4123702
Data Communications System
By
Ajarn Preecha Pangsuban
Part 1
Overview of Data Communications and
Networking
 Chapter 1
Brief description of data communication, networking,
and protocol standards
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 Chapter 2
Network models
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Chapter 1 - Introduction
Data Communication
 Why study data communications?
 Data communication is the exchange of data
between two devices via a transmission medium
 Effectiveness depends on:
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Delivery
Accuracy
Timeliness
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Communication System Components
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Data Representation
 Text – represented as a bit pattern, a sequence of
bit (0s or 1s); codes often used:

ASCII; Extended ASCII; Unicode; ISO
 Numbers – represented by binary equivalent
 Images – bit patterns representing pixels
 Audio
 Video
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A Communications Model
Transmission
Network
DTE
DCE
DCE
DTE
DTE: Data Terminal Equipment
DCE: Data communication Equipment
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Simplified Communications Model
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Simplified Data Communications Model
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Simplified Network Model
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Direction of Data Flow
 Simplex – unidirectional; one transmits, other
receives
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Direction of Data Flow (cont)
 Half-duplex – each can transmit/receive;
communication must alternate
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Direction of Data Flow (cont)
 Full-duplex – both can transmit/receive
simultaneously
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Networks

A network is set of devices (nodes) connected
by communication links (media)

A node can be a computer,printer,or other
device capable of sending and/or receiving data
Link connecting the devices are often called
communication channels

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Most network use distributed processing

Advantages?
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Network Criteria (Standard)
 Performance – depends on number of users, type
of medium, HW/SW
 Reliability – measured by freq of failure, recovery
time, catastrophe vulnerability
 Security – protection from unauthorized access,
viruses/worms
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Types of Connections
 Point-to-point – dedicated link
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Types of Connections (cont)
 Multipoint (Multidrop) – shared a single link
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Physical Topology
 The way in which a network is laid out physically
 4 basic types: mesh, star, bus, ring
 May often see hybrid
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Mesh Topology
 Dedicated point-to-point
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links to every other device
Has n(n-1)/2 physical
channel to link n devices
Devices have n-1 I/O
Advantages?
Disadvantages?
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Star Topology
 Dedicated point-to-point
links to central controller
(hub)
 Controller acts as
exchange
 Advantages ?
 Disadvantages ?
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Bus Topology
 Multipoint configuration
 One cable acts as a backbone to link all devices
 Advantages ?
 Disadvantages ?
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Ring Topology
 Dedicated point-to-point configuration to neighbors
 Signal is passed from device to device until it reaches
destination
 Each device functions as a repeater
 Advantages ?
 Disadvantages?
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Categories of Networks
 LAN – smaller geographical area
 WAN – large geographical area
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Local Area Network
 Usually privately owned and links the devices in a
single office, building, or campus
 LAN size is limited to a few kilometers.
 LANs are designed to allow resources to be shared
(hardware , software and data )
 Today LANs to have data rates of 100 Mbps to 10Gbps
 Backbone Networks (BN), have a scale of a few
hundred meters to a few kilometers. Include a high
speed backbone linking the LANs at various locations.
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Figure 1-2
Local Area Network (cont)
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Figure 1-2
Local Area Network (cont)
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Wide Area Network
 WAN provides long distance transmission of
data, voice, image, and video information over
large geographical areas
 Comprise a country, a continent, or even the
whole world (Interlink age of many LANs and
MANs)
 Low data transmission rate (below 1 Mbps)
 Unlimited number of miles example: Internet
Network
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Figure 1-4
Wide Area Network (cont)
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Internetworks
 Connection of two or more networks by the use of
internetworking devices which include routers and
gateways
 Internet is a generic term used to mean an
interconnection of networks
 The Internet is the name of a specific worldwide
network.
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Figure 1-5
Internetworks (cont)
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The Internet
 Collaboration of more than hundreds of
thousands interconnected networks
 1969 – started as ARPAnet, a small network of
connected computers
 1972 - Cerf and Khan – packet delivery and
Transmission Control Protocol (TCP)
 Shortly thereafter – evolution of TCP/IP
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The Internet today
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Protocols and Standards
 Why do we need them?
 A protocol is a set of rules that governs data
communication; the key elements of a protocol are

Syntax – data formats and Signal levels

Semantics – control information and error handling

Timing – speed matching and sequencing
 Standards are necessary to ensure that products
from different manufacturers can work together as
expected.
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Standards
 Types –
De jure (Formal) – legislated by an officially recognized
body
De facto – by convention or widespread use
 Standards Organizations
Committees – ISO, ITU-T, ANSI, IEEE, and EIA
Forums – special-interest groups that quickly evaluate
and standardize new technologies
regulatory agencies – FCC
 Internet Standards
Drafts (6 month lifetime)
RFC is an idea or concept that is a precursor to an
Internet standard.
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Credits
 All figures obtained from publisher-provided
instructor downloads
Data Communications and Networking, 3rd edition by
Behrouz A. Forouzan. McGraw Hill Publishing, 2004
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