The document summarizes the seven layers of the OSI reference model: 1) The physical layer is responsible for physical connections between devices and defines characteristics like data rates and topology. 2) The data link layer frames data and ensures error-free transmission between nodes through flow control and error checking. 3) The network layer handles packet routing and logical addressing between independent networks. 4) The transport layer manages reliable data transfer through segmentation, reassembly, and connection control using TCP or UDP.

1. Lecture-2
OSI Reference Model

2. OSI Model
 OSI stands for Open Systems Interconnection.
The ISO (International Standards Organization)
was introduced the OSI model in 1983.
 The OSI model is a standard reference model for
communication between two users in a network.
Each layer of this model does not depend on the
other layer, but the data is transmitted from one
layer to another layer.
 The OSI model describes how to send and
receive data in a network. All layers of the OSI
model have their different functions so that data
can be easily accessed from one system to
another.

3. OSI model divides into seven layers

6. Layer 1: The Physical Layer
 The first layer is the physical layer in the OSI model.
This layer is also called a bit unit. The physical layer
coordinates the functions needed to transmit a bit of
stream to a physical medium.
 It is responsible for the actual physical connection
between devices. The physical layer also describes
whether the communication will be wireless or wired.
The repeater is a device of the physical layer.

7. Major functions of the Physical
Layers
 Physical characteristics of interface and media: It
defines the interface properties between devices and
transmission devices. It also defines the type of
transmission medium.
 Line Configuration: There are two types of line
configuration Point-To-Point Configuration and Multipoint
Configuration. In a point-to-point configuration, the two
devices are connected through a separate link. In a
multipoint configuration, the link is shared between
multiple devices.
 Data rate: In data rate, the number of bits transmitted per
second, is also defined by the physical layer. In other
words, the physical layer determines how long the bit
lasts.
 Transmission mode: It defines which transmission mode
will be between two devices in the network.
 Topology: The physical layer defines how the different
devices are organized in a network such as a bus, star,
mesh, ring topology.

8. Layer 2: The Data Link Layer
 The second layer is the Data Link layer in the OSI
model. It is responsible for moving frames from one
node to another. This layer is also called the Frame
unit.
 In this layer, packets of data sent by the network layer
are decoded and encoded. This layer also ensures
the packets of data are error-free.

9. Major functions of the Data link
layer
 Framing: The data link layer divides the bit-stream
received from the network layer into manageable
pieces of data units called frames.
 Flow control: If the rate at which the receiver receives
the data is less than the rate at which the data is
produced at the sender end, the data link layer
establishes a flow control mechanism to avoid
overloading at the receiver end.
 Error control: It controls the error. CRC (cyclic
redundancy check) is added to the end of the frame
with the trailer so that the data is error-free.
 Access Control: When two or more devices are
connected to the same link, the layer will determine
which device is given access.

10. Layer 3: The Network Layer
 The third layer is the Network layer in the OSI model.
The data in the network layer is in the form of
packets. It is responsible for the delivery of individual
packets from the source to the destination. This layer
is also called the packet unit. Switching and routing
techniques are used in this layer.

11. Major functions of the Network
layer
 Routing: When independent networks are connected
to produce Internetworks, the connecting devices
change the packets to their final end. One of the roles
of this layer is to provide the mechanism.
 Logical addressing: The network layer defines a
logical address so that each device on the Internet
can be uniquely identified. The IP addresses of the
sender and the receiver are placed in the header
according to the network layer. This address
distinguishes each device individually and generically.

12. Layer 4: The Transport Layer
 The fourth layer is the Transport layer in the OSI
model. It is responsible for the transfer of a message
from one process to another. Although the network
layer monitors the distribution of individual packets
from source to destination, it does not recognize any
connection between these packets. The transport
layer also confirms a successful data transfer and re-
transmits the data if an error is detected.

13. Major functions of the Transport
Layer
 Segmentation and reassembly: When this layer receives a
message from the upper layers, it divides the message into
many abstract segments. Each segment has a sequence
number so that each segment can be easily identified.
 Connection control: It provides two types of connection: –
Connection-oriented and Connectionless. In
a connectionless, it is done by the user diagram protocol
(UDP). It sends the message directly to the destination without
established the connection. UDP is faster than TCP because
TCP does not establish the connection. In Connection-
oriented, it is done by the transmission control protocol (TCP).
In Connection-oriented, it establish the connection first, then
send the message to the destination. UDP is slower than TCP
because first UDP established the connection then sent the
message.
 Flow control: The transport layer is responsible for flow
control, similar to the data link layer. However, flow control in
this layer is done end to end instead of a single link.
 Error control: The transport layer is responsible for error
control, similar to the data link layer. However, error control in
this layer is done end to end instead of a single link.

14. Layer 5: The Session Layer
 The fifth layer is the Session layer in the OSI model,
which controls connections between many
computers. It manages, establishes, and terminates
the session connection between two communication
hosts.

15. Major functions of the Session
Layer
 Dialog control: In the session layer, it allows two
systems to enter a dialog that allows the
communication between two processes in either
half-duplex mode or full-duplex mode.
 Synchronization: It allows us to add the
checkpoints into the stream of data. For example,
suppose the system is sending a message of 1000
bits, it inserts the checkpoints after every 50 bits and
to ensure that each 50 bits unit is received and
acknowledged individually. In this case, if a crash
happens during the transmission of 453 bit, that only
needs to be a retransmission of 401 to 453.
Previous of 401 bits are not required to re-sent.

16. Layer 6: The Presentation Layer
 The sixth layer is the Presentation Layer in the OSI
model. In the Presentation layer, it deals with the
syntax and semantics information which exchanges
between two systems.

17. Major functions of the Session
Layer
 Translation: It converts a message to compatible
bit-streams before being transmitted because
various computers use various encoding methods.
It is responsible for interoperability between these
various encoding methods, which is changing the
message into a standard format.
 Encryption: Encryption means to transform the
original message into another form, that will not be
readable by others.
 Compression: In Compression, it reduces the
number of bits to be transmitted over the network.

18. Layer 7: The Application Layer
 The seventh layer is the Application Layer in the OSI
model. The end-user is the closest of application
layer. This layer enables user access to the network.
It allows remote logging into the application.

19. Major functions of the Session
Layer
 Network Virtual Terminal: It’s a physical terminal to
the software version. It provides remote host
services.
 File transfer, Access, and Management: It allows a
user to access, retrieve, and manage the files from a
remote host.
 Mail services: It provides email forwarding and
storage.