Lecture No. 8
Communications
CSE1720 Summer 2005 Lecture 8 / 1
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
CSE1720 Summer 2005 Lecture 8 / 2
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
CSE1720 Summer 2005 Lecture 8 / 3
Communications
There has been an very large increase in both the Demand
and Provision of Communication facilities in the past 5
years.
Australia’s $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, C&W Optus (?SingTel)
CSE1720 Summer 2005 Lecture 8 / 4
Communications
Transmission Options:
Asynchronous < 2000 bps
Synchronous
> 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
CSE1720 Summer 2005 Lecture 8 / 5
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
CSE1720 Summer 2005 Lecture 8 / 6
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 1’s
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.
CSE1720 Summer 2005 Lecture 8 / 7
Communications
System Components - there are 5
Step 1
Step 2
Step 3
…………
Step 1
Step 2
Step 3
…………
Protocol
Protocol
Message
Sender
Medium
Receiver
CSE1720 Summer 2005 Lecture 8 / 8
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 …..
CSE1720 Summer 2005 Lecture 8 / 9
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
CSE1720 Summer 2005 Lecture 8 / 10
Components of a Data Communications System
4
COMPUTER
Comms Control
Software
INPUT
1
OUTPUT
5
2
2
interface
Applications Software
interface
3
Storage, etc
communications
channels
CSE1720 Summer 2005 Lecture 8 / 11
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 ?
CSE1720 Summer 2005 Lecture 8 / 12
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 ?)
CSE1720 Summer 2005 Lecture 8 / 13
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
CSE1720 Summer 2005 Lecture 8 / 14
Communications - Networks
Applications
•
•
•
•
•
•
•
•
•
•
Marketing and Sales
Financial services
Manufacturing
Electronic messaging
Directory Services
Information services
Electronic data interchange
Teleconferencing
Cellular telephone
Cable television
CSE1720 Summer 2005 Lecture 8 / 15
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
CSE1720 Summer 2005 Lecture 8 / 16
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
CSE1720 Summer 2005 Lecture 8 / 17
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
CSE1720 Summer 2005 Lecture 8 / 18
Line Configurations
point to point
multipoint
CSE1720 Summer 2005 Lecture 8 / 19
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)
CSE1720 Summer 2005 Lecture 8 / 20
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)
CSE1720 Summer 2005 Lecture 8 / 21
Topologies - Star Network
computer
computer
computer
computer
computer
central computer
Primary-secondary relationship
CSE1720 Summer 2005 Lecture 8 / 22
Topologies - Tree Network
mainframe
node 6
node 5
node 9
node 8
Primary-secondary
relationship
node 7
node 3
node 2
node 4
node 1
CSE1720 Summer 2005 Lecture 8 / 23
Topologies - Bus Network
minicomputer
minicomputer
minicomputer
minicomputer
printer
Either peer-to-peer or primary-secondary
CSE1720 Summer 2005 Lecture 8 / 24
Topologies - Ring Network
computer
computer
computer
(Token Ring)
computer
computer
Peer-to-peer relationship
CSE1720 Summer 2005 Lecture 8 / 25
Topologies - Mesh
Peer-to-peer relationship
Every device has
a dedicated pointto-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
CSE1720 Summer 2005 Lecture 8 / 26
Transmission Modes
Simplex - one-way only
Half-Duplex
Both ways, but only one
way at a time
Full-Duplex
Both ways simultaneously
CSE1720 Summer 2005 Lecture 8 / 27
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)
CSE1720 Summer 2005 Lecture 8 / 28
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
CSE1720 Summer 2005 Lecture 8 / 29
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.
CSE1720 Summer 2005 Lecture 8 / 30
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
CSE1720 Summer 2005 Lecture 8 / 31
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
CSE1720 Summer 2005 Lecture 8 / 32
InterNetworks
R
LAN
LAN
R
R
R
WAN
LAN
R
LAN
R
R
LAN
R
MAN
R indicates Routers
CSE1720 Summer 2005 Lecture 8 / 33
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
CSE1720 Summer 2005 Lecture 8 / 34
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
CSE1720 Summer 2005 Lecture 8 / 35
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
CSE1720 Summer 2005 Lecture 8 / 36
Radio Communication Band
Frequency Range Wavelength
3KHz to 30 KHz 10km to 100km
30KHz to 300KHz 1km to 10km
300KHz to 3 MHz 100m to 1km
3MHz to 30MHz
10m to 100m
VLF
LF
MF
HF
Very low frequency
Low frequency
Middle frequency
High frequency
VHF
UHF
SHF
EHF
Very high frequency 30MHz to 300MHz 1m to 10m
Ultra high frequency 300MHz to 3GHz
10cm to 1m
Super high frequency 3GHz to 30GHz
1cm to 10cm
Extremely high frequency 30GHz to 300GHz 0.1mm to
10mm
CSE1720 Summer 2005 Lecture 8 / 37
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
CSE1720 Summer 2005 Lecture 8 / 38
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
CSE1720 Summer 2005 Lecture 8 / 39
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 selectionCSE1720 Summer 2005 Lecture 8 / 40
Analog / Digital Modulation
digital
signal
computer
analog
signal
modem
analog signal
modem
digital
signal
computer
CSE1720 Summer 2005 Lecture 8 / 41
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
CSE1720 Summer 2005 Lecture 8 / 42
I.S.D.N.
