Data Communications AND NetworkingData Communications AND Networking

2.1
Chapter 2
Network Models
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2.2
2-1 LAYERED TASKS
2-1 LAYERED TASKS
We use the concept of
We use the concept of layers
layers in our daily life. As an
in our daily life. As an
example, let us consider two friends who communicate
example, let us consider two friends who communicate
through postal mail. The process of sending a letter to a
through postal mail. The process of sending a letter to a
friend would be complex if there were no services
friend would be complex if there were no services
available from the post office.
available from the post office.
Sender, Receiver, and Carrier
Hierarchy
Topics discussed in this section:

2.3
Figure 2.1 Tasks involved in sending a letter

2.4
2-2 THE OSI MODEL
2-2 THE OSI MODEL
Established in 1947, the International Standards
Established in 1947, the International Standards
Organization (
Organization (ISO
ISO) is a multinational body dedicated to
) is a multinational body dedicated to
worldwide agreement on international standards. An ISO
worldwide agreement on international standards. An ISO
standard that covers all aspects of network
standard that covers all aspects of network
communications is the Open Systems Interconnection
communications is the Open Systems Interconnection
(
(OSI
OSI) model. It was first introduced in the late 1970s.
) model. It was first introduced in the late 1970s.
Layered Architecture
Peer-to-Peer Processes
Encapsulation

2.5
ISO is the organization.
OSI is the model.
Note

2.6
Figure 2.2 Seven layers of the OSI model

2.7
Figure 2.3 The interaction between layers in the OSI model

2.8
Figure 2.4 An exchange using the OSI model

2.9
2-3 LAYERS IN THE OSI MODEL
2-3 LAYERS IN THE OSI MODEL
In this section we briefly describe the functions of each
In this section we briefly describe the functions of each
layer in the OSI model.
layer in the OSI model.
Physical Layer
Data Link Layer
Network Layer
Transport Layer
Session Layer
Presentation Layer
Application Layer

2.10
Figure 2.5 Physical layer

2.11
The physical layer is responsible for movements of
individual bits from one hop (node) to the next.
Note

2.12
Figure 2.6 Data link layer

2.13
The data link layer is responsible for moving
frames from one hop (node) to the next.
Note

2.14
Figure 2.7 Hop-to-hop delivery

2.16
The network layer is responsible for the
delivery of individual packets from
the source host to the destination host.
Note

2.17
Figure 2.9 Source-to-destination delivery

2.18
Figure 2.10 Transport layer

2.19
The transport layer is responsible for the delivery
of a segment from one process to another.
Note

2.20
Figure 2.11 Reliable process-to-process delivery of a message

2.21
Figure 2.12 Session layer

2.22
The session layer is responsible for dialog
control and synchronization.
Note

2.23
Figure 2.13 Presentation layer

2.24
The presentation layer is responsible for translation,
compression, and encryption.
Note

2.25
Figure 2.14 Application layer

2.26
The application layer is responsible for
providing services to the user.
Note

2.27
Figure 2.15 Summary of layers

The TCP/IP Reference Model
 The TCP/IP reference model.
2

2.29
2-4 TCP/IP PROTOCOL SUITE
2-4 TCP/IP PROTOCOL SUITE
The layers in the
The layers in the TCP/IP protocol suite
TCP/IP protocol suite do not exactly
do not exactly
match those in the OSI model. The original TCP/IP
match those in the OSI model. The original TCP/IP
protocol suite was defined as having four layers:
protocol suite was defined as having four layers: host-to-
host-to-
network
network,
, internet
internet,
, transport
transport, and
, and application
application. However,
. However,
when TCP/IP is compared to OSI, we can say that the
when TCP/IP is compared to OSI, we can say that the
TCP/IP protocol suite is made of five layers:
TCP/IP protocol suite is made of five layers: physical
physical,
,
data link
data link,
, network
network,
, transport
transport, and
, and application
application.
.
Physical and Data Link Layers
Network Layer
Transport Layer
Application Layer

