> I have been reading with great interest the posts on trouble shooting > echo cancellation with *. Is it just coincidence that all of this > discussion has been with analog lines. Are PRI's susceptible to echo > problem like POTS lines. >
Keep reading. Echo _can_ occur whenever a two-wire circuit is converted to a four-wire circuit (eg, hybrid involved). There is no way for you to identify where (in a pstn call) that might occur even with a PRI. You could have a PRI (four-wire) leaving your facility, but the telco (or another pstn customer) may have a hybrid involved in that end-to-end call. Hybrids are most often associated with analog circuits, so you'll see a large number of postings relative to echo on X100P and TDM cards. But, that certainly does not support 'echo is only a problem on analog circuits'. It doesn't help that a majority of current telco technicians wouldn't recognize a hybrid (or echo canceller) if it hit them in the face. Add to that the number of readers that repeat other postings without any reasonable knowledge of what they are talking about, and you get a substantial variation in the cause of echo (and how to troubleshoot it). Its probably one of the most difficult telephony issues to troubleshoot as there aren't any real tools available to pin-point the source. It just happens that asterisk has more then its fair share of echo problems, and you'll find a large number of postings that suggest asterisk's echo canceller does not have a very wide operating range. (E.g., the exact same asterisk system moved from one location to another will exhibit different echo characteristics due to the differences in pstn lines, etc. Analog interfaces having more echo problems by far then digital PRI interfaces.) The companies that specialize in digital to analog interfaces (eg, channel banks, external pstn adapters) typically have more dedicated processing power to perform echo cancellation, plus those companies spend a fair amount of research and development time to make cancellation work correctly. If they didn't, they wouldn't be able to sell their products. A true end-to-end PRI will _never_ generate echo; but, the stuff that customer's (large and small) attach on the other end of a PRI certainly can cause echo. And, believe it or not, there are still a large number of telco switches that are purely analog, thus requiring a digital to analog converter (hybrid). _______________________________________________ Asterisk-Users mailing list [email protected] http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
