The Difference Between Hubs and Switches

Hubs and switches perform similar functions: They both sit in the center of a star topology network and thereby link several computers together. Unless the device is marked as a hub or switch, you can't normally tell them apart from their appearance. Both are fairly nondescript boxes that contain several RJ-45 jacks and some LEDs. Figure 17.8 shows a typical hub. The smallest hubs and switches support about four computers, but higher-end models support many more.

Hubs and switches differ primarily in the way they connect the computers on a network. Hubs are comparatively simple devices that echo any incoming packet out to all the other devices, as shown in Figure 17.9. This feature means that every computer on the network must wade through all the traffic on that network. For instance, in Figure 17.9, when polk sends data to pierce, the computers tyler, monroe, and madison must also examine the same packets. Fortunately, Ethernet frames include media access control (MAC) addresses, which uniquely identify Ethernet cards. The processing load on the network's computers of having to examine all frames is therefore extremely low, because frames can be accepted or rejected on the basis of the MAC address. In fact, in most cases the computer's CPU need not be involved in the process at all; the NIC can handle the task alone. The distribution of data to all computers on the network does mean that collisions are more-or-less inevitable, particularly on large or heavily used networks.

Part IV

Figure 17.8

Hubs and switches vary in size and shape, in part to accommodate differing numbers of connections.

Figure 17.8

Hubs and switches vary in size and shape, in part to accommodate differing numbers of connections.

Figure 17.9

A hub passes network traffic destined for a single computer (indicated by the dashed lines) to all machines.

Figure 17.9

A hub passes network traffic destined for a single computer (indicated by the dashed lines) to all machines.

Chapter 17

A switch, by contrast, is smarter than a hub. A switch can examine the MAC addresses of the Ethernet frames it receives and forward the frames on to only the intended recipient. This situation is illustrated in Figure 17.10. The result is less network congestion and less chance of collision. For instance, if polk transfers data to pierce, as shown in Figure 17.10, while tyler transfers data to monroe, there need be no collision. This is despite the fact that both polk and tyler transmit at the same time—a situation which would cause a collision with a hub.

polk polk

Figure 17.10

A switch forwards data only to the intended recipient computer.

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