The OSI Reference Model describes a 7-layer network architecture developed by ISO to standardize network communication globally. It defines separate protocols for each layer to define tasks and responsibilities. The physical layer is responsible for sending bits between systems by defining encoding, transmission rates, and hardware details. The data link layer provides error checking, frame creation, and hardware addressing. The network layer establishes logical connections between systems, performs routing, handles addressing, and switches packets between networks.

1. The OSI Reference Model
This layered network architecture model was developed by
International Organization for standardization (ISO)
located in Geneva, Switzerland.
It (OSI) was designed to develop common standards of
network architecture throughout the world.
It provides a general concept of inter process
communication in such a way that any open system may
communicate with another open system technically
without any problems.

2. It is a seven layered architecture model .
The model is designed in a highly structured way. It
defines a separate set of protocols for each layer
and hence each layer has a specific independent
function.
The OSI model does not perform any function in the
communication process. The actual work is done by
the appropriate software and hardware. The OSI
model simply defines which tasks need to be done
and which protocols will handle those tasks.
Fig. shows the seven layered Architecture of OSI
models along with protocols and interfaces.

4. In actual implementation of the seven layers, the first
three layers are likely to be in hardware, the next two
layers in the operating system, the presentation
layer in library subroutines in the user's address
space, and the application layer in the user's
program.

5. Physical Layer
This is the lowest layer in the model. This layer is
responsible for activating, maintaining and deactivating a
physical circuit between two end systems.
The Physical Layer is responsible for sending bits (bits
means binary digits, i.e. 1's and O's) from one computer to
another. That is, it may convert the sequence of bits into
electric signals, light signals, or electromagnetic signals,
depending on whether the two nodes are on a cable circuit,
fiber-optic circuit, or microwave/radio circuit,
respectively.

6. Even electrical details, such as how many volts to use for 0
and 1, how many bits can be sent per second, and whether
transmission can take place only in one direction or in both
directions simultaneously, are decided by the physical layer
protocols.
In addition, the physical layer protocols also deal with the
mechanical details, such as the size and shape of the
connecting plugs, the number of pins in the plugs, and the
function of each pin.
Hubs, terminators, couplers, cables, connectors, repeater,
multiplexers, transmitter and receives are devices associated
with physical layer. The position of the physical layer with
respect to the transmission medium and the data link layer is
shown in Fig.

8. Summary of Functions of the physical layer
1. Physical layer is responsible for sending bits from
source computer to destination computer.
2. It defines the bit transmission encoding i.e. how 0's
and 1's are changed to signals.
3. It defines the transmission rate (or data rate) i.e. the
number of bits transmitted per second.
4. It deals with line configuration
(a) Point-to-point (b) Multipoint
5. It defines the transmission mode between two
devices.
(a) Simplex (b) Half duplex (c) Full duplex

9. 6. Topology
It deals with physical topologies i.e. star, ring, bus,
hyprid or mesh etc.
7. Multiplexing
It deals with combining several data channels into one.
8. Media bandwith (Data transmission speed)
9. It deals with the synchronization of the transmitter
and receiver

10. Data Link Layer
The main task of the data link layer is to provide
error free transmission. The physical layer
simply transmits the data from the sender's node
to the receiver's node as raw bits. It is the
responsibility of the data-link layer to detect and
correct any errors in the transmitted data. It
accepts packets from the network layer and
splits the packets into frames which are
transmitted by sender through physical layer as
shown in fig.

12. It also provides flow control and prevents
overburdening of receiver with two much data at any
instant.
The data link layer can also detect when frames are lost
and request that those frames be sent again.
Bridges, intelligent hubs and network interface cards
are devices typically associated with data link layer.

13. Functions of data link layer are summarised as :
1. Framming
It divides packets received from the network layer into
manageable data units called frames.
2. Physical addressing
It adds a header to the frame to define the physical
address of the sender and receiver of the frame.
3. Error control
Error control is main task of the data link layer. It uses
the protocols to detect errors in frames and to ensure
transmission of correct frames.

14. 4. Flow control
It provides a flow control mechanism to
avoid a fast transmitter from overrunning a
slow receiver by buffering the extra bits.
5. Access control
The data link layer protocol determines which
device has control over the link at any given
time, when two or more devices are
connected to the same link.

15. Network Layer
The network layer establishes a logical connection
between sender and user by providing a logical path
between them. This layer switches and routes
message packets as necessary to get them to their
destination.
The layer is responsible for addressing and
delivering message packets.
Network layer makes routing decisions and forwards
the packets for devices that are farther away than a
single link.

16. The network layer also may break large
packets into small chunks if the packet is
larger than the largest data frame the data
link layer accept. The network reassembles
the chunks into packets at receiving end.
Routers and gateways operate in the
network layer.
The relationship of the network layer to the
data link and transport layer is shown in Fig.

18. The functions of the Network Layer are
1. Internetworking
This is the main duty of network layer. It
provides. the logical connection between
different types of networks.
2. Addressing
Addressing is necessary to identify each device
on the network uniquely. This is similar to a
telephone system.

19. 3. Routing
In a network, there are multiple roots available from a source to
a destination and one of them is to be chosen. Routing
algorithms are designed to find the most efficient paths
between the source and destination nodes of a message.
4. Packetizing
It breaks the larger packets into smaller packets if the packet is
larger than the largest data frame the data link layer accept.
This is called as packetizing.
5. Data Switching
The process of transferring packet (data blocks) from source to
destination node is called data switching. Hence, network layer
is concerned with the circuit, message or packet switching.

20. 6. Congestion control
Congestion in a network may occur when the load on the
network i.e. the number of packets sent to the network is
greater than the capacity of the network. Network layer
controls the congestion.
7. It provides connection services, including network
layer flow control, network layer error control and packet
sequence control.