The OSI Model

Note by Liam-Beckwith, updated more than 1 year ago
Created by Liam-Beckwith about 5 years ago


This set of notes will discuss the OSI Model and look each of the seven layers. It will also look at why the OSI Model with introduced as a way to set standards.

Resource summary

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StandardsWe have a lot of hardware; computers, network cards, motherboards, bridges, switches, cables, routers, firewalls, satellites and the list goes on. But how is all this managed? We have different network topologies with lots of data travelling around in packets. At the same time, we have different protocols; TCP/IP, FTP, TELNET, ICMP, ARP and again, the list goes on.All this is managed through 'standards' that define protocols and how we should communicate. A standard is required to ensure full compatibility and interoperability between devices. But what would happen if a 'standard' did not exist? Well, we have sometime called Big Endian and Little Endian in that situation.

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Big and Little Endian If we look at this number 101011. Must people would assume this is Base 2 (Binary) and would therefore equal 53. But it could also equate to 43, depending on the standard you are using.When using Big Endian, the Most Significant Bit (MSB) is stored in 0Ah, the lowest address. The other bits would then follow in decreasing order of significance.Little Endian, on the other hand, involves the Least Significant Bit (LSB) is stored in 0Dh, the lowest address. The other bytes then follow in increasing order of significance.But why do we have both? Why do we have VHS and Betamax? Why do we have NTSC and PAL? Very simply because they are competing technologies, fighting each other for business.

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The OSI ModelThis problem was addressed by the International Standards Organisation (ISO) in 1978 and the ISO Model became an international standard, agreed by all countries, in 1984.In short, the ISO set out to achieve the following objectives: To interconnect equipment, made by different manufacturers, through a standard interface To integrate software and hardware, and to be portable on differing systems Create a model which will be adhered to by all countries of the world. They basically wanted to create an open standardised networking model that all vendors would support. Headed up by the ISO, work began on what would be known as the Open System Interconnection (OSI) networking model. It simply had one goal: to standardise data networking protocols to allow communication between all computers across the entire planet.But this caused one more problem. How does information move between computers and applications? Well, the problem was divided into seven manageable problems or layers. Each layer soles one of the seven problems.

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A Basic Overview1. Physical LayerThe Physical Layer ensures the transmission of binary data via a medium. It defines cabling, or other methods of data carriage, connectors, electrical characteristics of the communication channel and the transmitted signal. It also defines the protocol to establish and terminate a connection between two 'directly' connected nodes.The NIC converts the binary data into electrical voltages (high and low). Devices that operate on the Physical Layer include network interface cards, hubs, switches, bridges and routers etc. However, some of these devices do work on other layers as well.2. Data Link LayerThe Data Link Layer provides a reliable link between two directly connected nodes. It does this by detecting and possibly correcting any errors that may occur in the Physical Layer. It also ensures that all of the data from the sending computer has been received, thereby, providing flow, error control, and synchronisation of the Physical Layer.Error detection prevents two computers from accessing the same media simultaneously i.e. collision detection. The Point-to-Point Protocol (PPP) is an example of the Data Link Layer in the TCP/IP stack.3. Network LayerThe Network Layer manages the transfer of packets that are to be forwarded on to different networks, between nodes on the same network. Network Addressing and determining the best path are two operations carried out on this layer. Data is routed through a network and can also be routed through interconnected networks. Splitting the data up for transmission and re-assembling upon reception. The IP part of TCP/IP also operates at this level.4. Transport LayerThe Transport Layer manages any end-to-end messages delivered on the network. It provides a reliable and sequential packet delivery through error recovery and flow control mechanisms. It can also keep track of the segments and provides the ability to resend any failed segments and acknowledge the successful delivery before sending the next packet of data.5. Session LayerThe Session Layer manages the communication between applications after a connection is made. It sets up the session, manages information exchanges, and then breaks it down when the session ends.The Session Inter-Host Communication sets up, maintains, and closes down a session. This operation provides an open communication path with another system.6. Presentation LayerThe Presentation Layer formats the data for exchange between the Application and Session layers. It masks the differences of data formats between dissimilar systems. It also encodes and decodes data, encrypts and decrypts data, compresses and decompresses data.Presentation Data Representation and Interpretation Translation may need to occur between two systems using different presentation standards e.g. different character sets or character codes. The Presentation layers allow data to be interpreted using a set of translations and can also add data encryption for security purposes.7. Application LayerFinally, the Application Layer provides the user interface for communication. These functions typically include; email, file transfer, file management, message handling and database query functions.Application Network Services to Application Programs provides network services to application programs such as electronic mail and file transfer. This includes Telnet, SSH, FTP, TFTP, etc.

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Accessing a Web PageSo how could the OSI Model be applied when someone wants to access a web page? Well let's start on the Application Layer and work down.So on the Application Layer, the Web Browser is used. Let's say the user is operating Internet Explorer. The application will give you the ability to select a web page from a server.Moving on to the Presentation Layer, the Hyper-Text Transfer Protocol (HTTP) could be used. The Web Browser handles the presentation by converting the files stored on the server into a type that can be displayed, such as UNICODE or ASCII (more about these later).The Session Layer will also use HTTP to open up a Transmission Control Protocol (TCP) connection via the browser into the server.Next, the Transport Layer uses TCP to connect to the server. The data is broken into chunks and is transported across the session which is made by the Session Layer.The Network Layer will use the Internet Protocol (IP) to access the unique addresses for the server and computer.The Data Link Layer will use the Medium Access Control (MAC) protocol. The request is handled by the NIC where it is transmitted on to the network.Finally, the Physical Layer will use Carrier-Sense Multiple Access with Collision Detection (CSMA/CD) to transmit the data bits on to the medium without any collisions.

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