Introduction to OSI Models
The OSI model by IBM is the biggest boon that systems developer can have had for their karma in the past life. It is actually a layered stack, with 7 layers. Each of these 7 layers is implemented by different protocols for each layer. But keep in mind; it’s not necessary that all these layers have to be used, when trying to device a system. A system can also make away with any of the 7 layers like say the Session Layer or the Application Layer. Consider a Router for example. A Router is a device, which works in the Network Layer of the OSI Model. Its main function is to route packets that arrive at its ports. Router takes a routing decision for each of the packet that arrives at its port, on the basis of Routing Tables.
Routing Tables are somewhat similar to the Time Table we follow while attending our courses in college. Depending on the subject scheduled in the timetable for a particular day, we decide whether to attend the lecture or not. In a similar way the Router looks at the destination address (which is the IP address) of the packet that is to be routed. Now remember that an IP address is made up of a Host ID and a Network ID (I will cover IP addresses in depth in the future articles).
The Router looks at the Network ID and compares it with the other Network ID’s present in its Routing Tables. If the particular Network ID is found to be present in the Router’s Table, the packet is routed to the appropriate destination successfully. If the Network ID of the packet is not present in the Router’s Routing Table, then the router forwards the packet to the next nearest router and this process continues till the packet reaches its intended destination. This is very similar to what you do when you are asked where someone stays, while walking on the road. You are shown the address by the person, who wants to reach a place (Consider this Address as the Destination IP address). Now what you do is directly look at the name of the street. This is because the name of the building is not something you will be immediately familiar with, as a building with the same name will surely exist in thousand different streets.
If you are very well aware of the geography of the place, then by looking at the Street (Network ID) you tell the person where the street is. (Similar to a Router finding that the Network ID field of the IP address of the packet is present in its Routing Table). If you don’t know the street or the address, you tell the person to ask some one else (that is you forward him to the next nearest Router). OSI model doesn’t say you do things this way. It just tells you “Hey keep in mind that these things should be covered while designing a feasible network solution “.
The OSI model is shown is figure below:
7 Application Application protocol Application
Interface
6 Presentation Presentation protocol Presentation
Interface
5 Session Session protocol Session
4 Transport Transport protocol Transport
Communication subnet boundary
3 Network Network Network Network
Internal subnet protocol
2 Data Link Data Link Data Link Data Link
1 Physical Physical Physical Physical
Host a Router Router Host B
Network layer host-router protocol
Data link layer host-router protocol
Physical layer host-router protocol
Introduction to TCP/IP Model
The TCP/ IP model superseded the OSI model. This is the model that is currently most widely used. The various layers in the TCP/IP model are:
1) Application
2) Transport
3) Internet work
4) Data Link
5) Physical
Sessions and Application Layer present in the OSI Model are absent in the TCP/IP Model.
In the 1990’s, TCP/IP has become firmly established as the dominant commercial architecture and as the protocol suite upon which the bulk of new protocol development is to be done. There are a number of reasons for the success of the TCP/IP model over the OSI model:
1). Internet is built on the foundation of the TCP/IP suite. The tentacles of the Internet and the World Wide Web have spread throughout the world and that is the main reason for the success of TCP/IP model over the OSI model.
2). TCP/IP protocols were initially researched under a project in the Department of Defense (DOD). DOD was committed to international standards and most of its operational requirements couldn’t be met by the OSI model. So it started to develop the TCP/IP. Since the DOD is the largest consumer of software products in the world, the vendors were encouraged to develop TCP/IP based products.
In the next article I will cover more about the various layers in the OSI Model and the TCP/IP Model.
The TCP/IP Model is shown as below:
Definition of OSI Models
“The Open Systems Interconnection (OSI) reference model for describing network protocols was devised by the Internet Standards Organization. It divides protocols into seven layers to standardize and simplify definition.”
Or
“OSI (Open Systems Interconnection) is a standard description or "reference model" for how messages should be transmitted between any two points in a telecommunication network”.
