OSI Reference Model

         Open System Interconnection Model 

Conceptual model define by the International Organisation for Standardization (ISO) describing how many  combination of devices can be connected for the purpose of communication. The OSI model divides the task into 7 functional layer, forming and hierarchy with applications at the top and the physical medium at the bottom and it defines the functions each layer must provided.

Advantages of reference models

The OSI model is hierarchical and the same benefits and advertisers can apply to any layer model. The primary purpose of all such model specially the model is to allow different vendors networks to interoperate.

Advantages of using the OSI layered model include, but are not limited to, the following:

# Allows multiple-vendor development through standardization of network components.

# Allows various type of network hardware and software to communicate

# prevents changes in one year from affecting other layers so it does not hamper development

The OSI reference model has 7 layers

. Application layer

. Presentation layer

. Session layer

. Transport layer

. Network layer

. Data link layer

. Physical layer

"Layer functions"

The upper layers

The Lower Layers

The Application Layer

The application layer of the OSI model marks the spot where user actually communicate to the computer.

The Presentation Layer

The presentation layer gets its name from its purpose:

It presents data to the application layer and it responsible for data translation and code formatting.

This layer is essentially a translator and provides coding and conversion functions. A successful data transfer technique is to adapt the data into the standard format before transmission.

The session layer

The session layer is responsible for setting up managing and then tearing down sessions

Between presentation layer entities.

This layer also provides dialogue control between devices or nodes.

It coordinates communication between systems and serve to organise their communication by offering three different modes:

1. Simplex. It support one way communication

2. Half Duplex. It support two way communication but not at a time one by one.

3. Full Duplex. Two way communication at a same time.

The Transport layer

The transport layer segments and reassembles data into a data stream. Service located in the Transport layer both segment and reassemble data from upper layer application and unique it into the same data stream.ju11

They provide end-to-end data transport services and can establish a logical connection between the sending host and destination Host on an internetwork.

The Network layer

The network layer manages device addressing tracks the location of devices on the network and determine the best way to move data, which means that the Network layer must transport traffic between devices that are not locally attached.

Routers are specified at the Network layer and provide the routing Services within an internetwork.

The Data Link Layer

The data link layer provides the physical transmission of the data and handles error notification, network topology, and flow control.

This means the data link layer will ensure that messages are delivered to the proper device on a LAN using hardware addresses, and translates messages from the Network layer into bits from the Physical layer to transmit.

The IEEE Ethernet Data Link Layer has two sublayers.

Media Access Control (MAC) 802.3

Defines how packets are placed on the media. Contention media is "first come/first served"  access where everyone share the same bandwidth hence the name. Physical addressing is define here as well as logical topologies.

What's a logical topology? It is a single path through a physical topology. Line discipline error notification order delivery of frames and optional flow control can also be used at this sublayer.

Logical Link Control (LLC) 802.2

Responsible for identifying network layer protocol and then encapsulating them. LLC header tells the data link layer what to do with the packet once of frame is received. It works like this: host will receive a frame and look in the LLC header to find out where the packet is designed for.

The switches and bridges both work at this layer and filter the network using hardware MAC addresses.

The Physical Layer

Finally arriving at the bottom we find that the physical layer does two things it send bits and receive bits come only in value of 0 or 1.

 This layer is responsible for converting data frames from the data link layer into electrical signals.

Physical layer protocols and Standards define for example the type of cables and connectors to be used including their pin assignments and the encoding scheme of signalling 0 and 1 values.