Layers of Trouble to Shoot

The preferred method of troubleshooting is known as "bottom-up" troubleshooting. The bottom-up approach to troubleshooting is a direct reference to the OSI reference model. As with the planning, design, and implementation steps involved in building a network infrastructure, the OSI reference model plays a very important part in troubleshooting issues that arise. Troubleshooting is a game of layers and logic. Different people are capable of exercising varying degrees of ability with both. It is said by many that there was a law enacted that dictates the placement of an OSI reference model graphic in every book relating to networking technologies. While this is doubtful at best, it is prudent to add such a graphic for reference purposes. Figure 7-1 serves to satisfy mythical and sarcastic legalities.

Figure 7-1 The OSIModel


Traffic Types (FTP, E-mail, IM, etc.)




Transmission Control Protocol (TCP) User Datagram Protocol (UDP)




Data Link

Frame Relay, Ethernet, ATM, PPP, etc.


PMD/CS, Bit Transmission

The ability to troubleshoot is a talent that many in the industry have developed over some years of experience and no small amount of pain. Many have learned the value of the bottom-down approach (which amounts to keeping one's bottom down and covered). There is some debate about whether the instinctive ability to stare at a complex network problem and pick the root cause out of the mix is something that can be taught. In any event, a process can be taught that may lead to the development of this ability over time.

Obviously, the focus of this chapter will turn to specific issues surrounding ADSL. With that in mind, the job becomes somewhat simpler in that the bottom pair of layers (physical and data link) of the OSI reference model are all that need be addressed.

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