Transcript CP2022 - Lecture 2 Computer communications 1

CP2022 - Lecture 2
Computer communications 1
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Introduction
• Information technology
– The convergence of computer systems and
communications technologies
• Now includes
– The Internet and World Wide Web
– PCs, mini-computers, mainframes
– The telephone network, ISDN, other networks
• How does it all work?
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Electronic communication
fundamentals
• What distinguishes new technology from
old?
• How is a system evaluated for
effectiveness?
• Which system is best to use in a particular
application?
• Which method is most cost effective?
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Computer Communication
principles
Computer communication is based on the
principles outlined in Lecture 1:
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Standards
Protocols
Error control, redundancy and accuracy
Channel
Context
Coding
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Analogue and digital signals
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Traditional systems (telephone, radio, TV) are all analogue
Computers use digital signals
Analogue = Continuous signals - Infinite levels
Digital = Discrete signals - Discrete levels
Digital signal
Analogue signal
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Analogue to digital conversion
• To use analogue channels (telephone, radio) digital
signals need conversion
• Digital channels need only format changes
– For example, voltage change and/or message
structuring
• Analogue input (e.g. video signals) require A to D
conversion
• Analogue output (e.g.. sound) requires D to A
conversion
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A to D sampling
Volts
Sampling levels
Time
Sampling interval
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Using the telephone system
POTS (Plain Old Telephone System) or
PSTN (Public Switched Telephone Network)
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Analogue system (at the end-points)
designed for voice
limited capacity for data
needs D-A/A-D conversion
• done by Modems
– MODEM - MODulator/DEModulator
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PSTN and Modem use
Digital link
Computer
Digital link
Computer
PSTN
Modem
Modem
Analogue links
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Capacity and Bandwidth
• Digital channels have a limit on the amount
of data that can be transmitted
• The channel’s Capacity
• On analogue channels similar restrictions
exist
• This is known as the channel’s Bandwidth
• These terms are now used interchangeably
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Multiplexing
• In many networks individual channels are
used by more than one communication
– different applications
– different users
– different data /media types
• This requires multiplexing
The use of a single channel by more than
one communication
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Multiplexing (2)
A
B
Multiplexer
A,B,C,D
C
D
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Multiplexing (3)
• Frequency Division Multiplexing
– Channel divided by frequency
– Limited by total bandwidth
• Time Division Multiplexing
– Channel divide in time
– Each use allotted a slot
– 1,2,3,4,1,2,3,4,etc.
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Protocols and error control
• Communication needs a structure this is a
Protocol
• A protocol needs to satisfy 8 principles
(see next slide)
• Protocols
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Provide framework for the communication
Allow error control mechanisms to be used
Monitor the communication
Provide communication of all data types
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Principles of protocol design
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Message format
Error control
Acknowledgements
Time-outs and re-tries
Sequence control
Flow control
Recovery
Data transparency
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Standards
• Communication and protocols are largely
determined by
Standards
• These are decided by expert groups,
manufacturers, user groups, international
committees etc.
• Use of standards simplifies the
communication process
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Standard models
• Standard models are used to simplify the
design of communication systems
• The Internet uses a four layer model
(or five including the physical layer)
• OSI is a seven layer model developed by
the ISO and ITU-T
– International Standards Organisation
– International Telecommunications Union
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Internet and OSI
communication models
Internet
OSI
Application
Application
services
Presentation
Session
Transmission
control
Internet
protocol
Transport
Network
Network
access
Data link
Physical
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Summary
• Computer communication is based on the
same principles as human communication
• Information can be analogue or digital
• Channels have capacity
• and can be multiplexed
• Protocols need to be defined
• Standards simplify the use of systems
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