20 March 2007

The popular Asterisk iPBX server is the reigning champion of the Voice over Internet Protocol (VoIP) set, but there are other worthy contenders as well, such as SipX. If Asterisk represents the first generation of software PBXes, SipX is second generation, with a different approach and design philosophy.

SipX, strictly speaking, is a Session Initiation Protocol (SIP) proxy that includes a raft of PBX functions like voice mail, sophisticated call routing and auto-attendants. SIP is becoming the most popular VoIP protocol. SIP is a general-purpose protocol designed to be adaptable for moving different kinds of traffic. As RFC 3261 says: "SIP is an agile, general-purpose tool for creating, modifying and terminating sessions that works independently of underlying transport protocols and without dependency on the type of session that is being established."