Ethernet 10BASET Cabling

The PCs and hub in Figure 3-5 typically use Category 5 UTP cables with RJ-45 connectors, as shown in Figure 3-2. The Ethernet cards in each PC have an RJ-45 connector, as does the hub; these connectors are larger versions of the same type of connector used for telephone cords between a phone and the wall plate in the United States. So, connecting the Ethernet cables is as easy as plugging in a new phone at your house.

The details behind the specific cable used to connect to the hub are important in real life as well as for the INTRO exam. The detailed specifications are covered in Chapter 11, and the most typical standards are covered here. You might recall that Ethernet specifies that the pair of wires on pins 1 and 2 is used to transmit data, and pins 3 and 6 are used for receiving data. The PC Ethernet cards do indeed use the pins in exactly that way.

The cable used to connect the PCs to the hub is called a straight-through cable, as shown back in Figure 3-2. In a straight-through cable, the wire connected to pin 1 on one end of the cable is connected to pin 1 on the other side, pin 2 is connected to pin 2 on the other end, and so on. Therefore, when Larry sends data on the pair on pins 1 and 2, the hub receives the electrical signal over the straight-through cable on pins 1 and 2. So, for the hub to correctly receive the data, the hub must think oppositely, as compared to the PC—in other words, the hub receives data on pins 1 and 2, and transmits on pins 3 and 6. Figure 3-6 outlines how it all works.

Figure 3-6 Straight-Through Ethernet Cable with Exaggerated RJ-45 Connectors

RJ-45 RJ-45

Figure 3-6 Straight-Through Ethernet Cable with Exaggerated RJ-45 Connectors

RJ-45 RJ-45

For example, Larry might send data on pins 1 and 2, with the hub receiving the signal on pins 1 and 2. The hub then repeats the electrical signal out the other ports, sending the signal to Archie and Bob. The hub transmits the signal on pins 3 and 6 on the cables connected to Archie and Bob because Archie and Bob expect to receive data on pins 3 and 6.

In some cases, you need to cable two devices directly together with Ethernet, but both devices use the same pair for transmitting data. For instance, you might want to connect two hubs, and each hub transmits on pins 3 and 6, as just mentioned. Similarly, you might want to create a small Ethernet between two PCs simply by cabling the two PCs together—but both PCs use pins 1 and 2 for transmitting data. To solve this problem, you use a special cable called a crossover cable. Instead of pin 1 on one end of the cable being the same wire as pin 1 on the other end of the cable, pin 1 on one end of the cable becomes pin 3 on the other end. Similarly, pin 2 is connected to pin 6 at the other end, pin 3 is connected to pin 1, and pin 6 is connected to pin 2. Figure 3-7 shows an example with two PCs connected and a crossover cable.

Figure 3-7 Crossover Ethernet Cable

RJ-45

RJ-45

RJ-45

RJ-45

Crossover Cable

--—^-r Cross-over Cable Conceptual View

Larry

Crossover Cable

--—^-r Cross-over Cable Conceptual View

Both Bob and Larry can transmit on pins 1 and 2—which is good because that's the only thing an Ethernet card for an end user computer can do. Because pins 1 and 2 at Larry connect to pins 3 and 6 at Bob, and because Bob receives frames on pins 3 and 6, the receive function works as well. The same thing happens for frames sent by Bob to Larry—Bob sends on his pins 1 and 2, and Larry receives on pins 3 and 6.

Most of the time, you will not actually connect two computers directly with an Ethernet cable. However, you typically will use crossover cables for connections between switches and hubs. An Ethernet cable between two hubs or switches often is called a trunk. Figure 3-8 shows a typical network with two switches in each building and the typical cable types used for each connection.

Figure 3-8 Typical Uses for Straight-Through and Crossover Ethernet Cables

Figure 3-8 Typical Uses for Straight-Through and Crossover Ethernet Cables

Building 1

Straight-

through

Cables

w Straight-through Cables

Building 1

Straight-

through

Cables w Straight-through Cables

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