Synchronous - Synchronous - AsynchronousAsynchronous

Data TransmissionData Transmission

AsynchronousAsynchronous

►The sender and receiver are not The sender and receiver are not Synchronised.Synchronised.

►The sender sends only one The sender sends only one character at a time.character at a time.

AsynchronousAsynchronous

►Each character needs a start bit Each character needs a start bit and a stop bit. and a stop bit.

►There can be idle time between There can be idle time between each charactereach character..

AsynchronousAsynchronous

► It is a slow and inefficient method of It is a slow and inefficient method of data transmission.data transmission.

► It is an inexpensive method for low It is an inexpensive method for low speed transmission.speed transmission.

SynchronousSynchronous

SynchronousSynchronous

►The senders and the receivers clocks The senders and the receivers clocks are synchronised.are synchronised.

►The sender sends a packet of data at a The sender sends a packet of data at a time.time.

►Synchronisation is achieved by Synchronisation is achieved by sending a ‘start’ frame and a ‘stop’ sending a ‘start’ frame and a ‘stop’ frame.frame.

SynchronousSynchronous

► It is a more efficient method of It is a more efficient method of transmission as only a start and stop transmission as only a start and stop frame are required with up to 8Kb of frame are required with up to 8Kb of data in the packet of data.data in the packet of data.

►Asynchronous has a much higher Asynchronous has a much higher overhead.overhead.

StopStop Data up to 8 KbData up to 8 Kb StartStart

FrameFrame FrameFrame

Parity CheckParity Check

►Used in Asynchronous transmission.Used in Asynchronous transmission.►Detects any errors in each character Detects any errors in each character

being sent.being sent.

Parity CheckParity Check

►A bit in each byte is set aside as the A bit in each byte is set aside as the parity bit.parity bit.

► In even parity, the bit is set to 1 or 0 to In even parity, the bit is set to 1 or 0 to ensure an even number of 1’s.ensure an even number of 1’s.

► In odd parity, the bit is set to 1 or 0 to In odd parity, the bit is set to 1 or 0 to ensure and odd number of 1’s.ensure and odd number of 1’s.

►The receiving system checks the The receiving system checks the system being used and the number of system being used and the number of 1’s1’s

ChecksumChecksum

►Used in Asynchronous Transmission.Used in Asynchronous Transmission.►Checks errors in each packet of Data Checks errors in each packet of Data

being sent.being sent.

ChecksumChecksum

►The checksum is generate by treating all The checksum is generate by treating all the bytes in the transmission as a number.the bytes in the transmission as a number.

► It adds up all the bytes in the block.It adds up all the bytes in the block.►The checksum is attached to the block and The checksum is attached to the block and

transmitted along with the data.transmitted along with the data.►The receiver does the same calculation The receiver does the same calculation

and if the checksums are the same the and if the checksums are the same the data was transmitted correctly, if not it’s data was transmitted correctly, if not it’s re-sent.re-sent.

Cyclic Redundancy CheckCyclic Redundancy Check

► The difference here is that the block of data The difference here is that the block of data is treated as one (very large) binary is treated as one (very large) binary number.number.

► This is then divided by an Integer agreed This is then divided by an Integer agreed between the sender and receiver.between the sender and receiver.

► The The Remainder only Remainder only from the calculation is from the calculation is sent along with the data.sent along with the data.

► The receiver can perform the same The receiver can perform the same calculation and request re-transmission if calculation and request re-transmission if there is a discrepancy.there is a discrepancy.