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Data & Computer Communications
MSCEG 425
Lecture 1
Introduction
Network Models
Fall 2007
Dr. L. Christofi
Fall 2007
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0. Overview
In this lecture we will cover the following topics:
1. Introduction
1.1
1.2
1.3
1.4
1.5
Introduction to data communications and networks
Networks
The Internet
Protocols & Standards
Summary (part 1)
2. Network Models
2.1
2.2
2.3
2.4
2.5
2.6
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Layered tasks
OSI model
Layers in the OSI model
TCP/IP protocol suite
Addressing
Summary (part 2)
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1.0 INTRODUCTION
The term telecommunication means communication at a
distance.
The word data refers to information presented in whatever
form is agreed upon by the parties creating and using the
data.
Data communications are the exchange of data between two
devices via some form of transmission medium such as a wire
cable.
Topics discussed in this section:
Components
Data Representation
Data Flow
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Components of data communication
A data communication system has 5 components:
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Message: info (data) to be communicated
Sender: device that sends the data message
Receiver: device that receives the message
Medium: transmission medium (physical path) by which a
message travels from the transmitter to the receiver
- Protocol: set of rules that governs data communications
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Data representation
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Text: bit pattern
ASCII: 128 different symbols (7 bits)
Extended ASCII: size of each pattern is 1 byte (8 bits)
Unicode: 65.536 symbols (16 bits)
ISO: 4.294.967.296 symbols (32 bits)
Numbers: decimal numbers converted directly to
binary
• Images: divided into a matrix of pixels
• Audio: representation of sound by an analog or a digital
signal
• Video: represented by an analog or digital signal
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Direction of data flow
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1.2 NETWORKS
A network is a set of devices (often referred to as
nodes)
nodes) connected by communication links.
links.
A node can be a computer, printer, or any other
device capable of sending and/or receiving data
generated by other nodes on the network.
Topics discussed in this section:
Network Criteria
Physical Structures
Categories of Networks
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Network criteria
• Performance
—Can be measured in may ways
• transit time: amount of time required for a message to
travel from one device to another
• response time: time elapsed between an inquiry and a
response
• Number of users
• Type of transmission medium
• Hardware capabilities and software efficiency
• Reliability
—A measure of frequency of failure and the time needed
to recover, network robustness
• Security
—Protecting of data from unauthorized users
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Physical Structures
Types of connections: point-to-point and multipoint
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Physical Topology
Physical topology refers to the way in which a network is
laid out physically.
Network topology is the geometric representation of the
relationship of all the links and linking devices (nodes)
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Topology categories
Star
Mesh
Number of links = n*(n-1)/2
Number of links = n
Ring
Bus
Number of links = n+1
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A hybrid topology
A star backbone with three bus networks
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Categories of networks
• Local Area Networks (LANs)
— Usually privately owned and links the devices in a single office, building
or campus
— Most common LAN topologies are bus, ring and star
— Speeds of 100Mbps-1Gbps
• Metropolitan Area Networks (MANs)
— Designed to extend over a city
— May be owned by a large private organization or a service provider
(telephone company)
• Wide Area Networks (WANs)
— Provide long-distance transmission of data, voice, image and video info
over large geographical areas that may comprise a country or a
continent
• Internetworks
— When two or more networks are connected they become an
internetwork or internet
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WAN examples
A switched WAN and a
point-to-point WAN
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An heterogenous network
made of 4 WANs and 2 LANs
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An heterogeneous network
made of four WANs and two LANs
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Comparison of LANs, MANs, & WANs
LANs: 1 – 1000 Mbps
MANs: 10 – 40 Gbps
WANs: Tbps
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1.3 THE INTERNET
The Internet has revolutionized many aspects of our daily
lives.
It has affected the way we do business as well as the way we
spend our leisure time.
The Internet is a communication system that has brought a
wealth of information to our fingertips and organized it for our
use.
Topics discussed in this section:
A Brief History
The Internet Today (ISPs)
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A brief history
• Mid-1960s
— Standalone devices
— ARPA (Advanced Research Projects Agency) was interested in finding a way
to connect computers to share information
— Backbones: None - Hosts: None
• 1967
— ARPA presented its ideas for ARPANET
— Backbones: None - Hosts: None
• 1969
— The first physical network was constructed
— Backbones: 50Kbps ARPANET - Hosts: 4
• 1972
— The first e-mail program was created by Ray Tomlinson of BBN
— Backbones: 50Kbps ARPANET - Hosts: 23
• 1973
— Development began on the protocol later to be called TCP/IP (by Vint Cerf
and Bob Kahn)
— Backbones: 50Kbps ARPANET - Hosts: >23
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Hierarchical organization of Internet
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The Internet today (ISPs)
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1.4 PROTOCOLS AND STANDARDS
In this section, we define two widely used terms: protocols
and standards.
standards. First, we define protocol, which is
synonymous with rule. Then we discuss standards, which
are agreedagreed-upon rules.
