Communication SystemsCommunication Systems

3.1) Characteristics of a Communication System

• 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

• Protocols are sets of rules that outline the transfer of data between computers

• 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.

• 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)

• 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. 1000111 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 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.

• 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]