Network Cabling: One Mark Questions, Book Back Answers and Notes

About Network Cabling

Medium ~60 min study

Wireless networks are everywhere, yet most offices, labs and server rooms still run on cables. A cable is harder to tap than a radio signal, gives steadier speed and does not drop out when a microwave oven is switched on. This unit looks at the hardware that carries data as electrical signals through copper or as light through glass fibre.

You start with the types of cable: coaxial cable of the kind used for cable TV, twisted pair cable used in most LANs, fibre optic cable for long and fast links, and serial and USB cables for connecting devices. Then you see how a LAN cable is actually built. The twisted pair patch cable is fitted with an RJ45 plug using a crimping tool, and the plug goes into the Ethernet port on the computer's network card.

This unit has a practical side, and the questions follow it. Be ready to name the number of pins on RJ45, RJ11 and RJ21, to arrange the steps of making a cable with a crimping tool, and to choose between straight-through, crossover and rollover wiring for a given pair of devices. The T568A and T568B colour orders are worth writing out a few times until you know them.

What you'll learn

• Describe coaxial, twisted pair and fibre optic cables and say where each is used.
• Name the parts of an Ethernet connection: patch cable, RJ45 connector, Ethernet port, Ethernet card and crimping tool.
• Write the T568A and T568B colour orders and state how they differ.
• Choose straight-through, crossover or rollover wiring for a given pair of devices.
• List in order the steps for making a patch cable with a crimping tool.
• State the pin counts and uses of RJ11, RJ14, RJ61, RJ21 and RJ45.

Before you start

• What a LAN is and how clients connect to a server.
• The common ports and cables on a desktop computer.
• The idea that data travels as signals, either electrical or light.

Network Cabling explained

Cables, Connectors and Wiring Methods

Types of Network Cable

A network cable is the physical path data takes between devices. The right type depends on distance, speed and how much electrical noise is around. Coaxial cable has a single copper conductor in the centre, surrounded by insulation, a metal shield and an outer jacket. It was used in early Ethernet and for cable TV, but it is slow by today's standards. Twisted pair cable has eight copper wires in four colour-coded pairs. Twisting each pair cancels out much of the outside interference. It comes as UTP (Unshielded Twisted Pair) and STP (Shielded Twisted Pair) and is the normal choice for LANs. Fibre optic cable carries pulses of light through thin strands of glass, so it is not affected by electromagnetic interference and works over long distances at high speed.

Parts of an Ethernet Connection

A wired Ethernet link needs four things. The patch cable is twisted pair cable graded by category, from CAT 3 through CAT 5, CAT 5e and CAT 6 up to CAT 7; higher categories support higher speeds. The RJ45 connector is the clear plastic plug on each end, with eight metal pins. It fits into the Ethernet port on the Ethernet card (network interface card), which may be built into the motherboard or added as an expansion card. The crimping tool presses the plug's pins down onto the wires so that each wire makes firm contact.

Registered Jacks

A Registered Jack (RJ) is a standard socket and plug design for telephone and data lines. RJ45 has eight pins and is used for Ethernet. RJ11 is the smaller plug used for landline telephones. It has six pin positions, of which usually only two carry the signal. RJ14 also has six positions and is used for telephones, while RJ61 has eight pins and uses twisted pair cable. RJ21 has 50 pins, 25 on each side, and is called a champ or Amphenol connector. It is used to bring many telephone lines into one cable.

Colour Codes and Wiring Methods

The eight wires must go into the RJ45 plug in a fixed order. Two standards set that order, T568A and T568B. The only difference between them is that the green and orange pairs swap places. For 10/100 Mbps Ethernet only pins 1, 2, 3 and 6 carry data: two pins transmit and two receive. A straight-through cable uses the same standard at both ends and connects different devices, such as a computer to a switch. A crossover cable uses T568A at one end and T568B at the other, which swaps transmit and receive, so two computers can be linked directly without a hub. A rollover cable reverses the pin order completely, pin 1 to pin 8 and so on. It is used as a console cable to configure a router or switch.

Common mistakes to avoid

• Mixing up RJ11 and RJ45. RJ11 is the smaller telephone plug with six pin positions; RJ45 is the larger Ethernet plug with eight pins.
• Untwisting too much of each pair when stripping the cable. Keep the untwisted length to about half an inch, otherwise interference increases.
• Choosing a crossover cable to connect a computer to a switch. Unlike devices need a straight-through cable; like devices, such as two computers, need a crossover cable.
• Pushing the wires into the plug in any order. Arrange them exactly by T568A or T568B before crimping, or the cable will not work.
• Skipping the test after crimping. Check the finished cable with a cable tester so a bad connection is found before it is installed.

Test yourself on these with the practice test, then check the worked reasoning in the solved MCQs.

Frequently asked questions

What is the difference between UTP and STP cables?

Both use twisted pairs of copper wire. UTP (Unshielded Twisted Pair) relies only on the twisting to reduce interference and is the usual choice for offices and homes. STP (Shielded Twisted Pair) adds a foil or braided shield around the pairs, which gives extra protection where there is heavy electrical noise, such as in factories.

Why is fibre optic cable better than copper cable?

Fibre carries data as light instead of electric current. Light is not disturbed by electromagnetic interference, so the signal stays clean. Fibre also supports much higher speeds and can run over far longer distances than copper before the signal becomes too weak.

When should I use a crossover cable instead of a straight-through cable?

Use a crossover cable to connect two devices of the same type directly, such as two computers. Use a straight-through cable to connect two different types of device, such as a computer to a switch or hub. Remember it as: same devices, crossover; different devices, straight-through.

What is a rollover cable used for?

A rollover cable, also called a console cable, has its pin order reversed from one end to the other. It does not carry normal network traffic. It connects a computer to the console port of a router or switch so that an administrator can configure the device.

What do the LED lights on an Ethernet port show?

When you plug a cable into the Ethernet port of a working network card, two LEDs light up, one green and one orange. The green light shows that a link has been made. The orange light blinks when the connection is active, showing that data is being sent and received.

What is an RJ21 connector and where is it used?

RJ21 is a connector with 50 pins, 25 on each side, which can terminate 25 pairs of wires. It is also called a champ or Amphenol connector, after the company that makes it. It is used in telephone systems to carry many phone lines through a single cable.

Why does Ethernet use only four of the eight wires?

For 10 Mbps and 100 Mbps Ethernet, only two pairs are needed: one pair to transmit and one to receive. These are the orange and green pairs, connected to pins 1, 2, 3 and 6 of the RJ45 plug. The other two pairs are left unused. Gigabit Ethernet uses all four pairs.