HSC IPT - Communication Systems Notes PDF

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HSC IPT Topic 3: Communication Systems (HSC Mark 94)
Still relevant to new 2020 syllabus...

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INFORMATION PROCESSES AND TECHNOLOGY – COMMUNICATION SYSTEMS Characteristics of Communication Systems • Communication system are used in information systems to enable people to send and receive data and information o E.g. telephone, radio and television are communication systems that have had a big impact on our society o E.g. when we withdraw money from ATMs, we access an information system and a communication system § Data and information is transferred between an ATM terminal and the bank’s central computer. • All communication systems have five basic components o Data source – produces data to be sent § E.g. person speaking in to the microphone o Transmitter – encodes the data into a signal suitable for a transmission medium § E.g. microphone and other associated electronics that change sound into a signal § E.g. keyboard o Transmission medium – a channel in which the signal is transmitted to the destination § E.g. twisted pair and fibre optic cables § E.g. space between the transmitting and receiving antennae o Receiver – decodes the signal back into the original data or an approximation of the data § E.g. the radio that converts the signal into the original sounds o Destination – the receiver of the information § E.g. the person listening to the radio • When computers and other devices are connected in some way, they form a network. • Modem converts digital data into analogue signals to send through the telephone line, to another modem which converts the analogue signal back to digital data for user to understand Good Communication systems • Accurate, reliable and secure transmission medium • Minimal delay in communicating between systems • Dependent on protocols, handshaking, speed of transmission, error checking, communication settings Protocols • Communication systems need protocols for communication • Protocol – a set of rules that governs the transfer of data between computers o Defines how a link is established o Defines how the information is transmitted and how errors are detected o Two computers must use the same protocols when communicating otherwise the data transfer may be unsuccessful • Numerous protocols have been developed for specific technologies • Protocols change over time and are the basis for the development of a particular product • Standardisation of data communication protocols has been the focus of a major international effort for years Handshaking • Data transmission between two devices can only be successful if handshaking occurs • Handshaking is an agreement about which protocol to use to accomplish the exchange of information o Series of signals that flow between devices during data transmission to indicate the type of protocol used o Handshaking is needed between devices as they have many different capabilities and may transfer data in different ways § Refer to COMMUNICATION SETTINGS (below) • Transmitting device will send a signal and wait for an appropriate response o Successful handshake à connection is made o Unsuccessful handshake à devices ‘hang up’ and try again • Methods of handshaking o Hardware flow control à uses a dedicated connection, such as a wire § Only practical when devices are close enough to link with a cable § Common hardware protocol is RTS/CTS (Request To Send / Clear To Send) o Software flow control à uses a special code sent with the data § Used for long distance communications § Common software protocol is XON/XOFF (X stands for transmit) • If a break in transmission is needed, XOFF command is sent • If transmission is ready to start again, XON command is sent

INFORMATION PROCESSES AND TECHNOLOGY – COMMUNICATION SYSTEMS Speed of transmission • Speed of data transmission is determined by the transmitting device and the bandwidth • Bandwidth – capacity of the channel or transmission medium o High bandwidth can transfer more data § When cable television is transmitted through fibre-optic cables, many different channels can be transmitted at the same time • Unit of speed of data transfer o Bits per second (bps) – maximum number of bits that can be transmitted in one second (aka. bit rate) § This measure of speed includes special bits used in asynchronous transmission and any error checking bits Transmission Errors • Data transmitted can end up being different from the data received • Transmission errors can be made due to: o Interference from dust and weather conditions (rain and extreme temperatures) o Power surges caused by lightning strikes and blackouts o Electromagnetic interference from devices like air conditioners and refrigerators turning on and off o Radio interference from televisions and radios using similar signal frequencies o The data being passed through a large number of different systems and devices to reach you o Old or damaged cables and equipment being used to carry the signal Error Checking • Parity check o A character error detection method that adds an extra bit to every character of byte transmitted o Sending and receiving devices must agree on which parity checking method will be used § Odd à a parity bit of 1 or 0 is added to the byte to make the total an odd number of 1s. § Even à a parity bit of 1 or 0 is added to the byte to make the total an even number of 1s. o Receiving device counts the number of 1s in the data received, including the parity bit § Odd parity à if the number of 1s counted is odd, data received is assumed to be correct § Even parity à if the number of 1s counted is even, data received is assumed to be correct o ISSUES § Unreliable method of data error detection (detects only half the errors that can occur when a byte is transmitted) à not commonly used in modern communication links § Bit swapping cannot be detected • If a 1 is swapped with a 0 in the data byte being transmitted, the parity will remain the same and thus the error would not be detected • Check sum o Block error detection method à relies on dividing a message up into blocks or ‘packets’ of bytes and testing the accuracy of each block instead of byte § Speeds up data flow § More reliable than parity check

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Header B1 B2 B3 B4 Trailer To calculate the Sender and receiver info and 37 90 82 75 28 checksum: packet details Checksum § Add the data byte values (37+90+82+75 = 284) § Checksum MUST be a single byte value between 0 and 255, if the total is greater than 255, divide the total by 256 and use remainder as the checksum (284/256 = 1 r28) Both the sending and receiving computers calculate the checksum individually, if the two checksums are different, then an error is detected. § Checksum is added to the end/trailer of the data packet ISSUES § Unreliable method of data error detection for long message blocks or larger blocks of data (detects only 90% of the transmission errors) à no longer used in modern communication protocols § Cannot detect which byte or bytes in the block is inaccurate so the entire block must be retransmitted to fix the error § If errors in data still give the same checksum, error will not be detected

INFORMATION PROCESSES AND TECHNOLOGY – COMMUNICATION SYSTEMS Error Checking • Cyclic redundancy check (CRC) o Block error detection method that cannot detect which byte or bytes in the block is inaccurate o Greater reliability than checksum à now the preferred error detection method in many communication protocols o Both sending and receiving computer performs the CRC calculation and compares the answers to detect errors o 32-bit CRC is more reliable than 16-bit CRC (99.99999977% compared to 99.998% detection of all possible errors) o CRC treats the entire block of data as a single binary number which is then converted to decimal for CRC calculations by dividing it by: § 69665 for 16-bit CRC – preferred for short message blocks...