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Impact of VAD?

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?

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values highlighted in red below are 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 ***

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Thank you.

Kevin

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4 REPLIES

Re: Impact of VAD?

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:

Voice Activity Detection

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).

http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a0080094ae2.shtml

Hailey

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New Member

Re: Impact of VAD?

Thank you, David.

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.)?

Kevin

Re: Impact of VAD?

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.

Hope my answers have helped some.

Hailey

Please rate helpful posts!

New Member

Re: Impact of VAD?

That helps clarify things somewhat, Hailey.  Thank you for taking the time to reply back.

Kevin

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