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There is a significant number of computers that is used by many global organizations and institutions in order to support their activities. For example, a corporation with many branches may have computers at each location to do some tasks . The point is that the administration of the company may have decides to connect those branches to communicate and transfer information. Some companies are interested in information communication technology to solve communication problems and difficulties of other organizations and they conduct some research about networks and communication technology because of the observed importance of them these days.
Computers which are unified, separated and interconnected are called computer networks which play a vital role in many places such as companies, universities and even personal use . They are a crucial part of computer science. There are some concepts of networks which mainly divided to two parts: The first is network hardware and the second is network software.
Communications between computers over networks have provided an opportunity for the emergence of protocols. A protocol is a set of rules to define the methods of data transmission through the means of transport in the network. Protocols facilitate communications between different computers. In the early stages of the development of networks, some companies had their own protocols which meant there were some difficulties of the interaction between computers of different vendors, if not sometimes impossible. To resolve this problem, the International Organization for Standardization (ISO) has developed a model named (OSI) in the late 1970s . Open Systems Interconnection reference model is based on the shortcut (OSI) which was formally published in 1984 . Subsequently, there was an emergence of another protocol to resolve some other problems named TCP / IP.
The first protocol known as the grandparents of all computer networks named ARPANET, and its inheritor, the worldwide Internet. It connected many universities and government institutes using a telephone line. When ARBANET has been developed using satellite and radio networks, there were some difficulties in connecting, hence there was need for having a new reference . Thus, to solve these problems, there was a protocol that had the ability to connect multiple networks together in a seamless way called TCP/IP which is able to facilitate access to the Internet in order to set up and use. In fact, it consists of a wide range of protocols, but the name came from the only two types of protocols, namely: The Transmission Control protocol (TCP) and the Internet Protocol (IP) .
Most networks are organized as a set of layers and levels, each one built above the other one below, the number of layers, the name of each layer, the contents of each layer and its functions vary from one network to another. However, in all types of networks, the purpose of any layer is to provide a certain service to the higher layers. Protocol is the rules which dealt with layers between machines . OSI has its own layers which is different than TCP/IP layers as well as the differences of the approach use in every layer in both of them. This paper will focus on the two reference models: the OSI and TCP/IP reference models in order to compare.
The OSI reference model
Figure 1 shows the OSI reference model. It contains seven layers which can be divided into two groups, upper layers and lower layers. The upper layers of the OSI model which are application, presentation and session deal with application issues and are only executed in software. The lower layers of the OSI reference model which are transport, network, data link and physics handle data transport issues. The physical and data link layers are implemented in hardware and software while the other layers are implemented only in software . In fact, the model is not used for commercial purposes but only for educational purposes to explain how a network works and that is still the current situation.
A real example of OSI
To explain how the OSI reference model works, here is a real example which provides the working idea clearly. There are two companies, one of which is Saudi and the other British. Each one has its manager as well as translators and secretaries who communicate with the world. The Saudi company sends an order to buy technological equipment from the British company. Thus, the order will go through many stages such as the preparation, drafting, translation and transmission up to the British company which will receive the order to process it. The stages in the example are like the layers in the OSI and the two companies are like two computers.
The benefit from the previous example is that there are some notes which were applied to arrive at the seven layers are as follows:
1- Each layer is independent of the preceding layer.
2- The common method of work which is provided between a first layer (the case of the secretary in Saudi company) with the same layer structure in the second one (the case of the secretary in British company), so there is a normative position to communicate with the lower layer (the transmitter) and the upper layer (the translator) and that so-called interface layer.
3- In the process of transmission, each layer relies on the information received from the upper layer which delivers the result to the lower layer.
4- In the process of reception, each layer depends on the information received from the lower layer to reach at the upper layer.
Data Encapsulation in the OSI model
Wrapping the date by adding a header that process flows down the OSI is called data encapsulation. In this situation, each layer may add its individual header to wrap it with the data received from above in order to pass it to the next layer below. There are five phases of data encapsulation in the OSI model: the upper layers (the Application, Presentation and Session layers) create data from inputs, the data is switched to segments by the Transport layer, the next layer which is Network layer will convert the segments into packets, the Data Link layer modifies the packets to frames, and finally those frames will be converted to bits by the physical layer.
The TCP/IP reference model
The Transmission Control Protocol and the Internet Protocol (TCP/IP) is not different than the general structure of the OSI model except that the number of layers is less than in the OSI as well as names of some layers have been changed. Of course, there are some variations in other aspects which will discuss later. Table 1 shows the complete TCP/IP stack . There are protocols at three layers which are Application, Transport and Internet layer. Nothing is specified in TCP/IP at the Host-to-Network layer. The Application layer has its protocols for example, a File Transport Protocol (FTP) which is largely responsible for the transfer of files, the Simple Mail Transfer Protocol (SMTP) that supports an e-mail service, and the Hyper Text Transfer Protocol (HTTP) used for web browsing .