Topics discussed in this section:
Protocols
Standards
Standards Organizations
Internet Standards
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Protocols
• A protocol is a set of rules that governs data
communications
• It defines what is communicated, how it is
communicated and when it is communicated
• Key elements of a protocol:
—Syntax
• Structure or format of data, meaning the order in which they
are presented
—Semantics
• Refer to the meaning of each section of bits, how a
particular pattern is interpreted and what action to be taken
—Timing
• Refers to when data should be sent and how fast can they
be sent
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Standards
• Standards are essential in creating and maintaining an
open and competitive market for equipment manufacturers
• Required to guarantee national and international
interoperability of data and telecommunications
technology and processes
• Categories of data communications standards
— De facto:
• Standards that have not been approved by an organizational body
but have been adopted through widespread use, eg. model TCP/IP)
— De jure:
• Those that have been legislated by an official recognized body, eg.
OSI model
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Standards organizations
• Standards creation committees
— ISO (International Organization for Standardization)
— ITU-T (International Telecommunications Union – Telecommunications
Standards)
• Initially known as CCITT (Consultative Committee for International
Telegraphy and Telephony)
— ANSI (American National Standards Institute)
— IEEE (Institute of Electrical and Electronics Engineers)
— EIA (Electronic Industries Association
• Forums
— Made up of representatives from interested corporations to speed
acceptance and use of new technologies in the telecom industry
• Regulatory Agencies
— Governmental agencies: to protect public interest by regulating radio,
TV and wire/cable communications
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Internet standards
• An Internet standard is a thoroughly tested
specification used by those who work with the
Internet
• A specification begins with an Internet draft
—Working document with no official status and a 6month lifetime
—Upon recommendation from the Internet authorities a
draft may be published as a Request for Comment
(RFC)
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1.5 SUMMARY (part 1)
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Data communication is the transfer of data from one device to another via some form of transmission medium.
A data communications system must transmit data to the correct destination in an accurate and timely manner.
The five components that make up a data communications system are the message, sender, receiver, medium,
and protocol.
Text, numbers, images, audio, and video are different forms of information.
Data flow between two devices can occur in one of three ways: simplex, half-duplex, or full-duplex.
A network is a set of communication devices connected by media links.
In a point-to-point connection, two and only two devices are connected by a dedicated link. In a multipoint
connection, three or more devices share a link.
Topology refers to the physical or logical arrangement of a network. Devices may be arranged in a mesh, star,
bus, or ring topology.
A network can be categorized as a local area network (LAN), a metropolitan-area network (MAN), or a wide area
network (WAN).
A LAN is a data communication system within a building, plant, or campus, or between nearby buildings.
A MAN is a data communication system covering an area the size of a town or city.
A WAN is a data communication system spanning states, countries, or the whole world.
An internet is a network of networks.
The Internet is a collection of many separate networks.
TCP/IP is the protocol suite for the Internet.
There are local, regional, national, and international Internet service providers (ISPs).
A protocol is a set of rules that governs data communication; the key elements of a protocol are syntax,
semantics, and timing.
Standards are necessary to ensure that products from different manufacturers can work together as expected.
The ISO, ITU-T, ANSI, IEEE, and EIA are some of the organizations involved in standards creation.
Forums are special-interest groups that quickly evaluate and standardize new technologies.
A Request for Comment (RFC) is an idea or concept that is a precursor to an Internet standard.
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2.0 NETWORK MODELS
• A network uses a combination of hardware and
software to send data from one location to
another
—Hardware consists of the physical equioment that
carries signals from one point of the network to another
—The task of sending a piece of information from one
point in the worls to another can be broken into several
tasks, each performed by a separate software package
• Each piece of software uses the services of another software
package o do its job
• At the lowest layer, a signal is sent from the source to the
destination computer
• In this part we will see a general idea of layers of
a network and discus the functions of each one
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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
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Tasks involved in sending a letter
• The task of transporting the
letter between the sender
and the receiver is done by
the carrier.
• The tasks must be done in
the order given in the
hierarchy.
carrier
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2.2 THE OSI MODEL
Established in 1947, the International Standards
Organization (ISO
(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)
OSI) model. It was first introduced in the late 1970s.
