Key Point
Star, ring, and bus are the most common LAN topologies.

Networks can have several different arrangements of links. The choice of topologies is often a matter of the technology being used for the network, or geographic considerations.

Bus Topology

A bus is a single electrical circuit to which all devices in the network are connected (although the bus might be made up of many individual pieces of wire). The Bus Topology Diagram illustrates this type of network layout.

A bus topology is a broadcast network. When a node transmits data, the signal travels down the bus in both directions. Each node connected to the bus receives the signal as it passes that connection point. However, a node ignores any signal that is not specifically addressed to it.

When the signal reaches the end of the bus cable, a terminator (resistor) prevents the signal from reflecting back from the end of the wire. If a bus network is not terminated, or if the terminator has the wrong level of resistance, each signal may travel across the bus several times instead of just once. This problem increases the number of signal collisions, degrading network performance.

If the bus cable breaks, the entire network may be disabled. In addition, it can be difficult to change the number and position of nodes on a bus network.

Star Topology

By far, the most common network topology is the star topology. In a star network, individual computers are connected to a central device, such as a hub or switch, as illustrated on the Star Topology Diagram. When a computer sends information to another computer, it is transmitted through the central device.

Like the bus topology, a hub-centered star topology is a broadcast network, because the hub copies each signal to all other computers attached to it. And, like a bus, the entire network may go down if the central hub fails.

Ring Topology

A "pure" ring topology is a collection of separate point-to-point links, arranged to make a ring. Each node's network interface card (NIC) has one input and one output connection, so each node is connected to two links.

When a node receives a signal on its input connection, its repeater circuitry retransmits that signal, immediately and without buffering, to its output connection. Thus, in many rings, data flows only in one direction, as illustrated on the Ring Topology Diagram.

To send a message, a node transmits new bits onto the ring. If a message is addressed to a node, that node copies bits off the ring as they go by. If a node receives a message that is not addressed to it, it repeats the message without copying it.

If a ring node malfunctions or is shut down, the ring is broken, and data transfer stops until the failed node is restored or removed from the ring. The ring can also be broken if any cable between nodes is damaged or broken. Therefore, some ring topologies such as Fiber Distributed Data Interface (FDDI) use a dual-ring structure. If one cable link fails, the other can immediately take over.

Ring topologies are often used as network backbones. A ring backbone often connects the floors of a multistory building or buildings in a campus network or MAN.

Star Ring Topology

A star ring topology combines a physical star configuration with a logical ring of information flow. The Star Ring Topology Diagram illustrates a typical star ring, such as a Token Ring LAN. In a star ring topology, wires run from each node to a central ring wiring concentrator, also called a multistation access unit (MAU). The star ring is a physical star configuration, but information travels from node to node in a logical ring as the MAU copies each signal to each of its nodes in turn. The MAU performs two other important functions:

Mesh Topology

In a mesh topology, point-to-point links directly connect every site to every other site. Mesh networks are usually built over time as new sites are added to the overall network. The Mesh Network Topology Diagram provides an example of a mesh configuration. A mesh topology is often used for MAN or WAN networks.

The number of point-to-point links increases sharply with the number of locations. Thus, if a network must connect more than a few sites, a mesh topology is usually too expensive.

Network Cloud

When an organization must connect more than a few sites over a metropolitan or wide area, a cloud network is usually more economical and flexible than a mesh of point-to-point links. The network cloud, shown on the Network Cloud Topology Diagram, represents a public mesh network of switching devices, often owned by a telephone company. Common types of cloud networks include the public telephone system, the Internet, or switched transmission services such as frame relay or ATM.

To use the services of a cloud network, a company subscribes to the service, then sets up a point-to-point connection between each location and a device at the edge of the cloud. The network provider is responsible for moving each message across the cloud to its destination.

   Activities

See the Activities and Extended Activities section in Unit 1 Lesson 1 in your textbook Local Area Networks to test what you have learned so far.

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