The first level of the European E-carrier digital hierarchy. An E-1 system comprises circuit terminating equipment in the form of a combination of a channel service unit (CSU) and data service unit (DSU) that jointly serve to interface a device to a full duplex (FDX) four-wire digital circuit and to perform various signal formatting, signal timing, monitoring, and diagnostic functions. E-1 operates at a signaling rate of 2.048 Mbps, which supports a frame rate of 8,000 frames per second (fps), with each frame comprising 32 eight-bit time slots, 30 of which are for user payload and 2 of which are set aside for signaling and control data. As illustrated in Figure E-1, time slot 0 (zero) is used for synchronization and alarming.Time slot 16 is used by the equipment for all signaling functions, such as circuit monitoring and diagnostics. At a rate of 8,000 fps, each time slot is repeated 8,000 times per second, which translates into a DS-O channel at 64 kbps (8 bits
A 2.048 Mbps point-to-point dedicated, digital circuit provided by the telephone companies in Europe. E1 is the European counterpart of the North American T1 line, which transmits at 1.544 Mbps, and E1 and T1 lines can be interconnected for international use. E2 through E5 lines provide multiple E1 channels. An E1 line uses two wire pairs (one for transmit, one for receive) and time division multiplexing (TDM) to interleave 32 64-Kbps voice or data channels. See DS and T1.