ISDL is a hybrid of ISDN and DSL (Digital Subscriber Line) technology that uses a little of each to deliver a service that is slightly faster than ISDN, but significantly slower than most DSL services. A person might opt for ISDL if standard DSL service is not available in his or her area.
Standard ISDN (Integrated Services Digital Network) service uses existing copper telephone lines to deliver data transfer speeds up to 128 kilobits per second (kbps). It also provides voice channels for telephone and fax, making it superior to standard dial-up, which cannot share an Internet connection with a telephone or external fax machine. Standard dial-up is also much slower than ISDN at about 53 kbps. However, ISDN is still a dial-up service, as opposed to an "always on" service like DSL.
IDSL uses the same technology as ISDN except that it routes the signals through different equipment at the phone company or Telco. While ISDN uses the standard switching equipment, IDSL uses DSL-like equipment. The difference is two-fold: an increase in speed to 144 kbps, and an "always on" connection.
IDSL can be expensive to set up, but may be worth it to businesses or individuals that require faster service without the option of DSL. One of the advantages of IDSL over DSL is that the former is not limited by distance from the Telco. To be a DSL subscriber, the client must be within the limit of 18,000 wire-feet (5,486 wire-meters) from the Telco hub. If the residence or business falls at the outer limits of this range, the maximum speed of the DSL connection will be significantly reduced and signal quality will not be optimal. The closer to the Telco hub, the better the DSL service. IDSL does not have this limitation, allowing clients at further distances to have an "always on" connection.
One drawback of IDSL is that it requires its own telephone line for the IDSL service, while a second line provides phone services. This makes it more expensive than ISDN.
Ascend Communications developed IDSL technology. If you think it might be the right solution for your needs, contact your local telephone company to ask about ISDN or IDSL services in your area.