Data Acquisition

The use of Thinnet cable was, and remains, very popular as a cheap and relatively easyway to set up a network. However, there are disadvantages with this approach. A cablefault can bring the whole system down very quickly. To avoid such a problem, the cableis often taken to wall connectors with a make-break connector incorporated. Theconnection to the node can then be made by ‘fly leads’ of the same cable type. It isimportant to take the length of these fly leads into consideration in any calculation oncable length! There is also provision for remote MAUs in this system, with AUI cablesmaking the node connection, in a manner similar to the Thicknet connection.The 10BaseT standard for Ethernet networks uses AWG24 unshielded twisted pair (UTP)cable for connection to the node. The physical topology of the standard is a star, withnodes connected to a wiring hub, or concentrator. Concentrators can then be connected toa backbone cable that may be coax or fiber optic. The node cable can be category 3 orcategory 4, although you would be well advised to consider category 5 for all newinstallations. This will allow an upgrade path as higher speed networks become morecommon, and given the small proportion of cable cost to total cabling cost, will be aworthwhile investment. The node cable has a maximum length of 100 meters; consists oftwo pairs for receiving and transmitting and is connected via RJ45 plugs. The wiring hubcan be considered as a local bus internally, and so the topology is still considered as alogical bus topology. Figure 9.5 shows schematically how the hub interconnects the10BaseT nodes.Figure 9.5Schematic 10BaseT systemCollisions are detected by the NIC and so an input signal must be retransmitted by thehub on all output pairs. The electronics in the hub must ensure that the stronger retransmittedsignal does not interfere with the weaker input signal. The effect is known asfar end crosstalk (FEXT), and is handled by special adaptive crosstalk echo cancellationcircuitsThe standard has become increasingly popular for new networks, although there aresome disadvantages that should be recognized:• The cable is not very resistant to electrical noise, and may not be suitable forindustrial environments.• While the cable is inexpensive, there is the additional cost of the associatedwiring hubs to be considered.• The node cable is limited to 100 m.