DataVoice

Characteristics of IT Cabling

Selection criteria for components for network cabling

nvp - the much quoted

The nvp value ranks just behind the maximum frequency on the popularity list. Nvp is an abbreviation for the English designation "normal velocity of propagation". This variable indicates how fast a signal runs along a line. It is specified in per cent of the speed of light in a vacuum or as a corresponding decimal number.

An nvp of 74 % (o 0.74) therefore means that an electrical signal runs along the line with 74 % of 300,000 km/s, i.e. with approx. 222,000 km/s. At a link length of maximum 100 metres it is therefore at the receiver after approximately 0,45 µs.

Whether the signal reaches the receiver a little earlier or later plays a rather insignificant role in practice. Depending on the line you will find nvp values of 0.69 to 0.79, partly also above or below.

The nvp value plays a much more important role in determination of the line length by field measuring instruments especially when these measurements are used for calculating laid lines.  By the simple formula "distance = speed x time" a field measuring instrument calculates the line length by multiplying the entered speed (nvp) by the measured time until the signal reaches the receiver. If a wrong nvp value is entered into the field measuring instrument, considerable errors can result in the length calculation.

There is also the effect that the mechanical length of a line is not equal to its electrical length. The mechanical length is the length that can be measured externally with a ruler. The electrical length is the length of a copper wire or wire pair, i.e. the length that an electrical signal actually covers. The mechanical and electrical lengths would be equal if you had two perfectly straight wires. However, since the wire pairs of a twisted pair are twisted, they are longer than the external measurement. This can be compared most simply with a ladder and a winding staircase: To climb up five metres, you do actually only have to climb five metres on a vertical ladder. However, the distance on a winding staircase is much longer.

So anyone who wants to have the exact line length will have no choice but to equip a line of exactly determined mechanical length (typically 50 m) with RJ45 modules and to use it as a length reference. The nvp values is then changed on the connected measuring instrument until it also shows 50 metres as the line length. Another difficulty is that the four wire pairs of a twisted pair are twisted at different strengths and therefore have different electrical lengths. Some planners therefore prescribe setting the nvp value so that 50 metres are measured on the shortest wire pair, others that 50 metres are measured on a previously defined wire pair (for example the orange-white pair) or that the arithmetic average of all four wire pairs adds up to 50 metres. This has to be agreed.