Traditionally, the Demarcation Point, sometimes referred to as the demarc, is the point at which the Public Switched Telephone Network (PSTN) ends and connects to the customer's on premises wiring. It is a physical location, which determines where the public infrastructure ends and where the private wiring beings. It is also a logical point which defines the dividing line which determines who is responsible for installation and maintenance of wiring and equipment: the customer/subscriber, or the telephone company/provider.
The idea of the demarc has evolved and expanded beyond the PSTN and is an important part of connections to ISPs as well. Where the actual location of the demarc is also depends upon the laws and rules that govern telecom companies in each country and area of the world. In some cases, equipment at or near the demarc is always provided by the telco. In other cases, it is provided by the customer. In some cases, it can be leased from the telco, but may exist on the customer side of the demarc.
As mentioned before, the demarc indicates what wiring and equipment belongs to the telco and what wiring and equipment belongs to the private entity being served. Various pieces of equipment are found near the demarc, and these include:
- Channel Service Unit/Data Service Unit (CSU/DSU)
- Data Terminal Equipment (DTE)
- Data Service Unit (DSU)