[Asterisk-Users] 5ESS central office question

[Steve Underwood]

Rich Adamson wrote:

>Anyone have a practical experience/knowledge relative to why a 5ESS 
>central office switch would require a "w" in the Dial statement to 
>handle analog pstn-fxo calls? 
>
>I fully understand what "w" is doing, just trying to better understand 
>why a 5ESS doesn't accept dtmf a little quicker then it does. Does
>that switch make use of dtmf receiver cards (or something) that
>might involve a delay in attaching it to an analog pstn line?
>
>Rich
>  
>
It depends, and I am not familar with specifics of the 5ESS, but.....

Older digital exchanges generally used shared resources for DTMF 
detection. Later ones tend to use something in the line card chip set 
for DTMF detection. When I say older and newer I mean of old or new 
manufacture rather than model. Exchanges change enormously in their 
implementation during the overall design's lifetime.

Usually when you pick up a phone there will be a short delay before 
dialtone appears. Generally it is only 100-200ms, so you don't really 
notice it. In general, the DTMF detector will not be functioning until 
that tone is playing. However, even if the tone appeared in the first 
millisecond, the DTMF receiver tends to be paralysed for a little while 
by the loop current sloshing around and settling down. 100-200ms should 
see that fairly well settled. So, one way or another you need at least 
100-200ms before the DTMF receiver can start detecting reliably. If the 
dial tone is a bit delayed it can be longer.

Most modern modems explicitly listen for dialtone before dialing. Older 
ones don't, but delay their dialing for a second or so after appling the 
loop.

Regards,
Steve