Troubleshooting Local
Contents
1. Figure 6 22 The most common causes of problems in fiber optic cabling infrastructures TROUBLESHOOTING LocAL AREA NETWORKS 6 Index p1 CABLE INFRASTRUCTURES IN LocaL AREA NETWORKS Index of chapter 6 A Air gap 163 Application classes for structured cabling 146 Attenuation budget 164 B BNC connectors 148 C Campus backbone 145 Category 7 cabling 155 Cheapernet 147 Coaxial cable 147 Color coding 159 D Dead zone 165 E EIA TIA 568A 145 Electromagnetic interference 158 EN 50173 145 F Failure of a segment 152 Fiber optic cable 163 Fiber optic cabling and connectors 164 G Grounding twisted pair cable 157 H Horizontal cabling 145 l IEC 874 1 165 Insertion loss 163 ISO IEC IS 11801 145 TROUBLESHOOTING LocAL AREA NETWORKS 6 Index p2 CABLE INFRASTRUCTURES IN LocaL AREA NETWORKS K Kinks 152 L Loss of connection 152 M Mated connections 163 Measuring cable lengths 152 Measuring electromagnetic interference 151 Measuring resistance in coaxial cable 149 Measuring the voltage in network cabling 149 MIL C 17G 147 N N type connectors 148 O OTDR measurements 165 P Pairs In Metal Foil PIMF 155 R RG 6 RG 11 147 RG 62 coaxial cable 147 S Screened STP S STP 155 Search for defects in coaxial cable 151 Shielded Twisted Pair STP 155 Short circuit 152 Solid wire 162 Stranded wire 162 T Testing the terminating resistor 150 Thick Ethernet 147 Thin Ethernet 147 Troub2. 500 terminating resistor Thin Ethernet cable with BNC connector BNC T connector 500 terminating resistor LAN cable resistance measurement Figure 6 9 Setup for testing a terminating resistor 6 1 2 4 Measuring Electromagnetic Interference When line noise is measured using a cable scanner or a spectrum analyzer the frequency of electromagnetic interference in the line can be identified Noise is measured between the shielding outer conductor and the core inner conduc tor If the noise exceeds permissible levels check the area around the affected cable route for possible sources of interference including production environ ments elevators photocopiers fluorescent tubes arc welding equipment and X ray devices If a spectrum analyzer is available you may try to determine the frequency spectrum of the interfering equipment Typical interference fre quency ranges are listed TROUBLESHOOTING LocAL AREA NETWORKS 5 152 CABLE INFRASTRUCTURES IN LocaL AREA NETWORKS Potential source of interference Operating frequency FM radio and TV signals 1 100 MHz GSM telephones 800 MHz Pagers 500 MHz PCs 100 400 MHz Fluorescent tubes electric motors 10 150 kHz 6 1 2 5 Measuring Cable Lengths Reflections and Short Circuits Cable length reflections and short circuits can be measured using a time domain reflectometer or TDR The TDR transmits a signal and then measures the time that elapses until t
3. In addition to observing segment length limitations 185 meters for 10Base2 500 meters for 10Base5 it is especially important to respect the minimum bending radius for coaxial cable 5 cm for 10Base2 25 cm for 10Base5 and to install pressure relief fixtures over the cables where necessary to prevent damage caused by equipment housings other objects resting on the cables or other mechanical stress 10Base2 10Base5 Maximum segment length Maximum bending radius 25cm Maximum number of nodes per segment 100 Maximum attenuation per segment 8 5 dB Maximum transmission delay per segment 2 165 ns Figure 6 5 Design guidelines for 10Base2 and 10Base5 networks IEEE 802 3 TROUBLESHOOTING LocAL AREA NETWORKS 5 149 CABLE INFRASTRUCTURES IN LocaL AREA NETWORKS 6 1 2 Troubleshooting Coaxial Cable 10Base2 10Base5 The first steps in tracking down the source of a problem in coaxial cable include checking the voltage resistance terminating resistors noise cable lengths reflections and attenuation When these tests are performed conscientiously they often lead directly to the cause of the problem 6 1 2 1 Measuring the Voltage in Network Cabling The easiest way to measure the voltage in a segment of network cabling is to use a voltmeter on an unused T connector or vampire tap If the voltage is less than 100 mV 200 mV at one end of the cable then there is probably nothing wrong in this area If the volt
4. Troubleshooting by Othmar Kyas Cable Infrastructures in Local Area Networks An Agilent Technologies Publication lt a Agilent Technologies Troubleshooting Local Area Networks Cable Infrastructures in Local Area Networks Always expect the worst and you will never be disappointed PETER WASTHOLM The main prerequisite for trouble free operation of a local area network is the use of a high quality cable infrastructure with specified data transmission para meters Estimating the overall transmission characteristics of network cable infrastructures is an extremely complicated task In practice the only way to design a physical foundation for reliable data communications is through strict adherence to accepted standards for the cabling The ISO IEC IS 11801 specifi cation from which the European standard EN 50173 is derived is an interna tionally recognized standard that describes customer premises cabling The corresponding North American standard is the EIA TIA 568A Commercial Building Telecommunications and Wiring Standard The ISO IEC IS 11801 specification distinguishes between three types of cable by range campus backbone for use over distances of up to 1 500 meters vertical backbone for up to 500 meters and horizontal cabling for connecting compo nents on the same floor over distances of up to 90 meters ISO IEC IS 11801 also Network Network Networ