Switched
Voice
Switched Fax
and Text
PABX
PABX
2Mb
Dedicated
Tie Line
ISDN
NODE
Common
Channel Signalling
2Mb
ISDN
NODE
Common Carrier - Telstra, Optus
CSE1720 Summer 2005 Lecture 8 / 43
Satellite Communications
satellite
point to point link
earth station
earth station
satellite
multiple receivers
transmitter
CSE1720 Summer 2005 Lecture 8 / 44
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)
CSE1720 Summer 2005 Lecture 8 / 45
Satellite Communications
Gas Emissions
JUPITER
Time of Flight Approx 6 Earth Years
Galileo
Digital Images
Arrival Date 6/12/95
Definitely not to scale
.
A question : Who has the Mars Lander ?
Earth
.
CSE1720 Summer 2005 Lecture 8 / 47
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
CSE1720 Summer 2005 Lecture 8 / 48
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 ISP’s are connected to a larger network service Network Service Providers (NSP’s) which are connected
using fibre optic cable OR satellite links.
CSE1720 Summer 2005 Lecture 8 / 49
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 C&W 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 ISP’s customers dial to
make an Internet connection.
CSE1720 Summer 2005 Lecture 8 / 50
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 customer’s premises and the exchange where
the length of copper wire is greater than 4.5Km.
• Watch the outcomes of the Federal Government’s legislation
under the Telecommunications (Consumer Protections and
Service Standards) Act 1999
• It’s a very interesting Act.
CSE1720 Summer 2005 Lecture 8 / 51
This Might Help
Satellite
Wholesale
Internet
Provider
NSP
NSP
Telstra
C&W Optus
OzEmail
Net.com
NSP
Network
Service
Provider
Internet Service Provider
Modem and phone line link
CSE1720 Summer 2005 Lecture 8 / 52
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
CSE1720 Summer 2005 Lecture 8 / 53
Multiplexor
terminals
aaa
aaa
bbb
cbacbacba
bbb
ccc
ccc
multiplexor
multiplexor
computer
CSE1720 Summer 2005 Lecture 8 / 54
Concentrator
terminals
concentrator
computer
buffer
CSE1720 Summer 2005 Lecture 8 / 55
Front End Processor
terminals
concentrator
front end
processor
concentrator
printer
disk unit
computer
CSE1720 Summer 2005 Lecture 8 / 56
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
CSE1720 Summer 2005 Lecture 8 / 57
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 ---- > TCP/IP - datagrams 65,535 bit packets)
SNA, Novell IPX/SPX, Decnet
TELNET Provides a simple asynchronous terminal
capability
CSE1720 Summer 2005 Lecture 8 / 58
Network Operating Systems
Novell Netware (Versions … 4.01)
Banyan Vines
3 COM
EasyLan
Network OS
CSE1720 Summer 2005 Lecture 8 / 59
Distributed Systems
Distributed
Processing
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
CSE1720 Summer 2005 Lecture 8 / 60
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
CSE1720 Summer 2005 Lecture 8 / 61
Communications Example1
signal
controllers
modem 1
Time Division
Multiplexor
300 bps dedicated
modem 2
Ballarat
modem 3
9600 bps
modem n
23 regional computer systems
1600 traffic signal controllers
1995
DDS Line
Footscray
Regional Computer
CSE1720 Summer 2005 Lecture 8 / 64
Communications Example
City Link and the Tollway system
Possibly the Mitcham-Frankston Tollway ?
- Pick up of E-tag at designated points
- Registration numbers video’d 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
CSE1720 Summer 2005 Lecture 8 / 65
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
CSE1720 Summer 2005 Lecture 8 / 66
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
CSE1720 Summer 2005 Lecture 8 / 67
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.)
CSE1720 Summer 2005 Lecture 8 / 68
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
CSE1720 Summer 2005 Lecture 8 / 69
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
CSE1720 Summer 2005 Lecture 8 / 70
Some things to do during
the (short) break until Monday!!
CSE1720 Summer 2005 Lecture 8 / 71