Enterprise VoIP Planet
The IT Manager's Guide to Voice over IP

Network World: Session controller
A session controller provides services between H.323 and SIP endpoints. In effect, this hardware/software combination operates simultaneously as an H.323 gatekeeper and SIP proxy server. In addition, it provides the SIP/H.323 interworking function that enables any-to-any connectivity between endpoints.

Quick Reference: Internet Telephony
Internet telephony can be basically broken down into three categories consisting of PC-to-PC telephony, PC to Phone, and Phone to Phone.