1
PART I: OVERVIEW
NETWORK MODELS
Subject: Computer Networks
Ch-2
Tutor: Bilal Munir Mughal
Content Outline
2
PROTOCOL LAYERING
Scenarios
Principles of Protocol Layering
Logical Connections
TCP/IP PROTOCOL SUITE
Layered Architecture
Layers in the TCP/IP Protocol Suite
Description of Each Layer
Encapsulation and Decapsulation
Addressing
Multiplexing and Demultiplexing
Content Outline
3
THE OSI MODEL
OSI versus TCP/IP
Lack of OSI Model’s Success
PROTOCOL LAYERING - Scenarios
4
In data communications and networking, a protocol
defines the rules that both the sender and receiver
and all intermediate devices need to follow to be
able to communicate effectively.
When communication is simple, we may need only one
simple protocol; when the communication is
complex, we may need to divide the task between
different layers, in which case we need a protocol at
each layer, or protocol layering.
PROTOCOL LAYERING - Scenarios
5
First Scenario
PROTOCOL LAYERING - Scenarios
6
Second Scenario
PROTOCOL LAYERING - Principles
7
Two principles of protocol layering
First Principle
If we want bidirectional communication, we need to make
each layer so that it is able to perform two opposite tasks,
one in each direction. e.g. talk and listen
Second Principle
The two objects under each layer at both sites should be
identical.
For example (Fig. 2.2), the object under layer 3 at both sites
should be a plaintext letter. The object under layer 2 at
both sites should be a cipher text letter. The object under
layer 1 at both sites should be a piece of mail.
PROTOCOL LAYERING - Logical Connections
8
TCP/IP PROTOCOL SUITE
9
TCP/IP is a protocol suite (a set of protocols
organized in different layers) used in the Internet
today.
It is a hierarchical protocol made up of interactive
modules, each of which provides a specific functionality.
The term hierarchical means that each upper level
protocol is supported by the services provided by one or
more lower level protocols.
The original TCP/IP protocol suite was defined as four
software layers built upon the hardware.
Today, however, TCP/IP is thought of as a five-layer
model.
TCP/IP PROTOCOL SUITE - Layered Architecture
10
TCP/IP PROTOCOL SUITE - Layered Architecture
11
TCP/IP PROTOCOL SUITE - Logical connections
12
TCP/IP PROTOCOL SUITE - Identical objects
13
TCP/IP PROTOCOL SUITE - Physical Layer
14
It is responsible for carrying individual bits in a frame
across the link.
The communication at physical layer is still a logical
communication because there is another, hidden
layer, the transmission media, under the this layer
Logical unit between two physical layers in two
devices is a bit, physical media carries signals e.g.
electrical, optical etc.
There are several protocols that transform a bit to a
signal.
TCP/IP PROTOCOL SUITE - Data-link Layer
15
The data-link layer is responsible for moving the
packet through the link. The link can be a wired/
wireless LAN, or a wired/wireless WAN.
TCP/IP does not define any specific protocol for the
data-link layer. It supports all the standard and
proprietary protocols.
Each link-layer protocol may provide a different
service e.g. complete error detection and correction,
or only error correction.
The data-link layer takes a datagram and
encapsulates it in a packet called a frame.
TCP/IP PROTOCOL SUITE – Network Layer
16
The network layer is responsible for host-to-host
communication and routing the packet through
possible routes.
The network layer in the Internet includes the main
protocol, Internet Protocol (IP), that defines the format
of the packet, called a datagram at the network
layer.
IP also defines the format and the structure of
addresses used in this layer.
TCP/IP PROTOCOL SUITE – Network Layer
17
IP is a connectionless protocol that provides no flow
control, no error control, and no congestion control
services.
The network layer also includes unicast (one-to-one)
and multicast (one-to-many) routing protocols.
A routing protocol does not take part in routing (it is
the responsibility of IP), but it creates forwarding
tables for routers to help them in the routing process.
TCP/IP PROTOCOL SUITE – Network Layer
18
The network layer also has some auxiliary protocols
that help IP in its delivery and routing tasks.
