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ISDN, or Integrated Services Digital Network, is a telecommunications standard that uses copper phone lines to transmit voice and data digitally, allowing for simultaneous phone calls and internet access. It is a less common, older technology that has been largely replaced by broadband services like DSL and fiber optics, but it is known for its reliability and capacity for high-speed (for its time) connections of up to \(128\) kbps by bundling lines. 

How it works 

Digital channels: ISDN divides its capacity into two types of digital channels: B (bearer) channels for voice and data, and a D (delta) channel for control signals.

Basic Rate Interface (BRI): A common version for home and small business use, BRI includes two \(64\) kbps B channels and one \(16\) kbps D channel (2B+D).

Primary Rate Interface (PRI): Used by larger businesses, PRI offers more channels, typically 23 B channels and one D channel (23B+D).

Simultaneous use: Since the B channels handle data and voice, and the D channel handles signaling, ISDN allows users to have a phone call and an internet connection at the same time.