The Case for Bridging and Switching

To appreciate the need for LAN switches and the logic behind LAN switches, you must learn about devices called transparent bridges. Vendors began offering transparent bridges in the marketplace long before switches. And because switches act like bridges in many ways, it helps your understanding of switches to first understand how bridges work and why they were created in the first place.

To appreciate the need for bridges, you must be reminded of the state of Ethernet networking before bridges came along. Once upon a time, there was no such thing as an Ethernet LAN. Then Ethernet was created, using a single electrical bus, and was cabled using coaxial cables between the Ethernet cards in the devices that needed to attach to the Ethernet.

As mentioned in Chapter 3, "Data Link Layer Fundamentals: Ethernet LANs," 10BASE-T was the next step in the development of Ethernet. 10BASE-T improved the availability of a LAN because a problem on a single cable did not affect the rest of the LAN, which did happen on 10BASE2 and 10BASE5 networks. 10BASE-T allowed the use of unshielded twisted-pair (UTP) cabling, which is much cheaper than coaxial cable. Also, many buildings already had UTP cabling installed for phone service, so 10BASE-T quickly became a popular alternative to 10BASE2 and 10BASE5 Ethernet networks.

Figure 9-1 depicts the typical topology for 10BASE2 and for 10BASE-T.

Figure 9-1 10BASE2 and 10BASE-T Physical Topologies 10BASE2, Single Bus

Larry

Solid Lines Represent* Co-ax Cable

Archie

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10BASE-T, Using Shared Hub - Acts like Single Bus

Hub 1

Archie

Solid Lines Represent Twisted Pair Cabling

When transparent bridges first were introduced, Ethernet networks were either 10BASE5, 10BASE2, or 10BASE-T. Each of these three types of Ethernet had some common characteristics that drove the need for a bridging device:

■ Any device sending a frame could have the frame collide with a frame sent by any other device attached to that LAN segment.

■ Only one device could send a frame at a time, so the devices were sharing the 10-Mbps bandwidth.

■ Broadcasts sent by one device would be heard by all other devices on the LAN.

When these three types of Ethernet first were introduced, a shared 10-Mbps of bandwidth was a huge amount of bandwidth! Before the introduction of LANs, people often used dumb terminals, with a 56-kbps WAN link being a really fast connection to the rest of the network—with that 56-kbps being shared among everyone in the building. So, getting to put your computer on a 10BASE-T Ethernet LAN was like getting a Gigabit Ethernet connection for your PC at your desk at work today—it was more bandwidth than you could imagine that you would need.

Over time, the performance of many Ethernet networks started to degrade. People developed applications to take advantage of the LAN bandwidth. More devices were added to each Ethernet. Eventually, an entire network became congested. The devices on the same Ethernet could not send (collectively) more than 10 Mbps of traffic because they were all sharing the 10 Mbps of bandwidth. However, with the increase in traffic volumes, collisions also increased. Long before the overall utilization approached 10 Mbps, Ethernet began to suffer because of increasing collisions.

Bridges solved the growing Ethernet congestion problem in two ways. First, they reduced the number of collisions that occur in a network. They also add bandwidth to the network. Figure 9-2 shows the basic premise behind an Ethernet transparent bridge.

The top part of the figure shows a 10BASE-T network before adding a bridge, and the lower part shows the network after it has been "segmented" using a bridge. The bridge creates two separate collision domains—two different sets of devices for which their frames can collide. For instance, Fred's frames can collide with Barney's, but they cannot collide with Wilma's or Betty's. If one LAN segment is busy, and the bridge needs to forward a frame, it simply holds the frame until the segment is no longer busy. By reducing collisions and assuming no significant change in the number of devices or the load on the network, network performance is greatly improved.

By adding a bridge between two hubs, the bridge really creates two separate 10BASE-T networks, one on the left and one on the right. So, the 10BASE-T network on the left has its own 10 Mbps to share, as does the network on the right. So, in this example, the total network bandwidth was doubled to 20 Mbps.

Figure 9-2 Bridge Creates Two Collision Domains, Two Shared Ethernets

Fred Wilma rrrrrrrrr

Barney

Barney

1 Collision Domain Sharing 10 Mbps

Betty

Wilma

Barney

Betty

Wilma

Barney

In summary, before bridges were created, 10BASE-T (and 10BASE2 and 10BASE5) network performance degraded as more stations and more traffic were introduced into the network. With the addition of bridges, an Ethernet network can add more capacity and increase performance.

Switches and bridges use the same core logic, as described in the next section of this chapter. Instead of using "bridges and switches" every time, I just refer to the devices as "bridges," but switches work the same way.

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