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Introduction to Networks

Types of Computer Network Topologies: Bus,Star, Ring Topology

What is a Network topology

A topology is the characteristic of a communication network that is concern bothwith the physical configuration of the cabling that is used to inter connectcommunicating system and the logical way in which system view the structure of thenetwork. Topology is the therefore physical or logical arrangement of computers

Factors to consider when selecting a topology

1. Cost whatever transmission medium is chosen for a Local Area Network hasto be physically installed in the building, this maybe a lengthy process including

the installation cables and raceways. For a Network to be cost effective onewould strive to minimize installation cost. This may be achieved by using theproper hardware linking the cables, good modems, cost effective computers toreduced cost

2. Flexibility one of the main benefit of a local Area Network is it ability to havethe data processing and peripheral nodes distributed around a given area. Thismeans that the computing power and equipment can be located close to theultimate users. Because the arrangements of furniture, internal walls etc in theoffices is often subject to change. The topology should allow for easyreconstruction of the network which involves moving existing nodes and addingnew nodes

3. Reliability The topology chosen for the network can help by allowing thelocation of the fault to be detected and to provide some means of isolating thefaults

Topologies

Networking topologies are organized by the way in which information "flows" acrossa network. Below are the basic topologies:

At the core of the Network+ exam and networking concepts in general is the idea oftopology, or more specifically, the manner in which data is exchanged over thenetwork. Network topology is a mainly conceptual topic - when we speak of "star"networks or "ring" networks, we are really speaking in terms of the manner in whichinformation is exchanged and not their physical setup. Remember that eachtopology/network type has its unique advantages and disadvantages that willbe tested on the Network+ exam in the form of asking you "which is the mostappropriate." Don't try to memorize the perks of each - rather, try to understand themanner in which each allows the exchange of information; then, the advantages anddisadvantages will seem only logical to you.

Bus - This is the most simplistic topology in which the nodes of the network are

individually linked to up to two successive othe nodes or another node and aterminating node or terminator. This is considered now an archaic topology, because

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of the difficulty of troubleshooting network issues (how do you know which node iscausing the connection issue?), redundancy issues (if one node fails, thenetwork as a whole can fail), the need for terminators, and the amount of trafficcreated (every node between A and B must receive the packet that A sends).The nodes linked in this topology are often referred to as "daisy-chained."

Ring - Similar to a bus network in that nodes are linked to each other, but dissimilarin that the ends of a ring network are not terminated because, well, there are noends! A ring network is something like a "circular" network in which each andevery node is linked to two other nodes. This shares many of the sameweaknesses as the bus topology, including troubleshooting difficulty, redundancyissues, and traffic created, and also adds an additional difficulty - the difficulty ofadding a node to a token ring network.

Star - This is the most typical and practical network setup. In a star network, eachnode maintains an individual connection to a switch, where all other nodes areconnected. Traffic between two known nodes, therefore, only goes through theswitch and not through other nodes. This increases the redundancy of the network(one computer faltering will not cause the network to fail), increases data privacy(unicast traffic does not travel through all nodes), and is a relatively easy-to-usesetup. Disadvantages include reliance on the switch (a fail-point) and the amountof wiring necessary.

Mesh/"Ad Hoc" - This is a rarely occurring configuration in which every node is

connected to every other node; it usually occurs only in wireless networks in "ad hoc"mode, which will be discussed later; in this mode, each wireless card maintains aconnection to each other wireless node it wishes to connect with, forming a "mesh"of a network. This is a relatively easy to understand option but is inefficient, requiresa large amount of overhead, and is difficult to manage.

Combined or Hybrid - This is simply a topology referring to the case where morethan one topology is utilized. For example, you may have three token ringnetworks connected to a central hub, forming a star of token rings. This is one ofmany possibilities of a hybrid network.

[edit section] Types of Networks (Access Models)

The Network+ examination is interested in your ability to identify network accessmodels, generally referred to as types of networks. This does not suggest the way inwhich network nodes are connected or the way that information flows (as dotopologies), but rather, the manner and mode in which nodes communicate witheach other and share information. There are three basic types:

Decentralized - Often referred to as "peer to peer" network, a decentralizednetwork does not contain any distinctions between client and server. In a

decentralized network, every node acts as a client and/or a server depending on thetask at hand. For example, many file sharing networks are considered

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"decentralized" because nodes both download and upload (serve) files. The ease ofadding nodes and the ease of setup is a drawing point of decentralized networks, butthe pivotal downfall of these networks is their difficulty of maintenance (a settingmust be changed on each node to reflect a setting change on the whole network).

Client-Server Access - In this type, nodes can either act as "clients" or "servers,"requesting or handing out information. Do not confuse the model with the startopology; though the star topology often utilizes the client-server access model, thisdoes not infer that every client-server network utilizes the star topology. In a client-server network, management is easy and the network can offer services thatdecentralized networks cannot, but this comes at the expense of difficulty in setup,setup cost, and server reliance.

Centralized - A centralized network is a modified client-server network in which

the clients have no individual control; that is, all maintenance and setup occurs atthe server level. The extreme ease of management and ability to micromanage isa drawing point of this access method, but the prohibitive cost and inflexibility ofthe method deter many from employing it.