Cs) These controllers connect calls to the Public Switched Telephone Network (PSTN) by translating protocols between public network trunks such as T1 and T3 trunks and IP networks. They also translate calls between networks that use incompatible IP signal- ing protocols. SBCs combine media gateway functionality with softswitch functionality. These are a new type of central office switch that handles calls between different networks. Older types of central office equipment switch an entire circuit (an electrical path between devices). SBCs transfer packets containing voice traffic, video sessions, and data between networks. Depending on the manufacturer, all of these functions can be combined onto a single multifunction server. In addition, applications such as unified communications in a large carrier's network can be installed on separate servers. See Figure 4-4 for an example of VoIP architecture. The Migration to VolP by Cable TV Companies versus Traditional Telephone Companies For the most part, cable TV providers began offering voice service over broadband at a time when VoIP was a trusted solution. Because it was already an accepted tech- nology, most cable providers have centralized softswitch architectures. They are not saddled with the burden of supporting the hundreds of local legacy switches. Legacy switches refer to the older, less-efficient circuit switches connected to subscribers have analog telephones. Furthermore, although they have traditionally used standards developed by the cable industry's standard setting organization, CableLabs, they now use standard IP protocols for their VoIP infrastructure. Convergence in Core and Metropolitan Networks Centralized VolP Architecture Intrastructure Network server Media server Application server Session border controller network Access network Metropolitan Area Network Figure 4-4 In a centralized Voice over Internet Protocol (VolP) architecture, the application server is the central "director" and contains applications. The network server has information f