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what important networking-troubleshooting tool is part of icmp
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Published Date:02-08-2017
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3 tools and troubleshooting Into the Wire Doesn’t she get my signal? Oh I do, but you’re not understanding my signal. How do you know when a network signal isn’t getting through a network cable? Often the first thing you’ll hear about it is when the network stops working effectively, but the trouble is, it’s hard to tell what’s wrong by just looking at a cable. Fortunately, there’s a raft of tools you can use that let you see deep into the heart of your network cables, down to the signal itself. Keep reading, and we’ll show you how to use these tools to troubleshoot your networks, and how to interpret the secrets of the signal. this is a new chapter 85 Download at agum and the super bowl - sweet Mighty Gumball won the Super Bowl contract Mighty Gumball is the leading vendor of a wide variety of candies and chocolates, and they’ve just won an exclusive contract to sell their products at the next Super Bowl. With sales on the day and all the extra publicity, Mighty Gumball stands to make millions. There’s just one problem. Mighty Gumball has recently been plagued by network problems, and it’s hitting gumball production. If they don’t get their network errors fixed soon, they won’t be able to produce enough gumballs in time for the Super Bowl, and they’ll lose the contract. We can’t lose this contract; we’ve got to get our network sorted. I’ll give 5,000 to whoever finds out what’s wrong. I’ll even throw in a year’s supply of gumballs. 86 Chapter 3 Download at a Mighty Gumball CEOtools & troubleshooting Am I missing the point here? Can’t we just use a toner and tracer? Won’t that find the problems? Toner and tracer sets can’t find every problem. So far, we’ve only looked at network problems that toner and tracers can help us find, and while they’re extremely useful, they can’t root out every kind of network problem. Here are some of the potential network problems that might be affecting Mighty Gumball. Can you see which ones a toner and tracer might not be able to detect? Why is each of these a problem? Why do you think you can’t use a toner and tracer set to detect all of these? you are here 4 87 Download at a Potential network cable problems Broken cables. Bad connectors. Mis-wired connectors. Too close to a power cable. Too long a cable. Wrong type of cable used.signal but not quality A toner and tracer can check for a signal... As we saw earlier, we can use a toner and tracer to listen out for signs of life from electrons. The toner sends a signal along the cable, and provided you hook it up to the same cable, the tracer picks up the signal from the electrons. If the tracer can’t pick up the signal at any point along the cable, this means there’s a break at that point. There’s no way I’m gonna jump that break. ...but can’t check for signal quality If there’s a connected wire going all the way through the cable, then the tracer will be able to pick up the signal along the entire length of the cable. The trouble is, it can’t tell you anything about the quality of the signal, or the speed with which it moves along the cable. All it can tell you is that the wire is continuous. So if the network problem’s down to signal quality, you need something more than a toner and tracer. But what? 88 Chapter 3 Download at a When there’s a break in continuity, the electrons can’t get through, so the tracer can’t pick up the signal. If the wire’s continuous, the tracer will pick up a signal all the way along the cable. What it can’t tell you is anything about the signal quality.tools & troubleshooting BE the Toner & Tracer Your job is to play like you’re the toner & tracer and imagine that you are facing the problems shown below. Say if you can help or not, and why or why not. Problem Can you help? Tell us more Yes The wrong cable is plugged into a device. No Yes The cable is wired improperly. No Yes The cable is longer than the recommended maximum for Ethernet (100 meters). No you are here 4 89 Download at acan you help? BE the Toner & Tracer Solution Your job is to play like you’re the toner & tracer and imagine that you are facing the problems shown below. Say if you can help or not, and why or why not. Problem Can you help? Tell us more You can hook the tracer on the correct Yes wire and use the toner to find the correct The cable is plugged to the plug on the opposite end. wrong end. No You can trace each wire of a cable and Yes find the incorrect wires and fix them. The cable is wired improperly. No A simple toner & tracer set will only tell Yes you if there is a connected wire or not. You The cable is longer than the recommended maximum for cannot tell the length with just a toner Ethernet (100 meters). No and tracer. 