f the cable. This time delay is then displayed as a a view to reducing the overall size conducive with the practicalities distance from the test point to joints, breaks or interference points. of working with the number of circuits involved. Some TDR instruments include a CRT oscilloscope type of dis- The advent of optical fibres has led to the arrival of very compact play which shows the amplitude and shape of the reflected signal optical mainframes which can handle around 100 fibres per 3U of against time. The distance along the X axis indicates the time delay 19" standard framing. The future could be that as fibre is used more for the reflection(s) and hence the distance(s) the cause of the into the local distribution network as well as the trunk network reflection is away from the instrument. By moving a cursor to the much of the size and space problems associated with the MDF will reflections an LCD display indicates the distance in metres. The disappear. instrument must be set up with the velocity factor for the cable under Today insulation displacement connection, IDC, is used most test. The shape of the echo gives an indication of the cause, which extensively as the connection method for all joints: incoming cable could be a short circuit, open circuit or joint, etc. termination; both ends of the jumper; and the equipment termina- The velocity factor or velocity of propagation is a measure of the tion. Traditional solder or wirewrap techniques are rarely used speed at which a signal travels along the cable as compared with free today. The velocity of a signal in a cable is a function of that cable 33.8.2 Building distribution frame depending mainly on the type of dielectric material and construc- tion. Commonly used dielectric materials such as polyethylene, The Building Distribution Frame (BDF) is the interface between the polyvinylchloride, polypropylene. polythene, cellular polythene, all private (automatic) business exchange PABX and the building wir- slow the progress of the signal by various amounts