ISDN is a digital version of the telephone system (POTS). It can transfer data at a rate much higher than the analog phone system because the line is much cleaner (higher clarity, less noise, and so on).
ISDN is an all-digital transmission facility that is designed to replace the analog PSTN on a worldwide basis. ISDN service is available in two forms: BRI and PRI. Basic Rate Interface (BRI), sometimes represented as 2B + D, operates at 144 kbps. Each (B)earer channel operates at 64 kbps, and the (D)ata channel operates at 16 kbps. Primary Rate Interface (PRI), sometimes represented as 23B + D, operates at 1.536 Mbps.
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