Asynchronous and Synchronous Transmission, Lecture notes of Data Communication Systems and Computer Networks

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DATA

COMMUNICATION

Annie Sealy-Emmanuel

Data Communications –

What is it?

 The distance over which data moves within a computer may vary from a few thousandths of an inch, as is the case within a single IC chip, to as much as several feet along the backplane of the main circuit board  however, data must be sent beyond the local circuitry that constitutes a computer  Unfortunately, as the distance between the source of a message and its destination increases, accurate transmission becomes increasingly difficult

So !!! Finally, A Definition



Data Communications concerns the

transmission of digital messages to

devices external to the message source.

"External" devices are generally thought

of as being independently powered

circuitry that exists beyond the chassis

of a computer or other digital message

source

Communication Channel

 A communications channel is a pathway over which information can be conveyed.  It may be defined by a physical wire that connects communicating devices, or by a radio, laser, or other radiated energy source that has no obvious physical presence.  Information sent through a communications channel has a source from which the information originates, and a destination to which the information is delivered.

TRANSMISSION MODES The transmission of binary data across a link can be accomplished in either parallel or serial mode. In parallel mode, multiple bits are sent with each clock tick. In serial mode, 1 bit is sent with each clock tick. While there is only one way to send parallel data, there are three subclasses of serial transmission: asynchronous, synchronous, and isochronous.

Data transmission and modes

Serial transmission

Serial Communications

 Most digital messages are vastly longer than just a few bits. Because it is neither practical nor economic to transfer all bits of a long message simultaneously, the message is broken into smaller parts and transmitted sequentially  Bit-serial transmission conveys a message one bit at a time through a channel.  Each bit represents a part of the message.  The individual bits are then reassembled at the destination to compose the message.

Serial Communications



Parallel (Byte-serial) transmission

conveys eight bits at a time through

eight parallel channels.



Although the raw transfer rate is eight

times faster than in bit-serial

transmission, eight channels are needed

Asynchronous vs. Synchronous Transmission  One major difficulty in data transmission is that of synchronising the receiver with the sender  this is particularly true in serial data transfer, where the receiver must be able to detect the beginning of each new character in the bit stream being presented to it.  If it is unable to achieve this, it will not be able to interpret the incoming bit stream correctly.  Two approaches exist to solve the problem of synchronisation: these are asynchronous and synchronous transmission.

Asynchronous Transmission



The diagram below shows one common

system, using two start bits and one stop

bit, to transmit "ABC":

Asynchronous Waveform

Asynchronous here means “asynchronous at the byte level,” but the bits are still synchronized; their durations are the same. Note

Synchronous

Transmission

 Transmits the message via a single channel.  There is no control information associated with INDIVIDUAL characters.  Instead the characters are grouped together in blocks of some fixed size and each block transmitted is preceded by one or more special synchronisation characters, which can be recognised by the receiver.