Lab 2

Networking and Telecomunications

Looking Inside Your Cable

One of the most commonly used types of local network cable is Category 5 unshielded twisted pair cable, commonly called “Cat 5.” Cat 5 (and an enhanced version called Cat 5e) are used in Ethernet LANs. If you have installed a LAN in your house or apartment, you probably used Cat 5 or Cat 5e. Figure 1 below shows a picture of a typical Cat 5 cable. Each end of the cable has a connector called an RJ-45 connector that enables the cable to be plugged into a computer or network device. If you look closely at the connector you will see there are 8 separate “pins.” You might think that this would mean the Cat 5 can transmit data in parallel, but it doesn't do this. Cat 5 is used for serial transmission.


RJ45.jpg

Figure 1 - CAT-5 with RJ45 connector

The instructor will supply a short piece of Cat 5 cable for each of you. Figure 2 shows the Cat 5 cable with the connector cut off. You can see why twisted pair is called twisted pair: a single Cat 5 cable contains four separate sets of twisted pair wires for a total of eight wires.


Cat5.jpg

Figure 2: CAT-5 Cable

Figure 3 lists the different colors of the wires and what they are used for under the EIA/TIA 568B standard (the less common 568A standard uses the pins in different ways). One pair of wires (connected to pins 1 and 2) is used to transmit data from your computer into the network. When your computer transmits, it sends the same data on both wires; pin 1 (transmit+) transmits the data normally and pin 2 (transmit–) transmits the same data with reversed polarity. This way if an error occurs, the hardware will likely detect a different signal on the two cables. For example, if there is a sudden burst of electricity with a positive polarity (or a negative polarity), it will change only one of the transmissions from negative to positive (or vice versa) and leave the other transmission unchanged. Electrical pulses generate a magnetic \x{fb01}eld that has very bad side effects on the other wires. To minimize this, the two transmit wires are twisted together so that the other wires in the cable receive both a positive and a negative polarity magnetic \x{fb01}eld from the wires twisted around each other, which cancel each other out.

Pin number Color (EIA/TIA 568B standard) Name
1 White with orange stripe Transmit +
2 Orange with white stripe or solid orange Transmit –
3 White with green stripe Receive +
4 Blue with white stripe or solid blue Not used
5 White with blue stripe Not used
6 Green with white stripe or solid green Receive –
7 White with brown stripe or solid brown Not used
8 Brown with white stripe or solid brown Not used

FIGURE 3.29 Pin connection for Cat 5 at the computer end

Figure 3 also shows a separate pair of wires for receiving transmissions from the network (pin 3 (receive+) and pin 6 (receive–)). These wires work exactly the same way as transmit+ and transmit− but are used by the network to send data to your computer. You’ll notice that they are also twisted together in one pair of wires, even though they are not side-by-side on the connector.

Figure 3 shows the pin functions from the viewpoint of your computer. If you think about it, you’ll quickly realize that the pin functions at the network end of the cable are reversed; that is, pin 1 is receive+ because it is the wire that the network uses to receive the transmit+signal from your computer. Likewise, pin 6 at the network end is the transmit− wire because it is the wire on which your computer receives the reversed data signal.

The separate set of wires for transmitting and receiving means that Cat 5 is designed for full-duplex transmission. It can send and receive at the same time because one set of wires is used for sending data and one set for receiving data. However, Cat 5 is not often used this way. Most hardware that uses Cat 5 is designed to operate in a half-duplex mode, even though the cable itself is capable of full duplex. You’ll also notice that the other four wires in the cable are not used. Yes, that’s right; they are simply wasted.

Assignment - Make a cable

Steps

Step 1

Make a Network Cable Step 1.jpg

Unroll the required length of network cable and add a little extra wire, just in case. If a boot is to be fitted, do so before stripping away the sleeve and ensure the boot faces the correct way.

Step 2

Make a Network Cable Step 2.jpg

Carefully remove the outer jacket of the cable. Be careful when stripping the jacket as to not nick or cut the internal wiring. One good way to do this is to cut lengthwise with snips or a knife along the side of the cable, away from yourself, about an inch toward the open end. This reduces the risk of nicking the wires' insulation. Locate the string inside with the wires, or if no string is found, use the wires themselves to unzip the sheath of the cable by holding the sheath in one hand and pulling sideways with the string or wire. Cut away the unzipped sheath and cut the twisted pairs about 1 1/4" (30 mm). You will notice 8 wires twisted in 4 pairs. Each pair will have one wire of a certain color and another wire that is white with a colored stripe matching its partner (this wire is called a tracer).

