An Integrated Services Digital Network (ISDN) modem is used to connect to the internet through a dedicated telephone line. While similar to dial up and DSL internet connections, ISDN connections do not often share a phone line with other devices like faxes or phones. Rather the ISDN is a dedicated line so that the internet connection is the only data connection on the line at any time. Usually phone companies provide the modem and the service to those interested in ISDN connections.


First you will need to see if your local phone service provider offers internet service in ISDN format. If so the company will need to install a dedicated new line in your home or office. The service is billed to you monthly on your phone bill. Usually the phone service provider will provide you with an ISDN modem which is usually an external modem. If not you’ll need to rent or purchase a modem. Make sure the chosen modem is ISDN and not cable.


Hook up the dedicated telephone line to the connection in port on the ISDN modem. Hook up another line from the ISDN modem to the computer. Depending on the modem design this will either be an Ethernet connection to the computer’s Ethernet card or a phone line connection to the computer’s dial up modem connection. In rare cases an ISDN modem may connect to older computers via a serial port connection. You will need to ensure you have the appropriate cables needed. For example Ethernet connections require an Ethernet cable while serial port connections may require a serial port adapter. 


Run any installation software on the computer that came from the internet provider or the modem itself. You can access the internet by clicking on your browser of choice such as Internet Explorer or Mozilla Firefox. You might be asked to enter your user name and password before connecting. This is your account information provided when you signed up for the internet service itself.


It is worth noting that ISDN modem technology is changing and many phone companies are moving from ISDN technology to DSL or even fiber optic technology. When signing up for high speed internet access you’ll want to check to see if a newer technology is offered by the phone company which will often ensure faster more stable internet connections. However, for those on a tight budget older technologies such as dial up and ISDN connections may still provide reliable (albeit slower) connections to the internet. 

External ISDN ModemCredit: By Johann H. Addicks-jha- at de.wikipedia [GFDL ( or CC-BY-SA-3.0 (], from Wikimedia Commons