Internet Control Message Protocol (ICMP) helps IP to
report some problems when routing a packet
Internet Group Management Protocol (IGMP) helps IP in
multicasting
Dynamic Host Configuration Protocol (DHCP) helps IP to
get the network-layer address for a host
Address Resolution Protocol (ARP) is a protocol that helps
IP to find the link-layer address of a host or a router
when its network-layer address is given
TCP/IP PROTOCOL SUITE - Transport Layer
19
The logical connection at the transport layer is also
end-to-end
It encapsulates data in a transport layer packet
(called a segment or a user datagram in different
protocols)
The main protocols are, Transmission Control Protocol
(TCP) and User Datagram Protocol (UDP)
TCP/IP PROTOCOL SUITE - Transport Layer
20
TCP provides flow control, error control, congestion
control
UDP is connectionless (each datagram is independent
and unrelated to the previous or the next one)
A new protocol, Stream Control Transmission Protocol
(SCTP) is designed to respond to new applications
that are emerging in the multimedia.
TCP/IP PROTOCOL SUITE - Application Layer
21
The two application layers exchange messages
between each other
Process-to-process communication is the duty of the
application layer.
The application layer in the Internet includes many
predefined protocols, but a user can also create a
pair of processes to be run at the two hosts.
TCP/IP PROTOCOL SUITE - Application Layer
22
Some of the well-known protocols are:
Hypertext Transfer Protocol (HTTP) is used for accessing the
World Wide Web (WWW)
Simple Mail Transfer Protocol (SMTP) is used in e-mail
File Transfer Protocol (FTP) is used for transferring files
Terminal Network (TELNET) and Secure Shell (SSH) are used
for accessing a site remotely
Simple Network Management Protocol (SNMP) is used to
manage the Internet at global and local levels
Domain Name System (DNS) is used by other protocols to find
the network-layer address of a computer
TCP/IP PROTOCOL SUITE - Encapsulation and
Decapsulation
23
TCP/IP PROTOCOL SUITE - Addressing
24
TCP/IP PROTOCOL SUITE - Multiplexing and
Demultiplexing
25
THE OSI MODEL
26
Established in 1947, the International Standards
Organization (ISO) is a multinational body
dedicated to worldwide agreement on international
standards.
An ISO standard that covers all aspects of network
communications is the Open Systems Interconnection
(OSI) model. It was first introduced in the late 1970s.
An open system is a set of protocols that allows any
two different systems to communicate regardless of
their underlying architecture.
THE OSI MODEL
27
The purpose of the OSI model is to show how to
facilitate communication between different systems
without requiring changes to the logic of the
underlying hardware and software.
The OSI model is not a protocol; it is a model for
understanding and designing a network architecture
that is flexible, robust, and interoperable.
The OSI model was intended to be the basis for the
creation of the protocols in the OSI stack.
THE OSI MODEL
28
The OSI model is a layered framework for the design
of network systems that allows communication
between all types of computer systems.
Away
All
People Pizza
Seem Sausage
To Throw
Need Not
Data Do
Processing Please
THE OSI MODEL - connections
29
THE OSI MODEL - Encapsulation and
Decapsulation
30
THE OSI MODEL versus TCP/IP
31
THE OSI MODEL versus TCP/IP
32
When we compare the two models, we find that two
layers, session and presentation, are missing from
the TCP/IP protocol suite.
These two layers were not added to the TCP/IP
protocol suite after the publication of the OSI model.
The application layer in the suite is usually considered
to be the combination of three layers in the OSI
model
THE OSI MODEL versus TCP/IP
33
Two reasons were mentioned for not including two
layers, session and presentation.
First, TCP/IP has more than one transport-layer protocol.
Some functionalities of the session layer are available in
some of the transport-layer protocols.
Second, the application layer is not only one piece of
software. Many applications can be developed at this
layer. If some of the functionalities mentioned in the
session and presentation layers are needed for a
particular application, they can be included in the
development of that piece of software
Lack of OSI Model’s Success
34
Most experts were at first excited and thought that
the TCP/IP protocol would be fully replaced by the
OSI model. Not quite!
There were several reasons that TCP/IP protocol suite
was not fully replaced with OSI model. Three of the
agreed upon reasons are mentioned here.
First, OSI was completed when TCP/IP was fully in place
and a lot of time and money had been spent on the suite;
changing it would cost a lot.
Lack of OSI Model’s Success
35
Second, some layers in the OSI model were never fully
defined. For example, although the services provided by
the presentation and the session layers were listed in the
document, actual protocols for these two layers were not
fully defined & described, and the corresponding
software was not fully developed.
Third, when OSI was implemented by an organization in
a different application, it did not show a high enough
level of performance to entice the Internet authority to
switch from the TCP/IP protocol suite to the OSI model.
Questions?
36
