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Practical Troubleshooting & Problem Solving Ethernet Networks

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Ethernet is fast becoming the obvious choice for industrial control networking worldwide. While the basic structure of Ethernet has not changed much, the faster technologies such as fast Ethernet and gigabit Ethernet have increased the complexity and choices you have available in planning and designing these systems. As Ethernet has become more complex, a number of misconceptions have arisen as to how Ethernet functions and how the system should be optimally configured. This workshop addresses these issues in a clear and practical manner, thus enabling you to apply the technology quickly and effectively in your next project. The workshop commences with a brief outline of the fundamentals of Ethernet and its operation. The method of access is discussed in depth and topics such as full duplex and auto negotiation are explained. The best methods of designing and installing the cabling systems are then explored with the discussion ranging from 10Base-T over twisted pair to gigabit Ethernet cabling. Methods of optimising Ethernet to obtain best performance are then defined. Finally, the all important topic of troubleshooting is examined with a summary of the typical problems you are likely to encounter from a two station network all the way up to a system comprising 30,000 PCs. http://www.idc-online.com/content/practical-troubleshooting-problem-solving-ethernet-networks-27

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Page 1: Practical Troubleshooting & Problem Solving Ethernet Networks

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Troubleshooting

Page 2: Practical Troubleshooting & Problem Solving Ethernet Networks

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Common Problems and Faults• Most problems will be experienced at the

commissioning phase

• Could theoretically be attributed to the cabling, the LAN devices (such as hubs and switches), the Network Interface Cards (NICs) or the protocol stack configuration on the hosts.

Page 3: Practical Troubleshooting & Problem Solving Ethernet Networks

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Common Problems and Faults• Wiring system should be installed

and commissioned by a certified installer

• Suppliers of high-speed Ethernet cabling systems will not guarantee their wiring if not installed by an accredited installer.

• This effectively rules out wiring problems for new installations, although old installations could be suspect.

Page 4: Practical Troubleshooting & Problem Solving Ethernet Networks

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Tools of the trade• It is relatively easy to isolate Ethernet related

problems with the help of the following tools:

Multimeters

• On coax cables, a simple multimeter can be used to check for continuity and cable resistance.

Handheld cable testers

• Many versions available, ranging from simple wiring continuity testers to sophisticated devices that comply with all the prerequisites for 1000BaseT wiring infrastructure tests.

Page 5: Practical Troubleshooting & Problem Solving Ethernet Networks

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Fiber optic cable testers• Simpler than UDP testers• Some UDP testers can be turned into fiber optic testers by

purchasing an attachment that fits onto the existing tester• For more complex problems such as finding the location of a

damaged section on a fiber optic cable, an alternative is to use a proper Optical Time Domain Reflectometer (OTDR) but these are expensive instruments and it is often cheaper to employ the services of a professional wire installer (with his own OTDR) if this is required.

Tools of the trade

Page 6: Practical Troubleshooting & Problem Solving Ethernet Networks

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Tools of the tradeTraffic generators• Can generate a pre-programmed data pattern on the network.

• Not used for fault finding, although they can be used to predict network behavior due to increased traffic

• Can be stand-alone devices or they can be integrated into hardware LAN analyzers.

Page 7: Practical Troubleshooting & Problem Solving Ethernet Networks

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Tools of the tradeRMON probes• A device that can examine a network at a given point and keep track of

captured information at a detailed level.

• Can monitor a network at a remote location. The data captured by the RMON probe can then be uploaded and remotely displayed by the appropriate RMON management software.

• Can create an RMON probe by running commercially available RMON software on a normal PC, although the data collection capability will not be as good as that of a dedicated RMON probe.

Page 8: Practical Troubleshooting & Problem Solving Ethernet Networks

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Tools of the tradeSoftware protocol analyzers• Software packages running on PCs and using either a general

purpose or a specialized NIC to capture frames from the network.

• The NIC is controlled by a so-called promiscuous driver, which enables it to capture all packets on the medium

• Simple analyzers such as Wireshark (Ethereal) are available for download from the Internet as freeware.

• Top-of-the-range software products such as Network Associates' ‘Sniffer’ or WaveTek Wandel Goltermann's ‘Domino’ suite have sophisticated expert systems that can aid in the analysis of the captured software.

Page 9: Practical Troubleshooting & Problem Solving Ethernet Networks

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Tools of the tradeHardware based protocol analyzers• Several manufacturers supply hardware based protocol

analyzers using their protocol analysis software running on a proprietary hardware infrastructure. This makes them very expensive but dramatically increases the power of the analyzer. For Fast and Gigabit Ethernet, this is probably the better approach.

