OSI open system interconnection LAYERS.pdf

TCP/IP Protocol Suite 1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chapter 2
The OSI
Model and the
TCP/IP
Protocol Suite

OBJECTIVES:
 To discuss the idea of multiple layering in data communication
and networking and the interrelationship between layers.
 To discuss the OSI model and its layer architecture and to show
the interface between the layers.
 To briefly discuss the functions of each layer in the OSI model.
 To introduce the TCP/IP protocol suite and compare its layers
with the ones in the OSI model.
 To show the functionality of each layer in the TCP/IP protocol
with some examples.
 To discuss the addressing mechanism used in some layers of the
TCP/IP protocol suite for the delivery of a message from the
source to the destination.

Chapter
Outline
2.1 Protocol Layers
2.2 The OSI Model
2.3 TCP/IP Protocol Suite
2.4 Addressing

2-1 PROTOCOL LAYERS
In Chapter 1, we discussed that a protocol is required
when two entities need to communicate. When
communication is not simple, we may divide the
complex task of communication into several layers. In
this case, we may need several protocols, one for each
layer.
Let us use a scenario in communication in which the
role of protocol layering may be better understood. We
use two examples. In the first example, communication
is so simple that it can occur in only one layer.

Topics Discussed in the Section
Hierarchy
Services

Assume Maria and Ann are neighbors with a lot of common ideas.
However, Maria speaks only Spanish, and Ann speaks only English.
Since both have learned the sign language in their childhood, they
enjoy meeting in a cafe a couple of days per week and exchange
their ideas using signs. Occasionally, they also use a bilingual
dictionary. Communication is face to face and Happens in one layer
as shown in Figure 2.1.
Example 2.1

Figure 2.1 Example 2.1

Now assume that Ann has to move to another town because of her
job. Before she moves, the two meet for the last time in the same
cafe. Although both are sad, Maria surprises Ann when she opens a
packet that contains two small machines. The first machine can
scan and transform a letter in English to a secret code or vice
versa. The other machine can scan and translate a letter in
Spanish to the same secret code or vice versa. Ann takes the first
machine; Maria keeps the second one. The two friends can still
communicate using the secret code, as shown in Figure 2.2.
Example 2.2

Figure 2.2 Example 2.2

2-2 THE OSI MODEL
Established in 1947, the International Standards
Organization (ISO) is a multinational body dedicated to
worldwide agreement on international standards.
Almost three-fourths of countries in the world are
represented in the ISO. 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.

Layered Architecture
Layer-to-layer Communication
Encapsulation
Layers in the OSI Model
Summary of OSI Layers

ISO is the organization;
OSI is the model.
Note

Figure 2.3 The OSI model

Figure 2.4 OSI layers

Figure 2.5 An exchange using the OSI model

The physical layer is responsible for
moving individual bits from one
(node) to the next.
Note

Figure 2.6 Summary of OSI Layers

2-3 TCP/IP PROTOCOL SUITE
The TCP/IP protocol suite was developed prior to the
OSI model. Therefore, the layers in the TCP/IP
protocol suite do not match exactly with those in the
OSI model. 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 with the layers named similarly to the ones in
the OSI model. Figure 2.7 shows both configurations.

Comparison between OSI and TCP/IP
Layers in the TCP/IP Suite

Figure 2.7 Layers in the TCP/IP Protocol Suite

Figure 2.8 TCP/IP and OSI model

Figure 2.9 A private internet

Figure 2.10 Communication at the physical layer
A
Physical
layer
Physical
layer
R1 R3 R4 B
Source Destination
Legend
011 ... 101
0
1
1
.
.
.
1
0
1
011 ... 101 011 ... 101
Link 3 Link 5 Link 6
Link 1

The unit of communication at the
physical layer is a bit.
Note

Figure 2.11 Communication at the data link layer
A
Physical Physical
Data link
Data link
R1 R3 R4 B
Source Destination Data
D Header
H
Legend
Link 1 Link 3 Link 5 Link 6
Frame
D2 H2
F
r
a
m
e
D
2
H
2
Frame
D2 H2
Frame
D2 H2

