When determining bandwidth capacity for VoIP-related call traffic, how does the voice_activity_detection (VAD) feature impact things? Is there a specific value included in the standard calculation (formula below) to account for how much savings in bandwidth VAD will provide?
valueshighlighted in red beloware represented in Bytes,
vPPS (voice-Packets-Per-Second) = codec_bit_rate/(voice_payload x 8)
VoIP bps_rate/call = (IP/UDP/RTP_header + voice_payload + L2_header + L2_flag) x 8 x vPPS
*** add 5% overhead for signaling (in bits) for TOTAL rate/call value ***
You may have seen the link I'll provide but I believe you may not. Essentially, VAD is a relative savings over a number of calls. It is not something that can be calculated on a call by call basis. Here is how VAD is addressed in terms of BW calculations:
With circuit-switched voice networks, all voice calls use 64 Kbps fixed-bandwidth links regardless of how much of the conversation is speech and how much is silence. With VoIP networks, all conversation and silence is packetized. With Voice Activity Detection (VAD), packets of silence can be suppressed.
Over time and as an average on a volume of more than 24 calls, VAD can provide up to a 35 percent bandwidth savings. The savings are not realized on every individual voice call, or on any specific point measurement. For the purposes of network design and bandwidth engineering, VAD must not be taken into account, especially on links that carry fewer than 24 voice calls simultaneously. Various features such as music on hold and fax render VAD ineffective. When the network is engineered for the full voice call bandwidth, all savings provided by VAD are available to data applications.
VAD also provides Comfort Noise Generation (CNG). Because you can mistake silence for a disconnected call, CNG provides locally generated white noise so the call appears normally connected to both parties. G.729 Annex-B and G.723.1 Annex-A include an integrated VAD function, but otherwise performs the same as G.729 and G.723.1, respectively.
In Cisco CallManager, VAD can be enabled (it is disabled by default) with these service parameters:
SilenceSuppressionSystemWide—This parameter selects the VAD setting for all skinny endpoints (for example: Cisco IP Phones and Skinny gateways)
SilenceSuppressionWithGateways—This parameter selects the VAD setting for all MGCP gateways. This does not have an effect on H.323 gateways. VAD on H.323 gateways must be disabled on the gateway.
You can find these service parameters under Cisco CallManager Administration (Service > Service Parameters > select_server > Cisco CallManager).
I have that very document you included in your response. Can I assume that there's no specific value to plug into the formulas used for determining bandwidth consumption when VAD is in play then, David? If yes, should I just be adding an addn'l 35% to the final BW formula result for any VAD-enabled links whose call traffic peak-time measures 24 or more simultaneous voice calls? Is it that simple?
Also, can VAD be enabled independent of whatever G7xx codec is being used, or is the VAD feature limited to specific G7xx codec types (e.g. G711, G729, etc.)?
I do not think it is that simple, no. Personally, I would use the BW calculator and leave the measurements - I would not add back any savings for VAD into the equation. Essentially, it is saying that you "might" save x % but you could save only y% and maybe you don't save any. It's a variant - that's why there is no calculation for it in the equations. As far as the specifics of VAD, I do not believe it is protocol specific; however, I don't tend to use VAD so I don't claim any authority there. It really is best practice to leave it disabled (default) unless you are routing over very low bandwidth, high cost links.
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The short answer is that you don't.... That isn't entirely true while at
the same time it kind of is, but for the most part you don't configure
the softkeys. You enable or disable them via TCL. Here is the long
answer. Be sure to read the whole thing or e...
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