OSI Model (Open System Interconnection)
OSI model is the conceptual model of how a message is sent from one system to another in a network.
1- Physical Layer
Physical Layer is the bottom-most layer
in the Open System Interconnection (OSI) Model which is a physical and electrical representation of the
system. It consists of various network components such as power plugs, connectors, receivers, cable types,
etc. Physical Layer sends data bits from one device(s) (like a computer) to another device(s). Physical
Layer defines the types of encoding (that is how the 0’s and 1’s are encoded in a signal). Physical Layer is
responsible for the communication of the unstructured raw data streams over a physical medium.
Functions Performed by Physical Layer :
Following are some important and basic functions that are performed by the Physical Layer of the OSI Model
–
1- Physical Layer maintains the data rate (how many bits a sender can send per second).
2- It performs Synchronization of bits.
3- It helps in Transmission Medium decision (direction of data transfer).
4- It helps in Physical Topology (Mesh, Star, Bus, Ring) decision (Topology through which we can connect the
devices with each other).
5- It helps in providing Physical Medium and Interface decisions.
6- It provides two types of configuration Point to Point configuration and Multi-Point configuration.
7- It provides an interface between devices (like PC’s or computers) and transmission medium.
8- It has a protocol data unit in bits.
9- Hubs, Ethernet, etc. device is used in this layer.
10- This layer comes under the category of Hardware Layers (since the hardware layer is responsible for all
the physical connection establishment and processing too).
11- It provides an important aspect called Modulation, which is the process of converting the data into
radio waves by adding the information to an electrical or optical nerve signal.
12- It also provides Switching mechanism wherein data packets can be forward from one port (sender port) to
the leading destination port.
Physical Topologies :
Physical Topology or Network Topology is the Geographical Representation of Linking devices. Following are
the four types of physical topology-
Mesh Topology: In a mesh topology, each and every device should have a dedicated point-to-point connection
with each and every other device in the network. Here there is more security of data because there is a
dedicated point-to-point connection between two devices. Mesh Topology is difficult to install because it is
more complex.
1- Star Topology: In star topology, the device should have a dedicated point-to-point connection with a
central controller or hub. Star Topology is easy to install and reconnect as compared to Mesh Topology. Star
Topology doesn’t have Fault Tolerance Technique.
2- Bus Topology: In a bus topology, multiple devices are connected through a single cable that is known
as backbone cable with the help of tap and drop lines. It is less costly as compared to Mesh Topology and
Star Topology. Re-connection and Re-installation are difficult.
3- Ring Topology: In a ring topology, each device is connected with repeaters in a circle-like ring
that’s why it is called Ring Topology. In Ring topology, a device can send the data only when it has a
token, without a token no device can send the data, and a token is placed by Monitor in Ring Topology.
4- Point to Point configuration : In Point-to-Point configuration, there is a line (link) that is fully
dedicated to carrying the data between two devices.
5- Multi-Point configuration : In Multi-Point configuration, there is a line (link) through which
multiple devices are connected.
Modes of Transmission Medium :
1- Simplex mode: In this mode, out of two devices, only one device can transmit the data, the other
device can only receive the data. Example- Input from keyboards, monitors, TV broadcasting, Radio
broadcasting, etc.
2- Half Duplex mode: In this mode, out of two devices, both devices can send and receive the data but
only one at a time not simultaneously. Example- Walkie-Talkie, Railway Track, etc.
3- Full-Duplex mode: In this mode, both devices can send and receive the data simultaneously. Example-
Telephone System, Chatting applications, etc.
2- Datalink Layer
The data link layer is responsible for
the node-to-node delivery of the message. The main function of this layer is to make sure data transfer is
error-free from one node to another, over the physical layer. When a packet arrives in a network, it is the
responsibility of DLL to transmit it to the Host using its MAC address.
Data Link Layer is divided into two sublayers:
1- Local Link Control
2- Media Access Control
The packet received from the Network layer is further divided into frames
depending on the frame size of NIC(Network Interface Card). DLL also encapsulates Sender and Receiver’s MAC
address in the header.
The Receiver’s MAC address is obtained by placing an ARP(Address Resolution Protocol) request onto the wire
asking “Who has that IP address?” and the destination host will reply with its MAC address.
The functions of the Data Link layer are :
1- Framing: Framing is a function of the data link layer. It provides a way for a sender to transmit a set
of bits that are meaningful to the receiver. This can be accomplished by attaching special bit patterns to
the beginning and end of the frame.
