352-001 · Question #527
An enterprise customer will deploy a centralized unified communications application to provide voice, video, and instant messaging to their branch offices. Some of the branch offices are located in…
The correct answer is B. Set voice activity detection to avoid sending packets when the conversations is silent F. Use a low bitrate codec such as G.729. On a constrained 1.5 Mbps WAN link carrying unified communications, using a low-bitrate codec like G.729 and enabling VAD are the most cost-effective ways to conserve bandwidth without additional infrastructure cost.
Question
An enterprise customer will deploy a centralized unified communications application to provide voice, video, and instant messaging to their branch offices. Some of the branch offices are located in remote locations and are connected via a 1.5 Mb/s Layer 3 VPN connection. Which two actions are the most cost-effective ways to ensure that this new application is implemented properly? (Choose two.)
Options
- AUse a low bitrate codec such as G.711
- BSet voice activity detection to avoid sending packets when the conversations is silent
- CEnable VRF-Lite on the CE router to create a separate voice VRF
- DSet LFI on the WAN connections to interleave the small voice packets with the large data packets
- ESet WAN optimization on the CE router to compress the voice packets for improved bandwidth
- FUse a low bitrate codec such as G.729
How the community answered
(31 responses)- A13% (4)
- B77% (24)
- C3% (1)
- E6% (2)
Why each option
On a constrained 1.5 Mbps WAN link carrying unified communications, using a low-bitrate codec like G.729 and enabling VAD are the most cost-effective ways to conserve bandwidth without additional infrastructure cost.
G.711 uses 64 kbps per call, which is a high-bitrate codec - the opposite of what is needed on a 1.5 Mbps link shared with data traffic.
Voice Activity Detection (VAD) detects silence during a conversation and suppresses packet transmission during those periods, reducing bandwidth consumption by approximately 35-50% with no additional hardware cost.
VRF-Lite separates routing table instances for traffic segmentation but does not reduce bandwidth consumption on the WAN link and adds configuration complexity without bandwidth savings.
Link Fragmentation and Interleaving (LFI) reduces serialization delay for small voice packets by interleaving them with fragmented data frames, but it does not reduce bandwidth consumption and requires additional QoS configuration.
Voice traffic encoded with G.729 or G.711 is already using lossy compression - applying WAN optimization compression to pre-compressed audio yields negligible bandwidth savings and adds router CPU overhead.
G.729 encodes voice at approximately 8 kbps compared to G.711's 64 kbps, reducing the bandwidth per voice call by roughly 87% and making it the most cost-effective codec choice for constrained WAN links.
Concept tested: VoIP codec selection and VAD for WAN bandwidth optimization
Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/voice/codec/configuration/xe-3se/vcodec-xe-3se-book/vcodec-xe-3se-book.html
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