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12 ipt 0301: Characteristics of a Communication System

HSC IPT Course - Topic 3.1

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12 ipt 0301: Characteristics of a Communication System

  1. 1. Communication SystemsCommunication Systems 3.1) Characteristics of a Communication System
  2. 2. • There are five main parts of any communication system: • a data source • a transmitter for encoding • a transmission medium • a receiver for decoding • a data destination • When a message is sent it goes through three levels of being: application, control and transmission • Protocols are sets of rules that outline the transfer of data between computers
  3. 3. • Handshaking is an agreement between computers as to which protocols to use • The speed of transmission is determined by the bandwidth. • Bandwidth is the capacity of a transmission medium. • The speed of transfer can be measured in baud rate or bits per second (bps) • Baud rate is the number of electrical signals that can be transmitted in a second, while bps is how many bits can be transmitted.
  4. 4. • One signal can contain multiple bits so bps can be greater than the baud rate • When data arrives at its destination it may contain errors so there needs to be ways of detecting these errors. • Three common error checking methods are: • parity checking • checksum • cyclic redundancy check (CRC)
  5. 5. • Parity checking involves adding an extra bit to a data packet that makes the total number of 0’s odd or even. • In even parity checking the total number of 0’s, including the parity bit, must be even; in odd parity it must be odd. • E.g. 1000111 is sent. If we are using even parity a 0 is added, if we are using odd a 1 is added.
  6. 6. • Practice: what would be the result if the data packet 1100110 is sent with even parity? • Answer: 11001100 • Practice 2: what would be the resulting transmission if 1100010 was sent with odd parity? • Answer 2: 11000100 • This simplistic method only works for an odd number of errors; an even number of errors cancel each other out.
  7. 7. • Checksum is a method of error checking that counts the number of bits in a data packet. • The count is attached to the data packet and then the packet is counted at the receiving end and if the two counts match then the message is accepted. • This method is about 90% effective.
  8. 8. • Cyclic redundancy check (CRC) is the method of checking using a division process. • The data is divided using a fixed number and the remainder is attached and sent with the data. • The same calculation is carried out at the receiving computer and the two remainders compared. • If the two remainders do not match then an error has occurred.
  9. 9. • CRC achieves 99.99% detection of errors. • There are several communication settings that are agreed upon during handshaking. • These include: • bits per second • data bits • parity • stop/start bits • flow control
  10. 10. • Bps is the speed of transmission. • Data bits are the number of bits sent in a data packet (usually 7 or 8 bits). • Parity is whether the data contains a parity bit for error checking. • Stop and start bits are used in asynchronous transmission to identify each byte. • Flow control is the type of software handshaking protocol used. [Complete L.A.’s 1-4, p.80]