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