Transcript CP2022 - Lecture 2 Computer communications 1

CP2022 - Lecture 2
Computer communications 1
CP2022- Lecture 2
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Introduction
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Information technology
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Now includes
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The convergence of computer systems and
communications technologies
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
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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
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To use analogue channels (telephone, radio)
digital signals need conversion
Digital channels need only format changes
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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
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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
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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
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In many networks individual channels are
used by more than one communication
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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)
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Frequency Division Multiplexing
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Channel divided by frequency
Limited by total bandwidth
Time Division Multiplexing
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Channel divide in time
Each use allotted a slot
1,2,3,4,1,2,3,4,etc.
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Broadband
Digital link
Computer
Broadband
Modem
BT
Wall socket
Filter
Splitter
Internet
Telephone
exchange
Telephone
Analogue links
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PSTN
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Broadband technology
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Uses a carrier to modulate digital signal
over telephone line in addition to voice
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Effectively a form of FDM
Works over an analogue PSTN line
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Computer has a digital link
Telephone has an analogue link
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Protocols and error control
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Communication needs a structure this is a
Protocol
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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
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Communication and protocols are
largely determined by
Standards
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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
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Standard models are used to simplify
the design of communication systems
The Internet uses a four layer model
(or five including the physical layer)
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OSI is a seven layer model developed
by the ISO and ITU-T
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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
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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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