5. Symptom Cause 1 Cause 2 Cause 3 Cause 4 Symptom Cause Symptom Cause 1 Cause 2 TROUBLESHOOTING LocAL AREA NETWORKS 5 CABLE INFRASTRUCTURES IN LocaL AREA NETWORKS Unusually High Number of Collisions Strong reflections in the cable resulting in collisions Check to see if terminating resistor is missing defective or out of tolerance Segment may have too many MAUs Segment grounded in more than one place Maximum cable length exceeded Intermittent or Frequent Collisions and Fragments Electromagnetic interference Check whether photocopiers pagers elevators or x ray devices are being used near the cabling No Connection or Intermittent Loss of Connection After New Installation High attenuation in newly installed cables variations in imped ance between connectors or in patch panels Impedance of new cable is outside tolerance limits wrong cable type installed Check tolerances and specifications of all new components Gathering Information Common Errors Most problems in cabling infrastructures arise in conjunction with external alterations Such changes may have been made intentionally or even inadvert ently as can happen during the course of other activities Specific information about the context of the problem can provide clues to its exact location and possible causes Questions to ask at this stage include e Has anyone connected or disconnected a PC laptop or desktop
6. arrive in rapid succession Regular maintenance of fiber optic cables is also important Cable maintenance tasks include regular measurements of the signal strengths of all active compo nents as well as checking the connector contacts for crossed fibers and contami nation TROUBLESHOOTING LocAL AREA NETWORKS 5 166 CABLE INFRASTRUCTURES IN LocaL AREA NETWORKS 6 3 3 Symptoms and Causes Fiber Optic Cable Symptom No Connection Cause Line break due to external shock or extreme bending in the cable The bending radius of the cable must not be less than 20 times the cable diameter Thus for internal fiber optic cables with a diam eter of 3 mm for example the minimum bending radius allowed is 6 cm Check all bend radii if necessary switch to an intact pair for the connection in question Use an OTDR to check for breaks in the cable If no suitable test device is available the lighter test can be used for cable seg ments of up to several hundred meters hold a lighter to the cross section of the fiber at one end and verify that light is visible with the naked eye at other end Symptom No Connection or Intermittent Connection Problems Cause 1 Attenuation too high due to poor workmanship on splices or too many splices Check the signal strength using an optical source and power meter Cause 2 Contamination of connectors dust fingerprints humidity etc Cause 3 Transmitter signal strength too low Increase light
7. or any other component to or from the network e Has anyone installed an interface card in a computer e Has anyone stepped on a cable e Has any maintenance work been performed in the building recently by a telephone company or building maintenance personnel for example e Has anyone including cleaning personnel moved any equipment or furni ture 154 TROUBLESHOOTING LocAL AREA NETWORKS 6 CABLE INFRASTRUCTURES IN LocaL AREA NETWORKS Figure 6 10 lists the most common causes of problems in coaxial cabling infra structures Cable continuity is interrupted at a T connector Continuity fault in a connector Connector has a short circuit Cable insulation is damaged shielding is visible Cable is kinked or bent too tightly Cable is too long Cable is not grounded Cable is grounded in two places Cable segments look similar but have different impedances Electromagnetic interference elevators electric machinery or mechanical stress doors furniture exists along cable route Impedance in new cabling exceeds tolerance limits or the wrong cable type has been installed A MAU is defective Newly installed vampire taps act as mini antennae and induce signals Network voltage exceeds allowable levels due to a defective MAU or lack of grounding New cables with high attenuation have been installed variations in impedance between connectors or in patch panels Physical connection to the netw
