Twisted-pair cable is the cable most commonly used in local area networks. It’s relatively easy to work with, flexible, efficient, and fast. As a network administrator, you should know how to identify the different types of twisted-pair cabling, as well as how to install twisted-pair cabling in a permanent fashion and as a temporary solution. It’s also important to know how to test twisted-pair cables in case one fails or as a way of proving that new installations work properly.
Twisted-pair cables are the most common of all copper-based cables. A single twisted-pair cable has eight wires; they are copper conductors that transmit electric signals. These eight wires are grouped into four pairs: blue, orange, green, and brown. Each pair of wires is twisted along the entire length of the cable, and all of the pairs are twisted together as well. The reason the wires are twisted is to reduce crosstalk and interference, which are described later
EXAMINE TWISTED-PAIR PATCH CABLES
In this exercise, you will examine a patch cable connected to either your computer or the central connecting device for your network.
1. Examine the back of your computer and locate the network adapter. There should be a twisted-pair patch cable that connects the network adapter to the network. If not, and if you use a wireless connection, examine the back of your central connecting device, whether it’s a router, switch, or hub. Identify the patch cable that connects to that device. If you decide to disconnect the cable, keep in mind that the Internet connection will be temporarily lost and any downloads will be stopped. The cable should look something like the one in Figure 3-1, which is shown from the side of the RJ45 plug. You can see where the cable itself enters the plug and where the plastic sheath is cut, exposing the individual wires. Also notice the teeth that bite into the plastic jacket (they are shown in the black rectangle). Once the plug is crimped onto the cable, these teeth ensure that the cable does not slip out of the plug.
Figure 3-1 Twisted-pair patch cable
If you have some extra twisted-pair cable handy, cut a six-foot piece with a sharp cutting tool. Then, strip away about two inches of the plastic jacket to expose the wires.(The plastic jacket is also known as a plastic or PVC sheath.) You should see somethingsimilar to Figure 3-2, which illustrates the four twisted-pair wires. Once again,these four pairs are blue, orange, green, and brown, also known as the BOGB colors.Each letter represents a color: B = blue, O = orange, and so on.
Figure 3-2 Twisted-pair cable with the wires exposed
3. Untwist each of the wires so that they are all separated. The wires should now look similar to Figure 3-3. In the figure, the wires are in the proper order for most of today’s twisted-pair networks. Table 3-1 summarizes the cabling standards when it comes to wire (or pin) orientation. Whereas the BOGB standard is where everything originates from, 568B is the most common, and 568A is an older standard. The proper name for 568B is TIA/EIA-568-B; this standard was developed by the Telecommunications Industry Association/Electronics Industries Alliance or TIA/EIA. When making a patch cable, the wires are placed in the RJ45 plug in order, and the plug is crimped once they are in place. If a particular wire is named white/orange, that means the bulk of the wire is white in color and it has an orange stripe. If the wire is named something like orange, it is a solid orange wire.
Figure 3-3 Twisted-pair cable with the wires straightened
There are two types of networking patch cables that you might work with. The first is a straight through cable. This is the most common type of patch cable, and it is the type that you would use to connect a computer to a central connecting device like a switch. It’s called “straight through” because the wires on each end of the cable are oriented in the same way.Generally, this is a 568B on each end. However, there is also another type of patch cable—the crossover cable. This type is used to connect like devices to each other, for example, a computer to another computer, or a switch to another switch. In this case, the patch cable is wired with the 568B standard on one side and the 568A standard on the other. To make a patch cable,you use a cutting tool, wire stripper, RJ45 crimper, RJ45 plugs, and a patch tester. These tools are illustrated in Figure 3-4.
