The Principle of Electronic DataThe Principle of Electronic DataSerial and Parallel Data Serial and Parallel Data CommunicationCommunicationTransmission RateTransmission RateBandwidthBandwidth Bit RateBit Rate Parity bitsParity bits

The Principle of Electronic The Principle of Electronic DataData

Involves sending and receiving Involves sending and receiving information data from one computer or information data from one computer or data processing device to another.data processing device to another.

Examples

E-mail, Supermarket EPOS (Electronic Point of Sale) Terminals, Cash Dispensers, Facsimile

Video Conferencing.

Serial Data Serial Data Communication.Communication.

Serial Data Transmission: bits are Serial Data Transmission: bits are sent via an interface one bit at a sent via an interface one bit at a time over a single wire, from the time over a single wire, from the source to the destination. source to the destination.

0 1 0 1 1 0

Serial Data Serial Data Communication.Communication.

(An example of a very high data (An example of a very high data transfer rate is through the use of transfer rate is through the use of fibre-optic cable, which has a data fibre-optic cable, which has a data transfer rate of 1000Mbs per transfer rate of 1000Mbs per second.) second.)

Parallel Data Parallel Data Communication.Communication.

Parallel Data transmission is used Parallel Data transmission is used inside the computer (using various inside the computer (using various computer buses) and for short computer buses) and for short distances (a few metres), so to distances (a few metres), so to prevent data being misread. prevent data being misread.

0

1

0

1

1

0

Parallel Data Parallel Data Communication.Communication.

A parallel port can send 8, 16, or A parallel port can send 8, 16, or 32 bits simultaneously down 32 bits simultaneously down separate lines. separate lines.

A printer uses the parallel port.A printer uses the parallel port.

Parallel Data Parallel Data Communication.Communication.

Limited to short distances of a few metres, Limited to short distances of a few metres, e.g. between a computer and printer.e.g. between a computer and printer.

Limitation because it is very difficult to Limitation because it is very difficult to produce copper wire with the exact same produce copper wire with the exact same resistance. resistance.

Simultaneous transmission of data over Simultaneous transmission of data over distances can result in skewing of data distances can result in skewing of data (bits not arriving in parallel) due to varying (bits not arriving in parallel) due to varying resistance of wires.resistance of wires.

Transmission RateTransmission Rate

The Baud Rate is named after Baudot The Baud Rate is named after Baudot who did a lot of work with electronic who did a lot of work with electronic signals.signals.

The Baud Rate is the rate that the The Baud Rate is the rate that the voltage changes and the speed at voltage changes and the speed at which the data is transmitted which the data is transmitted serially. serially.

0 1 0 1 1 0

Transmission RateTransmission Rate

The sender will generate one voltage The sender will generate one voltage to represent 1 and a different voltage to represent 1 and a different voltage to represent 0. to represent 0.

The sender has a clock which The sender has a clock which transmits bits at regular intervals. transmits bits at regular intervals.

At any point in time there will be a At any point in time there will be a number of bits being carried along the number of bits being carried along the wire. wire.

Generally one baud is one bit per Generally one baud is one bit per second.second.

Baud RateBaud Rate

Framing bits have to be transmitted with Framing bits have to be transmitted with each character to provide each character to provide – start and stop bits, start and stop bits, – error detection error detection – and communication controls.and communication controls.

A baud rate 56K does not mean that 7000 characters per second will be transmitted: the figure will be lower.

Why?

BandwidthBandwidth Bandwidth measures signal changes Bandwidth measures signal changes

per second, and is measured in Baud. per second, and is measured in Baud.

When a signal is transmitted along a wire (or any other medium) it is reluctant to travel as a square since waves are naturally in the form of sine waves (analogue signals).

BandwidthBandwidth

High Frequency Low Frequency

•All media are capable of transmitting a set of waves All media are capable of transmitting a set of waves that have a range of frequencies. that have a range of frequencies.

•The frequency of the wave is the rate at which the wave The frequency of the wave is the rate at which the wave repeats itself.repeats itself.

The higher the

bandwidth the greater the amount of data that

can be transmitted

.

BandwidthBandwidth

The range of the frequencies that a The range of the frequencies that a medium can transmit is the medium can transmit is the bandwidth.bandwidth.

If the bandwidth is wide then more If the bandwidth is wide then more

data can be transmitted.data can be transmitted.– A normal telephone wire has low bandwidth so A normal telephone wire has low bandwidth so

it is not possible to transmit large amounts of it is not possible to transmit large amounts of data.data.

– An optical fibre has a high bandwidth and is An optical fibre has a high bandwidth and is able to transmit large amounts of data.able to transmit large amounts of data.

