Open Systems Interconnection (OSI) Model: Tutorial

Some common protocols of the data link layer are

-PPP -ATM -DLC

Which reference model was the first to standardize network communication?

OSI

Logical Link Control (LLC) layer:

The LLC layer controls frame synchronization. It can also provide flow control and error checking mechanism.

Data link layer (layer 2):

The data link layer transfers data between adjacent networks. This layer is above the physical layer. It defines the encoding and decoding of data packets into frames. It also ensures that data frames reach their intended destination. This layer contains protocols that handle transmission control, flow control, and synchronization. The protocols prepare data bits. The protocols of the layer below transmit these data bits. The data link layer has two sub layers:

Network layer (layer 3):

The network layer provides protocols for the transfer of data sequences from one node to another over the same network. Protocols in this layer provide switching and routing, as well as path creation for transmission. These protocols also perform error handling, congestion control, and packet sequencing. The network layer facilitates connectionless communication. This refers to communication that can travel from a sender to a receiver, without an acknowledgement from the receiver. The network layer also provides a unique address to every host on the network. It contains protocols for message forwarding over subnetworks. Some common network layer protocols are DDP, IP, and IPX. In our example, data segments that the transport layer prepares go to the network layer. At the network layer they are properly addressed to the sender. The network layer also knows the address of the neighboring nodes in the networks. It then selects the path a packet will take and addresses the packet appropriately.

A simple analogy to understand the importance of the OSI reference model:

is that of sending a package through snail mail. If you want to send a gift to your grandmother, you can't just wrap it and place it in the post box. You would need to follow some guidelines, such as writing her address in a proper format and including adequate postage. Mail services also have constraints on the size and weight of packages. You would have to ensure that the package falls within these norms. Similarly, computer networks need guidelines and norms to allow different devices to communicate reliably. The OSI reference model sets these guidelines. The OSI model consists of protocols that specify all the aspects of network communication. These aspects include everything from packet size to address format. Thus, even new devices from any manufacturer, anywhere in the world, can connect to a network if the OSI reference model forms the basis for both of them

Evolution of the OSI Model

The Open Systems Interconnection (OSI) model standardizes the functions of communication networks. The OSI model emerged in the late 1970s to create a unified standard for network communication. The purpose of the OSI model was to restrict manufacturers to producing practical and standardized network devices and software. The structure of communication protocols formed the basis for the OSI model. This model allowed devices from different manufacturers to communicate on the same network. For example, in the image, the LAN network can communicate with the Internet, which has a different structure. The model consisted of two independent projects the International Organization for Standardization (ISO) project; and the International Telegraph and Telephone Consultative Committee (CCITT) project. These projects produced two documents that defined the new standard. In 1983, the two documents merged into a single standard the OSI reference model.

Physical layer (layer 1):

The physical layer is the bottom-most layer of the OSI model. This layer governs communication that uses bit streams. In simple terms, it handles communication via a connecting medium. Communication can be in the form of electrical pulses, light, or radio waves. It encompasses protocols that control hardware that send and receive data over various transmission media. The protocols define the establishment and termination of communication as well as flow control. The physical layer controls the most basic transmission mechanism of a network. It acts as the underlying communication layer for all the layers above it. This layer defines the transmission of raw bits of data (bit streams) over a physical medium. The physical medium carries bits of data in the form of signals (this varies according to the medium). For example, hubs, routers, and other network devices use these protocols to define communication over cables. They also define communication over pin layouts, voltages, and so on. Some common protocols of the physical layer are Ethernet, FDDI, and RJ45. In our example, the host transmits data frames prepared in the data link layer over a LAN network. This network uses a cable between nodes. The physical layer in the laptop's OSI stack will transmit a bit stream over the connecting cables to the closest node on the network. This node will then transmit the bit stream to the next, and so on, until the data reaches the recipient.

Presentation layer (layer 6):

The presentation, or syntax, layer consists of protocols that deal with the presentation of data. These protocols ensure the transmission of data from the sender to the receiver in the correct form. This layer translates data between applications and network formats. The protocols transform data into a form that the application layer can understand. The layer converts any form of data representation and transmitting it over the network. This function provides independence from data representation and prevents compatibility problems. This layer also checks for compatibility with the operating system at either end. The presentation layer encapsulates data into "envelopes" for transmission. Some of the common services this layer provides are data conversion, character translation, encryption and decryption, and compression. Some common protocols are ASCII, GIF, and MIDI. Let's assume the website you're viewing contains images. The presentation layer checks the compatibility of the image file formats with your browser and operating system. It also converts the data from "envelopes" that the network transmitted into the proper image format so you can view them.

