3.1 Serial Point-to-Point Links  
  3.1.3 Demarcation point  
The demarcation point, or "demarc" as it is commonly known, is the point in the network where the responsibility of the service provider or "telco" ends. In the United States, a telco provides the local loop into the customer premises and the customer provides the active equipment such as the channel service unit/data service unit (CSU/DSU) on which the local loop is terminated. This termination often occurs in a telecommunications closet and the customer is responsible for maintaining, replacing, or repairing the equipment.

In other countries around the world, the network terminating unit (NTU) is provided and managed by the telco. This allows the telco to actively manage and troubleshoot the local loop with the demarcation point occurring after the NTU. The customer connects a customer premises equipment (CPE) device, such as a router or frame relay access device, into the NTU using a V.35 or RS-232 serial interface.