Local Area Networks (LAN) - Topologies

A Local Area Network (LAN) is a group of computers located in the same room, on the same floor, or in the same building that is connected to form a single computer network. local area networks (LANs) allow users to share storage devices, printers, applications, data, and other network resources. They are limited to a specific geographical area, usually less than one kilometer in diameter.

What is a Local Area Network (LAN) system?

Typical LAN system in an organization with different departments namely financial, engineering, sales, operations, and publications each connected with shared storage devices and peripherals such as printers.

Local Area Networks (LAN) - Topologies

The above-given image shows the basic physical characteristics, information transfer, and shared device concepts underlying LAN operation. the line printer and magnetic tape storage are shared resources since any user attached to the network can access these devices through PCs acting as the resource manager, or server, as it is commonly called in LAN terminology. In some networks, the users can directly exchange data or files from Mainframe or mini computers. 

Local area networking is a form of computer communications generally used for the internal transfer of data and information within an organization.

Characteristics of LANs

Local area networks are a specialized form of communication systems, However, there are three primary characteristics of LANs that distinguish them from wide area networks such as Telnet, Tymnet, CompuServe, and the public-switched telephone network (PSTN). These are:
  1. LANs are designed to work in a restricted geographical area
  2. LANs operate at relatively high speeds when compared to the typical wide area networks currently in use. LAN data transfer speeds may be as high as 80 million bits per second (80 Mbps) or slightly less than 10 million characters per second (10 Mcps). compare this to the maximum data transfer speed of 56 Kbps (7000 Cps) for high-grade telephone company digital trunk lines, or the 1200/2400 bps (120/240 Cps) transmission speed used by most personal computers communications systems.
  3. LANs are private networks, not subject to tariffs or other regulatory controls, from the government.

Different Topologies used in LANs

Topology is the method by which networks are physically connected together. Topology determines the complexity of connecting computers and therefore the cost of network cable installation. cable installation can often be a major cost factor for the network systems. Topology also determines the strategy for physically expanding the network, in the future. Different topologies are described in the following sub-sections.

Various ways in which computers may be connected physically together (i.e topology) are described below. these topologies are named linear bus, circular, star, tree, and graph or mesh topologies. The most commonly used ones are linear bus topology or star topology in the networking systems. other types of topologies are used where very high network system reliability is required. however, the other types of topologies involve higher cost and therefore, are not commonly used.

Linear Topology or Bus Topology

In Linear bus topology, all computers are connected by a single length of cable with a terminator at each end. the bus topology is the simplest and most widely used local area network design. it is a passive topology which means only one computer at a time can send a message. Hence, the number of computers attached to a bus network can significantly affect the speed of the network. a computer must wait until the bust is free before it can transmit. each node is connected to two others except the machines at either end of the cable, which is connected only to one other node.

Linear bus topology
A linear bus topology

The network operating system keeps track of a unique electronic address for each node and manages the flow of data based on this addressing scheme. this topology has the advantage of not requiring that every computer be up and running in order for the network to function. but because a single cable is dedicated to all the information traffic performance can be slow at times. where one of the machines on the network is designated as a file server meaning that it is dedicated solely to the distribution of data files, and is not typically used for information processing.

Ring or Circular Topology

This layout is similar to the linear bus, except that the nodes are connected in a circle using cable segments. In this layout, each node is physically connected to only two other nodes. each node passes information along to the next until it arrives at its intended destination. Since each computer re-transmits what it receives, a ring is an active network and is not subject to the signal loss problems which a bus topology experiences. there is no termination because there is no end to this ring.

ring or circular topology
A ring or circular topology for token ring network

Performance on this type of topology is faster because each portion of the cabling system is handling only the data flow between two machines. this type of topology can be found in peer-to-peer networks, in which each machine manages both information processing and the distribution of data files.

In-Ring topology, computers are arranged in a circle. data travels around the ring in one direction with each device on the ring acting as a repeater. ring networks typically use a token-passing protocol.

Start Topology

Each computer on a start network communicates with a central hub re-sends the message either to all the computers (in a broadcast start network) or only to the destination computer (in a switched star network). The hub in a broadcast star network can be active or passive.

A star topology
A star topology

An active hub regenerates the electrical signal and sends it to all the computers connected to it. this type of hub is often called a multiport repeater. active hubs and switches require electrical power to run. a passive hub merely acts as a connector point and does not amplify or regenerate the signals. passive hubs do not require electrical power to run.

Ethernet 10BaseT is a popular network based on the star topology. you can use several types of cable to implement a star network. The hybrid hub can accommodate several types of cable in the same start network. you can expand a star network by placing another star hub where a computer might otherwise go, allowing several more computers or hubs to be connected to that hub. this creates a hybrid star network.

Tree Topology

This is a network topology containing zero or more nodes that are linked together in a hierarchical fashion. the topmost node is called the root. the root may have zero or more child nodes, connected by edges (links), the root is the parent node to its children. each child node can in turn have zero or more children of its own.

A tree topology
A tree topology

Nodes sharing the same parent are called siblings. every node in a tree has exactly one parent node (except for the root, which has none), and all nodes in the tree are descendants of the root node. these relationships ensure that there is always one and only one path from one node to any other node in the tree.

Graph or Mesh Topology

In this method is a connection, zero or more nodes are linked together in an arbitrary fashion. any two nodes in a graph may (or may not) be connected by a link.

A graph or mesh topology
A graph or mesh topology

Not all nodes in a graph need to be connected, but if a path can be traced between any two nodes, the graph is a connected one.

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