OSI Model

To address and resolve the issues associated with computer communication, ISO (International Organization for Standards) standardized the communication system by setting an International standard, OSI (Open Systems Interconnection model).

In OSI model, the important functions of the communication protocol were broken down into 7 layers:

1. Application Layer
2. Presentation Layer
3. Session Layer
4. Transport Layer
5. Network Layer
6. Data Link Layer
7. Physical Layer

The complicated network protocols get simplified with these layers. Every OSI Layer gets a particular service from its lower layer and every layer has to give a particular service to its upper layer. The conventions that administer the interaction between upper and lower layers are referred to as “interfaces”. The conventions that administer the interactions between layers are referred to as protocols.

Protocol layering is similar to modular development in computer software. The OSI reference model is ideal, as it proposes to implement all the modules from the first layer to the seventh layer and combine them to enable proper network communication. Through this layering suggested in the OSI model, there is an independent usage of every layer, which limits the effects of any layering alterations on the entire system. Thus, an extensible and flexible system is probable to be created. Further, as the communication functions are divided into layers, assigning separate protocols and certain responsibilities to each layer may require fewer efforts. Therefore, layering has many benefits. However, there are some disadvantages of layering as well, such as over modularity, demanding to process, and applying similar processing logic for each module.

Data is sequentially transmitted by the sender from Layer 7, Layer 6 to Layer 1, whereas data is transferred from Layer 1, Layer 2 to Layer 7 towards each upper layer. The data from the preceding layer can be processed at each layer using the “first” information that is needed by the existing hierarchical protocol. The end of the data is then obtained from the “first” and “content” of the separation, after which it is forwarded to the subsequent layer, after which the end of the data is transmitted back to the initial state.

Now, Let’s look at every layer of the OSI Model with an example:

If Sam wants to wish Estella “good morning” through email, what would the web do? We will perform an analysis from the top to bottom to understand what happens in this process.

1. Application Layer
   This OSI Layer offers services to the application and mentions the particulars of communication in the application, such as file transfer, email, remote login (virtual terminal), and other protocols.

   A new email message is written by User Sam on the host computer (typically in a browser), the recipient is specified as Estella and the message “good morning” is entered. The software used to send and receive emails can essentially be differentiated into two functional categories, where one is communication-related and the other is not. For instance, the non-communication-related function is when Sam writes “good morning” on the keyboard, while the communication-related function is when the message is sent by Sam to Estella. Therefore, the application layer equals to “writing the contents of the e-mail and then sending it to the target audience”.

   The processing of the application layer protocol starts from the point the user enters the message he wants to send and presses the “send” button. A header (label) is assigned by the protocol to the front end of the data that is to be transferred. In the first part, the message is recognized as “Good Morning” and the recipient as “Estella”. This data with the message entered is dispatched to host Estella, after which the mail-receiving software on the host receives the content through the “mail-receiving” function. Following the application on the host, the data sent by host Sam is received by Estella, and analysis of the data header and data body is carried out. The data will be then be saved onto a hard drive or some other non-volatile random-access memory, a storage device where it is not possible to lose data even during a power outage. If the mailbox space of the recipient Estella is full and is unable to receive new messages, the sender receives an error. These kinds of exceptions also face problems during their handling and have to be solved using the application layer.

2. Presentation Layer
   The information processed by the application is converted by this OSI Layer into an appropriate format for network transmission or the data from the next layer is converted into a format that can be dealt with by the upper layer. This indicates that the layer chiefly handles the data format conversion. To be more precise, the data can be formatted by the device into the network standard transmission format. There may be a variation in the interpretation of the same bitstream by various devices, which means that the chief role of this layer is increasing their consistency.

   “Presentation” in the presentation layer refers to representation and demonstration. Therefore, the focus is on robust presentations of data (most commonly, how data is assigned by the computer in memory differently, usually, large and small entities). Furthermore, when different software applications are used, there will be a performance of distinct kinds of data. For instance, files are generated by a few word processors that only a particular version of the software offered by the word processor machine can open and read.

   So what should you do if you face this kind of issue in your email? If the same mail client software is used by both Sam and Estella, then they may not face any issues in sending and receiving emails. However, this is not likely to happen in reality. It is very inconvenient to get all users to use the same client software in a cookie-cutter manner (in the present times, there are other devices apart from PCs that are also connected to the Internet, like smartphones). It is becoming increasingly important to become capable of reading communications shared between one another.

   This issue can be solved in various ways. The first is to use the presentation layer to transform data from “A computer-specific data format” to a “standard network data format” before it is dispatched. Once the data is received, it is restored by the receiving host in the network standard format into the “computer-specific “computer-specific data format”, after which it is appropriately processed.

   Data was transformed in the preceding example to a shared standard format before being processed, which creates consistency of data among the heterogeneous models. This is what the purpose of the presentation layer is. That is, it is a layer that changes the “uniform network data format” and the “data format specific to a computer or a part of software”.

   In this example, the message “Good Morning” is transformed into a “unified network data format”, corresponding to its encoding format, even if it is a simple stream of text that can be encoded in different complex formats. For example, consider Japanese, such as EUC-JP, Shift, ISO-2022-JP, UTF-8, UTF-16, as well as various other encoding formats. If you are unable to encode it in a particular format, then at the receiving end, the recipient of mail may also be garbled (in actual life, the situation when the receipt and delivery of mail become garbled is not uncommon. This typically happens when the presentation layer does not operate in the desired encoding format, or the encoding format is not set accurately).

   To determine the encoding format, the header information will be attached by the presentation layer, and this information will transmit the actual data that has been transferred to the subsequent layer for processing.

