Network Models

2.1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
 2-1 LAYERED TASKS

We use the concept of layers in our daily life. As an

example, let us consider two friends who communicate
through postal mail. The process of sending a letter to a
friend would be complex if there were no services
available from the post office.

Topics discussed in this section:

Sender, Receiver, and Carrier
Hierarchy

2.2
 Figure 2.1 Tasks involved in sending a letter

2.3
 2-2 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.

Topics discussed in this section:

Layered Architecture
Peer-to-Peer Processes
Encapsulation

2.4
 Note

ISO is the organization.

OSI is the model.

2.5
 Figure 2.2 Seven layers of the OSI model

2.6
 Figure 2.3 The interaction between layers in the OSI model

2.7
 Figure 2.4 An exchange using the OSI model

2.8
 2-3 LAYERS IN THE OSI MODEL

In this section we briefly describe the functions of each

layer in the OSI model.

Topics discussed in this section:

Physical Layer
Data Link Layer
Network Layer
Transport Layer
Session Layer
Presentation Layer
Application Layer

2.9
 Figure 2.5 Physical layer

2.10
 Note

The physical layer is responsible for movements of

individual bits from one hop (node) to the next.

2.11
 The physical layer is also concerned with the following:

Physical characteristics of interfaces and medium: The physical layer

defines the characteristics of the interface between the devices and the
transmission medium. It also defines the type of transmission medium.
Representation of bits:
The physical layer data consists of a stream of bits (sequence of Os or 1s)
with no interpretation. To be transmitted, bits must be encoded into
signals--electrical or optical. The physical layer defines the type of
encoding (how Os and Is are changed to signals).
Data rate:
The transmission rate-the number of bits sent each second-is also defined
by the physical layer. In other words, the physical layer defines the duration
of a bit, which is how long it lasts.
Synchronization of bits: The sender and receiver not only must use the
same bit rate but also must be synchronized at the bit level. In other words,
the sender and the receiver clocks must be synchronized.

2.12
 Line configuration:
The physical layer is concerned with the connection of devices to the media. In a
point-to-point configuration, two devices are connected through a dedicated link.
In a multipoint configuration, a link is shared among several devices.

Physical topology:
The physical topology defines how devices are connected to make a network.
Devices can be connected by using a mesh topology (every device is connected
to every other device), a star topology (devices are connected through a central
device), a ring topology (each device is connected to the next, forming a ring), a
bus topology (every device is on a common link), or a hybrid topology (this is a
combination of two or more topologies).

Transmission mode: The physical layer also defines the direction of

transmission between two devices: simplex, half-duplex, or full-duplex. In
simplex mode, only one device can send; the other can only receive. The simplex
mode is a one-way communication. In the half-duplex mode, two devices can
send and receive, but not at the same time. In a full-duplex (or simply duplex)
mode, two devices can send and receive at the same time.
2.13
 Figure 2.6 Data link layer

2.14
 Note

The data link layer is responsible for moving

frames from one hop (node) to the next.

2.15
 Figure 2.7 Hop-to-hop delivery

2.16
 Other responsibilities of the data link layer include the following:
Framing. The data link layer divides the stream of bits received from the network layer into
manageable data units called frames.

Physical addressing.
If frames are to be distributed to different systems on the network, the data link layer adds a
header to the frame to define the sender and/or receiver of the frame. If the frame is intended
for a system outside the sender's network, the receiver address is the address of the device
that connects the network to the next one.

Flow control. If the rate at which the data are absorbed by the receiver is less than the rate
at which data are produced in the sender, the data link layer imposes a flow control
mechanism to avoid overwhelming the receiver.

Error control. The data link layer adds reliability to the physical layer by adding
mechanisms to detect and retransmit damaged or lost frames. It also uses a mechanism to
recognize duplicate frames. Error control is normally achieved through a trailer added to the
end of the frame.

Access control. When two or more devices are connected to the same link, data link layer
protocols are necessary to determine which device has control over the link at any given
time.

2.17
 Figure 2.8 Network layer

2.18
 Note

The network layer is responsible for the

delivery of individual packets from
the source host to the destination host.

2.19
 Figure 2.9 Source-to-destination delivery

2.20
 Other responsibilities of the network layer include the following:
Logical addressing:

The physical addressing implemented by the data link layer handles the
addressing problem locally. If a packet passes the network boundary, we
need another addressing system to help distinguish the source and destination
systems. The network layer adds a header to the packet coming from the
upper layer that, among other things, includes the logical addresses of the
sender and receiver.

Routing:

When independent networks or links are connected to create internet works

(network of networks) or a large network, the connecting devices (called
routers or switches) route or switch the packets to their final destination. One
of the functions of the network layer is to provide this mechanism.

2.21
 Figure 2.10 Transport layer

2.22
 Note
The transport layer is responsible for the delivery
of a message from one process to another.

 The transport layer is responsible for process-to-process delivery of the entire

message.
 A process is an application program running on a host.
 Whereas the network layer oversees source-to-destination delivery of individual
packets, it does not recognize any relationship between those packets.
 It treats each one independently, as though each piece belonged to a separate message,
whether or not it does.
 The transport layer, on the other hand, ensures that the whole message arrives intact
and in order, overseeing both error control and flow control at the source-to-
destination level.

