Transcript Comp 445

Communications, Standards & Protocols
© Prof. Aiman Hanna
Department of Computer Science
Concordia University
Montreal, Canada
H istory of Communications
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For 1000s of years through auditory &
visual senses
• Visual senses:
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Face to face
Reading messages/letters/symbols/sketches
Seeing visual signals (smoke, fire, ..etc).
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• Auditory senses:
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Direct listening
Listening through messengers
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H istory of Communications
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Drastic change in 1837
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The invention of the telegraph by Samuel Morse
Characters of a message were translated into long
and short electrical impulses
These impulses were then transmitted over a copper
wire
The association between the characters and the
impulses was called Morse Code
1876: The Magical invention of Telephony by Alexander
Graham Bell
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Bell showed that voice can be converted directly into
electrical energy
This energy can then be transmitted over a wire
using continuously varying voltages
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H istory of Communications
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Switchboards
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Early telephones required a pair of wires that connects
telephones at both ends
That changed with the invention of the switchboard, a
switching device that connected lines between two phones
Establishing connections was activated through an operator
at the switchboard
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H istory of Communications
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1945: ENIAC
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(continue...)
Electronic Numerical Integrator And Calculator
The 1st electronic computer
Designed for computing ballistics for World War II
Although did not play a direct role in communication, it did
show that calculations and decision making could be done
electronically
1947: The Transistor
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Allowed smaller and affordable computers to be built
Computers applications developed during the 1960s made
routing and processing telephone calls economically
possible
The increased utilization of computers by businesses
resulted in a growing need to transfer information between
these computers as well
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H istory of Communications
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Electronic Communications The massive need in relatively short time
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The 1st form of communication between systems were done through
magnetic tapes
Information were copied from on system into a tape; which is transferred to
another system, possibly a remote one
The development of the Personal Computer (PC) in the 1980s was a
milestone in electronic communication
The infusion of millions of PCs into, virtually, everywhere created a further
massive need to make information easily accessible
At this point it was clear that the need for electronic communication was no
longer a luxury, rather a necessity and a must
1990s: The World Wide Web (WWW) & The Internet
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WWW is application that enabled information from around the world to
become easily accessible from one’s desk.
The Internet is an electronic network of computers that includes nearly
every university, government, and research facility in the world, as well as
many commercial sites
The Internet started with four interconnected computers in 1969 and was
known as ARPAnet
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F orms of Communications
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Transferring data between computers is just one form of
communication
Transferring television signals is just another example
TV antennas are becoming less common; cable TV and
direct reception via satellite dishes are becoming more
popular today
Satellite Transmission
Broadcast Tower
Broadcast Television
Reception
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F orms of Communications
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Other communications applications use local area
networks (LANs) and wide are networks (WANs)
systems that allow communication over short (LAN)
or long (WAN) distances.
Once connected to LANs or WANs, users can send or
receive data files, log into remote machine, send
Electronic Mails (E-mails), or connect to the
Internet
Other forms of communications include:
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Facsimile machines (Fax)
Voice and video communications over computers
Teleconferencing / Videoconferencing
Cellular telephones
Peer-to-peer networking
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Okay! Let us Connect,
but what about:
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Security
Performance
Medium of communication
Ease of use
Cost and cost-effectiveness
After all, is there a law?
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C omputer Networks
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A network is a system connecting different devices, such as
PCs, printers, scanners, ..etc.
A PC may be dedicated to manage a disk drive containing
shared files; such a PC is referred to as file server
Often, a network covers a small geographical area (up to
few kilometers), such a network is referred to as LAN
A network that connects large areas, such as state,
country, or the world, is referred to as WAN
Now, with many users connecting to a network it is
significant to avoid conflicts as well as provide service to all
of them with a minimal delay
Connections to the network hence must be made in a way
that achieves such requirements
The connection strategy is referred to as Network
Topology
The best topology depends on the network needs
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N etwork Topologies
Common Bus Topology
 Star Topology
 Ring Topology
 Fully Connected Topology
 Tree Topology
 Combined Topology
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C ommon Bus Topology
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Simply known as Bus Topology or Linear Bus
Topology
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Terminators are placed at both ends
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C ommon Bus Topology
Collision is possible
 A collision results on noise
 The interfaces detect the noise on the bus, stop transmission,
wait for a random amount of time then attempt transmission
again
 This process is called Carrier Sense Multiple Access with
Collision Detection (CSMA/CD)
 The Ethernet (original configuration) is an example of a
common bus network
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Advantages
 Disadvantages
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S tar Topology
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The central component is commonly called
hubs or switches
Advantages
 Disadvantages
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R ing Topology
The ring can either be unidirectional or bidirectional (dual)
 IBM’s Token Ring network is an example of a ring topology,
where a token (sequence of bits) is passed between the
devices.
