alk), and there- fore improves the likelihood that the telecommunications signal will make it through the cable in an undistorted form. The out- side interference is called EMI (Electro Magnetic Interference) or RFI (Radio Frequency Interference). The UTP is rated in terms of the number of turns or twists per foot, more twists being better. As mentioned above, the rate of twist varies on pairs within the same cable. There are different levels of cable which are UL (Underwrit- ers Laboratories) specified: Categories 1 through 6. All premise cable currently being installed is either Category 5E (also called The Telecom Handbook 14 247 CABLE CAT 5E) or Category 6 (CAT 6) which support gigabit applica- tions (very high speed for large quantities of data). Many telephone systems are still in place that use Category 3. The cat- egories also apply to the hardware that works with the cabling including jacks, patch panels and 110 blocks for terminating the cable. There are 25-pair CAT 5 cables that can be used for feeder cables to connect several floors. CAT 6 cables only come as single strands with 4 pairs of wires, SO the feeder or backbone cable in a CAT 6 installation will be fiber optic cable. NOTE: The speeds which can be supported are dependent upon both the category of cable used and the distance which the signals must travel. Figure 12.1 Unshielded Twisted Pair Cable (UTP) The Concept of Speed or Bandwidth All signals travel over copper cable at approximately the same speed. Speed, as it is used in this context refers to how much can be sent over a cable or a circuit within a second (100 mbps = one hundred million bits per second). So higher speed cables allow much more information (more bits) to be sent in one second than lower speed cables. Because more information is being sent in one second, the information appears to be traveling faster, but it is just that more is getting through in a given time interval. An- other term for this speed is bandwidth. UTP Configurations UTP is manufactured in a variety of conf