There are two basic categories of network topologies:
1. Physical topologies
2. Logical topologies
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.
Physical topology refers to the placement of the network's various components, including device location and cable installation, while logical topology shows how data flows within a network, regardless of its physical design. In other words, the logical topology, is the way that the signals act on the network media, or the way that the data passes through the network from one device to the next without regard to the physical interconnection of the devices. A network's logical topology is not necessarily the same as its physical topology.
The logical classification of network topologies generally follows the same classifications as those in the physical classifications of network topologies but describes the path that the data takes between nodes being used as opposed to the actual physical connections between nodes. The logical topologies are generally determined by network protocols as opposed to being determined by the physical layout of cables, wires, and network devices or by the flow of the electrical signals, although in many cases the paths that the electrical signals take between nodes may closely match the logical flow of data, hence the convention of using the terms logical topology and signal topology interchangeably.
Main Types of Physical Topologies
• Bus
• Star
• Tree (Expanded Star)
• Considerations When Choosing a Topology
Bus
A bus topology consists of a main run of cable with a terminator at each end. All nodes (file server, workstations, and peripherals) are connected to the linear cable. A signal from the source travels in both directions to all machines connected on the bus cable until it finds the intended recipient. If the machine address does not match the intended address for the data, the machine ignores the data. Alternatively, if the data matches the machine address, the data is accepted. Since the bus topology consists of only one wire, it is rather inexpensive to implement when compared to other topologies. Additionally, since only one cable is utilized, it can be the single point of failure.
Linear bus
The type of network topology in which all of the nodes of the network are connected to a common transmission medium which has exactly two endpoints (this is the 'bus', which is also commonly referred to as the backbone, or trunk) – all data that is transmitted between nodes in the network is transmitted over this common transmission medium and is able to be received by all nodes in the network simultaneously.
Note: The two endpoints of the common transmission medium are normally terminated with a device called a terminator which absorbs the energy that remains in the signal to prevent the signal from being reflected or propagated back onto the transmission medium in the opposite direction, which would cause interference with and degradation of the signals on the transmission medium.
Distributed bus
The type of network topology in which all of the nodes of the network are connected to a common transmission medium which has more than two endpoints that are created by adding branches to the main section of the transmission medium – the physical distributed bus topology functions in exactly the same fashion as the physical linear bus topology (i.e., all nodes share a common transmission medium).
Advantages of Bus Topology
• Easy to connect a computer or peripheral to bus.
• Requires less cable length than a star topology.
Disadvantages of 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.
Star
A star topology is designed with each node (file server, workstations, and peripherals) connected directly to a central network hub or switch. Data on a star network passes through the hub or switch before continuing to its destination. The hub or switch 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.
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 or switch fails, nodes attached are disabled.
• More expensive than linear bus topologies because of the cost of the hubs, etc.
Ring
A network topology that is set up in a circular fashion in which data travels around the ring in one direction and each device on the right acts as a repeater to keep the signal strong as it travels. Each device incorporates a receiver for the incoming signal and a transmitter to send the data on to the next device in the ring. The network is dependent on the ability of the signal to travel around the ring.
In a ring network, every device has exactly two neighbors for communication purposes. All messages travel through a ring in the same direction (either "clockwise" or "counterclockwise"). A failure in any cable or device breaks the loop and can take down the entire network.
To implement a ring network, one typically uses FDDI, SONET, or Token Ring technology.
Mesh (Point – to – Point)
The value of fully meshed networks is proportional to the exponent of the number of subscribers, assuming that communicating groups of any two endpoints, up to and including all the endpoints.
Fully connected
A fully connected network, complete topology or full mesh topology is a network topology in which there is a direct link between all pairs of nodes. In a fully connected network with n nodes, there are n(n-1)/2 direct links. Networks designed with this topology are usually very expensive to set up, but provide a high degree of reliability due to the multiple paths for data that are provided by the large number of redundant links between nodes.
The number of connections in a full mesh = n(n - 1) / 2.
Note: The physical fully connected mesh topology is generally too costly and complex for practical networks, although the topology is used when there are only a small number of nodes to be interconnected.
Partially connected
Partially connected mesh topology
The type of network topology in which some of the nodes of the network are connected to more than one other node in the network with a point-to-point link.
Note: In networks that are based upon the partially connected mesh topology, the data that is transmitted between nodes in the network takes many shortest paths between nodes using a technology like Shortest Path Bridging, except in the case of a failure or break in one of the links, in which case the network uses the remaining alternative paths to the destination. This requires that the nodes of the network possess some type of algorithm to determine the correct path to use at any particular time.
