/mcpfragmentation size set under the map−class. enabled. VAD A regular voice conversation consists of several moments of silence. A typical voice conversation consists of 40 to 50 percent silence. Since there is not any voice going through the network for 40 percent of a voice call, some bandwidth can be saved by deploying VAD. With VAD, the gateway looks out for gaps in speech. It replaces those gaps with comfort noise (background noise). Thus, an amount of bandwidth is saved. However, there is a trade−off. There is a small time (in order of milliseconds), before the codecs detect speech activity followed by a period of silence. This small time results in the front−end clipping of received voice. To avoid activation during very short pauses and to compensate for clipping, VAD waits approximately 200 ms after speech stops before it stops transmission. Upon restarting transmission, it includes the previous 5 ms of speech along with the current speech. VAD disables itself on a call automatically if ambient noise prevents it from distinguishing between speech and background noise. However, if the bandwidth is not an issue, turn the VAD off. Tune VAD Parameters There are two parameters that dictate the functioning of VAD. These are the music−threshold and voice vad−time commands. music−threshold An initial threshold is decided which governs when VAD needs to become active. This is controlled by defining the music−threshold threshold_value command on a voice−port, as shown in this example. The range for this is from −70 Decibels Per Milliwatt (dBm) to −30 dBm. The default value for this is −38 dBm. Configuring a lower value (towards −70 dBm) results in VAD becoming active at a much lower signal strength (the volume must drop really low before it is considered as silence). Configuring a higher value (closer to −30 dBm) results in VAD becoming active for even a small drop in voice signal strength. It drives the playout to play comfort noise packets more often. However, this sometimes leads to minor clipping of audio. 3640−6#configure terminal Enter configuration commands, one per line. End with CNTL/Z. 3640−6(config)#voice−port 3/0/0 3640−6(config−voiceport)#music−threshold ? WORD Enter a number b/w (−70 to −30) 3640−6(config−voiceport)#music−threshold −50 3640−6(config−voiceport)#end 3640−6# 3640−6#show run | be voice−portvoice−port 3/0/0 music−threshold −50 voice vad−time Once the VAD becomes active, the component of background noise and comfort noise is controlled by configuring the voice vad−time timer_value command under the global configuration, as shown in this example. This is the delay time in milliseconds for silence detection and suppression of voice packet transmission. The default value for the holdover time is 250 msec. This means that within 250 msec, comfort noise begins. The range for this timer is 250 msec to 65536 msec. If a high value is configured for this, comfort noise comes into play much later (background noise continues to be played). If this is configured for 65536 msec, then the comfort noise is turned off. A higher value for this timer is desired for smoother transition between background noise and comfort noise. The downside to configuring the voice vad−time command to a high level is that it does not achieve the desired 30 to 35 percent bandwidth saving.