[asterisk-users] Disconnection supervision: what about PBX

[Trevor G. Hammonds]

> From: Yuan LIU
> Sent: Tuesday, February 06, 2007 8:11 PM
> 
> After reading through several recent threads, I started to wonder why
> the
> Cisco document (and other VoIP documents) appears to present this issue
> as
> VoIP gateway specific.  Don't (plain old) PBX' face the same issue if
> they
> use analogue interfaces?  If there are analogue PBX' at all, how do
> they
> solve the problem?

Yuan,
Well engineered analogue PBXs typically do not use standard loop start
subscriber lines.  When digital trunks are not an option, they use analogue
PBX and/or DID trunks.  At the very least, ground start circuits are
preferred to avoid "glare".  The best call quality for analogue is achieved
by using four-wire E&M trunks that provide answer and disconnect
supervision.  There are two-wire trunks (which are probably more common), as
well as different signalling methods.  These trunks require special
interface hardware, and I am unaware of any that work with Asterisk.  As the
cards are typically very expensive, it is usually better to go with digital
if you require that functionality.  It would be nice to see a BRI interface
for Asterisk that works in North America, as BRI circuits are often
comparable in price to analogue lines.  

		Sincerely,
		Trevor Hammonds