Demarcation point
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In telephone networks, the demarcation point is the point at which the telephone carrier's local loop network ends and connects with the telephone system or wiring at the customer's premises.
In most cases, everything up to and including the demarcation point is owned by the carrier, and everything past it is owned by the property owner. This makes working with the demarcation point illegal even if one owns the property, unless one is authorized to work on it by the carrier.
Because the demarcation point is owned by the carrier, many alarm systems running over copper or coax will travel to the demarcation point directly, bypassing any intervening patch panels and wiring closets. This places the burden of liability on the carrier, rather than on the property owner.
The demarcation point varies from building type and service level. At a typical residential house, the demarcation point is a box normally mounted on the outside wall (in recent years, as close to the electrical ground as possible). Apartments and businesses with multiple lines would typically have a 'punch down block' in an equipment room, though recent changes in policies have resulted in a either the first jack serviced or a special jack designed for the function as a demarcation point in the premises. The special made jacks often resemble jacks made to mount wall mounted telephones, but with a short cord looped at the bottom to a separate plug. Unplugging that loop isolates the rest of the residence.
With the changes in phone repair services now placing the responsibility of repair and maintenance beyond the demarcation point in the hands of the customer, most residential and some large scale units now have a jack which allows the customer to disconnect their inside wires and check to see if their service is working properly from that point on.