This book will help in understanding VoIP and will also explain in increments how we can build software for simulating some basic errors occurring over the network, such as delay, jitter and packet loss. The first phase of the book gives a broad understanding of SIP (session initialization protocol), media compression and QOS (quality of service). It also sets out designs for the proposed system such as application system design, SIP design and QOS simulation design. The second phase of the book contains the implementation and testing of SIP and QOS simulations. Finally, the book ends with evaluating the software and suggesting any future work to be carried out. The system is capable of getting speech from microphone and playing it back through a simulated VoIP system. The 'network' may be perfect but can be downgraded by introducing delay, packet loss, varying jitter etc controlled by the user. The effect of this can be heard in the playback.