Communication Systems 3.1) Characteristics of a Communication System.

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Presentation transcript:

Communication Systems 3.1) Characteristics of a Communication System

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)

Parity checking involves adding an extra bit to a data packet that makes the total number of 1’s odd or even. In even parity checking the total number of 1’s, including the parity bit, must be even; in odd parity it must be odd. E.g is sent. If we are using even parity a 0 is added, if we are using odd a 1 is added.

Practice: what would be the result if the data packet is sent with even parity? Answer: Practice 2: what would be the resulting transmission if was sent with odd parity? Answer 2: This simplistic method only works for an odd number of errors; an even number of errors cancel each other out.

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.

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.

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

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]