Network topology, or layout, is the pattern by which the network is Topology should not be confused with the actual wiring path of a network, which is determined by the physical layout of walls and floors and other environmental factors. organized
One way to think of topology is to picture a map showing roads, rivers, railroads, cities, mountains, and other features. The relationship between the various locations can be understood by looking at the map. A diagram of a network's topology fune tions in much the same way. This allows a viewer to locate each network compo- nent, or node.
The common network topologies are bus, star, and ring.
Bus Topologies
In a bus topology, all computers (nodes) are linked by means of a single line of cable with two endpoints. The cable connection is called a bus. All commu- nications travel the length of the bus. Each computer has a network card with a transceiver, a device that sends messages along the bus in either direction. Messages contain data, error-checking code, the address of the node sending the message, and the address of the node that is to receive the message. As the com- munication passes, each computer's network card checks to see if it is the assigned destination point. If the computer finds its address in the message, it then reads the data, checks for errors in the transmission, and sends a message to the sender of the data acknowledging that the data was received. If the computer's network card does not find its address, it ignores the message. To prevent transmitted data from bouncing back and forth, a bus topology may include a terminator at both ends of the line. A terminator is a device that absorbs signals so they do not reflect back down the line. Figure 6-21 shows the layout of a bus topology.
Problems can occur if two or more computers send messages at the same time. This creates an interference pattern, and when one of the computers detects this pattern it jams the network, stopping all transmissions. Computers that are send- ing messages then wait and resend, a process that is repeated until a message gets through without being blocked. Another problem with linear bus topology is that a broken connection along the bus can bring down the whole network. Bus topologies commonly use coaxial or fiber-optic cables. They are less expensive than some other network layouts, but may be less efficient.
Star Topologies
In a star topology (also referred to as a hub-and-spoke topology), multiple com- puters and peripheral devices are linked to a central hub, in a point-to-point configuration resembling a star (Figure 6-22). The hub acts as a switching station, reading message addresses the nodes send and routing the messages accordingly. Companies with multiple departments needing centralized access to databases and files often prefer this topology.
The chief disadvantage of star topology is its dependence on the host computer. Because all communications must go through the hub, the network becomes inop- erable if it fails to function properly. On the other hand, the hub can prevent the data collisions that may occur with bus topologies, and the rest of the network can remain operational if a node's connection is broken.
Ring Topologies
In a ring topology there is no hub, and each computer is connected to two or more other devices in a circular path (Figure 6-23). A type of ring technology called token ring uses a single electronic signal, or token, to pass information from the source computer to the destination. A computer is bypassed if it isn't working messages As with bus topologies, ring topologies have the potential drawback that if two computers try to send communications at the same time, one or both may become garbled.
Hybrid Topologies
Some businesses prefer using one kind of topology throughout the organization. while others prefer to use several different kinds. Indeed, the more common prac tice is for companies to combine network layout types to suit their particular situation. A system that mixes topologies in a network is called a hybrid topology. For example, a company's plant in Ohio may use a ring topology, while another plant in North Carolina may set up a bus topology. When the company's two plants are connected, they form a hybrid topology.
Physical versus Logical Topologies
One of the elements that makes networking potentially confusing to beginners is that the term topology has two levels of meaning. On the one hand, it refers to the physical arrangement of LAN components, including PCs, peripherals, and cables as discussed in the preceding section. On the other hand, it refers to the way the data travels through the physical connections, which is called its logical arrangement. For example, workstations might be physically connected to a central hub, forming a star, but might pass data from PC to PC in a circular fashion, forming a logical ring.
There are two primary logical topologies: bus and ring. The main difference between them is in the way they avoid collisions in network traffic. In a logical bus topology, the network is like a telephone party line. A computer that wants to send some data listens to make sure no other PC is sending data, and then it sends. In a logical ring topology, the token system is used to prevent collisions. There is only one token in the ring, and only the PC in possession of it may send data. PCs do not monitor the network to see whether the line is free; instead they wait until they receive the token.