OSI REFERENCE MODEL AND THE

TCP/IP MODELOSI REFERENCE MODEL

Models and Standards in Communication• Communication• Established standards• Standards are known as protocols

• Implementation• A framework is helpful in the design of hardware and software for

communication• ISO-OSI Model serves this purpose• ISO-OSI supersedes the TCP/IP model

OSI REFERENCE MODEL

• International Standards Organization (ISO) is a multinational body dedicated to world wide agreement on international standards.

• Open System Interconnection (OSI) model is an ISO standard that covers all aspects of network communication.

• An open system is a set of protocols that allows any two different systems to communicate regardless of their underlying architecture.

• The model acts as a frame of reference in the design of communications and networking products

• The purpose of the OSI model is to show how to facilitate communication between different systems without requiring changes to the logic of the underlying hardware and software. The OSI model is not a protocol; it is a model for understanding and designing a network architecture that is flexible, robust, and interoperable.

• OSI comprises of seven layers namely:

OSI Reference Model: 7 Layers

OSI Model

Physical LayerProvides physical interface for transmission of information.Defines rules by which bits are passed from one system to another on a physical communication medium.Covers all - mechanical, electrical, functional and procedural - aspects for physical communication.Such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other similar attributes are defined by physical layer specifications.

OSI Model

2.6

Figure 2.5 Physical layer

Data Link LayerData link layer attempts to provide reliable communication over the physical layer interface. Breaks the outgoing data into frames and reassemble the received frames.Create and detect frame boundaries.Handle errors by implementing an acknowledgement and retransmission scheme.Implement flow control.Supports points-to-point as well as broadcast communication.Supports simplex, half-duplex or full-duplex communication.

OSI Model

2.8

Figure 2.6 Data link layer

Network LayerImplements routing of frames (packets) through the network.Defines the most optimum path the packet should take from the source to the destinationDefines logical addressing so that any endpoint can be identified. Handles congestion in the network.Facilitates interconnection between heterogeneous networks (Internetworking).The network layer also defines how to fragment a packet into smaller packets to accommodate different media.

OSI Model

2.10

Figure 2.8 Network layer

Transport LayerPurpose of this layer is to provide a reliable mechanism for the exchange of data between two processes in different computers. Ensures that the data units are delivered error free.Ensures that data units are delivered in sequence.Ensures that there is no loss or duplication of data units.Provides connectionless or connection oriented service.Provides for the connection management.Multiplex multiple connection over a single channel.

OSI Model

2.12

Figure 2.10 Transport layer

Session LayerSession layer provides mechanism for controlling the dialogue between the two end systems. It defines how to start, control and end conversations (called sessions) between applications.This layer requests for a logical connection to be established on an end-user’s request.Any necessary log-on or password validation is also handled by this layer.Session layer is also responsible for terminating the connection.This layer provides services like dialogue discipline which can be full duplex or half duplex.Session layer can also provide check-pointing mechanism such that if a failure of some sort occurs between checkpoints, all data can be retransmitted from the last checkpoint.

OSI Model

2.14

Figure 2.12 Session layer

Presentation LayerPresentation layer defines the format in which the data is to be exchanged between the two communicating nodes. Coding and conversion of Application layer data to ensure that data from the source device can be interpreted by the appropriate application on the destination device.This is the layer at which application programmers consider data structure and presentation Also handles data compression and data encryption (cryptography).

OSI Model

2.16

Figure 2.13 Presentation layer

Application LayerApplication layer interacts with application programs and is the highest level of OSI model.Application layer contains management functions to support distributed applications.Examples of application layer are applications such as file transfer, electronic mail, remote login etc.

OSI Model

2.18

Figure 2.14 Application layer

2.19

Figure 2.15 Summary of layers

TCP/IP MODEL• Transmission Control Protocol/Internet Protocol

• The Internet protocol suite is the networking model and a set of communications protocols used for the Internet and similar networks. TCP/IP is the communication protocol for the internet.• TCP/IP defines the rule computers must follow to communicate with each

other over the internet.• TCP/IP is TCP and IP working together.• TCP takes care of the communication between your application software

(i.e. your browser) and your network software.• IP takes care of the communication with other computers.• TCP is responsible for breaking data down into IP packets before they are

sent, and for assembling the packets when they arrive.• IP is responsible for sending the packets to the correct destination.

TCP/IP MODEL CONT’D

• It is commonly known as TCP/IP, because its most important protocols, the Transmission Control Protocol (TCP) and the Internet Protocol (IP) (192.168.1.50) were the first networking protocols defined in this standard. • It is occasionally known as the DoD model due to the foundational

influence of the ARPANET in the 1970s (operated by DARPA, an agency of the United States Department of Defense).• TCP/IP provides end-to-end connectivity specifying how data should be

formatted, addressed, transmitted, routed and received at the destination.

TCP/IP MODEL LAYERS

• It has four abstraction layers which are used to sort all related protocols according to the scope of networking involved. From lowest to highest, the layers are:• The link layer contains communication technologies for a single network

segment (link) of a local area network.• The internet layer (IP) connects independent networks, thus establishing

internetworking.• The transport layer handles host-to-host communication.• The application layer contains all protocols for specific data communications

services on a process-to-process level. For example, the Hypertext Transfer Protocol (HTTP) specifies the web browser communication with a web server.

2.23

Figure 2.18 Relationship of layers and addresses in TCP/IP

LINK BETWEEN OSI MODEL & TCP/IP MODEL• IP corresponds to the Network layer (Layer 3) in the OSI model, whereas

TCP corresponds to the Transport layer (Layer 4) in OSI. • In other words, the term TCP/IP refers to network communications where

the TCP transport is used to deliver data across IP networks. • The average person on the Internet works in a predominately TCP/IP

environment. Web browsers, for example, use TCP/IP to communicate with Web servers.

CONTRIBUTORS• MARANGE ALICE B1129595

• SIBANDA GUGULETHU B1129993

• MUNGAZI AGRIPPA B1129976

WE ARE HUMBLED BY YOUR COORPERATION, CONTRIBUTIONS AND QUESTIONS WHICH INCREASED OUR INSIGHT .

ABOVE ALL, GOD, OUR HELP IN THE AGES PAST AND OUR HOPE FOR THE YEARS TO COME.