Ask Professor Phil: Limits on Twisted Pair

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Dear Professor Phil,

Is it true that data, audio, and video can travel only about 300 feet on twisted pair because of external interference?

Jane, Pine Bluff, AR




Jane,

There is an element of truth in that statement but the 300 foot limitation has its roots from an entirely different set of conditions, which have only a slight relationship to signals and interference. First, I assume you are referring to unshielded twisted pair rather than shielded twisted pair because the first is a great deal more popular.

The use of unshielded twisted pair (UTP) soared when local area networks were first introduced in the early 1980’s. However, the three popular LAN systems (Arcnet, Ethernet and Token Ring) used different types of UTP. They had distinctly different distance limitation, connectors, and installation parameters. This situation created an immense for architects and building owners. Since the telecommunications industry had been responsible for most of the previous wiring and had rigid standards for their installation techniques, they created a committee to develop a rule for using twisted pair regardless of what the transmitted payload type might be. The resulting standard EIA/TIA-568, published by the Telecommunication Industry Association, became the basis for proper use of twisted pair in all commercial buildings.

The 300 foot limitation resulted from this 568 standard. However, that distance was chosen primarily because studies had shown that the vast majority of buildings could be wired if the distance from the wiring closet to the office outlet did not exceed 300 feet. For nearly three decades manufacturers have designed transmitter that use twisted pair assuming that the receiver will be within approximately 300 feet. Nothing precludes a manufacturer from designing devices that transmit further than that distance. It’s just not the standard practice.

Dr. Phil Hippensteel teaches at Penn State Harrisburg and is a regular AV Technology contributor.


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