Generally, Ethernet transmits data signals on the orange and green wires. This means pins one, two, three, and six. Other technologies use different pairs or possibly all four pairs of wires. Usually, twisted-pair networks are wired to the 568B standard. This means that all wiring equipment must comply with 568B, including patch panels, RJ45 jacks, patch cables,and the actual termination of wiring to each of these devices. To be more specific, the orange pair has a and – wire, also known as tip and ring (old telco terminology). The green pair is similar. The orange pair transmits data, and the green pair receives it. If the connection is half duplex, only one of these pairs works at any given time. But if the connection is full duplex, both pairs work simultaneously.
Network adapters normally have an MDI port; this stands for medium dependent interface.However, in order for computers to communicate with other devices, the wires have to cross somewhere. In any crossed connection, pin one crosses to pin three, and pin two crosses to pin six. But instead of using crossover cables to connect computers to central connecting devices such as switches, these central connecting devices are equipped with MDI-X ports (medium dependent interface crossover), which take care of the cross. This is how straight through cables can be used to connect computers to the central connecting device, which is much easier, plus these cables are cheaper to manufacture. This is why a crossover cable is needed if you want to connect one computer to another computer directly, or a switch to another switch directly. However, some switches have a special auto MDI/MDIX port that senses whether you’re trying to connect one switch to another switch with a straight through cable or a crossover cable. In other cases, the special port has a button that allows you to select whether it acts as a MDIX or a MDI port.
Patch cables are a temporary solution. They are meant to be unplugged and plugged in as necessary. Therefore, most companies also have permanent cabling solutions. For example, consider a connection between a patch panel in the server room and an RJ45 jack at a computer workstation. Figure 3-5 shows examples of both of these pieces of equipment.
The cable that connects these two pieces of equipment has the individual wires permanently punched down so that they are immovable. The front of a patch panel simply has a lot of RJ45 ports. The patch panel works great if a computer is moved to a different area of an office; the patch cable can simply be moved to the correct port on the patch panel.
The tools necessary to make the connections between patch panels and RJ45 jacks include a cutting tool, a wire stripper, a punch down tool, and a testing device known as a continuity tester, which tests all of the pins of a connection one by one. The tester lets you know whether any of the pins are mis-wired. It does this by testing the entire cable from end to end. The testing device is connected to one end of the run, and a terminating device connects to the other end; signals are bounced back and forth on every wire or pin. These last two tools are illustrated in Figure 3-6. Generally, twisted-pair cables can be run 100 meters before the signal degrades to such a point that it cannot be interpreted by the destination host. This is known as attenuation. If a cable needs to be run farther, a signal repeater, a hub, or switch can be used. Otherwise, fiber optic cable is the solution because it can be run much farther
than twisted-pair cable.
Twisted-pair cables are categorized according to the frequency at which they transmit signals and their data transfer rate or speed. Table 3-2 describes the different categories of twistedpair cable and the types of network speed they can accommodate.
Category 5e is usually rated at 350 MHz, but the actual speed varies depending on several different networking factors. Category 6 already has different versions that run at 250 MHz and 500 MHz. Due to the different types of category 5e and category 6, it is better to simply say that these are rated for 100 Mbps networks and gigabit networks. Take a look at one of your network cables now. Quite often, the category type is printed directly on the plastic jacket of the cable. For today’s networks, category 3 (and even category 5) is not adequate; category 5e or higher is necessary for current high-bandwidth applications.
Interference can be a real problem with twisted-pair networks, or any networks for that matter.Interference is anything that disrupts or modifies a signal that is traveling along a wire.There are many types of interference, but there are only a few you should know for the exam,including the following:
• Electromagnetic interference (EMI): This is a disturbance that can affect electrical circuits, devices, and cables due to electromagnetic conduction and possibly radiation.Just about any type of electrical device causes EMI: TVs, air conditioning units, motors, unshielded electrical cables (Romex), and so on. Copper-based cables and network devices should be kept away from these electrical devices and cables if at all possible. If this is not possible, shielded cables can be used, for example shielded twisted-pair (STP) cables. STP cables have an aluminum shield inside the plastic jacket that surrounds the pairs of wires. Alternatively, the device that is emanating EMI can be shielded. For example, anair conditioning unit could be boxed in with aluminum shielding in an attempt to keep the EMI generated by the AC unit’s motor to a minimum. In addition, electrical cables should be BX (encased in metal) and not Romex (not encased in metal); in fact, most states require this to meet industrial and office space building code.
