethernet bandwidth be per call?
Options
- A31.2 kbps
- B38.4 kbps
- C55.2 kbps
- D87.2 kbps
How the community answered
(47 responses)- A17% (8)
- B9% (4)
- C4% (2)
- D70% (33)
Explanation
To calculate total Ethernet bandwidth for a VoIP call, all protocol header overhead must be added to the voice payload. Given: voice payload = 160 bytes per 20ms packet, packetization rate = 1000ms ÷ 20ms = 50 packets/second. Header sizes: RTP = 12 bytes, UDP = 8 bytes, IP = 20 bytes, Ethernet header = 14 bytes, Ethernet FCS = 4 bytes. Total per packet = 160 + 12 + 8 + 20 + 14 + 4 = 218 bytes. Total bandwidth = 218 bytes × 8 bits × 50 packets/sec = 87,200 bps = 87.2 kbps. This matches answer D. The other options (31.2, 38.4, 55.2 kbps) result from incorrectly omitting header layers or miscalculating the packet rate. This overhead-inclusive calculation is why VoIP bandwidth planning must account for significantly more than just the raw codec bit rate (64 kbps).
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