[Asterisk-Users] 5ESS central office question

[Rich Adamson]

> >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.

I was aware of that, but more interested in specifics involving the
5ESS since it has been involved in two cases now that need the "w", and
I'm not familiar with internal design of the 5ESS.