|  | 5.1 | ISDN
| 5.1.2 | Basic ISDN components

ISDN components include terminals, terminal adapters (TAs), network-termination (NT) devices, line-termination equipment, and exchange-termination equipment. The table provides a summary of the ISDN components. ISDN terminals come in two types, Type 1 or Type 2, as shown in Figure . Specialized ISDN terminals are referred to as Terminal Equipment type 1 (TE1). Non-ISDN terminals such as Data Terminal Equipment (DTE) that predate the ISDN standards are referred to as Terminal Equipment type 2 (TE2). TE1s connect to the ISDN network through a four-wire, twisted-pair digital link. TE2s connect to the ISDN network through a TA. The ISDN TA can be either a standalone device or a board inside the TE2. If the TE2 is implemented as a standalone device, it connects to the TA via a standard physical-layer interface. Beyond the TE1 and TE2 devices, the next connection point in the ISDN network is the Network Termination type 1 (NT1) or Network Termination type 2 (NT2) device. These are network-termination devices that connect the four-wire subscriber wiring to the conventional two-wire local loop. In North America, the NT1 is a Customer Premises Equipment (CPE) device. In most parts of the world besides North America, the NT1 is part of the network provided by the carrier. The NT2 is a more complicated device, typically found in digital Private Branch eXchanges (PBXs), that performs Layer 2 and Layer 3 protocol services. An NT1/2 device also exists; it is a single device that combines the functions of an NT1 and an NT2.
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