Split pair one line from each of two pairs is

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Split pair: One line from each of two pairs is connected as if it were a pair (example: the Blue and White-Orange lines are connected to pins 4&5, White-Blue and Orange to pins 3&6). The result is excessive Near End Crosstalk (NEXT), which wastes 10Base-T bandwidth and usually prevents 16 Mb/s token-ring from working at all. 22.3 Length Tests Checking cable length is usually done using a time domain reflectometer (TDR), which transmits a pulse down the cable, and measures the elapsed time until it receives a reflection from the far end of the cable. Each type of cable transmits signals at something less than the speed of light. This factor is called the nominal velocity of propagation (NVP), expressed as a decimal fraction of the speed of light. (UTP has an NVP of approximately 0.59-0.65). From the elapsed time and the NVP, the TDR calculates the cable's length. A TDR may be a special-purpose unit such as the Tektronix 1503, or may be built into a handheld cable tester. 22.4 Testing for Impulse Noise The 10Base-T standard defines limits for the voltage and number of occurrences/minute of impulse noise occurring in several frequency ranges. Many of the handheld cable testers include the capability to test for this. 22.5 Near-End Crosstalk (NEXT) What's NEXT, you ask? Imagine yourself speaking into a telephone. Normally, as you speak you can hear the person on the other end and also hear yourself through the handset. Imagine how it would sound if your voice was amplified so it was louder than the other person's. Each time you spoke you'd be deaf to anything coming from the other end. A cable with inadequate immunity to NEXT couples so much of the signal being transmitted back onto the receive pair (or pairs) that incoming signals are unintelligible. Cable and connecting hardware installed using poor practices can have their NEXT performance reduced by as much as a whole Category. 22.6 Attenuation A signal traveling on a cable becomes weaker the further it travels. Each interconnection also reduces its strength. At some point the signal becomes too weak for the network hardware to interpret reliably. Particularly at higher frequencies (10MHz and up) UTP cable attenuates signals much sooner than does co-axial or shielded twisted pair cable. Knowing the attenuation (and NEXT) of a link allows you to determine whether it will function for a particular access method, and how much margin is available to accommodate increased losses due to temperature changes, aging, etc. Forthcoming updates to cabling standards call for a number of new tests which will add to this list. ------------------------------
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Data Communications Cabling FAQ Page 15 of 19 09/21/1999 Subject: 23.0 - 29.0 Not Used (Blank) These sections are blank for future topics. ------------------------------ Subject: 30.0 Sources of Additional Information AMP Addr: Harrisburg, PA 17105-3608 Tel: 1-800-722-1111 1-800-245-4356 (Faxback service, USA) (905) 470-4425 Canada (617) 270-3774 (Faxback service, Canada) Anixter (An international cable products distributor) see _Anixter 199x Cabling Systems Catalog_ Addr: Anixter, Inc 4711 Golf Road Skokie, IL 60076 Tel: (708) 677-2600 1-800-323-8167 USA
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  • BryanJensen
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