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Advantages
 Disadvantages
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F ully Connected Topology
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A direct link is there between each pair of
devices in the network; this design is extreme
Advantages
 Disadvantages
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T ree Topology
Combines characteristics of linear bus and star
topologies
 Allows for the expansion of an existing network
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Advantages
 Disadvantages
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C ombined Topologies
Used by many computer networks
 Groups of specialized users may need to have a separate
LAN
 Different LANs are connected through bridges/switches
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S tandards
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Computers have different architecture, understand
different languages, store data in different formats, …etc.,
so how can they communicate then!
For communication to occur, the system must use, and
agree upon, a protocol to communicate
A protocol is a set of rules that define an exact format and
rules for communication between systems
If two system agree to follow a protocol, but different
protocol, they might as well follow none!
If all systems agree to follow one same protocol, then this
protocol would become a standard
Fortunately, there is a standard; BUT
Unfortunately, there are few of them
Through the years, different goals and ideas have cause
this creation of various standards
In general, there are two types of standards: De facto
standards, and standards through standards organizations
De facto standards are those who exist by virtue of their
widespread use, such as many of IBM products
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S tandards Organizations
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Few standards organizations are important to the field of
computer networks and data communication
Such organizations are:
• ANSI: American National Standards Institute
(www.ansi.org) – ASCII, FDDI, …
• IEC: International Electrotechnical Commission
(www.iec.ch) – JPEG, …
• ITU: International Telecommunication Union
(www.itu.int) - X.25, …
• EIA: Electronic Industries Association
(www.eia.org) – optical cables, wires, …
• IETF: Internet Engineering Task Force
(www.ietf.org) – the next-generation Internet protocol
• IEEE: Institute of Electrical and Electronics Engineers
(www.ieee.org) – journals, 802 LAN Standard, …
• ISO: International Standards Organization
(www.iso.org) – OSI, …
• NIST: National Institute of Standards and Technology
(www.nist.gov) – DES, …
• IBM: International Business Machines
(www.ibm.com) – SNA, EBCDIC, …
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O pen Systems & the OSI Model
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Open System: A set of protocols that would allow any
two different systems to communicate regardless of
their underlying architecture
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The Open System Interconnect (OSI) Model:
• Introduced by ISO
• Employs 7 layers, where each layer performs
specific functions and communicate with the one
above and below it
• Higher layers deal more with the user applications,
services and activities
• Lower layers deal more with the actual transmission
of information
• Advantages of layered model
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O pen System Interconnect (OSI)
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O pen System Interconnect (OSI)
(continue...)
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O verview of OSI
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Layer 7: The Application Layer:
Interfaces directly with the user. At this layer:
• Communication partners are identified
• Quality of service is identified
• User authentication and privacy are considered
• This layer provides application services such as file
transfers & e-mail
• Telnet and FTP are examples of applications that
exist entirely in the application level
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O verview of OSI
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Layer 6: The Presentation Layer:
• Responsible for presenting data in a format that the
user can understand
• Provides data encryption for security reasons
• Sometimes, this layer is referred to as the Syntax
Layer
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O verview of OSI
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Layer 5: The Session Layer:
• Allows applications on two different computers to
establish a session, or in other words, a logical
connection
• A session is established when the communication
start and ended when the communication ends
• This layer also handles error recovery
• The session layer allows the insertion of
checkpoints in the middle of the stream
• This layer also brackets ([…], perform it as atomic)
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O verview of OSI
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Layer 4: The Transport Layer:
• Lowest layer that deals with end-to-end
communication; lower layers deal with the network
itself
• A computer may be connected to several networks;
this layers determines which network to use for
communication depending on many factors
• Ensures complete data transfer
• Provides end-to-end error recovery, and flow
control
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O verview of OSI
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Layer 3: The Network Layer:
• Deals with routing strategies
• In specific, this layer controls the communication
subnet (the collection of transmission media and
switching elements over that media required for
routing and data transmission)
• Provides congestion control
• May also contain billing and accounting routines –
information need to bill users based on their use of
the network
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O verview of OSI
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Layer 2: The Data Link Layer:
• Supervises the flow of information between adjacent
network nodes
• Uses error detection or correction techniques to ensure
that a transmission is error-free
• Controls how much information is sent at a time in order
to avoid both congestion and long delays
• The data link layer is divided into two sublayers: The
Media Access Control (MAC) layer and the Logical
Link Control (LLC) layer.