Session layer (layer 5):

The session layer (also called port layer) handles the communication between two computing systems. It has protocols that establish, manage, and terminate connections between applications. Different applications usually establish these connections to remote servers. The channel of communication between two end-user applications is a session. The session layer provides authentication, authorization, and session-recovery services. This layer responds to requests from the layer above it (the presentation layer). It then issues service requests to the layer below it (the transport layer). Some common session layer protocols are SQL, NetBios, and RPC. In our example, suppose you visit the website of your mail client and log in using your credentials. The browser then requests the creation of a session for a certain period. This session opens on your login credentials and allows communication between your browser and the mail server. The session layer also terminates the session. This termination would occur if you log out of the account or, in some cases, if the connection is idle for too long.

Transport layer (layer 4):

The transport layer facilitates the transfer of variable-length data segments from one host to the other host over a network. It also provides quality-of-service functions. This layer controls the reliability of a link and provides flow control, segmentation-desegmentation, and error control. As the name suggests, the transport layer contains protocols that are responsible for transporting data segments over a network. The TCP component of TCP/IP is an example of the transport layer protocol. Other examples include SPX and UDP. In our example, the protocol segments the data that the session layer transfers down to the transport layer. The segments are prepared for transmission. The host at the other end of the network has a corresponding transport layer. This layer desegments the data segments and checks for errors. These checked data segments then travel to the higher layers at the other host's end. In this example, the other host is the website host.

Application layer (layer 7):

This is the uppermost layer of the OSI model. This layer is closest to the end user, and it directly interfaces with the application. Application layer protocols mainly identify communication nodes, determine availability, and synchronize communication. The functions this layer performs are specific to the application using it. It provides services for mail, file transfer, and other network application services. It also displays information to the user. Some common application layer protocols include Telnet, FTAM, and CMIP. Let's assume you're using a browser to visit certain web resources. In this case, the browser would communicate using protocols in the application layer. The browser uses protocols such as FTP and HTTP to transmit data to and from the network to the user. Web pages communicate using the services that these protocols provide. Requests from the browser are then sent to the next layer.

Media Access Control (MAC) layer:

The MAC layer controls network access and transmission permissions.

Match each protocol with the layer to which it belongs. Pairs 1. application 2. presentation 3. session 4. transport 5. network 6. data link 7. physical

1. HTTP 2. GIF 3. RPC 4. SPX 5. IPX 6. ATM 7. FDDI

The OSI model:

was officially published in 1984. This model had two major components. The first was the Basic Reference Model, a seven-layer abstract model of networking. The second component was a set of protocols. Charles Bachman (Honeywell Information Services) developed the seven layers. According to his model, a network had many layers. Each layer defined the functioning of one or more components. Each component could communicate directly only with the layer immediately below it. The component could also provide facilities to the layer just above it. The protocols enabled communication between layers on different hosts.

more info:

After the bits reach the recipient's physical layer, this layer reassembles the data bits. The OSI model then runs the process in the opposite direction until the application layer gets the final message. The physical layer reassembles the electrical or optical signals received from the communication medium as bits. These bits are sent to the data link layer. This layer converts these bits back into frames. The frames are sent to the network layer. Protocols at this layer convert the frames into packets. The packets are sent to the transport layer. The transport layer converts the packets into segments. These segments are sent to the session layer for conversion into the required data. The session layer also checks whether the connections are proper and the session is valid. The data then passes to the presentation layer. This layer checks the data and converts it into the required format. Finally, the data reaches the application layer in the correct format. At this point, the prepared data directly interfaces with the application and performs the required action at the receiver host.

OSI Model - Layers

Communication devices implement the OSI model as several protocols spread across seven layers numbered from 1 to 7. Control passes from one layer to the next. Each layer controls a specific set of protocols and defines their communication. The end user usually is in direct contact with the topmost layer (layer 7). To understand the layers better, imagine that you are using your favorite browser to surf the Internet. Let's examine the various layers of the OSI model in this context, starting from the topmost layer.

Uses of OSI

OSI concepts are useful tools to explain network protocols and technologies. With the help of these tools, you can understand how networks and network protocols work. The OSI model has layers. Therefore, it helps define the communication between connected protocols within similar layers. It also defines which devices and protocols can interact with one another. The OSI model provides the following benefits: -It guarantees that any hardware or software that meets the standard will be able to communicate with any other device that follows this model. -It allows a wider choice of products to customers, because any OSI-compliant hardware or software will work together. -The OSI model is independent of country or region. -The OSI model is independent of operating system or platform. -Since it follows a layered approach, each layer has protocols and supports error handling. Each layer can also operate automatically.