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Use CSU ACE to lower db gain on Network side

I have a PRI coming out of an Edgewater 4550.  Problem is that we have severe echo using analog telephones and the PRI. Digital telephones and the PRI circuit are ok.

The PBX manufacturer says the PRI is coming into the PBX 'too hot' and want the service provide to adjust the db gain to -7.5.  PRI provider will not do that. 

PBX manufacturer says to add a CSU between the Edgewater and the PRI card.

I have an Adtran T1 CSU Ace (1st gen and 3rd gen).  Will either unit allow me to adjust db gain to -7.5 on the network side?  If not is there an Adtran CSU / DSU unit that will do such a thing?

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Re: Use CSU ACE to lower db gain on Network side

You are dealing with two completely different issues and are getting bad advice from someone who doesn't understand this.

PRI rides on an underlying T-1 digital circuit. This is carried as 4-wire, one pair in each direction. The signal level of the digital carrier can be adjusted by using line build-out (abbreviated LBO) on the transmit side to ensure that it is of sufficient level to drive the digital receiver. If the signal is too low, noise will cause bit errors. If too high, overload of the receive electronics can also cause errors. However, this has absolutely nothing to do with the echo. If your T-1 interface on the PBX isn't showing errors, tweaking the LBO won't help the performance and could make it worse.

Keep in mind that PRI/T-1 is digital. As long as the line isn't showing errors, the level of the incoming digital signal won't matter at all with regard to the analog signal level. The analog signal level is set by the codec on the analog line card of the PBX, not by the line build-out on the digital signal.

To control echo, you'll need to look at the configuration of the PBX analog line cards. Transmit and receive gain are adjustable there. Try lowering one or both. Usually running the analog gain too high results in echo at the far end, not heard by your customer but by the other party. Also check the line card configuration to make sure that they aren't optioned for fax or modem use. There are normally echo-cancelers built in for analog voice use, but these are typically disabled for fax and modem use. If analog lines are shared for voice and modem/fax, some equipment monitors for fax/modem tones and disables the echo-cancelers for data/fax only but leaves them in for voice.

In your situation, I'd look for a more knowledgeable PBX technician.

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Re: Use CSU ACE to lower db gain on Network side

Thank you for your excellent explanation. I'm glad I posted my question here.

I am the PBX technician!  I am relying on manufacturer support to get the problem resolved.  The CSU was the manufacturer recommendation and I've been skeptical of that solution which is why I posted it here.

We've adjusted the transmit and receive on the analog card with no positive result. I will check on some of the class of service settings to see if fax / modem is a option for us to disable.

Excellent help and thank you very much.

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Re: Use CSU ACE to lower db gain on Network side

Is the echo audible to your PBX user or to the party on the other end of the call?

If it's the local user, verify that echo suppression is enabled as well as possibly lowering TX and RX analog gain. If it's the other party, try tweaking your analog impedance settings.

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Re: Use CSU ACE to lower db gain on Network side

Echo only to the outside caller. No echo to inside user.  We are using NEC SL1100 which is a nice modern system. 

I lowered transmit and receive to -3.0, -4.0 and -5.0 db with no improvement.

NEC still says try a CSU in between Telco and the PBX.

Fax / Modem is an option, currently set to FAX (default). Only option is Modem but manuals say this applies to use of SIP Trunks.  I don't have much else to change.

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Re: Use CSU ACE to lower db gain on Network side

jimtgs wrote:

Echo only to the outside caller. No echo to inside user. We are using NEC SL1100 which is a nice modern system.

I lowered transmit and receive to -3.0, -4.0 and -5.0 db with no improvement.

NEC still says try a CSU in between Telco and the PBX.

Fax / Modem is an option, currently set to FAX (default). Only option is Modem but manuals say this applies to use of SIP Trunks. I don't have much else to change.

Echo heard by the remote party is usually a symptom of reflections on your end or lack of echo-cancellation on their end. Is it on calls to all numbers or just some?

Can you de-select both fax and modem? I'd try that if you're using analog voice phones. Any kind of data configuration, either fax or modem, will disable echo-cancellation and you'll have this problem on voice calls.

Also is there an impedance selection on the analog side, something like 600/900 ohms, complex, etc.? Changing this may help. It's kind of cut-and-try to match the characteristics of the specific phone you're using.

I very highly doubt that adding a CSU will make any difference whatsoever. According to the Edgewater 4550 spec sheet it is capable of line build-out to reduce the level of the PRI T-1 to 0, -7.5 or -15 so you can do that there without an external CSU. There's one built into the Edgemark. If it were me, I'd replace the Edgemark with an Adtran TA908e and use the Edgemark for a wheel chock. There are very likely some settings within the Edgemark that will help, but fortunately I haven't touched Edgewater products in a very long time.

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Re: Use CSU ACE to lower db gain on Network side

You are very kind to me!  I cannot deselect fax, it's either fax or modem. It is the only analog station in the office.

The edgewater belongs to the telco so I am hesitant to touch it and break it.

We had a Nortel system before and this was never a problem.  But we had an Atlas 550 between the telco and the Norstar (PRI to T1).

I am going to try a different analog card too.  I will keep you posted.

Thank you!

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Re: Use CSU ACE to lower db gain on Network side

CSU had no effect as you already know.

Different analog port on a different card had no improvement.

We are using a POTs for the time being. Not the greatest but we move on from there.

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