2.23
 Figure 2.11 Reliable process-to-process delivery of a message

2.24
 Other responsibilities of the transport layer include the following:
Service-point addressing:
Computers often run several programs at the same time. For this reason,
source-to-destination delivery means delivery not only from one
computer to the next but also from a specific process (running program)
on one computer to a specific process (running program) on the other.
The transport layer header must therefore include a type of address
called a service-point address (or port address). The network layer gets
each packet to the correct computer; the transport layer gets the entire
message to the correct process on that computer.
Segmentation and reassembly:
A message is divided into transmittable segments, with each segment
containing a sequence number. These numbers enable the transport layer
to reassemble the message correctly upon arriving at the destination and
to identify and replace packets that were lost in transmission.

2.25
 Connection control:
The transport layer can be either connectionless or connection oriented. A
connectionless transport layer treats each segment as an independent packet
and delivers it to the transport layer at the destination machine. A connection
oriented transport layer makes a connection with the transport layer at the
destination machine first before delivering the packets. After all the data are
transferred, the connection is terminated.

Flow control:
Like the data link layer, the transport layer is responsible for flow control.
However, flow control at this layer is performed end to end rather than across
a single link.
Error control:
Like the data link layer, the transport layer is responsible for error control.
However, error control at this layer is performed process-to process rather
than across a single link. The sending transport layer makes sure that the
entire message arrives at the receiving transport layer without error (damage,
loss, or duplication). Error correction is usually achieved through
retransmission.
2.26
Figure 2.12 Session layer

2.27
 Note

The session layer is responsible for dialog

control and synchronization.

2.28
 Specific responsibilities of the session layer include the following:

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. For example, if a system is
sending a file of 2000 pages, it is advisable to insert checkpoints after
every 100 pages to ensure that each 100-page unit is received and
acknowledged independently. In this case, if a crash happens during
the transmission of page 523, the only pages that need to be resent
after system recovery are pages 501 to 523. Pages previous to 501
need not be resent.
2.29
 Figure 2.13 Presentation layer

2.30
 Note

The presentation layer is responsible for translation,

compression, and encryption.

2.31
 Specific responsibilities of the presentation layer include the following:

Translation:
The processes (running programs) in two systems are usually exchanging
information in the form of character strings, numbers, and so on. The information must be
changed to bit streams before being transmitted. Because different computers use different
encoding systems, the presentation layer is responsible for interoperability between these
different encoding methods. The presentation layer at the sender changes the information
from its sender-dependent format into a common format. The presentation layer at the
receiving machine changes the common format into its receiver-dependent format.
Encryption:
To carry sensitive information, a system must be able to ensure privacy. Encryption means
that the sender transforms the original information to another form and sends the resulting
message out over the network. Decryption reverses the original process to transform the
message back to its original form.
Compression:
Data compression reduces the number of bits contained in the information. Data
compression becomes particularly important in the transmission of multimedia such as text,
audio, and video.

2.32
 Figure 2.14 Application layer

2.33
 Note

The application layer is responsible for

providing services to the user.

2.34
 Specific services provided by the application layer include the following:

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. The user's computer talks to the software terminal which, in
turn, talks to the host, and vice versa. The remote host believes it is communicating with
one of its own terminals and allows the user to log on.

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.

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.

2.35
 Figure 2.15 Summary of layers

2.36
 2-4 TCP/IP PROTOCOL SUITE

The layers in the TCP/IP protocol suite do not exactly

match those in the OSI model. The original TCP/IP
protocol suite was defined as having four layers: host-to-
network, internet, transport, and application. However,
when TCP/IP is compared to OSI, we can say that the
TCP/IP protocol suite is made of five layers: physical,
data link, network, transport, and application.

Topics discussed in this section:

Physical and Data Link Layers
Network Layer
Transport Layer
Application Layer
2.37
2.38
 Figure 2.16 TCP/IP and OSI model

2.39
2.40
 2-5 ADDRESSING

Four levels of addresses are used in an internet employing

the TCP/IP protocols: physical, logical, port, and specific.

Topics discussed in this section:

Physical Addresses
Logical Addresses
Port Addresses
Specific Addresses

2.41
 Figure 2.17 Addresses in TCP/IP

2.42
 Figure 2.18 Relationship of layers and addresses in TCP/IP

2.43
 Example 2.1

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.

2.44
 Figure 2.19 Physical addresses

2.45
 Example 2.2

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:

07:01:02:01:2C:4B

A 6-byte (12 hexadecimal digits) physical address.

2.46
 Example 2.3

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.

2.47
 Figure 2.20 IP addresses

2.48
 Example 2.4

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.

2.49
 Figure 2.21 Port addresses

2.50
 Note

The physical addresses will change from hop to hop,

but the logical addresses usually remain the same.

2.51
 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.

2.52