90 Chapter 3 Download at aFrank tools & troubleshooting Well, I don’t know about you guys, but I’m going after that 5,000 bonus. Joe: Yeah? Me too. There must be some pretty hairy problems if the toner and tracer can’t find them all. Jim: Well, I’m going to set my multimeter to work on the cables. I’ll soon be 5,000 richer. Frank: A multimeter? Are you crazy? What’s a multimeter going to tell you about the state of the cables? Jim: Just wait and see. You might learn something. Q: Q: Can a toner and tracer find a cable What signals can a toner send? Q: Does a tracer have to touch a that is too long? cable to hear the warbling signal? A: Generally, a toner can send a A: A: No. A toner and tracer can only check warbling type of signal that is used to trace No, that is the nice thing about it. You for whether there is a electrical connection. or find the end of a cable. can run it quickly over some wires and To check for the length, you need to actually hear as you get closer to the wire measure the resistance of the cable. A second signal is just a voltage that the that the toner is connected to. tracer checks for on the other end. This is used to check a cable for breaks. This is called continuity. you are here 4 91 Download at a Joe JimUse the selector knob to select what you want to measure. measure resistance Introducing the multimeter As you can probably tell from its name, a multimeter is a versatile tool that measures multiple things, such as voltage and resistance along a cable. To use it, turn the selector knob on the multimeter to whatever it is you want to to measure, and then touch the probes to opposite ends of the wire you’re testing. You then read the measurement off the multimeter display. Use a multimeter to measure resistance The thing that makes the multimeter really useful in networking is its ability to measure resistance along a cable. This is something that a toner and tracer can’t tell you, and it can shed important light on how a cable is functioning. So what’s resistance, and what effect does it have on your network? 92 Chapter 3 Download at a An analog multimeter A digital multimeter Read off the measurements from the output screen. You touch the probes to the opposite ends of a wire to test it.0 0 1 1 2 2 5 5 0 0 1 1 Here’s the readout from the multimeter where the resistance is low. tools & troubleshooting So what’s resistance? Resistance is a measurement of how hard it is for electrons to move through a wire. It’s measured in ohms, and represented by the Greek letter omega, Ω. The higher the number of ohms, the harder it is for electrons to move through the wire. When resistance is low When a cable has low resistance, this means that it’s easy for electrons to pass through the conductor. There’s very little stopping the electrons from flowing, and so they move quickly and easily along the cable. ΩΩ When resistance is high When a cable has resistance that’s high, this means that it’s difficult for electrons to move through the wire. And if the electrons can’t move well, your signal can’t either. ΩΩ Oh man, this is like molasses; it’s gonna take forever. So if you’re having network problems, you can use a multimeter to check out whether high resistance along the cable is slowing down your signal. you are here 4 93 Download at a 0 0 When resistance is low, electrons can move quickly and easily along the wire. When resistance is high, the electrons can’t move as quickly. 2 2 0 0 0 0 1 1 k k 1 1 ∞ ∞which tool? Tonight’s talk: Toner & Tracer vs. Multimeter Toner & Tracer: Multimeter: Whoa, what are you doing here, Multimeter? What do you mean? I do networking too, you know. I guess, like a screwdriver has uses in networking as well. We’re made for finding broken cables and tracing cables. What can you possible add? Well I can tell how long a cable is by measuring its resistance. I can find broken cables as well That resistance thing is nice, but we don’t think it’s used very much. Your biggest problem is that both ends of the cable must be right next to you, how lame’s that? You kind of got me there. We even work on cables where the ends are in different buildings. Tracer doesn’t even need to be touching the cable; we can keep tabs on each other with magnetic induction. But I have ways to measure cables like that. I can create a loop and then make measurements on that. Notice I said measurements. What is that supposed to mean? Just because you give some number readout doesn’t make you more