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CP2022 - Lecture 3

Computer communications 2

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Information quality and reliability A reliable communication implies

reliable information but.. Other factors influence its quality e.g.

Timeliness of the delivered information Speed or frequency of the information

provision Completeness of the information Selectivity of the information Relevance or specificity of the information

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Data compression

Data compression is the reduction of unstructured redundancy in data

Many schemes are used - For example:

JPEG images are compressed Fax documents are compressed Internet Audio is compressed

Compression ratios depend on scheme and redundancy

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Data compression 2

Two forms of compression Lossless and lossy LosslessLossless

Where the original data is recoverable Used for text, programs, and similar data

LossyLossy Where some information is lost used for some picture formats, sound &

video

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Communication systems

For systems to be effective they need to fit the purpose of the communication be able to transfer the amount of data

required Choice is not always available

Equipment can determine limits E.g. modem access limits data rate to 56 kbps

Compression + increasing system data rates multimedia communication

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Transmission Media

Various types are used Twisted Pair, Coaxial and Fibre

Twisted pair - can be unshielded or shielded

UTP - STP

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Transmission Media (2)

Central conductor

InsulatorsScreen conductor

Coaxial Cable

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Transmission Media (3)

Optical conductor

Optical claddingProtective plastic coating

Optical fibre - Uses laser light

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Characteristics of communication systems

Choice of system is determined by need Usually Capacity is over-riding concern

Pre-emptive choice can save future expense

E.g. using fibre now - allows later system upgrades

Actual system determines its characteristics

E.g. Typical system error rate determines the error control mechanism employed

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Typical systems

PSTN The telephone system

Designed for voice Data rate - up to 56 kbps

ISDN the digital system

Designed for Voice, data, fax etc. 2x64k bps channels =128Kbps minimum Can be aggregated for higher rates

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Systems

Cellular networks Two main digital systems GSM &PCN

Same technology - different frequencies used Now 3G also available

Data rates Data can be used simply at 9.6k or 14.4k bps GPRS can be faster (up to115kbps) EDGE claims up to 384kbps 3G phones claims speeds of 2Mbps

(asymmetric links – faster to handset than to server)

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Networks

Classified by size, technology and protocol Local Area Networks

Small area, High speed ring or bus, IEEE 802.X Wide Area Networks

Large area, slow speed sparsely connected, Internet, OSI or SNA

Metropolitan Area Networks City wide, fibre network, high speed technology.

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Local Area Networks

Under the control of a single authority e.g. a company or university

Typically a Ring, Star or Bus network Bus is commonest with IEEE 802.3 being

most common 10 Mbps protocol Higher speed networks

100baseT is now popular - uses hub concentrators/repeaters

Upgraded from older 10baseT networks Gigabit Ethernet also available (MI block)

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LAN topologies

Star

Bus

Ring

The three main topologies used in LANs

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LAN hub wiring

Hub

Computers

Bus

Typical 10 or 100 baseT configuration Star-like BUS system!

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Wide Area Networks

Typically sparsely connected nodes nodes e.g.

Network node

A

B

C

D

E

H

F

G

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High speed LANs

Standards at 100 –1000 Mbps Mostly Bus systems

can use similar cables as 10 Mbps but.. length and cable properties more critical

Other options include desktop integration of ATM at 155+ Mbps (switched)

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Switching

Switching allows full speed of network to individual computers

So 10 Mbps switched is 10Mbps per userMinimum expected speed = Minimum expected speed = Backbone Backbone

speedspeed

Number of Number of usersusers

E.g. For a 100 Mbps backbone with 24 users@10 Mbps

Minimum speed = 4.1 Mbps (not 10 Mbps)

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LAN standards

Local area network standards are made and published by IEEE (US organisation)

IEEE 802 series Examples:

802.3 - Ethernet (CSMA/CD bus systems)

802.5 - Token Ring 802.11 - Wireless networks 802.16 - Broadband Wireless MAN

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WAN standards

Most WANs now use Internet standards TCP/IP the underlying transport

protocols

Other network protocols have been used X.25 - Public switched networks SNA - IBM networks OSI - Government networks

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Narrow band restrictions

WANs share data links Total capacity in each link is shared Speed determined by slowest link Routing used to determine best route for

data Route choice needs to dynamically change

toadapt to network change

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Broadband systems

Increase in speed in WANs requires newer technology

ATM seems the popular choice Becoming more widely used

Allows 155+ Mbps switched links Simple technology - very high speed

switches Connection-oriented service Full speed end-to-end if required Useful for data-intensive multimedia

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The Internet

The Internet is the current network of choice for many users

A network of networks Partially administered by the Internet

Society (IAB, IETF) Becoming more regulated as its use

increases Access via Internet Service Providers

(ISPs)

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Summary

There are many ways to communicate between computers

Networks offer a mechanism for wider links

LANs, WANs and the Internet combine to give universal communication based onStandardStandard protocols

StandardStandard technologies

Standard Standard topologies