Communication Systems - aceh.b-cdn.net



Communication Systems45720014859000Enable users to send and receive data and information Functions performed within the communication systems in passing messages between source and destination: message creation organisation of packets at the interface between source and transmitter signal generation by the transmitter transmission Transmission speed is the time taken to transmit data from source to destination (bps). To calculate:Determine the size of the file being transferredDetermine the data transfer rateConvert the measurements into one unitCross multiply Bandwidth is the amount of data that can be transferred simultaneouslyBaud rate is the number of times per second the transmission medium changes state Parallel transmission (data bits are transferred simultaneously). Only suitable for short distances because of data skew.Serial Transmission (data bits are transferred sequentially, one at a time).Asynchronous – individual characters are sent with stop and start bits so receiving terminal knows difference between characters Synchronous – blocks of data are sent in which sending and receiving terminals use clocks to ensure data is transferred at same rate. Less stop and start bits used. Simplex (transmission in one direction only)Half duplex (in both directions, but not simultaneously)Full duplex (in both directions simultaneously)synchronising the exchange addressing and routing error detection and correction Retransmission – costly and wastes time if entire file is resentSymbol substitution – replaces the character containing the error with a symbolsecurity and management Noise is any interference during transmission Roles of protocols in communicationBefore communication can begin, ‘handshaking’ must occur between communicating devices. Signals are exchanged to define protocols so the transmitter and receiver are munication systems have different layers – protocols have functions in these layers e.g. Post Office Protocol (retrieves e-mail); File Transfer Protocol (log onto another server through the internet to download files to your computer) The client server model Role of the server: provide services in a networkPrint server – control files to be printedMail server – route emailsWeb servers – store web pages, cache websites, act as internet filterFile server – provide fast access to multiple filesRole of the client (workstation): allow users access to services provided by server. A request is sent by client software to the operating system on the server. Web browsers Mail clientsAdvantage: centralisationOnly authorised clients are granted access to certain data (security) Updates do not need to be made to each client (just the server)Disadvantages Simultaneous client requests can overload serversIf a critical server fails, no client requests can be fulfilledThin clients – rely on the server for their processingFat clients – perform the bulk of the processing themselves (as software is installed on each client). Examples of communication systems Teleconferencing systems People in different geographical locations can have a meeting using their telephones (distance education; business conference)For face-to-face communication, video conferencing is usedAdvantages: avoids expensive travel; worldwide connectivityDisadvantage: requires high speed connection Information technology: digital video camera, microphone, modem, speaker, fast processorsThe video and audio data is captured, digitised (by assigning a number to each pixel on the image to represent its colour), transmitted over the internet, and then decoded when received. Messaging systems Voice mailAudio messages are recorded and then transmitted by telephone system for storage (when someone is unavailable)Advantages: send and receive brief messages; security features (dial into system, enter account number and password)Information technology: telephone, voice mail accountEmail Use computers to send and receive text messages and attached documents (used in schools to send notices and make appointments)Advantages: fast; cheapDisadvantages: viruses and spamInformation technology: email address, email software, web browser, computer networked to internet Voice over Internet Protocol Make voice calls over a broadband internet connection Advantages: no charge for call; uses packet switching (more efficient as several calls can occupy the same amount of space as one call in circuit switching)Disadvantage: distorted if packet lossInformation technology: computer networked to internet, microphone, speaker, VOIP softwareSound waves are recorded by microphone, digitised by sampling, sent to the router which transmits the packets and then the receiving computer decodes transmission to sound waves. Systems dependent on communication technologyE-commerce (less labour costs, less paper work, wider market) EFTPOS Employee swipes card in a magnetic strip reader. Customer enters password. Card details collected and funds transferred from customer to retailer’s account. Advantage: no cash handled; secureDisadvantage: not practical for small purchasesElectronic banking (ATMs and internet banking)Advantage: convenient (don’t need to go to bank); secureDisadvantage: phishing scams; can lose bankcard/forget passwordInformation technology: telephone lines, bank’s database Transmitting and Receiving Wired transmission mediaTwisted pair (thin insulated copper wires with twists to reduce noise)Advantage: Cheap; high bandwidth; fast over short distance; installation easy (small connector size)Disadvantage: slow over long distancesCoaxial cable (thick copper wire surrounded by insulation)Advantage: