ckly and accurately locate the faulty device. on the network, focusing on the electrical characteristics of the transmission. This sort of network trouble shooting is of greatest importance when a network is first installed. Additionally, periodic health A more complex tool, the protocol analyser, records conversations checks can aid in pointing the engineer towards areas where correc- between devices on the network, and provides a detailed diagnosis tive action is needed well before network users experience any of the protocols used to exchange information. significant disruption. The most widely used measurement tool is the network monitor. The monitor continuously gathers information about the behaviour of the network, focusing on the data link and network layers. The 61.4.2 Statistical information monitor records the number and size of packets, the source and destination addresses, and uses this data to summarise the nature of Network monitors continuously collect statistics about the traffic on traffic on the network. the network, categorise the statistics, form tables, and finally pres- Information gathered by network measurement devices may be ent this information to the user in tabular or graphical form. The user useful to a great many people in an organisation. Each will require will derive the most benefit from the results of this process if he can differing measurement facilities according to their respective roles. apply a significant degree of control over each stage. Network maintenance engineers will concentrate on low level network characteristics, for fault detection and repair. Network managers will be more interested in security and control issues. 61.4.2.1 Categorisation Board level management will require reports on the usage and performance of the network to enable planning for future needs. Typically, network monitors will receive all packets transmitted on Many of the more common network monitors provide a rich set of the network and extract the following details for each packet: functions