In Detail OpenSER is, flexible free open-source VoIP server based on 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, but can handle 4 million users on a single processor server. It is adjusted, could feature as fast load balancer; sense of SIP servers: registration, location servers, proxy servers, redirect servers, gateway to SMS / XMPP; or advanced VoIP application server. Who is this book written for? This book is for readers who want to understand how to build a SIP provider from scratch using OpenSER.