computer network OSI layer

The Reference Model for network communication
2

Index
1. OSI model & function of each
Layer.
2. TCP/ IP model.
3. Connection oriented v/s
Connectionless approach.
4. Comparison of OSI & TCP/IP
Models.
3

1.OSI model & function of each Layer.
 There are many users who use computer
network and are located all over the world.
 For national and worldwide data
communication, systems must be developed
which are compatible to communicate with
each other.
 ISO has developed this.
4

 ISO stands for International organization of
Standardization.
 This is called a model for Open System
Interconnection (OSI) and is commonly
known as OSI model.
 The ISO-OSI model is a seven layer (7
Layer)architecture developed in 1984
5

 To reduce the design complexity, most of the
networks are organized as a series of layers or
levels, each one build upon one below it.
 The basic idea of a layered architecture is to
divide the design into small pieces.
 Each layer adds to the services provided by the
lower layers in such a way that the highest
layer is provided has a full set of services.
6

 The basic elements of a layered model are
 services
 protocols
 and interfaces.
 A service is a set of actions that a layer offers to
another (higher) layer.
 A Protocol is a set of rules that a layer uses to
exchange information.
 A Interface is communication between the
layers. 7

 Layer 1,2 and 3 that is physical ,data and
network layers are Network Support
Layers.
 Layers 5,6 and 7 that is session,presentation
and application layers are User Support
Layer.
 Transport Layer links the network support
layers and user support layers.
9

Physical Layer:Layer 1
 Physical layer is the lowest layer of all. It is
responsible for sending bits from one
computer to another. This layer is not
concerned with the meaning of the bits and
deals with the physical connection to the
network and with transmission.
 This layer defines electrical and physical
details represented as 0 or a 1
11

Functions of Physical Layer:
 Representation of Bits: Data in this layer
consists of stream of bits.
 Data Rate: This layer defines the rate of
transmission which is the number of bits per
second.
12

 Synchronization: It deals with the
synchronization of the transmitter and receiver.
 Interface: The physical layer defines the
transmission interface between devices and
transmission medium.
13

 Line Configuration: This layer connects
devices with the medium: Point to Point
configuration and Multipoint configuration.
 Topologies: Devices must be connected using
the following topologies: Mesh, Star, Ring
and Bus.
14

 Transmission Modes: Physical Layer defines
the direction of transmission between two
devices: Simplex, Half Duplex, Full Duplex.
 Deals with baseband and broadband
transmission.
15

Data Layer:Layer 2:
 Data link layer is most reliable node to node
delivery of data. It forms frames from the
packets that are received from network layer
and gives it to physical layer.
 Error controlling is easily done. Error detection
bits are used by the data link layer. It also
corrects the errors.
16

FUNCTIONS OF DATA LINK LAYER:
 Framing: Frames are the streams of bits
received from the network layer into
manageable data units.
 Physical Addressing: The Data Link layer
adds a header to the frame in order to define
physical address of the sender or receiver of the
frame, if the frames are to be distributed to
different systems on the network.
17

 Flow Control: A flow control mechanism to
avoid a fast transmitter from running a slow
receiver by buffering the extra bit is
provided by flow control. This prevents
traffic jam at the receiver side.
 Error Control: Error control is achieved by
adding a trailer at the end of the frame.
Duplication of frames are also prevented by
using this mechanism
18

 Access Control: Protocols of this layer
determine which of the devices has control
over the link at any given time.
19

Two Parts of Data Link Layer
 Data link layer is divided into two subparts
 1.Logical Link Control Layer (LLC)
 2.Media Access Control Layer (MAC)
20

Data Link Layer:LLC and MAC
 LLC:Logical Link Control layer is the upper
layer of the two in data link layer.
 Essential for the communications between
devices.
 Supports connection oriented and
connectionless services both.
 MAC:Media Access Control is the lower
layer.
 It is responsible for providing a method for
station to gain access to the medium. 21

Layer 3: The Network Layer :
 It routes the signal through different
channels from one node to other.
 It acts as a network controller. It manages
the Subnet traffic.It decides by which route
data should take.
 It divides the outgoing messages into
packets.
22

Functions of Network Layer:Layer 3
 It translates logical network address into
physical address.
 Routers and gateways operate in the
network layer. Mechanism is provided by
Network Layer for routing the packets to
final destination.
 Connection services are provided.
 Breaks larger packets into small packets.
23

