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TEST & MEASUREMENT
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Qualifiers for performance parameters What happens when no complex certification is required ? In contrast to conventional new installations , in-house system administrators are increasingly involved , and often don ’ t have a cabling certifier and the expertise required for it . It ’ s rather necessary here to test the functionality and localise faults quickly . The demands placed on the documentation of the test results , however , are similarly high as those placed on certification measurements performed with certifiers .
These challenges gave birth to a new generation of testers in the world of data networks . These devices are used to determine the transmission capabilities of structured cabling , which are now well known as qualifiers , and were developed with the objective to provide an easy and costefficient method to verify the performance parameters of data links after installation . Moreover , qualifiers should be able to perform simple tests during commissioning and troubleshooting in existing installations . Unlike a certification tester , these qualification testers don ’ t provide any service independent evaluation , as specified by the classic standards for structured cabling . Instead , they allow for simple OK / fault tests compliant with specific application standards . These qualifiers , just like the active network components available , were limited to a maximum transmission rate of 1Gbit / s .
New transmission rates As transmission rates have increased , so have the demands placed on the qualifiers . Today , we find a new generation of qualifiers on the market that enable the end user to quickly and easily qualify cabling links up to 10Gbit / s and to document this . As before , the manufacturers of network qualifiers were agreed on the supported data rates , however , they were not agreed on the required combination of tests and measurements . Simple cable tests , for example , are supported by all devices but there are considerable differences between the individual qualifiers with regard to tests aiming at qualification . Apart from the simple cabling tests , three different tests or rather measurements have established in the field of qualification :
Signal to Noise Ratio ( SNR )
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The level of the useful signals and of the noise signals in the transmission link are measured and compared . If the ratio becomes too small , the receiver of the data will no longer be able to distinguish the data from the noise . Particularly in unshielded cables , noise can quickly become a problem due to the lack of shielding and will ultimately impair the quality of data transmission .
Bit Error Rate Test ( BERT )
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The easiest method to test the transmission rate is the bit error rate test . While data is sent at maximum speed from a main unit to a remote unit and back , it analyses if and how many bits are erroneous or lost . In this test , 10 seconds have to pass without any erroneous bits to be considered as passed . Delay Skew
3 The difference in signal propagation delay between
the individual pairs is measured . Since Gigabit Ethernet has been put into operation , all four pairs have been used simultaneously for data transmission . The sender fragments the data packet and sends the individual pieces in parallel over the four pairs . The receiver then has to reassemble the packet . If the delay skew between the individual pieces of the packet is too large , the receiver will be unable to reassemble the packet correctly and will discard it .
The different approaches of these three tests clearly show that only a combination of all the three tests , in addition to the simpler cabling tests , and a length measurement can provide a reliable evaluation of the passive components used . The qualifiers currently available in the market , however , often support only one or two of the mentioned tests .
Power over Ethernet A test that ’ s getting increasingly important is the test of Power over Ethernet ( PoE ) supplies . Today , a growing number of network devices are powered by PoE . The benefit is that only one single cable , the network cable , is needed to connect each device , thus eliminating the need for separate power supplies . Power is often supplied via central injections and there are more and more switches offering this functionality .
Due to the complexity of the many PoE configurations that are possible , pure voltage measurements with conventional measurement devices are no longer sufficient . A problem frequently occurring in PoE supplies is that the amplifiers provide the correct voltage in idle state , which , however , will drop to a fraction as soon as a load is added . High-quality devices detect a voltage drop by applying loads to a PoE link . Moreover , which PoE classes ( modes of operation ) are supported needs to be tested . This helps to avoid the problem of the IP camera that was working perfectly during its configuration at the desk , won ’ t work at its final installation location .
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