Andrew Kohlsmith wrote:
On Saturday 10 July 2004 11:21, Rich Adamson wrote: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.
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. :-)
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
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