ISDN Digital Subscriber Line

ISDN Digital Subsriber Line (IDSL) is a cross between ISDN and xDSL. It is like ISDN in that it uses a single wire pair to transmit full-duplex data at 128 Kbps and at distances of up to RRD range (the specified distance for ISDN). Like ISDN, IDSL uses a 2B1Q line code to enable transparent operation through the ISDN "U" interface. Finally, the user continues to use existing CPE (ISDN BRI terminal adapters, bridges, and routers) to make the CO connections.

The user speed of ISDN and IDSL is the same. The big difference is from the carrier's point of view. Unlike ISDN, IDSL does not connect through the voice switch. A new piece of data communications equipment terminates the IDSL connection and shunts it off to a router or data switch. This is a key feature because the overloading of CO voice switches by data users is a growing problem for telcos.

The limitation of IDSL is that the customer no longer has access to ISDN signaling or voice services. But, for ISPs who do not provide a public voice service, IDSL is an interesting way of using POTS dial service to offer higher-speed Internet access, targeting the embedded base of ISDN users as an initial market.