Meaning "sound over distance," telephony refers to electronically transmitting the human voice. In the beginning, telephony dealt only with analog signals in the circuit-switched networks of the telephone companies. It later referred to a mix of analog and digital circuits, but still with the telephone industry. Starting in the 1990s, telephony began to embrace sending voice over internet networks, which is now deployed by common carriers, private enterprises and independent VoIP providers.
IP Telephony
The two-way transmission of voice over a packet-switched IP network, which is part of the TCP/IP protocol suite. The terms "IP telephony" and "voice over IP" (VoIP) are synonymous. However, the term VoIP is widely used for the actual services offered (see VoIP for more details), while IP telephony often refers to the technology behind it. In addition, IP telephony is an umbrella term for all realtime applications over IP, including voice over instant messaging (IM) and videoconferencing.
Starting in the late 1990s, the Internet and its TCP/IP protocol suite began to turn the data communications and telephone industry upside down. IP has become the universal transport for almost all data and video communications worldwide. It is increasingly becoming the infrastructure for voice traffic as well. Today, every communications carrier has built or is using an IP backbone for some or all of its voice services. In addition, large enterprises are either already using IP for some amount of internal voice traffic or have plans to implement it or create test beds.
Data Over Voice Became Voice Over Data
Data was first transmitted over telephone networks, starting in the 1960s, and by the late 1980s, data routinely traveled over digital voice circuits. By the 1990s, the majority of worldwide communications traffic had changed from voice to data, and as IP networks began to flourish, the economics of using IP for voice began to emerge.
Although the backbone of the global telephone network had been converted to digital for some time, the circuit-switched nature of the public switched telephone network (PSTN) is wasteful. Even though one person talks and the other listens, both "to" and "from" channels are always dedicated. In addition, newer voice codecs cut the digital requirement from the traditional 64 Kbps (PCM) down to 8 Kbps with respectable quality. Thus, the bandwidth requirement for voice on an IP network is 1/16th that of the PSTN's dedicated, digital circuits.
Early Days of PBX
The term PBX spawns from the original term PABX, which is an acronym for Private Automatic Branch Exchange. Essentially a PBX is a private telephony switch that resides inside the enterprise and is used for its internal employees to communicate both between themselves and the outside world. The original purpose of a PBX was to allow line pooling within an enterprise in order to grant that enterprise the ability to have more employees than telephone lines - taking advantages of the natural economies of scale that begin to occur as headcount rises.
Over time, the PBX has grown to incorporate all sorts of advanced features such as voicemail, unified messaging, auto attendant (IVR), automatic call distribution (ACD), call queuing, branch office support, telecommuters, softphones, CTI (integration with the PC), and more. With the advent of IP, the acronym PBX morphed into its latest incarnation, the IP PBX. An IP PBX is a PBX that supports packet-based transport protocols - commonly referred to as "VoIP". The most popular current protocol is SIP, which stands for "Session Initiation Protocol". Then, as the IP PBX began to rise in market share an even new label appeared called the "Hybrid IP PBX". A Hybrid IP PBX is an IP PBX that, along with VoIP, also supports legacy and analog switching protocols such as TDM. A Hybrid IP PBX will typically also support analog phones to complement its support of IP Phones. PhoneSource's PBXtra is a Hybrid IP PBX.
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