Programmer Help: ISO-OSI Layer Model + TCP/IP Model

ISO OSI and TCP/IP Model Comparison

OSI Model (Open Systems Interconnection)

This is reference model in this model we can identify architectural perspective for networks.

This is for how messages should be transmitted between any two points in a telecommunication network. (OSI model makes it easier to learn and understand the concepts involved) In this model have seven layers. They are,

· Application Layer – Layer 7

· Presentation Layer – Layer 6 User support layers(5,6,7)

· Session Layer – Layer 5

· Transport Layer – Layer 4

· Network Layer – Layer 3

· Data Link Layer – Layer 2 Network support layers(1,2,3)

· Physical Layer – Layer 1

Application Layer – Layer 7

This layer provides a user interface by interacting with the running application. E-mail, FTP, web browsers are network applications that run on this layer. Provide services and protocols to applications.

Presentation Layer – Layer 6

The data conversion takes place at this layer. The data that it receives from the application layer is converted into a suitable format that is recognized by the computer. For example, the conversion of a file from .wav to .mp3 takes place at this layer. Also concerned with the syntax and semantics of the information transmitted. And also doing Encapsulation of data for transmission through the network.

Session Layer – Layer 5

This layer is responsible to establish and terminate connections between two communicating machines. This connection is known as a session, hence the name. It establishes full-duplex, half-duplex and simplex connection for communication. The sessions are also used to keep a track of the connections to the web server. Session services include:

· dialog control (who transmits next)

· token management (who is allowed to attempt a critical action next)

· synchronization (check pointing long transactions so they can continue after a crash)

Transport Layer – Layer 4

This layer provides end-to-end delivery of data between two nodes or the transport layer is responsible for the delivery of a message from one process to another. It divides data into different segments before transmitting it. On receipt of these segments, the data is reassembled and forwarded to the next layer. If the data is lost in transmission or has errors, then this layer recovers the lost data and transmits the same. Or Provides reliable, transparent transfer of data between end points by

• Service port addressing

• Connection controlling

• Flow controlling

• Error controlling

Network Layer – Layer 3

The main function of this layer is to translate the network address into physical MAC address. Or the network layer is concerned with controlling the operation of the subnet. The data has to be routed to its intended destination on the network. This layer is also responsible to determine the efficient route for transmitting the Packets to its destination / determining how packets are routed from source to destination. While doing so, it has to manage problems like network congestion, switching problems, etc. The protocols used here are IP, ICMP, IGMP, IPX, etc.

Data Link Layer – Layer 2

The data link layer is responsible for moving frames from one hop (node) to the next. And Provides for reliable transfer of information across the physical link. The main function of this layer is to convert the data packets received from the upper layer into frames, and route the same to the physical layer. Error detection and correction is done at this layer, thus making it a reliable layer in the model. It establishes a logical link between the nodes and transmit frames sequentially.

Physical Layer – Layer 1

Physical layer coordinates the functions required to transmit a bit stream over a physical medium. It defines a number of network functions, not just hardware cables and cards. As the name suggests, this is the layer where the physical connection between two computers takes place. The data is transmitted via this physical medium to the destination's physical layer. The popular protocols at this layer are Fast Ethernet, ATM, RS232, etc.

2. TCP/IP (Transmission Control Protocol/Internet Protocol)

This is the Implementation of OSI model. TCP/IP is a set of protocols developed to allow cooperating computers to share resources across a network. The TCP/IP reference model is the network model used in the current Internet architecture.in this model main two protocols are Transmission Control Protocol and Internet Protocol. And so many other protocols are there. TCP/IP services can be divided into two groups:

• services provided to other protocols - IP, TCP and UDP

• services provided to end users directly – HTTP

In this model have four layers.

• Application Layer – Layer 4

• Transport Layer – Layer 3

• Internetwork Layer – Layer 2

• Network Interface Layer – Layer 1

Application Layer

In TCP/IP model, session or presentation layer are not present. Application layer is present on the top of the Transport layer. It includes all the higher-level protocols such as TELNET, FTP, DNS SMTP, SSH…

Transport Layer

This layer is responsible for providing datagram services to the Application layer. This layer allows the host and the destination devices to communicate with each other for exchanging messages, irrespective of the underlying network type. Error control, congestion control, flow control, etc., are handled by the transport layer. The protocol that this layer uses is TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). TCP gives a reliable, end-to-end, connection-oriented data transfer, while UDP provides unreliable, connectionless data transfer between two computers.

Internet Layer

This layer is also known as the Network Layer. The main function of this layer is to route the data to its destination. The data that is received by the link layer is made into data packets (IP datagrams). The data packets contain the source and the destination IP address or logical address. These packets are sent on any network and are delivered independently. This indicates that the data is not received in the same order as it was sent. The protocols at this layer are IP (Internet Protocol), ICMP (Internet Control Message Protocol), etc.

Network Interface Layer

This layer corresponds to the OSI's Physical and Data Link layers. It explains how the data is transmitted from the host, through the network. The physical connectors like the coaxial cables, twisted pair wires, the optical fiber, interface cards, etc., are a part of this layer. This layer can be used to connect different network types like ATM, Token ring, Ethernet, LAN, etc.

3. Comparison Between OSI and TCP/IP Model

3.1 Main Similarities between OSI and TCP/IP Model

• They share similar architecture

Both of the models share a similar architecture. This can be illustrated by the fact that both of them are constructed with layers

• They share a common application layer

Both of the models share a common "application layer". However in practice this layer includes different services depending upon each model

• Both models have comparable transport and network layers

This can be illustrated by the fact that whatever functions are performed between the presentation and network layer of the OSI model similar functions are performed at the Transport layer of the TCP/IP model

• Knowledge of both models is required by networking professionals

According to article obtained from the internet networking professionals "need to know both models"

• Both models assume that packets are switched

Basically this means that individual packets may take differing paths in order to reach the same destination

3.2 Main Differences Between OSI and TCP/IP Models

• The OSI model consists of 7 architectural layers whereas the TCP/IP only has 4 layers

• TCP/IP appears to be a more simpler model as it has fewer layers

• TCP/IP combines the presentation and session layer issues into its application layer

• TCP/IP combines the OSI data link and physical layers into the network access layer

• OSI Model supports connection oriented communication in transport layer, whereas in network layer it supports both connectionless and connection oriented

• The TCP/IP model has only one mode in the network layer but supports both modes in transport layer

• OSI and TCP/IP models are based on the concept of a stack of independent protocols

• Functions of the layers are more or less similar

• Three concepts are central to OSI Model: Services, Interfaces, and Protocols

– Services: This definition tells what the layer does. It defines the layers semantics.

– Interface: It tells the process above it how to access it.

– Peer Protocols: Protocols used in a layer are the layer’s own business.

• TCP/IP model did not clearly distinguish between service, interface and protocol

• The protocols in the OSI model are better hidden than in the TCP/IP model and can be replaced relatively easily as the technology changes

• OSI model was devised before the protocols were invented

– This ordering means that the model was not biased toward one particular set of protocols

– Downside of this ordering is that the designers did not have much experience with the subject and did not have good idea of which functionality to put in which layer

• With TCP/IP, the protocols came first and the model was really just a description of existing protocols.

– Protocols fit the model perfectly.

– But the model did not fit any other protocols stacks

• TCP/IP Protocols are considered to be standards around which the internet has developed.

• The OSI model however is a generic, protocol- independent standard.

3.3 Protocol port comparison