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Data Communications
Data – facts, concepts, and instructions Data – Information presented in whatever
form is agreed upon by the parties creating and using the data
Represented by binary information units (in the form of 0s and 1s)
Data Communications – exchange of data between two devices via some form of transmission medium such as a wire cable
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Effectiveness of Data Communications Effectiveness of a data communications system
depends on three fundamental characteristics Delivery : The system must deliver data to the correct
destination. Data must be received by the intended device or user and only by that device or user
Accuracy: The system must deliver the data accurately. Data that have been altered in transmission and left uncorrected are unusable
Timeliness: The system must deliver data in a timely manner. Data delivered late are useless. Delivering the data in the same order that they are produced and without significant delay (real time transmission)
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Five components of data communicationMessage: Information to be communicated.
Consist of text, numbers, pictures, sound or video - combination
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Five components of data communicationSender: device that sends the data
message- may be computer, workstation. telephone handset, video camera and etc.
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Five components of data communicationReceiver: the device that receives the message. Can be computer, work station, telephone handset, television and so on.
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Five components of data communicationMedium: Physical path by which a message
travels from sender to receiver – twisted pair wire, coaxial cable, fibre optic cable or radio waves
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Five components of data communicationProtocol: Set of rules that governs data
communication. It represents an agreement between the communicating devices. Without a protocol, two devices may be connected but not communicating
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Data Representation (1)Information Today comes in different formssuch as text, numbers, images, audio and video Text: Represented as a bit pattern, a
sequence of bits 0s or 1s. Different codes are used ASCII (7 bits per symbol) Extended ASCII (8 bits per symbol) Unicode (16 bits – supports different languages) ISO (32 bits)
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Data Representation (2)
Numbers Converted to a binary number – to simplify the
mathematical operations Images
Represented by bit patterns Each pixel is assigned a bit pattern Black and White Image – 1 bit per pixel Gray scale images – depends on number of levels in gray
scale Colour images: Each pixel has 3 bit patterns (RGB)
Audio / Video Converted in to Analog/Digital
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Applications (2)
Peer-Peer model: No fixed clients or servers Each host can act as both client & server
Examples: Napster, Gnutella, KaZaA
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Applications (3)
WWW Instant Messaging (Internet chat, text
messaging on cellular phones) Peer-to-Peer Internet Phone Video-on-demand Distributed Games Remote Login (Telnet) File Transfer
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Network Criteria
Performance – can be measured by transit time and response time. Affected by number of users, type of medium, connected HW/SW
Reliability – measured by frequency of failure, recovery time, robustness in a catastrophe
Security – protection from unauthorized access, viruses / worms
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Line Configuration The way two or more communication devices
attach to a link Link – Physical communication pathway that
transfer data from one device to another Two possible line configurations
Point-to-point Multipoint
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Topology
Physical or logical arrangement Topology of a network is the geometric
representation of the relationship of all the links and linking devices to one another
4 basic types: mesh, star, bus, ring May often see hybrid
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Mesh Topology Dedicated point-to-point links
to every other device n(n-1)/2 links an each device
will have n-1 I/O ports Advantages
Dedicated links – no traffic problems
Robust Privacy/Security Easy fault identification and
isolation Disadvantages
more amount of cabling and I/O ports requirement
Installation and reconnection is difficult
Expensive
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Star Topology
Dedicated point-to-point links to central controller (hub)
Controller acts as exchange Advantages
less expensive robustness
Disadvantages More cabling requirement
than ring and bus topologies
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Bus Topology
Multipoint configuration One cable acts as a backbone to link all devices Advantages : Ease of installation, less cabling Disadvantages : Difficult reconnection and fault
isolation, a fault/break in the bus cable stops all transmission
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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 : easy to install and reconfigure Disadvantages :limited ring length and no: of
devices; break in a ring can disable entire network
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Categories of Networks
Based on size, ownership, distance covered, and physical architecture Local Area Network (LAN) – smaller geographical
area Metropolitan Area Network (MAN) – network
extended over an entire city Wide Area Network (WAN) – large geographical
area
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MAN
Designed to extend over an entire city – May be a single network such as cable television network – may be a means of connecting a number of LANS into a larger network
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Metropolitan Area Network (MAN)
home
cable headend
cable distributionnetwork (simplified)
Typically 500 to 5,000 homes
A Cable TV Network is an example of a MAN
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WAN
Provides long distance transmission of data, voice, image and video information over large geographic areas – a country/continent/world
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Protocols and Standards
Protocols Set of rules that governs data communications Defines what is communicated, how it is
communicated, and when it is communicated Key elements
Syntax : Structure/ format of data –order in which it is presented
Semantics : meaning of each section of bits- how pattern to be interpreted – What action to be taken
Timing: When data to be sent and how fast they can be sent
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Protocols and Standards
Standards Essential in creating and maintaining an open and
competitive market for equipment manufacturers and in guaranteeing national and international interoperability of data and telecommunications technology and processes
De facto: Standards that have not been approved by an organized body but have been adopted as standards through widespread use
De jure: legislated by an officially recognized body
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Protocols and Standards
Standards Organizations International organization for Standardization
(ISO) International Telecommunication Union –
Telecommunication Standards Sector (ITU-T) American National Standards Institute (ANSI) Institute of Electrical and Electronics Engineers
(IEEE) Electronic Industries Assoctiation (EIA)
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