OSI Model, stands for Open System Interconnection model. It is a conceptual framework that was developed by International Organization for Standardization so as to give standard conventions for setting up powerful communication among network hubs. In essence, OSI layers provide its users with certain standards with which effective communication can be done between two nodes. The primary purpose served by these layers is interoperability amongst various software products. Fundamentally, an OSI model characterizes a coherent system that portrays the transfer of information packets among network nodes utilizing various layers of convention.
The OSI layers are composed of seven abstraction layers where each layer has a specific function to perform. These layers work in hierarchical models in a way that processing output of one layer is served to another for further processing. The OSI layers cooperate so as to transmit information from one network node then onto the next. Also, it is important to note that the OSI model is most often known as Seven Layer Model. Many successful protocols are developed using the approach of the OSI reference model. Also, it is one of the most common models followed to hold inter computer communications. Though OSI model is just a conceptual framework, yet it helps network administrators in troubleshooting various network issues. As, it works by dividing a large and complex problem into smaller chunks and helps in easy identification of the root causes. Thus, in this manner, the abstraction layers of the models help in avoiding certain network issues.
The main agenda of this article is to discuss the â€˜Open System Interconnection Modelâ€™ also known as OSI model. Basically, this is a conceptual model which is mainly used to scrutinize the flow of data with respect to its seven layers. Further, this model characterizes several computing functions directly into the universal set of rules as well as requirements so as to support interoperability amid software and products. In this article, a detailed information about key features of the OSI model with respect to its protocols is discussed. Along with this, the working of each layer and its protocols is given. Moreover, a comparison between TCP/IP model and OSI model concerning its functions and layers is also detailed in the article.
The Seven Layered Model (i.e. OSI Model) is composed of several capabilities where few of which are mentioned as below:
Used for establishing effective communication amongst various network nodes.
Easy troubleshooting of network problems.
Although, there are several layers of this absolute effective framework, yet the model is classified into two layers mainly, the upper and lower layers.
Provides interoperability among software products.
Breaks larger issues into smaller chunks. Every layer of the model is intended to perform explicit subset of a task
Easy Error identification
Enhanced security through encryption of information bundles
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There are a total seven abstraction layers of OSI model where each of these layers are specifically designed to perform a particular task. Each of these layers are explained in the underneath section as follows:
The Application Layer
This is the main layer which communicates with the client through software applications. These software applications may incorporate a few web browsers or email applications which utilizes the application layer for commencement of communication with other system nodes. However, it is important to understand that customer software applications differ from software applications. The conventions like HTTP or SMTP are considered under the application layer for starting email interchanges among the system hubs. The working of Application Layer can be portrayed in the accompanying outline.
The Presentation Layer
The primary purposes served by this presentation layer of OSI model includes the translation of data packets and applying necessary compression techniques in order to reduce the data size. It also makes the use of encryption techniques with a view to ensure that data integrity is maintained while the data is being transmitted over the network. In between all this, it is also imperative to ensure that the receiver is able to read the text contained in the data packet in its original format. The data which was amalgamated should be transformed in its original form. This transformation is done by this presentation layer and the working can be depicted in the below area.
The Session Layer
When a particular communication begins, there are certain sessions that are part of it. These include session establishment, monitoring and management and the end of session i.e. termination. Thus, the session layer is the one that manages the communication between network nodes. It also ensures synchronization of data packets so as to lessen the chances of data loss during the transmission process.
The Transport Layer
The transport layer is the one that chooses if information transmission should be done in parallel mode or single way. Other than this, it performs different functionalities that include multiplexing, portioning and division of larger messages into smaller parts. It also works by receiving the message from its predecessor layer i.e. session layer which are converted into smaller chunks and afterward these are passed onto the network layer. The basic role served by the transport layer is to break the message into smaller parts which are then passed to the network layer for additional handling.