The unit of communication at the data
link layer is a frame.
Note

Figure 2.12 Communication at the network layer
A
Physical Physical
Data link
Data link
R1 R3 R4 B
Network
Network
D Header
H
Legend
Datagram
D3 H3
Datagram
D3 H3

network layer is a datagram.
Note

Figure 2.13 Communication at transport layer
A
Physical Physical
Data link
Data link
R1 R3 R4
B
Network
Network
Transport Transport
D Header
H
Legend
Segment
D4 H4
Segment
D4 H4

transport layer is a segment, user
datagram, or a packet, depending on the
specific protocol used in this layer.
Note

Figure 2.14 Communication at application layer
A
Physical Physical
Data link
Data link
R1 R3 R4
B
Network
Network
Transport Transport
Application
Application Source Destination Data
D Header
H
Legend
Message
D5 D5
D5 D5
Message

application layer is a message.
Note

2-4 ADDRESSING
Four levels of addresses are used in an internet
employing the TCP/IP protocols: physical address,
logical address, port address, and application-specific
address. Each address is related to a one layer in the
TCP/IP architecture, as shown in Figure 2.15.

 Physical Addresses
 Logical Addresses
 Port Addresses
 Application-Specific Addresses

Figure 2.15 Addresses in the TCP/IP protocol suite

In Figure 2.16 a node with physical address 10 sends a frame to a node
with physical address 87. The two nodes are connected by a link (a LAN).
At the data link layer, this frame contains physical (link) addresses in the
header. These are the only addresses needed. The rest of the header
contains other information needed at this level. As the figure shows, the
computer with physical address 10 is the sender, and the computer with
physical address 87 is the receiver. The data link layer at the sender
receives data from an upper layer. It encapsulates the data in a frame.
The frame is propagated through the LAN. Each station with a physical
address other than 87 drops the frame because the destination address
in the frame does not match its own physical address. The intended
destination computer, however, finds a match between the destination
address in the frame and its own physical address.
Example 2.3

Figure 2.16 Example 2.3: physical addresses
Data
87 10
1 packet
accepted
Data
87 10
4

As we will see in Chapter 3, 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 2.4
07:01:02:01:2C:4B
A 6-byte (12 hexadecimal digits) physical address

Figure 2.17 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. So each router
has three pairs of addresses, one for each connection. Although it may
be obvious that each router must have a separate physical address for
each connection, it may not be obvious why it needs a logical address for
each connection. We discuss these issues in Chapters 11 and 12 when
we discuss routing. The computer with logical address A and physical
address 10 needs to send a packet to the computer with logical address
P and physical address 95. We use letters to show the logical addresses
and numbers for physical addresses, but note that both are actually
numbers, as we will see in later chapters.
Example 2.5

Figure 2.17 Example 2.5: logical addresses
Data
A P
20 10 Data
A P
20 10
Physical
addresses
changed
Data
A P
33 99
Data
A P
33 99
Physical
addresses
changed
Data
A P
95 66 Data
A P
95 66

Example 4
As we will see in Chapter 4, an Internet address (in
IPv4) is 32 bits in length, normally written as four
decimal numbers, with each number representing 1
byte. The numbers are separated by a dot. Below is an
example of such an address.
132.24.75.9
An internet address in IPv4 in decimal numbers

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

Figure 2.18 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 both computers are using
the same application, FTP, for example, the port addresses are
different because one is a client program and the other is a server
program, as we will see in Chapter 17.
Example 2.6

A Sender Receiver P
Internet
Figure 2.18 Example 2.6: port numbers
a Data
j
A P
H2
a Data
j
A P
a Data
j
Data
a Data
j
A P
H2
a Data
j
A P
a Data
j
Data

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

As we will see in future chapters, a port address is a 16-bit
address represented by one decimal number as shown.
Example 2.7
753
A 16-bit port address represented as one single number

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