A digital signal line is the generic designator for a digitally multiplexed T-carrier signaling scheme, rated DS-0 through DS-5, which was devised by Bell Labs. Multiplexing is a method by which multiple analog message signals or digital data streams are combined into one signal over a shared medium. The aim is to share an expensive resource, such as in telecommunications, where several telephone calls may be carried using one wire. Looking at the chart on this slide, the Digital Signal Designation is the classification of digital bit rates in the digital multiplex hierarchy. For example, at the lowest level the POTS (Plain Old Telephone Service) is a DS-0. It is one circuit and can transmit at only 64 kilobits per second. A DS-1 circuit is made up of twenty-four 8-bit channels, or DS-0s, each channel being a 64 kbit/s multiplexed carrier circuit. The terms "DS-1" and "T1" are often used interchangeably, but this is technically incorrect. DS-1 is the logical bit pattern used over a physical T1 line. DS-1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. Moving up the DS hierarchy, the DS-3 line is the digital signal line this presentation will be focused on. It has 672 circuits and can transmit 44.7 megabits per second.