UNIT-2
THE REFERENCES MODEL FOR
NETWORK COMMUNICATION
2
SYLLABUS
2.1 OSI model & function of each Layer
2.2 TCP/ IP model
2.3 Connection oriented v/s Connectionless
approach
2.4 Comparison of OSI & TCP/IP Models
2
2.1 OSI MODEL & FUNCTION OF EACH LAYER
Open Systems Interconnection Basic Reference Model
(OSI Reference Model or OSI Model) is an abstract
description for layered communications and computer
network protocol design.
It was developed as part of the Open Systems
Interconnection (OSI) initiative.
3
CONTINUE…
In its most basic form, it divides network
architecture into seven layers which, from top to
bottom, are the Application, Presentation,
Session, Transport, Network, Data-Link, and
Physical Layers.
It is therefore often referred to as the OSI Seven
Layer Model.
4
5
APPLICATION
PRESENTATION
SESSION
TRANSPORT
NETWORK
DATA LINK
PHYSICAL
APPLICATION
PRESENTATION
SESSION
TRANSPORT
NETWORK
DATA LINK
PHYSICAL
Application to Application
Application to Application
Application to Application
Process to Process
Hop to Hop Switch
Hop to Hop
Physical Medium
Hub and Repeater
Router Source to Destination
Source to Destination
OSI MODEL'S 7 LAYERS
HOST AND MEDIA LAYER
6
DATA, PROTOCOL & ACTIVITIES
7
TCP/IP Suit OSI Layers
Application Telnet, FTP, SMTP, HTTP, DNS, SNMP, Specific address etc…
Session
Presentation
Transport SCTP, TCP, UDP, Sockets and Ports address
Network IP, ARP/RARP, ICMP, IGMP, Logical address
Data Link IEEE 802 Standards, TR, FDDI, PPP, Physical address
Application
Session
Presentation
Transport
Network
Data Link
Activities
To allow access to network resources
To establish, manage, and terminate
session
To Translate, encrypt, and compress
data
To Provide reliable process-to-process
Message delivery and error recovery
To move packets from source to
destination; to provide internetworking
To organize bits into frames; to provide
Hop-to-hop delivery
Physical Medium, Coax, Fiber, 10base, Wireless
Physical To Transmit bits over a medium; to provide
Mechanical and electrical specifications
PHYSICAL LAYER
One of the major function of the physical layer is to move data in the form of electromagnetic signals across a transmission medium.
Its responsible for movements of individual bits from one hop (Node) to next.
Both data and the signals can be either analog or digital.
Transmission media work by conducting energy along a physical path which can be wired or wireless 8
10101000000010111 110
From data link layer
10101000000010111 110
To data link layer
Physical layer Physical layer
Transmission medium
CONTINUE…
Concerned:
Physical characteristics of interface and medium (Transmission medium)
Representation of bits (stream of bits (0s or 1s) with no interpretation and encoded into signals)
Data rate (duration of a bit, which is how long it last)
Synchronization of bits (sender and receivers clock must be synchronized)
Line configuration (Point-to-Point, Point-to-Multipoint)
Physical topology
Transmission mode (Simplex, half duplex, full duplex)
9
DATA LINK LAYER (HOST TO HOST)
Data link layer is responsible for moving frames from one hop (Node) to the next.
Concerned:
Framing (stream of bits into manageable data units)
Physical addressing (MAC Address)
10
Data H2 T2
From network layer
Data H2 T2
To network layer
To physical layer From physical layer
Data link layer Data link layer
Flow Control (mechanism for overwhelming the receiver)
Error Control (trailer, retransmission)
Access Control (defining master device in the same link)
11
NETWORK LAYER (SOURCE TO
DESTINATION)
The network layer is responsible for the delivery of individual packets from the source host to the destination host.
Concerned:
Logical addressing (IP Address)
Routing (Source to destination transmission between networks)
12
Data H3 Packet
From transport layer
To data link layer
Data H3 Packet
To transport layer
From data link layer
Network layer Network layer
TRANSPORT LAYER (PROCESS TO
PROCESS)
The transport layer is responsible for the delivery of a message from one process to another
Concerned:
Service-point addressing (Port address)
Segmentation and reassembly (Sequence number) 13
Segments
Data H4
From session layer
To network layer
Transport layer Data H4 Data H4
Segments
Data H4
From session layer
From network layer
Transport layer Data H4 Data H4
Connection control (Connectionless or connection oriented)
Flow control (end to end)
Error Control (Process to Process)
14
SESSION LAYER (DIALOG INITIATION)
The session layer is responsible for dialog control and synchronization
Concerned:
Dialog Control (Half Duplex/Full duplex)
Synchronization (Synchronization points, process inline within same page)
15
Data H5
From Presentation layer
To transport layer
Data Data
Syn Syn Syn
Data H5
To Presentation layer
From transport layer
Data Data
Syn Syn Syn
Session layer Session layer
PRESENTATION LAYER (DEPENDENCY)
The presentation layer is responsible for translation, compression and encryption
Concerned:
Translation (interoperability between different encoding system)
Encryption (Privacy schemes)
Compression (data compression)
16
Data H6
From application layer
To session layer
Data H6
To application layer
From session layer
presentation layer presentation layer
APPLICATION LAYER (USER LEVEL
SERVICE)
The application layer is responsible for providing services to the user.
