OpenSER is a flexible, free, open-source VoIP server based on the Session Initiation Protocol (SIP), an application-layer control (or signaling) protocol for creating, modifying, and terminating sessions with one or more participants, including internet telephone calls, multimedia distribution, and multimedia conferences.

Engineered to power IP telephony infrastructures up to large scale, OpenSER is written in pure C for Linux/Unix-like systems with architecture-specific optimizations to offer high performance; it is able to handle 4 million users on a single processor server. The server keeps track of users, sets up VoIP sessions, relays instant messages, and creates space for new plug-in applications. It can be used on systems with limited resources as well as on carrier-grade servers, scaling up to thousands of call setups per second. It is customizable, being able to feature as fast load balancer; SIP server flavors: registrar, location server, proxy server, redirect server; gateway to SMS/XMPP; or advanced VoIP application server.

This book is a well illustrated, step-by-step guide to building a SIP based network using OpenSER. This book is for readers who want to understand how to build a SIP provider from scratch using OpenSER. Telephony and Linux experience will be helpful but is not essential. Readers need not have prior knowledge of OpenSER. It shows how OpenSER can be used to implement features not available in Asterisk PBX.