Layer 4:Transport Layer
 It decides if data transmission should be on
parallel path or single path.
 It receives messages from the Session layer
above it, convert the message into smaller
units and passes it on to the Network layer.
 Transport layer breaks the message (data)
into small units so that they are handled
more efficiently by the network layer.
24

Function of Transport Layer:Layer 4
 Service Point Addressing : Transport
Layer header includes service point address
which is port address.
 Segmentation and Reassembling : A
message is divided into segments; each
segment contains sequence number, which
enables this layer in reassembling the
message.
25

26
 Flow Control : In this layer, flow control is
performed end to end.
 Error Control : Error Control is performed end to
end in this layer to ensure that the complete
message arrives at the receiving transport layer
without any error. Error Correction is done
through retransmission.

Layer 5: The Session Layer :
 Session layer manages and synchronize the
conversation between two different
applications.
 Transfer of data from source to destination
session layer streams of data are marked and
are resynchronized properly, so that the
ends of the messages are not cut and data
loss is avoided.
27

Functions of Session Layer:Layer 5
 Dialog Control : This layer allows two
systems to start communication with each
other in half-duplex or full-duplex.
 Synchronization : This layer allows a
process to add checkpoints which are
considered as synchronization points into
stream of data.
28

 Example: If a system is sending a file of 800
pages, adding checkpoints after every 50
pages is recommended. This ensures that 50
page unit is successfully received and
acknowledged. This is beneficial at the time
of crash as if a crash happens at page
number 110; there is no need to retransmit 1
to 100 pages.
29

Layer 6: The Presentation Layer :
 The primary goal of this layer is to take care
of the syntax and semantics of the
information exchanged between two
communicating systems.
 Presentation layer takes care that the data is
sent in such a way that the receiver will
understand the information (data) and will
be able to use the data.
30

Functions of Presentation
Layer:Layer 6
 Translation : Before being transmitted,
information in the form of characters and
numbers should be changed to bit streams.
 Encryption : It carries out encryption at the
transmitter and decryption at the receiver.
 Compression : It carries out data
compression to reduce the bandwidth of the
data to be transmitted. It is important in
transmitting multimedia such as audio,
video, text etc. 31

Layer 7: Application Layer :
 It is the topmost layer.
 Transferring of files disturbing the results to the user is
also done in this layer. Mail services, directory services,
network resource etc are services provided by
application layer.
 This layer mainly holds application programs to act
upon the received and to be sent data.
32

Functions of Application
Layer:Layer 7
 Mail Services : This layer provides the basis
for E-mail forwarding and storage.
 Directory Services : This layer provides
access for global information about various
services.
 File Transfer, Access and Management
(FTAM) : It is a standard mechanism to
access files and manages it.
33

Protocols at Application Layer
 HTTP:Hyper Text Transfer Protocol
 FTP:File Transfer Protocol
 SMTP:Simple Mail Transfer Protocol
 NFS:Network File Services
 NVT:Network Virtual Terminal
34

Merits of OSI
 OSI model distinguishes well between the
services, interfaces and protocols.
 Protocols of OSI model are very well hidden.
 Protocols can be replaced by new protocols
as technology changes.
 Supports connection oriented services as
well as connectionless service.
35

Demerits of OSI
 Model was devised before the invention of
protocols.
 Fitting of protocols is tedious task.
 It is just used as a reference model.
36

2.TCP/IP MODEL
 TCP/IP means Transmission Control
Protocol and Internet Protocol. It is the
network model used in the current Internet
architecture as well. Protocols are set of
rules which govern every possible
communication over a network. These
protocols describe the movement of data
between the source and destination or the
internet. These protocols offer simple
naming and addressing schemes.
37

 TCP/IP that is Transmission Control
Protocol and Internet Protocol was
developed by Department of Defence's
Project Research Agency (ARPA, later
DARPA) as a part of a research project of
network interconnection to connect remote
machines.
38

 Flexible architecture: Adding more
machines to a network was easy.
 Robust:Connections remained intact
untill the source and destination machines
were functioning.
 Communication:The overall idea was to
allow one application on one computer
send data packets another application
running on different computer.
39

Layer 1: Host-to-network Layer
 Lowest layer of the all.
 Protocol is used to connect to the host, so
that the packets can be sent over it.
 Varies from host to host and network to
network.
41

