Physical Topology :
It is a Way in which network is laid out physically, two or more devices connect to a link; two or more links form a topology. Topology is geometric representation of relationship of all the links & nodes to one another.
Every device has a dedicated pt 2 pt link to every other device. Dedicated means that link carries traffic only between two devices it connects. In mesh topology to find no of physical links in fully connected mesh network with n nodes we must consider that each mode is connected to every other node. We need n(n-1)/2 duplex mode links.
Advantage: – Eliminates traffic problem, it is robust, privacy.
Disadvantage: – Amt. of cabling & no. of I/O parts reqd. installation & re connection are difficult. Bulk of wiring can be greater than available space. Hardware reqd. to connect each link can be expensive.
Each device has a dedicated point to point link only to a central controller, usually called a hub. The devices are not directly linked to one another. Unlike a mesh topology a star topology doesn’t allow direct traffic between devices. The controller acts as an exchange: If one device wants to send data to one another, it sends the data to the controller, which then relays the data to the other connected device.
- It is less expensive than mesh topology.
- In star, each device needs only one link and one I/O port to connect it to any number of others. This makes it easy to install and reconfigure.
- Far less cabling needs to be housed and additions, moves, and deletions involves only on reconnection between the device and the hub.
- It is robust means if one link fails, only that link is affected.
It is the dependency of the whole topology on one single point, the hub. If the hub goes down, the whole system is dead.
Bus Topology is multipoint. One long cable acts as a backbone to link all the devices in a network. Nodes are connected running between the device & the main cable. A tap is a connector that either splices into main cable or punctures the sheathing of a cable to create a contact with metallic core. As a signal travels along the backbone, some of its energy is transformed into heat. It becomes weaker & weaker as it travels further & further. For this reason there is a limit on the number of taps a bus can support & on the distance between those tops.
Ease of installation, bus uses less cabling than mesh or star, redundancy is eliminated.
- Difficult reconnection & fault isolation.
- Difficult to add new devices.
Each device has a dedicated point to point connection with only the two devices on either side of it. A signal is passed along the ring in one direction, from device to device, until it reaches its destination. Each device in the ring incorporates a repeater. When a device receives a signal intended for a device, its repeater regenerates the bits and passes them along.
- A ring is easy to install and reconfigure. Each device is linked to only its immediate neighbors.
- To add or delete a device requires changing only two connections.
- Fault isolation is simplified.
- Unidirectional traffic can be a disadvantage.
- In a simple ring, a break in the ring can disable the entire network.
A network can be Hybrid for ex we can have a main star topology with each branch connecting several stations in a bus topology as shown in the fig. below.
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