NETWORKINGStandards and Protocols

STANDARDS AND PROTOCOLSThe OSI Model

WHAT IS THE OSI MODEL?

WHAT IS THE OSI MODEL? OSI Stands for Open Systems Interconnection. The OSI model is a layered, abstract

description for communications and computer network protocol design.

The Open Systems Interconnection model is a set of standard specifications that allows various computer platforms to communicate with each other openly.

It is concerned with the interconnection between systems – the way the systems exchange information – and not with the internal functions of the particular system

WHAT IS THE OSI MODEL? It divides the network architecture into seven

layers:1. Physical2. Data Link3. Network4. Transport5. Session6. Presentation7. Application

WH

AT IS THE O

SI MO

DEL?

Each of these different layers has its own set of functions and only communicates with the layers directly above and below and with its opposite layer on other computers.

WHY A LAYERED MODEL?

WHY A LAYERED MODEL?Change:

When changes are made to one layer, the impact on the other layers is minimized.

A layered model defines each layer separately. The layered approach reduces a very complex

set of topics, activities, and actions into several smaller, interrelated groupings.

This makes learning and understanding the actions of each layer and the model generally much easier.

Troubleshooting efforts to be pinpointed on the layer that is the suspected cause of the problem.

WHY A LAYERED MODEL?Standards:

Probably the most important reason for using a layered model is that it establishes a prescribed guideline for interoperability between the various vendors developing products that perform different data communications tasks.

Remember, though, that layered models, including the OSI model, provide only a guideline and framework, not a rigid standard that manufacturers can use when creating their products.

THE SEVEN LAYERS OF THE OSI MODEL

PHYSICAL LAYER

PHYSICAL LAYER The Physical layer is the lowest or first layer

of the OSI Model. This layer contains the physical networking medium, such as cabling, connectors, and repeaters.

The Physical Layer defines: Physical network structures Mechanical and electrical specifications for using

the transmission medium Bit transmission encoding and timing rules

PHYSICAL LAYER The following network connectivity hardware

are normally associated with the OSI physical layer: Network interface boards (NIC, adaptors, and so

on) Hubs, and repeaters that regenerate electrical

signals Transmission media connectors (cables, BNC

connectors, etc) Modems and codec's, which perform digital

analogue conversions.

DATA LINK LAYER

DATA LINK LAYER The second layer of the OSI Model, the Data

Link Layer, controls communications between the Network layer and the Physical layer.

Its primary function is to divide data it receives from the Network layer into distinct frames that can be transmitted by the Physical layer.

DATA LINK LAYER The basic purposes of the data link layer

protocol implementations are: Organise the physical layer’s bits into logical

groups of information called frames Detect and correct errors Control data flow Identify computers on the network

NETWORK LAYER

NETWORK LAYER The primary function of the Network Layer,

the third layer in the OSI Model has the main objective of moving data to specific network locations.

NETWORK LAYER Decides on the best route for the data to take

from sender to receiver. Similar to what the data link layer

accomplishes, however, data link layer addressing only operates on a single network

NETWORK LAYER The network layer describes methods for

moving information between multiple independent networks, called internetworks.

TRANSPORT LAYER

TRANSPORT LAYER The transport layer provides enhancements

to the services of the network layer. Its main tasks is to ensure that data sent

form one computer arrives reliably, in the correct sequence and without errors at the receiving computer.

TRANSPORT LAYER This layer is the last chance for error

recovery. The transport layer is also responsible for flow control.

It’s here that there rate of transmission is determined, based on how fast the receiving computer can accept the data packets being sent to it.

Data on the sending computer is broken down into packets that are the maximum size that the type of network can handle.

SESSION LAYER

SESSION LAYER The Session Layer is responsible for

establishing and maintaining communication between two nodes on the network.

SESSION LAYER The term session refers to a connection for

data exchange between two nodes. Often, this layer also helps the upper layers

identify and connect to the services that are available on the network.

If a communication session is broken, is the session layer that determines where to restart the transmission once the session has been reconnected

SESSION LAYER This layer is also responsible for determining

the terms of the communication session – it will determine which computer or node can communicate first and for how long

It is sometimes known as the ‘traffic cop’ of the network

PRESENTATION LAYER

PRESENTATION LAYER The Presentation Layer serves as a translator

between the application and the network. At the Presentation layer, data become

formatted in a schema that the network can understand; this format varies with the type of network used.

The Presentation Layer manages data encryption and decryption, such as the scrambling of system passwords.

APPLICATION LAYER

APPLICATION LAYER The top or seventh layer of the OSI Model is

the Application layer. The Application provides interfaces to the

software that enable programs to use network services.

APPLICATION LAYER The term “Application Layer” does not refer

to a particular software application, such as Microsoft Word, running on the network.

Instead, some of the services provided by the Application layer include file transfer, file management, and message handling for electronic mail.

APPLICATION LAYER Examples of common functions include:

Protocols for providing remote file services, such as open, close, read, write, and shared access to files

File transfer services and remote database access

Message handling services for electronic mail applications

Locate resources on a network A uniform way of handling a variety of system

devices