OSI Model

TUTORIAL 2 - OSI MODEL

OSI model is a reference tool for understanding data communication between any network systems. It consists of seven layers where one layer performs specific functions to support the layer above it.

LEVEL 1- PHYSICAL LAYER

·      Describes the electrical or optical signals used for communication. It is responsible for movement of bits from one node to the next. It is concerned with physical characteristics of electric or optical signalling techniques , representation of bits, data rate (number of bits sent per second and duration of the bit), synchronization of bit, line configuration( connection of devices to the media), physical topology and transmission mode(simplex, half-duplex, full duplex).                                  

LEVEL 2- DATA LINK LAYER

·    Transforms physical layer into a link making it error free to the layers above. It is responsible for moving frames i.e packets with address information. It is concerned with framing which divide bits into data units knows as frames, physical addressing of the frames, flow control of the data, error control which adds reliability to the link, and access control which determines which device has control over the link.

LEVEL 3- NETWORK LAYER

·    Source to destination delivery of packets (small chunks of broken data sent over a computer network) ensuring that it arrives at its destination intact. It is responsible for Logical addressing adding a header to the packet received from the above layer and Routing which sends the packet to its final destination.

LEVEL 4- TRANSPORT LAYER

·   Responsible for process to process delivery of the entire message ensuring that it arrives at the destination intact and in order. Other responsibilities include error control, flow control, connection control between devices and terminating it after data is transferred, segmentation and reassembly where a message is divided into segments that contain sequence numbers which allows for the message to be reassembled upon destination arrival and service point addressing also known as port address.

LEVEL 5- SESSION LAYER

·    It is responsible for developing, maintaining and synchronizing interaction between communication systems. It is also its responsibility to allow systems to dialog allowing processors to communicate using the same transmission mode and synchronization of the data.

LEVEL 6- PRESENTATION LAYER

·   This layer is responsible for the translation of the information to bits prior transmission, compression of data to reduce the number of bits contained, and encryption which transforms the original information into a different form before sending it forward.

LEVEL 7-APPLICATION LAYER

·      Allows user access to the network. It offers mail services, directory services that provide distributed database source and access to global information, file transfer, access and management, and network virtual terminal that allows a user to log onto a remote host.

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REFERENCE LIST

Simoneau,P.2009.The OSI model: Understanding the seven layers of computer networks(PDF).:Global Knowledge Training. Available: http://faculty.spokanefalls.edu/Rudlock/files/WP_Simoneau_OSIModel.pdf (Accessed 04 September 2015)

Forouzan, B,A.2007.Data Communication and networking(online).New York,USA.Alan R Apt.Available: http://iit.qau.edu.pk/books/Data%20Communications%20and%20Networking%20By%20Behrouz%20A.Forouzan.pdf  (Accessed 07 September 2015).

Network Topologies

SYSTEM SOFTWARE 1 MODULE 2

TUTORIAL 1 - NETWORK TOPOLOGIES

Topologies describe the physical and logical layout of a network. It is crucial to understand the concept of topologies so as to have better knowledge and be able to create new topologies when working with networks.

v  Logical topology refers to how the hosts access the medium and communicate on the network. It is commonly divided into two, mainly: Broadcasting where a host broadcast a message to all hosts on the same network segment on a first in, first out basis and Token passing that controls network access by passing an electronic token sequentially to each host.

v  Physical topology defines the way devices on a network are connected. Types of physical topologies include

·         Bus topology- all computers are interconnected using one cable. The example of a bus line through the city can be used as imagery for how the computers are connected.

The cable has a smaller cap allocated at the end called a Terminator that prevents signals from bouncing back and causing errors.

·         Ring topology has devices connected in a physically circular shape. This topology has no beginning or end.

·         Star topology has a central point which is usually a hub, switch or router.                                                                                                                             

Each device is connected directly to the central point by means of a cable segment. If any cable problem occurs, only the host is affected.

·         Hierarchical / Extended star topology is a star topology with a networking device connected to the main networking device. It is used by larger networks i.e universities and corporations.

·         Mesh topology connects all devices to each other. A failure in cabling or device connection does not affect the network

·         Hybrid topology is a combination of two or more basic network topologies. It can be implemented for a number of different network environments.

DIAGRAMS

REFERENCE LIST

Boger, P.ed.2013.IT ESSENTIALS:PC Hardware and software Companion Guide.FifthEdition.Indianapolis,USA:Cisco Press.

Image

Network topologies diagram.2015.Available: http://www.conceptdraw.com/examples/topologies.
(Accessed 07 September 2015)