Data Communications

Chapter 2
Network Models

Dr. Dalia Nashat

Information Technology Department
Faculty of Computers and Information
Outline
2.1 PROTOCOL LAYERING
2.2 TCP/IP PROTOCOL SUITE
2.3 THE OSI MODEL
PROTOCOL LAYERING
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.
❖ Scenarios
First Scenario
In the first scenario, communication is so simple that it
can occur in only one layer. Assume Maria and Ann are
neighbors with a lot of common ideas. Communication
between Maria and Ann takes place in one layer, face to
face, in the same language.
 Second Scenario
▪ Assume that Ann is offered a higher-level position in her company but needs to move to
another branch located in a city very far from Maria.
▪ The two friends still want to continue their communication and exchange ideas because
they have come up with an innovative project to start a new business when they both retire.
They decide to continue their conversation using regular mail through the post office.
However, they do not want their ideas to be revealed by other people if the letters are
intercepted.
▪ They agree on an encryption/decryption technique. The sender of he letter encrypts it to
make it unreadable by an intruder; the receiver of the letter decrypts it to get the original
letter. Now we can say that the communication between Maria and Ann takes place in three
layers.
Principles of Protocol Layering
❖ First Principle
The first principle dictates that 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. For example,
the third layer task is to listen (in one direction) and talk (in the
other direction). The second layer needs to be able to encrypt and
decrypt. The first layer needs to send and receive mail.
❖ Second Principle
The second principle that we need to follow in protocol layering is
that the two objects under each layer at both sites should be
identical. For example, the object under layer 3 at both sites
should be a plaintext letter. The object under layer 2 at both sites
should be a ciphertext letter. The object under layer 1 at both sites
should be a piece of mail.
 Logical Connections
▪ We have layer-to-layer communication.
▪ Maria and Ann can think that there is a logical (imaginary)
connection at each layer through which they can send the object
created from that layer.
TCP/IP PROTOCOL SUITE
▪ TCP/IP is 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.
Communication through an internet
Logical connections between layers of the TCP/IP
▪ The duty of the application, transport, and network layers is end-to-end.
However, the duty of the data-link and physical layers is hop-to-hop, in
which a hop is a host or router.
▪ In the top three layers, the data unit (packets) should not be changed by
any router or link-layer switch. In the bottom two layers, the packet
created by the host is changed only by the routers, not by the link-layer
switches.
 Identical objects in the TCP/IP protocol suite
▪ In the top three layers, the data unit (packets) should not be changed by any
router or link-layer switch.
▪ In the bottom two layers, the packet created by the host is changed only by the
routers, not by the link-layer switches.
▪ Although the logical connection at the network layer is between the two hosts,
we can only say that identical objects exist between two hops in this case
because a router may fragment the packet at the network layer and send more
packets than received.
Description of Each Layer
Physical Layer
▪ Carrying individual bits in a frame across the link.
▪ The communication between two devices at the physical layer is
still a logical communication because there is another, hidden
layer, the transmission media, under the physical layer.
Data-link Layer
▪ The routers are responsible for choosing the best links.
▪ Taking the datagram and moving it across the link.
▪ TCP/IP does not define any specific protocol for the data-link layer.
It supports all the standard and proprietary protocols.
▪ The data-link layer takes a datagram and encapsulates it in a
packet called a frame.
Network Layer
▪ Creating a connection between the source computer and the
destination computer.
▪ The routers in the path are responsible for choosing the best route
for each packet.
▪ Includes the main protocol, Internet Protocol (IP), that defines the
format of the packet, called a datagram at the network layer.
Transport Layer
▪ Gets the message from the application layer, encapsulates it
in a transport layer packet and sends it, through the logical
connection, to the transport layer at the destination host.
▪ Giving services to the application layer: to get a message
from an application program running on the source host and
deliver it to the corresponding application program on the
destination host.
▪ For examples : UDP, TCP, and SCTP.
Application Layer
▪ Communication at the application layer is between two processes (two
programs running at this layer).
▪ The Hypertext Transfer Protocol (HTTP) is a vehicle for accessing the
World Wide Web (WWW).
▪ The Simple Mail Transfer Protocol (SMTP) is the main protocol used in
electronic mail (e-mail) service.
▪ The File Transfer Protocol (FTP) is used for transferring files from one
host to another.
▪ The Domain Name System (DNS) is used by other protocols to find
▪ the network-layer address of a computer.
Encapsulation and Decapsulation
Encapsulation at the Source Host
At the source, we have only encapsulation.
Decapsulation and Encapsulation at the Router
At the router, we have both decapsulation and encapsulation because the
router is connected to two or more links.
Decapsulation at the Destination
At the destination host, each layer only decapsulates the packet received,
removes the payload, and delivers the payload to the next-higher layer
protocol until the message reaches the application layer. It is necessary to
say that decapsulation in the host involves error checking.
 Addressing
▪ Any communication that involves two parties needs two addresses:
source address and destination address.
▪ If we need five pairs of addresses, one pair per layer, we normally have only four
because the physical layer does not need addresses; the unit of data exchange at the
physical layer is a bit, which cannot have an address.
Multiplexing and Demultiplexing
Multiplexing
A protocol at a layer can encapsulate a packet from several next-higher layer
protocols (one at a time)
demultiplexing
A protocol can decapsulate and deliver a packet to several next-higher layer
protocols (one at a time)
THE OSI MODEL
▪ 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.
▪ ISO is the organization; OSI is the model.
Seven layers of the OSI model
OSI versus TCP/IP
▪ 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.
▪ Two reasons were mentioned for this
decision.
✔ First, TCP/IP has more than one
transport-layer protocol. Some of the
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.
Lack of OSI Model’s Success
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.
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, nor
were they fully 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.