Introduction to Topologies

Before you can even begin installing your networking software, you must have some type of hardware that connects all the computers. In this section, you will learn about network topologies and Ethernet, the most common network communication architecture for connecting network hardware. A topology is defined as the layout of the network. Topologies can be physical or logical:

10BASE-T

A network communication standard that uses Ethernet on unshielded twisted-pair cable in a star topology at 10Mbps.

Physical topology  Describes how the network physically looks or how the network is physically designed

Logical topology  Describes how data is transmitted through the network

The concept of a topology is important because each network card is designed to work with a specific topology. Conversely, if your network cable is already installed and you want to use existing wiring, you must select your network cards based on the preexisting physical topology.

Ideally, you can design your network from scratch. Then you can choose your topology, cabling, and network cards based on what best meets your needs.

This section will review the commonly defined topologies:

• Star topology

• Bus topology

• Ring topology

Star Topology

Physically, the star topology looks like a star. The hub is at the center of the star, and all devices attach to the hub via a cable.

Note 

The term hub is used generically to specify the central device in a star topology. In a 10BASE-T network, the hub is technically called a concentrator. In a Token Ring network, the hub is technically called a Multistation Access Unit, or MAU (pronounced mow).

Logically, the physical star topology operates as a logical bus topology (explained in the next section) by sending the data signal to all nodes at once. The hub at the center of the star works as a signal splitter, which means the signal is split and sent to all computers at the same time, with one exception-it is not sent back to the computer from which the signal originated. The signal is terminated at each network card, thereby preventing the signal from accidentally reentering the network. If this were to happen, data packets would travel the network endlessly-seriously slowing down network performance.

Benefits

The benefits of a star topology include the following:

• A star topology is more fault tolerant than other topologies, because a cable break does not bring down the entire network.

• Reconfiguring the network, or adding nodes, is easy because each node connects to the central hub independent of other nodes.

• Isolating cable failures is easy because each node connects independently to the central hub.

Drawbacks

The drawbacks of a star topology include the following:

• If the central hub fails, the entire network becomes unavailable.

• This topology is more expensive than others to install because of the additional cable and equipment involved.

Bus Topology

Physically, a bus topology uses a linear segment of cable to connect all network devices. Devices typically connect to the bus (the cable) through T-connectors. At each end of the bus are terminators. Each terminator absorbs the signal when it reaches the end of the cable. Without a terminator, a signal would bounce back and cause network errors.

Like the physical star topology, the physical bus topology uses a logical bus to transmit data on the cable in both directions. In a logical bus topology, only one transmission can occur at any given moment. Otherwise, two transmissions would collide and cause network errors. Termination ensures that the signal is removed from the cable when it reaches either end, preventing possible network errors.

Benefits

The benefits of a bus topology include the following:

• This is one of the least expensive topologies to install, because it uses less cable than the star topology and needs no hardware for a central device.

• It is an easy way to network a small number of computers.

Drawbacks

The drawbacks of a bus topology include the following:

• If there is a break in the cable, the entire network will fail.

• This topology can be difficult to troubleshoot because cabling problems often cannot be isolated to one computer.

• On a medium-sized to large network, reconfiguration is more difficult than the cable management of a star topology.

Note 

In the early days of networking, the bus topology was very popular. It used less equipment than other topologies and was therefore inexpensive to set up. But now Ethernet concentrators are inexpensive. The falling cost to build a star topology-based network, combined with its fault tolerance, has made 10BASE-T with the star topology the most popular Ethernet configuration.

Ring Topology

Physically, the ring topology is shaped in a ring. Cables pass from computer to computer until the ring is complete. When data is transmitted, each workstation receives the signal and then passes it on when the workstation is done with the data. Other than Fiber Distributed Data Interface (FDDI), no current networks use a physical ring topology, because a break in the ring makes the entire network unavailable.

Logically, a ring topology works by passing the signal, traditionally called a token, from one node to another until it reaches all the way around the ring. Token-passing schemes use the logical ring topology.

Benefits

A logical ring topology ensures access to the network without the risk of collisions, which can occur in logical star or bus topologies.

collisions

A problem on Ethernet networks that occurs when two computers transmit on the wire simultaneously, causing an electrical spike twice the strength than is normal for the network.

Institute of Electrical and Electronic Engineers (IEEE)

Pronounced I triple E, an international organization that defines computing and telecommunications standards. The LAN standards defined by IEEE include the 802-work-group specifications.

Drawbacks

The drawbacks of a ring topology include the following:

• If there is a break in the cable of a physical ring topology, the network becomes unavailable.

• Physical ring topologies are difficult to troubleshoot.

• Physical ring topologies are hard to reconfigure.

• There is limited support for ring networks.

• The costs for a ring network are significantly higher than for star or bus.