Tất cả các subchannels thấp sau đó được ghép vào một "ast" kênh kết nhà quản lý mạng lưới thu hút đầu vào của người dùng trên mạng xây dựng-outs, một trong những nhu cầu phổ biến nhất là tốc độ nhanh hơn. | Switching TDM and VoIP Networks 69 of the softswitch architecture occurs in the service creation environment. This is covered in greater detail in a following chapter. Applications for Softswitch IPPBX Perhaps the earliest and most popular application for enterprise VoIP giving rise to the softswitch was the installation of a VoIP gateway on the trunk side of a PBX. This gateway packetized the voice stream and routed it over an IP network which saved the business a lot of money in long-distance transport costs. This solution used the existing PBX s set of features conferencing call forwarding and so on . It also provided investment protection to the user by leveraging the legacy PBX into a VoIP solution. The intelligence in this solution was contained in software known as the gatekeeper. The gatekeeper was the precursor to the softswitch. Eventually software developers devised a soft PBX which could replace legacy PBXs. These soft PBXs Figure were considerably less expensive than a hardware PBX. They then came to be known as IP PBXs. An IP PBX can be thought of as an enterprise grade softswitch. IP phones Branch office IP phones IP-PBX Headquarters Figure IP PBX also known as soft PBX. 70 Voice over IP Centrex Just as the Centrex model followed the PBX in circuit switching it does the same in packet switching. Shortly after IP PBXs began to catch on in the market the regional Bell operating companies RBOCs began to realize a threat to their circuit-switched Centrex services from VoIP applications. Centrex accounted for about 15 of all business lines and many subscribers were locked into 5-year contracts with the RBOCs. As these contracts began to expire in the late 1990s many customers were actively evaluating less expensive alternatives to Centrex. If large companies could route their interoffice voice traffic over a corporate WAN using an IP PBX what would be the demand for their circuit-switched Centrex services With this threat in mind IP Centrex
