Lecture No. 8

1
Lecture No. 8
Communications
2
Lecture Objectives

• Today we are going to explore some of
  the aspects and features of
• COMPUTER BASED DATA COMMUNICATIONS
• and to obtain an overview of some of its
  forms, technology and terminology

3
Communications

• Telecommunications
• - Voice
• - Data
• - Images
• Telephone Radio Television Any others ?
• Teleprocessing
• Telecommunications Computer Based Processing
• Speeds Bits per second
• Frames per second
• Speed Bands Narrow Voice Broad

4
Communications

• There has been an very large increase in both
  the Demand and Provision of Communication
  facilities in the past 5 years.
• Australias A 1.2 billion link to the U.S.
  (Southern Cross Cable Network) is to provide a
  120 gigabits a second data capacity by the end of
  2000.
• The cable length is 29,000Km.
• Any problems ?
• And a link to Singapore has been developed
• The Applications driving these are multimedia
  and digital television, and the Customers are
• AAPT, Primus, OzEmail, CW Optus (?SingTel)

5
Communications
Transmission Options Asynchronous lt 2000
bps Synchronous gt 2000 bps Frame
transmissions and ATM (Asynchronous Transfer
Mode) are the current options. Methods
Packet and Packet Switching Public
Switched Network - Dial up network -
Leased lines Private networks Circuit
Switching Message Switching Packet
Switching Public Authorised Carrier -
Protocols, Standards
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Digital Data Transmission

• Asynchronous serial transmission
• Signal timing is unimportant.
• Information is received and translated by agreed
  patterns - the receiving device can retrieve
  information without worrying about the rhythm.
• Patterns are based on grouping the bit stream
  into bytes, and each byte is sent as a unit.
• The receiver cannot use timing to predict when
  the next group will arrive
• An extra bit is added to the beginning of each
  byte (normally 0) - known as the start bit.
  Another bit is added to the end of each byte
  (normally 1) - known as the stop bit.
• The receiver counts bits to ensure content

7
Digital Data Transmission

• Synchronous transmission.
• The bit stream is combined into frames of
  multiple bytes
• Each byte is sent without there being a gap
  between bytes
• Data is an unbroken stream of 0 and 1s
• There is no built-in device for synchronisation
• Timing is essential - the receiver has only the
  count of bytes
• Synchronous transmission is faster than
  asychronous transmission - It is more useful for
  high-speed applications (computer to computer
  links).
• The data link layer synchronises the bytes.

8
Communications

• System Components - there are 5

Step 1 Step 2 Step 3
Step 1 Step 2 Step 3
Protocol
Protocol
Message
Sender Medium
Receiver
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Communications

• The 5 components
• 1.The Message is the data (information) to be
  communicated. It can be text, numbers, pictures,
  sound, video - or any mix of these
• 2. The Sender is the device which sends the data.
  It can be a computer, workstation, telephone
  handset, video camera
• 3. The Receiver is the device which receives the
  message. It can be a computer, workstation,
  telephone handset, television ..

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Communications

• 4. The Medium is the physical path by which the
  message moves from the Sender to the Receiver. It
  can be a twisted wire pair, coaxial cable,
  fibre-optic cable, laser, or radio waves
  (terrestrial or satellite microwave)
• 5. The Protocol is a set of rules which govern
  data communication.
• It represents an agreement between the
  communicating devices.
• The protocol is essential to ensure that
  connected devices can also communicate - and with
  no confusion, or misinterpretations

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Components of a Data Communications System

4
COMPUTER
Comms Control Software
Applications Software Storage, etc
INPUT OUTPUT
1
5
2 2
interface
interface
3
communications
channels
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Communications - Networks

• A network is a set of devices (nodes) which are
  connected by media links.
• Typically a node can be a computer, a printer, or
  any other device which is capable of sending
  and/or receiving data generated by other nodes on
  the network. (the links are called communication
  channels).
• Networks use Distributed Processing. A task is
  divided amongst multiple computers. Instead of a
  single large machine being responsible for all
  aspects of a process, each separate computer
  (personal or workstation) handles a subset -
  electronic job sharing ?

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Communications - Networks

• What are the advantages of this ?
• 1. Security/encapsulation. The system designer
  (and manager) can limit the nature of of
  interactions of a given or all users)
• 2. Distributed Database. No one system needs to
  provide storage capacity for the entire database.
  Which is just as well, as the total information
  on the World Wide Web is beyond the capacity of
  any single device to store it.
• 3. Faster Problem Solving. Multiple computers
  working on parts of a problem can often result in
  the process being completed in a shorter time
  than a single machine could do
• (many computers make light work ?)

