In Chapter 11, we discussed the integrated services digital network (ISDN), which was developed in the 1970s to eventually replace the public switched telephone network (PSTN). As part of its development, ISDN was designed to provide both voice and data service. Although ISDN was considered a significant improvement to the PSTN, it was still an evolution of the PSTN and hence was circuit-switched. To support data applications in an efficient manner, a packet-switching component needed to be included with the ISDN standard. The only packet-switching technology available at the time was X.25 (see Appendix E), which did not support the ISDN model of keeping user data separate from control data. Consequently, frame relay was developed specifically as a packet-switching technology component of ISDN for data applications. Today, frame relay has emerged as a popular and robust standalone technology. In this chapter, we examine various concepts related to frame relay from both conceptual and technical perspectives. An outline of the main topics we address follows:

• Frame Relay Overview (Questions 1 3)

• Frame Relay's Physical Layer: Virtual Circuits and Committed Interface Rates (CIRs) (Questions 4 20)

• Technical Aspects and Operation of Frame Relay (Question 21)

• Frame Relay's Data Link Layer (Questions 22 28)

• Voice Over Frame Relay (Question 29)

• Frame Relay versus ATM, SMDS, and Gigabit Ethernet (Questions 30 31)

• Frame Relay in the News (Question 32)