If you've worked with network cabling or fiber, you know the longer the distance, the more the signal tends to degrade. That loss of signal as you extend the distance is referred to as attenuation. And it's usually a gradual diminishing of signal as you extend this over a longer distance. This attenuation occurs whether you're sending an electrical signal over copper, or light over fiber. Eventually, that signal is going to degrade so much that the receiver at the other end won't be able to interpret any of that signal. We often refer to the measurement of that signal as a Decibel, or a dB. A "deci-bell" is, technically, 1/10 of a Bell. That is a capital B in decibel. The capital B stands for Alexander Graham Bell. This is a logarithmic scale, which means the scale varies widely as you extend it out over a longer distance. This is a logarithmic scale, but you are able to add and subtract these losses and gains to get a final decibel value. If you looked at how the logarithmic scale is related to the amount of signal, you would see that three dBs would be twice the signal, 10 dBs would be 10 times the signal, and 20 dBs would be 100 times the signal. Taking it even further, 30 dBs would be 1,000 times the signal. You can see that logarithmically this scales up very quickly as you begin counting up additional decibels. This also works in the other direction. So if somebody mentions that you're losing 1/2 of the signal over a link, then you're losing 3 decibels. If you have dB loss, then you need to troubleshoot where that loss is occurring. If it's complete signal loss, with no connectivity and no signal, then you will have 0 dB of signal. If the signal is intermittent, then you may have some signal getting through, but other signals not able to be received on the other side. There might be just enough signal to show a link light, but not enough signal to be able to transfer any significant amount of data. If you look at the statistics on an interface, you'll be able to see if you're having a signal problem. If you see an increasing number of Cyclic Redundancy Check, or CRC, errors, that means that the data is being corrupted when you receive it, and you're not able to transfer data successfully through that connection. It's sometimes useful to get a measurement tool that can tell you just how much signal is being transferred through a copper or fiber connection. Not only do you have to worry about the distance of the cable, you also have to be concerned about things that may be affecting the signal inside of the cable. This would be EMI, or Electromagnetic Interference, and this EMI could be created by many different components. You first need to be sure that the cable you're using is one that is not damaged. You don't want to twist or pull or stretch it as you're installing it into the wall or the ceiling. You want to be sure that the bend radius does not exceed the manufacturer's specifications. And never use staples when fastening cable to any part of your building, make sure you use cable ties or Velcro. With copper cables, you'll find that electromagnetic interference comes from many different sources, including power cords, fluorescent lights, or anywhere there may be some type of power system. For example, you would never run a copper cable in a ceiling over a fluorescent light because you're certain to have an excessive amount of EMI. This is why it's very common after installing a copper or fiber connection that you use a monitoring tool to be able to examine how much signal you're able to get from one side of the cable to the other. Now that we've run these cables and tested them and made sure there was not an excessive amount of EMI, we need to add the connectors. But we need to make sure that we're using the correct pin-outs on those connectors. With those small wires and connectors, it's very easy to mix up some of the wires as you're crimping down that connector. So it's useful to have some type of testing device that can show you the exact pin-outs for everything on that cable. This is why it's common to use specialists who run a lot of cable because they know exactly how to punch down and add connectors, so that you don't run into these pin-out problems. If you were to look at the pin-outs of a patch cable, you would commonly see it as a straight through cable. Pin 1 connects to pin 1 on the other side, pin 2 to pin 2, pin 3 to pin 3, and so on. If you do have problems with some of those pin-outs, you may find that you have performance problems or connectivity issues. You may find that your gigabit connection will not connect at 1 gig but, instead, connects at 100 megabits per second. Or you may find that you're not able to get a link light at all. If you make just a minor problem with pin-outs and connect pin 1 to pin 2 and pin 2 to pin 1, you'll find that you have no connectivity on your ethernet connection. Sometimes the hardware of our devices will go bad, and you'll start to see interface errors. Most devices can show you what errors it may have found on a particular interface, so if you're troubleshooting a problem and you think it might be related to errors, you'll be able to get some detailed statistics to help you troubleshoot. You should also verify the ethernet configuration and make sure the speed, the duplex, and the VLAN configurations are correct on the switch and the workstation. If you do have a link light, you may want to confirm that you're able to send data in both directions down that cable to verify that there is, indeed, end-to-end connectivity. If you verify that the cable is wired correctly but you're still not able to get very good throughput in that connection, you may have an interface configuration problem. You need to look at the duplex and the speed and make sure they match on both sides. If you have a mismatch of those, you may see no connectivity at all. There's no link light on your network interface cards, and you're not able to transfer any data over that connection. Or maybe you do have a link light, and you do see some activity lights, but you're still not getting any connectivity. In that case, you may have a configuration issue with the VLAN or some other setting that's on your switch. When you're configuring your switch and your workstations, you have the option to tell it to use an automatic configuration or a manual configuration. This is more of a personal preference and, usually, the automatic configuration works fine. But if you do find that the automatic on your switch and your device are giving you different values on both sides, you