ASYNCHRONOUS AND

SYNCHRONOUS

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Prepared by : Parackattu.Akhil.B

Dept : Electronic And Communication

NEED OF SYNCHRONIZATION.

• Whenever an electronic device transmits digital (and sometimes analogue) data to another electronic device.

• There must be a certain rhythm established between the two devices, i.e., the receiving device must have some way of knowing, within the context of the fluctuating signal that it's receiving.

• where each unit of data begins and where it ends.

• So there two types synchronization.

i. Asynchronous.

ii. Synchronous.

ASYNCHRONOUS.

• In asynchronous the transmission of data is generally without the use of an external clock signal, where data can be transmitted intermittently rather than in a steady stream.

• Any timing required to recover data from the communication symbols is encoded within the symbols.

• The most significant aspect of asynchronous communications is that data is not transmitted at regular intervals, thus making possible variable bit rate.

• And that the transmitter and receiver clock generators do not have to be exactly synchronized all the time.

PHYSICAL LAYER.

• In asynchronous serial communication the physical protocol layer, the data blocks are code words of a certain word length.

• For example octets (bytes) or ASCII characters, delimited by start bits and stop bits.

• A variable length space can be inserted between the code words. No bit synchronization signal is required.

• This is sometimes called character oriented communication.

TWO CATEGORIES OF ASYNCHRONOUS.

• Strobe Control.

• Handshaking Control.

Strobe control.

• This is one way of transfer i.e. by mean of strobe pulse supplied by one of the units to indicate to the unit when the transfer has to occur.

Handshaking control.

• This method is used to accompany each data item being transferred with a control signal. That indicates presence of data into the bus.

STROBE CONTROL METHOD.

• Strobe control method of data transfer uses single control signal for each transfer.

• The strobe may be activated by either source unit or destination unit

SourceUnit

Destination Unit

Data bus

Strobe

HANDSHAKING CONTROL METHOD.

• In case source unit the data transfer under the strobe. The source unit has no way of knowing whether destination unit has received the data or not

• Similarly destination has no way of knowing whether source unit has place data on the data bus or not

• So handshaking mechanism solve this problem by introducing second control signal that provides a reply to unit that initiates to transfer.

Sourceunit

DataUnit

Data valid

Data Bus

Data Accepted

BLOCK DIAGRAM.

sender stop Data Start Receiver

Here the data is transmitted intermittently rather than in a steady stream.

APPLICATION.

• Asynchronous activities take place outside of real time system.

• As asynchronous does not require a constant bit rate.

• For examples file transfer, email and the World Wide Web.

DRAWBACKS.

• Overhead of start and stop bits.

• Asynchronous transmission is relatively slow due to the increased number of bits and gaps.

SYNCHRONOUS.

• In Synchronous the transmission of data is sent in a continuous stream at a constant rate.

• Synchronous communication requires that the clocks in the transmitting and receiving devices are synchronized running at the same rate.

• so the receiver can sample the signal at the same time intervals used by the transmitter. No start or stop bits are required.

• For this reason synchronous communication permits more information to be passed over a circuit per unit time.

TWO CATEGORIES OF SYNCHRONOUS

• Byte oriented protocols

• Bit oriented protocols

Byte Oriented Protocol.

• Early synchronous protocols were byte-oriented protocols, where synchronization was maintained by transmitting a sequence of synchronous idle characters.

• when the line was not actively transmitting data or transparently within a long transmission block.

• This protocol was developed by IBM in late 1960.

BIT ORIENTED PROTOCOL.

• Bit-oriented protocols are synchronous protocols that view the transmitted data as a stream of bits with no semantics, or meaning.

• Control codes are defined in terms of bit sequences instead of characters. Synchronization is maintained on an idle line by transmitting a predefined sequence of bits.

• Synchronous Data Link Control (SDLC) specifies that a station continue transmitting a sequence of '1' bits on an idle line.

BLOCK DIAGRAM.

Sender DATA Receiver

Here data is transmitted in a steady stream.

• Over time the transmitting and receiving clocks will tend to drift apart, requiring resynchronization.

• Synchronous communication is direct communication that occurs in real time.

• That take place face-to-face, and as technology has evolved, can take place irrespective of distance

• (ex. telephone conversations and instant messaging)

APPLICATION.

• Most common use is in the ASCII terminals.

• High speeds communication links established using synchronous modems.

DRAWBACKS.

• The clock frequency should be same at both the sending and receiving ends.

• No tolerance in clock frequency is allowed.

THANK YOU…