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 A+ Essentials (220-801) Cram Notes


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2.9 Compare and contrast network devices, their functions, and features

   1. Most commonly used network devices are hubs, switches (or bridges), and routers.

a. Router: A router for Internet sharing is normally configured using web browser. High-end routers may provide option for terminal connectivity, wherein you can connect a terminal, and issue commands for configuring the router.

b. Hub: A hub is basically a multi-port repeater. When it receives a packet, it repeats that packet out each port. This means that all computers that are connected to the hub receive the packet whether it is intended for them or not. It's then up to the computer to ignore the packet if it's not addressed to it. This might not seem like a big deal, but imagine transferring a 50 MB file across a hub. Every computer connected to the hub gets sent that entire file (in essence) and has to ignore it.

c. Bridge: A bridge is a kind of repeater, but it has some intelligence. It learns the layer 2 (MAC) addresses of devices connected to it. This means that the bridge is smart enough to know when to forward packets across to the segments that it connects. Bridges can be used to reduce the size of a collision domain or to connect networks of differing
media/topologies, such as connecting an Ethernet network to a Token Ring network.

d. Switch: A switch is essentially a multi-port bridge. The switch learns the MAC addresses of each computer connected to each of its ports. So, when a switch receives a packet, it only forwards the packet out the port that is connected to the destination MAC address. Remember that a hub sends the packet out every port, and you can see how much more efficient this it.

2.10 Given a scenario, use appropriate networking tools

a)      An assortment of Phillips-head, flathead, and Torx screwdrivers, as well as varying sizes of nut drivers.       

b)     Several types of small parts used on circuit boards, including various sizes of screws and nuts; extra jumpers (the tiny devices used to make connections between the pins on certain circuitry); screws; and stand-offs, small washer-type parts made of a nonconducting material, often nylon, used to ensure that a motherboard does not come in contact with a computer case.      

c)      Several sizes of slot covers, the metal brackets that cover the openings (slots) in the back of a PC for accommodating expansion cards. A slot cover serves to keep dust out when an expansion slot is empty and helps cooling airflow.       

d)     A parts grabber, also called an extending extractor. This pen-sized tool has a plunger at one end, which, when pressed, causes small, hooked prongs to extend from the other end of the tool. These are useful for retrieving dropped objects, such as jumpers or screws, from inside a computer. Be very careful not to touch any circuitry when using one.       

e)      An extension magnet, a long-handled tool with a magnet on the end. Use it like a parts grabber, only it has a magnet that attracts small objects that contain iron. This is handy for picking up objects that fall on the floor, but the potential dangers may not be worth the convenience. Never use an extension magnet near a computer or any peripherals that contain magnetic storage because the magnet can damage data.   

f)       A flashlight for illuminating dark places.

g)      A small container for holding extra screws and other small parts (a pill bottle works well).

h)     An ESD wrist strap to use when working on any component except the power supply, monitor, and laser printers.

i)        An ESD mat provides a static charge with a path to ground and is designed for the desktop or floor of a workspace. While this mat may not fit in your toolkit, it is something that should be available at any PC technicianís workbench.       

j)       Field replaceable units (FRUs) should be included in your hardware toolkit. An FRU is any component that you can install into a system onsite. This includes such items as memory modules, heat sinks, CMOS batteries, various adapter cards, hard drives, optical drives, keyboards, mice, fans, AC adapters, spare cables and connectors, power supplies, and even spare motherboards. Of course, all of this depends on the scope of your job and how cost-effective it is to have these items on hand.       

k)     A multimeter is indispensable in determining power problems from a power outlet or from the power supply. Youíll use this handheld device to measure the resistance, voltage, and/or current in computer components using two probes (one negative, one positive) that you touch to power wires in the equipment you are testing.

l)        A power supply tester is a specialized device for testing a power supply unit, and is a bit safer to use than a multimeter for this purpose. A power supply tester comes with connectors compatible with the output connectors on a standard power supply, rather than with just the simple probes of a multimeter. An LCD display or LEDs show the test results.      

m)   A cable tester will detect if a cable can connect properly end to end and determine if there is a short. Several types of cable testers are available, such as those for copper Ethernet and phone cables, fiber-optic cable testers, and coaxial cable testers. A toner probe is a cable tester that generates a tone on one end of a cable and evaluates the signal received on the other end.

n)     A loopback plug, a plug wired to send signals back to a specific port type (serial, parallel, USB, etc.) or device, such as an Ethernet adapter, as a test of the device. It reroutes the sending pins from the port or device to the receiving pins, thus allowing you to test the ability to send and receive without connecting to an external device or network.     

o)      A digital camera will enable you to document the condition of a computer before you begin troubleshooting. One important way we use a digital camera is to document the cabling and connectionsóboth external and internalóbefore making any changes, so that we can reconnect all components correctly. A camera is also handy for capturing low-level error messages that cannot be captured otherwise.       

p)     Wire cutters for cutting various types of wires, but especially for Ethernet cables.   

q)     A punch down tool is a hand tool with a screwdriver-type handle and one or more specialized blades used for inserting various types of wire into appropriate wiring panels. The wiring may be for electrical power or network cabling.       

r)      A crimper, also called a crimp tool, resembles a pair of pliers, but is used to terminate a multistranded cable into a connector, clamping each wire in place in the connector so that the wires line up with the wires in the connector.      

s)       A cable stripper is a cross between a pair of pliers and scissors and is designed to strip the insulation from around the wires in a cable.

t)       A POST card is an adapter card used to run a special diagnostic test on a computer as it powers up. These tests go beyond those performed by the computerís own BIOS-based testing that occurs as it starts up.

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