爱刨根知识网

  1. 首页 > 经验知识 > >

传播延迟和传输延迟的区别( 二 )

生活经验延迟


Lower NVP values will contribute to additional delayfor a given length of cable, just as an increase in end-to-end cable lengthwill cause a proportionate increase in the end-to-end delay. As with most othertransmission parameters, delay values are frequency dependent.
When multiple pairs in the same cable exhibitdifferent delay performance, the result is delay skew. Delay skew is determinedby measuring the difference between the pair with the least delay and the pairwith the most delay. Factors that affect delay skew performance include materialselection, such as conductor insulation, and physical design, such asdifferences in twist rates from pair to pair.
Although all twisted-pair cables exhibit delay skewto some degree, cables that are conscientiously designed to allow for variancesin the NVP and pair-to-pair length differences will have acceptable delay skewfor standard-compliant horizontal channel configurations. Some of thecharacteristics that could adversely affect delay skew performance includecables with poorly designed dielectric constructions and those with extremedifferences in pair-to-pair twist rates.
Propagation delay and delay skew performance arespecified by some local area network (LAN) standards for worst case 100 m channel configurations to ensure propersignal transmission. Transmission problems associated with excessive delay anddelay skew include increased jitter and bit error rates. Based on IEEE802-series LAN specifications, a maximum propagation delay of 570 ns/100m at 1 MHz and a maximum delay skew of 45ns/100m up to 100 MHz are under consideration byTIA for category 3, 4 and 5, 4-pair cables. TIA Working Group TR41.8.1 is alsoconsidering development of requirements to assess propagation delay and delayskew for 100 ohm horizontal links and channels that are constructed inaccordance with ANSI/TIA/EIA-568-A. As a result of TIA committee "LetterBallot" TR-41:94-4 (PN-3772) it was decided during the September 1996meeting to issue an "Industry Ballot" on a revised draft prior torelease. Still unresolved is the issue of whether or not the categorydesignations will change (e.g., category 5.1), to reflect differences betweencables that are tested for additional delay/delay skew requirements, and thosethat are not.
Although propagation delay and delay skew arereceiving much attention, it is important to note that the most significantcabling performance issue for most LAN applications remains to be attenuationto crosstalk ratio (ACR). Whereas ACR margins improve signal to noise ratiosand thereby reduce the incidence of bit errors, system performance is not asdirectly affected by cabling channels with significant delay skew margins. Forexample, 15 ns delay skew for a cabling channel will typically not result inany better network performance than 45 ns, for a system designed to tolerate upto 50 ns of delay skew.
For this reason, the use of cables with significantdelay skew margins are more valuable for the insurance they provide againstinstallation practices or other factors that may otherwise push delay skew overthe limit, rather than the promise of better system performance compared to achannel that only meets the system delay skew limits by several nanoseconds.
Because cables that use different dielectricmaterials for different pairs have been found to cause problems with delayskew, there has been recent controversy over the use of mixed dielectricmaterials in cable construction. Terms like "2 by 2" (a cable havingtwo pairs with dielectric material "A" and two pairs with material"B") or "4 by 0" (a cable having all four pairs made fromeither material A, or material B) that are more suggestive of lumber thancable, are sometimes used to describe dielectric construction.
Despite commercial hype that may mislead one tobelieve that only constructions having a single type of dielectric materialwill exhibit acceptable delay skew performance, the fact is that properlydesigned cables having either one dielectric material, or multiple dielectricmaterials are equally capable of satisfying even the most severe channel delayskew requirements specified by applications standards and those underconsideration by the TIA.

推荐阅读


上一篇：瑞纳大灯一般多大功率，瑞纳203款三厢大灯是多少瓦的呢

下一篇：联动云租车怎么注册，联动云共享汽车怎么开后备箱