may want to configure manual settings and make sure that the speed and the duplex are exactly the same. If you initially look at the interface and there is no lights whatsoever-- there's no link light, and there's certainly no activity light-- then you probably do have a problem with either the interface or the cable itself. It could be that the speed settings on both sides are set incorrectly, and if there is a mismatch of speed, you won't see any link light. You will see a link light if you have a mismatch of duplex, which means one side is set for full duplex, and the other side is set for half duplex. If you do have a duplex mismatch, you'll find that the speed will be much slower than normal, and there will be an increase in late collisions in your error counters. Sometimes when cables are moved around a lot, you'll find that the internal insulators will wear away, and wires will start to touch each other. We refer to that as a short circuit. And that will certainly cause some type of communication problem over your Ethernet connection. If you have a complete disconnection of that wire, we refer to that as an open circuit. That is a broken wire, and it will need to be repaired to have that link work again. In both the short circuit and an open circuit, you may have a complete interruption of signal. And it may be difficult to troubleshoot exactly which one of these might be occurring. If you're able to plug in the wire and wiggle it just the right way and, suddenly, see a link light occur, then you probably have either a short circuit, or you're able to somehow get that open circuit connected over a temporary period. In both of those cases, it may be easiest just to replace the cable, especially patch cables, which can be removed very quickly and replaced in just a matter of seconds. If the problem is in the wall or in the ceiling, this may be more difficult to troubleshoot. You may want to use a TDR which can tell you exactly how far down that cable you might have a short or an open. If you're using fiber, you may be using a transceiver in your switch or your router to connect that fiber into the device. You have to make sure that the transceiver matches the type of fiber you're using. So if you're using single mode fiber, you want to be sure you're using a single mode transceiver. You also have to make sure that the wavelength you're using for the transceiver matches the wavelength that's being sent over that fiber. So if you're using 850 nanometers, you need to make sure that your transceiver is for 850 nanometers. If you're using transceivers on both sides, you need to make sure that those transceivers match on that entire connection. If they don't match, you will have dropped frames. You'll have a loss of signal. And you may have no signal at all over that fiber connection. This is an example of how easy it is to have a mismatch of transceivers. You have a pocket full of these. You plug one into one side, and one into the other, and you connect your fiber. Except in this case, one of the transceivers is for 1,310 nanometers, and the other is for 850. Otherwise, these transceivers look exactly the same, and you wouldn't know unless you looked at the specifications on the transceiver itself. If you're configuring a gigabit ethernet connection, you may find that you have many options for speed. You might want to configure it for 10 megabit, 100 megabit, 1,000 megabit, or have it automatically configure the speed. On the duplex side, you have an option between half duplex, full duplex, or, again, have it autoconfigure the duplex. If you set your systems for autoconfiguration, you would expect that both sides would have exactly the same speed. But unfortunately, autoconfiguration sometimes will set one end to one speed, and the other end to something else. This will certainly give you less than expected throughput and, in some cases, will not provide you with any link whatsoever. A similar situation exists with the duplex. You want to be sure that both sides match the duplex. So if one side is set to full duplex, the other side should also be set to full duplex. If you do have a mismatch in duplex, you will see that you will have significant slowdowns. And if you were to look at the error counters on those devices, you'll probably see that the late collision counter continues to increase as you send data over that connection. Another easy mistake to make is reversing the transmit and receive pairs on your RJ45 connection. This is an example of the crossed pairs. You can see that pin 1 does not connect to pin 1, and pin 2 does not connect to pin 2, which means we are crossing some pairs in this connection. This may be occurring in the connectors on the end of the cable, or someone may have simply punched down the wrong wires into the punchdown block. This is very easy to find with a cable tester. It will show you exactly what your wire map happens to be. And then you'll be able to hunt down where the switch is occurring, somewhere between point A and point B. In some cases, you may not even realize that you've reversed transmit and receive because some ethernet devices will automatically correct this type of connection. This is called Auto-MDIX, and it's a standard with many ethernet devices. So although you made a mistake in the wiring and you have crossed pairs, one of your ethernet devices is uncrossing the signal, and it's able to work as normal. If you don't have a device that can automatically correct this issue, then you may find that you're getting no signal across the link whatsoever. You may want to try turning on Auto-MDIX, if your device supports it, to see if you, at least, can get a link light. Ultimately, you'll need to identify where the switch occurred. so you'll need to go to every step along the way, look at your patch cables, look at the punchdown blocks, and make sure from end-to-end that you have all of the pins connected to the same wires. If you're using fiber, which relies on light being sent over a connection, then you need to make sure the connectors are not dirty. When you first receive the fiber, you'll notice that there are dust caps over the end of the connection. And when you plug it into your device, you may want to clean it first with a fiber cleaner and then connect it to the device. This will remove any dirt from that connection and limit the amount of attenuation. Sometimes you can use external alcohol swabs or a device like this one, where you can simply plug in the fiber connection and then connect it to your device to keep that connector clean.