• Radio frequency interference (RFI): This is interference that can come from AM/FM transmissions and cell phone towers. It is often considered part of the EMI family and is sometimes even referred to as EMI. The closer a business is to one of these towers, the greater the chance of interference. The methods mentioned in the EMI bullet can be employed to help defend against RFI. In addition, filters can be installed on the network to eliminate the signal frequency being broadcast by a radio tower, although this will usually not affect standard wired Ethernet networks.
One serious issue with data networks, especially networks with copper-based cabling is data emanation (also known as signal emanation). This is the electromagnetic (EM) field that is generated by a network cable or network device, which can be manipulated to eavesdrop on conversations or to steal data. Data emanation is sometimes also referred to as eavesdropping in itself, although this is not accurate. Data emanation is the most commonly seen security risk when using coaxial cable, but it can also be a security risk for other copper-based cables such as twisted pair. There are various ways to tap into these (EM) fields in order to get unauthorized
access to confidential data. To alleviate the situation, you could use shielded cabling or run the cabling through metal conduits. You could also use electromagnetic shielding on devices that might be emanating an electromagnetic field. This could be done on a small scale by shielding the single device, or on a larger scale by shielding an entire room, perhaps a server room. This would be an example of a Faraday cage.
Another common type of interference is crosstalk. Crosstalk is when the signal that is transmitted on one copper wire or pair of wires creates an undesired effect on another wire or pair of wires. Crosstalk first occurred when telephone lines were placed in close proximity to each other. Due to the fact that the lines were so close, the signal could jump from one line to the next intermittently. If you have ever heard another conversation while talking on your home phone (and not a cell phone), then you have been the victim of crosstalk. If the signals are digital (e.g., Ethernet data transfers or voice over IP), you already have an environment that is less susceptible to crosstalk. Data can still bleed over to other wires, but it is less common.Sometimes this occurs because cables are bundled too tightly, which could also cause crimping or other damage to the cable. If this is the case, a continuity tester will let you know which cable has failed so that it can be replaced.
When it comes to twisted-pair cabling, crosstalk is broken down into two categories: near end crosstalk (NEXT) and far end crosstalk (FEXT). NEXT occurs when there is measured interference between two pairs in a single cable, measured on the cable end nearest the transmitter.FEXT occurs when there is similar interference, measured at the cable end farthest from the transmitter. If crosstalk is still a problem, even though twisted-pair cable has been employed and digital data transmissions have been implemented, shielded twisted pair (STP) could be used. Normally, companies opt for regular twisted-pair cabling, which is unshielded twisted pair (also known as UTP), but sometimes, there is too much interference in the environment to send data effectively, and STP must be utilized.
Cables that are installed inside walls or above drop ceilings where they cannot be accessed by sprinkler systems in the case of a fire should be plenum-rated or low-smoke rated. Plenumrated cables have a Teflon coating that makes them more impervious to fire. They are used in these situations because standard twisted-pair cables have a PVC jacket, which when burned can emit deadly gas into the air that ultimately gets breathed in as hydrochloric acid.
Finally, the physical plant should be grounded. Quite often, server rooms or wiring closets are the central connecting point for all the cabling. All of the cables are punched down to patch panels, which are screwed into data racks. These racks should be bolted to the ground and connected with 10 gauge or thicker grounding wire (usually with a green jacket) to a proper earth bonding point, such as an I-beam in the ceiling. This protects all of the cabling (and the devices it connects to) from surges, spikes, lightning strikes, and so on.
That was a lot of information about twisted-pair cabling. We could go on and on, but that should suffice for now.