8. power or replace the LED laser module Symptom No Connection After New Installation Cause 1 Faulty connection in wiring closet Cause 2 Poor splice attenuation too high Cause 3 Dirty connectors dust fingerprints humidity etc Gathering Information Common Errors As with copper cables comprehensive information about the context of a problem with fiber optic cable provides a detailed description of the symptoms and clues to possible causes Questions to ask at this stage include e Has anyone connected or disconnected a PC laptop or desktop or any other component to or from the network e Has anyone installed an interface card in a computer e Has anyone stepped on a cable e Has any maintenance work been performed in the building recently by a telephone company or building maintenance personnel for example e Has anyone including cleaning personnel moved any equipment or furni ture TROUBLESHOOTING LocAL AREA NETWORKS 5 167 CABLE INFRASTRUCTURES IN LocaL AREA NETWORKS The most common causes of problems in fiber optic cabling infrastructures are listed here e Excessive loss due to faulty splices or connectors too many splices or connectors e Fiber break due to mechanical stress or insufficient bending radius e Wrong fiber connected in splice tray or at patch panel e Insufficient transmitter power e Excessive loss due to contaminated connector e Excessive loss due to excessive cable length
9. with a multimeter is TROUBLESHOOTING LocAL AREA NETWORKS 5 158 CABLE INFRASTRUCTURES IN LocaL AREA NETWORKS sufficient to determine whether there is a difference in potential between the wall jack shielding and the patch cable shielding If a noticeable voltage level is detected the grounding of the individual components must be reviewed so that they all end at the same grounding block Fiber optic cabling is used to connect floor wiring closets to the building distributor This prevents current due to varying ground potentials This is even more important in connecting building distributors to the campus backbone cable because there can be significant differences in ground potentials from one building to another 6 2 1 2 Electromagnetic Interference It is important to make sure that the cable is installed at a sufficient distance from potential sources of electromagnetic interference especially when using unshielded twisted pair cables The North American specification EIA 569 de scribes the following guidelines for determining appropriate distances Minimum distance at line power gt 5kVA Minimum distance Minimum distance at line power lt 2 kVA at line power 2 to 5kVA Source of electromagnetic interference Unshielded power lines or electrical equipment near open or non metallic cable ducts Unshielded power lines 3 in or electrical equipment near metallic grounded cable ducts or ele
10. Figure 6 19 lists the most common causes of problems in twisted pair cabling infrastructures TROUBLESHOOTING LocAL AREA NETWORKS 6 163 CABLE INFRASTRUCTURES IN LocaL AREA NETWORKS e Wiring errors in patch panels e Wiring errors in connectors e Crosstalk due to split pairs e Near end crosstalk due to inferior cable unsuitable for high speed data e Continuity faults due to poor crimp connections in connectors e Continuity faults due to bends kinks loose connectors e Short circuits due to mechanical stress or material defects e Near end crosstalk due to faulty installation of patch panels or cable routing e Near end crosstalk due to untwisted patching in patch panels or TCUs e Impedance anomalies due to cramped cable bundles e Excessive attenuation because cables are too long e Electromagnetic interference elevators electric machinery or mechanical stress doors furniture Figure 6 19 The most common causes of problems in twisted pair cabling infrastructures 6 3 Fiber Optic Cable 6 3 1 Fiber Optic Cable Specification and Implementation In recent years fiber optic cabling has proven to be a sturdy high performance transmission medium for use in both primary campus and secondary build ing or vertical cabling The fiber optic cables most commonly used are multi mode cables with wavelengths of 850 nm or 1 300 nm At power levels of 10 dBm to 20 dBm signals can travel distances of some 5 kilometers The
11. KS 5 157 CABLE INFRASTRUCTURES IN LocaL AREA NETWORKS a attenuation NEXT near end crosstalk loss a dB 100m BSE re i tS I pti o wo S a o o wo os o a o o NEXT dB Figure 6 13 Specifications concerning attenuation and NEXT for cabling of Categories 3 5 and 7 Cable type Minimum Minimum Minimum bending radius bending radius one time during installation in installed state bending radius under tension 100 Q 120 Q 8 times the cable s 6 times the cable s N A twisted pair outside diameter outside diameter in backbones 4 times the cable s outside diameter workgroup areas 150 Q twisted pair N A 7 5 cm 2 cm Figure 6 14 Minimum bending radii for twisted pair cabling ISO IEC IS 11801 the cable is low enough for your installation requirements and to install protec tive fixtures where necessary to prevent undue mechanical stress on the cables 6 2 1 1 Grounding Twisted Pair Cable Infrastructures Proper grounding of the cabling infrastructure is absolutely essential Cable shielding must be grounded both in the patch panel which should have a ground bus connected to the building ground and in the wall jack Connecting cables and patch cables must be grounded in both the wall jack and the end device It is important that the potential in the wall jack ground is the same as that in the patch panel and terminal device A simple measurement
