The OSI Reference Model | CCNA |Techyami |

Introduction to the OSI Reference Model 

The International Organization for Standardization (ISO) defined the Open
Systems Interconnection (OSI) reference model to standardize networking
of devices from different vendors. The OSI reference model is mostly an
architecture blueprint that networking and computer device manufacturers
implement. The OSI model has never been implemented exactly as defined.
The TCP/IP protocol stack is the closest implementation available today.

Seven Layers

The OSI reference model is designed in seven functional layers. Each layer

has a precise mission, and each layer works fairly independently of the

upper and lower layers. Upper layers use the services provided by lower

layers, but the internal workings of each layer are not visible to other layers.

This independence is achieved through encapsulation and very clearly

defined interfaces between layers. Here are the layers in a nutshell:

✦ Top layers build an application data payload that is divided by the

lower transport layer into several small data chunks called segments.

Each segment is numbered so that the receiving host can reassemble

the application data.

✦ The transport segments are then forwarded down to the network layer,

which tags each segment with logical source and destination addresses

and some control information, and hands over the resulting shippable

data packet to the lower data link layer.

✦ The data link layer adds the physical source address of the sender and

the physical destination address of the receiver if the receiver is located

in the same local network as the sender. If the receiver is not located in

the same local network as the sender, the data link layer adds the physical

destination address of the gateway in the local network. The gateway

of a local network is usually a router that connects the local network to

other networks.

Layer 7: Application

This layer represents the various network applications such as e-mail

reader, Web browser, Hypertext Transfer Protocol (HTTP), File Transfer

Protocol (FTP), and Network File System (NFS).

✦ The application layer provides a user interface and processes network
data.

✦ The application layer on the sending host produces the network data to
be transmitted from the sender host.

✦ The application layer on the receiving host consumes the network data
produced and transmitted by the sender host.

Layer 6: Presentation

This layer is mostly concerned with data format. It converts the data
between different formats so that both the sender and the receiver can use
heterogeneous data. For example, mail messages contain various data formats:
Text, application attachments, video, audio, and graphical signature.

✦ The presentation layer on the sending host receives the data payload from the application layer.

✦ The presentation layer on the sending host converts the data into a format that is easily transportable over the network.

✦ The presentation layer on the receiving host converts the data from the network format back to its native format that can be easily interpreted, used, and displayed by the application layer above.

Layer 5: Session

Some applications need to open logical communication channels between
the computer hosts. Logical communication channels (sessions) maintain
data about the communication established between the network application
running on the sending host and the network application running on the
receiving host. The session layer does the following:

✦ Opens and maintains logical communication channels between network

applications running on the sending host and network applications

running on the receiving host.

✦ Handles authentication: Some network applications use authentication

mechanisms before they open a logical communication channel (session)

with a remote host.

Layer 4: Transport

The transport layer manages the transport of data between two hosts over a

network. In a nutshell, the transport layer does the following:

✦ Slices the data to be transmitted into small chunks called data segments
that can be easily sent over the network medium.

✦ Reassembles the data in order on the receiving host: Data segments

are not guaranteed to arrive in order at destination since they may use

different routes to reach the destination host. The transport layer is

responsible to reassemble the data in order on the receiving host..

Layer 3: Network

The network layer routes data packets across networks that link the sending

and the receiving host. In a nutshell, the network layer does the following:

✦ Chooses the best route to send packets between hosts.

✦ Assigns logical addresses to all devices in the network to be able to

identify each source host and each destination host, as well as each

network through which packets need to be routed.

Logical addresses are assigned at the network protocol level. Physical

addresses are assigned on a physical device, such as a network card.

✦ Receives each data segment from the transport layer on the sending

host and wraps it in a data packet along with routing data. The packet

is sent down to the data link layer to send it over the network physical

medium.

✦ On the receiving host, the network layer unwraps the packet received to

extract the data segment and sends it up to the transport layer.

Several protocols operate at the network layer, such as IP, IPX,

AppleTalk, and SNA, but the CCNA test is only concerned with IP.

The Internet Protocol (IP) is the TCP/IP implementation of the network layer.

IP addresses are logical addresses provided by the IP in TCP/IP.

Cisco routers are Layer 3 (network layer) devices.

Layer 2: Data link

The data link layer does the following:

✦ Transmits the data on the physical medium.

✦ Routes the data locally on the physical network medium. The data link

layer uses physical addresses assigned to each physical network device

in the local network to route data from one physical device to another.

✦ The data link layer receives each packet from the network layer on the

sending host and wraps it in a data frame along with local routing data.

✦ The data link layer sends each data frame down to the physical layer

to code an electrical or optical signal to transmit the data frame over a

wire or over the air (wireless transmission).

✦ On the receiving host, the data link layer unwraps the data frame

received to extract the packet and sends it to the network layer.

Layer 1: Physical

The physical layer provides the electrical, optical, or over-the-air connection

between the sending host device and the receiving host device. This typically

involves copper or fiber-optic cabling, or wireless radio connections, patch

panels, signal repeaters, submarine cables, or satellites.

CCNA certification does not require you to be a space science expert.

However, you do need to understand that data is always converted into bits

that can be transmitted over a medium using electrical current or optical

signals that simulate a 1 (signal) or a 0 (no signal).

In a nutshell, the physical layer defines mechanical, electrical, optical, radio,

procedural, and functional standards to enable the transmission of data-link

(Layer 2) frames over a certain transmission medium.

These standards define how a physical link is built, activated, maintained,

and deactivated to enable transmissions between DTE (data terminal

equipment) and DCE (data communications equipment).

DTEs are host devices. DCEs are network devices, that is, any device that

stands between two host devices.