• The MAC sublayer controls how a computer on the
network gains access to data and permission to transmit
data.
• The LLC layer controls flow control and error checking 29
O verview of OSI
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Layer 1: The Physical Layer:
• Transmits data bits over a network
• Concerns with the physical, or electrical, aspects of
data communication
• For example, it concern with the communication
medium. Is it copper cable, optical fiber, satellite,
...etc.
• Does not concern with the meaning of the data it
transmits; just receive the data from the data link
layer and deliver it to the data link layer without any
analysis of this data
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O verview of OSI
Brief Comparisons of the 7 Layers
Layer
Functions
7. Application
User services such as e-mail, file transfer, ...
6. Presentation
Translate data format, encrypt and decrypt data
5. Session
Synchronize communicating users, error recovery, bracket operations
4. Transport
Determine network, may assemble and reassemble packets
3. Network
Determine routes, manages billing information
2. Data Link
Detect or correct errors
1. Physical
Transmits physical data
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C onnection Strategies
Computers must be connected to communicate,
however
 the traversal of information though the network
is a design issue
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Figure 1.13 page 25
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C onnection Strategies
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(continue...)
There are 3 strategies:
• Circuit Switching
• Message Switching
• Packet Switching
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C onnection Strategies
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Circuit Switching:
• Once a connection is made, it is maintained until the
communication terminates
• In a telephone network, dialing the number makes
the connection
• In a computer system, the user enters appropriate
commands to connect to a specified location
Figure 1.14 page 26
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C onnection Strategies
(continue...)
Circuit Switching (continue):
• Circuit switching requires that the route be
determined and that the connection is made
before any information is transmitted
• The connection is then maintained until the
communication terminates
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C onnection Strategies
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Message Switching:
• Message: unit of information
• A route is established when a message is to be sent
• A sending node attach the message along with the destination address,
chooses a route, then forwards it to the first node in that route
• The message, in its entirety, is temporarily stored at that node, while
logic is working the next node in the following possible route
• Once the following node is known the message is routed to it
• This is know as Store-and-Forward
Figure 1.15 page 27
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C onnection Strategies
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Comparison between Circuit & Message Switching
Circuit Switching
No store-and-forward;
intermediate nodes act as
switches
Message Switching
Store-and-forward; this results
in a delay which makes this
strategy unsuitable for telephone
networks
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Connection must be established
prior to communication
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Route is dedicated to the
connection
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No connections are needed prior
to communication; messages are
sent and can be retrieved later on
No route is dedicated; that is
different communications can
utilize the same route, which
results in higher utilization
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C onnection Strategies
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Packet Switching:
• A message is divided into smaller units, called packets
• Each packet contains its destination address (or other indicators
to where it should go)
• Packets are then sent, possibly through different routes
• At the destination, packets are reassembled to from the original
message
Figure 1.16 page 28 (datagram in fact)
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C onnection Strategies
(continue...)
Packet Switching (continue):
• Similar to message switching, no connection is
maintained
• The small size of the packets reduces buffering
problems and long delays
• Packet switching has two common routing methods:
 Datagram
 Virtual Circuit
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L ayers of the OSI Model
Physical Layer:
• Deals with the transmission media and how signals
are transmitted
• Examples of transmission media include twisted pair,
coaxial cable, optical fiber, microwave, satellite,
infrared, radio waves
• The choice of the media depends on many factors
including interference chances, digital or analog
signals, ..etc.
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L ayers of the OSI Model
(continue...)
Data Link Layer:
• Makes sure that the physical layer works correctly
• Two nodes may try to send information over the
same link at the same time; this is referred to as
Contention
• Contention is handled by Collision Detection
• Contention can be avoided by using Carrier Sense
Multiple Access with Collision Detection
(CSMA/CD)
• CSMA/CD avoids contention but does not eliminate
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it
L ayers of the OSI Model
(continue...)
