The first computer networks contained only computers from the same manufacturer because of incompatibilities between the vendors.
For example, enterprise networks ran either a DECnet solution or an IBM solution, but not both. The International Organization for Standardization (ISO) decided to overcome this problem by creating a unified reference framework model, the Open Systems Interconnection (OSI) model.
This model defines standards and protocols that all vendors have to comply with in order to create interoperable network devices and software.
Therefore, even if nowadays computers and operating systems are manufactured by different vendors, they are still able to communicate with each other since they follow a common set of rules defined by the OSI model.
Since the OSI Model is the basic building block of computer networks, the Cisco CCNA 640-802 exam will definitely test your knowledge extensively on the OSI Model. So you have to study the OSI Model very carefully.
The OSI reference Model consists of seven layers with each layer describing a particular network function.
Each OSI layer contains a set of functions performed by networked systems to enable data flow from a source to a destination on a network.
You should memorize the seven layers of the OSI model and learn what happens at each layer and also give examples of protocols/applications/devices that run or belong to each particular layer. The seven OSI layers are:
Layer7: Application Layer:
This is the layer closer to the user. A computer user interacts with an application layer in the OSI model in order to use the network services. Some examples of Application layer are Telnet, FTP, email client (SMTP), Web Browser (HTTP).
Layer6: Presentation Layer:
The presentation layer is responsible to present the data received from the lower layers towards the application layer (and vice-versa) in the appropriate format.
It performs data formatting (ASCII, binary etc), translation between multiple data formats, possible encryption and compression of data etc. Some examples include ASCII, Binary, EBCDIC, GIF, JPEG, WAV, AVI, MPEG, QuickTime etc.
Layer5: Session Layer:
As the name implies, the session layer is responsible to establish, manage and terminate sessions between two communicating systems. It manages multiple communication connections between two networked hosts.
For example there might be multiple web clients communicating to the same web server. These multiple sessions are controlled by the session layer. Examples in this layer are NFS (Network File System) and RPC (Remote Procedure Call).
Layer4: Transport Layer:
It is used to ensure reliable data transport across the network. It segments the data at the sending host and reassembles the data at the receiving host.
It uses sequencing, acknowledgments, retransmissions, multiplexing and flow control to provide transport delivery. The TCP and UDP protocols work at this layer. The TCP protocol is reliable but the UDP protocol is unreliable.
Layer3: Network Layer:
This is where routing occurs. It provides logical addressing (e.g IP addresses) that routers use to determine the best path to a destination. Packet forwarding and packet filtering occurs at this layer. The protocols that belong to this layer are IP, IPv6, and IPX. Also routers work at this layer.
Layer2: Data Link Layer:
This layer provides physical addressing (e.g MAC address of an Ethernet network card) and facilitates the physical transmission of the data. It handles error detection and formats the message into data frames. LLC and MAC work at this layer. Switches also belong to this layer.
Layer1: Physical Layer:
This layer defines the electrical, mechanical and functional specifications of the physical link through which the data is represented as electronic signals. A Hub works at this layer.
An easy mnemonic to remember the seven layers of the OSI model is the following:
Please (Physical – Layer1)
Do (Data Link – Layer2)
Not (Network – Layer3)
Throw (Transport – Layer4)
Sausage (Session – Layer5)
Pizza (Presentation – Layer6)
Away (Application – Layer7)
Again, learning the OSI Model inside-out is a MUST if you want to pass your CCNA exam. If you want a recommendation for an excellent CCNA Training Package, I recommend the CCNA Video Training from TrainSignal. The instructor is Chris Bryant, a veteran in the Cisco Certification field and an awesome instructor. In fact I used Trainsignal when studying for my CCNP Certification and passed all exams with scores over 950.