Concerned:
Network virtual terminal (Software) 17
Data H7
USER
(Human or Program)
To presentation layer
Data H7
Application layer
Application layer
Message Message
X.500 FTAM X.400 X.500 FTAM X.400
From presentation layer
USER
(Human or Program)
CONTINUE…
File transfer, access and management
Mail services
Directory services (access to distributed database sources for global information about various objects and services)
18
2.2 TCP/ IP MODEL
The Internet Protocol Suite (commonly known as TCP/IP) is the set of communications protocols used for the Internet and other similar networks.
It is named from two of the most important protocols in it:
the Transmission Control Protocol (TCP) and
the Internet Protocol (IP), which were the first two networking protocols defined in this standard.
APPLICATION
This layer is comparable to the application,
presentation, and session layers of the OSI model
all combined into one.
It provides a way for applications to have access
to networked services.
This layer also contains the high level
protocols. The main issue with this layer is the
ability to use both TCP and UDP protocols.
For example TFTP uses UDP because usually on a
LAN the physical links are short enough to ensure
quick and reliable packet delivery without many
errors. SMTP instead uses TCP because of the error
checking capabilities.
Since we consider our email important information we
would like to ensure a safe delivery.
TFTP (Trivial File Transfer Protocol) -
Simplified version of the FTP protocol which has
no security features.
TRANSPORT
This layer acts as the delivery service used by the
application layer.
Again the two protocols used are TCP and UDP.
The choice is made based on the application's
transmission reliability requirements.
The transport layer also handles all error
detection and recovery.
It uses checksums, acknowledgements, and
timeouts to control transmissions and end to end
verification.
Unlike the OSI model, TCP/IP treats reliability
as an end-to-end problem
INTERNET LAYER(NETWORK
LAYER)
This layer is also known as Internet layer. The
main purpose of this layer is to organize or
handle the movement of data on network.
By movement of data, we generally mean
routing of data over the network. The main
protocol used at this layer is IP. While
ICMP(used by popular ‘ping’ command) and IGMP are also used at this layer.
HOST TO NETWORK INTERFACE
This layer is also known as network interface
layer
This layer normally consists of device drivers in
the OS and the network interface card attached
to the system.
Both the device drivers and the network
interface card take care of the communication
details with the media being used to transfer the
data over the network.
In most of the cases, this media is in the form of
cables.
Some of the famous protocols that are used at
this layer include ARP(Address resolution
protocol), PPP(Point to point protocol) etc.
2.3 CONNECTION ORIENTED V/S CONNECTIONLESS
APPROACH
Connection-oriented approach
Setup data transfer ahead of time (through
handshaking)
Internet’s connection-oriented service is TCP
(Transmission Control Protocol)
It provides reliable, in-order byte delivery
flow control
congestion control.
Applications using TCP: Email (SMTP), web
browsing (HTTP), and file transfer (FTP)
32
CONTINUE Connectionless approach
Internet’s connectionless service is UDP (User Datagram Protocol) .
It provides unreliable data transfer.
no flow control
no congestion control
Applications using UDP: streaming media, video
conferencing, and IP telephony
33
2.4 COMPARE OF OSI AND TCP/IP
OSI: Open Systems
Interconnection. It
was developed by ISO
as a first step toward
international
standardization of the
protocol used in
various layers. It
deals with connecting
open system..
TCP/IP: Transport
Control
Protocol/Internet
Protocol. TCP is used
in connection with IP
and operates at the
transport layer. IP is
the set of convention
used to pass packets
from one host to
another.
SIMILARITY
Both are based on the concept of a stack of
independent protocols.
The functionality of the layers is roughly similar.
DIFFERENCE
OSI makes the
distinction between
services, interfaces,
and protocol.
The OSI model was
devised before the
protocols were
invented. It can be
made to work in
diverse heterogeneous
networks.
TCP/IP does not
originally clearly
distinguish between
services, interface,
and protocol.
TCP/IP model was
just a description of
the existing protocols.
The model and the
protocol fit perfectly.
DIFFERENCE
(CONTINUE)
The OSI model
supports both
connectionless and
connection-oriented
communication in the
network layer, but
only connection-
oriented
communication in the
transport layer.
The TCP/IP model has
only one mode in the
network layer
(connectionless) but
supports both modes
in the transport layer,
giving the user choice.
DIFFERENCE
(CONTINUE)
OSI has seven layers TCP/IP has four layers
Application
Presentation
Session
Transport
Network
Data link
Physical
Application
Transport
Internet
Host-to-network
DIFFERENCE
(CONTINUE)
OSI emphasis on providing a reliable data transfer service, Each layer of the OSI model detects and handles errors, all data transmitted includes checksums. The transport layer checks source-destination reliability.
TCP/IP treats reliability as an end to end Problem. The transport layer handles all error detection and recovery, it was checksums, acknowledgments, and timeouts to control transmissions and provides end-to-end verification.
DIFFERENCE
(CONTINUE)
Host on OSI
implementations do
not handle network
operations.
TCP/IP hosts
participate in most
network protocols.