2- Physical addressing: After creating frames, the Data link layer adds physical addresses (MAC address) of
the sender and/or receiver in the header of each frame.
3- Error control: Data link layer provides the mechanism of error control in which it detects and
retransmits damaged or lost frames.
4- Flow Control: The data rate must be constant on both sides else the data may get corrupted thus, flow
control coordinates the amount of data that can be sent before receiving acknowledgement.
5- Access control: When a single communication channel is shared by multiple devices, the MAC sub-layer of
the data link layer helps to determine which device has control over the channel at a given time.
3- Network Layer
The network layer works for the
transmission of data from one host to the other located in different networks. It also takes care of packet
routing i.e. selection of the shortest path to transmit the packet, from the number of routes available. The
sender & receiver’s IP addresses are placed in the header by the network layer.
The functions of the Network layer are :
1- Routing: The network layer protocols determine which route is suitable from source to destination. This
function of the network layer is known as routing.
2- Logical Addressing: In order to identify each device on internetwork uniquely, the network layer defines
an addressing scheme. The sender & receiver’s IP addresses are placed in the header by the network layer.
Such an address distinguishes each device uniquely and universally.
4- Transport Layer
The transport layer provides services to
the application layer and takes services from the network layer. The data in the transport layer is referred
to as Segments. It is responsible for the End to End Delivery of the complete message. The transport layer
also provides the acknowledgement of the successful data transmission and re-transmits the data if an error
is found.
At sender’s side: Transport layer receives the formatted data from the upper layers, performs Segmentation,
and also implements Flow & Error control to ensure proper data transmission. It also adds Source and
Destination port numbers in its header and forwards the segmented data to the Network Layer.
Generally, this destination port number is configured, either by default or manually. For example, when a
web application makes a request to a web server, it typically uses port number 80, because this is the
default port assigned to web applications. Many applications have default ports assigned.
At receiver’s side: Transport Layer reads the port number from its header and forwards the Data which it
has received to the respective application. It also performs sequencing and reassembling of the segmented
data.
The functions of the transport layer are as follows:
1- Segmentation and Reassembly: This layer accepts the message from the (session) layer, and breaks the
message into smaller units. Each of the segments produced has a header associated with it. The transport
layer at the destination station reassembles the message.
2- Service Point Addressing: In order to deliver the message to the correct process, the transport layer
header includes a type of address called service point address or port address. Thus by specifying this
address, the transport layer makes sure that the message is delivered to the correct process.
The services provided by the transport layer :
A. Connection-Oriented Service: It is a three-phase process that includes
– 1- Connection Establishment
– 2- Data Transfer
– 3- Termination / disconnection
In this type of transmission, the receiving device sends an acknowledgement, back to the source after a
packet or group of packets is received. This type of transmission is reliable and secure
B. Connectionless service: It is a one-phase process and includes Data Transfer. In this type of
transmission, the receiver does not acknowledge receipt of a packet. This approach allows for much faster
communication between devices. Connection-oriented service is more reliable than connectionless Service.
5- Session Layer
This layer is responsible for the
establishment of connection, maintenance of sessions, authentication, and also ensures security.
The functions of the session layer are :
1- Session establishment, maintenance, and termination: The layer allows the two processes to establish, use
and terminate a connection.
2- Synchronization: This layer allows a process to add checkpoints which are considered synchronization
points into the data. These synchronization points help to identify the error so that the data is
re-synchronized properly, and ends of the messages are not cut prematurely and data loss is avoided.
3- Dialog Controller: The session layer allows two systems to start communication with each other in
half-duplex or full-duplex.
6- Presentation Layer
The presentation layer is also called the
Translation layer. The data from the application layer is extracted here and manipulated as per the required
format to transmit over the network.
The functions of the presentation layer are :
1- Translation: For example, ASCII to EBCDIC.
2- Encryption/ Decryption: Data encryption translates the data into another form or code. The encrypted data
is known as the ciphertext and the decrypted data is known as plain text. A key value is used for encrypting
as well as decrypting data.
3- Compression: Reduces the number of bits that need to be transmitted on the network.
7- Application Layer
At the very top of the OSI Reference
Model stack of layers, we find the Application layer which is implemented by the network applications. These
applications produce the data, which has to be transferred over the network. This layer also serves as a
window for the application services to access the network and for displaying the received information to the
user.
Example: Application – Browsers, Skype Messenger, etc.
The functions of the Application layer are :
1- Network Virtual Terminal
2- FTAM-File transfer access and management
3- Mail Services
4- Directory Services