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In ISDN terminology, a terminal adapter or TA is a device that connects a terminal (computer) to the ISDN network.The TA therefore fulfills a similar function to the one a modem has on the POTS network, and is therefore sometimes called an ISDN modem.

A TA may contain an interface and codec for one or more analog telephone lines , allowing an existing POTS installation to be upgraded to ISDN without changing phones.Terminal adapters typically connect to a basic rate interface (S0, sometimes also U0). On the terminal side, the most popular interfaces are RS-232 serial and USB.

Devices connecting ISDN to a network (e.g. Ethernet) commonly include routing functionality; while they technically include a TA function, they are referred to as (ISDN) routers.

Network Termination 1 -NT1

In ISDN, a functional grouping of customer-premises equipment that includes functions that may be regarded as belonging to OSI Layer 1, i.e. functions associated with ISDN electrical and physical terminations on the user premises.

R-Interface

R-interface defines the point between a non-ISDN device and a terminal adapter (TA) which provides translation to and from such a device.

(b) serves as a universal interface between ISDN terminals or terminal adapters and the network channel termination,

(c) allows a variety of terminal types and subscriber networks, such as PBXs, local area networks (LANs), and controllers, to be connected to the network, and

(d) operates at 4000 48 bit frames per second, i.e.192 kbit/s, with a user portion of 36 bits per frame, i.e. 144 kbit/s.

T-Interface

A T-interface is used for basic rate access in an ISDN environment. It is a User-Network Interface reference point that is characterized by a four-wire, 144 kbit/s (2B+D) user rate.

U-Interface

A basic-rate access in an ISDN environment, a user-to-network interface reference point that is characterized by the use of a 2-wire-loop transmission system that

(a) conveys information between the 4-wire user-to-network interface, i.e., the S/T reference point, and the local exchange,

(b) is located in the servicing central office, and

(c) is not as distance sensitive as a service using a T interface.