design issues

An IP PBX is kind of special PBX which can transform analog voice into digital data i.e. packets which can be sent over IP and, or any other network protocol and vice versa. This data may further be encoded to reduce bandwidth requirements with the help of standard and, or proprietary codecs (also known algorithms in programmers point of view)

Yate (http://YATE,null.ro/), OpenPBX

and, or Asterisk are good working examples of open source IP PBX's, whereas the later one's are much more mature and complex and the former one is somewhat newer, but is highly structured, object oriented and statefull.

The major design consideration is compatibility and, or implementation of protocols i.e. SIP, H.323, IAX, RTP/RTCP and, or SRTP for inter operation with other IP and, or network based devices which may optionally play various roles; these devices could be gateways, routers, switches, gatekeepers, proxies, registrars, session border controllers, media controllers and, or MCU's.

Try