FUNCTION OF OSI LAYERS

We will now discuss the purpose of each layer in the OSI model.

Application Layer

The Application layer is the top layer of the OSI model. It is used to allow applications to access network services. It handles general network access, flow control, and error recovery. Some of the functions it supports are:

	Accessing remote files
	Accessing remote printers
	Accessing remote databases
	Electronic mail

Presentation Layer

The next layer down is the Presentation layer which is located at layer 6. It is the translator for the network as it determines the format used to exchange data among the computers on the network. Some of its functions are:

	Protocol conversion
	Data translation
	Data encryption
	Data compression
	Conversion of the character set

Session Layer

Layer 5 is the Session layer, which establishes a communications connection between processes running on different computers. It performs name recognition and related functions; for example, user authentication and resource-access security that are needed to allow processes to communicate over the network. To ensure that all data gets sent, it uses checkpoints placed in the data stream to provide for synchronization. In the case of network failure the only data that needs to be sent again is the data after the last checkpoint. The Session layer is also responsible for controlling when and for how long each side transmits.

Transport Layer

Layer 4 is the Transport layer, which is responsible for ensuring that all the data is delivered in the correct sequence, error free, with no losses. It is the Transport layer that breaks up large messages into smaller packets for delivery. Some of the other functions provided by the Transport layer are:

	Error handling
	Flow control by notifying the transmitting computer to not transmit when the receiving computer has no available receive buffers

Network Layer

Layer 3, the Network layer, is responsible for determining the route that is taken from the transmitting computer to the receiving computer. It is also responsible for addressing messages and the translation of logical addresses into physical addresses. The Network layer determines what path the data should take based upon several factors, including the condition of the network and the priority of service.

Data Link Layer

Layer 2 is the Data Link layer, which is responsible for providing error-free transfer of frames from one computer to another using the Physical layer. A frame is a bundle of information sent as a single entity. Some of the other functions provided by the Data Link layer are:

	Transmitting and receiving frames sequentially
	Providing frame acknowledgment for frames it receives
	Retransmitting frames that are not acknowledged by the receiving computer

Physical Layer

The Physical layer is located at layer 1, the bottom of the OSI model. The Physical layer handles the unstructured, raw, bit-stream data that is transferred over a physical medium. It also defines how the physical medium, or cable, is attached to the Network Interface Card (NIC). It does that by determining how many pins are in the connector that is being used and also the function of each of the pins. The Physical layer sends out bits equaling 1s and 0s and determines how long each bit lasts and how it is translated into the suitable optical or electrical impulse for the network cable.

 KNOW HOW THEY WORK WITH FIGURE