12. age exceeds 100 mV especially if the reading is several volts then it is likely that the supply voltage of a media access unit MAU or other component on the cable segment is leaking into the network cable In this case disconnect the MAUs one at a time and repeat the voltage measurements after each unit is removed Because more than one MAU may be defective do not re connect the deactivated units to the network until the source of the problem has been isolated If the voltage is still too high even after all components have been disconnected the cable is probably grounded in more than one place with different electrical potentials at each ground Remove all grounds except one Sometimes a second ground is created by a defective network component that Inner conductor Multimeter 3 Veevi F ewn R Braided outer conductor Fade ANA F y are other devices connected to the LAN cable R und R are terminating resistors Do Cu RAR or 250 Figure 6 6 Measuring resistance in coaxial cable TROUBLESHOOTING LocAL AREA NETWORKS 6 150 CABLE INFRASTRUCTURES IN LocaL AREA NETWORKS does not electrically isolate network components from the data transport me dium If this is the case a simple network interface card may be the cause of the voltage problem 6 1 2 2 Measuring the Cable Resistance All network components must be switched off before performing resistance measurements Measure resistance at a T connector or
13. and the whole is encased in a plastic jacket Called coax two syllables for short coaxial cable can provide data speeds of several gigabits per second and is used for both digital data and television signals Inner conductor Dielectric Braided outer conductor Plastic jacket Figure 6 3 Structure of coaxial cable The types of coax most commonly used for data communications besides the traditional 50 Q cable of 10Base2 and 10Base5 Ethernet networks are RG 62 coaxial cable which has an impedance of 93 Q used to connect IBM 3270 terminals RG 6 and RG 11 coax with 75 Q impedance and twin axial cable with an impedance of 105 Q AS 400 IBM 36 38 series Detailed specifica tions for the various types of cable are found in the MIL C 17G standard see Figure 6 4 There are two ways to connect network components to coaxial cable in bus based Ethernet networks 10Base2 10Base5 The first method consists of cutting the cable and inserting a T connector The second method involves drilling a hole in the coaxial cable until the inner conductor is just reached and then inserting a special connector known as a tap or vampire tap This method is used with 10Base5 cabling yellow cable or thick Ethernet while T connec tors are commonly used in 10Base2 networks thin Ethernet or cheapernet The advantage of vampire taps is that new components can be installed without interruptin
14. ctrical equipment in grounded metallic shielding near metallic grounded cable ducts Transformers 3ft electric motors Flourescent tubes 1ft Power lines 6in 1ft Figure 6 15 Guidelines for minimum distances between cabling and potential sources of electromagnetic interference EIA 569 6 2 2 Troubleshooting Twisted Pair Cable The main task in troubleshooting twisted pair cabling infrastructures is the measurement of key operating parameters such as cable length attenuation NEXT ACR and signal to noise ratio SNR To eliminate the possibility of incorrect pin assignments right at the outset however it is a good idea to begin by comparing the pin assignments in the patch panel with those in the corre sponding wall jacks This type of wiring fault can result from incorrect manual wiring but may also be due to the use of different color coding systems The TROUBLESHOOTING LocAL AREA NETWORKS 159 CABLE INFRASTRUCTURES IN LocaL AREA NETWORKS 6 specifications TIA EIA 568A and TIA EIA 568B for example define different systems of color coding for twisted pair wiring Figure 6 16 points out typical wiring errors as well as the color coding systems used in TIA 568A and in TIA 568B Pair r 2 TIA EIA 568A color code TIA EIA 568A color code Pair Pair Pair option 1 option 2 pe l pas ee Fi Le ees ee Pin Color Pin Color l i l i T 1 white green 1 blue We
15. dynamic range of such segments is between 15 dB and 20 dB With the trend toward data speeds of 622 Mbit s and 2 4 Gbit s however even LAN infrastruc tures are increasingly coming to rely on single mode fiber optic cabling with a wavelength of 1 550 nm Two types of connectors are used in fiber optic LANs those that make contact between one fiber and the next fiber mated connections and those that leave an air gap between the fiber end and the connector unmated connections Unmated connections are easier to work with because they are not as easily contaminated as mated connections but they have a higher insertion loss TROUBLESHOOTING LocAL AREA NETWORKS 6 164 CABLE INFRASTRUCTURES IN LocaL AREA NETWORKS Figure 6 20 lists the types of fiber optic cabling and connection systems used in various LAN topologies Ethernet FOIRL LAN topologies 10Base F Token Bus Token Ring FDDI multimode Gigabit Ethernet single mode Channel Wavelength 1300 1300 850 nm 1270 1380 1270 1340 multimode Cat 1 13 000 1290 1330 single mode Cat 2 Signal power 12 to 15 14 to 20 dBm Cat 1 15 to 37 Cat 2 Receiver 9to 27 12to 32 5 31 41 sensitivity active to 11 21 dBm 27 to 41 passive Maximum network 4 5 4 5 length km Maximum 1 1 200 Mbit s 0 55 segment gt 10 multimode 5 length
