Troubleshooting  

Trouble Shooting  Structured Cabling Systems

Structured cabling systems account for between 30 and 70% of all network problems, yet only attract between 0.5 and 5% of the LAN budget.  A professionally designed and installed cabling system will certainly pay for itself.  But if it doesn’t work, what are the likely problems?

First of all how do you know it doesn’t work?

If the cable system has already been installed and working then the first indication of a problem is a failure of the LAN to communicate to all of its stations.  This will be caused by:

failure of LAN hardware or software

mechanical damage to the cabling system

unauthorised change or disconnection

intrinsic failure of the cabling components

poor installation practice

external EMC/EMI problem 

Problems                                     Most likely cause

Complete failure of the LAN Major server/switch/software fail
Individual stations disconnected Local failure of LAN cards/switches
Localised mechanical damage to the cabling
Unauthorised tampering / disconnection at patchpanel
Cables too long
Long response times External electrical noise, EMC
Failure to autonegotiate speed Cable grade too low

 

To differentiate between active and passive equipment failure then the common practices of swapping cards and ports can be used to isolate the problem.  There are specialist test equipment devices that can send packets of data around a LAN to stress and exercise the system.  There is also a device called a Bit Error Rate Tester, BERT, which will send blocks of data and compare what has been received to what has been sent.  The discrepancy is known as the bit error rate.  ATM and gigabit Ethernet require a bit error rate of 1010 or better, i.e. for every ten thousand million bits of data sent, not more than one bit is received in error.

Having decided that it is the cable plant at fault, what are the likely causes;

Mechanical damage Cut cable due to inadvertent removal, insufficient cable containment, sharp edges, cable stepped on etc.
Flood damage from burst pipes, leaking roofs
Cable ties too tight Nylon cable ties are often applied too tightly and dig into the cable.  If this is coupled with very narrow ties, e.g. 2 mm, then this will degrade high speed performance.
Bend radius too small High speed LAN cables cannot go through 90 degree bends.  The manufacturer’s specification must be adhered to e.g. 25 mm is typical.
Incorrect termination The 8 wires of the cable must be correctly connected at each connector, with a maximum of 13 mm ‘untwist’ on each pair.
Cables too long Category 5 and 6 cables are limited to 90 metres to guarantee all protocols will be transmitted,  Optical fibre also has length restrictions for different protocols.
Unauthorised disconnection Patchpanels can be very vulnerable places, and it is very easy to mistakenly change the wrong patchcord and disconnect a user or group of users.  At the work station end, patchcords can also be easily pulled out if too exposed.  The solution is to lock all patching cabinets and telecom closets.
Long response times Outside electrical interference will cause the LAN protocol to keep on requesting retransmission of the data packets.  The user will see this as long response times from the server or even unexplained logging off.  Ensure all data cables are separated from power cables, e.g. 75 mm and 130 mm from fluorescent lights.  Severe problems will need screened cable or optical fibre.
Failure to autonegotiate Many users now have 10/100 NICs in their PCs but are only communicating at 10 Mb/s to the hub/switch.  When the switch is upgraded to 100 Mb/s, e.g. 100BaseTX, all the attached terminals will try to autonegotiate up to the higher speed.  If the cabling is not up to this requirement then this exercise will fail and the PCs will fall back to 10 Mb/s.

Modern structured cabling needs to be 100% tested to ensure compliance with the latest standards. These are now Cat5e (TIA 568A –A5, ISO 11801 Class D Amd. 1 & 2, EN 50173 Class D Amd. 1) or Category 6, Class E, to the latest draft standards e.g. ISO 11801 2nd Ed 2001 Class E.  There is also a standard for testing cabling currently installed to see if it is gigabit Ethernet compliant, TIA/EIA 568-A TSB 95.

There are five manufacturers of suitable copper cable test equipment.  The cable must be tested from both ends to all the requirements of the above standards or to IEC 61935.

Optical cables can be tested with an optical power meter and calibrated light source.  This will tell if the cable link is satisfactory.  If there is a problem then the power meter will identify that there is a problem but not where it is.  For this an instrument called an OTDR, Optical Time Domain Reflectometer, is required.  This device will give a complete ‘picture’ of the fibre link and identify any problem areas, but only after expert interpretation.

IEC 61935         Generic specification for the testing of balanced generic cabling in accordance with ISO/IEC 11801

The tests required are;

 

IEC 61935

Wire Map

X

Attenuation

X

NEXT pair to pair

X

NEXT Powersum

X

ELFEXT pair to pair

X

ELFEXT Powersum

X

Return Loss

X

Propagation Delay

X

Delay Skew

X

DC Loop Resistance

X

Cable length and ACR are also useful additions to this set of tests.

 

The best way to ensure success in a structured cabling installation is to use properly trained people to design, implement and test the system.  The RCDD qualification from BICSI is the only qualification which covers all aspects of structured cabling design and implementation.

The above information is offered as a summary of ISO 11801 and related standards.  It is not a definitive design guide and does not replace study and implementation of the Standards themselves.  The publisher accepts no responsibility for inaccuracies or omissions.  To purchase the full Standards go to your national standards body, e.g. British Standards Institution, Nederlands Normalisatie Instituut etc. or ISO.

  Copyright Ó annor Ltd   2004