Customer Information Control System is an online transaction processing program. It generally refers to the telephone company's switching equipment or the building in which the switching equipment is housed. The C.O. is located in your town or in a nearby town. The client is a computer that relies on the server to fulfill certain functions. In a LAN where computers are either servers or clients .
Clients depend on the server.
The server is usually a much more powerful computer. With ISDN the D-Channel is used for signaling only. Direct Inward Dialing. A feature that allows an outside caller to dial a phone number that will ring directly to a specific party instead of calling a main phone number. Each workstation is assigned a direct phone number.