Centrex or PBX : IP Telephony

For several years the term Voice over Internet Protocol (VoIP) has been used to identify the transmission of voice signals over any IP-based network. In the business context, VoIP now refers to the transmission of voice and/or video communications over a managed WAN or LAN.

Some consultants and one or two telecom manufacturers have used the term telephony over Internet protocol (ToIP) to describe the switching of real-time, conversational traffic in systems that are attached to IP data networks.

The words "IP telephony" have emerged as the umbrella term that covers both VoIP and ToIP, which can be delivered either by an IP-PBX (housed on the customer's premises) or by IP-Centrex (for which the call processor is owned and accommodated by the carrier).

Some organizations have been using VoIP to reduce the cost of longdistance service, particularly international service, for several years. In countries that have highly competitive interexchange carriers it is generally no longer worthwhile to use VoIP, after allowing for the cost of gateway hardware and software. Several international carriers, such as the T-Systems division of Deutsche Telekom, have specifically promoted the use of IP networks for international traffic.

IP telephony has been validated since the year 2000 by the availability of some IP-PBXs. An IP-PBX system or IP-Centrex service may deliver only VoIP (through appropriate trunk and line interfaces), only ToIP (by retaining interfaces for the existing analog and digital devices), or both in the form of IP telephony. Now that IP has become, by far, the most popular protocol for data transmission and is widely deployed in networks of all sizes, from one room to worldwide, organizations have access to the appropriate transport technology to gain the advantages of convergence. Full convergence between multimedia and data applications requires the availability of both VoIP and ToIP, which are both inherent to the concept of IP-Centrex.

Interface Standards
The key to success with IP-Centrex will be a high level of interoperability between devices and systems or applications. This will be a major move away from the proprietary interfaces that have kept acquisition costs up and made application implementations complex with PBX systems. The acceptance of a limited number of open standards will also facilitate more competition between a broader range of manufacturers and service providers.

The earliest call control standard for mapping users' names or telephone numbers into and IP source or destination addresses was H.323, which was adopted by the International Telecommunications Union (ITU). H.323, was originally intended to define how multimedia communications were to be transmitted over a data network between teleconference units and, as used for VoIP, defines only a restricted feature set, with different enhancements being made by various manufacturers. H.323 does not define transmitted voice quality, is considered to be too processing intensive, and depends on the use of intelligent workstations.

A working group of the IETF created the session initiation protocol (SIP) to lessen call setup times and take better advantage of the Internet infrastructure than H.323. SIP is most likely to become the interface standard of choice between telephone sets and computers with ToIP systems.

A third standard, officially known as H.248 but more generally as MGCP or Megaco, is being jointly developed by the IETF and ITU, with support from some, but not all, major manufacturers. The H.248/ MGCP protocol addresses the needs of multimedia conferencing and is intended for use with media gateway controllers (MGC).

Every communicating device on an Internet protocol-conforming network must have an IP address, so that a desktop with a telephone, a PC, and a softphone (within the PC) or a video terminal needs to be allocated three addresses. There must be a process with the network for mapping telephone numbers to corresponding IP addresses.

Also, any endpoint's address must be known in order to be accessible; in some circumstances, this requirement becomes a security concern. This situation can be problematic in that multiprotocol label switching (MLPS), which is widely used in managed wide area networks (WANs), does not allow for any "spoofing" (i.e., the alteration and retransmission of any part of a signal in order to hide the address contents and therefore discourage hacking).