Central Office or CO is a brief borrowed from the English name for an exchange or a PoP. The term CO is often used in the DSL context. In such a CO subscriber lines run together, connecting the subscriber terminals to the switch. There they are according to the contract switched to the telephone network and / or to a DSLAM / DSL -AC.
In order to enable short subscriber lines, these central switching centers are no longer connected for many years as a rule directly with subscriber terminals, but primarily through fiber-optic cables (formerly with copper wires ) with a large number of local exchanges. One can find these devices in control cabinets at the roadside or mounted underground or in buildings. The subscriber terminal is then connected to the nearest local exchange. The English term for this is Local Office or shortly LO. From the perspective of the subscriber terminal there is no difference between CO and LO.