Upgrading and Repairing Networks

Using a Star Topology

Instead of linking workstations in a linear fashion along a single cable, the hub acts as a wiring concentrator, providing a central point in the network where all nodes connect. Figure 13.2 shows a simple LAN connected to a hub in a star configuration.

	The chapter "Bridges, Repeaters, and Hubs" on the upgradingandrepairingpcs.com Web site and Chapter 8 discuss the star topology, which was introduced after 10BASE-T was developed.

All data that travels from one node to another must pass through the hub. A simple hub merely repeats incoming transmissions on all other ports, whereas more complex hubs can perform functions that strengthen the signal or correct minor problems.

The star topology was continued when switches were developed. A central switch looks just about the same as a hub, but it limits the collisions in a LAN dramatically. Switches, which are covered in Chapter 8, do this because they don't rebroadcast a frame on every other port. They transmit the frame only onto the port that will get it delivered to its destination.

The star topology has only a few shortcomings when compared to the bus: More cabling is required, and the hub becomes a single point of failure. However, the benefits that the star topology has over the bus are many:

• Installing wiring for this type of network (twisted-pair cables, similar to telephone cables but of a higher quality) is easier than installing coaxial cabling used for the bus. Although more cable is required, the cables are also less expensive and more flexible for routing throughout a building.

• It is easier to detect errors in the LAN through LEDs on the hub/switch or by using a hub/switch that incorporates management software.

• One workstation or cabling segment that experiences problems does not disrupt the entire network.

• Adding and removing nodes from this type of LAN is a simple matter of plugging the cable into a free socket on the hub. Modern hubs don't require you to place terminators on unused ports.

• If a hub fails, it can be replaced quickly with a spare by simply unplugging cables and inserting them into the new hub/switch. Alternatively, in a wiring closet with multiple hubs/switches, you could simply move users from a disabled unit to free ports on other hubs/switches until repairs could be made.

Over the years, hubs became more intelligent, and finally switches were developed for use in local area networks. A switch works similarly to a hub, in that it centralizes the wiring of the LAN. The main difference, however, is that the switch doesn't broadcast every frame it receives on all the other ports after it learns where a particular computer is located. A switch is similar to putting multiple bridges into one device. After the switch learns the locations of all the computers attached to it, LAN traffic can be switched between ports at a very fast rate, eliminating the collisions that would occur in a high-traffic environment using a hub.

In a modern network, switches are the preferred wiring concentrator. If you have a small home or departmental network that doesn't generate a lot of network traffic, a cheap hub might have been a good solution a few years ago. However, you probably won't be able to find one today at your local computer store. Instead, switches are the wiring concentrator used today. Hubs are discussed in this chapter mainly to show you how Ethernet technology has advanced.