同轴电缆测试课件.ppt

小结 正确认识验证测试和认证测试。 正确理解基本链路、永久链路和信道的概念及它们的关系。 了解测试标准。 熟悉双绞线和光纤测试的性能指标。 熟悉双绞线链路故障类型 了解光纤OTDR测试的方法。 能熟练使用验证测试仪表和认证测试仪表测双绞线链路。 会用HDTDX和HDTDR分析NEXT和RL故障。 会用光功率计等仪表测光纤衰减。 * ELFEXT is simply the difference between the FEXT signal and the attenuated signal at the receiver. ELFEXT is simply how much the FEXT signal is present compared to the attenuated signal. ELFEXT = FEXT - attenuation and is an indicator of the signal to noise ratio a 100Base-T receiver can expect. ELFEXT is like ACR: ACR = NEXT - attenuation and was an indicator of the signal to noise ratio a 100Base-T receiver could expect. Generally, the requirements related to FEXT will be stated in terms of ELFEXT. ELFEXT stands for “Equal Level Far End Crosstalk”. The words “Equal Level” are used to indicate that all FEXT signals are to be considered with the same attenuation. No matter where a crosstalk contribution occurs in the link, it is always attenuated by roughly the same amount. Cross-talk from multiple pair or “disturbers” results in performance requirements in terms of “Power Sum”. * When the length test is performed on each pair, the period of time it took for the pulse to travel down and back on the cable pair was measured, calculated and the result was the length. This propagation time is used to also determine the variation between each of the pair measurements. This is important for signaling schemes using all four pairs as signals need to arrive at the receiver within a window of time. The results of the propagation delay is used to determine the skew or difference from the fastest pair compared to the other three pairs. * This illustrates the time difference between tested pairs of a UTP cable. The most variation or skew is shown to be 13 nanoseconds. The pins 7/8 pair was the fastest pair measured, so the other pairs are compared to it and used for comparison to the test limits. * * Back when we discussed the signaling schemes used for Ethernet, we referred to the 4 Level system that was