central office (USA) (CO)
The central office(CA) is a local exchange that forms the transition between the optical network and the customer lines. The subscriber lines(ULLs), usually in the form of copper twisted pairs, terminate in the central office.
The central office contains the switching technology used to switch the connections between the optical network and the local loop network. Between the central office and the local loop, there is often another distribution point, the serving area interface( SAI), which is connected by feeder cable. Such a feeder cable may contain up to a thousand pairs of cables. The subscriber lines are bundled and routed to the serving area interface.