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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

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2.11
Figure 2.3 The interaction between layers in the OSI model

2.12
Figure 2.4 An exchange using the OSI model

2.13
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

2.14
Figure 2.5 Physical layer

2.15
The physical layer is responsible for movements of
individual bits from one hop (node) to the next.
Note

2.16
 Physical characteristics of interfaces and medium.
 Representation of bits.
 Data Rate (number of bits sent each second).
 Synchronization of bits.
 Line configuration (P-P, multipoint)
 Physical topology
 Transmission mode (Simplex, HD,FD)
The Major duties of the Physical layer
are as follows.

2.17
Figure 2.6 Data link layer

2.18
The data link layer is responsible for moving
frames from one hop (node) to the next.
Note

2.19
 Framing (manageable data units).
 Physical addressing.
 Flow Control.
 Error Control.
 Access Control.
The Major duties of the Data Link layer
are as follows.

2.20
Figure 2.7 Hop-to-hop delivery

2.22
The network layer is responsible for the
delivery of individual packets from
the source host to the destination host.
Note

2.23
 Logical Addressing.
 Routing.
The Major duties of the Network layer
are as follows.

2.24
Figure 2.9 Source-to-destination delivery

2.25
Example 1
Example 1
In Figure 2.11 we want to send data from a node with
network address A and physical address 10, located on
one LAN, to a node with a network address P and
physical address 95, located on another LAN. Because
the two devices are located on different networks, we
cannot use physical addresses only; the physical
addresses only have local jurisdiction. What we need here
are universal addresses that can pass through the LAN
boundaries. The network (logical) addresses have this
characteristic.

2.27
Figure 2.10 Transport layer

2.28
The transport layer is responsible for the delivery
of a message from one process to another.
Note

2.29
 Port addressing/Service point addressing .
 Segmentation and reassembly.
 Connection Control.
 Flow Control.
 Error Control.
The Major duties of the Transport layer
are as follows.

2.30
Example 2
Example 2
Figure 2.14 shows an example of transport layer
communication. Data coming from the upper layers have
port addresses j and k (j is the address of the sending
process, and k is the address of the receiving process).
Since the data size is larger than the network layer can
handle, the data are split into two packets, each packet
retaining the port addresses (j and k). Then in the network
layer, network addresses (A and P) are added to each
packet.

2.32
Figure 2.11 Reliable process-to-process delivery of a message

2.33
Figure 2.12 Session layer

2.34
The session layer is responsible for dialog
control and synchronization.
Note

2.35
 Dialog Control: The session layer allows two systems to
enter into a dialog. It allows the communication between
two processes to take place in either half-duplex (one
way at a time) or full-duplex (two ways at a time) mode.
 Synchronization: The session layer allows a process to
add checkpoints, or synchronization points, to a stream
of data.
The Major duties of the Session layer
are as follows.

2.36
Figure 2.13 Presentation layer

2.37
The presentation layer is responsible for translation,
compression, and encryption.
Note

2.38
 Translation
 Encryption
 Compression
The Major duties of the Presentation
layer are as follows.

2.39
Figure 2.14 Application layer

2.40
The application layer is responsible for
providing services to the user.
Note

2.41
It provides user interfaces and support for services such as
electronic mail, remote file access and transfer, shared
database management, and other types of distributed
information services.
 Network Virtual Terminal: A network virtual terminal is
a software version of a physical terminal, and it allows a
user to log on to a remote host. To do so, the application
creates a software emulation of a terminal at the remote
host.
 File Transfer, Access, and Management: This
application allows a user to access files in a remote host
(to make changes or read data), to retrieve files from a
remote computer for use in the local computer, and to
manage or control files in a remote computer locally.
The Major duties of the Application

2.42
 Mail Services: This application provides the basis for e-
mail forwarding and storage.
 Directory Services: This application provides distributed
database sources and access for global information
about various objects and services.
The Major duties of the Application

2.43
Figure 2.15 Summary of layers

2.44
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

2.45
Figure 2.16 TCP/IP and OSI model

2.46
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

2.47
Figure 2.17 Addresses in TCP/IP

2.48
Figure 2.18 Relationship of layers and addresses in TCP/IP

2.49
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 2.3

2.50
Figure 2.19 Physical addresses

2.51
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 2.4
07:01:02:01:2C:4B
A 6-byte (12 hexadecimal digits) physical address.

2.52
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 2.5

2.54
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 2.6

2.55
Figure 2.21 Port addresses

2.56
Example 2.5
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.

2.57
The physical addresses change from hop to hop,
but the logical and port addresses usually remain the same.
Note

2.58
Some applications have user-friendly addresses that are
designed for that specific address. Examples include the e-
mail address (for example, kashif@pafkiet.edu.pk) and the
Universal Resource Locator (URL) (for example, www.
Taleem.greatnow.com). The first defines the recipient of an
e-mail, the second is used to find a document on the World
Wide Web.
Note

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