16. e of optical transmission media Bandwidth ranges and typical applications are also defined for each class Class A applications for example require bandwidths of up to 100 kHz this class includes such applications as X 21 V 11 and ISDN S Class F applications can require up to 600 MHz bandwidth and include Gigabit Ethernet and 622 Mbit s ATM see Figure 6 2 Application classes Definition per ISO IEC IS 11801 Class A Applications up to 100 kHz analog telephony X 21 ISDN S etc Class B Applications with bandwidths of up to 1 MHz X 21 ISDN S Class C Applications with bandwidths of up to 16 MHz ISDN S 10Base T 4 16 Mbit s Token Ring Class D Applications with bandwidths of up to 100 MHz 100Base TX ATM 155 Mbit s Gigabit Ethernet Class E Applications with bandwidths of up to 200 MHz ATM 155 Mbit s Gigabit Ethernet Class F Applications with bandwidths of up to 600 MHz ATM 155 Mbit s Gigabit Ethernet Optical data transmission Data communications over multimode and single mode optical fiber Figure 6 2 Application classes for structured cabling per ISO IEC IS 11801 TROUBLESHOOTING LocAL AREA NETWORKS 5 147 CABLE INFRASTRUCTURES IN LocaL AREA NETWORKS 6 1 Coaxial Cable 6 1 1 Coaxial Cable Specification and Implementation Coaxial cable has a copper inner conductor at the center and an outer conductor that acts as shielding Between the two conductors is an insulator
17. eS SEE TS 2 green 2 orange 3 white orange 3 black 4 blue 4 red 5 white blue 5 green T568A 6 orange 6 yellow 7 white brown 7 brown 8 brown 8 gray TIA EIA 568B color code TIA EIA 568B color code option 1 option 2 Pin Color Pin Color 1 white orange 1 black 1E2374 5 6 78 2 orange 2 yellow 3 white green 3 blue 4 blue 4 red 5 white blue 5 green T568B 6 green 6 orange F white brown 7 brown 8 brown 8 gray Pin 8 Correct wiring Reversed pair Split pair Transposed pair q sx 1 1 eee 1 1 Po 4 1 om il o WA N 2 2 22 2 2 2 2 MN Dd Seo 3 3 3 3 3 UU 3 ANA 33 Be 4 4 4 4 hg Os A 4 14 5 5 5 5 5 5 5 4 5 AA af 6 6 6 6 6 6 6 w6 oe ao eee 8 8 8 8 8 8 8 8 Figure 6 16 Color coding and wiring errors in twisted pair cabling TROUBLESHOOTING LocAL AREA NETWORKS 5 160 CABLE INFRASTRUCTURES IN LocaL AREA NETWORKS To test for faulty pin assignments in the cable connect the wall plug to a number of different terminating resistors in turn and test the wire pairs coming from the patch panel with each of the resistors Reversed pairs are the most common wiring errors and are caused by a simple reversal of the wires usually at the patch panel Some wiring errors such as split pairs cannot be detected using this procedure Split pairs can only be detected indirectly when NEXT tests produce unusually poor results Split pairs are usually the result of incorrect installation by technicians unfamiliar with twisted pair Transpo
18. ffset the influence of the test location and the test connector Once the reference values have been recorded the same measurements are performed on the cable to be tested see Figure 6 21 Test cable test connector Transmission path under test g Qo fe CJ C Step 1 Step 2 Calibrating the test setup Measurement attenuation over the transmission path Figure 6 21 Attenuation measurement with test setup calibration in accordance with IEC 874 1 OTDR measurements are used to detect individual components of the overall attenuation along a segment of fiber optic cable and to precisely locate each component When performing OTDR measurements keep in mind that most OTDR devices have a dead zone at close range The dead zone is the range that cannot be reliably evaluated due to reflections in the device connector and an overdriven OTDR The sensitivity of the OTDR determines the maximum ampli tude difference that the testing device can detect between the signal pulse and the reflected signal Over longer cable segments a higher dynamic means the ability to measure greater distances With the relatively short distances found in LANs high dynamics are also an advantage because they allow the use of shorter pulses resulting in higher resolution The shorter the signal pulse the easier it is to detect separate reflections that
