What is (SIP) IP PBX ? 

IP PBX means Intranet Private Branch eXchange. An Intranet Private Branch eXchange is a telephony solution for a business or other agency where the primary means of exchanging voice internal to the system is by using Voice over IP (VoIP).

Most of these IP PBX systems have the same or similar features to their PBX counterparts. Many have enhanced features, such as integrating voice mail with e-mail, or the other enhanced features that VoIP can provide.

IP PBX Private Branch Exchange. A telephone, data and video switching system, usually located on customer premises and belonging to the user. Makes use of technologies like VoIP, H.323 and Ethernet.

What is IP PBX ?

A Private Branch eXchange (also called PBX, Private Business eXchange or PABX for Private Automatic Branch eXchange) is a telephone exchange that serves a particular business or office, as opposed to one a common carrier or telephone company operates for many businesses or for the general public.

Overview

PBXs are often differentiated from "Key Systems" in that users of Key systems manually select their own out-going lines while PBXs select the out-going line (trunk) automatically. Hybrid Systems combine features of both.

Initially, it was private since in general it served a user company, which wanted to have its own branch to save some money in, for example, internal calls. This was done by having the exchanging or switching of circuits done locally, inside the company. Because fax machines, modems (though the PBX may degrade line quality) and many other communication devices can be connected to a PBX, extension developed to a generic term describing all devices connected to a PBX. The PBX's are connected to the outside world by a number of lines called trunk lines.

After the PBX business took off, PBX's started offering services to small businesses and home users that were not available in the operator network. The status quo was changed after several decades by two significant developments during the 1990's. One was the massive growth of data networks (and the concept of packet switching entering public consciousness) and the other one was the trend of focusing on core competence. Companies needed packet switched networks anyway for data networks, so using them for telephone calls was tempting. The Internet - and its low price on global communications - was the final straw, and the VoIP PBX was created. Technically nothing was being eXchanged anymore, but the term PBX was very well recognized, and there apparently were no major pushes for a different acronym.

On the other hand, many companies realized that handling their own telephony was not their core competence. Also, PBX services had always been hard to arrange for smaller companies. These considerations gave rise to the concept of Hosted PBX, where the PBX was actually managed by the telephone company, and the customer company just had to sign up for a service rather than buy an expensive piece of hardware and then maintain it. This essentially removed the branch from the private premises, making it centrally managed. Some simply gave up the PBX for Centrex service.

"The Session Initiation Protocol (SIP) is an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences." (cit. RFC 3261). It was originally designed by Henning Schulzrinne (Columbia University) and Mark Handley (UCL) starting in 1996. The latest version of the specification is RFC 3261 from the IETF SIP Working Group. In November 2000, SIP was accepted as a 3GPP signaling protocol and permanent element of the IMS architecture. It is widely used as a signaling protocol for Voice over IP, along with H.323 and others.

SIP has the following features:

    * Lightweight, in that SIP has only six methods, reducing complexity.
    * Transport-independent, because SIP can be used with UDP, TCP, ATM & so on.
    * Text-based, allowing for humans to read SIP messages.
 

What is VOIP ( Voice Over IP)

Voice over Internet Protocol, also called VoIP, IP Telephony, Internet telephony, Broadband telephony, Broadband Phone and Voice over Broadband is the routing of voice conversations over the Internet or through any other IP-based network.

Protocols which are used to carry voice signals over the IP network are commonly referred to as Voice over IP or VoIP protocols. They may be viewed as commercial realizations of the experimental Network Voice Protocol (1973) invented for the ARPANET.ce providers. Some cost savings are due to utilizing a single network to carry voice and data, especially where users have existing underutilized network capacity they can use for VoIP at no additional cost. VoIP to VoIP phone calls on any provider are typically free, while VoIP to PSTN calls generally costs the VoIP user.

There are two types of PSTN to VoIP services: DID (Direct Inward Dialing) and access numbers. DID will connect the caller directly to the VoIP user while access numbers require the caller to input the extension number of the VoIP user. Access numbers are usually charged as a local call to the caller and free to the VoIP user while DID usually has a monthly fee. There are also DID that are free to the VoIP user but is chargeable to the caller.

 Wikipedia