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1 Network Problems and Tools Part 2 ITEC 370 George Vaughan Franklin University.

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1 1 Network Problems and Tools Part 2 ITEC 370 George Vaughan Franklin University

2 2 Sources for Slides Material in these slides comes primarily from course text, Guide to Networking Essentials,Tomsho, Tittel, Johnson (2007). Other sources are cited in line and listed in reference section.

3 3 TCP/IP and OSI Models

4 4 Making Use of Problem Solving Tools Digital Volt Meter (DVM) Time-Domain Reflectometer (TDR) Basic Cable Testers Advanced Cable Testers Oscilloscopes Network Monitors Protocol Analyzers

5 5 Digital Voltmeter (DVM) Measures a cables resistance Can determine if a cable break occurred Can also be used to identify short circuits –A short circuit (or short) prevents network traffic from traversing the cable and requires repair or replacement of that cable

6 6 Time-Domain Reflectometer (TDR) A TDR, like a DVM, can determine whether theres a break or short in a cable Measures the time it takes for signal to return and estimates how far down the cable the fault is located –A high-quality TDR can determine the location of a break within a few inches TDRs are available for fiber-optic as well as electrical cables TDR function is standard in most advanced cable testers Use a TDR to document actual lengths of all cables

7 7 Basic Cable Testers Basic cable testers cost less than $100 Typically test only the correct termination of a twisted- pair cable or continuity of a coaxial cable Excellent tools for checking patch cables and testing for correct termination of a cable at the patch panel and jack Can only verify that the cable wires are terminated in correct order or that there are no breaks in the cable Cant check a cable for attenuation, noise, or other possible performance problems in your cable run

8 8 Advanced Cable Testers Advanced cable testers not only measure where a break is located in a cable, but can also gather other information, including a cables impedance, resistance, and attenuation characteristics Functions at both the Physical and Data Link layers of the OSI model –Can measure message frame counts, collisions, congestion errors, and beaconing information or broadcast storms –They combine the characteristics of a DVM, a TDR, and a Protocol Analyzer

9 9 Oscilloscopes Oscilloscopes are advanced pieces of electronic equipment that measure signal voltage over time When used with a TDR, an oscilloscope can help identify shorts, sharp bends, or crimps in a cable, cable breaks, and attenuation problems

10 10 Network Monitors Network monitors are software packages that can track all or part of the network traffic –By examining packets sent across the network, they can track information such as packet type, errors, and traffic –Can collect this data and generate reports/graphs –E.g., Windows Server 2000/2003 Network Monitor, WildPackets EtherPeek, Network Instruments Analyst/Probe, and Information Systems Manager Inc.s PerfMan

11 11 Protocol Analyzers A protocol analyzer evaluates the networks overall health by monitoring all traffic –Also captures traffic and decodes received packets –Some combine HW and SW in a self-contained unit May include built-in TDR to help determine the networks status –E.g., Network General Sniffer, Ethereal, WildPacket EtherPeek, Fluke Network Protocol Inspector Experienced network administrators rely on protocol analyzers to establish baselines for network performance and to troubleshoot their networks

12 12 TCPDUMP Unix/Linux command line protocol analyzer (packet sniffer) used for: –Debugging networks –Debugging applications that depend on networking. –Monitoring traffic Ported to Windows –Windump Supports user defined filters Command Line syntax: tcpdump -v –e Check Man page for other options

13 13 TCPDUMP (Cont.) Example: Ping (Internet Control Message Protocol) –16:23: :15:f2:4d:52:19 > 00:20:ed:73:b7:1d, ethertype IPv4 (0x0800), length 74: IP (tos 0x0, ttl 128, id 8475, offset 0, flags [none], proto 1, length: 60) > : icmp 40: echo request seq –Timestamp = 16:23: –Source (MAC Address) = 00:15:f2:4d:52:19 –Destination (MAC Address) = 00:20:ed:73:b7:1d –Source IP = –Destination IP = –Protocol = icmp

14 14 TCPDUMP (Cont.) Example: arp (Address Resolution Protocol) –16:22: :15:f2:4d:52:19 > Broadcast, ethertype ARP (0x0806), length 60: arp who-has tell –Timestamp = 16:22: –Source (MAC Address) = 00:15:f2:4d:52:19 –Destination (MAC Address) = Broadcast –Protocol = arp who-has tell

15 15 TCPDUMP (Cont.) example: Web Request –16:22: :20:ed:73:b7:1d > 00:16:b6:21:71:d1, ethertype IPv4 (0x0800), length 74: IP (tos 0x0, ttl 64, id 42626, offset 0, flags [DF], proto 6, length: 60) > www8.cnn.com.http: S [tcp sum ok] : (0) win 5840 –Timestamp = 16:22: –Source (MAC Address) = 00:20:ed:73:b7:1d –Destination (MAC Address) = 00:16:b6:21:71:d1 –Source IP = –Destination IP = www8.cnn.com.http

16 16 Wireshark (Ethereal) Name change in 06/2006 due to trademark issues. GUI based protocol analyzer Available for Unix, Linux, Windows Open Source application –www.ethereal.comwww.ethereal.com –Documentation: Can be used to analyze raw data files from TCPDUMP tool. Supports user-defined filters.

17 17 Ethereal (Cont.)

18 18 ARP Broadcast ARP Unicast Response TCP 3-way Handshake SYNC SYNC-ACK ACK ARP Broadcast Broadcast Message

19 19 Common Troubleshooting Situations This section outlines some common network problems and possible solutions

20 20 Cabling and Related Components Majority of networking problems occur at the Physical layer First, determine whether the problem lies with the cable or the computer –Make sure you use the same type of UTP cable throughout the network –Check cable lengths to make sure you dont exceed the maximum length limitation –If you suspect a faulty or misconfigured NIC, check the back of the card –If the NIC seems functional and youre using TCP/IP, use Ping to check connectivity to other computers

21 21 Power Fluctuations Power fluctuations in a building can adversely affect computers Verify that servers are up and functioning –Remind users that it takes a few minutes for servers to come back online after a power outage You may eliminate effects of power fluctuations by connecting devices to UPSs Some packages perform shutdowns automatically, eliminating the need for human intervention when power failures or severe power fluctuations occur

22 22 Upgrades When you perform network upgrades, remember three important points –Ignoring upgrades to new software releases and new HW can lead to a situation in which a complete network overhaul is necessary because many upgrades build on top of others Keep current and do one upgrade at a time –Test any upgrade before deploying it on your production network –Dont forget to tell users about upgrades

23 23 Poor Network Performance When performance problems appear, answering these questions should help pinpoint the causes –What has changed since the last time the network functioned normally? –Has new equipment been added to the network? –Have new applications been added to computers? –Is someone playing electronic games in the network? –Are there new users on the network? How many? –Could any other new equipment, such as a generator, cause interference near the network?

24 24 References Tomsho, Tittel, Johnson (2007). Guide to Networking Essentials. Boston: Thompson Course Technology. Odom, Knott (2006). Networking Basics: CCNA 1 Companion Guide. Indianapolis: Cisco Press Wikipedia (n.d.). OSI Model. Retrieved 09/12/2006 from


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