higher bandwidth than twisted pair; better shielded from noiseDisadvantage: more expensiveOptic fibre (thin strands of glass fibre internally reflecting light pulses)Advantage: very high transmission speed and bandwidth; not affected by corrosion or noiseDisadvantage: very expensiveWireless transmission media Microwaves and radio waves (wireless signals in air)Advantage: high bandwidth over longer distanceDisadvantage: affected by thunderstorms and needs repeater stationsSatellite (object in space functioning as a repeater)Advantage: very high bandwidth over very long distancesDisadvantage: very expensive to launch and need satellite dishes on Earth Infrared (wireless signals in air)Advantage: add devices without cablingDisadvantage: only for very short distance transmission (signal cannot penetrate walls) Communication Protocols Application level protocols HTTP (hypertext transfer protocol) – used for sending request to download hypertext files (from browser to server)SMTP (simple mail transfer protocol) – used for sending emailSSL (secure sockets layer) – used for encrypting data for secure transmissionCommunication control and addressing level protocolsTCP (transmission control protocol) – responsible for breaking data down into packets and reassembling packets when receivedIP (internet protocol) – responsible for routing packet to correct destination Transmission level protocols Ethernet – each node listens for quiet period before transmitting (Carrier Sense). If no signal, each node has equal chance to transmit (Multiple Access). If two nodes transmit at same time, they must detect that fact and quit (collision detection).Token Ring – string of bits which signifies permission to transmit. Token Ring continuously passed around the network until a node wishing to transmit captures token ring and replaces it with its message. If message is long, only part of it will be sent – ensuring each node has equal chance for transmission. Network Topologies Star network (one central node radiates distant nodes)Messages pass through central nodeAdvantage: fast if central computer fast; non-centralised failure doesn’t affect networkDisadvantage: Network won’t work if central node breaks down and central node needs upgrade as more nodes added Bus network (several nodes connected to one central cable)For temporary or small networkAdvantage: cheaper; easy setup and trouble shootDisadvantage: only one computer can transmit at a time to avoid collision and network won’t operate with broken bus line or terminator at cable endsRing (nodes are points on a circumference of a circle)Message sent in one direction around the ringAdvantage: cheaper; no collisionsDisadvantage: network will not work if any part of the ring is disconnectedWireless (nodes are connected using radio frequencies)When cabling is not possible or inconvenientAdvantage: gives users mobility; no drilling Disadvantage: quality of transmission not high (disconnections common); signal can be hacked; signal not available over wide area; devices require wireless adapter cards Hybrid (made up of many topologies)Disadvantage: difficult to trouble shoot as network is complicated Functions performed by hardware components in communication systems Hubs – multiport device allowing many computers to be connected to one network pointPasses network traffic to all connected devices; cheaper Switches – multiport device allowing many computers to be connected to one network pointPasses traffic directly from source to intended receiver (reducing bandwidth competition and improving security); more expensive Routers – directs data traffic between different networksSelects best path on network to route data (efficient)Modems – converts between digital and analog data to allow transmission over phone lines for internet usersBridges – connect two LANs with the same protocol into one networkGateways – connect networks which use different protocols by translating data in one protocol to another (important for internet) Network interface cards – links the computer processor to the networkMobile phones – uses radio waves to connect to telephone networkProvides portable access to text and video messaging, e-commerce and social networking sitesCables – directly connects pieces of hardware into a networkWireless access points – hardware allowing wireless communication devices to be connected to a wireless networkAdvantage: devices can be added without cabling; used by businesses to attract customers (as they provide free internet)Disadvantage: small range for wireless signal; suffers interference from bad weather. Bluetooth protocol – enables communication between Bluetooth enabled devices such as mobiles and laptopsAdvantage: devices can be added without cablingDisadvantage: small range for wireless signal Network operating software Allows all of the hardware and software in the network to function as an organised system.It performs file management, administration and security like operating systems on individual workstations, but also:Performs these services in a complex environment, running many tasks simultaneously for different users on multiple machinesControls recovery when entire network goes downEstablishes links between network nodesInternetIntranetExtranetInternational public network allowing any individual accessPrivate corporate network allowing access to individuals only within the organisationPrivate corporate network allowing access to individuals within the organisation and secure access to those outside but related to the organisationMany LANsOne LANOne LAN and external networkUses browser software to display HTML documentsAllows