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High-Level Data Link Control (HDLC) is a bit-oriented code-transparent synchronous data link layer protocol developed by the International Organization for Standardization (ISO). The standard for HDLC is ISO/IEC 13239:2002.

HDLC provides both connection-oriented and connectionless service.

HDLC can be used for point-to-multipoint connections via the original master-slave modes Normal Response Mode (NRM) and Asynchronous Response Mode (ARM), but they are now rarely used; it is now used almost exclusively to connect one device to another, using Asynchronous Balanced Mode (ABM).

High-Level Data Link Control - Wikipedia

Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network.

Since its introduction in 1881, the twisted pair copper line has been installed for telephone use worldwide, with well over a billion individual connections installed by the year 2000. Over the first half of the 20th century, the connection of these lines to form calls was increasingly automated, culminating in the crossbar switches that had largely replaced earlier concepts by the 1950s.

ADSL quickly replaced ISDN as the customer-facing solution for last-mile connectivity. ISDN has largely disappeared on the customer side, remaining in use only in niche roles like dedicated teleconferencing systems and similar legacy systems.

Integrated Services Digital Network - Wikipedia