Layer 2: Internet layer
 Selection of a packet switching network
which is based on a connectionless
internetwork layer is called a internet layer.
 It is the layer which holds the whole
architecture together.
 It helps the packet to travel independently
to the destination.
 Order in which packets are received is
different from the way they are sent.
 IP (Internet Protocol) is used in this layer.
42

Layer 3: Transport Layer
 It decides if data transmission should be on
parallel path or single path.
 Functions such as multiplexing, segmenting
or splitting on the data is done by transport
layer.
 The applications can read and write to the
transport layer.
43

 Transport layer adds header information to
the data.
 Transport layer breaks the message (data)
into small units so that they are handled
more efficiently by the network layer.
 Transport layer also arrange the packets to
be sent, in sequence.
44

Layer 4: Application Layer
 The TCP/IP specifications described a lot of
applications that were at the top of the
protocol stack. Some of them were TELNET,
FTP, SMTP, DNS etc.
 TELNET is a two-way communication
protocol which allows connecting to a
remote machine and run applications on it.
45

 FTP(File Transfer Protocol) is a protocol,
that allows File transfer amongst computer
users connected over a network. It is
reliable, simple and efficient.
 SMTP(Simple Mail Transport Protocol) is a
protocol, which is used to transport
electronic mail between a source and
destination, directed via a route.
 DNS(Domain Name Server) resolves an IP
address into a textual address for Hosts
connected over a network.
46

Sr
no
Protocol Full Form Function
1 IP Internetwork Protocol Used by TCP/IP
2 ICMP Internet Control Message Protocol Control and error
messages
3 IGMP Internet Group Message Protocol Unicasting and
multicasting
4 ARP Address Resolution Protocol To know physical
address
5 RARP Reverse Address Resolution Protocol To know internet
address
6 TCP Transmission Control Protocol Connection
oriented and
reliable
7 UDP User Datagram Protocol Connectionless
and unreliable
49

8 SMTP Simple Mail
Transfer
Protocol
Deals with e-
mails
9 FTP File Transfer
Protocol
Deals with files
10 TFTP Trivial File
Transfer
Protocol
Read and write
operations only
11 SNMP Simple Network
Management
Protocol
Managing the
network
12 TELNET Terminal
Network
Remote Login
50

3.Connection Oriented and
Connectionless Services
 These are the two services given by the layers to layers
above them. These services are :
1. Connection Oriented Service
2. Connectionless Services
51

Connection Oriented Services
 There is a sequence of operation to be
followed by the users of connection oriented
service. These are:
 Connection is established
 Information is sent
 Connection is released
52

 In connection oriented service we have to
establish a connection before starting the
communication. When connection is
established we send the message or the
information and then we release the
connection.
 Connection oriented service is more reliable
than connectionless service. We can send
the message in connection oriented service
if there is an error at the receivers end.
Example of connection oriented is TCP
(Transmission Control Protocol) protocol. 53

Connectionless Services
 It is similar to the postal services, as it
carries the full address where the message
(letter) is to be carried. Each message is
routed independently from source to
destination. The order of message sent can
be different from the order received.
54

 In connectionless the data is transferred in
one direction from source to destination
without checking that destination is still
there or not or if it prepared to accept the
message. Authentication is not needed in
this. Example of Connectionless service is
UDP (User Datagram Protocol) protocol.
55

Difference between Connection
Oriented and Connectionless
Services
Sr
no.
Connection
Oriented Services
Connectionless
Services
1. It needs
authentication.
It doesnot need
authentication.
2. It gurantees a
delivery
It doesnot gurantee a
delivery
3. It is more reliable It not that reliable
4. Connection Oriented
is stream based
Connectionless is
message based.
56

4.Comparison of OSI Reference
Model and TCP/IP Reference Model
Sr
no
OSI TCP/IP
1. OSI model
Developed after
TCP/IP
TCP/IP model
Developed before OSI
2. OSI model has 7
layers
TCP/IP has 4 layers
3. OSI model
developed before
protocols of OSI
were developed
TCP/IP model
developed after
protocols of TCP/IP
were developed 57

4. OSI differentiates
services,protocols and
interfaces.
TCP/IP doesn’t
differentiate
them.
5. Connection oriented and
connectionless in network
layer and connection
oriented in transport layer
in OSI model
Connection
oriented and
connectionless in
transport layer and
connection oriented
in network layer in
tcp/ip model
58

computer network OSI layer

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