OSI(open system Interconnection)Model
OSI model
OSI (Open Systems Interconnection) model was created by the International Organization for Standardization (ISO), an international standard-setting body. It was designed to be a reference model for describing the functions of a communication system. It has seven layers, with each layer describing a different function of data traveling through a network.
Here is the graphical representation of these layers:
The layers are usually numbered from the last one, meaning that the Physical layer is consider to be the first layer. It is good to learn these layers, since there will certainly be a couple of questions on the CCNA exam regarding them. Most people learn the mnemonic Please do not tell sells people anything ".
So, what is the purpose of these layers?
They are most commonly used by vendors. They enable them to implement some functionality into a networking device, which then enables easier interoperability with devices from other vendors.
what is PDU?
A protocol data unit (PDU) is an open-system interconnection (OSI) term used in telecommunications that refers to a group of information added or removed by a layer of the OSI model. Each layer in the model uses the PDU to communicate and exchange information, which can only be read by the peer layer on the receiving device and is then handed over to next upper layer after stripping.
Here is a brief description of each of these layers.
Physical – defines how to move bits from one device to another. It details how cables, connectors and network interface cards are going to work and how to send and receive bits.
Data Link – encapsulates a packet in a frame. A frame contains a header and a trailer that enable devices to communicate. A header, most commonly, contains a source and a destination MAC address.It makes decision on the basis of usually MAC (media access control) address or physical address. A trailer contains the Frame Check Sequence field, which is used to detect transmission errors.
PDU: frame
The data link layer has two sub layers:
1. Logical Link Control – used for flow control and error detection 2. Media Access Control – used for hardware addressing and controlling the access method
Network – defines device addressing, routing, and path determination. Device (logical) addressing is used to identify a host on a network (e.g. by its IP address).
PDU:Packet
Transport – segments great chunks of data received from the upper layer protocols. Establishes and terminates connections between two computers. Used for flow control and data recovery.
This layer breaks up the data from the sending host and reassembles it in the receiver .
It is also used to ensure the reliable transport across the network.
PDU : segment
Session – defines how to establish and terminate a session between the two systems.
eg:you use facebook, twitter , or any online shopping sites at the same time in different tabs within the same browser the session is created for different tabs that you are opening and as soon as you are closed the browser or terminate the tab and signed out from the facebook your session is closed .
PDU :Formatted data
Presentation – This layer is responsible for presenting the data in the required format. Compression and encryption are defined at this layer.
PDU: Formatted Data
Application – this layer is the closest to the user. It enables network applications to communicate with other network applications.This layer deals with network applications.
eg. Email, Web Browser.
PDU: User data