The Network Layer
The network layer performs data transmission by sending the data packets from source location to the destination location in an encrypted format. Besides this, it manages the network traffic and based upon network traffic, it selects an optimal path or route which is followed for the transmission process. The network layer sends the data in smaller packets and on the receiver side, it assembles the arrived data packets which could be anticipated by the receiver node. The network layer works as follows:
The Data Link Layer
The Data Link Layer is in charge of handling the errors that may arise during the data transmission procedure. Synchronization is performed by this layer on the physical layer for avoiding data loss of frames transmitted. Acknowledgements are sent and received by this layer so as to ensure that data packets have successfully arrived at the receiver side without any error. In case, if the receiver does not revert back then, this layer resends the acknowledgement for its confirmation. Also, this layer manages the transmission of data packets from source to destination as if the frame buffer is completed, then it automatically terminates the transmission process. The working of the data link layer can be shown as follows.
The Physical Layer
This layer is responsible for providing suitable medium using which data bits will be transmitted onto the network. It is also responsible for maintaining a physical connection amongst network nodes. Not only this, the data speed at which the bits are to be transmitted over the network is managed by this network layer. Further, it converts the signals i.e. analog signals in the form of digital signals.
Each Layer has its own working and process the data in an effective manner. Functions of each layer in the OSI model are describes below.
The main function of this layer is line configuration which defines how devices can connect physically.
There are multiple transmission modes available, however, the main function of this layer is to define it. For instance, half-duplex, full-duplex and simplex are transmission modes.
Two types of signals are available, this function aids to determine the signal type which has been used for data transfer.
As the name suggests, this layer mainly translates streams into packets so that it can be transmitted easily. Although header and trailer are added to the frame which renders hardware source and destination address.
The next after framing is addressing where the header is integrated to the frame which embraces a destination address.
Error, Flow and Access Control
After all the process of addressing and framing, three functions are performed by the data link layer, namely error control, flow control and access control. The main use of error and flow control is to make sure that data is transmitted in its actual form without any tampering. Despite this, in case data frames are corrupted, retransmission acknowledgement can be generated. Furthermore, in case of access control, it is used to determine the link of devices connected to the network.
This layer primarily integrates source and destination address to frame, so that it can transmit further and reaches its destination.
Routing and Packetizing
In order to identify the best optimal trajectory, routing is performed by this layer and the process of packetizing is also performed.
Segmentation despite reassembling
When this layer mainly receives information from its predecessor, the process of segmentation is initiated. This is done by allocating sequence numbers to messages so that it can be identified easily when reassembling of packets is done.
In this layer, control of connection-oriented and fewer services is also done so that connection can be established securely.
Dialogue Control and Synchronization
In this layer, it primarily acts as a dialogue box for the purpose of creating dialogue amid two processes. Furthermore, the integration of checkpoints is also done so that data can be transmitted in a secure environment.
The process of translation is initiated from here where several strings are converted in another form.
Encryption and Compression
In order to sustain security, encryption is performed so that data cannot be sniffed by the attackers. Also, with encryption, a data compression process is initiated where the number of bits which are going to be transmitted are abridged.
FTAM and Mail services
An acronym used for FTAM is ‘File transfer Access and Management’ which is certainly done by the application layer. This layer enables users to send and receive files without facing any problem. In addition to this, mail services are also accessed by users with the aid of this layer.
There is a close relation between each layer. Let us understand it with the aid of an example, the below-given table represents two hosts i.e. Host A and host B, which are communicating with each other. When a data communication session is established, the protocol unit is changed among the layers which shows the expansion of data. As shown below, communication amid host A and host B is given along with its protocol data unit.