2.30
Figure 2.16 TCP/IP and OSI model

 SMTP – Simple Mail Transfer Protocol
 FTP – File Transfer Protocol
 HTTP – Hyper Text Transfer Protocol
 DNS – Domain Name Server
 SNMP – Simple Network Management Protocol
 SCTP - Stream Control Transmission Protocol
 TCP – Transmission Control Protocol
 UDP – User Datagram Protocol
 ARP - Address Resolution Protocol
 RARP - Reverse Address Resolution Protocol
 ICMP - Internet Control Message Protocol
 IGMP - Internet Group Management Protocol
 IP – Internet Protocol
2.31
Full Forms: TCP/IP and OSI model

Multiplexing and
Demultiplexing
2.32

2.33
2-5 ADDRESSING
2-5 ADDRESSING
Four levels of addresses are used in an internet employing
Four levels of addresses are used in an internet employing
the TCP/IP protocols:
the TCP/IP protocols: physical
physical,
, logical
logical,
, port
port, and
, and specific
specific.
.
Physical Addresses
Logical Addresses
Port Addresses
Specific Addresses

2.34
Figure 2.17 Addresses in TCP/IP

2.35
Figure 2.18 Relationship of layers and addresses in TCP/IP

TCP/IP v/s OSI
 4 Layers
 Did not clearly
distinguish
between service,
interface and
protocol.
 Protocols in TCP/IP
model are not
hidden and tough
to replace if
technology
changes.
 7 Layers
 Distinction between
these three concepts
are explicit.
 Protocols in the OSI
model are better
hidden than in the
TCP/IP model and can
be replaced relatively
easily as the
technology changes.
3

 The protocols came
first, and the model
was really just a
description of the
existing protocols.
 Designers have
much experience
with the subject and
have clear idea of
which functionality
to put in which
layer.
 The model was not
biased toward one
particular set of
protocols, a fact that
made it quite
general.
 Designers did not
have much
experience with the
subject and did not
have a good idea of
which functionality to
put in which layer.
3

2.38
In Figure 2.19 a node with physical address 10 sends a
frame to a node with physical address 87. The two nodes
are connected by a link (bus topology LAN). As the
figure shows, the computer with physical address 10 is
the sender, and the computer with physical address 87 is
the receiver.
Example 2.1

2.39
Figure 2.19 Physical addresses

2.40
As we will see in Chapter 13, most local-area networks
use a 48-bit (6-byte) physical address written as 12
hexadecimal digits; every byte (2 hexadecimal digits) is
separated by a colon, as shown below:
Example 2.2
07:01:02:01:2C:4B
A 6-byte (12 hexadecimal digits) physical address.

2.41
Figure 2.20 shows a part of an internet with two routers
connecting three LANs. Each device (computer or
router) has a pair of addresses (logical and physical) for
each connection. In this case, each computer is
connected to only one link and therefore has only one
pair of addresses. Each router, however, is connected to
three networks (only two are shown in the figure). So
each router has three pairs of addresses, one for each
connection.
Example 2.3

2.43
Figure 2.21 shows two computers communicating via the
Internet. The sending computer is running three
processes at this time with port addresses a, b, and c. The
receiving computer is running two processes at this time
with port addresses j and k. Process a in the sending
computer needs to communicate with process j in the
receiving computer. Note that although physical
addresses change from hop to hop, logical and port
addresses remain the same from the source to
destination.
Example 2.4

2.44
Figure 2.21 Port addresses

2.45
The physical addresses will change from hop to hop,
but the logical addresses usually remain the same.
Note

2.46
Example 2.5
As we will see in Chapter 23, a port address is a 16-bit
address represented by one decimal number as shown.
753
A 16-bit port address represented
as one single number.

2.47
The physical addresses change from hop to hop,
but the logical and port addresses usually remain the same.
Note

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