There has been a clear division between two types of networks for a lot of years:

- Voice Networks, based on circuit switching, a circuit is reserved and the routing during the communication is always made by the same path. Example: Public Switched Telephone Network (PSTN)

- Data networks, based on packet switching, the information is divided into packets and each packet can travel by a different path. Example: Internet
In order to send to the information by the data networks (Internet) based in packet-switching, it is necessary to adopt protocols that are the responsible of transmit and recover the information.

The problem with the circuit switching technology is that it requires a great amount of bandwidth for each call. The circuit is not used efficiently because the same channel is using during a call and most of the phone calls have a lot of silence moments.

The data networks, on the contrary, only transmit information when it is necessary, using the bandwidth efficiently. The delay, the variations in delay of packet delivery and the loss of packages should not be a disadvantage, due to the final system has procedures to recover the original information. However, voice and video streaming are very sensitive to these parameters. Therefore, networks and protocols with a high degree of QoS (quality of service) are required.

Voice over IP (VoIP) defines the necessary routing systems and protocols for transmit voice conversations over Internet. Internet is a packet switching network based on TCP/IP protocol.

At this moment there are, mainly, two architectures of VoIP for the voice transmission by Internet that are very used:

SIP (Session Initiation Protocol)

SIP are the acronym of Session Initiation Protocol , being a standard developed by IETF, identified like RFC 3261, 2002. SIP is a protocol of signalling to establish calls and conferences in IP networks. The beginning of the session, change or term of the session, is independent of the type of application that it is being used in the call; a session can include several data types, including audio, video and many other formats

H.323

H.323 was the first international standard of communications multimedia, that facilitated the convergence of voice, video and data. Initially it was thought for packet circuit networks, in which it found his strength to integrate itself with IP, being a protocol very used in VoIP.