ISDN functional identifiers include the following:
• Terminal Equipment 1 (TE1)— A device that is ISDN compatible.
• Terminal Equipment 2 (TE2)— A device that is not ISDN compatible and requires a terminal adapter (TA).
• Terminal Adapter (TA)— A device that converts non-ISDN signals into ISDN compatible signals so that non-ISDN devices can connect to an ISDN network.
• Network Termination Type 1 (NT1)— A device that connects 4-wire ISDN subscriber units to the conventional 2-wire local loop facility. In the U.S. the NT1 is part of the CPE; in Europe and Japan, it is part of the local exchange facility.
• Network Termination Type 2 (NT2)— A device that performs switching and concentration of non-ISDN signals, often a PBX. Requires a TA to interconnect with the ISDN network.
• Line Termination (LT)— A device that is located on the local exchange carrier (LEC) side of the subscriber line that functions as an NT1.
• Exchange Termination (ET)— Subscriber cards in the ISDN exchange.
ISDN reference points include the following:
• User reference point (U)— Located between the NT1 and LT. Corresponds to a subscriber line.
• Terminal reference point (T)— Located between the NT1 and NT2 or between the NT1 and TE1 or TA, if no NT2 device exists. Same characteristics as the S reference point.
• System reference point (S)— Located between the NT2 and TE1 or T1 connecting the terminal to the ISDN network. Same characteristics as the T reference point.
• Rate reference point (R)— Located between TA and TE2. The TE2 connects to the TA via a standard physical interface, such as EIA/TIA-232, V.24, X.21, and V.35.
The ITU I-series and G-series documents specify the ISDN physical layer. The U interface that the LEC provides for BRI is a 2-wire, 160 kbps digital connection. Echo cancellation is used to reduce noise and data encoding schemes (2B1Q in North America, 4B3T in Europe) and permit this relatively high data rate over ordinary single-pair local loops.
The ISDN network layer is specified by the ITU Q-series documents Q.930 through Q.939. Layer 3 is used for the establishment, maintenance, and termination of logical network connections between two devices.
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