Most telephone systems operate in basically the same manner and have the same components. They are the power supply, the processor, the software, the interface cards and the telephones. The power supply, processor , software and interface cards are normally refered to as the KSU, or Key Service Unit. The telephones and telephone lines (numbers from the phone company) connect to the KSU either by plugging directly into the KSU, or more commonly by connecting to a System Interface, called a 66 block.
Interface cards connect the KSU to the various outside devices. Interface cards that connect the telephone numbers from the telephone company to the KSU are typically called trunk cards, line cards or CO cards (CO stands for Central Office, referring to the building the lines come from). Station cards or digital station cards are the cards that run the actual keysets, or key telephones. Analog or single line cards can run portable phones, fax machines and voicemails.