Considerations When Choosing a Topology
• Money. A bus network may be the least expensive way to install a network.
• Length of cable needed. The bus network uses shorter lengths of cable.
• Future growth. With a star topology, expanding a network is easily done.
• Cable type. The most common cable is unshielded twisted pair, which is most often used with star topologies.
Hybrid
Hybrid networks use a combination of any two or more topologies in such a way that the resulting network does not exhibit one of the standard topologies (e.g., bus, star, ring, etc.). For example, a tree network connected to a tree network is still a tree network topology. A hybrid topology is always produced when two different basic network topologies are connected. Two common examples for Hybrid network are: star ring network and star bus network
Tree (star bus topology)
A tree topology (hierarchical topology) can be viewed as a collection of star networks arranged in a hierarchy. This tree has individual peripheral nodes (e.g. leaves) which are required to transmit to and receive from one other node only and are not required to act as repeaters or regenerators. Unlike the star network, the functionality of the central node may be distributed.
Each star network is a local area network (LAN) in which there is a central computer or server to which all the workstation nodes are directly linked. The central computers of the star networks are connected to a main cable called the bus. Thus, a tree network is a bus network of star networks.
Tree Topology Features
• There will be at least three levels of hierarchy in the Tree Network Topology and they all work based on the root node.
• The Tree Topology has two kinds of topology integral in it, the star and the linear way of connecting to nodes.
• The Tree Topology functions by taking into account the total number of nodes present in the network. It does not matter how many nodes are there on each level. Nodes can be added to any level of the hierarchy and there are no limitations a far as the total number of nodes do not exceed.
• The higher levels in the hierarchy are expected to perform more functions than the lower levels in the network.
The Tree Topology follows a hierarchical pattern where each level is connected to the next higher level in a symmetrical pattern. Each level in the hierarchy follows a certain pattern in connecting the nodes. Like the top most level might have only one node or two nodes and the following level in the hierarchy might have few more nodes which work on the point to point connectivity and the third level also has asymmetrical node to node pattern and each of these levels are connected to the root level in the hierarchy. Think of a tree that branches out in various directions and all these branches need the roots and the tree trunk to survive. A Tree Structured network is very similar to this and that is why it is called the Tree Topology.
As in the conventional star network, individual nodes may thus still be isolated from the network by a single-point failure of a transmission path to the node. If a link connecting a leaf fails, that leaf is isolated; if a connection to a non-leaf node fails, an entire section of the network becomes isolated from the rest.
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.
1. Physical topologies
2. Logical topologies
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.
Physical topology refers to the placement of the network's various components, including device location and cable installation, while logical topology shows how data flows within a network, regardless of its physical design. In other words, the logical topology, is the way that the signals act on the network media, or the way that the data passes through the network from one device to the next without regard to the physical interconnection of the devices. A network's logical topology is not necessarily the same as its physical topology.
The logical classification of network topologies generally follows the same classifications as those in the physical classifications of network topologies but describes the path that the data takes between nodes being used as opposed to the actual physical connections between nodes. The logical topologies are generally determined by network protocols as opposed to being determined by the physical layout of cables, wires, and network devices or by the flow of the electrical signals, although in many cases the paths that the electrical signals take between nodes may closely match the logical flow of data, hence the convention of using the terms logical topology and signal topology interchangeably.
Main Types of Physical Topologies
• Bus
• Star
• Tree (Expanded Star)
• Considerations When Choosing a Topology
Bus
A bus topology consists of a main run of cable with a terminator at each end. All nodes (file server, workstations, and peripherals) are connected to the linear cable. A signal from the source travels in both directions to all machines connected on the bus cable until it finds the intended recipient. If the machine address does not match the intended address for the data, the machine ignores the data. Alternatively, if the data matches the machine address, the data is accepted. Since the bus topology consists of only one wire, it is rather inexpensive to implement when compared to other topologies. Additionally, since only one cable is utilized, it can be the single point of failure.
Linear bus
The type of network topology in which all of the nodes of the network are connected to a common transmission medium which has exactly two endpoints (this is the 'bus', which is also commonly referred to as the backbone, or trunk) – all data that is transmitted between nodes in the network is transmitted over this common transmission medium and is able to be received by all nodes in the network simultaneously.
Note: The two endpoints of the common transmission medium are normally terminated with a device called a terminator which absorbs the energy that remains in the signal to prevent the signal from being reflected or propagated back onto the transmission medium in the opposite direction, which would cause interference with and degradation of the signals on the transmission medium.
