The physical topology of a network refers to the configuration of cables, computers, and other peripherals. Physical topology should not be confused with logical topology which is the method used to pass information between workstations. Logical topology was discussed in the Protocol chapter.
Main Types of Physical Topologies
The following sections discuss the physical topologies used in networks and other related topics.
Linear Bus
A linear bus topology consists of a main run of cable with a terminator at each end [See fig. 1]. All nodes [file server, workstations, and peripherals] are connected to the linear cable.
Fig. 1. Linear Bus topology
Advantages of a Linear Bus Topology
- Easy to connect a computer or peripheral to a linear bus.
- Requires less cable length than a star topology.
Disadvantages of a Linear Bus Topology
- Entire network shuts down if there is a break in the main cable.
- Terminators are required at both ends of the backbone cable.
- Difficult to identify the problem if the entire network shuts down.
- Not meant to be used as a stand-alone solution in a large building.
Star
A star topology is designed with each node [file server, workstations, and peripherals] connected directly to a central network hub, switch, or concentrator [See fig. 2].
Data on a star network passes through the hub, switch, or concentrator before continuing to its destination. The hub, switch, or concentrator manages and controls all functions of the network. It also acts as a repeater for the data flow. This configuration is common with twisted pair cable; however, it can also be used with coaxial cable or fiber optic cable.
Fig. 2. Star topology
Advantages of a Star Topology
- Easy to install and wire.
- No disruptions to the network when connecting or removing devices.
- Easy to detect faults and to remove parts.
Disadvantages of a Star Topology
- Requires more cable length than a linear topology.
- If the hub, switch, or concentrator fails, nodes attached are disabled.
- More expensive than linear bus topologies because of the cost of the hubs, etc.
Tree or Expanded Star
A tree topology combines characteristics of linear bus and star topologies. It consists of groups of star-configured workstations connected to a linear bus backbone cable [See fig. 3]. Tree topologies allow for the expansion of an existing network, and enable schools to configure a network to meet their needs.
Fig. 3. Tree topology
Advantages of a Tree Topology
- Point-to-point wiring for individual segments.
- Supported by several hardware and software venders.
Disadvantages of a Tree Topology
- Overall length of each segment is limited by the type of cabling used.
- If the backbone line breaks, the entire segment goes down.
- More difficult to configure and wire than other topologies.
5-4-3 Rule
A consideration in setting up a tree topology using Ethernet protocol is the 5-4-3 rule. One aspect of the Ethernet protocol requires that a signal sent out on the network cable reach every part of the network within a specified length of time. Each concentrator or repeater that a signal goes through adds a small amount of time. This leads to the rule that between any two nodes on the network there can only be a maximum of 5 segments, connected through 4 repeaters/concentrators. In addition, only 3 of the segments may be populated [trunk] segments if they are made of coaxial cable. A populated segment is one that has one or more nodes attached to it . In Figure 4, the 5-4-3 rule is adhered to. The furthest two nodes on the network have 4 segments and 3 repeaters/concentrators between them.
NOTE: This rule does not apply to other network protocols or Ethernet networks where all fiber optic cabling or a combination of a fiber backbone with UTP cabling is used. If there is a combination of fiber optic backbone and UTP cabling, the rule would translate to a 7-6-5 rule.The speed of networking switches is vastly improved over older technologies, and while every effort should be made to limit network segment traversal, efficient switching can allow much larger numbers of segments to be traversed with little or no impact to the network.
Considerations When Choosing a Topology
- Money. A linear bus network may be the least expensive way to install a network; you do not have to purchase concentrators.
- Length of cable needed. The linear bus network uses shorter lengths of cable.
- Future growth. With a star topology, expanding a network is easily done by adding another concentrator.
- Cable type. The most common cable in schools is unshielded twisted pair, which is most often used with star topologies.
Summary Chart
| Twisted Pair Coaxial Fiber | Ethernet |
| Twisted Pair Fiber | Ethernet |
| Twisted Pair Coaxial Fiber | Ethernet |
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The way a network's links and nodes are connected is referred to as network topology. Actual network topology refers to the physical signal transmission medium, whereas logical network topology refers to the manner in which data passes through the network between devices, regardless of the physical connectivity of the devices.
Star Topology
A star network is a type of computer network that uses the spoke–hub distribution concept.
In a star network, each host is connected to a central hub. One central hub serves as a channel for messages in its most basic form.
The hub and hosts and the transmission lines that connect them form a star-shaped graph. A star network's data goes via the hub before proceeding to its final destination.
The hub is in charge of managing and controlling all network functions. It also performs the role of a data flow repeater.
The star topology reduces the impact of a transmission line failure by connecting each host to the hub separately. Each host can communicate with the others by transmitting to and receiving from the hub.
If a transmission line connecting any host to the hub fails, that host will be isolated from the rest of the network, but the remainder of the network will be unaffected.
Bus Topology
All devices [or nodes] in a bus topology are linked together via a shared link known as the bus. The network traffic is received by each node on the bus.
A station is a name for a host on a bus network. Every station in a bus network receives all network traffic, and each station's traffic has equal transmission priority.
The collision domain and network segment of a bus network are the same. Nodes employ a medium access control system such as carrier-sense multiple access [CSMA] or a bus master to share the bus.
There is a primary wire in a bus topology, and all of the devices are connected to it by dropline. The drop lines is connected to the primary wire by a device known as a tap.
Since all the data is transferred across the main cable, the number of drop lines and the maximum distance the main cable can go are limited. In a bus topology, each computer in the network communicates with another computer independently.
All computers can share the whole bus capabilities of the network. The flow of data from one point in the network to another is shared among the devices.
The following table highlights the major differences between Start topology and Bus topology −
| A central hub connects all the devices. | Each device is connected to the backbone, which is a single cable. |
| If the central hub fails, the entire network will fail. | The failure of a network cable will result in the failure of the entire network. |
| Data is serially delivered from one device to another when devices are coupled as a result, data transfer speed is quite slow. | The hub sends data straight to the relevant peripheral device, resulting in very fast transmission. |
| Non-linear orientation | Linear structure |
| Because of the central hub and additional cables needed for connectivity, it has a high implementation cost. | Less expensive |
Published on 19-Aug-2021 11:58:38