TREE TOPOLOGY

The tree topology is a generalization of the bus topology. The transmission medium is a branching cable with no closed loops. The tree layout begins at a point known as the headend, where one or more cables start, and each of these may have branches. The branches in turn may have additional branches to allow quite complex layouts.

Again, a transmission from any station propagates throughout the medium and can be received by all other stations. Two problems present themselves in this arrangement. First, because a transmission from any one station can be received by all other stations, there needs to be some way of indicating for whom the transmission is intended. Second, a mechanism is needed to regulate transmission.

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BUS TOPOLOGY

For the bus, all stations attach, through appropriate hardware interfacing known as a tap, directly to a linear transmission medium, or bus. Full-duplex operation between the station and the tap allows data to be transmitted onto the bus and received from the bus.

A transmission from any station propagates the length of the medium in both directions and can be received by all other stations. At each end of the bus is a terminator, which absorbs any signal, removing it from the bus.

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ring topology

In the ring topology, the network consists of a set of repeaters joined by point-topoint links in a closed loop. The repeater is a comparatively simple device, capable of receiving data on one link and transmitting them, bit by bit, on the other link as fast as they are received, with no buffering at the repeater. The links are unidirectional; that is, data are transmitted in one direction only and all are oriented in the same way. Thus, data circulate around the ring in one direction (clockwise or counterclockwise).


Each station attaches to the network at a repeater and can transmit data onto the network through that repeater. As with the bus and tree, data are transmitted in frames. As a frame circulates past all the other stations, the destination station recognizes its address and copies the frame into a local buffer as it goes by. The frame continues to circulate until it returns to the source station, where it is removed. Because multiple stations share the ring, medium access control is needed to determine at what time each station may insert frames.

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STAR NETWORK

In the star LAN topology, each station is directly connected to a common central node. Typically, each station attaches to a central node, referred to as the star coupler, via two point-to-point links, one for transmission and one for reception. In general, there are two alternatives for the operation of the central node. One approach is for the central node to operate in a broadcast fashion. A transmission of a frame from one station to the node is retransmitted on all of the outgoing links.

In this case, although the arrangement is physically a star, it is logically a bus; a transmission from any station is received by all other stations, and only one station at a time may successfully transmit. Another approach is for the central node to act as a frame switching device. An incoming frame is buffered in the node and then retransmitted on an outgoing link to the destination station.

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LAN topology

LAN topology

There are four basic types of LAN topology.




* STAR
* RING
* BUS
* TREE

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What is the OSI model ?

The OSI model:

* Allows various “open” systems to communicate.
* The Open Systems Interconnection model was created by the International
Standards Organization in the late 1970's.
* Serve as a blueprint for all network communication technologies.
* Dividing up all the processes of networking activity into seven layers.
* Each layer has its own distinct functions and services.


The OSI model consists of seven layers which are:


1. The Physical Layer: transmits raw data bits over a communication channel (mostly
mechanical and electrical issues)
2. The Data Link Layer: guarantees to the network layer that there are no
transmission errors by breaking the input data stream up into frames and sending
back acknowledgement frames
3. The Network Layer: controls the operation of the involved subnet; main issues
are routing (determine a way from source to destination) and dealing with
problems of heterogeneous networks, e. g. different size requirements of
transmitted data blocks
4. The Transport Layer: splits up data from the session layer if necessary
(segmentation) and ensures that the pieces arrive correctly
5. The Session Layer: allows users on different computer systems to establish a
session between them, i. e. they are able to transfer files or log into a remote
system; the conditions of communication are laid down, for example full-duplex
or half-duplex
6. The Presentation Layer: unlike the layers before it is concerned with the syntax
and semantics of the transmitted information; it is concerned with all aspects
of information representation such as data encoding, data compression and
encryption
7. The Application Layer: contains a variety of commonly needed protocols like
handling with different terminal types and file systems; a label to identify the
communication process, its origin and destination application is added to the
transmitted information

Layers 4 to 7 are true end-to-end layers, i. e. the layer on the source system carries on a communication process with the same layer on the destination system. In the lower layers the protocols are between a system and its immediate neighbour, for example the source system and a system "on the way" to the destination.

Some of the functions of the physical and the data link layer are combined in the Medium Access Control (MAC) sublayer, which in particular is important to Local Area Networks (LANs). It determines how devices attached to the network gain access to the transmission medium.
Note that the OSI model does not lay down the specific protocols used to communicate between two computers on a specific layer. Although ISO recommends which protocols to use with the OSI model, the model itself is in proper speaking no standard of computer networking. Which protocol in a single layer is actually used, depends on several factors like the physical network, the needed reliability, etc. ictglobal.com

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The Importance of Computer Networks

Describes why and how computer networks support successful work

Information and communication are two of the most important strategic issues for the success of every enterprise. While today nearly every organization uses a substantial number of computers and communication tools ( telephones, fax, personal handheld devices), they are often still isolated. While managers today are able to use the newest applications, many departments still do not communicate and much needed information cannot be readily accessed.

To overcome these obstacles in an effective usage of information technology, computer networks are necessary. They are a new kind (one might call it paradigm) of organization of computer systems produced by the need to merge computers and communications. At the same time they are the means to converge the two areas; the unnecessary distinction between tools to process and store information and tools to collect and transport information can disappear. Computer networks can manage to put down the barriers between information held on several (not only computer) systems. Only with the help of computer networks can a borderless communication and information environment be built.


Computer networks allow the user to access remote programs and remote databases either of the same organization or from other enterprises or public sources. Computer networks provide communication possibilities faster than other facilities. Because of these optimal information and communication possibilities, computer networks may increase the organizational learning rate, which many authors declare as the only fundamental advantage in competition.

Besides this major reason why any organization should not fail to have a computer network, there are other reasons as well:

* cost reduction by sharing hard- and software resources
* high reliability by having multiple sources of supply
* cost reduction by downsizing to microcomputer-based networks instead of using
mainframes
* greater flexibility because of possibility to connect devices from various
vendors

Because of the importance of this technology, decisions of purchase, structure, and operation of computer networks cannot be left to technical staff. Management as well has a critical need for understanding the technology of computer networks. ictglobal.com

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