[Asterisk-Users] T1 Hardware Echo Can

[Steve Underwood]

Andrew Kohlsmith wrote:

>On Saturday 10 July 2004 11:21, Rich Adamson wrote:
>  
>
>>If you install a T1 card and an external T1 mux (with fxo cards), the
>>echo can function already exists within the mux and/or cards. Don't
>>really need 'another' external echo can box unless you actually
>>purchased a T1 mux that didn't have echo can in the first place (and
>>they do exist).
>>    
>>
>
>As Steve already said, generally speaking echo cancellation hardware on T1/E1 
>interfaces is an option adder.  If it doesn't mention it, it doesn't have it.
>
>  
>
>>If you install a PRI (or related types of channelized T1 arrangements),
>>you don't need an external echo can function as those interfaces are
>>generally 4-wire to 4-wire interfaces already. If echo exists, its
>>generally the result of other interfaces (located somewhere else) and
>>those locations should be addressing the corrective actions needed to
>>resolve the issue.
>>    
>>
>
>You will often hear echo from the far-end hybrid, even on PRI, as I have found 
>out.  Normal KSU/PBX systems with T1/PRI interfaces have echo cancellation 
>hardware within the KSU itself.  I am purchasing a T1 echo canceller in order 
>to try and eliminate the echo we hear (i.e. far-end echo) -- something I 
>didn't think I'd need to do.  Our telco (Bell Canada) seems oblivious to any 
>knowlege about echo cancellation for T1 within the CO, but I continue to 
>press, because every now and again you hear a glimpse of "oh yeah we can do 
>that, your line just wasn't engineered with that equipment" kind of 
>blurb.  :-)
>  
>
I think you are missing something important about how traditional 
telephone networks function. In the days before echo cancellation was 
practical, it was vital to avoid the need for them. They couldn't avoid 
the echo, so they avoided significant delays. Within almost any country, 
the physical delay is so short the echo from the far end appears as 
pleasant reverberation, and not nasty echo. International circuits have 
always been a pain, as significant delay is unavoidable there. It is 
packetising voice that really introduced delay as a broad issue. First 
in digital cellular networks, where codecs process voice in blocks, and 
inherently introduce at least a one block (say 20ms) delay. Now VoIP 
broadens the issue further.

Equipment makers specifically design traditional network equipment to 
minimise delay. When I was developing DSP processing within the PCM 
network I was only allowed 375us (3 samples) delay - one sample to 
de-serialise the PCM stream, one to process it, and one to re-serialise 
the result. Delay budgets are always set as tight as possible.

Bottom line: the traditional PSTN has always had echo, and it is 
normally irrelevant. Telcos, have no need and no interest in removing it.

Regards,
Steve