DSL (for Digital Subscriber Loop or Digital Subscriber Line) is a way to transmit digital data over a telephone line. Telephone lines only transmit a limited spectrum of signals (roughly 20 Hertz to 20,000 Hertz, for voice). This means that the other frequencies can be used to transmit data. The data is combined or multiplexed onto the telephone line. At both ends, a device called a "Splitter" (or DSL filter) separates the data part and the telephony part. DSL provides the physical layer, the lowest layer of the OSI model that is used for understanding how the different parts of a telecommunications network can connect with each other. In this model, ATM or Ethernet are two communications protocols used as the data link layer (layer 2) and IP is used at the network layer (layer 3).