Step 3

Make a Network Cable Step 3.jpg

Inspect the newly revealed wires for any cuts or scrapes that expose the copper wire inside. If you have breached the protective sheath of any wire, you will need to cut the entire segment of wires off and start over at step one. Exposed copper wire will lead to cross-talk, poor performance or no connectivity at all. It is important that the jacket for all network cables remains intact.

Step 4

Make a Network Cable Step 4.jpg

Untwist the pairs so they will lay flat between your fingers. The white piece of thread can be cut off even with the jacket and disposed (see Warnings). For easier handling, cut the wires so that they are 3/4" (19 mm) long from the base of the jacket and even in length.

Step 5

Arrange the wires based on the wiring specifications you are following. There are two methods set by the TIA, 568A and 568B. Which one you use will depend on what is being connected. A straight-through cable is used to connect two different-layer devices (e.g. a hub and a PC). Two like devices normally require a cross-over cable. The difference between the two is that a straight-through cable has both ends wired identically with 568B, while a cross-over cable has one end wired 568A and the other end wired 568B. [1] For our demonstration in the following steps, we will use 568B, but the instructions can easily be adapted to 568A.

  • 568B - Put the wires in the following order, from left to right:

    Make a Network Cable Step 5Bullet1.jpg

    • white orange
    • orange
    • white green
    • blue
    • white blue
    • green
    • white brown
    • brown
  • 568A - from left to right:

    Make a Network Cable Step 5Bullet2.jpg

      • white/green
      • green
      • white/orange
      • blue
      • white/blue
      • orange
      • white/brown
      • brown

Step 6

Make a Network Cable Step 6.jpg

You can also use the mnemonic 1-2-3-6/3-6-1-2 to remember which wires are switched.

Step 7

Make a Network Cable Step 7.jpg

Press all the wires flat and parallel between your thumb and forefinger. Verify the colors have remained in the correct order. Cut the top of the wires even with one another so that they are 1/2" (12.5 mm) long from the base of the jacket, as the jacket needs to go into the 8P8C connector by about 1/8", meaning that you only have a 1/2" of room for the individual cables. Leaving more than 1/2" untwisted can jeopardize connectivity and quality. Ensure that the cut leaves the wires even and clean; failure to do so may cause the wire not to make contact inside the jack and could lead to wrongly guided cores inside the plug.

Step 8

Make a Network Cable Step 8.jpg

Keep the wires flat and in order as you push them into the RJ-45 plug with the flat surface of the plug on top. The white/orange wire should be on the left if you're looking down at the jack. You can tell if all the wires made it into the jack and maintain their positions by looking head-on at the plug. You should be able to see a wire located in each hole, as seen at the bottom right. You may have to use a little effort to push the pairs firmly into the plug. The cabling jacket should also enter the rear of the jack about 1/4" (6 mm) to help secure the cable once the plug is crimped. You may need to stretch the sleeve to the proper length. Verify that the sequence is still correct before crimping.

Step 9

Make a Network Cable Step 9.jpg

Place the wired plug into the crimping tool. Give the handle a firm squeeze. You should hear a ratcheting noise as you continue. Once you have completed the crimp, the handle will reset to the open position. To ensure all pins are set, some prefer to double-crimp by repeating this step.

Step 10

Make a Network Cable Step 10.jpg

Repeat all of the above steps with the other end of the cable. The way you wire the other end (568A or 568B) will depend on whether you're making a straight-through, rollover, or cross-over cable (see Tips).

Step 11

Make a Network Cable Step 11.jpg

Test the cable to ensure that it will function in the field. Mis-wired and incomplete network cables could lead to headaches down the road. In addition, with power-over-Ethernet (PoE ) making its way into the market place, crossed wire pairs could lead to physical damage of computers or phone system equipment, making it even more crucial that the pairs are in the correct order. A simple cable tester can quickly verify that information for you. Should you not have a network cable tester on hand, simply test connectivity pin to pin.

Deliverable

  1. Create a straight through cable. Test it out with the tester device. Show the instructor that the cable work.
  2. Turn in a write-up on your experience. What worked? What didn't? What was your biggest struggle? Did you learn anything new? What?
Topic attachments
I Attachment Action Size Date Who Comment
Jpgjpg Cat5.jpg manage 4.4 K 2014-09-13 - 03:05 JimSkon CAT-5 Cable
Jpgjpg RJ45.jpg manage 4.3 K 2014-09-13 - 03:04 JimSkon Cat-5 with RJ45 connector
Topic revision: r2 - 2014-09-14 - JimSkon
 
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