 overall foil shield. This often has a larger sufficient for most uses of the main proprietary cabling schemes. overall diameter to compensate for the detrimental effect of the There is a fire regulation which requires a fire exit to be within 90 shield on crosstalk. metres of the workplace. The building risers are generally located Shielded twisted pair (STP) is made up from individually near fire escape stairways. screened pairs in a common outer sheath which often includes an Four factors combine to reduce the office area that can be served overall shield. from a single closet below that of a circle of 90m radius. They are: The outer diameter of the cable is an important consideration in structured cabling systems. The packing density of the cable is 1. The use of 'street and avenue' horizontal cable routes dependent on the diameter. This is one of the advantages of PVC 2. The need to route cables around obstacles such as service cores. insulated UTP cable. 3. The need to avoid congestion in the areas where the cables Low smoke zero halogen fire resistant cables should also be approach and enter the closets. These areas are often densely considered because of the PVC fire hazard. packed with other services as well. The electrical and transmission characteristics of the commonly 4. The need to allow some cable length at the patch panel, and used data cables are shown in Table 33.3. sometimes, from the service space to the presentation. A cable is defined by the characteristics: near end crosstalk; attenuation; characteristic impedance; conductor resistance; insula- The area that can be served is further reduced when the shape of tion resistance; velocity factor; and mutual capacitance. the building is considered. This often leads to the location of a The characteristics of the link between the wiring closet and the telecommunications closet near a corner of the space to be served. floorbox outlet must meet the most stringent standard required by In practice, it is quite difficult to serve areas of more than 1,0