IgNiTeD SoUL

Before delving into VoIP configurations, a brief introduction with terminology is necessary:

• Public Switched Telephone Network (PSTN) —PSTN is the world’s collection of interconnected public voice telephone networks. It is also known as the Plain Old Telephone Service (POTS). It is set up and managed by the government and commercial organizations. It has evolved from the early days of Alexander Graham Bell to mostly digital, circuit-switched telephone network.
• Private branch exchange (PBX) —This is a device located within an organization that routes telephone calls to internal extensions or to the PSTN. It provides additional features such as voicemail and call-forwarding. A PBX is less expensive than connecting an external line to every telephone. Numbers within the PBX (internal numbers) can be dialed using the last few numbers of the entire phone number and without going through the PSTN. A PBX usually has more than 125 ports.
• Key telephone system —This is used like a PBX in small offices where far fewer phones are required. Each key telephone system supports up to a hundred ports.
• Software IP phones —These consist of a headset that plugs into the USB or serial interface of a PC. The PC needs client software that supports IP telephony.
• Hardware IP phones —These look like regular telephone sets, but they are plugged into a LAN switch. Most IP phones get power from the switch (power over Ethernet or PoE) and encapsulate voice data into IP frames for transmission over the LAN.
• H.323—This was approved by the International Telecommunications Union (ITU) in 1996 as a standard for multimedia and audiovisual transmission across disparate networks. In 1998, it was followed by version 2. It also includes several functions such as bandwidth management, call control, multimedia management, and interoperability between different network types. H.323 has come to be the most popular protocol for VoIP.
• Session initiation protocol (SIP)—SIP is IETF’s standard for multimedia communication over IP networks. It is an application-layer control protocol that initiates, manages, and terminates calls between two or more terminals. It is picking up as an alternative to H.323.