IP telephony – protocols and operations

In this section, we talk about the IP telephony protocols, also known as the VoIP protocols, but before directly moving on to the protocols, let's first understand what VoIP is, how VoIP works, and its various derivatives.

VoIP

VoIP means telephonic communication that happens over an internet line and not the old PSTN-based telephone lines. The VoIP protocol is responsible for delivering voice communications and streaming live sessions over the internet. A very basic example is Skype, a Microsoft product that works over the VoIP protocol. VoIP has its advantages and disadvantages, but before moving on to that, let's understand VoIP in depth.

The following diagram depicts VoIP communication:

Figure 16.1 – VoIP communication

The complete VoIP communication works like this:

1. Client 1 using some intermediate VoIP server (here PBX) sends an INVITE request to Client 2.
2. Client 2 accepts...