. Table 33.7 ing network. shows the velocity factors associated with these materials. The BDF is analogous to the vertical distribution frame in the Table 33.7 Effect of dielectric on velocity of propagation structured cabling system. It provides: 1. Cross connection flexibility point. 2. Termination point for cable. Dielectric material Velocity factor Velocity of 3. Interface between PABX and building wiring. propagation With the building distribution frame many of the space and size Free air problems exist in the same way as with the PTT MDF's. In the voice Semi polythene area, the use of fibre optics is some way off but may be hastened when legalisation allows voice and data in one cable sheath. Polythene In the case of a small PABX handling 200-300 extensions and Polyethylene their associated incoming lines from the PTT, size is not a problem with the use of the modern IDC connectors available. As the instal- Polyvinylchloride lation increases in size to 10,000 extensions, which is not unusual, When using TDR's the velocity factor must be entered into the the same problems as with MDF's exist. instrument if it is known or can be measured using a known length Another demand on the BDF is that on one side the cables are of good cable with an open end. terminated from a structured or flooded cable scheme with a The shape of a reflection as displayed on a TDR CRT screen will planned life of many years (future proof). On the other the PABX indicate the cause of the reflection. Some modern TDR's can ana- wiring to a switch which will have an expected service life of often lyse the signal and indicate the cause in words on a liquid crystal only a few years. This is owing to the rate of technical advancement display. For example "short circuit at 25 metres". and the need for business users to maximise their voice installation efficiently. These differing requirements often mean that a separate frame directly associated with the switch is provided and changed Voice distribution frames when the switch is changed. This 