useful. We’re simple and effective at our job. Yeah, your job. Notice my name has multi in it. That means I can do multiple things, not like you. I can also measure how fast the electrons are flowing, or current. Can you do that? Can you? Well... Anyway, gotta dash, I’m needed for some resistance training. See you around, guys. 94 Chapter 3 Download at a0 0 0 0 1 1 1 1 2 2 2 2 5 5 5 5 0 0 0 0 1 1 1 1 tools & troubleshooting Match up the wire with the resistance than an electron moving through these wires would feel. A really long wire ΩΩ A really thick wire ΩΩ ΩΩ A broken wire ΩΩ A short wire you are here 4 95 Download at a 0 0 0 0 2 2 2 2 0 0 0 0 0 0 0 0 1 1 1 1 k k k k 1 1 1 1 ∞ ∞ ∞ ∞0 0 0 0 1 1 1 1 2 2 2 2 5 5 5 5 0 0 0 0 1 1 1 1 which resistance? Match up the wire with the resistance than an electron moving through these wires would feel. A really long wire ΩΩ A really thick wireΩΩ ΩΩ A broken wire ΩΩ A short wire 96 Chapter 3 Download at a 0 0 0 0 2 2 2 2 0 0 0 0 0 0 0 0 1 1 1 1 k k k k 1 1 1 1 ∞ ∞ ∞ ∞tools & troubleshooting BE the Multimeter Your job is to play like you’re the multimeter and imagine that you are facing the problems shown below. Say if you can help or not, and why or why not. Problem Can you help? Tell us more Yes The cable is longer than the recommended maximum for Ethernet (100 meters). No Yes Noise on the cable produced by power, RF, loose connector, bare No cables, and shorts. Yes Connector is mis-wired. No you are here 4 97 Download at aresistance isn’t futile BE the Multimeter Solution Your job is to play like you’re the multimeter and imagine that you are facing the problems shown below. Say if you can help or not, and why or why not. Problem Can you help? Tell us more You can use the resistance setting on the Yes multimeter to read the resistance of the The cable is longer than the recommended maximum for cable. A longer cable will have a higher Ethernet (100 meters). No resistance. The multimeter will not show noise on a line, Yes Noise on the cable with the exception of a higher than normal produced by power, RF, voltage. loose connector, bare No cables, and shorts. You can use the resistance setting on a Yes multimeter to check for continuity, just Connector is mis-wired. like a toner and tracer, but with a readout No instead of a tone. 98 Chapter 3 Download at atools & troubleshooting So how well did the multimeter do? Jim tested the cables at Mighty Gumball, and found six with mis-wired connectors, and twelve that were too long. So does this mean that Jim takes away the 5,000 bonus? You’re kidding me, right? We’ve still got a ton of network problems, and nobody’s getting any money til they’re all fixed. So what’s next? Q: Q: Q: So, it’s called a multimeter. What What is current then? Is a multimeter used that much by else does it measure? network people? A: If you think about voltage like the A: Great question Most multimeters can pressure in a water line, current is how fast A: Not that much. We are using it here to measure voltages, both AC and DC, as well the water flows. So electrical current is how show how to check cables and to show how a current. fast the electrons are flowing in the wire. resistance measurements work. Sometimes, you will need to check for a Q: Q: What’s AC & DC voltage? Are there any big differences specific voltage, and then they come in real between digital and analog multimeters? handy. A: DC stands for direct current. That is the type of voltage you get from a battery. It A: Not really. In most cases, an analog is just steady at one voltage, like 9 volts. multimeter uses just electrical components such as resistors, capacitors, and inductors. AC stands for alternating current. This current alternates in the direction it is The digital multimeters use integrated flowing. Your house current is AC. circuits along with the electrical components. This is why they are more expensive. you are here4 99 Download at aFrank the right tool for the job... So, still feeling confident about multimeters, Jim? Jim: Yeah, well maybe the problems are a bit bigger than I thought. Frank: I still think I have a shot at the 5,000, though. I’m sure my oscilloscope will be able to sniff out the remaining problems. Jim: A what? Frank: An oscilloscope. You see, if you use a multimeter to track down resistance problems, you’ll only be able to troubleshoot cables with that sort of problem. I think we need to look a bit wider than that. Jim: Such as... Frank: Well, I think we need to look for voltage problems too. Joe: And you think an oscilloscope will find everything? Frank: Of course it will. You just wait and see. So what’s an oscilloscope? 