CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [405]
No standard governs how NIC manufacturers use their lights; as a result, they come in an amazing array of colors and layouts. When you encounter a NIC with a number of LEDs, take a moment to try to figure out what each one means. Although different NICs have different ways of arranging and using their LEDs, the functions are always the same: link, activity, and speed.
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EXAM TIP Though no real standard exists for NIC LEDs, CompTIA will test you on some more-or-less de facto LED meanings. You should know that a solid green light means connectivity, a flashing green light means intermittent connectivity, no green light means no connectivity, and a flashing amber light means there are collisions on the network (which is sometimes okay). Also, know that the first things you should check when having connectivity issues are your NIC’s LEDs.
Fiber Optic Ethernet
Fiber optic cable is a very attractive way to transmit Ethernet network packets. First, because it uses light instead of electricity, fiber optic cable is immune to electrical problems such as lightning, short circuits, and static. Second, fiber optic signals travel much farther, up to 2000 meters (compared with 100 meters on UTP) with some standards. Most fiber Ethernet networks use 62.5/125 multimode fiber optic cable. All fiber Ethernet networks that use these cables require two cables. Figure 23-13 shows three of the more common connectors used in fiber optic networks. Square SC connectors are shown in the middle and on the right, and the round ST connector is on the left.
Figure 23-13 Typical fiber optic cables with connectors
Like many other fiber optic connectors, the SC and ST connectors are half-duplex, meaning data flows only one way—hence the need for two cables in a fiber installation. Other half-duplex connectors you might run into are FC/PC, SMA, D4, MU, and LC. They look similar to SC and ST connectors but offer variations in size and connection. Newer and higher-end fiber installations use full-duplex connectors, such as the MT-RJ connectors.
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EXAM TIP ST, SC, LC and MT-RJ fiber connectors will likely be questioned on both exams.
Light can be sent down a fiber optic cable as regular light or as laser light. Each type of light requires totally different fiber optic cables. Most network technologies that use fiber optics use LEDs to send light signals. These use multimode fiber optic cabling. Multimode fiber transmits multiple light signals at the same time, each using a different reflection angle within the core of the cable. The multiple reflection angles tend to disperse over long distances, so multimode fiber optic cables are used for relatively short distances.
Network technologies that use laser light use single-mode fiber optic cabling. Using laser light and single-mode fiber optic cables allows for phenomenally high transfer rates over long distances. Except for long-distance links, single-mode is currently quite rare; if you see fiber optic cabling, you can be relatively sure it is multimode.
The two most common fiber optic standards are called 1000BaseSX and 10GBaseSR. The major difference is the speed of the network (there are also some important differences in the way systems interconnect, and so on). Fiber optic cabling is delicate, expensive, and difficult to use, so it is usually reserved for use in data centers and is rarely used to connect desktop PCs.
Coax/BNC
Early versions of Ethernet ran on coaxial cable instead of UTP. Coax consists of a center cable (core) surrounded by insulation. This in turn is covered with a shield of braided cable. The inner core actually carries the signal. The shield effectively eliminates outside interference. The entire