Private Branch Exchange PBX - device that automatically transmits a call signal from one telephone to another. The system of Private Branch Exchange provides for establishing, maintaining and breaking connections between devices.

  Currently in use two types of PBX
  Digital PBX , analog PBX currently no longer produced so Digital PBX usually referred to simply as the PBX.
  IP- PBX - digital PBX using IP as a transport network (can be implemented in both appliance and software) .

  Analog phones - cheap and simple as possible - while digitizing the analog signal for subsequent transmission occurs on ports FXS, PBX or Gateway.
Digital system phones - are different in that the line between the subscriber and the switch is set digitally, allowing besides high voice quality, as well as advanced functionality to use the call control (typically use proprietary signaling).
VoIP telephone - is a digital device, but using different IP- packet transport for digitized voice and signaling (used codecs standardized signaling protocol SIP at the moment) .

 PBX elements
   Digital PBX is typically a single hardware-software complex to implement circuit switching. The design is Chassis PBX is necessarily completed the required fees, most of the space usually occupied by the subscriber interface board sets - which can be installed directly to the PBX chassis and specially designed for it anymore. Design and configuration, as well as functional PBX different manufacturers can be very different from each other, but at the same time PBX serving different sites is performing using standard protocols - for example, SS7 .
Elements of IP-PBX
   IP- PBX - is a server or cluster of servers with specialized software for call control and management of traffic in packet-switched IP network. To communicate with external networks such as the PSTN and to provide specific ports - for example: FXS for fax connection or G703 to connect to the stream E1 uses separate unit or the server board - voice gateways allow to recoding signaling and interface type .

   The main advantage of traditional PBX is a low cost analog ports while providing sufficient communication range of the two-wire line . Easy device analog phones and their simplicity makes this type of PBX competitive in cases where required to provide only basic services . IP-ATC in this case is more promising in terms of providing additional services up to commit video calls using VoIP phone supports this functionality .

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