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Communications - Networks

• Network Criteria. There are some interesting
  aspects associated with networks.
• Data Communication
• Network Criteria

Performance Reliability Security
Factors Involved Number of Users Type of
transmission medium Hardware Software
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Communications - Networks

• Applications
• Marketing and Sales
• Financial services
• Manufacturing
• Electronic messaging
• Directory Services
• Information services
• Electronic data interchange
• Teleconferencing
• Cellular telephone
• Cable television

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Mobile and Wireless Applications

• Mobile Computing
• Personal Digital Assistants, cell phones -
  networked communications and applications
• Online transaction processing - order entry,
  customer account charge
• M-Commerce Internet access, Information
  searches, Collaborate and make joint decisions,
  but/sell from ???
• Retail - inventory detail, sales transactions
• Field service/sales - dispatching, online
  diagnostics, parts ordering
• Banking Purchasing, selling, inquiry, brokerage

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Standards Organisations

• On previous overheads (7 and 9), you saw the term
  protocol
• Some of the Standards creation Committees you
  will meet are
• The International Standards Organisation (ISO)
• The International Telecommunications Union
  (ITU-T, which was formerly the CCITT)
• The American National Standards Institute (ANSI)
• The Institute of Electrical and Electronics
  Engineers (IEEE, also known as I triple e)
  not to be confused with I Claudius
• Bellcore (Bell Communications Research)
• and a few others which popped up in the
  e-commerce lecture

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Some Concepts

• What are some of the relationships between
  communicating devices ?
• There are 5 general concepts
• 1. Line configuration
• 2. Topology
• 3. Transmission mode
• 4. Categories of networks
• 5. Internetworks

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Line Configurations
point to point
multipoint
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Topologies

• 2 relationships are possible in a topology
• (1) peer-to-peer (the devices share the
  link equally)
• (2) primary-secondary (one device controls
  traffic and the others must transmit
  through it)

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Bands and Speeds

• Narrow Band - Low Speed Channel
• Up to 300 bits per second
• (Telex, Low speed printers)
• Voice Band - Medium Speed channels
• Up to 4800 bits per second
• (Video Terminals, Microcomputers, Medium speed
  printers, )
• Can be conditioned to 9600 bps
• Broadband or High Speed Channels
• - 20,000bps to 100Mbps
• (high speed transmissions, Computer to
  Computer
• or High speed packed data
  transmission)

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Topologies - Star Network
computer
computer
computer

computer
computer
central computer
Primary-secondary relationship
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Topologies - Tree Network
mainframe
node 6
node 5
node 7
node 3
node 4
node 8
node 9
Primary-secondary relationship
node 1
node 2
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Topologies - Bus Network
minicomputer
minicomputer
minicomputer
minicomputer
printer
Either peer-to-peer or primary-secondary
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Topologies - Ring Network
computer
computer
computer
(Token Ring)
computer
computer
Peer-to-peer relationship
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Topologies - Mesh
Every device has a dedicated point- to-point link
with every other device. The link carries traffic
only between the 2 devices it links. There are
n(n-1)/2 physical links for n devices
Peer-to-peer relationship
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Transmission Modes
Simplex - one-way only Half-Duplex Both ways,
but only one way at a time Full-Duplex Both
ways simultaneously
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Categories of Networks

• There are 3 primary categories
• 1. Local area networks (LAN)
• 2. Metropolitan area networks(MAN) also known
  as Medium Area networks
• 3. Wide area networks (WAN)

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Local Area Network

• This is normally a privately owned network and
  links the devices in a single office, a building,
  a campus
• A LAN can be 2 computers and a printer (many
  users have this at home)
• It may extend through a Company and include
  voice, sound, and video peripherals.
• Generally the cable reach is about 2 kilometres
• It is common for one computer to be a server -
  which will store all of the software and user
  accounts
• Speeds are in the 4 to 16Mbps range. 100Mbps and
  nGbps are in sight

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Medium Area Networks

• A medium area network is normally designed to
  extend over a city. It can be a single network
  (cable television network) or it can be a means
  of connecting a number of LANS so that resources
  may be shared LAN to LAN, as well as device to
  device.
• A MAN may be wholly owned and operated by a
  private company, or it may be a service provided
  by a public company.