|Host A (Sender)||Protocol Data Unit||Host B (Receiver)|
|Application||Application Protocol Data Unit, Message||Application|
|Presentation||Presentation Protocol Data Unit||Presentation|
|Session||Session Protocol Data Unit||Session|
|Transport||Transport Protocol Data Unit, also known as Segment||Transport|
|Network||Network Layer host-router Protocol, Packet||Network|
|Data Link Layer||Data Link layer host-router Protocol, Frame||Data Link Layer|
|Physical Layer||Physical layer host-router Protocol, Frame||Physical Layer|
The OSI model has definitely a number of uses. However, along with the benefits, there are a number of disadvantages also. Both the advantages and disadvantages are discussed below:
As OSI is a conceptual model with no implementation, there are some benefits of this model which aids users and organizations to an extensive scale. First of all, this layered architecture is also considered as the standard model in networking as it provides in-depth knowledge about functionality of each layer with respect to its protocols. The below listed are several benefits of this model which helps in implementing a network easily.
The foremost advantage of this model is that all layers in this model architecture are distinguished concerning its interfaces, services as well as protocols which are provided by it.
This model allows implementing any arbitrary protocol so that working and functionality of protocol can be tested easily. In simple words, this model has the flexibility to adapt several protocols which are the major advantage of this model.
Essentially, there are two types of services available, namely Connection-oriented and Connectionless services. With the aid of this model, both services can be supported in order to test data transmission from its origin to destination.
The working of this architectural model depends on two techniques, namely Divide and Conquer techniques. As services are categorized under seven layers, the task for administration as well as maintenance can certainly be done easily without facing any difficulty.
There are multiple principles available out of which, abstraction principle is mainly considered by this model. In case there are some changes in any layer, it can certainly be done without affecting the working of other layers.
In terms of security, this model provides an additional protective mechanism as working of each protocol is independent. Despite this, the OSI model is likely to be adaptable as compared to other models.
Network troubleshooting is one of the most significant tasks in networking. However, it can be easily done with this model. Moreover, each layer is defined with a set of rules which aids in communicating data.
Some additional advantages for this model are such as it reduces complexity, it facilitates modular engineering, accelerates evolution, and provides interoperability among vendors.
There are encryption and decryption services available in this model where expansion, as well as compression of message, can be done easily. Furthermore, this ensures that data is reached to its destination without any tampering.
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Despite advantages, there exist several disadvantages for this model which can severely impact network performance. This is the reason this model is still used as a reference model. The following are disadvantages of this model which hinders the working of the network.
Open system interconnection is a hypothetical model which has not been implemented till date. The implementation of this model is not done as it does not support several emerged technologies. Instead of this model, Transmission control protocol/Internet protocol is used.
This model was launched in the 1990's which was not an appropriate time period. However, during that time, protocol of TCP/IP suite was implemented and several organizations adopted it. This is the major disadvantage of the OSI conceptual model.
Among incorporation of multiple protocols, there exists a major issue related to its architecture which is complex. There are seven layers in this model which makes the structure of this model more complex and thus, affects time and cost.
From the top, there are two major layers which are used in the network, namely Presentation Layer and Session layer. Here, the use of both layers is less as compared to the other one still, there are two layers which are not combined. Also, when this model is practically implemented, the functionalities for both these layers were less as compared to other layers.
It is often seen that the same service is offered by multiple layers which should not be done. For instance, services such as error control, addressing and flow control are performed by other layers too.
The standard on which this model is implemented is not adequate as no appropriate solutions are offered for implementing this model in real-time.
When a comparison of OSI is done with TCP/IP model, the practical needs for this model does not pass the test as service of quality is poor. However, with this failure, it is said that this model provides inferior quality.
The working of layers in this model can affect overall network performance as layers do not work parallelly when data request is generated. In this case, one layer has to wait until the data is received by the other layer.
Sometimes, implementation and configuration of the new protocol in this model are a challenging task as invention for this model was done when no protocols were implemented. Therefore, it becomes difficult for new protocols to adopt the environment offered by this model.
The number of layers in OSI model are several which means data needs to pass from total seven layers to reach its destination end. However, at the receiving side also, data will again pass through seven layers so that it can be accessed by receiver. This affects the network performance as a total of 14 rounds needs to be taken by data to completely reach its destination. On the other hand, in the TCP/IP protocol suite, there are merely 4 layers which provide reliability and higher data transmission rate.