19. g network operation This can be an important factor especially in TROUBLESHOOTING LocAL AREA NETWORKS 6 148 CABLE INFRASTRUCTURES IN LocaL AREA NETWORKS RG 58 10Base2 cable thin Ethernet Cheapernet 10Base2 50 4 6 at 10 MHz RG 8A U 10Base5 cable thick Ethernet yellow cable 10Base5 5042 1 7 at 10 MHz Cable designation Use Impedance Q Attenuation dB 100 m Velocity factor 0 77 0 83 0 86 Inner conductor mm 0 94 2 7 Insulation mm 2 52 6 15 PVC outer jacket mm 4 62 10 3 Bending radius cm 5 25 Figure 6 4 Specifications for RG 58 and RG 8A U coaxial cable 10Base2 and 10Base5 production networks Care must be taken in using this method however as the cable may break if the hole is drilled too deep If the hole is not deep enough the connection will be unstable In either case serious network problems can result 10Base5d cable is usually equipped with N type connectors 10Base2 cable with BNC connectors To prevent reflections each cable end must be connected to a 50 Q terminating resistor It is essential that one of the terminating resistors be grounded and equally important that the second one not be grounded If neither terminating resistor is grounded this could result in an electrical charge through out the entire network If both are grounded and the two grounds have different electrical potentials considerable current on the line may result
20. he signal s reflection is received Any irregularity in a given cable segment will cause reflections of greater or lesser intensity depending on the magnitude of the irregularity The TDR can be set to wait either for the largest reflection that occurs or to measure smaller reflections such as those that amount to only 30 percent of the original signal The latter method allows you to determine the distance to impedance anomalies along the cable To determine the overall cable length remove the far terminating resistor and set the TDR to measure the greatest reflected signal If the cable is broken at any point along its length however the reflection will come from the break not from the far end As is the case with most cable measurement procedures the cable segment to be tested must be taken out of operation completely before performing TDR measurements 6 13 Symptoms and Causes Coaxial Cable Symptom Intermittent Loss of Connection Cause 1 Poor or no physical connection to the network cable due to a loose BNC connector or a vampire tap that does not reach the inner conductor Cause 2 Terminating resistors exceed tolerance limits Symptom Complete Failure of a Segment Cause 1 Short circuit in the cable outer conductor touching inner conduc tor due to kinks defective cable or vampire tap drilled too deep Cause 2 Voltage on the network exceeds permissible levels due to defective MAU or lack of grounding 153
21. inction is made between shielded twisted pair STP and unshielded twisted pair UTP STP has braided shielding metal foil or a combination of the two wrapped around each wire pair In addition to blocking external EMI this shielding also significantly reduces interference emitted by the wire pairs themselves Metal foil is more effective than braided shielding against high frequency EMI while braided shielding is more effective in absorb ing low frequency radiation In high performance Category 7 cabling for ex ample which is specified to 600 MHz each pair of wires is wrapped in metal foil Cable with this type of shielding is also referred to as pairs in metal foil PIMF Another cable type known as screened STP S STP has shielding around each wire pair plus an additional wrapping of braided shielding around all pairs Shielded Unshielded Pair shielding Twisted Pair STP Twisted Pair UTP Cable shielding All four wires twisted twisted quad Each pair twisted Each pair twisted Figure 6 11 Structure of copper twisted pair cabling TROUBLESHOOTING LocAL AREA NETWORKS 6 156 CABLE INFRASTRUCTURES IN LocaL AREA NETWORKS The ISO IEC IS 11801 standard divides twisted pair cabling into seven catego ries according to frequency ratings Twisted pair cabling is specified for fre quencies of up to 600 MHz Figure 6 12 shows the different categories with the maximum distances that they can span Cable type Class A C
22. k Component Component Component Campus G Buildung F Floor wiring E distributor distributor closet End system aemm ik aina NEI esos Wall jack a eoi ioa toi 1 RE 1500 m pead 500 m tat 90 m l D campus Gl building Bl floor i A po backbone E backbone oa cabling i l i l l l 1 l A B E lt 10m C D lt 20m F G lt 30m Figure 6 1 Campus vertical and horizontal cabling according to ISO IEC IS 11801 TROUBLESHOOTING LocAL AREA NETWORKS 5 146 CABLE INFRASTRUCTURES IN LocaL AREA NETWORKS defines the maximum permissible lengths for cabling systems as well as the upper limits for work area and device cabling see Figure 6 1 The transmission media described in the cable performance categories are twisted copper wire and single mode and multimode fiber optic cabling Coaxial cable or coax is used in 10Base2 and 10Base5 networks but is not suitable for structured cabling and thus not in the specifications mentioned previously The only standard that defines coaxial cabling is EIA TIA 568 which describes 50 Q coax as an option for horizontal cabling The different types of twisted pair data cable available are divided into seven performance categories numbered 1 through 7 Twisted pair cabling can be used over various distances in any of the six ISO IEC application classes A through F defined for copper cabling depending on the transmission char acteristics of each category A seventh application class has been defined for the us
