Propagation Delay and Delay Skew

To several telecommunications professionals, concepts such as ‘propagation delay’ and ‘delay skew’ bring to mind painful memories of high school physics class. In reality, the effects of delay and delay skew on signal transmission are easily explained and understood.

Delay is a property that is known to exist for all types of transmission media. The propagation delay is equivalent to the amount of time that passes between when a signal is transmitted and when it is received on the other end of a cabling channel. The effect is akin to the delay in time between when lightning strikes and thunder is heard-except that electrical signals travel much faster than sound. The actual delay value for twisted-pair cabling is a function of the nominal velocity of propagation (NVP), length and frequency.

NVP varies according to the dielectric materials used in the cable and is expressed as a percentage of the speed of light. For example, most category 5 polyethylene (FRPE) constructions have NVP ranges from0.65cto0.70c(where “c” represents the speed of light ~3 x108 m/s) when measured on finished cable. Teflon (FEP) cable constructions range from0.69cto0.73c, whereas cables made of PVC are in the0.60cto0.64crange.

Lower NVP values will contribute to additional delay for a given length of cable, just as an increase in end-to-end cable length will cause a proportionate increase in the end-to-end delay. As with most other transmission parameters, delay values are frequency dependent.

When multiple pairs in the same cable exhibit different delay performance, the result is delay skew. Delay skew is determined by measuring the difference between the pair with the least delay and the pair with the most delay. Factors that affect delay skew performance include material selection, such as conductor insulation, and physical design, such as differences in twist rates from pair to pair.

Cable Propagation Delay

Although all twisted-pair cables exhibit delay skew to some degree, cables that are conscientiously designed to allow for variances in the NVP and pair-to-pair length differences will have acceptable delay skew for standard-compliant horizontal channel configurations. Some of the characteristics that could adversely affect delay skew performance include cables with poorly designed dielectric constructions and those with extreme differences in pair-to-pair twist rates.

Propagation delay and delay skew performance are specified by some local area network (LAN) standards for worst case100 mchannel configurations to ensure proper signal transmission. Transmission problems associated with excessive delay and delay skew include increased jitter and bit error rates. Based on IEEE 802-series LAN specifications, a maximum propagation delay of 570 ns/100mat 1 MHz and a maximum delay skew of 45ns/100mup to 100 MHz are under consideration by TIA for category 3, 4 and 5, 4-pair cables. TIA Working Group TR41.8.1 is also considering development of requirements to assess propagation delay and delay skew for 100 ohm horizontal links and channels that are constructed in accordance with ANSI/TIA/EIA-568-A. As a result of TIA committee “Letter Ballot” TR-41:94-4 (PN-3772) it was decided during the September 1996 meeting to issue an “Industry Ballot” on a revised draft prior to release. Still unresolved is the issue of whether or not the category designations will change (e.g., category 5.1), to reflect differences between cables that are tested for additional delay/delay skew requirements, and those that are not.

Although propagation delay and delay skew are receiving much attention, it is important to note that the most significant cabling performance issue for most LAN applications remains to be attenuation to crosstalk ratio (ACR). Whereas ACR margins improve signal to noise ratios and thereby reduce the incidence of bit errors, system performance is not as directly affected by cabling channels with significant delay skew margins. For example, 15 ns delay skew for a cabling channel will typically not result in any better network performance than 45 ns, for a system designed to tolerate up to 50 ns of delay skew.

For this reason, the use of cables with significant delay skew margins are more valuable for the insurance they provide against installation practices or other factors that may otherwise push delay skew over the limit, rather than the promise of better system performance compared to a channel that only meets the system delay skew limits by several nanoseconds.

Because cables that use different dielectric materials for different pairs have been found to cause problems with delay skew, there has been recent controversy over the use of mixed dielectric materials in cable construction. Terms like “2 by 2″ (a cable having two pairs with dielectric material “A” and two pairs with material “B”) or “4 by 0″ (a cable having all four pairs made from either material A, or material B) that are more suggestive of lumber than cable, are sometimes used to describe dielectric construction.

Despite commercial hype that may mislead one to believe that only constructions having a single type of dielectric material will exhibit acceptable delay skew performance, the fact is that properly designed cables having either one dielectric material, or multiple dielectric materials are equally capable of satisfying even the most severe channel delay skew requirements specified by applications standards and those under consideration by the TIA.

Under some conditions, mixed dielectric constructions may even be used to offset delay skew differences that result from different twist rates. Figures 1 and 2 illustrate representative delay and skew values obtained from a randomly selected 100 meter cable sample having a “2 by 2″ (FRPE/FEP) construction. Note that the maximum propagation delay and delay skew for this sample are 511 ns/100mand 34 ns, respectively in the frequency range from 1 MHz to 100 MHz.