1. ISDN • Integrated Services Digital Network • 2B+D Basic Rate Interface (BRI)to the desktop • Two 64-kbps B channels • Can be bonded for 128 kbps service • One 16-kbps D channel, usually for supervisory signals 64 kbps BRI 2B+D 64 kbps ISDN Modem

2. ISDN ISDN • Primary Rate Interface (PRI) • Connection between firm and ISDN carrier • 23B+D (on a T1 line) • 30B+D (on an E1 line) • One 64 kbps D channel for supervision 2B+D 23B+D BRI PRI

3. ISDN • Circuit-Switched • Dedicated capacity • Expensive for data • Dial-Up Connection • Must connect each time you wish to communicate • Other PSDNs are dedicated (always on) • Unreliable • Only Popular PSDN that is either circuit-switched or dial-up

4. ISDN • Never achieved strong market use • Being overtaken by PSDNs that are both faster and less expensive • Often, ISDN is spelled out as “It still does nothing” • However, there is enough ISDN in use that you must know it • Also, if connectivity is only needed a short time each day, ISDN is still a good choice for low-speed transmission

5. X.25 • First Packet-Switched PSDN Standard • Developed in the 1970s • Now obsolete • But still used, especially in third-world countries and Europe • Slow: Usually 64 kbps or slower • Some faster X.25 services are available • Reliable, so costs of switches are high • So cost of service is high • But works even if transmission lines are poor