Page 10: Practical Troubleshooting & Problem Solving Ethernet Networks

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Problems and Solutions

•Noise •AUI problems

•Thin coax problems •NIC problems

•Thick coax problems •The origin of bad packets

•UTP problems •Host related problems

•Fiber optic problems •Hub related problems

Page 11: Practical Troubleshooting & Problem Solving Ethernet Networks

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Problems and SolutionsThin coax problems • Incorrect cable type

• Loose connectors

• Excessive number of connectors

• Overlong cable segments

• Stub cables

• Incorrect terminations

• Invisible insulation damage

• Invisible cable break

Page 12: Practical Troubleshooting & Problem Solving Ethernet Networks

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Problems and Solutions

Thick coax problems • Loose connectors• Dirty taps • Open tap holes• Tight cable bends

• Excessive loop resistance

Page 13: Practical Troubleshooting & Problem Solving Ethernet Networks

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Problems and Solutions

UTP problems • Incorrect wire type (solid/stranded) • Incorrect wire system components • Incorrect cable type • ‘Straight’ vs. crossover cable• Excessive untwists• Stubs• Damaged RJ-45 connectors

Damaged RJ45 plug - Network sockets damaged due to a faulty plug

Page 14: Practical Troubleshooting & Problem Solving Ethernet Networks

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Problems and Solutions

Fiber optic problems • Incorrect connector installation • Dirty cable ends • Component ageing

AUI problems • Excessive cable length• SQE test

Page 15: Practical Troubleshooting & Problem Solving Ethernet Networks

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Problems and Solutions

NIC problems • Basic card diagnostics• Incorrect media selection• Wire hogging• Jabbers• Faulty CSMA/CD mechanism • Too many nodes • Improper packet distribution • Excessive broadcasting • Bad packets

Page 16: Practical Troubleshooting & Problem Solving Ethernet Networks

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Problems and Solutions

The origin of bad packets • Runts• CRC errors• Late collisions• Misaligned frames • Faulty auto-negotiation • 10/100 Mbps mismatch

• Full/half-duplex mismatch

Page 17: Practical Troubleshooting & Problem Solving Ethernet Networks

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Problems and Solutions

Host related problems • Incorrect host setup• Failure to log in

Hub related problems• Faulty port• Faulty hub• Incorrect hub interconnection

Page 18: Practical Troubleshooting & Problem Solving Ethernet Networks

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Troubleshooting switched networks

• Troubleshooting in a shared network is fairly easy since all packets are visible everywhere in the segment

• The protocol analysis software can run on any host within that segment

• In a switched network the situation changes radically since each switch port effectively resides in its own segment

• Packets transferred through the switch are not seen by ports for which they are not intended

Page 19: Practical Troubleshooting & Problem Solving Ethernet Networks

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Troubleshooting switched networks

• Many vendors have built traffic monitoring modules into their switches

• These modules use either RMON or SNMP to build up statistics on each port and report statistics via the switched management software

• Capturing packets on a particular switch port is also a problem

• Packets are not forwarded to all ports in a switch so there is no place to plug in a LAN analyzer and view the packets

Page 20: Practical Troubleshooting & Problem Solving Ethernet Networks

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Troubleshooting switched networks

• One solution is port aliasing, also known as port mirroring or port spanning.

• The aliasing has to be set up by the user • The switch copies the packets from the port under

observation to a designated spare port. • This allows the LAN user to plug in a LAN analyzer

onto the spare port in order to observe the original port.

Page 21: Practical Troubleshooting & Problem Solving Ethernet Networks

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Troubleshooting switched networks

• Another solution is to insert a small, inexpensive shared hub in the segment under observation, i.e. between the host NIC and the switch port to which it is normally connected.

• The LAN analyzer can then be connected to the hub in order to observe the passing traffic.

Page 22: Practical Troubleshooting & Problem Solving Ethernet Networks

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Troubleshooting Fast Ethernet • Most diagnostic software is PC based and uses a NIC with a

promiscuous mode driver• This makes it easy to upgrade the system by simply adding a

new NIC and driver • Most PCs are not powerful enough to receive, store and

analyze incoming data rates • It might therefore be necessary to consider the purchase of a

dedicated hardware analyzer • Most of the typical problems experienced with Fast Ethernet

include:– a physical network diameter that is too large (for half-duplex networks)– the presence of Cat3 wiring in the system– mismatched 10BaseT/100BaseTX ports, and noise.

Page 23: Practical Troubleshooting & Problem Solving Ethernet Networks

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Troubleshooting Gigabit Ethernet

• Gigabit Ethernet packets arrive so fast that they cannot be analyzed by normal means

• A Gigabit Ethernet link is capable of transporting around 125 MB of data per second

• Few analyzers have the memory capability to handle this

• Gigabit Ethernet analyzers minimize storage requirements by filtering and analyzing capture packets in real time, looking for a problem.

Page 24: Practical Troubleshooting & Problem Solving Ethernet Networks

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