X.25/LAPB Continued
An SVC is like a phone call. It is setup then disconnected. A PVC behaves like a leased line. It is always available.
X.25 was created for existing analog lines but can be used over digital lines such as ISDN.
X.25 is associated with high overhead due to error correction and flow control that was built into the protocol. Errors are checked at every node not just end nodes like TCP/IP.
X.25 is considered reliable but not efficient. It operates at transmission speeds of 64 kbps to 2 mbps but a significant portion of this bandwidth is used for error checking.
X.25 has its own model which is older than the OSI reference model.