100 Chapter 3 Download at a Joe JimTime base Screen where the adjustment signal shows up tools & troubleshooting An oscilloscope shows voltage changes An oscilloscope is another tool you can use to help you troubleshoot your network. It shows you the voltage changes on a wire over time, measured in milliseconds. Here’s an example readout from the oscilloscope showing you the voltage changes in a cable over time. Being able to analyze the voltage changes in this way gives you important troubleshooting information about your cables. Let’s see how. you are here4 101 Download at a An oscilloscope takes some training to use properly. An oscilloscope is a complex piece of test equipment requiring advanced training and some knowledge of electrical principles to use properly. Channel 1 signal input The highs and lows on the display correspond to the highs and lows of the voltage. Channel 1 voltage level adjustmentResistance is anything that might slow down the electrons, like the width of the garden hose. voltage is electrical pressure Voltage is really electrical pressure Let’s start by taking a look at what voltage is. In simple terms, voltage is the pressure that tries to make electrons flow. It’s the push felt by the electrons. Think of voltage as the force that moves the electrons in a circuit, and resistance as anything that might slow them down, such as a broken wire. If we compare electron flow in a cable to water flow down a garden hose, voltage is like the pressure of the water coming out the faucet, and resistance is like the width of the garden hose. So how does this help us troubleshoot problems? As we’ve seen, an oscilloscope allows us to see voltage changes over time. The signal on a network cable is just the change in voltage over time. This means that an oscilloscope effectively allows us to see a network signal. More importantly, it allows us to see how clear and distinct the network signal is, or whether there are any extraneous voltages changing the signal and causing network problems. We call extraneous voltages noise. 102 Chapter 3 Download at a ...and a network signal A clear clean network signal... distorted by noise on the wire. Voltage is electrical pressure, like the pressure of water coming out a faucet. T here are ways to take care of noise. First make sure that you don’t untwist your cable too much, that twist takes care of a lot of noise. Also, keep some distance between your network cables and things that create noise. tools & troubleshooting Where does noise on network cables come from? Noise is unwanted signals on your network cables. It comes from lots of different sources of electronic energy. Most things that use electricity leak electromagnetic energy. This is especially true of electric things that move or have moving parts. Match up the things that create noise on network lines with how they create noise. I cause noise when I do not have a connection to Bad Grounding what I am plugged into. I cause noise when I move cables. Radio Frequency Interference I cause noise because I have revolving magnets. Cable Crosstalk I cause noise by broadcasting waves of Bad Connector electromagnetic energy. I cause noise when signals from me cable creep on to Physical Vibration another cable next to me. I cause noise with voltage differences. Electric Motor Answers on page 107. you are here 4 103 Download at amore tool sparring Tonight’s talk: Multimeter vs. Oscilloscope Multimeter: Oscilloscope: Hey Oscilloscope, it is good to see another real electrical tool here. Did you have a bad morning or something? No, I just got done talking with Toner & Tracer. They were wondering why I was here. I guess you’re in the same boat. We pretty much do the same thing. Hold your horses now. I think we do some different stuff. I mean, you can’t really see a signal now, can you? I guess it’s “pick on Multimeter” day. No, I can’t do that trick of seeing the signal on a cable. I can measure the voltage, but that’s about it. But I can tell how long a cable is, can you? It goes down well at parties. That resistance thing you do is cool. But seeing the signal is more than a trick. If there’s noise or extraneous power on a cable I can help see that. Well, I can measure extraneous power as well, even AC power which is how noise shows up, right? True, but if it’s transient or very quick, you won’t catch it. I will. Well, only when you’re setup correctly. Isn’t it true that it takes some pretty special training to run you? I wouldn’t say special training as such. But yes, someone using me will have to spend some time understanding all of my powerful skills. You’re just a toy by comparison. I think I would rather be a “pick up and use” type of portable tool. Anyway, gotta go. 104 Chapter 3 Download at a