Definition of TCP/IP Models
“A protocol for communication between computers, used as a standard for transmitting data over networks and as the basis for standard Internet protocols.”
Or
“A communications protocol for computer networks, the main protocol for the Internet.”
Comparison between OSI Model and TCP/IP
| OSI Models | TCP/IP Models |
| Protocols in the OSI model are better hidden and can be replaced relatively easily as the technology changes, which is one of the main objectives of layered protocols. | Service, interface and protocol are not clearly defined. For example, the only real services offered by the Internet layer are
|
| In this case, the protocols have been invented before models, so the functionalities are perfectly described. | Because models were invented before protocols, functionalities put in each layer are not very optimized. |
| Only four layers. | Seven layers, Network (Internet), Transport and Application layers being similar to TCP/IP |
| Only one mode in the network layer (connectionless) but both modes in the transport layer are supported, giving the users a choice. | Both connectionless and connection-oriented communications are supported in the network layer, but only connection-oriented communication in the transport layer. |
| OSI Models | TCP/IP Models |
| The OSI model however is a "generic, protocol- independent standard."
| TCP/IP Protocols are considered to be standards around which the internet has developed. |
| Networks are not usually built around the OSI model as it is merely used as a guidance tool. | TCP/IP is considered to be a more credible model- This is mainly due to the fact because TCP/IP protocols are the standards around which the internet was developed therefore it mainly gains creditability due to this reason. |
| Open Systems Interconnect (OSI) is a general model for communications. | TCP/IP is a specific four-layer protocol stack that fulfills those functions. |
| Basically OSI-Open System Interconnection is a model designed to explain the characteristics/way to achieve data communication using different layers (7 Layers )never been physically used to make communication. Because it is just a model designed by ISO. | TCP/IP- Transmission Control Protocol/ Internet Protocol contains only 5 layers And is the system using which data communication can be achieved practically. |
| OSI model is a 'reference model' that describes how the protocols should interact with each one another invented by the Department of Defense. | TCP/IP model becomes one of the standards that enabled the Internet what it is today... the application layer of TCP/IP model can handle the responsibilities of the layers 5 6 7 in OSI model. The transport layer in TCP/IP does not always guarantee reliable delivery of packets at the transport layer while the OSI model does. TCP/IP also offers an option called UDP that does not guarantee reliable packet delivery. |
| Protocols enable an entity in one host to interact with a corresponding entity at the same layer in another host. Service definitions abstractly describe the functionality provided to an (N)-layer by an (N-1) layer, where N is one of the seven layers of protocols operating in the local host. | The TCP/IP model consists of four layers (RFC 1122). From lowest to highest, these are the Link Layer, the Internet Layer, the Transport Layer, and the Application Layer. |
| OSI Models | TCP/IP Models |
| The OSI model is based on layers. The system is set up on the basis of layers. Protocols are selected on the basis of requirement for the system for which we want to set up the network. | TCP/IP suite based on protocols. It means that the model is created on the basis of protocol. If any network is needed to set up then first the protocols of that system are selected and on the basis of these protocols the layers are created. So, this model is simply a protocols based model. |
| OSI uses the CLNS (Connection Less Network Service) protocols ES-IS for communication of an end system to an intermediate system and IS-IS for communication between intermediate systems. | TCP divides messages in datagrams of up to 64k length. Each datagram consists of a header and a text part. Besides some other information, the header contains the source and the destination address of the datagram. IP routes these datagrams through the network using e.g. the protocol OSPF (Open Shortest Path First) or RIP (Route Information Protocol) for path calculation purposes. The service provided by IP is not reliable. Datagrams may be received in the wrong order or they may even get lost in the network. |
| In the internet protocol suite, a layer represents a reasonable packaging of function.
| In the TCP/IP protocols, a given protocol can be used by other protocols within the same layer, whereas in the OSI model two separate layers would be defined in such circumstances. Examples of such "horizontal dependencies" are FTP, which uses the same common representation as TELNET on the "application layer," and ICMP, which uses IP for sending its datagrams on the "internet work" layer. |
| OSI was designed as a total solution by a committee, and you don't see that much of it. | TCP/IP was muddled together and works, and then was enhanced and continued to grow organically over time. |
Which one is best OSI Models and TCP/IP Models and why?