Topics discussed in this section:
Layered Architecture
Peer-to-Peer Processes
Encapsulation
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Seven layers of the OSI model
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PeerPeer-toto-peer processes
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Encapsulation
• The process starts at layer 7
(application layer), then moves
from layer to layer in descending,
sequential order.
• At each layer, a header is added
to the data unit.
• At layer 2, a trailer is added as
well.
• When the formatted data unit
passes through the physical layer
(layer 1) it is changed into an
electromagnetic or optical signal
and transported along a physical
link
• At the destination the reverse
process is performed
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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
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Physical layer
Note
The physical layer is responsible for movements of
individual bits from one hop (node) to the next.
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Data link layer
Note
The data link layer is responsible for moving
frames from one hop (node) to the next.
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HopHop-toto-hop delivery
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Network layer
Note
The network layer is responsible for the
delivery of individual packets from
the source host to the destination host.
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SourceSource-toto-destination delivery
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Transport layer
Note
The transport layer is responsible for the delivery
of a message from one process to another.
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Reliable processprocess-toto-process delivery
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Session layer
Note
The session layer is responsible for dialog
control and synchronization.
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Presentation layer
Note
The presentation layer is responsible for translation,
compression, and encryption.
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Application layer
Note
The application layer is responsible for
providing services to the user.
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Summary of layers
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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: hosthost-toto-network,
network, internet,
internet, transport,
transport, and
application.
application.
However, when TCP/IP is compared to OSI, we can say
that the TCP/IP protocol suite is made of five layers:
physical,
physical, data link,
link, network,
network, transport,
transport, and application.
application.
Topics discussed in this section:
Physical and Data Link Layer
Network Layer
Transport Layer
Application Layer
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TCP/IP layers
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TCP/IP and OSI model
OSI
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TCP/IP model
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2.5 ADDRESSING
Four levels of addresses are used in an internet employing
the TCP/IP protocols: physical,
physical, logical,
logical, port,
port, and specific.
specific.
Topics discussed in this section:
Physical Addresses
Logical Addresses
Port Addresses
Specific Addresses
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Addresses in TCP/IP
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Physical & Logical address
• Physical address
— In computer networks a physical address means a MAC
(Medium Access Control) address
• Also known as Ethernet Hardware Address (EHA) or
hardware address or adapter address
— It is a number that acts like a name for a particular network
adapter, eg. the network cards
• Logical address
— In computer networks, a logical address refers to a
network layer address such as an IP address
— An IP address (Internet Protocol address) is a unique
address that certain electronic devices use in order to
identify and communicate with each other on a computer
network utilizing the Internet Protocol standard (IP)
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Port & specific address
• Port address
— TCP and UDP are transport protocols used for communication
between computers via ports
— The port numbers are divided into three ranges.
• The Well Known Ports are those in the range 0–1023.
• The Registered Ports are those in the range 1024–49151.
• The Dynamic and/or Private Ports are those in the range
49152–65535. These ports are not used by any defined
application.
• Specific address
— This address is used by application processes
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Relationship of layerslayers-addresses in TCP/IP
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Example: Physical address
In the following figure, 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.
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Example: Physical address
As we will see in a later lecture, most LANs use a 48-bit (6byte) 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.
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Example: IP addresses
This figure 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.
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Example: Port addresses
This figure 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.
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Note
The physical addresses change from hop to hop,
but the logical and port addresses usually remain the same.
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2.6 SUMMARY (part 2)
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The five-layer model provides guidelines for the development of universally
compatible networking protocols.
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The physical, data link, and network layers are the network support layers.
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The application layer is the user support layer.
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The transport layer links the network support layers and the user support layer.
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The physical layer coordinates the functions required to transmit a bit stream over a
physical medium.
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The data link layer is responsible for delivering data units from one station to the
next without errors.
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The network layer is responsible for the source-to-destination delivery of a packet
across multiple network links.
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The transport layer is responsible for the process-to-process delivery of the entire
message.
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The application layer enables the users to access the network.
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References
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B.A. Forouzan, Data Communications and Networking,
4th edition, McGraw-Hill, 2007
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W. Stalling, Local and Metropolitan Area Networks,
6th edition, Prentice Hall, 2000
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W. Stallings, Data and Computer Communications,
7th edition, Prentice Hall, 2004
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F. Halsall, Data Communications, Computer Networks and
Open Systems, 4th edition, Addison Wesley, 1995
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