Chapter 10. Troubleshooting Serial Links

Terms you'll need to understand:

• CSU/DSU Channel Service Unit/Digital Service Unit

• Frame Relay

• Encapsulation

• HDLC High-level Data Link Control

• Loopback

• PVC Permanent Virtual Connection

• SVC Switched Virtual Connection

• X.25

Techniques you'll need to master:

• Troubleshoot Frame Relay problems

• Troubleshoot X.25 problems

• Troubleshoot serial link problems

• Determine whether serial link problems are hardware or software issues

• Perform a local or remote loopback test

• Troubleshoot IOS serial link configuration issues

• Use IOS commands that will aid in isolating problems

Many different protocols can use a serial link. Serial links send data bit by bit over their links or circuits. Frame Relay and X.25 are all protocols used on serial links. These protocols have many common troubleshooting commands, as well as techniques to help you, the administrator, locate configuration and hardware problems.

Frame relay is a telecommunication service designed for cost-efficient data transmission for intermittent traffic between local area networks (LANs) and between end-points in a wide area network (WAN). Frame relay puts data in a variable-size unit called a frame and leaves any necessary error correction (retransmission of data) up to the end-points, which speeds up overall data transmission.

Before Frame Relay was adopted as a standard, the X.25 protocol was adopted as a standard by the Consultative Committee for International Telegraph and Telephone (CCITT). The X.25 protocol allows computers on different public networks to communicate through an intermediary computer at the network layer level.

This chapter will focus on what you need to know to troubleshoot serial links and the protocols used on serial links. You learn techniques for troubleshooting serial links, including CSU/DSU Loopback Testing and common serial link troubleshooting commands. The chapter also discusses frame relay troubleshooting and techniques for troubleshooting X.25. The information you learn here will help in your preparation for the CIT Exam.

An in-depth discussion of configuring Frame Relay and X.25 is quite lengthy and beyond the scope of this book. If you would like to learn more about configuring Frame Relay and X.25, I recommend reading CCNP BCRAN Exam Cram 2 by Eric Quinn and Fred Glauser, ISBN: 0-7897-3020-0.