23. km 800 Mbit s single mode Sc Connector types sc ST sc E2000 Mini BNC F SMA Escon SC Duplex Duplex MIC FDDI roy UUUUT lol onnni Figure 6 20 Fiber optic cabling and connectors in local area networks The permissible length for a fiber optic cable is derived from the available attenuation budget which in turn depends on the performance of the transmit ting device and the sensitivity of the receiver Each splice in a cable increases the attenuation by about 0 1 dB each connector by up to 0 5 dB 6 3 2 Troubleshooting Fiber Optic Cable The most important tasks in diagnosing problems with fiber optic cable involve checking the power and attenuation and taking measurements with an optical time domain reflectometer OTDR Attenuation is measured in order to determine whether or not the total attenu ation of a cable segment exceeds the attenuation budget The first step is to set TROUBLESHOOTING LocAL AREA NETWORKS 5 165 CABLE INFRASTRUCTURES IN LocaL AREA NETWORKS up the testing conditions in accordance with IEC 874 1 Method 6 This involves inserting a reference cable in place of the cable segment to be tested between the transmitter and receiver of the measuring instrument A reference measure ment is taken from this cable and compared with the results measured on the actual network cable The reference values serve to o
24. l Complete Failure of a Station Faulty cable bent cable Faulty connection between cable and connector due to defective or low quality crimp components TROUBLESHOOTING LocAL AREA NETWORKS 6 162 CABLE INFRASTRUCTURES IN LocaL AREA NETWORKS Twisted pair cable can have either solid or stranded wire If RJ 45 plugs designed for stranded wire cables primarily used as patch cables are used for solid wire the crimp contacts just touch the surface of the wire and lose contact over time see Figure 6 18 Figure 6 18 Crimping solid and stranded wire cables Cause 8 Short circuit in the cable due to kinks excessive bending defec tive material or a nail inadvertently driven through the cable Gathering Information Common Errors As with coaxial cable comprehensive information about the context of a prob lem with twisted pair cable provides a detailed description of the symptoms and clues to possible causes Questions to ask at this stage include e Has anyone connected or disconnected a PC laptop or desktop or any other component to or from the network e Has anyone installed an interface card in a computer e Has a telephone or fax machine been installed Has anyone stepped on a cable e Has any maintenance work been performed in the building recently by a telephone company or building maintenance personnel for example e Has anyone including cleaning personnel moved any equipment or furni ture
25. lass B Class C Class D Class E Class F Optical 100 kHz 1 MHz 16 MHz 100 MHz 200 MHz 600MHz links Category 3 2km 500m 100m symmetrical copper cable Category 4 3 km 600m 150m symmetrical copper cable Category 5 3 km 700m 160m 100m symmetrical copper cable 1509 3 km 1 km 250m 150m symmetrical copper cable Category 6 100m symmetrical copper cable Category 7 100m symmetrical copper cable Multimode 2 km fiber Single mode 3 km fiber Figure 6 12 Performance categories for structured cabling according to ISO IEC IS 11801 The key operating parameters of twisted pair cabling are the signal propagation speed attenuation near end crosstalk NEXT and the attenuation to crosstalk ratio ACR see Figure 6 13 Essential factors for trouble free network operation include not only the use of high quality cable of the appropriate category but also connectors of the right category When components of different categories are used in the same net work the lowest performance component on a given transmission path deter mines the overall transmission characteristics of that path As with coaxial cable it is also important to make sure the minimum bending radius allowed for TROUBLESHOOTING LocAL AREA NETWOR
26. leshooting coaxial cable 149 Troubleshooting fiber optic cable 164 Troubleshooting twisted pair cable 158 Twisted pair cable 155 TROUBLESHOOTING LocAL AREA NETWORKS 6 Index p3 CABLE INFRASTRUCTURES IN LocaL AREA NETWORKS U Unmated connections 163 Unshielded Twisted Pair UTP 155 V Vampire tap 152 Vertical backbone 145 W Wiring errors 159 Y Yellow cable 147
