CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [402]
Introducing Ethernet
A consortium of companies including Digital Equipment, Intel, and Xerox invented the first network in the mid 1970s. More than just creating a network, they wrote a series of standards that defined everything necessary to get data from one computer to another. This series of standards was called Ethernet, and it is the dominant standard for today’s networks. Ethernet comes in two main flavors defined by cabling type: unshielded twisted pair and fiber optic. Because all flavors of Ethernet use the same packet type, you can have any combination of hardware devices and cabling systems on an Ethernet network and all of the PCs will be able to communicate just fine.
Most modern Ethernet networks employ one of three technologies (and sometimes all three), 10BaseT, 100BaseT, or 1000BaseT. As the numbers in the names suggest, 10BaseT networks run at 10 Mbps, 100BaseT networks run at 100 Mbps, and 1000BaseT networks—called Gigabit Ethernet—run at 1000 Mbps, or 1 Gbps. All three technologies—sometimes referred to collectively as 10/100/1000BaseT or just plain Ethernet—use a star bus topology and connect via a type of cable called unshielded twisted pair (UTP).
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NOTE Earlier forms of Ethernet used coaxial cable: 10Base5 used RG-8 coaxial cables with DB-15 connectors called AUI connectors and could run up to 500 meters; 10Base2 used RG-62 cable with BNC connectors and could run up to 185 meters.
Star Bus
Imagine taking a bus network and shrinking the bus down so it will fit inside a box. Then, instead of attaching each PC directly to the wire, you attach them via cables to special ports on the box (Figure 23-5). The box with the bus takes care of all of the tedious details required by a bus network. The bus topology would look a lot like a star topology, wouldn’t it?
The central box with the bus is called a hub or switch. The hub provides a common point for connection for network devices. Hubs can have a wide variety of ports. Most consumer-level hubs have four or eight, but business-level hubs can have 32 or more ports. A hub is the old-style device, rarely used in today’s networks. A switch is a far superior and far more common version of a hub. Figure 23-6 shows a typical consumer-level switch.
Different networks come in different speeds. One common speed is 100 Megabits per second (Mbps). We call this speed a bandwidth. If you put 32 PCs on a 32-port 100 Mbps hub, you have 32 PCs sharing the 100 Mbps bandwidth. A switch addresses that problem by making each port its own separate network. Each PC gets to use the full bandwidth available. The bottom line? Swap out your old hubs for newer switches and you’ll dramatically improve your network performance.
Figure 23-5 Star bus
Figure 23-6 A switch
Cheap and centralized, a star bus network does not go down if a cable breaks. True, the network would go down if the hub itself failed, but that is rare. Even if a hub fails, replacing a hub in a closet is much easier than tracing a bus running through walls and ceilings and trying to find a break!
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EXAM TIP Although Token Ring is very rarely seen today, CompTIA wants you to know a little of its terminology. Just as in Ethernet, the creators of Token Ring decided to move to a star topology and put the ring (as opposed to Ethernet’s bus) in a box that looks like a hub/switch. They call the box the media access (or sometimes attachment) unit (MAU). Some techs call it a Multistation Access Unit (MSAU), but CompTIA uses MAU, so remember that for the exam!
Unshielded Twisted Pair
Unshielded twisted pair (UTP) cabling is the specified cabling for 10/100/1000BaseT and is the predominant cabling system used today. Many types of twisted pair cabling are available, and the type