Table1: TCP/IP reference model
TCP/IP layer TCP/IP model Example protocols
1 Application FTP, SMTP, HTTP, DNS, Telnet.
2 Transport TCP, UDP
3 Internet IP, OSPF, PIP, BGP
4 Host-To-Network ---------------
To understand TCP/IP (Transmission Control Protocol, Internet Protocol), It is important to consider its individual components and their functions. When combined into a suite of protocols and applications, the overall purpose of TCP/IP is to provide a structure for transmitting data across the vast majority of networks worldwide as they interconnect to compose the Internet. One of its important components is routers which act as gateways among networks and arrange data packets to arrive at their last target effectively. Another one is Internet protocol (IP) that controls data in its way from computer to another. Also, Transmission Control Protocol (TCP) that is required to guarantee consistent data delivery.
Base of the comparison of the OSI and TCP/IP reference model
It is important to show the relationship between the two reference models to understand every part in each model that will help to compare them straightforwardly. As a result of that, there is a figure shown below that indicates the correlation between the OSI and TCP/IP reference models. The topic would be based on the diagram below.
O S I TCP/IP
1 Application 1 Application
2 Presentation 2 Transport
3 Session 3 Internet
4 Transport 4 Host-to-network
6 Data link
A comparison of the Upper Layers
As shown in table 2, there are three layers in the OSI reference model which are Application, Presentation and Session layer, but there is only one layer in the TCP/IP which is Application.
Table2: The Upper Layers
The Session Layer
The Session Layer in the OSI permits users on dissimilar computers to set up sessions between them . It also provides enhanced services helpful in some application and allows two parties to hold ongoing communications called a session across a network as the transport layer does. The Session Layer is not supplied in the TCP/IP model because its characteristics are provided by the TCP protocol in the Transport Layer, and the Session Layer does not work in all application types, so there is no need to obtain The Session Layer in the TCP/IP.
The Presentation Layer
The Presentation Layer in the OSI model executes definite functions that are applied for handling data format information for network communications. These functions are done by converting data into general format that can be understood by sides, sender and receiver. The TCP/IP model does not contain the Presentation Layer because those functions are provided by the Application Layer.
The Application Layer
The application Layer is the top of both reference models: the OSI and the TCP/IP where most people, both users and developers interact with computer networks. It consists of a variety of protocols that are normally required for all kinds of networks as well as its different functions which provides many services such as Message Handling Service (X.400), virtual terminal and File Transfer Access and Management (FTAM) .
The notion of application service element is common to both, the OSI and TCP/IP model. In spite of that, According to Cassell  “There is a difference in philosophy and approach between the OSI model and the TCP/IP environment. Both paradigms are important”.
There are some methods which illustrate the OSI approach used in constructing entities. The OSI approach is sometime called ‘Horizontal Approach'. The OSI model declares that distributed applications operate over a firm hierarchy of layers. The applications are created from a general tool kit of standardized application service elements. In the OSI model, each distributed application service decides on functions from a large common toolbox of Application Service Element (ASEs) and harmonize these with application service elements that execute specific functions to given end-user service.
As the OSI model has its methods that display its approach and , there is different approach for the TCP/IP. It is occasionally called ‘vertical approach' in the TCP/IP. The concept of the approach in the TCP/IP is that each application body is collected of what set of function it requests beyond end to end transport to prop a distributed communications facility. Most of these application processes build on what it needs and supposes only that a basic transport system (data package or connection) will be presented.
A comparison of the Lower Layers
There are four layers in the OSI reference model, the Transport, Network, Data link, and Physics layer and in the TCP/IP model has three layers, The Transport, Internet, and Host-to-Network layer.
The Transport Layer in the OSI model
Receiving and accepting the data from the Session Layer is the basic function of the transport Layer, and then break the data into small packets and pass them to the network layer as well as make sure that the arrival of the pieces occurs properly by the Transport Layer at the receiving node . Moreover, this process must be done in an efficient manner by isolating the upper layers of the predictable changes in the hardware equipments. Another function is to provide a signaling service for the distant node, so that the receiving node notifies the sending node when its data is received successfully. Also, the Transport Layer is responsible for choosing the type of service to provide the Session layer and, eventually, the network users. An error-free point-to-point channel is the most popular kind of transport connection that conveys the message or bytes in the same order in which they were sent by . In contrast, the other types do not guarantee receiving the message in order.
The Transport Layer in the TCP/IP model
The basic function of the Transport layer in the TCP/IP is as the same in the OSI transport layer, it is designed to permit senders and receivers to carry on a conversation and it has two end-to-end protocols. The first one is TCP (Transmission Control Protocol) that permits a byte stream on one machine to deliver without fault on any other device in the internet as well as handling flow control to make sure that happens smoothly. The second protocol is UDP (User datagram Protocol) that implements an unreliable data stream, it is connectionless protocol and useful in many applications for instance, where data needs to be broadcasted or client-server type request-reply quires. Both of the protocols introduce the concept of ports such as FTP, Talent and POP3.
Comparing the Transport layer for OSI and TCP/IP
Most of the requirements for the transport layer in the OSI model are identical to the features of TCP and UDP protocols which defined in the TCP / IP model and most of the TCP and UDP functions and specifications map to the OSI Transport Layer.
At the Transport layer, the TCP/IP and OSI reference models both utilize all connection and connectionless models, but the internet architecture refers to the two models in TCP/IP as basically connections and data-grams. But the OSI reference model uses the terms connection-mode and connection-oriented for the connection model and the term connectionless-mode for the connectionless model.