Chapter 14. Frame Relay Technology BackgroundFrame Relay technology includes a combination of hardware, software, standards, and architectures to provide a variety of services. They include data, Voice over Frame Relay (VoFR), Frame Relay multicast, Internet Protocol (IP) multicast over Frame Relay, Frame Relay compression, and others. It is beyond the scope of this book to include all available features of Frame Relay, so the content of Part IV, "Frame Relay," only includes the technology background, common design and configuration solutions for enterprise remote access, and proven troubleshooting techniques and scenarios. This chapter also provides a more detailed background of Frame Relay technology, including the following:
One of the fundamental reasons for creating Frame Relay technology was the need for higher speeds. The other driving factor was the industry change from mainly text exchange, to graphical exchanges, which requires peak bandwidth or dynamic bandwidth and lower response time. Widespread digital facilities demand more reliability and less overhead, and the ability to handle bursty traffic. This combination of factors drove the development of new technology at the end of the 1980s. Frame Relay combines the statistical multiplexing and port sharing of X.25 with the high-speed and low-delay characteristics of time-division multiplexing (TDM) circuit switching. Conceived as a derivative from X.25, Frame Relay eliminates the Layer 3 protocols of X.25 and concentrates the addressing and multiplexing in Layer 2. The architecture model is more compliant with Open System Interconnection (OSI), where the second layer deals with frames and not with packets. Only a few Layer 2 functions are used in permanent virtual circuit (PVC) solutions, and they are known as the core aspects such as checking for valid error-free frames but not requesting retransmission if an error is found. Therefore, many high-level protocol functions such as sequencing, windowing, acknowledgments, and supervisory frames are not duplicated within Frame Relay.
The lack of certain protocol functions in Frame Relay dramatically increases throughput because each frame requires much less processing time. Table 14-1 summarizes the characteristics of these protocols, comparing X.25 switching and Frame Relay1.
Frame Relay uses a variable length framing structure, which ranges from a few to thousands of bits. This feature affects delay-sensitive user data because the delay is a function of the packet size. Although this is a feature in Frame Relay compression, it is a disadvantage when carrying voice traffic. However, Frame Relay has been adapted to handle the voice traffic, as defined in FRF11.1 (see Table 14-3). The two factors that make Frame Relay a desirable choice for data transmission are the following:
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