A three level serial line code for transmitting binary digits. A format in T1 transmission systems in which binary digits are sent with equal time periods and in which zeros have zero amplitude, and ones (marks) have equal amplitudes that alternates in polarity (positive and negative).

A term that is commonly used for a circuit that is used to carry signals from one point in a network back to another point, For example, a signal may be carried from the Base Transceiver Station (BTS) site back to the Mobile Switching Center (MSC).

Cyclic Redundancy Check. A bit pattern determined by a formula, applied to a block of data, and used to verify data integrity over a communications link.

Channel Service Unit or CSU/DSU (where DSU is Data Service Unit) is a device that terminates a digital channel on a customer’s premises. A CSU is located between the incoming digital line and the customer’s equipment.

Refers to a “D4” channel bank, but is commonly used to refer to the signal framing originally used by the D4 channel bank. Also called Super Frame.

Digital Cross-connect. A manual cross-connect point that primarily serves as a test access point for DS1signals and for substituting operational equipment when necessary.

Extended Super Frame is a framing format that is employed in T1 systems. The ESF format consists of 24 consecutive T1 frames (each T1 frame containing 24 DS0 signals). This format provides FPS bits for framing, CRC bits for error checking, and FDL bits for maintenance‑signaling information that is out-of-band from payload data.

Facility Data Link refers to the bits that are included as part of ESF framing format and that provide an out‑of‑band path for communication of performance data.

The digital bit stream is organized into fixed units, called frames. A frame typically consists of a block of data (payload) plus overhead bits that are used for frame synchronization and for out‑of‑band communication.

Line Build Out. An electrical network that is used to simulate a length of cable.

Local Exchange Carrier. Local service provider.

A type of diagnostic test in which the transmitted signal is returned to the sending device after passing through a network or devices. A transmission facility may be said to be in loopback when it is set up in a condition such that a received signal is returned towards the sender.

A multiplexer in the digital hierarchy that multiplexes 28 DS1 signals into a single DS3 signal.

Network Interface Unit, may also be called a Network Interface Device (NID). The NIU is a device that is placed at the demarcation point between the network and the customer premises. It typically provides regeneration, isolation, and testing loopback capabilities. It may also provide performance monitoring information, which is available either locally or via the FDL on ESF circuits.

Pulse Code Modulation is a common method of encoding an analog voice signal into a digital bit stream.