By Greg Niemiera
In September 1997, the first addendum to tia/eia-568a, the revised commercial building cabling standard, was released. tia/eia-568a-1, "Propagation Delay and Delay Skew Specifications for 100-ohm, 4-pair Cable," establishes additional performance requirements for unshielded twisted-pair (utp) and screened twisted-pair cables that are critical for applications that use multiple pairs in a cable for parallel transmission. The Telecommunications Industry Association (tia--Arlington, VA) continues to investigate requirements for multipair utp cable, defined as cable with more than four twisted pairs.
Propagation delay, expressed in nanoseconds per meter of cable, is the time a signal is delayed when propagating through a length of wire. Both the geometry and materials of an insulated conductor affect propagation delay. Delay skew is the difference in propagation delay between any two pairs within the same cable sheath.
Delay skew became an issue when a shortage of fluorinated ethylene propylene (fep), a material used in conductor insulations, led some manufacturers to substitute other jacketing materials for one or more wire pairs within the cable sheath. Such constructions were called 3 x 1 if one of the pairs in a 4-pair cable was insulated in a substitute material, and 2 x 2 if two pairs were so insulated.
Since insulating material affects the speed at which a signal travels along a copper wire, it is conceivable that these 3 x 1 and 2 x 2 cable constructions could introduce transmission errors into networks that use more than one wire pair to transmit and receive signals. Such errors might result from the fact that signals transmitted in sequence over different wire pairs would not necessarily arrive at the receiver in the same sequence.
According to Addendum 1 to tia/ eia-568a, delay skew between the fastest and slowest pair in a cable must not exceed 45 ns/100 m. Further, the skew between all pair combinations cannot vary more than 10 ns over a temperature range of 20o to 60oC.
The addendum establishes maximum delay and delay skew, as well as minimum velocity of propagation along a cable, for frequencies from 1 to 100 megahertz, covering the full range of Category 3, 4, and 5 cables. The measurements apply to a standard channel configuration as defined in tia/eia-568a, but measurement methods are for the laboratory, and the addendum does not cover field-testing of these cable parameters.
As noted, the geometry of the cable and the jacketing materials used are key factors affecting propagation delay and delay skew. Cable geometry comes into play because conductor pairs, even when insulated with the same material, will show slight variations between pairs in phase delay because of differences in the concentricity of the conductors and length variations between wire pairs resulting from the lay schemes used by manufacturers.
(To reduce crosstalk, manufacturers twist each pair in a 4-pair cable at a slightly different rate; this is called "lay." The different twist rates lead to each pair in a cable being of a slightly different length from the other pairs.)
For example, in a Category 5 4-pair cable with a National Electrical Code cmr/mpr fire rating and jacketed in polyvinyl chloride, even if all four pairs are jacketed with the same material, a delay skew of as much as 0.15 ns/m might be expected.
Different insulating materials also produce differing phase delays. For example, operating at a frequency of 10 MHz, a wire pair insulated with fep has a nominal phase delay of 463 ns/100 m and a velocity of propagation (Vp) of 72%. A pair insulated with polyolefin has a nominal phase delay of 490 ns/100 m and a Vp of 68%.
Delay skew between cable pairs insulated with different materials will always be greater than delay skew between pairs insulated with the same material.
In parallel transmission schemes, digital signals consisting of a series of constant voltage pulses representing bits of information are split apart and transmitted down multiple pairs. They are recombined at the receiver. A clock is used to synchronize these high-speed signals. If delay skew is greater than the value prescribed for a transmission scheme, then signals transmitted over multiple pairs in a predetermined sequence may not arrive in the same sequence, leading to timing errors called bit-error-rate, packet collision, or clocking errors.
This article is adapted from a technical advisory update on propagation delay and delay skew published by Mohawk/cdt. Greg Niemiera can be reached at tel: (800) 422-9961 or (978) 537-9961 and fax: (978) 537-4358.
Greg Niemiera is technical support manager at Mohawk/cdt (Leominster, MA).
