A demarcation point is the physical point at which a telecommunications company’s public network ends and the customer’s private network begins. The demarcation point is often the point at which the cable physically enters the building, but this varies from one country to another.The demarcation point defines where the telephone company’s responsibility for maintenance ends and the consumer’s responsibility begins. The demarcation point contains a surge suppressor to protect the wiring and connected equipment in a customer's home from external or internal damage. It also permits consumers to disconnect from the telephone company’s wiring for troubleshooting.The demarcation point is also referred to as a network terminating interface or demarc.
Demarcation points differ depending on the building type and service level. They include a junction block where telephone extensions join to connect to networks. The junction block includes a lightning arrestor. Demarcation points have user-accessible RJ-11 jacks connected to the telephone network, and small loops of telephone cords connecting to jacks by modular connectors.In the United States, the demarcation point is a device defined by Federal Communications Commission rules designed to provide for the safe connection of third-party telephone customer premises equipment and wiring to public switched telephone networks.The modern demarcation point is the network interface device (NID) or intelligent network interface device (INID) also known as a “smartjack”. The NID is the telco’s property. The NID may be outdoors (typically, mounted on the building exterior in a weatherproof box) or indoors.
The NID is usually placed for easy access by a technician. It also contains a lightning arrestor, fuse and test circuitry which allows the carrier to remotely test whether a wiring fault lies in the customer premises or in the carrier wiring, without requiring a technician at the premises. The demarcation point has a user accessible RJ-11 jack (a “test jack” or “demarcation jack”), which is connected directly to the telephone network, and a small loop of telephone cord connecting to the jack by a modular connector.
When the loop is disconnected, the on-premises wiring is isolated from the telephone network and the customer may directly connect a telephone to the network via the jack to assist in determining the location of a wiring fault. In most cases, everything from the central office to and including the demarcation point is owned by the carrier and everything past it is owned by the property owner.
As the local loop becomes upgraded, with fiber optic and coaxial cable technologies sometimes replacing the original unshielded twisted pair to the premises, the demarcation point has grown to incorporate the equipment necessary to interface the original premises POTS wiring and equipment to the new communication channel.
Demarcation points on houses built prior to the Bell System divestiture usually do not contain a test jack. They only contained a spark-gap surge protector, a grounding post and mount point to connect a single telephone line. The second wire pair was usually left unconnected and were kept as a spare pair in case the first pair was damaged.
DEMARCs that handle both telephony and IT fiber optic internet lines often do not look like the ones pictured above. In many places several customers share one central DEMARC for a commercial or strip mall setting. Usually a DEMARC will be located indoors if it is serving more than a single customer. This may impede access. Outdoor ones provide easier access, without disturbing other tenants, but call for weatherproofing and punching through a wall for each new addition of wires and service.
Typically indoor DEMARC’s will be easily identified by a patch panel of telephone wires on the wall next to a series of boxes with RJ48 jacks for T-1 lines. Each business or individual customer can expect their own separate box for internet access T-1 lines.