Star Topology

Star Topology
Unlike those in a bus topology, each computer in a star topology is connected to a central point by a separate cable or wireless connection. The central point is a device known by such names as
hub, MAU, concentrator, switch, and access point, depending on the underlying technology.

Real World Scenario
    A bus sounds good, but . . .

Despite the simplicity of the bus topology, there are some inherent disadvantages to this design. For example, what happens if the wire breaks or is disconnected? Neither side can communicatewith the other, and signal bounce occurs on both sides. The result is that the entire network is down. For this reason, bus topologies are considered to have very little fault tolerance. Sometimes, because a cable is inside a wall, you cannot physically see a break. To determine if a break has occurred, you can use a tool known as a Time Domain Reflectometer, or TDR (also called a cable tester). This device sends out a signal and measures how much time it takes to return. Any break in the cable will cause some portion of the signal to return prematurely, thus indicating the presence of, and the distance to, a break in the cable. Programmed with the specifications of the cable being tested, it determines where the fault lies with a high degree of accuracy.We’ll discuss cable testers in Chapter 6, “Wired and Wireless Networks.”

 Although this setup uses more cable than a bus, a star topology is much more fault tolerant than a bus topology. This means that if a failure occurs along one of the cables connecting to the hub, only that portion of the network is affected, not the entire network. Depending on the type of device at the other end of that cable, this may affect only a single device. It also means that you can add new stations just by running a single new cable. Figure 1.6 shows a typical star topology.

Example:

A typical star topology with a hub

The design of a star topology resembles an old wagon wheel with the wooden spokes extending from the center point. The center point of the wagon wheel would be considered the hub.
Like the wagon wheel, the network’s most vulnerable point is the hub. If it fails, the whole system collapses. Fortunately, hub failures are extremely rare.

As with the bus topology, the star topology has advantages and disadvantages. The increasing
popularity of the star topology is mainly due to the large number of advantages, which
include the following:

• New stations can be added easily and quickly.
• A single cable failure won’t bring down the entire network.
• It is relatively easy to troubleshoot.
• The disadvantages of a star topology include the following:
• Total installation cost can be higher because of the larger number of cables, but prices are constantly becoming more and more competitive.
• It has a single point of failure (the hub, or other central device). There are two subtle special cases for the star topology, the point-to-point link and the wireless

access point. If you think of a point-to-point connection as one spoke of a star-wired network,
with either end device able to play the role of the hub or spoke device, then you can begin
to see the nature of any star-wired topology. What about when there is no wire, though? It takes
a firm understanding of what the devices making up the wireless network are capable of to be
able to categorize the wireless topology. Wireless access points, discussed in detail in Chapter
6, are nothing more than wireless hubs or switches, depending on capability, that are able to act
as wireless bridges by establishing a wireless point-to-point connection to another wireless

access point. Either use is reminiscent of the wired star/point-to-point topologies they emulate.