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

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 ISDN BRI PRI

ISDN Circuit-Switched Dial-Up Connection Unreliable 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

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

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