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Last Updated: Mar 16th, 2009 - 15:42:05 |
Architecture of a High-Density VoIP Gateway
A Voice over Internet Protocol (VoIP) gateway bridges the circuit-switched telephone network to the packet-switched data network. Such a gateway permits calls coming from an ordinary telephone to be carried over an IP network to a compatible computer client terminal or bridged back to another ordinary telephone. Connected together, multiple gateways create a virtual unified network that lets users to make and receive telephone calls over IP data networks. The issue of gateway interoperability is being resolved by the VoIP Forum through development of an Implementation Agreement (IA). The VoIP IA defines a set of required functionalities and protocols that allow gateways from various manufacturers to interoperate. The major functional blocks of a VoIP gateway are application, directory services, IP subsystems, IP call control, IP data transport, Public Switched Telephone Network (PSTN) subsystem, PSTN call control, and PSTN compression and packetization. Meanwhile, the issue of gateway scaleability is planted in system design and architecture, both hardware and software. The design tradeoffs made while designing a gateway may limit or enhance one's ability to scale upwards. Furthermore, design decisions can result in implementations that are appropriate for a given target density or port count.
Although debate over IP telephony's primary benefits and future course continues, there is general agreement that the technology is driving the convergence of worldwide PSTN and IP data networks -- both Intranets and the Internet. The IP telephony market is burgeoning thanks to initiatives and products from a range of contributors, from the small, innovative companies that pioneered the technology, to the major players of the telecommunications and data communications industries.
A key piece of the IP telephony puzzle is the PSTN-to-IP, or VoIP, gateway. A VoIP gateway bridges the circuit-switched telephone network to the packet-switched data network. Such a gateway allows calls originating from an ordinary telephone to be carried over an IP network to a compatible computer client terminal, or bridged back to another ordinary telephone.
Linked together, multiple gateways create a virtual unified network. Such a network allows users to make and receive telephone calls over IP data networks, and to do so with ordinary telephones or computers, just as easily as users can place calls over the PSTN today.
At present, the industry needs to clear several technology hurdles. These include setting standards for gateway interoperability, and making it possible to scale gateways from several ports to thousands of ports. For IP telephony to become ubiquitous, very high-density, interoperable gateway technology must be developed and then deployed by service providers, ISPs, and large private networks.
The issue of gateway interoperability is being addressed by the VoIP Forum through the development of an Implementation Agreement, or IA. The VoIP IA defines a set of required functionality and protocols which allows gateways from different manufacturers to interoperate. The VoIP Forum has built the IA on existing standards wherever possible and applicable, primarily the ITU H.323 standard for multimedia communications over non-guaranteed quality-of-service (QoS) LANs. The IA is, therefore, an amalgamation of these standards with extensions for requirements which are unique to VoIP. The major functional blocks of a VoIP gateway are described in the remainder of this section.
The future of VoIP gateway technology seems clear: high-density, scaleable, open platforms must be developed and deployed to enable the hundreds of millions of installed telephones and fast-growing number of H.323 computer clients, like Microsoft's NetMeeting and Netscape's Communicator, to communicate over IP. Hopefully, many vendors will create interoperable gateways based on innovative architectures which meet the demands of carriers, service providers, and large corporate network clients.
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