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 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

3. 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

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