for sharing of information between users Other information processes Collecting and displaying The phone acts as the collection and display device in voice mailEFTPOS terminal acts as collection and display device for electronic bankingBrowser software collects text, sound and graphics in email and also displays web pages. Processing Encoding and decoding analog and digital signals Digital data is stored in two possible states (1 or 0)Analog data is represented as waves and has continuously variable statesCommunication systems use both analog and digital data. For this reason, it is often necessary to encode and decode data. Modulation: Digital to analogDemodulation: Analog to digital Formation of data packets Convert data into smaller units (packets) Add control characters to packet: header (source and destination) and tail (error checking codes). Routing In packet switching, a permanent link does not need to be open between transmitting devices as packets are routed through different routes. This improves security (no single link carries entire message); and is more efficient (many devices can use the one link)Encryption and decryption (for data security)Error checking Parity: an additional bit is added to a byte. Only works if an odd number of errors otherwise parity will be detected as correct.Checksum: sum of data bits in a packet. Used to compare sent and received data to ensure checksum matches. Cyclic Redundancy Check: checksum number is divided by a constant and the remainder sent to the receiver which performs the same calculation. If remainders do not match, there is an error. Managing communication systemsNetwork administration tasks: adding/removing users client installationinstallation of software assigning users to printersgiving users file access rightsmanaging back-ups and security logon and logoff proceduresWhere user enters username and password to validate access. A user session is opened and the user can access resources they have rights to. Log-off disconnects that user form network – important for security as open network can be used by unauthorised users. Issues in communication systems Teleconferencing systemsChanging the nature of work as individuals around the world can work on a project and share informationMay lose interpersonal skills as communication via microphones and speakers Messaging systems Cyber bullyingMisinterpretation (may not understand abbreviation, cannot see facial expressions)PrivacyEmployers have rights to read employee emails sent from workSocial networking companies may sell user information without user’s knowledgeChild grooming for sexual assault (using fake Facebook accounts)Diminishing of interpersonal skills E-commerceHacking Loss of jobs as machine replace humansGlobalisation: access to unreliable sellers – loss of moneyElectronic banking Jobs and bank branches close as ATM machines replace bank tellersExtra costs for businesses who need to train staff in ITE-crime more common (phishing scams, key logging)Issues related to communication systems The need for ethical behaviour when using the internetDo not attack others through insulting messages – cyber bullyingDo not copy and paste information without proper referencing – plagiarism Do not impersonate others to extract personal data – follow netiquette These behaviours make the internet safe and enjoyable for everyone.Issues that have arisen from internet useAvailability of material normally restrictedInternet saturated with pornography and children with internet access can easily view sites if no filter (leading to sexualisation of children)Internet used to spread child pornography in file sharing sites (abuse of children)Australian Government implementing a mandatory filter: will this restrict material that is legally available? Domination of content and control of access to internet Previously, large media organisations decided what news people would receive. But individuals have greater control and power through blogs – where they can report news.Access to internet controlled by a few large companies. There is less competition, leading to higher internet prices, which limits access to the poor. Changing nature of work (working from home)Advantage Flexibility over when to work when productivity is high and when it suits private life (taking care of children)Save costs on travelAllows disabled and rural workers to find jobsReduced office needs for employersDisadvantage Need self motivated staff who can cope with isolationVirtual communities Network of individuals who pursue similar interests e.g. forums created in Facebook and online dating sitesAdvantage: people from around the world can communicateDisadvantage: Young people can become addictedLoss of social skillsCurrent and emerging trends in communications BlogsWikisOnline personal journal which other users can read (entries are dated and has a journal design)An internet resource allowing individuals to seek answers to questionsOnly updated by blog ownerCan be updated by any internet user with a wiki accountGives individual the power of the media as they can share opinion – but is the information accurate?Powerful educational resource – but is the information accurate? RSS feeds Delivers changing web content to users Podcasts (Uploading an MP3 file to a webpage)Cheap to create and can be downloaded for listening anywhere, anytime. Online TV, radio Stream live broadcastsVideo on demandPay TV service3G technologies for wireless mobile communication Higher speed internet and better multimedia access but expensive and need compatible device. ................
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