A protocol analyzer is a tool that listens to network packets on a shared segment and decodes them into human-readable format. There are two types of protocol analyzers: packet analyzers and statistical analyzers. Freeware and commercial packet analyzers are available. The neat thing about packet analyzers is that they run on most PCs if you have the right type of network cardthat is, a "promiscuous" network card, which is able to listen to all network packets.
Depending on the analyzer, you can expect to see many functions that will help you analyze the raw data that the analyzer captures. Some of these functions include capture filtering, sortable statistical displays, "expert" analysis of data, and customizable views.
There's more than one analyzer available on the market because there's more than one problem out there. Different analyzers are good for various things, and some analyzers have better decodes of certain protocols.
Knowing what and when to filter is a really important part of learning how to use an analyzer. After you learn how to take small manageable trace files, you'll be able to quickly go through them and find what you need in order to vanquish your problemsor to entice your vendor to help out.
Analyzers, like any tool, have limitations, such as the need to use a passive tap, hub, or switch port mirroring when switching is in the picture. Still, if you have your wits about you, they're a powerful addition to your troubleshooting arsenal.