Private Branch Exchange (PBX) is a telephone switch used by enterprises and located at the premises of a company. The traditional PBX based on the TDM technology is reaching the end of its lifecycle due to the emergence of IP-PBX. The IP-PBX, based on the VoIP technologies, offers easier user administration and advanced applications. With an IP-PBX, the Local Area Network is the platform for connecting smart IP phones logically over a shared packet network to the call manager. This unifies the data applications and the voice network, but places demands on the packet prioritization aspects of the LAN infrastructure to ensure user satisfaction with the quality of audio. The key benefits of IP-PBXs are:

• easy to administer users since all users are provisioned like a PC
• support telephone user mobility with wireless LAN technologies
• provide unified messaging services
• lower total cost of ownership of the voice systems

The key component of IP-PBX are:

• Call manager (or softswitch) is for call control, signaling and management
• Analog Station Gateway allows plain old telephone service (POTS) phones and fax machines to connect to the IP PBX network
• Analog Trunk Gateway allows the IP PBX to connect to the PSTN or PBX.
• Digital Trunk Gateway supports both digital T1/E1 connectivity to the PSTN or transcoding and conferencing.
• Converged Voice Gateway allows you to connect standard POTS phones with IP or any H.323-compliant telephony devices.
• IP Phone is the end customer device replacing the traditional telephone set.

How does IP-PBX work? An IP phone connects to a LAN either through a hub port or a switch port. The phone can talk with CallManager and registers itself. CallManager stores the IP-address-to-phone-number mapping (and vice versa) in its tables. When a user wants to call another user, the user keys in the called party's phone number. The CallManager translates the phone number to an IP address and generates an IP packet version of ring tone to the called IP phone through the TCP connection. When the called IP phone receives the packet, it generates a ring tone. When the user picks up the phone, CallManager instructs the called IP phone to start talking with the calling party and removes itself from the loop. From this point on, the call goes between the two IP phones. When any change occurs during the call due to a feature being pressed on one of the phones, or one of the users hanging up or pressing the flash button, the information goes to CallManager through the control channel.

If a call is made to a number outside of the IP PBX network, CallManager routes the call to an analog or digital trunk gateway which in turn routes it to the PSTN.

Over years of development and improvement, many technology hurdles of VoIP for the deployment of IP-PBX, such as QoS and inter-operability between different vendor products, have been resolved. IP-PBX is ready for prime time deployment.

IP-PBX: Private Branch Exchange using VoIP Technologies

Related Terms: PBX, VoIP, SIP, IP Telephony, Call Manager, Trunk Gateway, Voice Gateway