IDSL
Encyclopedia
ISDN Digital Subscriber Line (IDSL) uses ISDN
Integrated Services Digital Network
Integrated Services Digital Network is a set of communications standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network...

-based technology to provide a data communication channel across existing copper telephone lines at a rate of 144 kbit/s, slightly higher than a bonded dual channel ISDN connection at 128kbit/s. The digital transmission bypasses the telephone company's central office equipment that handles analogue signals. IDSL uses the ISDN grade loop without 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...

 in ISDN transmission mode. The benefits of IDSL over ISDN are that IDSL provides always-on connections and transmits data via a data network rather than the carrier's voice network.

IDSL also avoids per-call fees by being generally billed at a flat-rate.

IDSL is not available in all countries.

ISDN digital subscriber 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 kbit/s and at distances of up to RRD range. 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 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 central office 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 Internet service providers, who do not provide a public voice service, IDSL is an interesting way of using 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....

dial service to offer higher-speed Internet access, targeting the embedded base of more than five million ISDN users as an initial market.
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