Circuit switching is the most common technique used for building communication networks worldwide. It establishes dedicated channels for the duration of a transmission. Integrated services digital network (ISDN) efficiently sends data and sound, and provides various communication services through a circuit. TCP/IP combines basic Internet communication protocols, connecting TCP to IP. A intraswitch trunk is a fiber-optic link that transports VLAN traffic, and frames on trunk links contain VLAN identification information. The central office switch accepts incoming calls and digitally connects to destinations.