Distributed bus
The type of network topology in which all of the nodes of the network are connected to a common transmission medium which has more than two endpoints that are created by adding branches to the main section of the transmission medium – the physical distributed bus topology functions in exactly the same fashion as the physical linear bus topology (i.e., all nodes share a common transmission medium).
Advantages of Bus Topology
• Easy to connect a computer or peripheral to bus.
• Requires less cable length than a star topology.
Disadvantages of 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.
Star
A star topology is designed with each node (file server, workstations, and peripherals) connected directly to a central network hub or switch. Data on a star network passes through the hub or switch before continuing to its destination. The hub or switch 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.
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 or switch fails, nodes attached are disabled.
• More expensive than linear bus topologies because of the cost of the hubs, etc.
Ring
A network topology that is set up in a circular fashion in which data travels around the ring in one direction and each device on the right acts as a repeater to keep the signal strong as it travels. Each device incorporates a receiver for the incoming signal and a transmitter to send the data on to the next device in the ring. The network is dependent on the ability of the signal to travel around the ring.
In a ring network, every device has exactly two neighbors for communication purposes. All messages travel through a ring in the same direction (either "clockwise" or "counterclockwise"). A failure in any cable or device breaks the loop and can take down the entire network.
To implement a ring network, one typically uses FDDI, SONET, or Token Ring technology.
Mesh (Point – to – Point)
The value of fully meshed networks is proportional to the exponent of the number of subscribers, assuming that communicating groups of any two endpoints, up to and including all the endpoints.
Fully connected
A fully connected network, complete topology or full mesh topology is a network topology in which there is a direct link between all pairs of nodes. In a fully connected network with n nodes, there are n(n-1)/2 direct links. Networks designed with this topology are usually very expensive to set up, but provide a high degree of reliability due to the multiple paths for data that are provided by the large number of redundant links between nodes.
The number of connections in a full mesh = n(n - 1) / 2.
Note: The physical fully connected mesh topology is generally too costly and complex for practical networks, although the topology is used when there are only a small number of nodes to be interconnected.
Partially connected
Partially connected mesh topology
The type of network topology in which some of the nodes of the network are connected to more than one other node in the network with a point-to-point link.
Note: In networks that are based upon the partially connected mesh topology, the data that is transmitted between nodes in the network takes many shortest paths between nodes using a technology like Shortest Path Bridging, except in the case of a failure or break in one of the links, in which case the network uses the remaining alternative paths to the destination. This requires that the nodes of the network possess some type of algorithm to determine the correct path to use at any particular time.
Considerations When Choosing a Topology
• Money. A bus network may be the least expensive way to install a network.
• Length of cable needed. The bus network uses shorter lengths of cable.
• Future growth. With a star topology, expanding a network is easily done.
• Cable type. The most common cable is unshielded twisted pair, which is most often used with star topologies.
Hybrid
Hybrid networks use a combination of any two or more topologies in such a way that the resulting network does not exhibit one of the standard topologies (e.g., bus, star, ring, etc.). For example, a tree network connected to a tree network is still a tree network topology. A hybrid topology is always produced when two different basic network topologies are connected. Two common examples for Hybrid network are: star ring network and star bus network
Tree (star bus topology)
A tree topology (hierarchical topology) can be viewed as a collection of star networks arranged in a hierarchy. This tree has individual peripheral nodes (e.g. leaves) which are required to transmit to and receive from one other node only and are not required to act as repeaters or regenerators. Unlike the star network, the functionality of the central node may be distributed.
Each star network is a local area network (LAN) in which there is a central computer or server to which all the workstation nodes are directly linked. The central computers of the star networks are connected to a main cable called the bus. Thus, a tree network is a bus network of star networks.
Tree Topology Features
• There will be at least three levels of hierarchy in the Tree Network Topology and they all work based on the root node.
• The Tree Topology has two kinds of topology integral in it, the star and the linear way of connecting to nodes.
• The Tree Topology functions by taking into account the total number of nodes present in the network. It does not matter how many nodes are there on each level. Nodes can be added to any level of the hierarchy and there are no limitations a far as the total number of nodes do not exceed.
• The higher levels in the hierarchy are expected to perform more functions than the lower levels in the network.
As in the conventional star network, individual nodes may thus still be isolated from the network by a single-point failure of a transmission path to the node. If a link connecting a leaf fails, that leaf is isolated; if a connection to a non-leaf node fails, an entire section of the network becomes isolated from the rest.
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.
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