Chapter-2
Network Models
2.1
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
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
Seven layers of the OSI model
2.6
Figure 2.3 The interaction between layers in the OSI model
2.7
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
Figure 2.6 Data link layer
2.12
Note
The data link layer is responsible for moving
frames from one hop (node) to the next.
2.13
Hop-to-hop delivery
2.14
Figure 2.8 Network layer
2.15
Note
The network layer is responsible for the
delivery of individual packets from
the source host to the destination host.
2.16
Figure 2.9 Source-to-destination delivery
2.17
Figure 2.10 Transport layer
2.18
Note
The transport layer is responsible for the delivery
of a message from one process to another.
2.19
Figure 2.11 Reliable process-to-process delivery of a message
2.20
Figure 2.12 Session layer
2.21
Note
The session layer is responsible for dialog
control and synchronization.
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.
2.22
Figure 2.13 Presentation layer
2.23
Note
The presentation layer is responsible for translation,
compression, and encryption.
2.24
Figure 2.14 Application layer
2.25
Note
The application layer is responsible for
providing services to the user.
2.26
Figure 2.15 Summary of layers
2.27
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-tonetwork, 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
2.28
Figure 2.16 TCP/IP and OSI model
2.29
At the transport layer, TCP/IP defines three protocols: Transmission
Control Protocol (TCP), User Datagram Protocol (UDP), and Stream Control
Transmission Protocol (SCTP).
At the physical and data link layers, TCPIIP does not define any
specific protocol. It supports all the standard and proprietary protocols.
At the network layer (or, more accurately, the internetwork layer),
TCP/IP supports the Internetworking Protocol. IP, in turn, uses four
supporting protocols: ARP, RARP, ICMP, and IGMP.
The application layer in TCP/IP is equivalent to the combined session,
presentation, and application layers in the OSI model. Many protocols are
defined at this layer.
2.30
Internetworking Protocol (IP) The Internetworking Protocol (IP) is the
transmission mechanism used by the TCP/IP protocols.
The Address Resolution Protocol (ARP) is used to associate a logical
address with a physical address.
The Reverse Address Resolution Protocol (RARP) allows a host to
discover its Internet address when it knows only its physical address.
The Internet Control Message Protocol (ICMP) is a mechanism used
by hosts and gateways to send notification of datagram problems back to
the sender. ICMP sends query and error reporting messages.
The Internet Group Message Protocol (IGMP) is used to facilitate the
simultaneous transmission of a message to a group of recipients.
2.31
The User Datagram Protocol (UDP) is the simpler of the two standard
TCP/IP transport protocols. It is a process-to-process protocol that adds
only port addresses, checksum error control, and length information to the
data from the upper layer.
The Transmission Control Protocol (TCP) provides full transport-layer
services to applications. TCP is a reliable stream transport protocol. The
term stream, in this context, means connection-oriented: A connection must
be established between both ends of a transmission before either can
transmit data.
The Stream Control Transmission Protocol (SCTP) provides support
for newer applications such as voice over the Internet. It is a transport layer
protocol that combines the best features of UDP and TCP.
2.32
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
2.33
Figure 2.17 Addresses in TCP/IP
2.34
Figure 2.18 Relationship of layers and addresses in TCP/IP
2.35
Physical Addresses
The physical address, also known as the link address, is the
address of a node as defined by its LAN or WAN. It is included in
the frame used by the data link layer. It is the lowest-level
address.
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.
2.36
Figure 2.19 Physical addresses
2.37
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.
2.38
Logical Address
 Logical addresses are necessary for universal communications that are independent of
underlying physical networks.
 Physical addresses are not adequate in an internetwork environment where different
networks can have different address formats.
 A universal addressing system is needed in which each host can be identified uniquely,
regardless of the underlying physical network.
 The logical addresses are designed for this purpose.
 A logical address in the Internet is currently a 32-bit address that can uniquely define a
host connected to the Internet.
 No two publicly addressed and visible hosts on the Internet can have the same IP
address.
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.
2.39
Figure 2.20 IP addresses
2.40
Port Address
 The IP address and the physical address are necessary for a quantity of data to
travel from a source to the destination host. However, arrival at the destination
host is not the final objective of data communications on the Internet.
 The end objective of Internet communication is a process communicating with
another process.
 Label assigned to a process is called a port address.
 A port address in TCP/IP is 16 bits in length
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.
2.41
Figure 2.21 Port addresses
2.42
Note
The physical addresses will change from hop to hop,
but the logical addresses usually remain the same.
2.43
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.
2.44
Specific addresses




2.45
Some applications have user-friendly addresses that are
designed for that specific address.
Examples include the e-mail address (for example,
forouzan@fhda.edu) and the Universal Resource Locator
(URL) (for example, www.mhhe.com).
The first defines the recipient of an e-mail; the second is
used to find a document on the World Wide Web.
These addresses, however, get changed to the
corresponding port and logical addresses by the sending
computer