NETWORK TOPOLOGIES
The network topologies are a structure defined by the interconnecting scheme. This structure can be viewed as a graph with vertices representing the transmission channels.
The choice of topology involves many interrelated factors:
* Expected traffic.
* Routing method.
*Channel capacity
* Cost.
The various topologies are:
*Mesh topology(fully connected topology):
With the mesh technology, the node may be fully connected, in which each node is directly connected or mesh topology, each device has a point-to-point link, with each other devices .so if there are N devices, then n(n-1)/2 full duplex links are required,and each device requires(n-1) input/output ports.Thus the cost of the system, in terms of installation and I/O hardware, increase the square of the number of devices.
*Advantages:
(1) This topology provides alternative routes between Nodes.(2) Fastest transmission.
Disadvantages:
(1) the total branch length can be significantly increased.
(2) the cost may become too high to be justified.
Usage
* Public and private data Communication networks.Ring topology
* The most common implementation for LANs.
* It consists of a set of repeaters connected by point to point links in a closed loop.
*Each station is connected into the ring at the repeater.
*The repeater is capable of receiving data on one link and transmitting it, bit by bit, on the other link as it is received, with no buffering at the repeater.
* The links are unidirectional, thus data are circulated around the ring in one direction.
*Data are broadcast in the form of packets which are circulated round in one direction to the various stations or transmitted to the next node.
* Only one station is allowed to transmit at any one time.
*Advantages
A ring is more economical.*Disadvantages
(1) the failure of one station will cause the system to break down as the link will be broken.(2) The unidirectional transmission also means a low data rate.
Usage
(1)in LANs
*BUS Topology
* Consists of a common channel over which data packets can be transmitted, either to a specific destination.or in the broadcast mode.* It allows flexible networking as devices can be easily attached.
* It is economical because of a single cable is involved.
*Advantages
(1) New Nodes can be added easily, all nodes can communicate directly with any other node on the bus.(2) DATA packets from one station can be received by all other stations.
Disadvantages
(1) since all the stations share the channel, only one station can transmit at any one time.
(2) the possibility of data collision
Usage
In LANsTree topology
*Also known as the hierarchical or vertical network.
*Control may be assigned to one or more nodes.
*There may be more than one centre node.
* With intermediate nodes, this topology enables the reduction of the total branch length relative to the star topology.
*If one of the centres fails, the network can continue to be partially operational.
*Advantages
(1) the central node need not be directly connected to all other nodes there is a saving of the total branch length and cost.(2)Its reliability is enhanced if the network has more than one centre node.
*Disadvantages
(1) with this topology, there is one route between any 2 nodes. If any part of the route is used by another pair of node.there is one route between any 2 nodes, there is no alternative path.Usage
(1) remote access networks.Star topology
*All the nodes are connected to a central controller which supervise the operation of the entire network.
*Central site is responsible for routing traffic and fault isolation.
*There are the potential bottleneck and failure problems at the Central site.
*Advantages
(1) With the centralized control, data sequencing is easier as the routing is being done by the central node.(2) security schemes can also be implemented more easily.
(3) When one station fails, other stations will not be affected as control is centralized.
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