Hosted service lets enterprise users save on operational and capital expenditures, and reap the benefits of next-generation voice-over-IP services.\nSome companies that want to implement IP telephony lack the staff required to manage complex networks or can't afford to invest in an IP PBX. IP Centrex could be just the remedy, as the hosted service lets enterprise users save on operational and capital expenditures, and reap the benefits of next-generation voice-over-IP services.\nWith IP Centrex, the carrier handles call control, service logic and all responsibility for network equipment, customer provisioning, operations, administration and maintenance.\nAbundance of features\nSubscribers to IP Centrex have access to more than 300 features, such as click-to-dial, add and transfer, find me\/follow me, instant messaging and IP conferencing. In addition, customers gain traditional voice applications such as call blocking, forwarding, hold, transfer, call waiting, voice mail, caller ID, distinctive ringing, message waiting, three-way conferencing, and restrictions such as 900-blocking.\nIP Centrex encompasses a number of IP telephony services that offer voice to companies that transmit calls as packets across a broadband access network. A single broadband access network is used to carry the packetized voice streams for multiple calls. When calls are not active, more bandwidth can be allocated to high-speed data sessions such as Internet access over the same broadband connection. This represents a significant cost savings for large companies whose IP bandwidth can become scarce.\nCarriers can implement IP Centrex via a Class 5 switch or softswitch-based architecture. They can roll out new services using existing Class 5 switches by adding network gateways. The Class 5 approach is actually an IP-enabled hybrid called Centrex IP.\nIn the second scenario, the Class 5 switch is replaced with a softswitch. The softswitch architecture uses an application server instead of a network gateway.\nHere are the components that link customer networks to a service provider:\n\n\nCentrex network gateway: Used only in a Class 5-based architecture, this access element acts as a bridge between the circuit-switched and packet-based distribution network, and appears as a gateway. It lets packet-based phone subscribers access existing traditional Centrex features on the Class 5 switch and provides IP Centrex features to enterprise-class subscribers terminated on IP endpoints. The user interface is Web-based and provides an intuitive method to customize service offerings.\n\n\n\n\nCentrex application server: In a softswitch architecture, the Centrex features reside on an application server that interfaces or integrates with a softswitch for call control via Session Initiation Protocol or H.323, and gateways that interface to the public switched telephone network.\n\n\n\n\nCustomer gateway: Common to both architectures, the IP Centrex interface supports various endpoints such as analog IP, and traditional desktop phones if the user is not equipped with an IP endpoint.\n\n\nIP Centrex comes together\nWith these elements in place, IP Centrex delivers telephony services over any means of transport, including hybrid fiber coaxial, fiber or DSL. It provides a level of network efficiency that increases mobility and accessibility, and lets carriers bundle voice, video and data packages with an easy migration to enhanced services.\nTavares is senior product manager for Excel Switching. He can be reached at email@example.com.