3. Session Layer
   This OSI Layer handles the creation and disconnection of the communication links (logical path of data flow) and data transfer related tubes, like data partitioning manager.

   We should now examine how to have efficient interactions with data between the session layers of the hosts on the two sides of data transmission. For example, consider a situation where five new e-mails have been prepared by Sam to dispatch to user Estella. It is possible to send these five messages in any order. For instance, a connection can be established (a communication connection), which is disconnected every time a message has been sent. It is also possible to send 5 emails in a sequence after a connection has been established. Five connections can also be established at the same time, and five emails can be sent to the recipient at the same time. The session layer is mainly responsible for determining the most appropriate connection method.

   A header or label is attached by the session layer (similar to the application or presentation layer) to the front side of the data it receives before it is sent to the subsequent layer. Information is present in these headers or tags regarding the order followed during transmittal of data.

4. Transport Layer
   This OSI Layer serves the purpose of a dependable transmitter. Processing is not only carried out on the router but also on both sides of the communication.

   It is made certain by the Host that there is communication with host Estella and data can be dispatched readily. This process is known as “making connections”, where the e-mail dispatched by host Sam is received by host Estella, and the final data is received by the Mail Handler of host Estella. Furthermore, the connection should be disconnected after communication transmission is complete.

   As mentioned earlier, processing should not be connected or disconnected (it should be remembered that the session layer decides when the connection should be established and disconnected, whereas the actual connection and disconnection processing is carried out by the transport layer). The transport layer is mainly responsible for generating logical communication among the two hosts. Furthermore, the arrival of the transmitted data at the destination address between computers at the two ends of the communication is validated by the transport layer, which then transmits the data again if it has not reached yet.

   For instance, a “Good Morning” message is sent by a host to Estella. The data may become compromised at this point for some reason, or because of a certain network exception that is part of the data going towards the destination address. If Host Estella does not receive the complete message, missing out on the “morning” part, then it will be informed that it has not received the “morning” part of the data. Also, Sam will be informed about the incomplete message sent, and will resend the data to host Estella, and will be asked to verify again if the complete message was received by the other party.

   This is like people asking others what they just said in routine conversations. This shows that the fundamental principles of computer communication protocols are closely related to our everyday life. Therefore, a critical role of the transmission layer is to make sure that data transmission is reliable. Reliability can be ensured by attaching a header to the data that is to be transferred at this layer to determine the hierarchical data. Practically, however, the network layer performs the tasks of transmitting data to the other party.

5. Network Layer
   Through this OSI Layer, the data reaches the destination address, which may be a single address, connected to multiple networks through a router. This is why the network layer chiefly handles addressing and routing.

   The network layer transmits data from the sender to the recipient in a network-to-network connected setting. Though there are several data links among the two end hosts, the network layer offers the ability to transfer data from host Sam to recipient Estella.

   When data is being dispatched, an important element is the Destination Address that is used for communication. The network indicated the unique ordinal number. Consider this as a phone number that is used in our routine lives. After identifying the destination address, the computer used to dispatch the data to the destination address can be determined from several computers. Based on this address, there is a dispatching and process of packets at the network layer. The address and network layer packet sending processing can be used to dispatch data to any interconnecting device across the globe. The data and address information acquired from the upper layer is also sent by the network layer to the data link layer beneath for subsequent processing.

6. Data Link Layer
   The physical level of interconnection, communication between the nodes is handled by this OSI Layer. For instance, in the communication between two nodes connected to one Ethernet network, the 0, 1 sequence is divided into meaningful data frames and transmitted to the opposite end (generation and reception of data frames).
7. Physical Layer
   This OSI Layer deals with the 0, 1-bit current (0, 1 sequence) and voltage levels, light flash between the exchange.

   Physical transmission media is used to achieve transmission communication. The data link layer is where data is processed between the devices connected through a transmission medium. Within the physical layer, there is the conversion of 0 and 1 of the data into voltage and pulse light for being transferred to the physical transmission medium, whereas connected devices directly use the address for transmission. This address is referred to as a Mac (Media Access Control) address or a physical or hardware address. The purpose of using the MAC address is to determine devices linked to the same transmission medium. Hence, in this hierarchy, the initial part including the Mac address information is connected to the data dispatched from the network layer to the network. Data is sent by the network layer and the data link layer to the receiver based on the destination address; however, it is the responsibility of the network layer to transfer the whole data to the ultimate destination address, whereas only the data within a segment is dispatched by the data link layer.

The way processing is carried out on host Estella is contradictory to that of host Sam. The processing commences from the physical layer and the data received is dispatched to the preceding layer for processing. Therefore, the mail client software can be viewed by user Estella where the mail sent by user Sam is received. User Estella is now able to read the content received as “good morning”. The functions of a communication network can be considered by the reader in layers. The protocol on every layer defines the format of the data header in that layer and the order in which processing of the header and the data takes place.

Relationship between Transport and Network Layer

Under the various network configurations, at times the network layer is unable to guarantee data accessibility. For instance, in an IP protocol equal to the TCP/ IP Network Layer, it is not guaranteed that data will be sent to the recipient. Hence, there will be a significant increase in data loss, risk of confusion, and other issues during the data transmission process. In this kind of network layer, in the absence of no transmission requirements, it may be the responsibility of the transport layer to offer “appropriate transmission data handling”. There is a collaboration between the network layer and the transport layer in TCP/IP to make sure that there is a dependable transmission of packets worldwide. When the role and responsibility of every layer are clear, then the specification and implementation (software coding particular protocols to enable them to work on a computer) of these protocols becomes easier.