27. lfunctions 6 2 3 Symptom Cause 1 Cause 2 Cause 3 Cause 4 Cause 5 Symptom Cause 1 Cause 2 Symptom Cause 1 Cause 2 Cause 3 Symptom Cause 1 Cause 2 Symptoms and Causes Twisted Pair Cable Diminished Network Performance Collisions Frame Check Sequence FCS Errors Crosstalk due to split pairs untwisted cable segments in patch panels or Token Ring concentrators Crosstalk due to insufficient cable quality for the data speeds used Crosstalk and reflections due to unsuitable connector systems connectors wall jacks etc for high data speeds Electromagnetic interference cables near photocopiers power lines x ray systems pagers production environments or other source of EMI Cable and wall plug shielding not grounded or grounded to differ ent potentials Unusually High Number of Collisions and Fragments Cable impedance exceeds tolerance limits This can be caused by the poor quality of the cable itself or by poor installation cabling bundled too tightly or bent Faulty wiring in patch panel faulty or loose connectors Intermittent Loss of Connection or No Connection After New Installation Excessive attenuation in newly installed cables variations in im pedance between connectors and cables or in patch panels inad equate wiring through several patch panels Wiring faults reversed pairs split pairs transposed pairs Patch faults at patch pane
28. ork cable is poor or does not exist due to loose BNC connectors or to vampire taps that do not reach the inner conductor The segment is grounded in more than one place Short circuit in the cable This can be caused by any of the following kinks cable defects vampire tap drilled too deep outer conductor touching copper core Terminating resistor is missing or defective Terminating resistors exceed tolerance limits Terminating resistor is missing defective or not to specification Too many MAUs exist on the segment Voltage induced by electromagnetic interference Check whether photocopiers pagers elevators or x ray devices are in use near the affected cable route Figure 6 1 O The most common causes of problems in coaxial cabling infrastructures TROUBLESHOOTING LocAL AREA NETWORKS 6 155 CABLE INFRASTRUCTURES IN LocaL AREA NETWORKS 6 2 Twisted Pair Cable 6 2 1 Twisted Pair Cable Specification and Implementation Whereas coaxial cabling is unbalanced because the outer conductor is always at ground potential twisted pair is balanced cabling because the signals on each wire of a pair have opposite potentials Twisted pair cable can have two or four wire pairs which are twisted together with 6 to 26 rotations per meter of cable Twisting the conductors together improves the cable s immunity to electromag netic interference EMI Cable with four wires twisted together is also referred to as twisted quad A dist
29. sed pairs usu ally result from counting pins from the wrong side of the jack Special testing equipment can perform most measurements on twisted pair cables Cables are directly connected to a tester using an RJ 45 connector In some cases it is also necessary to perform TDR measurements to create a detailed impedance profile for a given length of cable This requires an oscillo scope which can easily be connected to a wire pair by means of an RJ 45 breakout box The pulse for the TDR test can be produced by most cable testing devices Figure 6 17 shows typical TDR measurement results Each deviation in Pulse generator Cable tester test FX Cable under Reflected Reflected Reflected pulse pulse pulse Test pulse Test pulse Test pulse Discontinuity Connector problem Short circuit Figure 6 17 TDR measurements on twisted pair cables 161 TROUBLESHOOTING LocAL AREA NETWORKS 5 CABLE INFRASTRUCTURES IN LocaL AREA NETWORKS impedance along the cable under test interruptions connector problems short circuits etc causes a reflection of the test pulse The oscilloscope display shows the type and location of the impedance deviation Often a number of minor impedance anomalies are detected none of which would cause a problem on its own but which are compounded with other errors to cause serious ma
30. vampire tap see Fig ure 6 6 6 7 If the values measured are between 24 0 and 26 6Q for 10Base5 or between 24 4 and 26 6Q for 10Base2 then the level of resistance in the cable is not the root of the problem Thin Ethernet cable with BNC connector or T connector LAN cable resistance measurement Figure 6 7 Test setup for measuring the resistance of coaxial cable If the resistance is high the cable is broken somewhere along its length In this case isolate the defective cable segment by repeating the resistance measure ments on neighboring T connectors or vampire taps until you detect normal values If the resistance is very low the cable is short circuited You can quickly pinpoint the location of the short using a time domain reflectometer TDR 6 1 2 3 Testing the Terminating Resistor If you suspect that you have a defective terminating resistor make sure you test the T connector and the terminating resistor simultaneously It is possible that the terminating resistor is in perfect condition but connected to a defective T connector see Figure 6 9 TROUBLESHOOTING LocAL AREA NETWORKS 5 151 CABLE INFRASTRUCTURES IN LocaL AREA NETWORKS Rat e e AMM e A 7 500 R high R high R high R normal lt gt ma resistor Faulty segment of LAN cable O 0 Figure 6 8 Systematic search for defects in coaxial cable BNC connector
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