OSI LAYER

OSI stands for Open Systems Interconnection.

1. Physical Layer (Layer 1) :

The lowest layer of the OSI reference model is the physical layer. It is
responsible for the actual physical connection between the devices. The
physical layer contains information in the form of bits. It is responsible for
transmitting individual bits from one node to the next. When receiving data, this
layer will get the signal received and convert it into 0s and 1s and send them to
the Data Link layer, which will put the frame back together.
The functions of the physical layer are as follows:
1. Bit synchronization: The physical layer provides the synchronization of the
bits by providing a clock. This clock controls both sender and receiver thus
providing synchronization at bit level.
2. Bit rate control: The Physical layer also defines the transmission rate i.e.
the number of bits sent per second.
3. Physical topologies: Physical layer specifies the way in which the different,
devices/nodes are arranged in a network i.e. bus, star, or mesh topology.
4. Transmission mode: Physical layer also defines the way in which the data
flows between the two connected devices. The various transmission modes
possible are simplex, half-duplex and full-duplex.
2. Data Link Layer (DLL) (Layer 2) :
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 sub layers:
1. Logical Link Control (LLC)
2. Media Access Control (MAC)
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 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 (Layer 3) :

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 (Layer 4) :

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
–ConnectionEstablishment
–Data Transfer
– 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 (Layer 5) :

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 (Layer 6):

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 :
 Translation: For example, ASCII to EBCDIC.
 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.
 Compression: Reduces the number of bits that need to be transmitted on
the network.

7. Application Layer (Layer 7) :

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

TCP/IP LAYER
The layers are:
1. Application Layer
2. Transport Layer(TCP/UDP)
3. Network/Internet Layer(IP)
4. Data Link Layer (MAC)
5. Physical Layer

1. Physical Layer (Layer 1) :

The lowest layer of the OSI reference model is the physical layer. It is
responsible for the actual physical connection between the devices. The
physical layer contains information in the form of bits. It is responsible for
transmitting individual bits from one node to the next. When receiving data, this
layer will get the signal received and convert it into 0s and 1s and send them to
the Data Link layer, which will put the frame back together.
The functions of the physical layer are as follows:
5. Bit synchronization: The physical layer provides the synchronization of the
bits by providing a clock. This clock controls both sender and receiver thus
providing synchronization at bit level.
6. Bit rate control: The Physical layer also defines the transmission rate i.e.
the number of bits sent per second.
7. Physical topologies: Physical layer specifies the way in which the different,
devices/nodes are arranged in a network i.e. bus, star, or mesh topology.
8. Transmission mode: Physical layer also defines the way in which the data
flows between the two connected devices. The various transmission modes
possible are Simplex, half-duplex and full-duplex.

2. Data Link Layer (DLL) (Layer 2) :

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:
3. Logical Link Control (LLC)
4. Media Access Control (MAC)
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 functions of the Data Link layer are :
6. 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.
7. 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.
8. Error control: Data link layer provides the mechanism of error control in
which it detects and retransmits damaged or lost frames.
9. 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.
10. 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 (Layer 3) :

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 :
3. Routing: The network layer protocols determine which route is suitable from
source to destination. This function of the network layer is known as routing.
4. 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 (Layer 4) :

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:
3. 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.
4. 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
–ConnectionEstablishment
–DataTransfer
– 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. Application Layer (Layer 5) :

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 :
5. Network Virtual Terminal
6. FTAM-File transfer access and management
7. Mail Services
8. Directory Services