ubvoice, and telegraph. circuit switching The temporary direct connection of two or more channels between two or more points in order to provide the user with exclusive use of an open channel with which to exchange information. A discrete circuit path is set up between the incoming and outgoing lines, in contrast to message switching and packet switching, in which no such physical path is established. cladding In fiber-optic cable, a low-refractive-index material that surrounds the core and pro- vides optical insulation and protection to the core. clear-forward/clear-back signal A signal transmitted from one end of a subscriber line or trunk, in the forward/backward direction, to indicate at the other end that the established con- nection should be disconnected. Also called a disconnect signal. CLEC (competitive local exchange carrier) A telephone company that competes with an ILEC. CLECs in the United States today focus mainly on delivering dial tone to business customers. client A computer that requests network or application services from a server. A client has only one user; a server is shared by many users. client/server model The model of interaction in a distributed system in which a program at one site sends a request to a program at another site and awaits a response. The requesting pro- gram is called a client; the program satisfying the request is called the server CLNP (Connectionless Network Protocol) The OSI protocol for OSI connectionless net- work service. CLNP is the OSI equivalent to IP. clocking The use of clock pulses to control synchronization of data and control characters. closed user group A group of users in a network who are permitted to communicate with each other but not with users outside the group. cluster Two or more terminals connected to a single point or node. cluster controller A device that handles the remote communications processing for multiple (usually dumb) terminals or workstations. CMIP (Common Management Information Protocol) The OSI management infor- mation protocol fo