IP Telephony - NBX
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Silence Supression

Frequent Advisor

Silence Supression

I am using PCxset on computers located in India. The NBX is in Connecticut.

Once the network was set to use priority, EF class. then I noticed static and whoosing sounds in the silence between words.

I removed silense suppression and these noise went away.

Is silence suppression  recognized accross a WAN?

Cisco routers are in use

Super Advisor

Re: Silence Supression

Silence suppression should be off on all devices . Not sure why you would use it to begin with .

Frequent Advisor

Re: Silence Supression

I have had silence suppression on since I started using VTL's.

The book says it reduces packets by 40%.

Why wouldn't I use it? or When should it be used?

Super Advisor

Re: Silence Supression

Silence suppression has been used in the past to reduce a white noise that can show up on the voice . In over 9 years , I have not used it as it can cause issues . I am not sure what you would use it for , eg , reduce packets , but I can see it removing some of the packets . Compression can be used to reduce overhead on the packets . You can prob troubleshoot your network and find the root cause of why the change sin your settings caused the issue but again , I do not know of anyone that uses Silent Supression anymore !

Page 501 of the manual states for a condition known as congestion , silence suppression would be used to remove some packets .


You can reduce NBX system’s bandwidth

requirements by enabling 

“silence suppression,” but doing so

compromises audio quality. NBX

Telephones generate voice frames at

regular intervals for the duration of

a connection. These frames normally

continue when no one is speaking.

When silence suppression is enabled, the

NBX system sends a “silence

indicator” when the NBX Telephone

senses the start of a silent period.

When another NBX device receives this

indicator, it inserts “white noise”

until it receives the next frame that

contains real voice. All subsequent

“voiceless” frames are suppressed

during the silent period. However,

most telephone users will notice the

difference between genuine silence

and generated silence.









This type of silence suppression applies to Layer 2 Ethernet transfers. At

Layer 3, the ConneXtions software achieves a similar result by not

sending empty packets during a silent period. The receiving ConneXtions

gateway generates a silence indicator or sends frames filled with silence,

depending on the silence suppression mode.



Users notice congestion when audio “breaks up” during a call.

Congestion can occur anywhere on the

network, for example, at an

overloaded LAN (local or remote), at an

overloaded router or firewall, or

within an overloaded internet. Because

voice packets are only significant

during a conversation, IP networks

respond to congestion by discarding

data packets they cannot accommodate.

Resending or delaying packets is

not an effective solution.

At the local level, congestion symptoms can be subtle. For example,

routers from different vendors can

respond differently to congestion

because of the way they prioritize their

response to packet congestion.

When considering communications

problems, it is important to maintain

reserve capacity and to use a systematic

approach that considers the

entire, end-to-end, connection.

This message was edited by merlin on 2-9-10 @ 8:03 AM
Frequent Advisor

Re: Silence Supression


Silence suppression is off on all my units, including VTL's connections also.  Right?

Super Advisor

Re: Silence Supression

yes , I would

Then retest everything and I'm sure you will be happy with the results !


Frequent Advisor

Re: Silence Supression

thank you!!

Super Advisor

Re: Silence Supression

My Pleasure !

Email / call / IM if needed !