Bit RateBit Rate

Is the term given to the rate that Is the term given to the rate that the bits are transmitted.the bits are transmitted.

RememberRemember the baud is the rate the baud is the rate the voltage changes.the voltage changes.

If the voltage changes 10 times every If the voltage changes 10 times every second the baud is said to be ten.second the baud is said to be ten.

Bit RateBit Rate

If four voltages were generated If four voltages were generated instead of two, each change in signal instead of two, each change in signal could be used to represent two bits. could be used to represent two bits.

Therefore, if the signals are Therefore, if the signals are transmitted 10 times a second, the transmitted 10 times a second, the baud rate is still 10 but 20 bits per baud rate is still 10 but 20 bits per second are being transmitted.second are being transmitted.

RememberRemember

Baud Rate Baud Rate refers to the rate that refers to the rate that the signals are transmitted along a the signals are transmitted along a wire.wire.

Bit Rate Bit Rate refers to the rate that bits refers to the rate that bits are transmitted.are transmitted.

Parity bitsParity bits

A parity bit is an extra bit added to the A parity bit is an extra bit added to the character code character code

Set to 0 or 1 Set to 0 or 1 so thatso that total number of 1’s in code is total number of 1’s in code is

even, for even parityeven, for even parity or or total number of 1’s in code is odd, total number of 1’s in code is odd,

for odd parity.for odd parity.

Parity bitsParity bits

Parity checks are used to determine Parity checks are used to determine whether the parity of a data item is correct whether the parity of a data item is correct when it is being sent to or from an input, when it is being sent to or from an input, output or storage device, or being sent output or storage device, or being sent over a communications line. over a communications line.

Odd parity will detect the failure of a Odd parity will detect the failure of a transmission line, because all the bits will transmission line, because all the bits will then be set to zero. then be set to zero. 

Note that the character code, including the Note that the character code, including the parity bit, is an example of a parity bit, is an example of a self checking self checking codecode..

Parity bitsParity bits

Add a parity bit to the end (LHS) of each of the Add a parity bit to the end (LHS) of each of the following codes to maintain odd parity:following codes to maintain odd parity:

110101110101 001101 001101

Remember:Remember:– Set to 0 or 1 Set to 0 or 1 – so thatso that total number of 1’s in code is even, for even parity total number of 1’s in code is even, for even parity– or or total number of 1’s in code is odd, for odd parity.total number of 1’s in code is odd, for odd parity.

Parity bitsParity bits

The ASCII codes forThe ASCII codes for P is 1010000P is 1010000 Q is 1010001Q is 1010001 In an even parity transmission system what will In an even parity transmission system what will

be the value and parity bit for P and Q?be the value and parity bit for P and Q?

Synchronous and Asynchronous Synchronous and Asynchronous transmissiontransmission

Two types of serial mode for sending dataTwo types of serial mode for sending dataAsynchronous transmissionAsynchronous transmission one character at a time, with a one character at a time, with a start and stop bit and parity start and stop bit and parity

bitbit sent with each character. sent with each character. For each character sent, 10 bits are transmittedFor each character sent, 10 bits are transmitted Start bit alerts the receiving device and synchronises the clock Start bit alerts the receiving device and synchronises the clock

inside the receiver ready to receive the character.inside the receiver ready to receive the character. Baud rate at receiving end has to be set up be same as Baud rate at receiving end has to be set up be same as

senders.senders. Usually used by PC’s, fast and economical for relatively small Usually used by PC’s, fast and economical for relatively small

amounts of data.amounts of data.

Synchronous transmissionSynchronous transmission

Enables whole blocks of data to be sent in timed Enables whole blocks of data to be sent in timed sequences sequences

Timing signals (usually computer’s internal clock) Timing signals (usually computer’s internal clock) control rate of transmissioncontrol rate of transmission

There is no need for start and stop bits to There is no need for start and stop bits to accompany each character.accompany each character.

System is less error prone than asynchronous System is less error prone than asynchronous Much faster – usually used on mainframe computersMuch faster – usually used on mainframe computers

HandshakingHandshaking

The exchange of signals between devices to establish The exchange of signals between devices to establish their readiness to send or receive data.their readiness to send or receive data.– E.g. between a computer and printerE.g. between a computer and printer

One method of ensuring both sender and receiver are One method of ensuring both sender and receiver are ready before transmission beginsready before transmission begins

The conversation between two devices along lines of:-The conversation between two devices along lines of:-– Device 1:Device 1: “Are you ready to receive some data?” “Are you ready to receive some data?”– Device 2:Device 2: “Yes, go ahead.” “Yes, go ahead.”– Device 1:Device 1: (sends data) (sends data)– Device 2:Device 2: “Message received, thanks!” “Message received, thanks!”