helves With Circuit Boards Inside the PBX are shelves with slots into which circuit boards are placed. These circuit boards have different functions. Some control the telephones, some control the outside lines, but all work together to carry out the functions of the PBX. The Telecom Handbook 23 THE PBX A Look Inside The PBX Cabinet When referring to the telephone system, we include the tele- phones, the cable and the PBX control cabinet, but here the focus is on the cabinet and its contents. The equipment housed in this cabinet connects the telephones within the office to the telephone lines and the world outside the office. PBX Shelf Each cabinet or cabinet module contains one or more shelves. Avaya (formerly Lucent Technologies) sometimes calls the shelf a carrier, but most other companies call it a shelf. The shelf is made up of a group of slots into which the circuit boards fit. Some systems have different slots for different types of circuit boards fit. Other systems may have universal slots that can accept more than one type of circuit board. Each PBX has a numbering scheme SO that the location of each shelf and slot (and cabinet, if more than one) can be identi- fied. The numbering scheme also includes the actual ports on each circuit board. Note: These numbers are typically labeled on each cable connecting the PBX to the telephones (in the tele- phone equipment room). For example 01-04-08 may represent the 8th port on the 4th circuit board on the first shelf. Some sys- tem cabinets come equipped with all of the shelves and others do not, requiring future purchase of additional shelves as needed. A shelf can cost several thousand dollars or more, SO it is important to understand how each PBX is put together when it is installed to avoid surprises when its time to expand or make changes. Many PBX manufacturers build their system in stackable mod- ules. A module combines the exterior cabinetry with the shelfinside. This is becoming more common with newer systems than the older "refrigerator size" cabinet, al