 that go some way towards satisfying the requirements of these groups. The more specialised tools are appropriate for specific 1. The length in bytes. tasks, such as installing a new network or development of new 2. The physical address of the source of the packet. communication software. 3. The physical address of the intended destination of the packet. 61.4.1 Physical measurement The monitor may optionally determine the following for each packet: Cable testers work by first transmitting a pattern onto the network and then listening for a short period afterwards. The nature of the 1. Low level protocol type or identity. signal received during the listening period indicates the state of the 2. Logical network addresses for the source and destination. cable. Analysis of the physical characteristics of the network is best performed by more sophisticated devices. Having extracted this information, the monitor may then record it in a form specified by the user. At its simplest, this could involve 61.4.1.1 Electrical characteristics merely accumulating the total number of bytes transmitted on the network and then taking periodic samples of this total in order to Attaching an oscilloscope or logic analyser to a network enables an form an indication of how the load on the network varies over time. engineer to see exactly what the signals on the cable look like. A This would then be displayed as a histogram. network analyser supplements the view of the electrical signals by By recording the total number of bytes transmitted in a table, with comparing the characteristics against a table of technical specifica- one entry for each network device, the monitor forms a more tions for that network technology. In particular, the analyser will complete record of the traffic on the network. Some network moni- look at voltage levels, the rise and fall times of signals and the tors record even more information for each station: presence of electrical noise. By comparing key parameters against expected values and tolerances, the anal