input and crc errors on serial interface with integrated csu/dsu

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Sep 7th, 2007
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I keep getting input and crc errors on serial interface. I have changed WIC card on 2811 router and the cable connecting smart jack to the router. Other end of p2p T1 is clean. Any ideas what could be wrong?



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scottmac Sat, 09/08/2007 - 05:50
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It could be a network problem.


The carrier should be able to loop & test; first to the smartjack/NIU and then to the CSU/DSU.


If the carrier still gets errors after the SJ/NIU, then the most common problems tend to be:


** Extended Demarc: If the active smartjack is some distance from the router / DSU, then it's common to put in an "extended demarc", which is just a cabling extension from the SJ/NIU to another termination near the router. Bad extended demarcs are pretty common and explain why providers can "Test OK to the SJ/NIU" and still show errors to the CSU/DSU (when the CSU/DSU is proven good).


** Cabling / Inside Wireing (IW). A variation of the extended demarc. If you have a long run from the SJ/NIU to the router/DSU, and it's not terminated well (as in properly, and "by the book" and in good condition) then it can generate all sorts of errors.


Another similar issue is that Cat{anything} cabling (UTP) is not spec for T1/DS1. There is a specific cable type ('Premises cable", now also called "T1 Cable") that is individually shielded pair inside a shielded jacket that is the true spec cable for T1.


By spec, you can go 655 feet from the SJ to the DSU .... but only on Prem cable/T1 Cable...you'll never make it (without errors) on UTP/Cat{anything}.


I know it seems odd that cabling that can carry GIgEthernet (or now, 10GigEthernet) cannot carry a simple T1 ... but the signaling is different; T1 is pure baseband pulses (noisy and subject to massive crosstalk) and Ethernet is differential signaling (less likely to interfere and be interfered with).


** Noise sources on the path. If your IW or extended demarc passes near noise sources (like motors/compressors, noisy flourescent baluns, furnaces with spark-gap ignition ...) they can interfere.



So, short story: Make sure your cabling is good, make sure it's terminated properly, if it's long, make sure it's prem cable, and make sure it's not passing noisy stuff.


Good Luck


Scott


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