LAN Topologies

LAN Topologies

LAN physical topology defines the geographical arrangement of networking devices. Topologies are driven fundamentally by two network connection types:

A point-to-point connection is a direct link between two devices. For example, when you attach your computer to a printer, you have created a point-to-point link. In networking terms, most of the today’s point-to-point connections are associated with modems and PSTN (Public Switched Telephone Network) communications because only two devices share point-to-point connections, it defeats the purpose of a shared network.

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A multipoint connection, on the other hand, is a link between three or more devices. Historically, multipoint connections were used to attach central CPUs to distributed dumb terminals.

In today’s LAN environments, multipoint connections link many network devices in various configurations.

The major topologies of LAN are:

1. Bus Topology
2. Ring Topology
3. Star Topology
4. Mesh Topology
5. Cellular Topology
6. Hybrid Topology

Bus Topology

The physical bus topology is the simplest and most widely used of the network designs. It consists of one continuous length of cabling (trunk) and a terminating resistor (terminator) at each end. The data communications message travels along the bus in both directions until it is picked up by a workstation or server NIC.

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If the message is missed or not recognized, it reaches the end of the cabling and dissipates at the terminator. All nodes in the bus topology have equal access to the trunk – no discriminating here. This is accomplished using short drop cables or direct T-connectors.

This design is easy to install because the backbone trunk traverses the LAN as one cable segment. This minimizes the amount of transmission media required. Also, the number of devices and length of the trunk can be easily expanded.

Advantages of Bus Topology:

1. It uses established standards and it is relatively easy to install.
2. Requires fewer media than other topologies.

Disadvantages of Bus Topology:

1. The bus networks are difficult to reconfigure, especially when the acceptable number of connections or maximum distances have been reached.
2. They are also difficult to troubleshoot because everything happens on a single media segment. This can have dangerous consequences because any break in the cabling brings the network to its knees.

Ring Topology

As its name implies, the physical ring topology is a circular loop of point-to-point links. Each device connects directly or indirectly to the ring through an interface device or drop cable. Messages travel around the ring from node to node in very organized manner. Each workstation checks the messages for a matching destination address.

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If the address doesn’t match, the node simply regenerates the message and sends it on its way. If the address matches, the node accepts the message and sends a reply to the originating sender. Initially, ring topologies are moderately simple to install; however, they require more media than bus systems because the loop must be closed.

Once your ring has been installed, it’s a bit more difficult to reconfigure. Ring segments must be divided or replaced every time they’re changed. Moreover, any break in the loop can affect all devices on the network.

Advantages of Ring Topology:

1. They are very easy to troubleshoot because each device incorporates a repeater.
2. A special internal feature called becoming, allows the troubled workstation to identify themselves quickly.

Disadvantages of Ring Topology:

1. It is considerably difficult to install and reconfigure ring topology.
2. Media failure on unidirectional or single loop causes complete network failure.

Star Topology

The Physical star topology uses a central controlling hub with dedicated legs pointing in all directions – like points of a star. Each network devices has a dedicated point-to-point link to the central hub. This strategy prevents troublesome collisions and keeps the line of communication open and free of traffic.

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Star topologies are somewhat difficult to install because each device gets its own dedicated segment. Obviously, they require a great deal of cabling. This design provides an excellent platform for reconfiguration and troubleshooting.

Changes to the network are as simple as plugging another segment into the hub. In addition, a break in the LAN is easy to isolate and doesn’t affect the rest of the network.

Advantages of Star Topology:

1. Relatively easy to configure.
2. Easy to troubleshoot.
3. Media faults are automatically isolated to the failed segment.

Disadvantages of Star Topology:

1. Requires more cable than most topologies.
2. Moderately difficult to install.’

Mesh Topology

The mesh topology is the only true point-to-point design. It uses a dedicated link between every device on the network. This design is not very practical because of its excessive waste of transmission media. This topology is difficult to install and reconfigure.

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Moreover, as the number of devices increases geometrically, the speed of communication also become slow. ATM (Asynchronous Transfer Mode) and switched Hubs are the example of high-speed Mesh implementation.

Advantages of Mesh Topology:

1. Easy to troubleshoot because each link is independent of all others.
2. You can easily identify faults and isolate the affected links. Because of the high number of redundant paths, multiple links can fail before the failure affects any network device.

Disadvantages of Mesh Topology:

1. It is difficult to install and reconfigure especially as the number of devices increases.

Cellular Topology

A cellular topology combines wireless point-to-point and multipoint designs to divide a geographic area into cells. Each cell represents the portion of the total network area in which a specific connection operates. Devices within the cell communicate with a central station or hub. Hubs are then interconnected to route data between cells.

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The cellular topology relies on the location of wireless media hubs. Cellular networks exhibit interesting characteristics since this topology do not depend on cables. Troubleshooting is easy because each hub interacts independently with each device. A cellular installation depends on the accessibility hub locations.

Advantages of Cellular Topology:

1. It is relatively easy to install.
2. It does not require media reconfiguration when adding or removing users.
3. Fault isolation and troubleshooting is fairly simple.

Disadvantages of Cellular Topology:

1. All devices using a particular hub are affected by a hub failure.

Hybrid Topology

By modifying or combining some of the characteristics of the ‘pure’ network topologies, a more useful result may be obtained. These combinations are called hybrid topologies. Some of the hybrid topologies are:

1. Tree network

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2. Star-Ring or interconnected

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References:

Khanal, R.C. Khanal, R.C. Computer Concept for XII. Pashupatigriha Marga, Thapathali, Kathmandu, Nepal: Ekta Books Distributors Pvt. Ltd., 2010. 77-82.

Adhikari, Deepak Kumar.,et.al., Computer Science XII,Asia Publication Pvt.Ltd