VoIP and SIP are often confused for the same thing, but they are actually different. VoIP defines the kind of phone call (over Internet), and SIP refers to the method used to enable VoIP calls between different devices. Let me explain.
Voice Over Internet Protocol is a rather broad term that discerns Internet calls from landline calls. VoIP operates over the Internet by digitally transmitting the voice signal to another telephone. VoIP usually operates with a mobile or browser-based application via an Internet Telephony Service Provider (ITSP).
One of the communications protocols used to transmit VoIP calls is SIP. Phone calls always have two endpoints, and simply put, SIP defines the messages and governs essential elements of the phone call. SIP can also enable video conferencing, media distribution, and instant messaging.
The Workings of VoIP and SIP
Simply put, SIP is the protocol and VIP is the conduit. But how do they work?
VoIP condenses audio by way a codec into data packets. After that, it compresses them across an IP network, where it de-compresses them into audio at the other end of the connection. Dedicated desktop VoIP phones, softphone applications, and mobile devices (i.e. cell phones) are typical examples of VoIP endpoints.
In the IP protocol, the IP packets move through the Internet via nodes (devices and routers discovered on the way from source to destination).
IP telephony differs from traditional telephony in the sense that in traditional telephony the line or circuit between source and destination is always dedicated and reserved (also known as circuit switching). In other words, traditional telephony tends to be more expensive because of the overhead.