ch02 - Network Models- powerpoint-lecture

2.1
Chapter 2
Network Models
Edited by:
Dr. Essam Alnatsheh
efalnatsheh@utb.edu.bh
College of Engineering

2.2
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.
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
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.
Layered Architecture
Peer-to-Peer Processes
Encapsulation

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

2.6
OSI Network Model
 The OSI model is a layered framework for
the design of network systems that allows
communication between all types of
computer systems.
 Consists of 7 layers

2.7
Figure 2.2 Seven layers of the OSI model

2.8
Layered Architecture
 As the message travels from A to B, it may pass
through many intermediate nodes. These
intermediate nodes usually involve only the first
three layers of the OSI model.
 Within a single machine, each layer calls upon the
services of the layer just below it. Layer 3 for
example, uses services from layer 2 and provides
services for layer 4.
 Between machines, layer x on one machine
communicates with layer x on the other machine.
This is called a peer to peer process.

2.9
Process
 At layer 1 the entire package is converted to a
form that can be transmitted to the receiving
device. At the receiving machine, the message is
unwrapped layer by layer, with each process
receiving and removing the data meant for it.
 For example, layer 2 removes the data meant for
it, then passes the rest to layer 3. Layer 3 then
removes the data meant for it and passes the rest
to layer 4, and so on.

2.10
Figure 2.3 The interaction between layers in the OSI model

2.11
Organization of the Layers
 Layers 1, 2, and 3-physical, data link, and network-are the
network support layers; they deal with the physical aspects of
moving data from one device to another (such as electrical
specifications, physical connections, physical addressing, and
transport timing and reliability).
 Layers 5, 6, and 7-session, presentation, and application-can
be thought of as the user support layers; they allow
interoperability among unrelated software systems.
 Layer 4, the transport layer, links the two subgroups and
ensures that what the lower layers have transmitted is in a
form that the upper layers can use.

2.12
Process
 At the sending device, beginning with the upper
layers, each layer adds information to the data
and sends it to the layer below it.
 At layer 1, the physical layer, the entire package
is converted to electromagnetic signals and sent
across a transmission media.
 At the receiving device, the message is sent
back up through the layers, unwrapping the
information layer by layer.

2.13
Figure 2.4 An exchange using the OSI model

2.14
Encapsulation
 The data portion of a packet at level N - 1 carries the whole
packet (data and header and maybe trailer) from level N.
 A packet (header and data) at level 7 is encapsulated in a
packet at level 6. The whole packet at level 6 is encapsulated
in a packet at level 5, and so on.
 The concept is called encapsulation; level N - 1 is not aware
of which part of the encapsulated packet is data and which
part is the header or trailer.
 Encapsulation is the process of wrapping the data while the
decapsulation process is a process of opening packs.

2.15
2-3 LAYERS IN THE OSI MODEL
In this section we briefly describe the functions of each
layer in the OSI model.
Physical Layer
Data Link Layer
Network Layer
Transport Layer
Session Layer
Presentation Layer
Application Layer

2.16
Physical Layer
 The physical layer coordinates the functions
require to carry a bit stream over a physical
medium.
 Other responsibilities of the physical layer include
the following:
 Physical characteristics of interfaces and medium.
 Example: Cat 5, Coax, Fiber etc.
 Representation of bits – defines the type of encoding.
 (how the 0s and 1s are changed into signals).
 Data Rate – The transmission rate in bits per second
(duration of a bit).

2.17
Physical Layer
 Synchronization of bits – sender and receiver clocks must
be synchronized.
 Line configuration – point to point or multipoint.
 Physical Topology – how are the devices connected to
make a network.
 Example: Mesh, Star, ring, bus.
 Transmission Mode – direction of transmission
 Simplex, half-duplex or full duplex.

2.18
Figure 2.5 Physical layer

2.19
The physical layer is responsible for movements of
individual bits from one hop (node) to the next.
Note

2.20
Data Link Layer
 The Data link layer is responsible for transmitting
frames from one node to the next.
 Other responsibilities of the data link layer include
the following:
 Framing – divides stream of bits received from the
network layer into data units called frames.
 Physical Addressing – Data link adds header to the
frame to define the Physical address of the source
and/or the destination of the frame.
 Flow Control – prevents overwhelming the receiver with
too much information.

2.21
Data Link Layer
 Error Control – Mechanism to detect and
retransmit lost or damaged frames. This is
achieved through a trailer added to the end of
the frame.
 Access Control – If 2 or more devices are
connected to the same link, data link protocols
determine which device has control over the link
at any given time.

2.22
Figure 2.6 Data link layer

2.23
The data link layer is responsible for moving
frames from one hop (node) to the next.
Note

2.24
Figure 2.7 Hop-to-hop delivery

2.25
Network Layer
 The Network Layer is responsible for source–to-
destination delivery of a packet possibly across
multiple networks (links).
 Whereas the data link layer oversees the delivery of
the packet between two systems on the same
network (links), the network layer ensures that each
packet gets from its point of origin to its final
destination.
 Other responsibilities of the network layer include
the following:
 Logical Addressing
 Routing

2.26
Network Layer
 Logical Addressing - 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 intemetworks (network of networks)
or a large network, the connecting devices (called routers
or switches) route or switch the packets to their final
destination.

2.28
The network layer is responsible for the
delivery of individual packets from
the source host to the destination host.
Note

2.29
Figure 2.9 Source-to-destination delivery

2.30
Transport Layer
 The Transport Layer is responsible for process to
process delivery of the entire message.
 A process is an application running on a host.
 Ensures that the whole message arrives intact and in
order, overseeing both error control and flow control
at the source-to-destination level.
 Other responsibilities of the transport layer include
the following:
 Service-Point Addressing: The network layer gets each
packet to the correct computer, the transport layer gets
the entire message to the correct process on that
computer.

