Computer network, data communication in computer Network ,Network Criteria - Physical Structure Physical Topology Type of Connection, Star Topology ,Bus Topology ,Ring Topology
Star Topology :
In a star topology, 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 does not allow direct traffic between devices. The controller acts as an exchange: If one device wants to send data to another, it sends the data to the controller, which then relays the data to the other connected device.
- The star topology is used in local area network(LAN's).
- High-speed LANs often use a star topology with a central hub.
Advantages :
- A star topology is less expensive than a mesh topology . In a star, each device needs only one link and one I/O port to connect it to any number of others. This factor also makes it easy to install and reconfigure. Far less cabling needs to be housed, and additions, moves, and deletions involve only one connection: between that device and the hub.
- Robustness : If one link fails, only that link is affected. All other links remain active. This factor also lends itself to easy fault identification and fault isolation. As long as the hub is working, it can be used to monitor link problems and bypass defective links.
Disadvantages :
- The dependency of the whole topology on one single point, the hub . If the hub goes down, If the hub goes down, of the whole topology on one single point, the hub the whole system is dead .
- Although a star requires far less cable than a mesh, each node must be linked to a central hub. For this reason, often more cabling is required in a star than in some other topologies (such as than in some other topologies (such as ring or bus).
Bus Topology :
- A bus topology is multipoint. One long cable acts as a backbone to link all the devices in a A bus topology is multipoint. One long cable acts as a backbone to li network.
- Nodes are connected to the bus cable by drop lines and taps. A drop line s a connection running between the device and the main cable.
- A tap is connector a that either splices into the main cable or punctures the sheathing of a cable to create a contact with the metallic core. As a signal travels along the backbone some of its energy is transformed into heat. Therefore, it becomes weaker and weaker as it travels farther and For this reason
- there is a limit on the number of taps a bus can support and on the distance between those taps.
Advantages :
- Ease of installation - Backbone cable can be laid along the most efficient path, connected and then to the nodes by drop lines of various lengths. In this way, a bus uses less cabling than mesh or star topologies.
- In a star, for example, four network devices in the same room require four lengths of cable reaching all the way to the hub. in a bus, this redundancy is eliminated Only the backbone cable stretches through the entire facility. Each drop line has to reach only as far as the nearest point on the backbone.
Disadvantages :
- Difficult in reconnection and fault isolation A bus is usually designed to be optimally efficient at installation. It can therefore be difficult to add new devices. Signal reflection at the taps can cause degradation in quality. This degradation can be controlled by limiting the number and spacing of devices connected to a given length of cable.
- Adding new devices may therefore require modification or replacement of the backbone.
- In addition, a fault or break in the bus cable stops all transmission, even between devices on the same side of the problem. The damaged area reflects signals back in the direction of origin, creating noise in both directions. Bus topology was the one of the first topologies used in the design of early local area networks. Traditional Ethernet LANs can use a bus topology.
Ring Topology :
In a ring topology, 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
another device, its repeater regenerates the bits and passes them along.
Advantages :
- A ring is relatively easy to install and reconfigure.
- Each device is linked to only its immediate neighbors (either physically or logically).
- To add or delete a device requires changing only two connections. The only constraints are media and traffic considerations (maximum ring length and number of devices).
- Fault isolation is simplified. Generally, in a ring a signal is circulating at all times. If one device does not receive a signal within a specified period, it can issue an alarm. The alarm alerts the network operator to the problem and its location.
Disadvantages :
- Unidirectional traffic can be a disadvantage. In a simple ring, a break in the ring (such as a disabled station) can disable the entire network. This weakness can be solved by using a dual ring or a switch capable of closing off the break.
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