introduction to data communications chapter 1. data comm development of pc –tremendous changes in...
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Introduction to Data Communications
Chapter 1
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DATA COMM• Development of PC
– Tremendous changes in sciences, industry ,education etc– No more domain of technical personnel only– Requirement of transfer of data quickly
• TELECOMM– Tele means far in Greek– Tele includes Telephony, Telegraphy & TV– Local Comm– Remote Comm
• DATA COMM– Data means facts, concepts & instructions– Exchange of data between 2 devices– Transfer takes place as 0s & 1s– Effectiveness depends upon
• Delivery: To correct destination• Accuracy: Not to be altered• Timeliness must be delivered in time
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Data Communication System Components
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Networks• Set of devices connected by media links.
– E.g. Computer / Printer– Links connecting them also knows com. channels
• Network Criteria: Performance, Reliability, Security• Performance measured by
– Transit Time – Response Time– Depends upon
• No of users• Type of Transmission medium• Hardware• Software• Throughput and Delay
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Reliability• Frequency of failure• Recovery time of Network after failure• Catastrophe• Security
– Unauthorized access• Sensitive data must be protected• Lowest level, user name and password• At higher level encryption methods
– Viruses
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• Line configuration• Topology• Transmission mode• Categories of networks• Internetworks
Basic Concepts
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Line Configuration
• Two or more comm devices attached to a link• Link is a physical communicating path to transfer data
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Point – to – Point
• Dedicated link b/w two devices• Entire capacity reserved between two devices• Normally actual path line of wire but microwaves & satellites
links also possible– E.G Remote of a TV
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Multi Point Configuration• Also called multidrop• More than two devices share common link• Capacity of channel shared• If devices use link simultaneously
– Spatially shared-in terms of space– Time shared-devices take turns
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Topology• Way a Network is laid out physically or logically• Two or more devices connect to a link• Two or more links form a topology• Geometrical representation of relationship of all links
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Possible Relationships
• Peer to Peer : Devices share link equally
– Ring
– Mesh• Primary-Secondary :One device controls traffic
& other must transmit through it
– Star
– Tree
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Mesh Topology
• Every device connected with every device through dedicated link
• n(n-1)/ 2 - Physical channels• Every device on NW must have (n-1) I/O ports• Each connection carry its own data • Robust , Privacy & Security• Fault identification and isolation easy but Too many cables • Installation difficult and HW requirement expensive
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Mesh Topology
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Star Topology
• Pt – to - Pt link to a central controller called “hub”• Does not allow direct traffic but through hub• Less expensive• Robustness - if one device fails only it is effected,
all other remains active• Since each node must be linked to hub more
cabling is required• If hub fails whole network goes down.
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Star Topology
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Tree Topology• Variation of star• Not every device connects to central hub• Majority of devices connect to secondary hub which
connects to central hub• Central hub is active hub. It contains a repeater that
regenerates the received bit pattern before sending them out.
• Secondary hub may be active or passive. Passive hubs are simple physical connector.
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Tree Topology
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Bus Topology• Multipoint, Backbone - to link all the devices• Drop lines to connect the nodes to back bone• Tap is a connector that splices into main cable• Attenuation
– Limit on number of taps a bus can support and distance between two taps
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Bus Topology
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Bus• Advantages
– Easy installation– Less cabling required
• Disadvantages– Fault isolation difficult– Difficult to add new devices– Fault in bus stops all transfers
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Ring Topology• Pt – to - pt link• Each device has devices on either sides• Easy to install• Signal is passed in one direction• Each device incorporates repeater• Disadvantages
– Media and traffic consideration– Main ring length– Number of devices– Break in unidirectional ring can disable entire ring
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Ring Topology
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HYBRID TOPOLOGY
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SIMPLEXInformation flows in only one direction
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HALF-DUPLEXInformation flows in both directions, but only in one direction at a time.
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FULL-DUPLEXInformation flows in both directions at the same time
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LOCAL AREA NETWORK
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Local Area Networks• Smaller scope
– Building or small campus. – Usually owned by same organization that owns
the attached devices• Network maintenance is solely user responsibility• Data rates are much higher & usually in packets• Medium shared - only one node transmit at a time• Usually broadcast systems rather switching
approach • High speed switches are being introduced making
Higher data rates possible (Gigabits)• Run, installed by network administrators
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LOCAL AREA NETWORK
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MAN• Designed to extend over a city• May be a single Network such as cable TV
Network or no of LANs• Owner
– Private company or – Public such as Local Tel Company
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METROPOLITAN AREA NETWORK
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WIDE AREA NETWORK
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Wide Area Network• Across a continent (10km to 10000 km) or large
geographical area• In contrast to LANs may utilize public, leased or
private communication devices• WAN wholly owned by service providers around
the globe• Data Rate:
• Typically1.5 Mbits/sec to 2500 Mbits/sec• Run, installed by telephone companies (in most
cases)
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INTERNETWORK (INTERNET)
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The Internet
• Loosely administered network of networks• Agreed procedures for access and
intercommunication• Internetworking uses gateways, routers and
firewalls• Gateways: convert data traffic from one network
format to another. They link LANs to WANs and WANs to WANs
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Protocols • Set of rules that govern data communications. It
defines – What is communicated?– How it is communicated?– When it is communicated?
• key elements– Syntax - Structure or format of data– Semantics – Meaning of each section of bits– Timing - When data should be sent
• How fast it should be sent
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Standards• A standard provides model for development
– Ensure product to work regardless manufacturer– Essential for competition
• Data comm standards
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Defacto Standards• Defacto: Not approved by organized body but
adopted through widespread use• Proprietary
– Originally invented by commercial org for its products
– Also called closed standards• Nonproprietary
– Produced by groups and given to public domain– Also called open standards
• Dejure: Legislated by officially recognized body
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Standards Organizations• ISO• ITU-T• ANSI• IEEE• EIA• IETF• Regulatory Agencies (PTA in Pakistan)