ISDN

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 ones a modem

Modem

A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...

 has on the POTS

Plain old telephone service

Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....

 network, and is therefore sometimes called an ISDN modem. The latter term, however, is partially misleading as there is no modulation

Modulation

In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...

 or demodulation

Demodulation

Demodulation is the act of extracting the original information-bearing signal from a modulated carrier wave.A demodulator is an electronic circuit that is used to recover the information content from the modulated carrier wave.These terms are traditionally used in connection with radio receivers,...

 performed.

There are devices on the market that combine the functions of an ISDN TA with those of a classical modem (with an ISDN line interface). These combined TA/modems permit connections from both ISDN and analog-line/modem counterparts. In addition, a TA may contain an interface and codec

Codec

A codec is a device or computer program capable of encoding or decoding a digital data stream or signal. The word codec is a portmanteau of "compressor-decompressor" or, more commonly, "coder-decoder"...

 for one or more analog telephone lines (aka a/b line), allowing an existing POTS installation to be upgraded to ISDN without changing phones.

Terminal adapters typically connect to a basic rate interface

Basic rate interface

Basic Rate Interface is an Integrated Services Digital Network configuration intended primarily for use in subscriber lines similar to those that have long been used for plain old telephone service...

 (S0

S interface

S interface : For basic rate access in an Integrated Services Digital Network environment, a User–network interface reference point that* is characterized by a Four-wire circuit, 144 kbit/s user rate,...

, sometimes also U0

U interface

U interface: For basic-rate access in an Integrated Services Digital Network environment, a user-to-network interface reference point that is characterized by the use of a 2-wire-loop transmission system that conveys information between the 4-wire user-to-network interface, i.e., the S/T...

). On the terminal side, the most popular interfaces are RS-232

RS-232

In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...

 serial and USB; others like V.35 or RS-449

RS-449

The RS-449 specification, also known as EIA-449 or TIA-449, defines the functional and mechanical characteristics of the interface between data terminal equipment and data communications equipment....

 are only of historical interest.

Devices connecting ISDN to a network (e.g. Ethernet

Ethernet

Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....

) commonly include routing functionality; while they technically include a TA function, they are referred to as (ISDN) routers.

Mobile Networks

In Mobile networks, the terminal adapter is used by the Terminal equipment

Terminal equipment

In telecommunication, the term terminal equipment has the following meanings:* Communications equipment at either end of a communications link, used to permit the stations involved to accomplish the mission for which the link was established....

 to access the Mobile termination, using AT commands (see Hayes command set

Hayes command set

The Hayes command set is a specific command-language originally developed for the Hayes Smartmodem 300 baud modem in 1981. The command set consists of a series of short text strings which combine together to produce complete commands for operations such as dialing, hanging up, and changing the...

).

In 2G

2G

2G is short for second-generation wireless telephone technology. Second generation 2G cellular telecom networks were commercially launched on the GSM standard in Finland by Radiolinja in 1991...

 (such as GSM or CDMA), the terminal adapter is a theoretically optional while in 3G

3G

3G or 3rd generation mobile telecommunications is a generation of standards for mobile phones and mobile telecommunication services fulfilling the International Mobile Telecommunications-2000 specifications by the International Telecommunication Union...

 (such as W-CDMA

W-CDMA

W-CDMA , UMTS-FDD, UTRA-FDD, or IMT-2000 CDMA Direct Spread is an air interface standard found in 3G mobile telecommunications networks. It is the basis of Japan's NTT DoCoMo's FOMA service and the most-commonly used member of the UMTS family and sometimes used as a synonym for UMTS...

), the terminal adapter is mandatory and is part of the Mobile Termination.

Automation industry

In the automation industry, a terminal adapter is a passive device that converts a connector like the 8P8C (RJ-45) modular connector

Modular connector

Modular connector is the name given to a family of electrical connectors originally used in telephone wiring and now used for many other purposes. Many applications that originally used a bulkier, more expensive connector have now migrated to modular connectors...

 or 9 pin D-Sub into a terminal block to facilitate wiring. It is often used when daisy-chain wiring is necessary on a multi-node serial communication network like RS-485 or RS-422.