s. The first figure Ethernet Multiport transceiver/hub unit is the core size and the second is the outer cladding thickness in Ethernet Multiport transceiver/hub unit microns. For overlay services or general future use, fibre can be extended to the desk using a combination copper/fibre cable and terminated 33.4.1 Patch panels in the floor box cassette. Patch panels give flexibility to the structured cabling system. They allow any service to be routed along any cable to any desk. Patch Services panels ar used for two functions: Overlay services are often specified by the client and use data rates 1. Horizontal patching, which allows re-routing of services be- that far exceed the capabilities of the UTP. They are usually carried tween desks. in coax or fibre. For example, some banking, insurance, video 2. Vertical patching, which allows re-routing of services between conference, satellite and international information and new services floors. can be handled as overlay services. Examples of these services are: REUTERS, TOPICS, CHAPS, Patch panels fall into four categories: BACS, MegaStream links, KiloStream links, TV, video and teletext. Table 33.1 shows the range of cable types which are recom- 1. Wire patching, where linking is performed by individual wires. mended by the computer manufacturers. A structured cabling sys- 2. Cable patching, where linking is performed by 2,4,6, or 8-core tem replaces these with a common UTP cable type. patch leads with plugs on both ends. The characteristic impedance (Z) of a cable is the ratio of the 3. Soft patching, where linking is performed by software con- applied voltage and resultant current at the same point. Because of figured, electronic equipment. the capacitive and inductive effects within the cable, the charac- 4. Fibre patching, where linking is performed by fibre patching teristic impedance varies with frequency. Changes in characteristic leads. impedance along a cable length will give rise to signal distortion and reflection. The length of patch leads is an importan