As there are seven layers of OSI model, each layer embraces its own protocol which has different functionalities. Name of the layer, with respect to its name and protocol, are mentioned below in an appropriate manner.
|Layer 7||Application||SMTP, HTTP, FTP, SNMP and POPv3|
|Layer 6||Presentation||SSL, TLS, MPEG, and ASCH|
|Layer 5||Session||SAP and NetBios|
|Layer 4||Transport||TCP and UDP|
|Layer 3||Network||IP version 4, IP version 6, IPsec, MPLS and ARP|
|Layer 2||Data Link Layer||Fibre Cable, Frame Relay, ATM and PPP|
|Layer 1||Physical Layer||Etdernet, 100 base TX and ISDN|
There are two types of models available which can be used for data communication among two or more networks. These models are named as OSI and TCP/IP Model. An acronym used for OSI is ‘open system interconnection’ whereas TCP/IP is entitled as ‘Transmission Control Protocol/Internet Protocol’. Out of both models, only TCP/IP is implemented in real-life networks while OSI is just used for reference. Mainly, OSI conceptual model characterizes as well as standardizes different software and hardware components which are involved in a network communication session. The working of this model depends on 7 layers while TCP/IP works on four layers. There are several similarities among both these models, but implementation of TCP/IP is effective, as the number of layers in this model are less as compared to OSI.
In addition to this, data processing also depends on the number of layers, for instance, the speed of data transmission will be faster in TCP/IP as compared to OSI as data only needs to pass through four layers. Whereas in case of OSI model, the data needs to pass from all seven layers which can affect its performance.
The below-given table represents a detailed comparison between both the models with respect to its layers.
|OSI Model||TCP/IP Model|
|Application Layer||Application Layer|
|Network Layer||Transport Layer|
|Data Link Layer||Internet Layer|
|Physical Layer||Network Access Layer|
In the above-given table, it can be seen that layers of TCP/IP model are less as compared to OSI model. In TCP/IP suite, layers of OSI are amalgamated with application layer which works on protocol HTTP, SMTP, Telnet and even many more are available. Furthermore, the central layer of both models is said to be the transport layer as it mainly works on TCP and UDP protocol which are connection-based and connection-oriented protocols.
Also, network layer of OSI can be determined as an Internet layer in TCP/IP suite as working of both layers are similar to each other. The major protocols which are used in this layer are ARP, ICMP, and IGMP. Moreover, the Data Link layer and physical layer of the OSI model is replaced by network access layer which has the functionality of both protocols.
The main agenda of using TCP/IP model in real-life instead of OSI is because of fewer layers. In each model, data needs to pass through each layer, however, it can severely impact the network performance. In case of TCP/IP suite, data can easily pass through four layers and receiver can receive it in less time period as compared to another model.
The below-given table represents protocols which are used by both protocols in order to transmit data from origin to destination.
|TCP/IP Model||Protocols/Services||OSI Model|
|Application Layer||Hypertext Transfer Protocol (HTTP), Telnet, DHCP service, Ping service, and File Transfer Protocol||
|Transport Layer||Transmission Control Protocol and User Datagram Protocol||Transport Layer|
|Network Layer||Internet Protocol, Internet Control Message Protocol, and Address Resolution Protocol||Network Layer|
|Network Access Layer||Works on Ethernet||
The OSI model is mainly brought into use for research purposes rather than its practical implementation. Additionally, all the functionalities of OSI model are superseded by TCP/IP model which can be executed in real-time. Along with this, the number of layers are less in TCP/IP model as compared to OSI model. Moreover, working on TCP/IP is much more efficient than the OSI model. The main reason behind this is because data needs to be transmitted among four layers, whereas in the OSI model, it gets transmitted from seven layers.