For CLEC provided DSL (ie, DSL through Covad, or Northpoint), it is the process of taking the line from the drop, where the telco left it, to a socket on your wall.
This may involve running new lines at your premises, but this process is no more painful than adding another phone extension.
For ADSL, inside wiring is handled by the Telco as part of the whole install deal. Where the Telco and the CLEC must cooperate with wiring, there can sometimes be extensive delays and difficulties, none very technical, usually a lack of coordination.
For older apartment buildings, there may be much bigger problems. If there are no spare pairs available for non-Telco DSL, new wires may have to be run in the building, adding to the expense, time delays and trouble.
•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.
More photos available here:
Bell Canada Demarcs
Here are some examples of individuals that were able to vastly improve the performance of their DSL service simply by doing some simple repairs themselves:
»Bad wiring story...
»[HSE] I've conquered what I thought was never possible!
»DSL problems (long)
»Is Bell Obligated to Provide me Demarcation Outlet?