2.31
Transport Layer
 Segmentation and reassembly- The message is divided
into transmittable segments, each segment containing a
sequence number. The Transport Layer is responsible for
the “reassembly” of the message when it arrives at the
destination.
 Connection Control – The transport layer can be either
connectionless or connection-oriented.
 Flow Control – similar to the flow control at the data link
layer. Flow control at Transport layer is performed end to
end rather than a single link.
 Error control-similar to error control at the data link layer.
Error control is end to end rather than the single link.
Error correction is usually achieved through
retransmission.

2.32
Figure 2.10 Transport layer

2.33
The transport layer is responsible for the delivery
of a message from one process to another.
Note

2.34
Figure 2.11 Reliable process-to-process delivery of a message

2.35
Session Layer
 The Session Layer is the network dialog controller.
 It establishes, maintains, and synchronizes the
interaction among communicating systems.
 Specific responsibilities of the session layer include
the following:
 Dialog Control- Allows communication between two
processes to take place either in half duplex or full duplex.
 Synchronization- Allows a process to add checkpoints into
a stream of data (or synchronization points).
 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.

2.36
Figure 2.12 Session layer

2.37
The session layer is responsible for dialog
control and synchronization.
Note

2.38
Presentation Layer
 The Presentation Layer is concerned with the syntax
and semantics of the information exchanged
between two systems.
 Specific responsibilities of the presentation layer
include the following:
 Translation – Because different computers use different
encoding systems, the presentation layer is responsible for
interoperability between these different encoding methods.
 The presentation layer of the sending machine changes
information into a common format that can be translated at
the presentation layer of the receiving machine into receiver
dependent code.

2.39
Presentation Layer
 Encryption- Sending presentation layer transforms the
original data into another form and sends the message out
over the network. Decryption reverses the original process
to transform the message back to its original form. This
ensures privacy of sensitive information, while it travels
across the network.
 Compression- Data compression reduces the number of
bits to be transmitted.

2.40
Figure 2.13 Presentation layer

2.41
The presentation layer is responsible for translation,
compression, and encryption.
Note

2.42
Application Layer
 The application layer enables the user to access the
network.
 It provides user interfaces and support for services such
as electronic mail, remote file access and transfer, etc.
 Specific services provided by the application layer include
the following:
 Network Virtual terminal – allows for login to remote host.
 File Transfer, Access, and Management (FTAM) – Allows the
user to access files in a remote computer, to retrieve files
from a remote computer and to manage or control files
from a remote computer.
 Mail services – provides a basis for email forwarding and
storage.

2.43
Figure 2.14 Application layer

2.44
The application layer is responsible for
providing services to the user.
Note

2.45
Figure 2.15 Summary of layers

2.46
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.
Physical and Data Link Layers
Network Layer
Transport Layer
Application Layer

2.47
Figure 2.16 TCP/IP and OSI model

2.48
TCP/IP Model 5 layers
 Physical and Data Link Layers: TCP/IP
does not define any specific protocol. It
supports all the standard and proprietary
protocols.
 Network Layer: TCP/IP supports the
Internetworking Protocol. IP, in turn, uses
four supporting protocols: ARP, RARP,
ICMP, and IGMP.

2.49
 Application Layer – 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.

2.50
 Transport Layer: IP is a host-to-host protocol,
meaning that it can deliver a packet from one
physical device to another. UDP and TCP are
transport level protocols responsible for delivery
of a message from a process (running program)
to another process.
 The User Datagram Protocol (UDP) is the simpler of
the two standard TCP/IP transport protocols.
 The Transmission Control Protocol (TCP) provides full
transport-layer services to applications.
 The Stream Control Transmission Protocol (SCTP)
provides support for newer applications such as voice
over the Internet.

2.51
2-5 ADDRESSING
Four levels of addresses are used in an internet employing
the TCP/IP protocols: physical, logical, port, and specific.
Physical Addresses
Logical Addresses
Port Addresses
Specific Addresses

2.52
Figure 2.17 Addresses in TCP/IP

2.54
Physical Address
 Known as link address
 Lowest level of address
 Usually the MAC address
 6-byte (48-bit)
 Imprinted by the manufacturer of the NIC

2.55
Figure 2.19 Physical addresses

2.56
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

2.57
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.

2.58
Logical Addresses
 Necessary for universal communication
that are independent of physical network.
 IP address – currently used on the
Internet. It is unique and universal.
 IPv4 uses 32 bit address
 192.62.48.12
 Used by the routers in internetworking
 No two hosts will have the same address.

2.59
The physical addresses will change from hop to hop,
but the logical addresses usually remain the same.
Note

2.61
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

2.62
Port Address
 The IP address and the physical address
are necessary for a quantity of data to
travel from a source to destination.
 Computers run multiple processes, such as
FTP,TELNET,HTTP etc
 Data needs to be delivered to the correct
process (port address).

2.63
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

2.64
Figure 2.21 Port addresses

2.65
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.
443
A 16-bit port address represented
as one single number.

2.66
Specific Address
 Some Applications have user-friendly
addresses that are designed for that
specific address.
 URLs, email address
 www.utb.edu.bh
 helpdesk@utb.edu.bh
 The port and logical address get changed
by the sending computer.

2.67
Figure 2.18 Relationship of layers and addresses in TCP/IP

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