Data Link Layer (continue):
• Token Passing is another contention scheme that
prevents collision
• A token is a bit stream that traverses between nodes
• Token network often use token passing
• What happen if the token is lost?
• Token passing is not limited to token networks; any
topology may number the nodes and pass the token
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among these nodes in order
L ayers of the OSI Model
(continue...)
Figure 1.17 page 31
Data Link Layer (continue):
• Data is sent over the physical link from one node and received
by another, however have they arrived correctly
• The data link layer performs error detection and if errors are
detection then it performs error correction
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L ayers of the OSI Model
(continue...)
Data Link Layer (continue):
• Parity Bit is one of the simplest methods for error
detection
• An additional bit is attached to each sequence of
data bits
• Even parity or odd parity
• What is the problem with Parity Bit technique?
Figure 1.18 page 31
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L ayers of the OSI Model
(continue...)
Network Layer:
• Contains algorithms to find out the best route (cost, efficiency, ..)
between two points
• This is not usually an easy task since it needs to respond to
changing conditions
• A good route may attract a lot of traffic, ending it in being
congested
• Detouring can be used in order to take a better route. Is that
always good?
Figure 1.19 page 32
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L ayers of the OSI Model
(continue...)
Transport Layer:
• This layer, and the three layers above it, provide user services
• They execute at the sending and receiving nodes to ensure
information arrival at the destination as well as acknowledgment
to the sending node
Figure 1.20 page 33
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L ayers of the OSI Model
(continue...)
Transport Layer (continue):
• Provides reliable and efficient communication
• In practice, networks are unreliable; data can be lost
or delayed
• This layer insulates the session layer from many
details of the network
• TCP (Transport Control Protocol) is one of the
most famous transport layer protocols
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L ayers of the OSI Model
(continue...)
Transport Layer (continue):
• Transport functions include multiplexing, buffering and
connection management
• Multiplexing: Several transport users share a single node; for
example three users may connect to the same node (login,
download a file, and view a website)
• The transport layer ensures that the proper data is routed to
the appropriate user
Figure 1.21 page 34
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L ayers of the OSI Model
(continue...)
Transport Layer (continue):
Buffering: it may occur at either the destination or the
source
• Data received from the session layer is broken into
Transport Protocol Data Units (TPDUs)
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L ayers of the OSI Model
(continue...)
Transport Layer (continue):
Connection Management: the protocol by which the transport
establishes and releases connections
• Two-Way Handshake may seem easy, but will it always work?
Figure 1.22 page 35
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L ayers of the OSI Model
(continue...)
Session Layer:
• Contains the protocols necessary for establishing and maintaining a connection,
or session, between end users
• Well, then what did the transport layer do?!
Figure 1.23 page 36
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L ayers of the OSI Model
(continue...)
Session Layer (continue):
• Note: do not interpret the figure as a form of multiplexing; a transport
connection serves only one session at a time. A second session can use
the transport connection when the 1st one is finished
Figure 1.24 page 37
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L ayers of the OSI Model
(continue...)
Presentation Layer:
• Computers do not speak the same language; language here
refers to the way information is represented
• There is a difference between data and information
• A problem exists because different computers may have
different ways to represent the same information
• For example, integers may have different sizes (sometimes,
transformation is impossible), 2-D array can be stored based on
rows or columns,
• Thus, it is not enough to provide effective data communication;
rather, effective communication of information is needed
• The presentation layer performs that task
• Data encryption and data compression can also be performed
at this layer
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L ayers of the OSI Model
(continue...)
Presentation Layer (continue):
Figure 1.25 page 38
Figure 1.26 page 39
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L ayers of the OSI Model
(continue...)
Application Layer:
• Communicates directly with user/user
applications
• Contains network applications, such as:
 FTP,
 Telnet, E
 E-mail,
 Virtual terminal,
 …
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I nternet Layers
• The Internet has a
similar, not
identical layers to
the ones of the
OSI
• The lower 4 layers
roughly correspond
to the lower 4
layers of OSI
• Functionality of the
highest 3 layers of
OSI are either
omitted, or
incorporated in the
5th layer of the
Internet
• Layer 4: TCP or
UDP
• Layer 5: HTTP, FTP,
..etc
Figure 1.29 page 43
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