15.2. Network Monitoring Tools
Troubleshooting problems within a single computer can be difficult, but at least all of the potential problem-causing elements are physically located in the same place and are available for inspection and examination. In a network of interconnected computers, this is no longer true. Application, communication, and performance problems can occur as a result of misbehavior from a large number of interacting devices distributed over a wide geographical area. Traffic appearing on a single LAN can comprise interspersed application communication streams from a variety of different systems. A problem in one device (for example, an internetwork router or LAN switch) can affect many different users in ways that are almost unpredictable. Troubleshooting a network is thus considerably more complex than troubleshooting a single computer system. It demands tools that can observe the totality of network activity rather than the activity of any single device.
One important weapon in the network troubleshooter's arsenal is the protocol analyzer. As depicted in Figure 15-5, a protocol analyzer is a device that can monitor the traffic activity on a given LAN. By receiving and inspecting all LAN frames, the protocol analyzer can:
 A protocol analyzer is often called a network sniffer, although strictly speaking Sniffer is a trademark of Network Associates (formerly Network General), which makes protocol analysis products under this name.
Collect network performance ...