[Asterisk-Users] Anyone using * in a live production environment?

[Stephen R. Besch]

Dave Weis wrote:

>On Tue, 4 Nov 2003, Stephen R. Besch wrote:
>  
>
>>    5) Attempt to balance the hybrid at the 2-line to 4 line interface.
>>            Why:  99% of the time, this is where the echo originates and 
>>this is where is should be fixed.  Unfortunately, this is not for the 
>>faint of heart, but if your line card has a hybrid balance adjustment 
>>(many don't), use it.  Also, with multiple simultaneous calls, this may 
>>be the only real solution.  Part of the problem arises from the use of 
>>lower impedance telephone wiring nowdays. The typical characteristic 
>>impedance of Cat5 twisted pair is about 100 ohms and many line cards are 
>>optimized for a 600 ohm line. This is made worse if the DC resistance of 
>>the wiring to the CO switch is relatively low.  I haven't tried this 
>>myself, but you might try something as simple as a 500 ohm variable 
>>resistor in series with the ring line and adjust for minimum echo.  If 
>>it gets worse, you haven't lost anything, just take the resistor out of 
>>the line. If it works, measure the value of the resistor when set for 
>>minimum echo and replace it with a fixed value resistor.
>>    
>>
>
>I've had to do similar things to lower loop current. Be sure you get at 
>least 1 watt resistors and put the same size on the tip and ring. Putting 
>resistance on only one wire will throw other things off.
>
>dave
>
>  
>
The 1 watt recommendation is a good one.  The split resistor will 
probably not make any difference on a voice only line, as the 
transformed impedance of the phone line on the other side of the hybrid 
won't really care.  On the other hand, a big R imbalance will affect 
transmission line characteristics on the line, which will have a 
negative impact on data transmission.  The bottom line is that resistors 
are cheap and using two is probably worth the extra pennies.  Just be 
sure to divide the resistor value that minimizes echo in half so that 
the total resistance insterted is the measured value.