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Connecting to the Voice World April 17, 2000 By Darrin Woods Circuit switching is dead. Well, maybe not dead, but it certainly has a terminal disease. Will packet-switching-based phones be installed in our homes and offices tomorrow? Will PBX equipment that's in most phone closets become obsolete overnight? The answer to both questions is a resounding "No." Even though the rudder has been put hard over, the ship of technology is going to take some years to start heading in the direction of packetized voice. Just as a ship can't turn on a dime, neither can most businesses. Old technologies are rarely thrown out for newer ones overnight. There is usually a natural progression from old to new that includes a period of coexistence. This time can be painful for network engineers, given the need to ensure that everything works together. One millimeter to the left, and the old technology curls up into a fetal position; one millimeter to the right, and the new technology goes four paws to the sky. VoIP (voice over IP) is in this predicament. Vendors have been clamoring to introduce products and move customers away from circuit-switched voice technology. Businesses are considering the costs of main- taining two separate networks--one for voice and the other for data. VoIP solutions are being integrated into existing corporate PBXes to provide myriad services to their users, from low-cost long distance to non-U.S. destinations, to simple office-to-interoffice communication. The number of circuits brought into a facility also can be reduced by providing some services over the data network, thereby lessening the need for separate PSTN circuits. Integrating VoIP technologies with installed PBX technologies helps users through the transition from traditional telephones to computer telephony or Ethernet-based phones. Several WAN technologies can be used to provide these services. ATM has long been used to transmit voice over data networks, with frame relay being a slightly newer entrant. New methods incorporate the emerging DSL market to provide voice services as a separate channel or integrated as data packets.
PBXes can be thought of as the same type of switches used in a central office by a local exchange carrier, only much smaller. Connections coming into the PBX from the outside world are referred to as FXO (foreign exchange office) connections, and those running to individual phones are called FXS (foreign exchange station) connections. Connections between PBXes are most often referred to by the signaling used--E&M--instead of as tie trunks.
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Most businesses have at least one PBX that connects all phones in the office to the trunks that go to the PSTN. Offices located in different cities also have their own PBXes handling calls. Calls between offices usually are sent to the PSTN and routed via a long-distance carrier to the other city and back to the remote office. In larger setups, companies link PBXes over long-distance connections using their own tie trunks. This eliminates paying long-distance charges for calls to other offices, requiring only a flat monthly payment for the tie trunks.Best of the Web
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