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ISDN Terminal Adapters

Integrated Service Digital Network (ISDN) is another form of high-speed Internet access. It delivers digital services (in 64Kbps channels) over standard

telephone copper pairs. The device you must hook up to your computer to access ISDN services is properly known as an ISDN Terminal Adapter. It’s not a modem in the truest sense of the word because a modem changes from digital to analog for transmission, but an ISDN TA isn’t changing from digital to analog. It’s just changing between digital transmission formats.

The box itself is about the size of a modem and looks similar to one. But, like DSL modems, there is a phone jack and an Ethernet jack. You connect a phone cord from the phone jack to the wall jack where your ISDN services are being delivered. Then, you connect an Ethernet cable from your PC to the ISDN TA’s Ethernet jack.


The Channel Service Unit/Data Service Unit (CSU/DSU) is a common device found in equipment rooms when the network is connected via a T-series data connection (i.e., a T1). It is essentially two devices in one that are used to connect a digital carrier (the T-series or DDS line) to your network equipment (usually to a router). The Channel Service Unit (CSU) terminates the line at the customer’s premises. It also provides diagnostics and remote testing, if necessary. The Data Service Unit (DSU) does the actual transmission of the signal through the CSU. It can also provide buffering and data flow control.

Both components are required if you are going to connect to a digital transmission medium, such as a T1 line. Sometimes, however, one or both of these components may be built into a router. If both components are built into a router, you only have to plug the T1 line directly into the router.

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