The OSI "protocols" are dead, they practically never existed. But the OSI "model" is alive and well and serves as a guideline for standardized, layered protocol architecture. With TCP/IP the inverse is true; the TCP/IP "protocols" are alive and well and here to stay, but the TCP/IP "model" is really just a description of how the protocols map to the OSI layers. The TCP/IP model has 4 layers, the "host-to-network" layer covers OSI layers 1/2 physical/data link. Then IP is the OSI layer 3 network layers, TCP is the OSI layer 4 transport layer, and then there's the application layer.
As for some advantages of TCP/IP protocols themselves: 1) they are everywhere! It's the common worldwide standard now for networking. 2) Interoperability: different type’s computers from different vendors can communicate seamlessly if they speak the same TCP/IP language. 3) Built-in intelligent mechanisms for error and flow control. 4) Many others, just Google advantages of TCP/IP.
Not sure if this is what you were after, so before I go down the wrong rat whole, add details to clarify the question and we'll get you more info.
TCP/IP is a suite of protocols. The acronym TCP/IP means "Transmission Control Protocol/Internet Protocol" and is pronounced "T-C-P-I-P". It comes from the names of the two major protocols in the suite of protocols, i.e. the TCP and IP protocols).
In some ways, TCP/IP represents all communication rules for the internet and is based on the IP addressing notion, i.e. the idea of providing an IP address for each machine on the network so as to be able to route data packets. Given that the TCP/IP protocol suite was originally created with a military purpose, it is designed to respond to a certain number of criteria, including:
- Splitting messages into packets;
- Use of an address system;
- Routing data over the network;
- Error detection in data transmissions.
Knowledge of the collection of TCP/IP protocols is not essential for a simple user, just as a viewer does not need to know how his television or audiovisual networks work. However, for people who want to administrate or support a TCP/IP network, knowledge of it is vital.
The difference between standard and implementation
Overall TCP/IP brings together two notions:
- The notion of standard: TCP/IP represents the way in which communications are carried out over a network.
- The notion of implementation: the designation TCP/IP is often extended to software based on the TCP/IP protocol. TCP/IP is in fact a model which network application developers use. The applications are therefore implementations of the TCP/IP protocol.
TCP/IP is best because of:
TCP/IP is a layer model
So as to be able to apply the TCP/IP model to any machine, i.e. independently of the operating system, the TCP/IP system of protocols has been broken down into several modules each carrying out a specific task. Furthermore, these modules carry out these tasks one after the other in a specific order, so there is stratified system, this is the reason we talk about a layer model.
The term layer is used to reflect the fact that the data which travels over the network crosses several protocol levels. So, data (information packets) which circulates over the network is processed successively by each layer, which adds an item of information (called a header) then sends it to the next layer.
The TCP/IP model is very close to the OSI model (model comprising of 7 layers) which was developed by the international standards organization (ISO) in order to standardize communications between computers.
Presentation of the OSI model
OSI means Open Systems Interconnection. This model was established by ISO in order to implement a communications standard between computers on a network that is the rules which manage communications between computers. In fact, when networks began, each manufacturer had its own system (we talk of a proprietary system) so many incompatible networks coexisted. This is the why it was necessary to establish a standard.
The role of the OSI model is to standardize communication between machines so that different manufacturers can develop compatible (software or hardware) products (as long as they strictly observe the OSI model).
The importance of a layer system
The aim of a system in layers is to separate the problem into different parts (the layers) depending on their level of abstraction.
Each layer of the model communicates with an adjacent level (above or below it). Each layer therefore uses the services of the lower layers and supplies them to that of the upper layer.
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