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Wide Area Networks

• These provide long-distance transmission of data,
  voice, image and video information over large
  geographical areas.
• These may comprise a country, continent or the
  world.
• WANS may utilise public, leased or private
  communications devices.
• They have an unlimited span.
• A WAN which is wholly owned and used by a single
  Company is known as an enterprise network

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InterNetworks

• Two or more networks can be connected.
• They then become an internetwork or internet.
• Individual networks are joined into the
  internetworks by internetworking devices.
• These devices include routers and gateways.
• The term Internet (notice the I) is the name of a
  specific worldwide network

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InterNetworks
LAN
LAN
R
WAN
LAN
LAN
LAN
MAN
indicates Routers
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Transmission Media

• Guided Media A conduit from one device to
  another
• Twisted pair Telephone wire
• Co-Axial Cable - Baseband
• all devices share 1 communications
  channel
• - Broadband
• provides 10 times the number of
• baseband cable
• Fibre Optics - Signals transmitted by light
  energy
• 12.5mm dia f.o. cable can carry up to
• 50,000 channels
• - expensive

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Transmission Media

• Unguided Media No physical conductor
• Communications Satellite
• - Ground collectors
• - Ground stations
• Microwave signals to 200 Mbits/second
• (expected increase to 1000 Mbits/second)
• Cellular radio
• - Divides geographic area into cells
• (e.g. suburbs)
• Each cell is serviced with a low power
• transmitter / receiver

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Frequencies

• A few short words about frequencies and
  wavelengths
• Frequency is measured in cycles per second
  (called hertz).
• Wave forms are described by their length and the
  common measures of wavelength are measured in
  kilometres, metres, millimeter and micrometers

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Radio Communication Band

• 
  Frequency Range Wavelength
• VLF Very low frequency 3KHz to 30 KHz 10km to
  100km
• LF Low frequency 30KHz to 300KHz 1km to 10km
• MF Middle frequency 300KHz to 3 MHz 100m to 1km
  
• HF High frequency 3MHz to 30MHz 10m to
  100m
• VHF Very high frequency 30MHz to 300MHz 1m to
  10m
• UHF Ultra high frequency 300MHz to 3GHz 10cm
  to 1m
• SHF Super high frequency 3GHz to 30GHz
  1cm to 10cm
• EHF Extremely high frequency 30GHz to 300GHz
  0.1mm to
• 
  10mm

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Radio Communication Band

• The characteristics of communications channels
  are (broadly)
• Bandwidth - which is the range of frequencies a
  circuit can support
• Transmission Rate - the number of bits per second
  which can be transmitted in a circuit
• Direction of flow - (simplex, half duplex,
  duplex)
• Medium or circuitry used - twisted pair,
  microwave, broadcast circuit
• Circuit Conditioning - reduction (or attenuation)
  - filtering processes - as in a stereo which
  filters out turntable rumbling below 50MHz. A
  10dB attenuation at 100MHz means that sounds are
  reduced by 10 decibel units in the 100MHz range

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Transmission Modes

• Analogue - Based on Telephone (voice) system
• Convertor needed to
• 1. change input device mode to channel carrier
  mode
• 2. change from channel carrier mode to receiving
  device mode
• Known as Modulating and Demodulating modem
• (amplitude modulation
• angle modulation - covers phase and
  frequency)
• Typical speeds are 9.6KBps, 14.4KBps, 28.8KBps,
  56KBps

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Modems

• Types
• stand alone
• board level - plug in
• acoustic coupler
• intelligent
• Intelligent Modem facilities
• Simultaneous voice and data transmission
• Transmission error detection
• Automatic dialup
• Conversion from ASCII to EBCDIC and reverse
• Automatic testing
• Automatic transmission speed selection

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Analog / Digital Modulation
digital signal
analog signal
computer modem
analog signal digital
modem signal
computer
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Transmission Modes

• Digital Digital Communication Networks
• offer - high speeds
• - low error rate
• - broad band (range of speeds)
• - no modems
• - accommodation of voice carrier systems
• I.S.D.N. Integrated Services Digital Networks
• Voice, Data, Image, Video, Text,
• It is a circuit switched network
• Telstra ISDN - Voice Data Text
• speeds from 2400bps top 64000 bps (64KBps)
• Hybrid channels offer 1536Kbps

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I.S.D.N.
Switched Voice
Switched Fax and Text
PABX
PABX
Dedicated Tie Line
2Mb
2Mb
Common Channel Signalling
ISDN NODE
ISDN NODE
Common Carrier - Telstra, Optus
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Satellite Communications
satellite
point to point link
earth station
earth station
satellite
multiple receivers
transmitter
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Satellite Communications

