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Lecture No. 8

Lecture No. 8. Communications. 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. Communications.

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Lecture No. 8

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  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. 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)

  5. 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

  6. 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 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.

  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

  9. 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 Senderisthe 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 …..

  10. 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

  11. 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

  12. 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 ?

  13. 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 ?)

  14. 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

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

  16. 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

  17. 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

  18. 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

  19. Line Configurations point to point multipoint

  20. 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)

  21. 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)

  22. Topologies - Star Network computer computer computer computer computer central computer Primary-secondary relationship

  23. Topologies - Tree Network mainframe node 6 node 5 node 7 node 9 node 8 node 3 node 4 Primary-secondary relationship node 1 node 2

  24. Topologies - Bus Network minicomputer minicomputer minicomputer minicomputer printer Either peer-to-peer or primary-secondary

  25. Topologies - Ring Network computer computer computer (Token Ring) computer computer Peer-to-peer relationship

  26. 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

  27. Transmission Modes Simplex - one-way only Half-Duplex Both ways, but only one way at a time Full-Duplex Both ways simultaneously

  28. 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)

  29. 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

  30. Medium Area Networks Amedium 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.

  31. 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

  32. 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

  33. R R R R R R R R InterNetworks LAN LAN R WAN LAN LAN LAN MAN indicates Routers

  34. 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

  35. 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

  36. 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

  37. Radio Communication Band Frequency RangeWavelength 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

  38. 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

  39. 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

  40. 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

  41. Analog / Digital Modulation digital signal analog signal computer modem analog signal digital modem signal computer

  42. 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

  43. I.S.D.N. Switched Voice Switched Fax and Text PABX PABX 2Mb Dedicated Tie Line 2Mb Common Channel Signalling ISDN NODE ISDN NODE Common Carrier - Telstra, Optus

  44. Satellite Communications satellite point to point link earth station earth station satellite multiple receivers transmitter

  45. 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)

  46. 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 ?

  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

  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.

  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.

  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.

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