•It is the point that defines the end of the telephone company's wiring, and the beginning of your wiring.
•It defines where the telephone company's responsibility for maintenance ends, and your responsibility begins.
•It contains a surge suppressor to help protect the wiring and connected equipment in your home from damage
•It allows you to temporarilly disconnect your wiring from the telephone company's wiring for troubleshooting purposes.
Generally, the demarc is located on the exterior of your home in a grey or black box. Sometimes the demarc may be inside your home in the basement if you live in an older home. If you live in an apartment or condominium, the demarc is usually located in the main telephone room, although in some buildings it may be located in your suite, behind a blank outlet cover plate or in a closet or utility room for example. If you live in a townhouse, the demarc may be located in a common cabinet outside at the end unit.
Maintenance of the demarc itself is the responsibility of the telephone company.
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