• 3 Satellite Constellations (microwave, straight
  line signal propogation)
• Low Earth Orbit (LEO) not synchronised
• Medium Earth Orbit (MEO) not synchronised
• Geosynchronous Earth Orbit (GEO) (35,200 km)

Mobile Satellite Systems Satellite Personal
Communication Systems Small portable handsets
with access to anywhere on Earth Estimated
100,000,000 waiting for telephone services (world
base)
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Satellite Communications

• Earth Stations
• High Capacity Stations - Costly, complex
• 
  Services communities large enough to require
• 
  feeder line access to the Earth Station
• Mid-Capacity SES - Used by Corporations
  for Private Networks
• 
  Videoconferencing, electronic mail, data,
• 
  Voice Services. Each earth station can be
• 
  connected to any other station in the network.
• VSAT Network - One Master Earth
  Station and many VSAT
• 
  terminals sharing the MES. Limited to data
• 
  transmissions, digital voice and digital video.
• Receive-Only Earth Station - Voice only, Data
  only, TV Receive only or a
• 
  combination.

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Satellite Communications
Gas Emissions
JUPITER
Time of Flight Approx 6 Earth Years
Galileo
Digital Images
Arrival Date 6/12/95
Definitely not to scale
.
Earth
.
A question Who has the Mars Lander ?
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So What is The Internet ?

• It is a complex web of networks.
• Each network can have millions of inter-connected
  computers which use telephone infrastructure to
  communicate
• No one owns the Internet - Its backbone,
  through which information flows, is owned by a
  number of private companies
• Messages are transmitted through the Internet via
  a computer language called transmission control
  protocol - TCP/IP.
• Each message travels as a packet and has a coded
  address which tells the network its destination
  and also has a block of data content

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So What is The Internet ?

• This packet is transmitted via variety of routes
• Data travels more efficiently using digital
  techniques
• Telstra has been (slowly) digitising its public
  switched telephone network
• How do home computers link to the Internet ?
• Normally via an Internet Server Provider (ISP) -
  a monthly fee is charged for connection and
  maintenance
• These ISPs are connected to a larger network
  service - Network Service Providers (NSPs) which
  are connected using fibre optic cable OR
  satellite links.

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So What is The Internet ?

• There are about 700 ISPs operating in Australia
• Telstra has a large share of the Internet
  wholesale market - others are CW Optus, OzEmail,
  connect.com
• Most ISPs are small and operate a single point of
  presence or POP - this is a location which
  comprises modems and a network connection into
  which an ISPs customers dial to make an Internet
  connection.

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So What is The Internet ?

• High speed Internet requires ISDN - but there is
  a catch !
• This is not possible (at present) as it cannot
  be provided between a customers premises and the
  exchange where the length of copper wire is
  greater than 4.5Km.
• Watch the outcomes of the Federal Governments
  legislation under the Telecommunications
  (Consumer Protections and Service Standards) Act
  1999
• Its a very interesting Act.

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This Might Help
Satellite
Telstra CW Optus OzEmail Net.com
Wholesale Internet Provider
Network Service Provider
NSP
NSP
NSP
Internet Service Provider
Modem and phone line link
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Communications Hardware

• Multiplexor
• single channel to carry simultaneous
  transmissions
• Communications Control Unit
• multiterminal use of channel
• code and decode
• error functions
• Concentrator
• microprocessor - collects and bundles data for
  high speed transmission to CPU and reverse
• Front End Processor
• large systems (number of terminals, users,
  processing)
• access controls - priorities
• network statistics - message routing

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Multiplexor
terminals
aaa
aaa
bbb cbacbacba
bbb
ccc
ccc
multiplexor multiplexor
computer
55
Concentrator
terminals concentrator

computer
buffer
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Front End Processor
terminals
concentrator
front end processor
concentrator
printer
disk unit computer
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Data Communications Software

• Includes Communications Access Programs
• establish connections between terminals and
  Computers AND the link between application
  programs and the communications network
• Network Control Programs
• A communications network function manager which
• Connects / Disconnects communications links
• Detects and corrects errors
• Polls terminals
• Queues data and program tasks
• Routes messages
• Logs statistics - errors, users, usage,
  facilities

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Network Architectures

• Combination of hardware and software used to
  implement the communications function
• Protocols set of conventions which govern the
  exchange of data between 2 entities
• Syntax - data format, coding, signal levels
• Semantics - control information for coordination
  and error handling
• Timing - speed matching and sequencing
• TCP Transmission Control Protocol
• IP Internet Protocol ---- gt TCP/IP -
  datagrams -
  65,535 bit packets)
• SNA, Novell IPX/SPX, Decnet
• TELNET Provides a simple asynchronous terminal
  capability

