Network Architectures : Layers of OSI model and TCP/IP model.
A Network is a conceptual framework that describes how data and network information are communicated from an application on one computer through network media to an application on other computers in terms of different layers. Network architecture is also known as Reference model. There mainly two classifications of Reference models and are Open and closed. Open model is one which is open for everyone and no secrecy is there. In a closed model, also known as proprietary system the architecture is kept secret from users. OSI model is an open model while IBM's SNA 7 layer model is a closed system. Here Let's discuss two main reference models OSI model and TCP/IP model.
International Organization for standardization (ISO) developed OSI (Open Systems Interconnection) model in late 1970s. In this data communication functions are defined in terms of 7 layers which are organized in the chronological order of events occurring during a communications session. The top 3 layers explains how applications with in end stations communicate each other and also with users. The bottom 4 layers defines end to end data transmission. Different Layers and their functions are described below
OSI Model Layers
- Application Layer - Layer 7 : This is the layer where users communicate with computer. The main functions are file transfers, email, enabling remote access and network management activities. The common protocols in this layer are HTTP, FTP, TFTP, Telnet etc.
- Presentation Layer - Layer 6 : Also known as 'Syntax layer' . This layer presents data to application layer in format that can be processed by an end user. This is actually a translator that provides coding and conversion functions. Main tasks are data compression, decompression, encryption and decryption.
- Session Layer - Layer 5 : This layer is responsible for setting up, managing and then tearing down session between two computers. The main function is to manage flow of data communication during a connection between 2 computers. Main services offered are Dialogue control, Token management and Synchronization. Dialogue control refers to organizing a data communication session between two computers via 3 modes namely simplex, half duplex and full duplex. In token management during a session between 2 computers if a critical operation is to be done they passes tokens and the one holding the token will do that operation. Synchronization is used to manage timing signal between session for the functions like insertion of check points.
- Transport Layer - Layer 4 : Provides end to end data transfer and establishes logic connections between sending host and destination host. Main functions are Flow control, Sequencing , error detection and recovery. Important protocols in this layer are TCP and UDP
- Network Layer - Layer 3 : This layer is the domain of WAN. Switches and Routers are devices of Network layer. Functions are Logical addressing and Routing. The common protocols used by routers are RIP, EIGRP , OSPF etc.
- Data-Link Layer - Layer 2 : This functions the the transfer of data between the ends of a link. it has two sub layers namely Media Access Control Layer and Logical Link Control Layer. Packets received from network layer are transformed into frames. Devices in this layer are switches and bridges. Functions are Framing packets, Sequence control, Error control, Flow control, Physical addressing of devices in network, switching of LAN. Main protocols in this layer are HDLC, SDLC, PPP.
- Physical Layer - Layer 1 : Physical layer specifies electrical, mechanical, procedural and functional requirements for activating, maintaining and deactivating a physical link between end systems. The main function is to send bits and receive bits. Hub is the device in this layer. It also includes hardware interfaces to connect physical media.
TCP/IP Model (Internet Architecture)
TCP/IP ( Transmission Control Protocol/Internet Protocol) defines a large collection of protocols that allow computers to communicate.It has a 4 layer architecture. TCP/ IP defines each of these protocols inside document called Requests For Comments (RFCs). By implementing the required protocols in TCP/IP RFCs, a computer can be relatively confident that it can communicate with other computers that also implement TCP/IP. Different layers and functions are described below.
- Application Layer : Provide services to the application software running on a computer. Application layer provides an interface between software running on a computer and the network itself. Example for TCP/IP application is web browser. Example protocols are HTTP, POP3, SMTP etc.
- Transport Layer : Consists of mainly two protocol options. Transmission control protocol (TCP) and User datagram protocol (UDP). Each layer provides a service to the layer above it.In same layer interaction on different computes, the two computers use protocol to communicate with the same layer on another computer. The protocol defined by each layer uses a header that is transmitted between the computers t, to communicate what each computer wants to do. While In Adjacent layer interaction on the same computer, one layer provides a service to a higher layer. The software or hardware that implements the higher layer requests that the next lower layer perform the needed function.
- Internetwork Layer : Internet protocol (IP), works much like the postal service. IP defines logical addressing so that each host computer can have a different IP address. Similarly, IP defines the process of routing so that routers can choose where to send data correctly.
- Network Interface Layer : Defines the protocols and hardware required to deliver data across some physical network. The term network interface refers to the fact that this layer defines how to connect the host computer, which is not part of the network, to the network. It is the interface between the computer and network. Ethernet is one example protocol at the TCP/IP network interface layer. Ethernet defines the required cabling, addressing and protocols used to create an Ethernet. IP relies on the network interface layer to deliver IP packets across each physical network. IP understands the overall network topology, things such as which routers are connected to which networks, and what the IP addressing schemes looks like.
Comparison of OSI and TCP/IP Models
- Both of them use a layered architecture to explain data communication process in computer networks.
- Each layer performs well-defined functions in both models.
- Similar types of protocols are used in both models.
- OSI and TCP/IP reference models are open in nature.
- Both models give a good explanation on how various types of network hardware and software interact during a data communication process.
- Data hiding principle is well maintained on each layer in the two models. The core level functional details of each layer are not revealed to other layers.
- Transport layer defines end-end data communication process and error-correction techniques in both the models.
- OSI and TCP/IP reference models process data in the form of packets to perform routing.
1. 7 layers present in the architecture.
Only 4 layers are present.
2. Not practically implemented yet.
3. Layering aspects, functions of each layer and division of responsibilities are specifically presented by this model.
Division of responsibilities on each layer is not so specific.
4. The concept of services, interfaces and protocols are well explained.
No clear distinction between the three
5. Model was devised first and protocols were latter fitted to appropriate layers.
The protocols came first and model was just explanation of protocols based on 4 layers.
6. Widely used as a standard reference model in the design of computer networks.
Not considered as a design standard due to the failure in distinguishing services, interfaces and protocols.
7. Connectionless and connection oriented services are there in Network layer but only connection oriented services in Transport layer.
Connectionless and connection oriented services in transport layer but only connectionless service in Network layer.
8. This is a protocol independent model.
This is a protocol specific model.
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