Bi-Daily Question 6-14-2008

Luckycharms

How many and exactly what codec's use VAD??

Luckycharms

54 view and not even a comment??

Luckycharms

Wow... 74 views Not even an answer??


Well here is you answer...

G.729B and G.723.1A

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. Using 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 may 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 should 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 appe


ars 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 enable (it is disabled by default) with the following service parameter:

*

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).
RTP Header-Compression or Compressed RTP (cRTP)

bwidth_consume.gif

All VoIP packets are made up of two components: voice samples and IP/UDP/RTP headers. Although the voice samples are compressed by the Digital Signal Processor (DSP) and may vary in size based on the codec used, these headers are a constant 40 bytes in length. When compared to the 20 bytes of voice samples in a default G.729 call, these headers make up a considerable amount of overhead. Using cRTP, these headers can be compressed to two or four bytes. This compression offers significant VoIP bandwidth savings. For example, a default G.729 VoIP call consumes 24 Kb without cRTP, but only 12 Kb with cRTP enabled.

Because cRTP compresses VoIP calls on a link-by-link basis, both ends of the IP link need to be configured for cRTP.

In Cisco IOS Software Releases 12.0.5T and earlier, cRTP is process-switched, severely limiting the scalability of cRTP solutions due to CPU performance. Most of these issues have been resolved through various cRTP performance improvements introduced in Cisco IOS Software Releases 12.0.7T through 12.1.2T. A summary of the history follows.

*

cRTP is process-switched in Cisco IOS Software Release 12.0.5T and earlier.
*

In Cisco IOS Software Release 12.0.7T, and continuing in 12.1.1T, fast-switching and Cisco Express Forwarding-switching support for cRTP is introduced.
*

In Cisco IOS Software Release 12.1.2T, algorithmic performance improvements are introduced.

Moving cRTP into the fast-switching path significantly increases the number of RTP sessions (VoIP calls) that VoIP gateways and intermediate routers can process.

After reading my question I guess I should have put natively at the end... ooh well... I guess you guys either are not down with the bi daily questions or you need more time in between questions or this was just something you guys didn't care enough to look up.

Either way I will post one more and see the response and go from there...