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Network Operating Systems

• Novell Netware (Versions 4.01)
• Banyan Vines
• 3 COM
• EasyLan
• Network OS

60
Distributed Systems

• 
  Processing
• Distributed
  Data Base
• 
  Controls
• Distributed Processing Processes (jobs) to have
  processing capability at a number of sites
• Some Options Permanent, temporary Linkage
• Controls required to Ensure
• 1. INTEGRITY 2. AVAILABILITY 3.
  SECURITY
• 4. RECOVERY 5. COMPATABILITY 6.
  CONTINUITY

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Communications Interface Functions

• Error monitoring, Diagnosis and Correction
• Modulation / Demodulation
• Data Compression
• Data Coding and Decoding
• Protocol Conversion
• Message Switching
• Buffer Storage
• Port Contention
• Concentration

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Communications Interface Functions

• Handshaking The exchange of Predetermined
  Signals and
• characters in order to establish
  connection
• between the two data communication
  terminals
• PROTOCOL LAYER INFORMATION
• Data Link Control Information
• Packet Transport Control Information
• Switching Network Services Control
  Information
• Protocol Network Application Control
  Information
• Layers USER DATA
• Data Link Control Information

63
SNA Communications Layers

• Layer No. Function
• 5 Translates data between the formats used for
• communications and those used for the
  applications programs
• 4,3 Concerned with the mechanics of controlling
• data flow between the source and destination
• 2 Responsible for routing data between the
  source and destination
• 1 Responsible fore ensuring successful and
  orderly transmission of data of some type of
  data link
• (light wave, satellite, twisted pair)

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Communications Example1
signal controllers
Time Division Multiplexor
modem 1
300 bps dedicated
modem 2
Ballarat
modem 3
9600 bps
DDS Line
modem n
23 regional computer systems 1600 traffic signal
controllers
Footscray Regional Computer
1995
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Communications Example

• City Link and the Tollway system
• Possibly the Mitcham-Frankston Tollway ?
• - Pick up of E-tag at designated points
• - Registration numbers videod and
  converted to
  digital image
• - E-tags and registrations linked to
• owner details
• - Debiting system for E-tags
• - Day trippers and other pass forms
• - Reconciliation system for others
• - Detection of unauthorised users
• and follow up

66
Communications

• Telstra services and e-commerce
• A 9600bps connection ISP time-charged on time
  taken to down load - e.g. image.
• The result very slow and costly performance
• In 1985/8, Telstra introduced the pair-gain
  multiplexer facility over a single cable
  facility. (phone to exchange line ratio
• is 4 1)
• This means that instead of each connection having
  a copper wire to an exchange, 4 connections are
  fed into one cable

67
Communications

• The IP (Internet Protocol) requires as much
  bandwidth to an exchange as is available - with
  minimum multiplexers
• Normally, 8 phone connection wires enter a pair
  gain multiplexer - and 2 wires go back to the
  exchange
• In practice this means that a 56,000bps modem can
  only provide at a maximum 28,800 bps
• A 3rd user will achieve 9600bps (28,800/3
  9600)
• A 4th user would achieve 28,800/4 7,200bps

68
Communications

• You have probably heard of
• CDMA Code Division Multiple Access (useful for
  mobile phones as a substitute for Analog
  transmission)
• and the latest development
• ASDL Asymmetric Digital Subscriber Line (uses
  1KHz to 4KHz for telephone service and 20KHz to
  2.2MHz for data transfer over copper wire.)

69
Some Terms

• A Network - A facility which provides a data
  transfer service among stations attached to a
  network
• An Internet A collection of communication
  networks interconnected by bridges or routers
• A subnetwork A member unit of a network.
• IWU - Interworking Unit - A device used to
  connect two subnetworks and which permits
  communication between end systems attached to
  different subnetworks
• Bridge An Interworking Unit which is used to
  connect two LANs which use identical protocols.
  No data modifications are incurred
• Router A device used to connect two networks
  which need not be similar

70
Definitions - Medical

• Artery - The study of Art
• Caesarean Section - A neighbourhood of Rome
• Colic - A sheep dog
• Dilate - To live a long time
• Fester - Quicker
• Labour Pain - Getting hurt at work
• Outpatient - A person who has fainted
• Node - Was aware of
• Recovery Room - A place to do upholstery
• Seizure - A Roman Emperor
• Postoperative - A letter carrier
• Sciatic - An attic with a view of the sky

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